Mesozoic reworking of the Paleozoic subducted continental crust beneath the south-central margin of the North China Block: Geochemical evidence from granites in the Xiaoqinling-Xiong’ershan region

Abstract

Mesozoic granites are common in the south-central margin of the North China Block, beneath which the North Qinling microcontinent was subducted northwards in the early Paleozoic to subarc depths for ultrahigh-pressure (UHP) metamorphism. Although the continental deep subduction would have produced UHP eclogite facies metamorphic rocks in the North Qinling orogen, it is intriguing whether the Mesozoic granites record reworking of the crustal rocks from the subducted North Qinling microcontinent. This issue is addressed by an integrated study of zircon in-situ U-Pb ages and Hf-O isotopes as well as whole-rock geochemistry for four typical granitic plutons from the Xiaoqinling-Xiong’ershan region in the south-central margin of the North China Block. These plutons are metaluminous to weakly peraluminous, and belong to high-K calc-alkaline to shoshonitic series. They are enriched in LREE and LILE but depleted in HREE and HFSE. In terms of trace element and Sr-Nd-Hf isotope compositions, these granites can be categorized into two groups. Group I granites from the Huashan, Wenyu and Heyu plutons are high in Sr concentrations and Sr/Y and (La/Yb)N ratios without any significant Eu anomalies and thus similar to adakitic rocks, and have more enriched Sr-Nd-Hf isotope compositions. In contrast, Group II granites from the Funiushan pluton have low Sr concentrations and Sr/Y and (La/Yb)N ratios with negative Eu anomalies, and show less enriched Sr-Nd-Hf isotope compositions. Although zircon U-Pb dating for syn-magmatic zircons from the two groups of granite gives consistent emplacement ages of 120 to 137 Ma, relict zircons in them show different U-Pb ages. In detail, relict zircons from Group I granites exhibit Neoarchean to Paleoproterozoic U-Pb ages, with Hf-O isotope compositions similar to those of the ancient basement such as the Taihua Group and the Xionger Group in the south-central margin of the North China Block. In comparison, relict zircons from Group II granites display not only Paleoproterozoic U-Pb ages but also Neoproterozoic and early Paleozoic U-Pb ages. The Paleoproterozoic relict zircons exhibit Hf-O isotope compositions similar to those of the ancient basement in the south-central margin of the North China Block, whereas the younger ones have Hf-O isotope compositions consistent with those of the Neoproterozoic protoliths of metamorphic rocks and early Paleozoic granitoids in the North Qinling orogen. Therefore, Group I granites were derived from partial melting of the ancient basement in the south-central margin of the North China Block, whereas Group II granites were produced by partial melting of a mixing source that was composed of not only the ancient crust from the North China Block but also the juvenile crust from the North Qinling microcontinent. This provides the geochemical evidence for late Mesozoic reworking of the crustal rocks from the North Qinling microcontinent through its northward subduction in the early Paleozoic beneath the south-central margin of the North China Block.