Paleoproterozoic ultrahigh-temperature granulites in the North China Craton: Implications for tectonic models on extreme crustal metamorphism

Abstract

Ultrahigh-temperature (UHT) metamorphic rocks associated with the ‘Khondalite Belt’ within the Inner Mongolia Suture Zone (IMSZ) provide robust evidence for extreme thermal metamorphism in the North China Craton (NCC). The IMSZ marks the collisional suture between the Yinshan Block to the north and the Ordos Block to the South as the NCC was incorporated within the Columbia supercontinent amalgam during Paleoproterozoic. Here we present a synthesis of the salient features of the UHT rocks from the NCC including petrologic indicators, fluid characteristics, and monazite and zircon chronometry on the extreme crustal metamorphism. The granulites carry diagnostic UHT mineral assemblages including sapphirine+quartz, low Zn/Fe3+ spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature mesoperthite. The stability fields of the typical mineral assemblages, conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these UHT rocks experienced metamorphic temperatures up to or in excess of 1000°C at ca. 10kbar, followed by an isobaric cooling segment. The rocks were exhumed along a near-isothermal decompression path. Microstructures, mineral reactions and phase equilibria modeling suggest an anti-clockwise P–T path, similar to those displayed by metamorphic orogens developed in subduction-collision settings. The dominant category of fossil fluids preserved within the major UHT minerals is CO2, consistent with the stability of the broadly anhydrous mineral assemblage in these rocks. Both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the UHT event as around 1.92Ga. The Paleoproterozoic high grade metamorphism younging from 1.95Ga in the western domain to 1.92Ga in the eastern domain of the Khondalite Belt might suggest a scissor-like closure of oblique collision between the Yinshan and the Ordos Blocks.The salient features of the UHT metamorphism in the NCC include: (1) extreme metamorphic temperatures at moderate pressures, (2) dominantly anhydrous nature of the mineral assemblages, typically the stability of orthopyroxene, (3) common presence of CO2-rich fluid inclusions as the trace of the ambient fluid, (4) regional extent of the UHT granulites, and (5) the association of the UHT orogen with an accretionary belt in a continental collisional suture. We evaluate the diverse models on the generation of UHT orogens including their formation in thickened and inverted back-arcs, orogen self-heating through heat producing elements, heat and CO2 input by plume impingement below a carbonated tectosphere, and asthenospheric upwelling through ridge subduction and slab-window process or during post-collisional slab break-off. The ultra-hot and dry UHT rocks in the NCC provide one of the well preserved examples from the Paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm.