Petrogenesis of the Caledonian granites in the northern part of Miaoershan pluton, southwestern Hunan Province: Evidence from petrology, geochemistry and zircon U-Pb age

The Miaoershan pluton, located at the junction of southwestern Hunan and Northern Guangxi, is a composite batholith composed of Jinningian, Caledonian, Indosinian and Yanshanian granites. Among them, Caledonian granites constitute the main body of the composite batholith, which can be divided into six intruding episodes. Through zircon LA-ICP-MS dating, the ages of the first, second and sixth episodic granites were measured at (428.1±3.6) Ma, (420.3±3.4)-(421.3±3.2) Ma and (408.3±3.5) Ma, respectively. The average (⁸⁷Sr/⁸⁶Sr)_i, ε_Nd(t) and the δ¹⁸O values of the Caledonian granites are 0.71765, -9.7, and 9.8, respectively. Caledonian granites are generally crust derived weak peraluminous-strong peraluminous granites, formed in post-collisional tectonic environment characterized by weakened compression and relaxed stress after the peak period of collision and extrusion. Their material and heat sources are different in the early and late stages. The early stage granites were derived from upper crustal metamorphic mudstone, psammite and/or metamorphic igneous rocks, and their formation temperature is relatively high(> 875℃). The late stage granites were derived from upper crustal metamorphic mudstone with lower formation temperature (< 875℃), showing the characteristics of typical S-type granites.