PYROXENITES AND MEGACRYSTS IN ALKALINE BASALTIC MAGMAS FROM NORTHERN VICTORIA LAND (ANTARCTICA): CONSTRAINT ON THE THERMAL EVOLUTION OF SUB-CONTINENTAL LITHOSPHERE

Abstract

Cenozoic alkaline basaltic magmas of northern Victoria Land, Antarctica yield a wide variety of ultramafic xenoliths including clinopyroxene-rich xenoliths and augite and kaersutite megacrysts. Pyroxenites and megacrysts were collected in Mt Melbourne Volcanic Province, respectively at Browning Pass, Baker Rocks (both sites are near the coast of the Ross Sea) and at the base of Mt. Overlord, about 80 km inland. Sixteen samples have been petrographycally examined and chemically analyzed, including trace element analyses of minerals by ion microprobe. The dominant assemblage of pyroxenites consists of clinopyroxene + olivine ± spinel ± amphibole with strongly variable modal proportions of minerals. Rare interstitial plagioclase is found in Browning Pass samples. All pyroxenites belong to Al-augite Group (Frey and Prinz, 1978); the collection includes wehrlites, clinopyroxenites and rare dunites and olivinewebsterite. Moreover, Baker Rocks xenoliths are sometimes composite showing wehrlite and depleted lherzolite lithologies in sharp contact. Their typical texture is cumulitic and in only few cases deformation fabrics and polygonal texture may be found. Mt Overlord samples show widespread metasomatic effects, evidenced by clinopyroxene-amphibole replacement and by variably re-crystallized melt pools. In the Browning Pass pyroxenites the metasomatic effects are incipient as evidenced by rare amphibole lamellae in substitution of the clinopyroxene. In few samples from Baker Rocks and Mt. Overlord clinopyroxenes show orthopyroxene exsolutions. Major and trace element compositions of bulk rock and minerals indicate that pyroxenites and megacrysts are related to Cenozoic alkaline magmas and that formed through polybaric fractionation processes affecting McMurdo alkaline magmas. We evaluated the P and T of segregation of clinopyroxenes from parent basaltic melts. The obtained data depict a P-T gradient of 15°C/kbar for the whole area in contrast with widespread volcanic activity. If we hypothesize that pyroxenite veins represent the walls of the paths followed by adiabatically rising basaltic melts, we can assess the temperature of the ambient mantle, if we know the mantle potential temperature (Tp) at some depth. From the onset of crystallization in primitive melts we get Tp =1300-1350°C at P= 2- 2.5GPa. In this way we obtain, for the present day geothermal gradient of the area, a more realistic value of 30°C/kbar. This value is considerably higher than the 8°C/kbar obtained from the equilibration coarse crystals of lherzolite nodules in the same area and allows to clearly identify the heating of the mantle due to Cenozoic magmatism. The different thermal profile obtained from the pyroxenites with respect to those obtained by spinel-peridotite xenoliths, thus seems to be linked to the geodynamic evolution in this area. In particular the computed geothermal gradient fits with the heat-flow values of this area that range between 66 e 114 mW/m2 (Blackman et al., 1987; Della Vedova et al., 1992). The gradient that we propose seems to be compatible with a static rift geotherm of 90mW/m2 . See figure.