The relationship between tectonism and composition of granitoid magmas, Yarlarweelor Gneiss Complex, Western Australia

Abstract

The northwestern edge of the Archaean Yilgarn Craton in Western Australia was intensely deformed and metamorphosed during the Palaeoproterozoic Capricorn Orogeny (D 1n and D 2n) to form the Yarlarweelor Gneiss Complex. High-grade metamorphism and crustal thickening in the complex at ca. 1812 Ma during D 1n was accompanied by voluminous veins and sheets of I-type granite and pegmatite. Most of these rocks are leucocratic, and are characterised by low Rb, FeO T+MgO, Y, Zr, Th, U and total rare earth elements (∑REE), and high SiO 2, Ba, K/Rb and Ba/Rb, and chemically resemble leucosomes from stromatic migmatites (metatexites). The granite and pegmatite formed during compression and represent low to moderate degree melts of late Archaean and Palaeoproterozoic granitic rocks. At ca. 1800 Ma, dextral strike-slip faulting during D 2n followed exhumation of the Yarlarweelor Gneiss Complex, and was accompanied by intrusion of dykes and steeply dipping sheets of I-type granites. These granites have high K 2O/Na 2O ratios, high Rb, Zr, ∑LREE, Y, Th and U contents, and low K/Rb ratios similar to high-K granite melts and diatexite migmatites derived by large-scale biotite dehydration melting of igneous rocks. The source of the syn-D 2n granites was Palaeoproterozoic meta-igneous rocks. The change in granite chemistry between ca. 1812 and ca. 1800 Ma reflects a change in the nature of melting and melt extraction, which were in turn controlled by the style of deformation.