Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as pa rt of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P-T conditions observed in this study are remarkably similar t o the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates e xperienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the ass emblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clino pyroxene+/-quartz, scapolite+/-anorthite+/-calcite+clinopyroxene+wollastoni te. These assemblages constrain the peak metamorphic conditions to 830+/-20 degrees C, 6.8+/-0.5 kbar and X-CO2>0.46. During retrogression, highly flu oric humite-group minerals (humite, clinohumite, chondrodite) replaced fors terite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of ano rthite. Metamorphic conditions were about 650 degrees C, 4.5+/-0.7 kbar, 0. 2<X-CO2(fluid)<0.36, and the co-existence of garnet, clinopyroxene, wollast onite and quartz constrains f(O2) to FMQ-1.5 log units. Mineral textures in dicate a very limited influx of H2O-rich fluid during amphibolite facies re trogression and point to significant variations of fluid composition in mm- sized areas of the rock. Gypsum was observed in two samples; it probably re placed metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosit es, A-type granitoids) and the character of metamorphism between 610 and 51 0 Ma suggest that the crustal thermal structure was characterized by a long -lived (50-100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.