10:00 AM - 10:15 AM
[SMP35-04] Pressure-temperature-time evolution of high-grade metamorphic rocks from Tenmondai Rock in the northeastern part of the Lützow-Holm Complex, East Antarctica
Keywords:the Lützow-Holm Complex, P-T-t evolution, zircon U-Pb dating, rare earth element
Tenmondai Rock area is one of exposures in the amphibolite-granulite transition zone in the complex. It is dominantly composed of garnet-biotite gneisses and migmatites with layers of garnet-biottie gneisses and amphibolites as well as intruded granitoids. Some previous studies reported limited petrological and geochronological data from this region such as the peak P-T conditions of ca. 750°C and 7.2-7.5 kbar (Hiroi et al., 1983) and Rb-Sr whole rock isochron age of 780±70 Ma (Nakajima et al., 1988). In this study, we performed detailed petrological and geochronological investigations of various metabasites and garnet-biotite-sillimanite gneiss from this region to unravel P-T-t evolution of the lower-grade part of the LHC.
The metabasites are dominantly composed of hornblende and/or orthopyroxene, plagioclase, garnet, and biotite. Orthopyroxene + plagioclase symplectite surrounding garnet in one sample suggests isothermal-decompression probably along a clockwise P-T path. Garnet-biotite-sillimanite gneiss (metasediment), which comprises garnet, plagioclase, quartz, K-feldspar, biotite and sillimanite, contains intercalated kyanite-bearing quartzo-feldspathic leucosome possibly formed by prograde partial melting.
The application of hornblende-plagioclase geothermometer and garnet-hornblende-plagioclase-quartz geobarometer for one metabasite (Ts11021001A) yielded 747-855°C at 8 kbar and 8.02-8.62 kbar at 800°C, respectively. Similar P-T ranges (805-847°C at 8 kbar, and 6.5-9.25 kbar at 800°C) were obtained from other metabasite sample (Ts11021106A) using garnet-orthopyroxene geothermometer and garnet-orthopyroxene-plagioclase-quartz geobarometer. The results of ternary feldspar geothermometer for garnet-biotite-sillimanite gneiss (Ts11021008B) and kyanite-bearing leucosome (Ts11021008A) also show a temperature range of 700-850°C.
U-Pb dating for magmatic cores of zircon in metabasites by LA-ICP-MS yielded early to middle Neoproterozoic (ca. 900-750 Ma) ages. In addition, the detrital cores of zircon in garnet-biotite-sillimanite gneiss (Ts11021008B) and kyanite-bearing leucosome (Ts11021008A) show predominant Meso- to Neoproterozoic (ca. 1200-800 Ma) ages with small amount of Paleoproterozoic (ca. 2000 Ma) ages. These results suggest Neoproterozoic magmatism and deposition of sediments derived from predominant Meso- to Neoproterozoic provenances and potential older (Paleoprotrozoic ages) crust. On the other hand, late Neoproterozoic-Cambrian (ca. 620-500 Ma) metamorphic ages have been yielded from homogeneous rims of magmatic zircon grains from both metabasites and pelitic gneisses. Kyanite-bearing leucosome also contains slightly older metamorphic zircons (ca. 640-630 Ma). The age range is consistent with the results of previous studies, and may suggest multiple growth of zircon during high-grade metamorphism.
This study will also discuss REE chemistry of metamorphic zircon and garnet, and time scale of metamorphism at the lower-grade part of the LHC.