Petrogenesis of the Middle Devonian syenites in the Miyun–Huairou Region of Beijing and its tectonic implications

Abstract

Three Devonian magmatic rocks have been identified from the mid‐western part of the Yanshan Thrust‐and‐Fold Belt (YTFB). Here, we present new geochronological, Sr–Nd–Hf isotopic, and whole‐rock chemical data. Our new LA‐ICP‐MS U–Pb dating recognized syenite plutons of ca. 385 Ma, which include Laogongying, Shicheng, and Qiaomaiyu, respectively, around the Yunmeng Mountain in the Miyun–Huairou area of Beijing. Rocks from three plutons are characterized by low SiO2 and high alkali contents, especially high K2O, and they show syenite and foid syenite compositions. They display slight–middle light REE enrichment, no negative or positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf, and Ti, enrichment in Ba, K, and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios from 0.708203 to 0.710477, εNd(t) values from −12.26 to −13.28, and εHf(t) values from −19.92 to −17.96. This study shows the Middle Devonian intrusions were produced by fractional crystallization of a parental magma, which was derived by interaction between mantle‐derived alkaline magmas of a type I‐enriched mantle and pre‐existing continental crust. Tectonic background was likely emplaced in a back‐arc extension environment related to southward subduction of the Palaeo‐Asian oceanic plate or during the cessation of the subduction. Feldspar geothermometer and aluminium‐in‐hornblende barometry studies of the Middle Devonian plutons yielded emplacement temperature of ~700°C and pressure of ~6.5 kbar, indicating the emplacement depths of about 24 km. Combined with previous geobarometry results on the Middle Devonian plutons within the Inner Mongolia Palaeo‐uplift (IMPU) on the northern margin of the NCC, the strong uplift and exhumation the IMPU were identified during Middle Devonian to Late Triassic times. The exhumation was more than 4–5 km than those of the IMPU, resulting in widely absent of Palaeozoic strata within the YTFB.