Petrogenesis of ca. 1.95Â Ga meta-leucogranites from the Jining Complex in the Khondalite Belt, North China Craton: Water-fluxed melting of metasedimentary rocks

Anatexis occurs in response to changing P-T-fluid conditions during orogenesis, is thus crucial for understanding the tectonic evolution of orogenic belts. Here we report an integrated petrological, geochemical, whole-rock oxygen isotopic and zircon U-Pb-Hf-O isotopic study of meta-leucogranites from the Jining Complex, North China Craton, to unravel its petrogenesis and constrain the tectonic evolution of the Paleoproterozoic Khondalite Belt. The meta-leucogranites are weakly peraluminous with homogeneous minimum-melt compositions, and are characterized by low Rb contents and Rb/Sr ratios, high Ba and Sr contents and Sr/Y ratios, and moderately fractionated REE patterns without obvious Eu anomalies. These trace-element characteristics differ from that of leucogranites derived from fluid-absent melting. Considering their low whole-rock Zr saturation temperatures (746-780 °C) and comparable compositions with experimentally generated melts produced by water-fluxed melting, we infer that the meta-leucogranites are low-temperature granites, and were generated by water-fluxed melting of metasedimentary rocks at 6-8 kbar. SIMS zircon U-Pb analyses suggest that the meta-leucogranites were emplaced at ∼1.95 Ga or slightly earlier, and were subsequently metamorphosed and cut by a pegmatite dike at ∼1.92 Ga. The abundant inherited zircons yield nearly concordant 207Pb/206Pb ages ranging from 2.24 to 1.98 Ga, which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the Khondalite Belt. The Hf isotopic compositions of inherited and magmatic zircons (εHf(t) = +0.2 to +7.7) closely overlap with the time evolved εHf range of detrital zircons from metasediments in the Khondalite Belt. Different types of zircons (inherited, magmatic and metamorphic zircons) have similar δ18O values of 7.2-9.2‰, which is lower than that of metamorphic zircons (10.0-13.6‰) from surrounding metasediments. Thus, we propose that the meta-leucogranites were formed by water-fluxed melting of metasedimentary rocks from the Khondalite belt at ∼1.95 Ga during the prograde upper amphibolite-facies metamorphism.