Monday, October 26, 2015

Discovery of Hadean Crustal Materials in SW China

Discovery of Hadean–Mesoarchean crustal materials in the northern Sibumasu block and its significance for Gondwana reconstruction


Li et al


The micro-continental blocks in SE Asia are thought to be derived from the East Gondwana which has a basement as old as Hadean–Mesoarchean. However, such old crustal materials have not been found anywhere in SE Asia. In this paper, we report the occurrence of the Hadean–Mesoarchean crustal materials in the northern Sibumasu block, SW China. Our finding is based on inherited zircon U–Pb ages and Hf isotope model ages of co-magmatic zircon crystals from early-Paleozoic S-type granitoids in the northernmost Sibumasu. LA-ICP-MS zircon U–Pb isotopic analysis reveals that some S-type granitoids in this region formed between 468 Ma and 447 Ma. These rocks are strongly peraluminous, with high A/CNK ratios > 1.2 and normative corundum content > 2 wt%, and have low CaO/Na2O ratios less than 0.3 which indicates that they formed by anatexis of metapelitic crustal rocks. The ca. 470–450 Ma S-type granitoids contain inherited zircon crystals as old as Mesoarchean (∼3.1 Ga). The ɛHf(t) values of zircon crystallized from the magmas of these rocks vary from −49 to +16, with major peaks approximately at −46, −35 and −27. The corresponding model ages for the formation of the source crust are ∼4.39 Ga, ∼3.62 Ga and ∼3.12 Ga. This, together with the discovery of ∼3.1 Ga inherited zircon in the granitoids, indicates that the northern Sibumasu block has Hadean–Mesoarchean crustal materials. A Gondwana-wide comparison of crustal formation time data reveals that these Hadean–Mesoarchean crustal materials show similar age distribution with the crusts of the Pilbara and Yilgarn Cratons, Western Australia. Their derivation analysis provides a new line of evidence for the majority view that the Sibumasu block was attached to NW Australia before its breakup from Gondwana.

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