Utilization of “Landsat” images and conventional aerial photographs in the delineation of some...
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A14 THE UYAIJAH RING STRUCTURE: THE PLUTONIC ROOTS OF A PRECAMBRIAN EPICONTINENTAL CALDERA F.C.W. DODGE and H.T. MILLARD Jr. U.S. Geological Survey, Menlo Park, Calif. (U.S.A.) A multitude of granitic intrusions occur as circular masses or as ring complexes and are the youngest major group of plutonic rocks of the Arabian Shield. The Uyaijah ring structure is one of several of the complexes in the eastern Shield. This structure covers an area of 400 km 2 and consists of a central granite core encircled by a younger, red, leuco- granite ring dike, both of which have been intruded into a circular-foliated granodiorite batholith. Configuration of the stock and dike were controlled by the steeply inward-dipping primary foliation in the batholith. Form of the structure is similar to that inferred for Valles-type calderas at depth. Contents of major elements, Sc, total rare-earth elements (REE), Cr, and Co show well- defined, gradational trends from granodiorite through granite to leucogranite but Rb, Cs, Ta, Zr, Hf, and Sb do not. Average Sr and Ba contents of the granodiorite and granite are 460 and 890 ppm, respectively, whereas the leucogranite contains only 20 and 80 ppm, respectively. Thorium and U are considerably greater in the leucogranite than in the other rocks. The granodiorite and granite have similar fractionated REE patterns (La/Yb 30) with no appreciable Eu anomalies (Eu/Eu* = 0.7--1.2). The leucogranite has a relatively flat pattern (La/Yb = 5) with a large, negative Eu anomaly (Eu/Eu* = 0.04). The chemical data indicate that the core stock resulted from differentiation of batholithic magma, with fractional crystallization of hornblende, a common mineral in the granodiorite. The ring dike crystallized from a melt of ternary minimum composition. Although related volcanic rocks are not exposed in the general region, the ring structure is believed to re- present the roots of an epicontinental caldera exposed approximately 3 km beneath its original extrusive surface. UTILIZATION OF "LANDSAT" IMAGES AND CONVENTIONAL AERIAL PHOTO- GRAPHS IN THE DELINEATION OF SOME ASPECTS OF THE GEOLOGY OF THE CENTRAL EASTERN DESERT, EGYPT H.A. EL-ETR and M.S.M. YOUSIF Ain Shams University, Cairo (Egypt) The study area lies between lat. 25 ° 30' and 26 ° 30'N and is bounded to the west by the Qena bend of the Nile and to the east by the Red Sea. In the eastern part, exposures of Basement Complex rocks predominate, whilst in the western part Nubia Sandstone and younger Cretaceous to Palaeocene limestones and shales are present. The Red Sea coastal strip is covered mainly by Neogene to Recent sedimentary exposures. The drainage pattern delineated represents parts of the Red Sea and the River Nile drainage basins. It is characteristically integrated, dense, and, to variable degrees, structurally controlled. A total of 604 linear structural elements was detected including faults, fractures, and air-photo lineations. Several rounded to elliptical intrusive bodies were also recognized. ERTS data were then assessed in the light of the more detailed conventional aerial photographs and mosaics. It was found that: (a) satellite images help
Transcript of Utilization of “Landsat” images and conventional aerial photographs in the delineation of some...
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THE U Y A I J A H RING S T R U C T U R E : THE PLUTONIC ROOTS OF A P R E C A M B R I A N E P I C O N T I N E N T A L C A L D E R A
F.C.W. DODGE and H.T. M I L L A R D Jr.
U.S. Geological Survey, Menlo Park, Calif. (U.S.A.)
A mul t i tude of granitic intrusions occur as circular masses or as ring complexes and are the youngest major group of plutonic rocks of the Arabian Shield. The Uyaijah ring structure is one of several o f the complexes in the eastern Shield. This s t ructure covers an area of 400 km 2 and consists of a central granite core encircled by a younger, red, leuco- granite ring dike, bo th of which have been intruded into a circular-foliated granodiori te batholi th. Configurat ion of the stock and dike were control led by the steeply inward-dipping primary foliat ion in the batholi th. F o r m of the s tructure is similar to that inferred for Valles-type calderas at depth.
Contents o f major elements, Sc, total rare-earth e lements (REE), Cr, and Co show well- defined, gradational trends f rom granodiori te through granite to leucogranite bu t Rb, Cs, Ta, Zr, Hf, and Sb do not. Average Sr and Ba contents of the granodiori te and granite are 460 and 890 ppm, respectively, whereas the leucogranite contains only 20 and 80 ppm, respectively. Thor ium and U are considerably greater in the leucogranite than in the o ther rocks. The granodiori te and granite have similar f ract ionated REE patterns (La/Yb 30) with no appreciable Eu anomalies (Eu/Eu* = 0.7--1.2). The leucogranite has a relatively flat pat tern (La/Yb = 5) wi th a large, negative Eu anomaly (Eu/Eu* = 0.04).
The chemical data indicate that the core s tock resulted f rom different ia t ion of bathol i thic magma, with fractional crystal l ization of hornblende, a c o m m o n mineral in the granodiorite. The ring dike crystall ized f rom a mel t of ternary min imum composi t ion. Al though related volcanic rocks are not exposed in the general region, the ring s t ructure is believed to re- present the roots of an epicont inental caldera exposed approximate ly 3 km beneath its original extrusive surface.
UTILIZATION OF " L A N D S A T " IMAGES AND C O N V E N T I O N A L A E R I A L PHOTO- GRAPHS IN THE D E L I N E A T I O N OF SOME ASPECTS OF THE G E O L O G Y OF THE C E N T R A L E A S T E R N DESERT, EGYPT
H.A. EL-ETR and M.S.M. YOUSIF
Ain Shams University, Cairo (Egypt)
The s tudy area lies be tween lat. 25 ° 30' and 26 ° 30 'N and is bounded to the west by the Qena bend of the Nile and to the east by the Red Sea. In the eastern part, exposures o f Basement Complex rocks predominate , whilst in the western part Nubia Sandstone and younger Cretaceous to Palaeocene l imestones and shales are present. The Red Sea coastal strip is covered mainly by Neogene to Recent sedimentary exposures.
The drainage pat tern del ineated represents parts o f the Red Sea and the River Nile drainage basins. It is characterist ically integrated, dense, and, to variable degrees, s t ructural ly control led. A total o f 604 linear structural e lements was detected including faults, fractures, and air-photo lineations. Several rounded to elliptical intrusive bodies were also recognized. ERTS data were then assessed in the light of the more detailed convent ional aerial photographs and mosaics. It was found that : (a) satellite images help
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the recognition and the delineation of the surface distribution of super-regional to regional lithologic units (e.g., major intrusions, sedimentary belts, cultivated strips, etc. ), and (b) satellite imagery has the advantage of recording the larger regional structural lines that may be dispersed over different photo strips in normal lower-altitude photos.
THE FEIRAN--SOLAF GNEISS BELT, SOUTHWESTERN SINAI, EGYPT
S. EL-GABY and A.H.A. AHMED
Assiut University, Assiut (Egypt)
The Feiran- Solaf gneiss belt is 5--10 km wide and extends for about 40 km in a NW direction. Metamorphism and migmatization took place during the main (i.e., second) orogeny that shaped the basement of the Eastern Desert and Sinai. The gneisses are variably migmatized but an attempt is made here to reconstruct their lithostratigraphy.
Structurally, the gneisses form two doubly plunging anticlines (Feiran anticline to the west, and Solaf anticline to the east) trending NW--SE and separated by a thrust fault; and an overturned western limb of an incomplete anticline thrust over the Solaf anticline. The vergence of the folds is to the SW.
The Feiran anticline comprises two formations: a lower psammo-pelitic Nidia El-Samra Formation, and an upper calcareous pelitic Aleiyat Formation. The lower formation consists of granitic gneisses, surrounded by fine-grained biotite and biotite--hornblende gneiss. The upper formation is differentiated into two members. The lower member is mainly medium- grained banded and homogenized biotite--hornblende gneisses; the latter encloses patches of migmatitic granites and orbicular gneiss. The upper member is formed of fine-grained biotite--hornblende gneisses intercalated with thin biotite gneiss bands sometimes with garnet, sillimanite or cordierite.
The Solaf anticline comprises two formations: a lower, pelitic and calc-pelitic Khali' Formation and an upper, arenaceous Um Tart Formation. The lower part of the Khali' Formation is composed of medium-grained biotite and biotite--hornblende gneisses that locally develop into augen gneiss. The upper part consists of fine-grained biotite gneisses interbanded with biotite--hornblende gneisses. The Um Tarr Formation is composed (base to top) of psammitic gneiss, medium-grained quartzofeldspathic granulites with fine-grained biotite gneiss intercalations, and fine-grained quartzofeldspathic granulites.
The Sheikh Formation forms the western limb of the overturned anticline. It is stratigraphically lower and consists of coarse-grained biotite--hornblende gneisses and medium-grained hornblende--biotite gneisses.
The following evolutionary trends are constructed:
Pelites: bi-sill cord cord-- med. gr. augen schist ~ gneiss -* plag--bi ~ bi -~ gneiss
gneiss gneiss
Calc-pelites: fine gr. med. gr. banded bi--hb ~ banded bi--hb gneiss gneiss
tonalitic granodioritic migma-granite migma-granite
t reed. gr. gneissic homogenized ~ granite bi--hb gneiss
Psammites : fine gr. med. gr. psammitic nebulitic granulites -~ granulites -~ gneiss -~ gneiss
