TY - JOUR
T1 - Origin of fine-scale sheeted granites by incremental injection of magma into active shear zones
T2 - Examples from the Pilbara Craton, NW Australia
AU - Pawley, M. J.
AU - Collins, W. J.
AU - Van Kranendonk, M. J.
N1 - Funding Information:
Research was supported by a University of Newcastle Research Scholarship (M. Pawley). The Geological Survey of Western Australia provided logistical support. Angela Melville is thanked for assistance in the field. Jürgen Streit and Aaron Yoshinobu provided invaluable comments on earlier drafts of the manuscript. Keith Benn and Robert Miller are thanked for thorough reviews. This paper is published with the permission of the Director of the Geological Survey of Western Australia.
PY - 2002
Y1 - 2002
N2 - In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magmadominated, dextral shear zone. It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
AB - In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magmadominated, dextral shear zone. It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
KW - Fabrics
KW - Granite magma
KW - Shear zones
KW - Shear-sense indicators
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U2 - 10.1016/S0024-4937(02)00076-2
DO - 10.1016/S0024-4937(02)00076-2
M3 - Article
AN - SCOPUS:0036263203
VL - 61
SP - 127
EP - 139
JO - Lithos
JF - Lithos
SN - 0024-4937
IS - 3-4
ER -