Hk mohamed haneef , a person who left this world unrecognised
A previously unrecognised major orogenic front in … previously unrecognised major orogenic front...
Transcript of A previously unrecognised major orogenic front in … previously unrecognised major orogenic front...
A previously unrecognised major orogenic front in Argentina1Monash University, School of Geosciences, 3800, Clayton, Victoria, Australia ([email protected]; [email protected])
2Universidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina
Melanie Finch1, Maria Gabriela Fuentes2, Pavlína Hasalová1, Raul Becchio2, Nicholas Hunter1, and Roberto Weinberg1
RecrystallisedQtz+Fsp+Bt
RecrystallisedQtz+Fsp+Bt
RecrystallisedQtz+Fsp+Bt
RecrystallisedQtz+Fsp
Kfs(ii)
Pl(i)
Pl(i)
Pl
Pl
(iii)recrystallised
Pl
Pl
Kfs(ii)
Pl
Pl
Kfs
Kfs
Kfs
Qtz
Qtz
Qtz
Qtz
Fig. 12. Qtz ribbons wrapping around feldspar porphyro-clasts in protomylonites in (a) XPL and (b) PPL. Feld-spars show a variety of deformation mechanisms includ-
ing brittle fracture (i), free grain rotation (ii), and partial to complete dynamic recrystallisation (iii and yellow arrows). Qtz ribbons (orange arrows), formed through high temperature grain boundary migration, are occasionally isoclinally folded as a result of wrapping around the por-phyroclasts. When a feldspar porphyroclast wrapped in a quartz ribbon recrystallises and is sheared the result is a fine-grained mixture of feld-spar and quartz - this process destroys the compositional layering in the mylonite and connectivity of the phases resulting in a more homog-enously mixed matrix. Section parallel to stretching lineation.
1 cm
Brittle overprint of ultramylonites
Fig. 11. Breccia of silicified granite.
Fig. 9. Pegmatite clasts disaggregated through ductile shearing during mylonitisation faulted during the late brittle event.
Fig. 10. Pseudotachylyte in mylonitic gran-ite.
Fig. 4. Progressive disaggregation of pegmatite dykes (top) forms disconnected dykelets (bottom) and eventually discrete porphyro-clasts (middle). Rock face is vertical and parallel to stretch lineation.
Fig. 6. Asymmetric folds in Opx-Grt leucogranite in a mylonitic Grt+Crd+Opx+Sil migmatite. Top-to-SW thrusting (rock face is vertical and parallel to stretching lineation).
Opx+Grt leucosome
Fig. 7. Top-to-SW shear in mylonitic Opx migmatite (rock face is parallel to stretching lineation.
The El Pichao shear zone
Kfs
(i)Kfs
(i)Kfs
(iii)recrystallised
Kfs
Kfs
Qtz
recry
stallis
ed
Qtz+Kfs
Qtz
2 mm
Recrystallised Qtz+Kfs+Bt
Recrystallised Qtz+Kfs+Bt
1010
010
00S
ampl
e/ R
EE
cho
ndrit
e
REE Chondrite (Boyton, 1984)
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
SQ74
SQ86
SQ75
0.01
0.1
110
Sam
ple/
Ave
rage
cru
st
Average crust (Weaver & Tarney, 1984)
Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y Tm Yb
a) b)
Fig. 16. a) Chondrite normalised REE abun-dances and b) average crust normalised incom-patible element abundances of mylonites, gran-ites, migmatites, and metasedimentary rocks. REE patterns indicate that the mylonites are en-riched in REEs compared to granites proximal to the shear zone. Samples of the protomylonite, mylonite, and ultramylonite (green lines) are identical to each other and coincide with the field in grey corresponding to the composition of the
regional metasedimentary rocks (the Puncoviscana Formation). These results indicate that ultramyloniti-sation was not caused by mass loss or the infiltration of a fluid - that is, the El Pichao shear zone is a prod-uct of closed system shearing.
Geochemistry: closed system shearing
Cafayate granite
El Pichao mylonites (SQ30A-protomylonite; SQ80-ultramylonite; SQ77a-mylonite)
Mu+bi schist(SQ24-El Pichao; SQ33- Colalao del Valle)
Granites close to Sierra de Quilmes
Puncoviscana Formation near Sierra de Quilmes (punco1)
Puncoviscana Formation samples from Sierras Pampeanas (n = 157)
Sierra de Quilmes granites(SQ74 SQ75 San Pedro-Cafayate granite; SQ86 Tolombon tonalite)
Pun
covi
scan
aFo
rmat
ion
sam
ples
G
rani
ticsa
mpl
es
2 mm
El Pichao shear zone: key points
Opx+Grt leucosome
Mylonitic migmatite
Fig. 8. Geological map of El Pichao shear zone showing thrusting of granulite facies migmatites onto amphibolite facies schists. Waypoints are marked by black circles. The main shear plane dips to the NE with a down-dip stretching lineation (stereonets). All stereographic projections are lower hemi-sphere equal-area, the mean plane (x) indicated with a great circle and mean pole with a gray circle.
2 cm
C'C'
CC
SS
Pegmatite dyke
Disaggregated pegmatite dykeDisaggregated pegmatite dyke
Pegmatite dyke
ProtomyloniteProtomylonite
Fig. 1. Palinspastic schematic representation of the Gondwa-nan continents during the Terra Australis orogeny. El Pichao shear zone formed during the Pampean (555–515 Ma) and Famatinian orogenies (~490–350 Ma). Modified from Schwartz et al (2008).
Fig. 15. Feldspar δ - clast in ultramylonite showing top-to-SW shear (xpl). Tails of delta clast are progressively disaggregated and recrystallised to form porphyroclasts. Large delta clast shows brittle fracture along twinning plane, recrystallisation at margins, and ro-tation. Section parallel to stretching lineation
Within the ultramylonitic core of the shear zone there is an 150 m-thick band of faulted breccia and pseudotachylyte, marking a period of brittle deformation that post-dated ductile thrusting and mylonitisation.
Fig. 2. The Sierras Pam-peanas mobile belt and the Sierra de Quilmes. Location of the studied El Pichao shear zone shaded in grey in the inset and shown in detail in Fig. 8.
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Nooutcrop
37
60
58
40
59
25
4044
26
4136
4634
41
5348
4545
605960
453045
4750
6164
38
14
45
50
43
38
72
65
48
24
34
78
3937
23
42
3232
5288
29
48
38
80
4136
4546
60
4436
2438
2125
4630
5731
23
4520
20
2214
4544
3328
24
Foliation
lineation
x = 091/39n = 46
x = 073/41n = 36
x = 092/39n = 28
Stretch
Felsic volcaniclastic rock
OpxGrt-CrdGrt Grt-Crd-Opx-Sil
Granite MigmatiteUltramylonite
Mylonite
ProtomyloniteGranite Migmatite
Granite MigmatiteOrthogneiss
Granite
Peritectic minerals in Tolombóncomplex migmatites
Breccia and pseudotachylyte
Grt-pelite
SchistMyloniteUltramylonite
Boundary between Opx-present and Opx-absent migmatites
Ultramylonitic core
Managua river
Anchillo river
Tolombón complex
500 metresN
Agua del Sapo complex
Tolombón complex
Tolombón complex79
41
29
No outcrop
Stretchlineation
High strain zone
3.5 km thick
Foliation
454546
49x = 083/41n = 73
Ultramylonitic core
1 km thick
The El Pichao shear zone (PSZ) is part of a system of thrust shear zones of the Sierras Pampeanas which outcrop discontinuously in NW Argentina (Fig. 2). This system is inter-preted as the major orogenic front of the Pampean and Famatinian orogenies at the western margin of Gondwana (Fig. 1).
The PSZ contains a high strain zone >3.5 km thick and a 1 km thick ultramylonitic core that overprints a granitic protolith (Fig. 8).
Ultramylonitic shear zones of this thickness are very rare. Other shear zones of comparable thickness include the Tres Arboles shear zone of the Sierras Pampeanas (15 km thick; Fig. 2; Whitmeyer & Simpson, 2003), the Grease River shear zone of the western Canadian shield (<1 km thick; Dumond et al., 2008), the Main Central Thrust of the Himalaya (~650 m thick; Srivastava & Srivastava, 2010), and the shear zones related to the Pan-African orog-eny in NW Africa (3 – 400 m thick; e.g., Arthaud et al., 2008; Ferkous & Leblanc, 1995).
The thick mylonites of the western Canadian shield have been previously reported to be a result of high temperature recrystallisation of feldspar porphyroclasts (Hanmer et al 1995).
Feldspar porphyroclasts of the PSZ mylonites show three main behaviours: (a) syn-shearing brittle fracture, (b) dynamic recrystallisation, or (c) grain rotation (Figs. 12, 15). This indicates that feldspar was a hard phase and therefore ultramylonitisation was not attributable purely to high temperature recrystallisation. This implies very high strain rates (γ >100; Norris and Cooper, 2003) and large displacements (>100 km), comparable to that of major shear zones globally.
Fig. 5. δ- (top) and σ- porphyroclasts in ultra-mylonite resulting from prolonged shearing of pegmatite dykelets. View is parallel to stretching lineation.
N 100 km
ARG
ENTI
NA
La Rioja
Tucumán
Tres Arbolesshear zone
CH
ILE
64° W68° W
28º S
High-grade
Bandedschist
Puncoviscana Formation
Qui
lmes
Fiam
balá
Am
bato
San Luis
Aconquija
San Juan
Cafayate
Altautina
Jujuy
Cafayate
Qui
lmes
Córdoba
Anca
sti
Calchaquies
Cumbres
SaltaSalta
32º S
Argentina
Chi
le500 km
Argentina
Chi
le
ParaguayBoliviaBolivia Paraguay
UruguayUruguay
The Sierras Pampeanas mobile belt
El Tigreshear zone
La Chilcashear zoneLa Chilca
shear zone
Precordillera exotic
terrane
Cordillera frontal
Low-grade
Colalaodel Valle
Tolombon
Cafayate
San Antonio
Santa Maria
Ruinas de Quilmes
26º 20'
26º 40'
26º 00'
66º 00'66º 20'
10 km
Tolombón Complex
Agua del SapoComplex
N
EL PICHAOSHEAR ZONE
El Divisidero
OvejeriaAnchillo
The Sierra de Quilmes
Quilmes
Managua
Fig. 13. Mica-, and feldspar- fish, and garnet with Fsp strain shadows indi-cating top-to-SW thrusting (section parallel to stretching lineation; PPL).
100 µm
Qtz ribbons
Qtz ribbons Kfs-fish
Bt
Sil
100 µm
CrdBt
CrdBt
GrtGrt
NENE
NENENENE
NENE
SWSW
Antarctica
India
Australia
West Gondwana
Pampean and Famatinian
orogens
Paleo-Pacific Ocean
East Gondwana
Terra Australis orogen
South America
Africa
NENE
NENE
NENE
NENE
SWSW
SWSW
SWSW
SWSW
SWSW
SWSWSWSW
Opx leucosomeOpx leucosome
Ultramylonitic coreFoliation
1 mm
Fig. 14. Crd-fish partially re-placed by Bt and Sil, indicating top-to-SW shear and Qtz ribbon showing subgrain rotation recrysta l l i sa t ion (white arrows; sec-tion parallel to stretching lineation; XPL).
Microstructures of the El Pichao shear zone
Outcrop structures of the El Pichao shear zone
Fig. 3. Ultramylonite grading into proto-mylonite with asymmetric dykelets indicating top-to-SW shear. Pegmatite dykes are sheared into discontinuous lenses, and the feldspar porphyro-clast percentage is ~20% in mylonites and >50% in protomylonites. Rock face is vertical and paral-lel to the stretching lineation.
References: Arthaud, et al (2008) in Pankhurst, R., et al eds., Geol. Soc. Lon., v. 294, p. 49-67; Boynton (1984) in Henderson, P. ed, Rare Earth Element Geochemistry, 63-114; Dumond et al (2008) Chem. Geol., v. 254, p. 175-196; Ferkous & Leblanc (1995) Min. Dep., v. 30, p. 211-224.; Hanmer et al (1995) J. Str. Geol., v. 17, no. 4, p. 493-507; Norris & Cooper (2003) J. Str. Geol. v. 25, no. 12, p. 2141-2157; Schwartz et al (2008) J. Geol., v. 116, no. 1, p. 39-61; Srivastava and Srivastava (2010) J. Geol. Soc. India, v. 75, p. 152-159; Weaver & Tarney (1984) Nature, v. 310, p. 575-577; Whitmeyer & Simpson (2003) J. Str. Geol., v. 25, no. 6, p. 909-922.
ProtomyloniteProtomylonite
MyloniteMylonite
UltramyloniteUltramyloniteSWSW NENENENESWSW
El Pichao shear zone of western Gondwana
ProtomyloniteProtomylonite
UltramyloniteUltramylonite
UltramyloniteUltramylonite
(ii)Kfs
a)a) b)b)