Paleoproterozoic orogenesis of the eastern Alxa Block, North China Craton: Evidences from metamorphic evolution and zircon U–Pb geochronology of garnet-bearing mafic granulites

Abstract

High-pressure (HP) granulites provide valuable insights into the geodynamic processes of the lower continental crust and continent-collisional orogenesis. In this study, the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb analysis, phase equilibrium modelling, and geothermobarometry are presented for the HP mafic granulite in the Bayanwulashan complex of the eastern Alxa Block, North China Craton (NCC). These results are used to reveal the characteristics of granulite-facies metamorphism during the Paleoproterozoic orogenesis of the eastern Alxa Block, as well as their tectonic implications. The HP mafic granulites are thought to have undergone three distinct stages of metamorphism. The first stage (M1) is characterized by the inclusions of amphibole (Amp-1) + plagioclase (Pl-1) + clinopyroxene + ilmenite within garnet. In the second stage (M2), garnet is accompanied by matrix minerals consisting of clinopyroxene + coarse-grained amphibole (Amp-2) + cores of coarse-grained plagioclase (Pl-2) + quartz + rutile + ilmenite + melt. The third stage (M3) is marked by the presence of plagioclase corona (Pl-3) around garnet and the absence of rutile in the matrix. Phase equilibrium modelling constrained the metamorphic P–T conditions of the second stage (M2) and third stage (M3) to be 858–897 °C/11.1–11.5 kbar and 810–900 °C/8.0–11.1 kbar for garnet-bearing mafic granulite sample ALS2149, respectively, which are similar to those of the garnet-bearing mafic granulite sample ALS2122. Grt–amp–pl–qz thermobarometry with compositions of mineral rims, yielded metamorphic conditions of 741–777 °C/7.7–7.8 kbar and 755–761 °C /6.6–7.2 kbar for the samples ALS2149 and ALS2122, respectively, which are lower than those constrained by P–T pseudosection modelling, and are considered to represent the retrograde metamorphic conditions. As a result, a clockwise P–T evolution characterized by near-isothermal decompression and later decompressional cooling, was constructed for the HP mafic granulites in the Bayanwulashan complex of the eastern Alxa Block, NCC. LA–ICP–MS U–Pb analysis on two types of metamorphic zircons yielded nearly consistent ages within uncertainty, which were combined to give a continuous apparent 207Pb/206Pb ages of ca. 1985–1740 Ma. These continuous and long age spectrums are likely related to a slow retrograde cooling following rapid exhumation. In conjunction with metamorphic data from the Diebusige complex, we suggest the eastern Alxa Block likely experienced a collisional orogenesis during late Paleoproterozoic. Additionally, our new data also suggest the affinity between the Alxa Block and the Khondalite Belt.