S. Piazolo et G. Markl, Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions, J METAMORPH, 17(1), 1999, pp. 91-107
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.