Studies On The Geochronology, Geochemistry Of The High-Grade Gneiss Of The Larsemann Hills And Its Adjacent Region, East Antarctica

The Larsemann Hills and its adjacent region , East Anatrctica, is critical for understanding the assembly and breakup of Rodinia and Gondwana. the thesis concerns geochemical and geochronological study of magrnatic and metamorphic rocks from the area. the main progress states as following:1. The mafic-felsic composite gneiss lithologicat units, is interpreted as a possible bimodal metavolcanic unit based on geochemical data. U-Pb SHRIMP zircon and Sm-Nd data both felsic gneiss and mafic gneiss indicate that basement rocks of the region are Mesoproterozoic in age.The crystallization ages of metavolcanic rocks were determined at c.1100Ma, the mafic-felsic composite gneiss has experienced two major episodes of granulite facies metamorphism, one at c.990Ma (M1), the other at c.550Ma (M2).2. The geochemistry of the pelitic granulites of the Larsemann Hills suggests the arkosic to shaley nature of the protolith. The rocks are characterized by a high maturity index. The geochemical criteria imply active or passive continental margin nature.3. Grandidierite Kornerupine and tourmaline occur in high-grade pelitic gneiss from the Larsemann Hills,the borosilicates are restricted to a single layer several meters thick that can be traced for several hundred meters,. Boron isotope data suggest the boron-rich series originaly contained non -marine evaporites.4. Sparsely distributed ultramafic granulite lenes and boudins occur in metapelitic and quartzo-feldspathic gneiss in the Larsemann Hills and the Bolingen Hills. The rocks have characters of high Mg/Mg+Fe ratio (＞0.7), and high Cr (up to 1000ppm) and Ni (up to 500ppm) content. Sm-Nd isotope data demonstrate a mixing of sources from the depleted mantle and the crust.5. The metamorphic evolution of the different lithological units as deduced from the SHRIMP zircon work and Sm-Nd data is summarized in Tablel.Table 1. Summary of the lithological and metamorphic evolution as determined by U-Pb 6. The geochronological data from the Larsemann Hills and adjacent region is readily correlated with published data from the Mawson Coast, northern Prince Charles Mountains and Rayner Complex in the east Antarctic, and the East Ghats of India (Boger et al.2000). All yield syn-orogenic ages of between 990 and 900 Ma. 990-900Ma orogenesis has been recognised in the Larsemann Hills- northern Prince Charles Mountains- Mawson Coast- Rayner Complex- Eastern Ghats provinces in east Antarctica and India. The new data provide impotant constraints in reconstructions of the supercontinent Rodinia.