Geochemistry, Zircon U-Pb Dating, And Hf Isotopes Composition Of Carboniferous Granitoids In Xing'An Block, Ne China: Record Of Slab Break-Off Event In Eastern Central Asian Orogenic Belt

The Guang-Tou-Shan (GTS) granites, located in the Xing'an Block (XB), record Late Palaeozoic magmatism during the stage of convergence between micro-blocks in the eastern Central Asian Orogenic Belt (CAOB). We performed a detailed geochronological and geochemical study of GTS granites intending to obtain better constraints on the subduction processes of the Palaeo-Asian Ocean (PAO) in eastern CAOB. LA-ICP-MS U-Pb dating of zircons from GTS granitic rocks in western XB yielded emplacement ages of 332 +/- 2 Ma and 301 +/- 2 Ma, respectively, indicating a two-stage emplacement process. Geochemical analyses show that the Early Carboniferous monzogranite (332 +/- 2 Ma) have low values of the A/CNK and A/NK, are enriched in LILE (Rb, Sr, Ba) and LREE (LREE/HREE = 5.6-10.8), and depleted in HFSE (Nd, Ta, Zr, Hf) and P, Ti, which belong to I-type granites. The Late Carboniferous granodiorite (301 +/- 2 Ma) also displays LILE, LREE enrichment, and HFSE depletion with negative Nb-Ta anomalies, and contains relatively high Zr + Nb + Ce + Y contents (495-524 ppm) and high zircon crystallization temperature, indicating the Late Carboniferous granodiorite shows geochemical characteristics similar to those of typical A-type granites. The two-stage granitic rocks have epsilon(Hf)(t) values of +7.7 to +11.6 and yield a two-stage depleted mantle Hf model age of 0.66-0.88 Ga, suggesting that the GTS granitic rocks were derived from partial melting of juvenile arc-related mafic to intermediate rocks. The results, combined with regional magmatic information, indicate that the eastern CAOB experienced a subduction setting in the Early Carboniferous and extension setting in the Late Carboniferous. This change was most likely caused by the subduction slab break-off of PAO from the Late Carboniferous.