Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China

摘要

The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U-Pb and Hf isotopic compositions and whole-rock geochemical and Nd-Sr iso-topic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO + Na2O + K2O) = 1.01-1.46 molecular ratios), with 0.6-5.6 wt% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu~* = 038-0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393 Ma, consistent with a period of intensive magmatic activities and high temperature metamorphism in the Chinese Altai. While in situ zircon Hf isotopic analyses for these granites give predominantly positive ε_(Hf)(t) values (+0.8 to +12.8), a few inherited zircons yield negative ε_(Hf)(t) values from - 12.5 to - 153. The U-Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative ε_(Nd)(t) values (- 33 to - 0.5) and relatively high initial ~(87)Sr/~(86)Sr ratios (0.7079-0.7266), distinct from those of the 1-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5 kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672-861 °C The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.