Chemical evolution of granitic rocks in the East Antarctic Shield, with particular reference to post-orogenic granites

Abstract

Granitic intrusives of the East Antarctic Shield range in age from early Archaean ( ∼ 3930 Ma) to Cambrian and show systematic variations in chemical and isotopic characteristics. Many Archaean orthogneisses are of tonalitic to granodioritic composition and represent new continental crust derived from the mantle by a two-stage process involving hydrous melting of mafic crustal rocks. Syn-orogenic granitic rocks are generally of more potassic composition and were derived by intracrustal melting under lower P H 2O conditions. They are mostly of more typical I-type (derived from igneous source materials), but there is an increasing proportion of S-types (derived by melting of sedimentary materials), with time. Late and post-orogenic intrusives are mostly granites (s.s.) with A-type (anorogenic) affinities and were derived by melting of relatively anhydrous granulite-facies lower crustal rocks. They are characterised by marked enrichment of Y, Zr, Nb, La and Ce, relative to P, Ti and Sr, and commonly have high F, Fe Mg , Zn Fe and Ga Al . Plagioclase, apatite and Ti minerals were probably residual phases during partial melting, as well as being involved in fractional crystallisation, whereas other minor phases, such as zircon, were apparently relatively soluble in the high-temperature melts. Emplacement of A-type granites at about 500 Ma may have been associated with internal fracturing of Gondwanaland before eventual break-up.