Permian-Triassic Highly Fractionated I-Type Granites In The Baituyingzi Mo-Cu Area, Southeastern Inner Mongolia, Ne China: Petrogenesis And Tectonic Implications

The Permian-Triassic plutons along the Chifeng-Bayan Obo Fault, southeastern Inner Mongolia, help to provide important constraints on the craton-orogenic boundary of the North China Craton (NCC) and the eastern Central Asian Orogenic Belt (CAOB) and the regionally coeval Mo-Cu mineralization environment. New LA-ICP-MS zircon U-Pb data indicate that two granites from the Baituyingzi Mo-Cu area in Chifeng city were emplaced at ca. 274 Ma and ca. 242 Ma. They have high SiO2 (73.44-77.82 wt%) and Na2O + K2O (8.49-10.28 wt%) contents and are high-K calc-alkaline, with very high differentiation indexes (DI = 97.1-98.3) and low Zr saturation temperatures of whole rocks (703-738 degrees C). They are also characterized by (La/Yb)(N) ratios of 4.09-9.30 and enrichments in Rb and Th relative to Nb and Ta, with pronounced negative Eu (Eu/Eu* = 0.14-0.50), Ba, Sr, P, and Ti anomalies. Comparatively, the Permian granite has slightly higher zircon epsilon Hf(t) values (+1.7 to +4.6) than those of the Triassic granite (epsilon Hf(t) = -3.4 to +0.1) and is less alkaline-rich. These results, combined with petrographic data, indicate that they are highly fractionated I-type granites. The granites were likely derived from Mesoproterozoic juvenile crust that had experienced subduction-related fluid metasomatism, and the resulting magmas then underwent extensive fractional crystallization. In combination with regional geology, we propose that: (a) the granites have tectonic affinity with the CAOB and serve as spatial markers for monitoring the juxtaposition of the NCC and the CAOB, and (b) the studied granites and coeval porphyry deposits likely formed in a syn- to post-collisional setting.