A latest Jurassic A-type granite in the Middle of Inner Mongolia: Petrogenesis and tectonic implications

Abstract

The Late Jurassic tectonic environment of western part of northern margin of the North China Craton (NNCC) remains poorly understood due to weak magmatism in the Jurassic period and complications by supposedly superimposed effects of activities related with the Mongol-Okhotsk and the Paleo-Pacific tectonic regimes. In this study, we for the first time discovered and reported an A-type granite (Yingongshan granite) in the western NNCC in the Late Jurassic period. The Yingongshan granite is dated at 150 Ma years old. The Yingongshan granite is characterized by high contents of total alkalis (Na2O + K2O = 8.23–9.51%) and high field strength elements (Zr + Nb + Ce + Y = 289–705 ppm) and Ga/Al ratios (10,000 × Ga/Al = 3.01–4.05). Based on these geochemical characteristics and differentiation trend, it can be reasonably inferred that initial composition of the Yingongshan granitic melt exhibited an A-type magmatic affinity. In addition, results of zircon saturation thermometer show high magmatic temperature (> 887 °C) for the initial magma, which is consistent with its A-type granite affinity. Furthermore, the Yingongshan granite showed negative εNd(t) values (−14.5 to −13.3) and zircon εHf(t) values (−13.6 to −17.4) and contain mafic magmatic enclaves. Compared with the host Yingongshan granite rocks, the enclaves showed even lower εNd(t) values (−14.9 to −14.6) and zircon εHf(t) values (−16.9 to −19.5). Such “reversed isotope” characters indicate injection of a magma derived from the enriched lithospheric mantle during formation of the Yingongshan A-type granite. Therefore, we suggest that the latest Jurassic Yingongshan A-type granite was derived from partial melting of preexisting calc-alkaline granitoids in the crust at low pressure and high temperature under influence of magma intrusion derived from the enriched lithospheric mantle. Discovery of the Yingongshan A-type granite in this study indicates that local extension occurred as early as 150 Ma in the western NNCC, although previous studies suggest that extensive extension occurred since 132 Ma. In addition, based on the data obtained in this study combined with statistics of published zircon U–Pb age data and distribution localities of magmatic rocks in the NNCC from the literatures, we suggest that the latest Jurassic local extension in the western NNCC revealed in this study may have resulted from the collapse of the Mongol-Okhotsk orogen.