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 meltrelated processes. The compositional and textural information of ternary feldspars in felsic orthogneisses can be therefore potential tools for understanding UHT crustal processes.