Basement Tectonics 13: Proceedings of the Thirteenth International Conference on Basement Tectonics...

VOLUME 7
The titles published in this series are listed at the end of this volume.
BASEMENT TECTONICS 13
held in Blacksburg, Virginia, U .S.A., June 1997
Edited by
SPRINGER SCIENCE+BUSINESS MEDIA, B.V.
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-94-010-6015-8 ISBN 978-94-011-4800-9 (eBook) DOI 10.1007/978-94-011-4800-9
Cover: Schematic cross section of the Grenville basement in the Blue Ridge of Virginia (from Sinha and Bartholomew, 1984 in The Grenville Event in the Appalachians, Geological Society of America Special Paper 194)
Printed on acid-free paper
Originally published by Kluwer Academic Publishers in 1999 Softcover reprint ofthe hardcover Ist edition 1999
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CONTENTS
Geotectonics and Characteristic Features of Fertile and Non-Fertile Younger Granites, Eastern Desert, Egypt F.S. Bakhit, H.A. Hussein, M.M. Ali ................................................................... 1
Structural Associations of the Basement and Sedimentary Cover of the Georgian Part of the Caucasus L.Basheleishvili .................................................... ........................................... 25
The Eastern Edge of the Rio de la Plata Craton: A History of Tangential Collisions N. Campal, A.Schipilov ....................................................... ............................. 33
New Data and Interpretations for the Precambrian, Midcontinent USA M.P. Car/son, S.B.Treves, R.J. Goble, A. Xu ...................................................... .49
Allochthonous Units in the Variscan Belt of NW Iberia: Terranes and Accretionary History l.R.Martinez Catalan, R.Arenas, F.Diaz Garcia, l.Abati .......................................... 65
Early Compression and Late Dextral Transpression Within the Grenvillian Event of the Hudson Highlands, NY, USA A.E. Gates .... ................................................................................................... 85
Crust-Forming Processes: Basement and Basements in the Southern Appalachian Orogen R.D. Hatcher ................................................................................................... 99
Petrogenesis of Pan-African Granitoids, Gabal Hamra Area, Southwestern Sinai, Egypt H.A.Hussien, A.G.El Shazly, I.E. El Assy, M.M. El Galy .................................... 119
Geology of the Blue River Gneiss, Eastern Arbuckle Mountains, Oklahoma E.G. Lidiak, R.E. Denison ............................................................................... 139
vi
Zircon Ages of Basement Orthogneisses from the Northern Segment of the Araguaia Belt, Brazil C.A. V.Moura, H.E.Gaudette ............................................................................ 155
Damodar Graben - A Centre of Contrasting Magmatism in the Eastern Indian Shield Margin D.Mukherjee, N.C.Ghose ................................................................................ 179
A New Tectonic Belt in the Baltic Sea Region - Farther Interpretation of the Deep Seismic Results from the "Baltic Sea" and "Babel-B" Profiles A.A. Ostrovsky ............................................................................................. 203
A Geochemical Reconnaissance of the Roseland Anorthosite Complex, Virginia, and Comparisons with Andesine Anorthosites from the Grenville Province, Quebec B.E.Owens, R.F.Dymek .................................................................................. 217
Basement Tectonic Structures Delineated from Aeromagnetic Survey Data South Eastern Desert, Egypt S.I. Rabie, A.F. Khalil.. ................................................................................. 233
Appalachians in the Time Interval Between the Grenville Orogeny and Variscan Collision N.Rast, J. W.Skehan ....................................................................................... 257
Geodynamics of the Early Precambrian: Evidence for the Baltic Shield E. V.Sharkov ................................................................................................. 277
Lead Isotope Mapping of Crustal Reservoirs Within the Grenville Superterrane: II. Adirondack Massif, New York A.K.Sinha, J.M.McLelland .............................................................................. 297
Paleomagnetism of the Boot-Phantom Pluton and the Amalgamation of the Juvenile Domains in the Paleoproterozoic Trans-Hudson Orogen, Canada D. T.A.Symons, C.D.MacKay .......................................................................... 313
Dengying Formation Gas System of the Sichuan Basin, Southwest China: Model for Precambrian Indigenous Hydrocarbon Accumulation G.Zhang, A.B.Dickas, J.Song ...... .................................................................... 333
Morpholog~ and Isotopic Age of Zircons from Shear-Zones Within Granitoids of the Belomorian Tectonic Zone, Baltic Shield, Russia T.F.Zinger, V.S.Baikova, B. V.Belyatsky, S. V. Klepinin, J.Gotze, O.A.Levchenkov, I. K. Shuleshko .... ........................................................................................... 3 4 5
TRUSTEES International Basement Tectonics Association, Inc.
Chief Trustee Chairman of the Board of Trustees
M.Charles Gilbert School of Geology and Geophysics
University of Oklahoma Norman, OK 73019 USA
Trustee Treasurer
Urbana, IL 61801 USA
Virginia Polytechnic Institute and State University Blacksburg, VA 24061 USA
Trustee Marvin Carlson
Lincoln, NE 58588 USA
Past Chief Trustee MJ. Bartholomew
Earth Sciences and Resources Institute University of South Carolina Columbia, SC 29208 USA
vii
FOREWORD
The Thirteenth International Conference on Basement Tectonics was held on the campus of Virginia Polytechnic Institute and State University in Blacksburg, Virginia from June 2 - 6, 1997. The oral presentations and discussions over three days covered a wide range of topics, and provided the international audience with a perspective on scientific efforts underway around the world. The conference participants were able to attend two separate field trips: (I) a pre-conference trip guided by Professor Robert Hatcher of the University of Tennessee, Knoxville, examined the Basement rocks in the North Carolina - Tennessee region of the Appalachian Mountains, and (2) a mid-conference field trip guided by A.K. Sinha, convener of the conference, allowed participants to examine the complex rock associations and structures of the> 1000 m.y. old basement rocks in Virginia. Both the field trip guidebooks and abstract volumes were published for the conference.
The meeting brought together scientists from more than 14 countries. Their participation, and the fiscal success of the meeting would not have been possible without the support of the Department of Geological Sciences, the College of Arts and Sciences (VPI&SU) and the Basement Tectonics Association. Their support is gratefully acknowledged.
As Chairman of the Organizing Committee, I would like to thank Margie Sentelle, Jay Thomas, Peter Welch, and Barry Robinson for the smooth operation of the conference. Jyl Smithson - Riehl, Assistant Director of Program Development at VPI&SU provided the support necessary to host an International meeting in a university setting. Kim Bevan's ability to assemble and edit the manuscripts that constitute this volume is gratefully acknowledged. On behalf of the participants, I also thank the Trustees of the International Basement Tectonics Association, Inc. for their permission to hold the conference in Blacksburg. Finally, a thank you to all the participants who have provided the manuscripts for this volume.
A. Krishna Sinha, Convener Professor of Geology Virginia Polytechnic Institute and State University Blacksburg, VA 24061
ix
GEOTECTONICS AND CHARACTERISTIC FEATURES OF FERTILE AND NON-FERTILE YOUNGER GRANITES, EASTERN DESERT, EGYPT
F.S. Bakhit, H.A. Hussein and M.M. Ali Nuclear Materials Authority, Cairo, Egypt. P.O. Box: 530 EI-Maadi - Cairo
Abstract: The present article is concerned with the geochemistry of red-pink granites in the Late Orogenic Plutonites and their relationship with uranium content in order to discriminate the fertile from non-fertile types. Chemical analysis has been carried out, including the major oxides, trace elements and REE, to detennine their geotectonic affinity, and the relationship between U and major oxides, trace elements and rare earth elements.
Geotectonically, the study revealed that the post orogenic red-pink granites are either within-plate granites or volcanic arc granites. Chemically, the granites are subalkaline and peraluminous to metaluminous. Also, the younger granites occur in three phases. The highly differentiated phase is the fertile type which is characterized by high silica content, with respect to alumina, impoverished in calcium, as well as in ferromagnesian elements (Fe203+MgO) and is enriched in alkali elements (Na20+K20). Moreover, the fertile type ;s characterized by high Rb and low Sr. Also, direct relation between Si02 and Rb is obtained, while with Ba and Sr do not correlated well. The REE illustrate that the fertile granites possess high negative Eu value relative to the normal type.
The relationship between U and La, Ce, Sm, Pb, Rb, Zr, F and Mo was found to be direct relation, while Co, Sr, Eu, and Ba was found to be a weak correlation ..
1. Introduction
The study area including six younger granitic plutons namely; Gabal Hadrabiya, Gabal EI Dob, Gabal El Urf, Gabal El Erediya, Gabal EI Missikat aild Gabal Kab Amiri. They lie within the limits oflatitudes 26 18' and 26 43' N and longitudes 33 IS' and 33 40' E in the Central Eastern Desert of Egypt. These six granite plutons lie just at the north and south the midway of Qena-Safaga road (Fig. I ).
Aerial radiometric and magnetic surveys were carried out for an area covering about 4500 km" including the six mentioned granitic plutons in the studied area (Fig.2). The aerial survey indicated the presence of significant radioactive anomalies scattered in the area (Ammar, 1973). The subsequent field "'ork for ground checking and follo,\ing these airborne anomalies was carried out and some highly radioactiyc occurrences with visible uranium mineralization were discovered ( EI-Kassas, 1974 ; Bakhit, 1978).
A. K. Sinha (ed.), Basement Tectonics 13, 1-24. @ 1999 Kluwer Academic Publishers.
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G E
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4 F. S. BAKHIT ET AL.
Detailed studies showed that the highly radioactive bodies display great similarities in their type of mineralization, host rocks, country rocks and structures. The general geology, structure and radioactivity are discussed in previous works by El-Shazly et al. (1981), Bakhit, (1978), Bakhit and Guirguis (1983), Bakhit and Mostafa (1987), Bakhit et al.(1985), Bakhit and El-Kassas (1989), Abu Dief(1985), El-Kassas and Bakhit (1989 and 1992), EI-Tahir (1985), Hussein et al. (1986), Meleik and Bakhit (1990), Abdel Monem et al. (1990) and Bakhit and El-Kassas (1992).
The area surrounding these granitic plutons is an active region where different mineralized deposits of copper, gold, molybdenite, wolframite, iron-ores, tin and tungsten occur ( El-Kassas and Bakhit, 1983).
The aerial radiometric survey revealed the presence of different radioactive levels among the scattered younger granitic plutons. The different radiometric levels attracted the authors' attention to study this phenomenon. Many authors attributed differences to magmatic differentiation. The main goal of this study is to explore the exact fertile phase among the other different phases of younger granites.
2. Geology and Structure
The selected six younger granite masses were emplaced during the post-tectonic episode in Egypt, 603-575 Ma (Greenberg, 1981). They were emplaced near the northern edge of the central tectonic block of the Eastern Desert where rocks with strong oceanic affinity are exposed (Stem, 1981 and Stem et al., 1984). The central tectonic block is characterized by the presence of linear belts of ultramafics and serpentinites that are interpreted as suture zones resulting from island-arc-continental collision (Shackleton et al., 1980; EI-Bayoumi, 1984; Hassanain, 1984 and Ries et al., 1985). The presence of domal structures encompassing large crustal sections (Migif Hafafit and Meatiq gneiss dome) are characteristic features indicative of convergent continental margin and collisional tectonics (Bentor, 1985 and Kroner, 1985). The six younger granite masses are intruded into basement rocks comprising parts of metamorphosed ophiolite-island-arc sequences (Fig. 1). Gabal El-Missikat and Gabal EI-Erediya are of the six granite masses associated with uranium-mineralization.
Structurally, the position of the radioactive chalcedony and jasperoid veins is determined by open spaces along rock contact. The uplift of igneous intrusive masses at the contact with the country rocks may have resulted in mineralization as has been documented throughout the Eastern Desert (such as at Gabal El-Missikat and Gabal EI-Erediya), which is one of the most important structural control in the Eastern Desert (Bakhit, 1987). Dominant trends of the uranium-bearing veins are in the northeast to easterly direction such as in Gabal El-Misslkat (Bakhit, 1978) and less pronounced in the N-S and/or NNE as in Gabal El-Erediya (El-Kassas, 1974), and as fracture - filling in the NW and NE direction in Gabal Kab Amiri (Bakhit, 1987). The different mineralizations of fissure-filling or shear zones occur in three main sets of fractures, mainly in the NW-SE. ENE-WSW and N-S directions ( EI-Kassas and Bakhit,1983).
FERTILE AND NON-FERTILE YOUNGER GRANITES
None of these uranium deposits occur far away from the edge of the granite bodies. Moreover, they invariably lie within 50-200 meters of the contact (Bakhit, 1987). Also, veins containing uranium are commonly deformed during the introduction of gangue and ore deposits. These processes produced a series of small-scale fractures or breccias that may have localized any late uranium minerals.
3. Geochemistry
3.1. CHEMICAL CLASSIFICATION
Twenty representative samples were analyzed for major, trace and rare earth elements. The results of the chemical analysis of the examined granites are listed in Table (1), together with CIPW-norm values. The data are compared with corresponding geochemical data of the Egyptian younger granites reported by Ali and Mostafa (1984), Greenberg (1981), EI-Gaby (1975), as well as the World wide average for granitic rocks ofLe Maitre (1976), Table (2).
3.2. ANALYTICAL TECHNIQUE
Quantitative chemical analysis for major oxides were carried out following the method of Shapiro and Branock (1962). Si~, Al2~' Fe2~, Ti~, P20 S and MnO were determined spectrophotometrically. Na20 and K20 were determined by flame photometry. CaO and MgO were determined using EDT A titeration and FeO was determined volumetrically. Uranium was determined using laser fluourometric technique which allow the determination of uranium with high accuracy. Fluorine and chlorine were determined using chromatography technique.
The other trace elements were determined by inductively couple plasma emission spectroscopy which allow simultaneous multi-elements analysis using HF and H2S04
opening technique. The analytical system used was Philips PV 821d. (1.5 m airpath spectrometer) linked with Philips 852 computer. The standards G-2 and GSPI were used for comparison with the data obtained
3.3. MAGMA TYPE
- The An-Ab-Or ternary diagram (Fig.3 a) shows that these rocks fall within the field of granite (O'Connor, 1965 and Barker, 1979).
- On the alkali-silica diagram (Fig.3 b) ofIrvine and Baragar (1971), the examined granitic rocks are subalkaline.
- The AFM diagram (Fig.3 c) shows that these rock samples are alkali rich and magnesium-iron poor. On the same diagram and according to the dividing line of Irvine and Baragar (1971) they are of calc-alkaline affinity.
- On Al20 3 / (CaO+NazO+K20) versus Al20 3 / (Na20+K20) molar diagram (Maniar and Piccoli, 1989) these granitic rocks are of peraluminous to metaluminous nature (Fig.3 d).
5
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50 5581.111 75 lOIS SiG2Iwt·'.)
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3~------~--------------~
~l~----~~~--__________ ~ •• ct
Figure 3.
a. AN-Ab-Orternary diagram (O'Conner, 1965 and Barker, 1979). b. SiOz vs NazO+KzO diagram (Irvine and Baragar, 1971). c. AFM ternary diagram (dashed curve after Irvine and Baragar, 1971). d. AlzO/(KzO+Nazo+caO) vs AlzOf{NazO+KzO) diagram
(Maniar and Piccoli, 1989). e. Rb-Ba-Sr ternary diagram (HI bouseily and EI Sukkary, 1975)
A. Highly differentiated granites. B. Normal granites. C &. F. Anomalous granites. D. Granodiorites. E. Quartz-diorites. --+ Differentiated trend.
9
10 F. s. BAKHIT ET AL.
- Rb-Sr-Ba ternary diagram (EI-Bouseily and EI-Sukkary, 1975) indicates that the studied granitic masses fall within the nonnal granites and anomalous granites field "F" (Fig.3 e). Table (1) shows the ratios of Na20/CaO, Na20IK20, MgOlFeO*, MgOIMnO,
AhOy'(Na20+K20r mol. Granitoid rocks are described (using Shand's index) as peraluminous; AlCNK 1.0, Metaluminous; AINK 1.0 and AlCNK 1.0 and peralkaline AINK 1.0, with all ratios being molar ratios and A=Alz03, C=CaO, N=Na20 and K=K20. The studied granitic masses are of peraluminous and metaluminous nature as indicated by Shand's index.
CIPW-norms (Table 1) illustrate that the fertile granites contain very little amounts of fluorite dispersed in the granite which ranges from 0.01 to 0.09 %, while the non fertile type does not have this phenomenon. Also, the fertile type contains high zircon content relative to the non-fertile type as shown in Table ( 1 ). In addition, the fertile granite is characterized by large negative Eu anomalies, and high REE as compared with the non-fertile type (Table 1). On the other hand, some of the trace and rare earth elements can be used as indicators for the fertile type, such as La, Sm, Zr, F, Pb and Mo which fonn direct relation with uranium content while with Co, Sr, Eu, and Ba have not well correlation. Also, the ratio of LaNIYbs in the fertile granite is markedly higher than that in the non-fertile granite as shown in Table ( 1 ).
3.4. TECTONIC SETTING
The tectonic setting of the studied granitoids could be deduced from the following diagrams: - Si02 versus K20 diagram (Fig.4 a) ofManiar and Piccoli (1989) indicates that the
studied granitic rocks fall in the field of island arc granitoids (lAG), continental arc granitoids (CAG), continental collision granitoids (CCG), continental epeirogenic uplift granitoids (CEUG), and post orogenic granitoids (POG).
- Si02 versus FeO*/( FeO*+MgO) diagram (Fig.4b) of Maniar and Piccoli (1989) shows that the analyzed samples fall in the field of post orogenic granitoids (POG) and three samples fall in rift-related granites and continental epeirogenic uplift granitoids (RRG & CEUG).
- Al20 3 I(CaO+Na20+K20) versus Al2031'(Na20+K20) molar diagram (Fig.4c) of Maniar and Piccoli (1989) illustrates that the granitic rocks fall within the field of post orogenic granitoids (POG), continental arc granitoids (CAG) and continental collision granitoids (CCG). '
Greenberg (1981) reported that the Egyptian granites are mildly peralkaline and according to Maniar and Piccoli (1989), the studied granitic masses are post orogenic granitoids. - Si~: versus Ah03 diagram (Fig.4 d) of Maniar and Piccoli (1989) indicates that the
studied granitic samples fall in the field of post orogenic granites (POG). - Y+Nb versus Rb diagram (Fig. Sa) of Pearce et al. (1984) shows that the studied
granitic samples fall in the field of within plate granites (WPG).
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- Y versus Nb diagram (Fig.5 b) of Pearce et aI. (1984) illustrates that these granitic masses faIl within the field of within plate granites (WPG) and two samples in the field of volcanic arc granites (VAG) and syn-collision granites (syn-COLG).
- The plot diagram (Fig.Sc) Rl = [4Si-ll(Na+K)-2(Fe+Ti)] versus R2 = (6Ca+2Mg+Al) of Patchelor and Bowden (1985) shows that the studied granitic masses fall within the field of post orogenic granites.
Post orogenic granitoids are intruded during the end of an orogeny, generally, after regional deformation has ceased. These granitoid rocks are associated with the post orogeny in space and time (Maniar and Piccoli, 1989). It has been suggested (Rogers and Greenberg, 1981 a and b) that they represent the transitional phase of the continental crust undergoing stabilization follo\\ing the orogeny. - Rb versus Sr diagram (Fig.5 d) of Condie (1973) indicates that all twenty samples of
the six granitic plutons fall within the field of thickness ranging from 20-30 kIn. This conclusion is in concordance with many authors (e.g. Takla et al., 1991 and El Nashar, 1991).
3.5. REE AND TRACE ELEMENTS GEOCHEMISTRY
Post-magmatic, deuteric and hydrothermal processes are shO\\on to have great effect on the distribution of REE, because the fluids generated at such stages are highly enriched in complexing agents as C03 -, cr, and F -anions. Fluids rich in cr -anions are capable of enriching LREE (La-Sm), whereas fluids rich in C03 - groups are accompanied by extreme HREE enrichment (Tb-Lu) (Maclennan and Taylor, 1979). Mineyev (1963), suggested that although the REE group has similar ionic species, they form at high pressure complexes in the form ofNa (REE) F4 , with the stability of the HREE-complexes being greater than those for the LREE. Under such conditions fluoride-rich fluids could deplete the rock in HREE. Thus the application of REE geochemistry to hydrothermal systems may be useful to elucidate its chemical history as well as the mechanism responsible for metal transport.
It is generally recognized that a proportion of the REE do not substitute in trace amounts in essential minerals in granites (McCarthy and Robb, 1978). Jefferies (1985) demonstrated that monazite is the principal host for LREE in biotite granite. HREE are strongly partitioned into the accessory minerals such as xenotime, apatite and zircon. The previously mentioned accessory minerals are present in the six studied granitic plutons in very small variable amounts. The behavior of the REE during granite differentiation is controlled by the behavior of the radioactive accessory minerals, (Jefferies, 1985). Pagel (1982) mentioned that sphene plays a major role in controlling the REE geochemistry. Also, thorite has little effect on the geochemistry of REE compared with sphene or allanite.
The transfer of uranium from the more refractory accessory minerals to uraninite during deuteric evolution is a critical step which not only increases the uranium coment in the altered rocks, but also makes the uranium more readily leachable by later hydrothermal solutions (Cuney, 1978). The uraninite crystals are stable in surface samples, as occur in EI-Erediya occurrence, probably due to high thorium content.
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The data of the REE elements are listed in Table ( 1 ). The chondrite-normalized rare earth elements pattern for the fresh samples of the six granitic masses, are shown in Figure (6).
The average total REE content for Gabal Hadrabiya, Gabal EI-Dob, Gabal EI-Urf, Gabal EI-Erediya, Gabal El-Missikat and Gabal Kab Amiri are 272.4,239.4,236,401, 430.1 and 318.9 ppm respectively as shown in Table (1). The first three granitic masses are close to the average of the nonnal granite (Hermann, 1970). The chondrite nonnalized pattern (Fig. 6) indicates that the granitic rocks are more enriched in LREE than HREE with average LawYbN ratios 5.94, 4.99, 5.43, 8.80,6.60, and 6.12; namely for Gabal Hadrabiya, Gabal EI-Dob, Gabal EI-Urf, Gabal EI-Erediya, Gabal El-Missikat and Gabal Kab Amiri respectively. Table (1) shows that the REE content is generally less than 1 percent for all the studied granitic masses.
Figure (6) shows that the fertile granitic masses of Gabal EI-Erediya, Gabal EI Missikat and Gabal Kab Amiri have higher LREE content than the non-fertile granite masses, namely; Gabal Hadrabiya, Gabal EI-Dob and Gabal EI-Urf. In addition, the above mentioned fertile granitic masses have large negative Eu anomalies, as compared with the non-fertile granitic masses. This illustrates that the fertile granites are highly differentiated. Eu is depleted in this differentiation between level 8-1 ~ resulting in a decrease in the negative Eu anomaly (which is conventionally expressed as EulEu·) from 0.21 to 1.3.
3.5.1. Ba, Rb and Sr Relationships
Since the only one major element of comparable ionic size to barium is potassium, barium is included as a trace element in K-feldspar and biotite. The rules of trace element distribution in igneous processes (Onuma et al., 1969) predict that Rb should replace K. Rubidium forms no mineral of its own, being always incorporated in potassic minerals; in granitic rocks it is in muscovite, biotite, and K-feldspar. The size of strontium ion indicates that it can proxy for either calcium or potassium, being partitioned most strongly into calcic minerals. Mason and Moore (1982) indicated that strontium in igneous rocks is present mostly in plagioclase and K-feldspars.
The barium, rubidium, and strontium contents in granitic rocks were examined on a variation diagram (Fig. 7 a). In figure (7 a), silica content increases from 72.3 to 74.8; the sum of the other oxides must then decrease from 27.7 to 25.2 % and if no differentiation occurs except for the increase in silica, each oxide should diminish in this ratio. In other words, Just from the method of plotting, we should expect each oxide to show a decrease from left to right 72.2% of its initial concentration; the reactions that lead to differentiation will be significant only if they cause a markedly smaller or larger change. Figure (7 a) shows a narrow range of SiCh content ranging between 72.2 and 74.8 % without obvious relation with Ba and Sr while the value of Si02 increases by the increase of Rb.
Inspection of the diagrams show that Gabal EI-Erediya, Gabal EI-Missikat and Gabal Kab Amiri possess high Rb and high silica content as compared with the other
FERTILE AND NON-FERTILE YOUNGER GRANlTES 15
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a Gabal Hadrabiya ~ b. Gabal El Dob "" .."
C. Gabal El Urf 3 ~
d. Gabal Kab Amiri
~ e. Gabal El-Missikat f. Gabal El-&cdiya " ..., g. AVcr:lgcs of each granite masses. : ,
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Figure 7.
a. Si02 vs Rh, Ba, and Sr variation diagrams. b. Sr vs Rh binary diagram. c. Ba vs Rb binary diagram. d. Sr vs Ba binary diagram.
FERTILE AND NON-FERTILE YOUNGER GRANITES 17
three granite plutons, in which Gabal Hadrabiya pluton has the least Rb and silica content among all the plutons.
Rb-Sr binary diagram (Fig.7 b) shows a reverse relation as well as Rb-Ba (Fig.7 c). The arrows on both figures represent the predicted trend of differentiation.
Lehmann (1982) has regarded the Rb/Sr ratio as a measure of the degree of magmatic differentiation, being higher with increasing differentiation.
Ba-Sr binary diagram (Fig.7 d) does not show a significant trends however the spaced clusters may represent different phases of granite.
3.5.2. Relation between Uranium and some Trace and Rare Earth Elements U versus Rb, Pb, Zr, Ce, Mo, F, Sm and La form direct correlations with fertile
granite and indirect relation not well correlated \lith non-fertile granite, (Figs. 8 and 9). U versus Ba, Sr, Co, and Eu form indirect relation with the fertile type (Fig. 9). The above relations suggest that in all cases of direct relations and/or indirect relations that the fertile granites and non-fertile granites are completely (or markedly) separated into two groups, as shown in Figures (8 and 9). Also, each type, either fertile or non fertile, mostly clustered into two groups. The younger granites are mostly differentiated at least into three phases or more.
3.6. GRANITE DIFFERENTIATION
The late orogenic granites emit high gamma radioacti\ity (maximum 200 cps) in the Eastern Desert relative to the surrounding country rocks of basement complex. Field radiometric work was carried out by many authors in the Central Eastern Desert (El Kassas, 1969 and 1974, Boutros, 1973, EI-Ghawaby, 1973, Assaf, 1973, Meleik et al., 1981, Bakhit, 1984 and Bakhit et al., 1984) and reYealed great variations in the background radiometric levels (ranges between II and 30 RIb) in the red-pink granite. Thus reflecting mostly the age offormation (Bakhit et al., 1984). Accordingly, the writer attributed these variations due to magmatic differentiation and/or to their intrusion at different lapse of time of the same magma. Many red-pink granite masses in the Central Eastern Desert possess high background radiometric levels such as Gabal El-Missikat and Gabal EI-Erediya, in which their arithmetic means (x) are 82.2 cps and 85 cps respectively (Bakhit and Guirguis, 1983). Meanwhile, some similar red pink granite masses scattered elsewhere in the Central Eastern Desert recorded relatively low background, such as Gabal El-Urf, Gabal EI-Dob and Gabal Hadrabiya (Fig. 1), with arithmetic means 45.9 and 53.2 cps respectively (Bakhit et al., 1984). The electronically adjusted scintillometer used in this study is the Canadian type Gr- 101 (Scintrex). The younger granite generally displays high magmatic characters. Magmatic differentiation seems to control the further evolution of the younger granites and also lead to the development of certain alkaline characteristics probably during their late phase of formation.
El-Ramly and Akaad (1960) separated the granites in the Central Eastern Desert of Egypt into major groups namely; older grey granite and younger granites of red-pink colours. These are equivalent or belong to EI-Shazly's (1964) S)n-orogenic plutonites
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40 ~-- I··· · 6 .. ' 2
x Ie d' + • x
24 + III: ••• .. ,& 100 • ... .. 20 • eo -. • • 10
•• 60 111 10 0 10
U(pp ... ' UCpPIII)
• i 15 ++ ~ • 't. •• 10 • • .-.: . 5 ~,..l
0 10 20 0
Figure 8.
U vs La, Sm, Pb, 'h:, and Mo variation diagrams a. Gabal Hadrabiya (e) b. Gaba! El Dob (A) c. Gabal El Urf (_) d. Gabal Kab Amiri (.)
e. Gabal El-Missikat (+) e. Gabal El-Erediya (x)
220 360 + D x b
+ 200 300 • Ie
";'160 • /:L - Q.
U .. • u: 11.0
a •• --- .. ~ 80 • dl250 va
UCppm' U(ppm' 8 22
e f • .. E 6 • _ 1-8 •• • /:L e • • /:L I Q. •
I. Q. ••
2 .- + + 1 + • • 0 10
UlppmJ 20 0 10
UlppmJ 20
Figure 9. U vs Ce, F, Ba, Sr, Co and Eu
and late-orogenic plutonites respectively. A much younger post-orogenic granite group is distinguished by EI-Shazly (1964) for the porphyritic granite of Aswan. Since then
much work was done on the Egyptian granite aiming to determine the different parameters for the larger granite bodies in many localities of Egypt. EI-Gaby (1975) introduced a trial to survey the petrochemistry and geochemist!), of some granites from Egypt, and he concluded that the studied granites represent one continuous granite series. The early members are of tonalite composition, whereas the content of potash feldspars increases progressively in the late members. The last phases acquired alkaline or peralkaline affinities. Generally, the early members of synorogenic granitoids and the late members of the younger granites are quite distinct in the field. El-Gaby (1975) considered the post orogenic granites as the younger red- pink granites.
Akaad and Noweir (1980) recorded three different phases of the younger granites, each phase possessing its own field, petrographic and petrochemical characteristics. Phase I is of calc-alkaline, hornblende-biotite granodiorite composition, rich in xenoliths which are variably digested. Phase II is of alkaline character and adarnellitic composition and contains fewer xenoliths which are undigested. Phase III is of leucogranite with thick chilled margins of rnicrogranite which is devoid of xenoliths and dykes. Greenberg (1981) mentioned that the younger granites apparently were formed at a time when north Eastern Africa was being converted from an area of compressive tectonics and crustal instability to an area of stable continental craton. The younger granite magmas appear to have been derived by very limited partial fusion rather than as end products of differentiation of calc-alkaline magma series. He also stated that the younger granites are generally siliceous (65-77 %, Si02) and rich in alkalies (7.8-9.3 %, Na20+K20). He classified the Egyptian granites into three groups. Plutons near the Si02-rich end point are classified as "Group I " (mainly hypersolvus) and include EI-Erediya and EI-Sibai. Plutons near the mafic end point (65-70 %) are classified here as " Group III" (mainly subsolvus) and include Fawakhir, ... , .... They contain moderate amounts of biotite and hornblende. Intermediate plutons are classified as "Group II " (mainly transsolvus) and include Kadabora, .. , .... Group I is separated from Group II on the basis of Ti02 and MgO values, with Group I less than 0.1 Ti02 and less than 0.08 % MgO and Group II values above those values. In general, Group I and Group II are rich in K-Feldspar and have K20 in excess of Na20, while Group III are more sodic. Typical Group III averages are 3.64 % K20 and 4.57 % Na20 while those of Group I are 4.71 % K20 and 4.04 % Na20. Accordingly, the six studied granite plutons are equivalent to Group II of Greenberg (1981) as shown from table (1).
Hussein et al. (1982) proposed a new classification of the Egyptian granitoids, where the older granitoids "syn-orogenic" plutonites are designated G I granites and denote subduction related granites formed by partial fusion of the mantle wedge with little or without crustal melt contributions. Moreover, the younger granite, red-pink granites "late and post" orogenic plutonites are further classified into two subgroups namely, G II and G III granites. G II granites denote granitic type formed by partial melting of the lower crust.
FERTILE AND NON-FERTILE YOUNGER GRANITES
G III granites on the other hand, denote granitic type fonned by melting of pre existing crustal rocks (i.e. intraplate anorogenic rocks).
21
Pagel (1982) found a decrease ofThIU ratio corresponding to slight decrease of Th content whereas: there is an increase of uranium content at the end of differentiation. This situation is more comparable with the granite masses of Gabal El-Missikat, Gabal EI-Erediya and Gabal Kab Amiri plutons in which Th content is nearly absent as compared with the other studied granite plutons (Table 1). Accordingly, the high radioactive granite among these red-pink granites represent the end of differentiation.
Bakhit (1988) suggested the presence of at least three red-pink granites phases or more, based on the radiometric levels. He attributed the difference in the value of radiometric levels, as a result of granite differentiation. Also, he mentioned that all the three phases or types of younger granite without clear contact. Besides, he compared his study with Akaad and Noweir (1980) on the younger granites and concluded that Gabal El-Missikat, Gabal EI-Erediya and Gabal Kab Amiri granite plutons (Fig.2 ) could belong to phase II. As well as, he attributed that the three phases of the younger granites represent the differentiation process of Group II which mentioned by Hussein et al.(l982).
Takla et al. (1991) classified the younger granites petrographically, beginning with the oldest, as biotite-hornblende granites, perthitic leucogranites and graphic granites, while the porphyritic granites occur occasionally on the outer zone of perthitic leucogranites
Guirguis and Bakhit (in press) mentioned that the simultaneous rise in Rb content together with drop in Sr content for these granites is indicative of differentiation.
4. Discussion and Conclusions
This study reveals that the fertile younger granites are post orogenic granites and are one of the different phases of the younger granites, characterized by high differential index, which intum represents the latest stage of the successive younger granite intrusions.
By correlation of the chemical analyses of the well kno\\n fertile granites from different points of view, which include Gabal El-Missikat, Gabal EI-Erediya and Gabal Kab Amiri, with the other three non-fertile granite masses, namely; Gabal Hadrabiya, Gabal EI-Dob and Gabal EI-Urf. It is observed that the chemical analyses of the fertile granites indicate a saturation with respect to silica and alumina. Moreover, the fertile granites tend to be impoverished in Ca, as well as, in the ferromagnesian elements (Fe203+MgO) and are enriched in alkali elements (Na20+K20). Besides, it is clear from the analyzed samples including trace elements that they are characterized by high Rb content with drop in Sr content.
The fertile granites contain very little amounts of fluorites, while the non-fertile type does not have this phenomenon. Also, the fertile type contains high zircon content relative to the non-fertile type. In addition, the fertile granite is characterized by large negative Eu anomalies, high REE and high ratio of LaN/ ~ as compared to the non-fertile type. On the other hand, some of the trace elements and REE elements can
be used as indicators for the fertile type, such as La, Ce, Sm, Pb, Rb, Zr, F and Mo which form direct relation with uranium content and reverse relation with Co, Sr, Eu and Ba.
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STRUCTURAL ASSOCIATIONS OF THE BASEMENT AND SEDIMENTARY COVER OF THE GEORGIAN PART OF mE CAUCASUS
L.B.BASHELEISHVILI Geological institute of Georgian Academy of Science MAlexidze Sir.] Build. 9 Tbilisi, 380093 Georgia
Abstract: Investigations in the Caucasian region have clearly shown the two-staged structure of the upper part of the earth's crust: the pre-Mesozoic crystalline basement, and the overlying thick sedimentary cover composed of terrigenous, carbonate, volcanogenic, and molasse formations of Mesozoic-Cenozoic age. Structurally, the basement is characterized by mosaic block structure. Kinematics of basement blocks and their relative vertical and horizontal movement along faults are expressed in the sedimentary cover in the from of zones of intensive structure formation. Structural associations between tectonic forms in the basement and overlying sedimentary cover are clearly visible. The study of morphology and characteristic patterns of spatial interposition of tectonic forms helps to determine kinematics of transverse faults and consequently, the displacement of individual blocks of the basement.
L Introduction
The upper part of the Earth's crust of the Caucasus provides good evidence of a two stage division-the older unit represented by pre-Alpine crystalline basement and overlying, thick Mesozoic-Cenozoic sedimentary cover. The structure and composition of the basement is known from its not numerous outcrops which mainly are linked with the Transcaucasian transverse uplift (Central Caucasian, Dzirula, Loki, Khrami salients).
The basement consists of the pre-Cambrian schists, gneisses. amphibolites, migmatites, Lower and Middle Paleozoic metamorphites, Hercynian age gneissic granitoids, Paleozoic basic rocks, and upper Paleozoic volcano-sedimentary rocks of quartz porphyric composition (Gamkrelidze 1984).
The sedimentary cover is represented by thick complex of terrigenous-carbonate, volcanogenic, volcano-sedimentary, molassic formations of Mesozoic-Cenozoic age.
A. K. Sinha (ed.), Basement Tectonics 13, 25-32. © 1999 Kluwer Academic Publishers.
25
26 L. BASHELEISHVILI
Figure 1. Location of the main outcorps of pre-Jurassic crystalline basement in Caucasian region. I-Zone of Transcaucasian Tramversai uplift. 2-pre Jurassic ofbasement G.G-Greater Caucasus.
D-Dzirula. K- Khrami . L-Loki
2. Structural-cinematic features of basement and sedimentary cover
Structurally, the basement is characterized on the whole by mosaic-block structure due to the development of the fault systems, predominantly meridional, latitudinal and near latitudinal direction located at different depths of the lithosphere (fig. 2).
Figure 2. Scheme of the main structural units and faults of the basement I -Alpine mollase depression. 2- pre-Jurassic Outcrops ofbasement. 3- nappes.
4- faults in the basement observed by geologic-geophysical data.
Numerous data have appeared lately (Ioseliani 1982, Basheleishvili 1987, 1989) which permit the creation of a physical model of the present mOIphostructure of the surface of the pre-Jurassic basement, especially within the molasse depressions of the Transcaucasian median massif.
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS 27
According to seismic data the Ctystalline basement of the Kolkheti depression subsides from east to west up to 8-9 kIn depth (in coastal zone of the Black Sea). However, on the gradual westward subsidence several local uplifts have been discovered through both geophysical surveys and deep drilling. Missing stratigraphic sections and substantial decrease in their thickness are associated with these uplifts.
The seismogeologica1 profile Dzirula-Gldani gives the idea of the structure of the sedimentaIy cover and basement within the Kartli basin. The basement here has block type composition where three blocks have been distinguished.
The largest dip angles (about 20 0) of basement surface are known on the eastern slope of the Dzirula salient. An abrupt subsidence is observed at the distance of 16-18 kIn. Then there is nearly horizontal occurrence of basement lying at the depth of3.2-3.4 kIn.
Further east the basement is subject to rigid subsidence to the depth of 6.5-7 kIn accompanying by a meridional normal fault to the east of which (east of Thilisi) the basement again rises to the depth of about 4 kIn.
The character of pre-Jurassic basement within the Kartli basin shows that its surface forms a wide open to the east trough. From the north it is delimited by large zone of long faults of Caucasian (NW -SE) trend. Here, two different zones may be established in the basement. The western one is characterized by complicated relief with local structural forms elongated predominantly in meridional direction. To the east, the relief of the basement is more gentle and here gentle sloping sublatitudinal structures are developed. Three an echelon deepening have been established in this part of the basin indicating the step-like structure of the basement subsiding from W to E.
According to gravitational modeling (Yusupkhodjaev et al, 1986) the lithosphere with in the Crimea-Caucasian region along the different fragments of the geotraverse Varna Tashkent is subdivided into 8 layers decreasing within the limits of the Black and Caspian seas to 4-5 layers. As a result of this modeling it has been established that the Moho boundary, along this profile (fig. 3), is characterized by local rises under Central Black Sea and South Caspian areas and relative subsidence beneath the Dzirula salient (zone of Transcaucasian transversal uplift).
0
40
CASPIAN SEA
The comparison of this profile with factual material concerning the step-like subsidence of the crystalline basement to the west and east from the Trans-caucasian transversal uplift alone, the listric faults shows two plans of structural symmetry, of defonnations: I) meridional-caused by compressional tectonics due to northward propagation of the Afro Arabian projection; 2) latitudinal-characterized mainly by tensional structures in the form oflistric faults dipping toward maximum subsidence. (Basheleishvili 1993).
Analysis of maps of total thicknesses of Meso-Cenozoic sediments (Moshashvili, 1990) strongly promotes in the reconstruction of the picture of inversional nature of blocks of basement and, correspondingly, activity of faults delineating the latters.
Most of the Kolkhida depression is covered by the Quaternary sediments and only in its flanks Cretaceous and Paleogene rocks in carbonate and molasse facies are exposed. In the axial part of the depression under the Quaternaty sands and conglomerates (about 300m thick) drill-holes discovered Neogene sand-clayey sediments unconfonnablv overlying Mesozoic rocks. Passing in the Cretaceous rocks more than 2000m and about 450m in the Upper Jurassic variegated shales the Chaladidi drill-hole was stopped at the depth of 4450m. The Samtredia drill-hole shows somewhat reduced thickness of the Mesozoic: at the depth of lOOOm it entered the clayey-volcano-terrigenous sediments of the Cretaceous, at the depth of 1700m - tuffogene Upper Jurassic, and at the depth of 2835 - Middle Jurassic porphiritic series continuing to the bottom of the hole (3045m).
Thus, towards the Black Sea basin abrupt increases in thicknesses of both the Neogene Quaternaty and Mesozoic sediments takes place. It is characterized by the absence of the Paleogene from the section. Towards the flanks of the depression in Abkhaz-Megrel (in the north) and Guria (in the south) subzones the section of the Tertiary, complex becomes more complete and thick (up to 4-5 km). The intensity of rock defonnation is increasing in the same direction.
In the Kartli basin from west to east noticeable increase of thickness of the Cenozoic rocks at the expense of the Mesozoic is observed. In the western periphery of the basin the thickness of the Cenozoic does not exceed 500m (with complete absence of Pliocene Quaternaty) and the total thickness of the Cretaceous (drilled to the Upper Barremian) is more than 3 km. To the east in the Shindisi reference hole the Cenozoic molasse is about 2.5 km thick whereas the whole Cretaceous reduces up to 400m.
In the vicinity of Tbilisi (eastern part of Kartli basin) the thickness of only Pliocene Eocene sediments exceeds 4.7 km. There are no data of the thickness of Mesozoic as only, upper 400m of Cretaceous are drilled by holes.
The Alazani depression is located on the raised block of granite-metamorphic basement which occurs here at the depth no more than 2.5-3 km and is delineated by faults.
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS
The Kakheti depression is supposed to be made up of Mesozoic (5-6 Ian) and Tertiary (about 9 Ian) sequences of sedimentary rocks.
29
Kinematics of blocks of the basement and their relative vertical and horizontal displacement along faults are expressed in the sedimentary cover as zones of intensive structure formation strongly differing from the general style of deformations in the region. Structural associations (disjunctive and folds) in basement and sedimentary cover are mostly conditioned by realization of dynamic-kinematic parameters along basic faults.
The Odishi block corresponding to the Megrelian syncline is limited from all sides by basement faults having strike-slip character. Some of them are dextral, such as Tsaishi fault (figA) whose nature is conditioned by above-fault en echelon Vrta and Satandjo anticlines, whereas sinistral character of the Poti-Abedati fault are conditioned by above fault en echelon Eki, Nokalakevi and Abedati anticlines.
Figure 4. The tectonic scheme of the Odishi block I-Quaternary. 2- post-Eocene. 3- Cretaceous. 4- Bajossian volcanics.
5- reverse faults. 6- faults in the basement.
The establishment of sense of motion along faults with the help of above-fault and folds has its experimentally supported basis. In this case, strike-slip displacement in the basement induces two Gmdients of deformation in overlying sedimentary stmta horizontal and vertical. It is the existence of these two gradients that determines the development of folds in the fault zone.
As it has been indicated by Camond 1.F. et al, (1984) geometry and structure of folds formed by this manner are chamcterized by following specific features: 1) en echelon pattern- 2) periclinal closure ofbeds-, 3) certain position (at the angle of 30-40·) to the basic fault
In the case of disjunctive structures the situation is somewhat different as may be seen at the example of the fault bounding the Gurian depression in soutb-east. Detailed study of the near-fault zone showed that the Chokhatauri thrust that earlier was considered as a surface expression of the basement Fault (fig.5) in reality consists of four en echelon faults of reverse fault-shear nature replacing one another and steeply dipping (60-80 0 )
to south-east. Along these faults Middle Eocene volcanics of the Adjara-Trialetian zone overthrust the Oligocene-Miocene clays and marls.
The band ofleft-Iateral stepped-like en echelon reverse fault-shears is controlled by right-lateral strike-slip displacement in the basement.
v v v v v
O ~ fVVl 1 ~2 L.:!U3
04~5~6
Figure 5. The tectonic scheme of the Gurian depression and its surrounding's. 1. Quaternary, 2- post-Paleogene. 3- Eocene volcanics of the AIljara - Trialetian zone. 4- reverse faults and reverse fauh-shears. 5- front of nappes. 6- faults in the basement.
A typical example of interaction of structures in the basement and sedimentary cover represents the large flexure extending along the junction of the southern slope of the Greater Caucasus and the Georgian mass (fig. 6). Here the rotation of blocks causes formation of shear zones in narrow graben-like depression in the sedimentary cover. Formation of the Racha graben-syncline is a direct result of faulting in the basement. As for stepped-like shape of the southern limb of this graben, it is caused by syntectonic faults in the northward rising basement. Besides, the southern limb is subject to secondary folding. One of the forms of structural associations of the basement and sedimentary cover are cloaking structures, or structures in the sedimentary cover reflecting the morphologically expressed projections in basement. Such conditions for the formation of tectonic structures existed in the marginal zone of the Dziryla salient of the crystalline basement (fig. 7).
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS
---~-~-~---~--------- ---------,
Figure 6. The tectonic scheme of the zone of junction of the fold system of the southern slope of the Greater Caucasus and the Georgian Block.
1- Miocene sediments. 2- flexures. 3- faults. 4- axes anticlines. 5- axes of secondary anticlines. 6-axes of secondary synclines.
31
This salient developed in the Late Miocene-Quaternary time, and during the general northward propagation of the Adjara-Trialetian zone served as a rigid stamp bending which caused a different character of displacement on the Northern Adjara-Trialetian fault. It is also reflected in the character of smaller structures. Systems of transversal dextral shears, zones of en echelon near-fault folds, folds with vertical crest (horizontal folds) have been formed here.
+ o 10 km + + '---L-.J + +
Figure 7. The scheme of the Dzirula salient and its surroundings. l-postpPaleogene. 2-Eocene volcanics of the Adjara-Trialetian zone. 3-Mesozoic complex. 4-Paleozoic granites.
5-axes of main anticlines. 6-nappes and thrusts 7-reverse faults. 8-shears.
The Kakheti molasse depression is characterized by development of south-vergent nappe-thrust structures that have subhorizontal slip planes. Here, main displacement is connected with Oligocene-Lower Miocene clayey sediments. Locally, on the background of horizontal slips stepped-like deformations of this plane can be observed.
The latter are the direct consequence of those projections in basement above which in the process of plastic mass material injection narrow dyapiric anticlinal folds have fonned.
3. Conclusion
Studies carried out in different regions of the Caucasus showed the presence of various defonnational tectonic forms in the upper crust that is genetically connected with displacement of rocks of the rigid substratum. These dislocations are caused by a great number of factors.
Thus, through all the observations of combination of structures in crystalline basement and overlying sedimentary cover, regarding their genesis, morphology and dynamo kinematic parameters, give us a possibility to suggest a structural associations or paragenesis of the two regions of the Earth's crust. As a consequence, two different positions may take place: 1 )activation and re-activation of basement structures (predominantly, faults and blocks of basement) inducing deformations and structure fonnation in the sedimentary cover, 2) passive behavior of basement structures under active deformations in the sedimentary cover.
4. References
Bsheleishvili, L.B. (1987), Kinematics of the alpine defonnation of the Transcaucasian median massif II, national youth school, Sofia, 215-222.
Bsheleishvili, L.B. (1989), Tectonics of the zone of jWlction of Adjara-Trialetian fold zone and the Georgian Block. Geotecionics, NI, Moscow, 77-86.
Bsheleishvili, L.B. (1993), Extension structures of COOSQlidated crust of the Transcaucasion intermontane depression (latitude profile). Annales Geophysical. European Geophysical Society, c56.
Gamkrelidze, I.P.(1984), The tectonic structure and Alpine geodynamics of the Caucasus and acljacent areas. Otkmezuri., Z. W. (edit), Tectonics and Metalogeny of the Caucasus. Tbilisi, Metsniereba publish., 105-127.
Gamond, I.F., Odonne, F. (1984), Some identification criteria of deep wrench fault induced folds: analog, model and field data. Bulletin Sos. Geol. France, NI. tome XXII, 115-127.
IoseJiani, M.S. (1982), Deep geological structure of the Intermontane depression of Georgia and adjacent areas on geophysic data. Doclotare thesis Tbilisi. 1-29.
Yusupkbodjaev, Kh.l., Mindeli, P.Sh., Kartvelishvili, KM., (1986), Lithosphere model of the Crimea-Caucasian Mediterranean region, Dynamics and evolution oflithoshphere. Moscow, Nauka publish., 129-139
Moshashvili, AB. (1990), Comparative analysis of geotectonical evolution of the South Caspian and Black sea basins. Doctorate1heses. Baku. 1-505.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON: A HISTORY OF TANGENTIAL COLLISIONS
1. Abstract
N. CAMPAL and A. SCHIPILOV Universidad de la Republica Facultad de Agronomfa - Cdtedra de Geologfa Av. Garzon 780 - 11900, Montevideo, Uruguay.
The aim of this paper is to present the arguments in support of the idea that the Rio de la Plata craton as a whole was not involved in a Panafrican orogeny.
On the other hand, based on field relations, petrography, and only a few available geochronological data, we began to find evidences for the identification of a Middle Proterozoic (Grenvillian) event involving sedimentation, metamorphism, and large-scale thrusting. Intense dextral shear with the development of thick mylonitic bands (up to 20 km wide), associated with this episode also suggest oblique collision occurred here.
As we understand the evolution of the Uruguayan crystalline basement, the assembly of the Rio de la Plata craton, and the Panafrican (Brasiliano) terrane was completed during Cambrian and after the deposition of the Arroyo del Soldado Group. The main formations of this unit were considered up to now to belong to the Panafrican molasse.
2. Introduction
The name Rio de la Plata craton was initially introduced by ALMEIDA (1971) and more recently redefined by some with a different intent. The word "Craton" has a strict tectonic-position sense in relation to an orogenic belt. It thus was applied to the Rio de la Plata crystalline basement on the assumption that these rocks were the parts of the "Kratos" in relation to the Panafrican orogeny.
Afterward and without jUdging this assumption, different models implying collision with eastward (RAMOS, 1988) or westward (FRAGOSO CESAR et al., 1987) subduction or inclusively double subduction (TOMMASI & FERNANDES, 1990) were applied to explain the geology of Uruguay and southern Brazil.
All geological data involved in this work were obtained in Uruguay, and although we propose some possible correlations, no discussion about South American Geology is attempted.
To describe the observed geological features, we use the nomenclature proposed by BOSSI & CAMP AL (1992), with the subdivision of the Uruguayan crystalline basement
A. K. Sinha (ed.), Basement Tectonics 13, 33-48. © 1999 Kluwer Academic Publishers.
33
34 N. CAMPAL AND A. SCHIPILOV
into three main units: Piedra Alta Terrane (TPA), Nico Perez Terrane, (NPT) and Cuchilla Dionisio Belt (CDB) as shown in Fig. 1.
w [][] SYPL
-1780 My rapakivi batholith
ieo Perez terrane (Grenvillian ?)
[!]j (2)
Granitic intrusives (Cambrian)
Sicrra Ballcna lineament
Figure 1. Uruguay: geological sketch and main subdivision of the crystalline basement
The word "terrane" is here used as an informal subdivision, following the suggestions of CONEY et al. (1980). It will be used to describe some geological entities that have stratigraphic associations with different geological histories such as other terranes or neighboring stable areas.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON 35
3. The Sarandi del Yi - Piriapolis Lineament (SYPL)
During 1987-1990 a detailed survey of an extensive mafic dike produced an accurate geological (l :50,000) map with each dike located on aerial 1 :20,000 photographs (BOSSI & CAMPAL, 1991) (Fig. 2). More than 100 samples were petrographically analyzed, and 58 were geochemicaly (BOSSI et al., 1993a). The ages of these rocks were determined by different methods and the results are shown in table I.
TABLE I: KI Ar data of the "Rio de la Plata" basic dike swarm
Age Method Author 1604 ± 40 My KlAr whole rock GOMEZ RIFAS (1988) 1373 ±33 My KlAr whole rock GOMEZRIFAS (1988) 1393 ± 44 My KlAr whole rock GOMEZ RIFAS (1988) 1786 ± 26 My KlAr contact's biotite BOSSI et al. (1993a) 1828 ± 21 My KlAr contact's biotite BOSSI et al. (1993a)
,r:: ~, :: ';~" j;'~.;~ '; .' / ., , ' 0 Piedra Alta terrane (PAT)
',' '0 bn ~ Mafic dike swarm .... """""O:=~i!o!!""",;"=~ 1m Phanerozoic sediments
Figure 2, Rio de la Plata mafic dike swarm and details of its eastern edge, as can be seen in aerial photographs
As a consequence, the Sarandf del Yf - Piriapolis lineament was identified and subsequent studies were conducted to find some equivalent dikes in the eastern side of the lineament. Nevertheless, in the detailed survey of this area, no dikes of this age were found, but a rapakivi complex partially deformed and sharply delimited by the SYPL was discovered (CAMPAL & SCHIPILOV, 1995). Its age is broadly the same of the dike swarm, but important differences in the geological context at both sides of the lineament can be established and are summarized in table II.
TABlE II. Principal differences between PAT and NPT Piedra Alta Terrane (PAT)
Rio de la·Plata dike swarm No equivalent rocks No magmatic activity post basic dike swarm
Nico Perez Terrane (NPT)
No equivalent rocks llIescas rapakivi complex Intense Middle Proterozoic and Cambrian magmatism
Post-dike swarm deformation is rare and brittle Widespread mylonites and thrusting except at its eastern border post- rapakivi granite emplacement Metamorphism of Transamazonian Pre-rapakivi rocks of granulite facies and Pre-Transamazonian rocks from including meta-Banded Iron Formations (BIF) greenschist up to amphibolite facies with string perthites No Vendian sedimentary cover identified Vendian sedimentary rocks widespread No Middle Proterozoic-age rocks Common Middle Proterozoic metamorphic rocks
This lineament seems to have evolved in at least two main stages. The first one is a dextral shear and the sense can be well identified considering the macro structures in the PAT as well as in internal micro- and meso/structures. This deformation style can be easily detected in a 20-kIn wide band in the PAT near to SYPL, but is partially obliterated at NPT.
Geological evidence is that this stage is older than the Vendian cover because basal conglomerates and sandstones of the lower formation of "Arroyo del Soldado Group" (Vendian sedimentary sequence) are in sedimentary contact with high temperature granitic mylonites, which are the source of detrital elements in this unit.
Discrete mylonitic and ultra-mylonitic bands 10,000m wide identify a second stage with sinistral shear sense. These shear bands are generalized on NPT. They characterize internal deformation of the Brasilian Mobile Belt, and affect the Vendian meta/sedimentary cover and also the intruded granites.
4. Piedra Alta Terrane (PAT)
The PAT consists of a central granitic-gneissic region and three metamorphic belts that trend nearly EW. Except for some Rb/Sr ages that are at the Archean-Proterozoic boundary, the tectono/thermal event with higher age recorded in this Uruguayan basement portion is the Transamazonian (ca. 2000 My, CINGOLANl et ai., 1997).
This basement would be correlated with the Tandil hills basement of CINGOLANI et ai., (1997). In PAT, aside the small outcrop area of the Piedras de Afilar Formation of uncertain age, neither sedimentary deposition nor other metamorphic units were produced after the "Rio de la Plata" mafic dike swarm intrusion, until the deposition of Devonian sediments which uncomformably overlie granitic rocks and are tectonically undisturbed.
5. Nico Perez Terrane (NPT)
This area seems to be the key to understanding the geological evolution from the Early Proterozoic to the Cambrian because it combines the best sedimentary units with well
exposed structures. Until 1995, published papers treated this part of the crystalline basement as a Transamazonian (Archean ?) complex reworked by the Panafrican orogeny. Field work conducted during the last two years allows the separation of a metamorphic complex that will be discussed in detail below.
5.1. GEOLOGY OF THE NICO PEREZ TERRANE
The Nico Perez Terrane (NPT) has two major outcrops: one in North Uruguay (Rivera's crystalline island), which is surrounded by younger sedimentary rocks, and the other, the largest, in the central part of the country with well exposed relationships with neighboring geological units.
The central-Uruguay outcropping area comprise an old nucleus (Early Proterozoic - Archean?) occupied by granulite-facies metamorphic rocks, which are surrounded eastward and southward by a medium-grade metamorphic complex, being the whole partially covered by a Vendian to Cambrian metasedimentary unit.
The high-grade nucleous (named Valentines Complex by BOSSI et aI., 1965) is made up of meta-BIF (banded iron formations, here magnetite-augite quartzites), string perthite gneisses, and pyroxenites. These rocks are polydeformed and were intruded by granitic stocks of uncertain age.
The most important granitic intrusion is an extensive rapakivi complex named "Illescas Batolite" by CAMPAL & SCHIPILOV (1995). Their intrusion possesses several petrological facies, from quartz-bearing syenite to granophyres and granites with high quartz content. All of them have a distinctive and unequivocal feature: iridescent bluish euhedral quartz as an early crystallization phase. Rounded oligoclase-mantled potash feldspars is widespread; these along with the few existing chemical analyses, help to define the rapakivi nature of the intrusion.
The ages of this granite were obtained by whole rock Rb/Sr as 1760 ± 32 My (Ro=0.704; BOSSI & CAMPAL, 1991); and by 207PbrPb on zircon (1751 My, 5%
discordance); LARRY HEAMAN per. com., in CAMPAL & SCHIPILOV, 1995). The age, structure, and stratigraphy of the medium-grade metamorphic complex as a
whole have not been discussed until now. As all previous writers assumed that the NPT was the craton adjacent to a Brazilian orogenic belt, the analysis was focused on the position and nature of the margin of the belt. Consequently these rocks were mapped as a part of the Brazilian mobile belt (Lavalleja Group) by BOSSI et al. (1975) and PRECIOZZI et at. (1985); or as Transamazonian reworked rocks (FERRANDO & FERNANDES, 1971; BOSSI et at, 1965; FRAGOSO CESAR & SOLIANI, 1984; PRECIOZZI et aI., 1985; FRAGOSO CESAR et at., 1987; FRAGOSO CESAR et at., 1993; HALINAN et aI., 1993; MACHADO & FRAGOSO CESAR, 1987). In fact, the model proposed by these authors implies a Transamazonian plate with no tectonic activity existed between the Early Proterozoic and the Neoproterozoic Brazilian orogeny. With this in mind, lithologies that are similar to those founded in the Brazilian mobile belt were mapped as Upper Proterozoic or inclusively considered as isolated portions of hypothetical thrusts (FRAGOSO CESAR, 1986).
Near the eastern border of the NPT, medium-grade metamorphic rocks and others corresponding to the Vendian metamorphic sequence (here more folded) were not
subdivided by previous authors. One of the most important features of the NPT is the structure within thrust sheets
that verge toward the southeast (Fig. 3 and Fig. 4), and implies the medium grade metamorphic complex and its basement. These thrusts preceded the generation of wide dextral shear zones that we consider simultaneous with the SYPL's first stage of development. Another element that contributes to difficult the tectonostratigraphy of the NPT's rocks, is the intense Cambrian sinistral shear strain that affects preexisting structures: these effects can be clearly detected in the Vendian cover and its magmatic intrusions.
NW
A
D Piedra aha lerrane (PAT)
~ Rio de la PlaIa mafic dike swarm
I@l Nico Perez lerrane (high grade rocks)
~'\\\ I Nico Perez lerrmc (medium grade rocks)
t.>:i Blue quanz rapakivi balholilh
o 10 20
Vendian sedimenlary cover
"<;.' ~ Cuchill. Dionisio beh (CDB)
# d iTranseurrcnl faults I Thrusts Grenvillian: dextral shear & SE vergence Bruilian : sinistral shear &, W 'Vergence
Figure 3. Schematic cross section ofNico Perez terrane showing the relation between older core, the medium grade metamorphic complex and the Vendian cover.
SE
Some local cross sections will be described to illustrate the nature of the more common lithological associations (locations are showed in Fig. 4):
5.1.1. Location 1: Puntas del Santa Luda 1- muscovite quartzite interbedded with metaconglomerate and muscovite
schist. 2- recrystallized BIF's 3- gray-yellowish to pinkish carbonate, highly recrystallized, interbedded with
muscovite schist. At the top of this unit are 100 m of stromatolitic limestones with synsedimentary breccias that may have been caused by storms or tsunamis. SPRECHMANN et al. (1994) defined three types of stromatolites in this unit:
- planar stromatolites (cryptalgallaminites). - laterally linked hemispheroids (LLH). - vertically stacked hemispheroids (SH-V) with variable basal radii .
A WRAMIK (1992) pointed out that the relatively high diversity is typical of the Early and Middle Proterozoic.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON
x
x
x
Valenlines complex
D Peralumious graniles (ca. 1200 My) --an o D
ieo Perez mafic dike swarm (co 600 My)
Brazilian-age graniles (03 550 My)
Vendian sedimentary cover
39
33'
Ultramanc melamophie rocks (Ialc-schisl) 0 Low Crelaceous basalts Grenvillian: dextral shear & SE vergence
Grenvillian.agc graniccs ~.J Transcurrent faults I Thrusts Brasilia" : sinistral shear & NW vergence
Figure 4. Geologic map of the studied area and location of available geochronological data
5.1.2. Location 2: Piraraja 1- muscovite quartzite 2- Strongly deformed quartz-matrix metaconglomerate. This rock has elongated
pebbles with geometric relations between major and minor axes of 511 to 1011, and constitute a marker horizon in the complex stratigraphy that permits better resolution and structural geometry in this block.
3- red clastic limestone with variable amount of metamorphic muscovite. 4- leucocratic gneiss.
5.1.3. Location 3: Maria Albina 1- garnet-bearing biotite-muscovite schist 2- staurolite-bearing biotite-muscovite schist 3- garnet-bearing grunerite BIF 4- muscovite-biotite gneiss 5- muscovite quartzite 6- muscovite limestone
5.1.4. Location 4: Manso's Quarry 1- biotite - muscovite schist interstratified with garnet-bearing impure
limestone. 2- tremolite schist 3- spilitic metabasalt and metakeratophyre.
In this work we are proposing a Middle Proterozoic Grenvillian age for this metamorphic sequence on the basis of:
A- Muscovite KlAr data from the peraluminous synkinematic granite yield an age of 40 -6 ccSTP
1253.1 ± 32.2 My; K (%) = 8.7500; 75%ERR=1.9038; Arrad (10 'g)=614.61; Aralm
(%) = 6.16; Err = 2.6% (CINGOLANI, C. per. com.).
B- The only one post-intrusion thermal record obtained by 39 Arr Ar in the "Rio de la Plata mafic dike swarm" yielded a Middle Proterozoic age (RENNE & TEXEIRA, per. com., in CAMPAL et al., 1995)
C- Vergence of all observed structures systematically indicates mass transportation toward the southeast. These structures are intimately related to dextral shear zones, deduced from macro- and mesolkinematic indicators (Fig. 5). This fact contrast with the vergence and shearing from the Brasiliano mobile belt.
D- It is very difficult to explain how rocks systematically displaying medium-grade metamorphism ~;;d at least 3 phases of deformation could be the source area for the Arroyo del Soldado Group with a palaeontological age of about 570 My (see E), and can be a part of the Brasiliano mobile belt, which indicates broadly the same age for the main metamorphic peak and granitic intrusions (700 - 570 My; GOODWIN, 1991). This is specially relevant, taking into account the nature of the Arroyo del Soldado sequence.
Figure 5. (a) aerial photo of a macro kinematic indicator developed in the first stage of movement
P~~_LI.!..._~d::J~:c.;;;;::;;===::=====-- of Grenvillian age. (b) detail of a protomylonite ~ in this zone.
E- The ages obtained for the carbonate deposition of the Grenvillian metamorphic complex on the basis of the stromatolite morphology yield values between the Early Proterozoic and Middle Proterozoic.
In fact, the Brasilian ages of these metamorphic rocks were assumed by previous authors solely on the basis of RblSr or KiAr geochronological data, obtained on intrusive granitic stocks andlor highly deformed rocks that were supposed to be synorogenic with the Brasilian orogenic cycle. These rocks are really synkinematic with a transpressional accretional episode that happened at the end of the Cambrian Period.
In most places of the NPT the vergence, tectonic style, and lithologies of the medium grade metamorphic complex, give us reasonable criteria for its separation from both the Vendian cover and the Transamazonian basement, despite the imbrication of these units and the low relief of the area. Deformation and slab structure with basement rocks increass eastward into the Vendian group. Much field work to determine how these structures developed is needed.
Until now, the eastern edge of the Nico Perez terrane has not been well defined. The Sierra BaHena Lineament (SBL) is certainly the most important structure that separates Nico Perez terrane and CuchiHa Dionisio belt, but there is a thin belt of rocks between them for which there is no criterion that defines its age and origin.
5.2. GEOCHRONOLOGICAL DATA (NPT)
One of the principal contributions of this work has been a detailed geological survey at locations where samples for geochronological analysis were gathered by other authors. Consequentelly some isolated data acquired a new geological significance (Fig. 4).
TABLE m. Available geochronological data in Nico Perez terrane. Point 1: 515 ± 10 My (UMPIERRE & HALPERN, 1971) Rb/Sr- whole·rock Point 2: 510 ± 35 My (UMPIERRE & HALPERN, 1971) Rb/Sr- whole·rock Point 3: 1253.1 ± 32.2 My (CINGOLANI, Per. com., KlAr - muscovite Point 4: 665 ± 203 My (RIV ALENTI et al. , 1995) Rb/Sr - whole-rock Point 5: 550 ± 15 My (UMPIERRE & HALPERN, 1971) Rb/Sr - whole-rock & isolated mineral Point 6: 532 ± 16 My (CINGOLANI et aI., 1990) KI Ar - illite Point 7: 518 ± 15 My(CINGOLANIetal.,1990)KlAr-illite PointS: 1786 ± 26 My (BOSSIetal. , 1993a)KlAr-contact'sbiotite Point 9: 520 ± 2 My (BOSSI et aI., 1993b) Rb/Sr - whole-rock
Some ages deserve special emphasis; data 1 belong to a granitic mylonite. This rock was considerated by BOSSI et al. (1975) and PRECIOZZI et at. (1985) as an intrusive granitic body. Using detailed structural analysis we can confirm that the protolith of this mylonite is the -1800 My rapakivi granite, which has as a special feature: euhedral blue quartz crystals. The blue coloration is due to the presence of fine rutile needles and disappears when the rock is subject to hard strain (CAMPAL & SCHIPILOV, 1995). In the mylonite in the nuclei of quartz ribbon we can recognize small pieces of the original blue rutilated quartz.
At the point 2 (Cerro Palma) the age corresponds to a mylonite just recognized by PRECIOZZI et al. (1985). At the point 3 the age obtained by KlAr on muscovite (CINGOLANI Per. com.) from a garnet-muscovite-bearing granite, synkinematic with overthrusting in the Grenvillian basement. Fig. 6 is a geologic cross section near the sample location.
w granite E Amphibolite Garnet-mica-schist
Figure 6. Schematic cross section near sample 3 location.
Point 4 will be discussed later, it being a mafic dike swarm (Nico Perez dike swarm) that was studied by RIV ALENTI et al. (1995).
The age at point 5 (Polanco granite; UMPIERRE & HALPERN, 1971) establishes the upper limit for Vendian-Cambrian sedimentation. In fact, this granite intrudes the basal formations of the Arroyo del Soldado Group, producing a 5 to 10m wide contact metamorphic

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