Paleoproterozoic ultrahigh-temperature mafic granulites with a high-pressure prograde path from the Alxa Block: Implications on the tectonic evolution of the Khondalite Belt, North China Craton

Abstract

Ultrahigh-temperature (UHT) granulites, a prominent feature of Paleoproterozoic orogenic belts, preserve a record of geodynamic processes during the Precambrian (Archean–Paleoproterozoic). Quantitative pressure–temperature–time (P–T–t) paths of these UHT granulites can constrain the tectonic processes and metamorphic evolution in such a tectonic regime. Here, UHT mafic granulites with a high-pressure (HP) prograde path are first reported in the Diebusige Complex in the Alxa Block, western part of the Khondalite Belt (KB), North China Craton (NCC). The detailed petrographic studies show that two mafic granulite samples preserve corona textures around relict garnet or garnet pseudomorphs (completely replaced by plagioclase), and a third mafic granulite sample has a relatively simple mineralogy with a granoblastic-polygonal texture. These mafic granulites have similar peak (T_max) assemblages of clinopyroxene + orthopyroxene + plagioclase + ilmenite ± garnet ± amphibole ± quartz + melt. Phase equilibrium modelling and Ti-in-amphibole and rare earth element (REE)-based thermometries all constrain similar peak conditions of ~880–950°C/~8.5–10 kbar implying an ~100°C/kbar apparent geothermal gradient for these mafic granulites. Based on the corona textures or pseudomorphs of garnet and mineral assemblages, we identified a P_max (~14 kbar) prograde stage before the T_max stage. Thus, a clockwise P–T path with heating and decompression followed by near-isobaric cooling (IBC) is recorded from these UHT mafic granulites. In addition, zircon and apatite SHRIMP or LA–ICP–MS U–Pb dating yields an age interval of ~1.81–1.7 Ga, which is interpreted as representing the cooling time from ~900–800°C to ~575°C at the middle-upper crustal levels (<25 km deep) for these mafic granulites, with an ~1.5–2.5°C/Myr cooling rate. The new P–T–t path of these rocks includes high-pressure prograde, UHT peak, and slow cooling retrograde processes, which implicates a post-collisional tectonic setting for UHT metamorphism in the KB and the processes of collision, exhumation, and cooling of the KB.