Paleoproterozoic metamorphism of high-grade granulite facies rocks in the North China Craton: Study advances, questions and new issues

Abstract

High-grade granulite facies rocks (one of extreme granulites) refer in particular to a part with high metamorphic temperature (HT) and high metamorphic pressure (HP) within granulite facies. A part of basement rocks of the North China Craton (NCC) experiences high-grade metamorphisms up to HP-HT granulite-facies or granuite-eclogite transition facies in Paleoproterozoic, which offers ideal natural laboratories to probe into early Precambrian tectonic evolution of the Earth. The studies of Precambrian high-grade metamorphic rocks have made steady progresses on distributions and occurrences of high-grade granulite facies rocks, their mineral interactions (including micro-inclusion mineral), metamorphic conditions, metamorphic chronologies, and metamorphic processes and P-T paths in recent decades. The concept of Incipient Plate Tectonics was proposed by Zhai (1997, 2004, 2012, 2014) and Zhai and Santosh (2011). However, some intractable and controversial scientific problems still exist, which are challenging and significant issues to understand early tectonic evolution of continent crust and the Earth.This paper reviews study advances of Paleoproterozoic high-grade granulite terranes in the NCC and overviews its prospect. Three fundamental characteristics are distinct: 1) Paleoproterozoic high-grade metamorphic rocks seemingly occur in area or broad pan-belt, which widely spread in the northwest NCC, middle NCC, south NCC, eastern NCC and Korean peninsula, in almost all outcrops of Precambrian basements of the NCC (Sino-Korean Craton) except for some coverage districts of Phanerozoic rocks; Three kinds of high-grade rocks have been distinguished, which are HP mafic granulite, HT pelitic granulite and ultramafic granulite. As their country rocks, some tonalite-trondhjemite-granodiorite gneisses (TTGs) could have undergone metamorphism under the same P-T conditions. 2) The HP mafic granulites and HT pelitic granulites are commonly spatially-associated with concordant occurrences, showing the both underwent contemporaneous deformation and metamorphism since peak metamorphic stage. The HP mafic granulites are suggested to be metamorphosed gabboric dykes by previous studies. Detail district geological mappings in some terranes also demonstrate that the gabbroic dykes intruded into sedimentary rocks and had been in position before peak metamorphic stage. In Jiaobei region of northern East Shandong Province, ultramafic granulites are distributed associated with HP mafic granulites and/or HT pelitic granulites. Metamorphism studies demonstrate that all the three granulite facies rocks record uplift processes and they were uplifted to surface as a whole. 3) All the three types of granulite facies rocks underwent three major metamorphic stages, which are peak HP-HT granulite facies, moderate granulite facies and amphibolites facies. The corresponding metamorphic ages are ∼1980–1900Ma, ∼1890∼1820Ma, and ∼1800Ma, respectively. Some sedimentary rocks underwent ultra-high temperature (UHT) metamorphism with the indicating mineral assemblages of sappirine+quartz. Eclogite facies minerals were found in a minor amount of HP mafic granulites, which were presented by pseudomorphed omphacite±rutile±quartz preserved as inclusion minerals in garnets, or by some matrix clinopyroxenes broken down to wormy symplektite of albite+Na-poor clinopyroxene. Base on P-T conditions of the metamorphism and the corresponding geochronologic data, it is calculated that the Paleoproterozoic metamorphic terranes in the NCC have obviously high geothermal gradient of about 14–28°C/km (average 21°C/km), and abnormally slow uplift rate of about 0.18–0.24mm/yr, both of which are remarkably different from Phanerozoic continent collisional belts.The concept of incipient plate tectonics is established for Paleoproterozoic metamorphism in the NCC. Being clearly different from tectonic style of Archean high-grade region and greenstone belt, Paleoproterozoic terranes are seemingly more similar to tectonic style of Phanerozoic continent-continent collision. Metamorphic pressures of ∼10–14Kbar in the HT pelitic granulites indicate that the supracrustal rocks had sunk into lower crust with depth of 40–50km and then were uplifted to the surface again. This implies that the NCC had attained a considerable thickness and an extra drive force could be necessary for so great vertical displacement of supracrustal rocks. In consideration of the broad distributions of these high-grade metamorphic rocks, some “quasi-rigid” blocks were proposed to have preliminarily formed in Paleoproterozoic, which could facilitate sinking and uplifting motion of these rocks by subduction-collision-like processes between micro-blocks. In other words, the principle of plate tectonics might have been operated at that time. However, these blocks are limited in rigidity for their disability to sink into depth of boundary between lithosphere and asthenosphere but activation only in crustal scale. In this regard, a process of “hot” subduction-collision is considered, which comprehensively includes various elements, such as smaller continent blocks, high geothermal gradient, tiny density distinction between subducted slab and wall rock (lower crust), which together define a motion limited in crustal scales with lacking in enough strengths of thrust force and buoyancy. The incipient plate tectonics have displayed lateral compressional structure and kinematic interaction between blocks, but it is much smaller in scale relative to Phanerozoic collisional orogeny, although the tectonic style is more or less similar.