Petrogenesis of the Early Paleozoic Jiergalangtu pluton in Inner Mongolia:Constraints from geochronology, geochemistry and Nd-Hf isotopes

Early Paleozoic granites are distributed on the northern margin of Xingmeng orogenic belt discretely, and the genesis of these granites has important implications for the reconstruction of regional Paleozoic tectonic setting and orogenic evolution. Jier-galangtu pluton outcropped in northern Abag Banner and located in the middle of the early Paleozoic granite belt is the biggest com-posite pluton in the granite belt. Zircon LA-ICP-MS U-Pb dating shows that the early stage of the pluton was emplaced during 455.0~495.6Ma. Geochemically, the Jiergalangtu granites show moderate SiO₂ values (59.49%~68.22%) and depletion of magnesium and iron, with K₂O/Na₂O ratios less than 1 (0.64~0.85) and A/CNK values being 0.96~1.09; in addition, most of the samples present subalkaline and weak per-aluminous signature. The trace element data of these rocks display enrichment of Cs, Rb, Th, U, Pb and depletion of Ba, Sr, P and HFSE such as Nb, Ta, the pluton exhibits indistinct negative europium anomalies(δEu=0.04~0.25) with moderate total REE content. Jiergalangtu pluton shows depleted isotopic compositions, (⁸⁷Sr/⁸⁶Sr)_i=0.7053~0.7034, ε_Nd(t)=0.39~4.29, and both samples for Hf isotopic analysis have positive ε_Hf(t) values, being 7.6~10.8 and 3.7~7.9 respectively. Integrated geochemical, geochronology and Sr-Nd-Hf isotopic data suggest that the Jiergalangtu granites were generated in early Paleozoic and resulted from the subduction of Paleo-Asian Ocean along Sunid-Xilinhot. The granites were formed by partial melting of juvenile crust which was modified by subduction slab fluids. The back-arc extension and opening of Hegenshan Ocean possibly resulted in the separation of Jiergalagntu pluton from predominant Sunid-Xilinhot island arc. Along with the closure of Paleo-Asian Ocean, the pluton ultimately occurred on the northern margin of Xingmeng orogenic belt and was separated from the subduction zone by He-genshan ophiolite complex.