U–PB AND HF ISOTOPIC STUDY OF ZIRCONS FROM GNEISSIC ROCKS FROM THE CHINESE ALTAI: IMPLICATIONS FOR PALEOZOIC RIDGE SUBDUCTION?

The Chinese Altai is a type region of the Central Asian Orogenic Belt. Gneissic rocks in the Chinese Altai Mountains have been interpreted as either Paleozoic metasedimentary rocks or Precambrian basement. Our petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from I- type granites formed in a subduction environment. Zircons from the banded paragneiss cluster predominantly between 466 and 528 Ma, some give Neoproterozoic ages, and a few yield discordant Paleoproterozoic to Archean ages. In contrast, zircons from the gneissic granitoids define single populations for each sample with mean ages between 380 and 453Ma. These data suggest an Early Paleozoic arc setting for the Chinese Altai (e.g. Sun et al., 2008).

Hf isotopic compositions indicate that both juvenile/mantle and crust materials were involved in the precursor magma of zircons older than 420 Ma, whereas juvenile materials were overwhelmingly dominant in generating magma after 420 Ma. This may imply that the lithosphere was significantly modified in composition by a rapid addition of melt from the mantle at ~420 Ma (Sun et al., 2009). The formation of voluminous granitoids, mafic rocks with complex compositions, and the association of adakite + high-Mg andesite + boninite + Nb-enriched basalt, and the coeval high-T, low-P metamorphism at this time suggest that the dramatic Hf isotopic change was possibly due to the onset of ridge subduction in the evolution of the Chinese Altai (Windley et al., 2007).

This study is supported by Hong Kong RGC grant (HKU704307P).

References

Sun M., Yuan C., Xiao W., Long X., Xia X, Zhao G., Lin S., Wu F._, Kröner A., 2008. Zircon U–Pb and Hf isotopic study of gneissic rocks from the Chinese Altai: progressive accretionary history in the early to middle Palaeozoic. Chemical Geology, 247, 352-383.

Sun M., Long X., Cai K., Jiang Y., Wong P.W., Yuan C., Zhao G., Xiao W., Wu F., 2009. Early Paleozoic ridge subduction in the Chinese Altai: insight from the abrupt change inzircon Hf isotopic compositions. Science in China, Series D, 39, 1-14.

Windley B.F., Alexeiev D., Xiao W.J., Kroner A., Badarch G., 2007. Tectonic models for accretion of the Central Asian Orogenic Belt. Journal of the Geological Society 164, 31-47.