Feldspar in felsic orthogneiss as indicator for UHT crustal processes

Abstract

Felsic orthogneisses with tonalitic and granodioritic compositions are dominant in the ultrahigh－temperature (UHT) metamorphic terrane of the Archaean－early Proterozoic Napier Complex, East Antarctica. Antiperthitic or mesoperthitic ternary feldspar occurs in addition to Qtz, Opx with/without Cpx in these felsic orthogneisses. Such feldspar grains commonly display zonal structure with respect to the volume of exsolution lamella; lamellae－enriched core and lamellae－free rim. Pre－exsolution one－phase compositions have been recovered separately for the feldspar core (an:ab:or = 20－28:53－56:17－28 in tonalitic samples and an:ab:or = 15－17:37－ 48:36－48 in granodioritic samples) and for the whole feldspar grain (an:ab:or = 24－29:58－63:13 in tonalitic samples and an:ab:or = 18－20:45－56:24－37 in granodioritic samples). Application of feldspar thermometry for these recovered one－phase compositions and the feldspar cores yield a temperature range of 940－1100 °C, which is relatively higher temperatures than the whole feldspar grain giving > 850－1070 °C. These compositional differences between core and rim of single feldspar grains are probably due to either sub－solidus or melt－ related processes. The compositional and textural information of ternary feldspars in felsic orthogneisses can be therefore potential tools for understanding UHT crustal processes.