Four phases of Orosirian metamorphism in the north North China Craton (NNCC): Insights into the regional tectonic framework and evolution

Abstract

The early Precambrian tectonic framework and evolution of the North China Craton (NCC) remain diversely contentious. This paper presents an overview mainly of the progresses in metamorphic studies for the NNCC and proposes alternative litho-tectonic division and tectonic scenarios for the Paleoproterozoic NNCC based on the recognized four phases of metamorphism. Five litho-tectonic units can be divided in the NNCC, including the Yinshan Block (YB), Wulashan Reworked Belt (WRB), Jining Mobile Belt (JMB), Huai'an Reworked Belt (HRB) and the Longgang Block (LB). All of them underwent different metamorphic modification during the Orosirian period at 1.98–1.80 Ga. The Orosirian metamorphism in the NNCC includes four phases. The first-phase metamorphism (M-I) occurred at >1.95 Ga, is the medium-P/T type marked by high-pressure (HP) granulites. It shows clockwise P–T paths characterized by the pre-Pmax (peak pressure) prograde and post-Pmax decompression, indicating a crustal thickening orogeny. The second-phase metamorphism (M-II) occurred during 1.94–1.90 Ga, is the low-P/T type marked by ultrahigh-temperature (UHT) granulites. It shares clockwise P–T paths characterized by the pre-Tmax (peak temperature) decompression-heating from HP granulite conditions and post-Tmax isobaric cooling. This UHT metamorphism may ascribe to conductive and advective heating from mantle upwelling and hyperthermal mafic magma intraplating in a post-orogenic setting with back-arc extension. The third-phase metamorphism (M-III) is the high-P/T type characteristic of eclogites occurred at >1.88 Ga, which, together with ∼1.90 Ga ophiolitic mélanges, suggests oceanic subduction beneath the northern margin of the NCC. The fourth-phase metamorphism (M-IV) is also the medium-P/T type with clockwise P–T paths, occurred during 1.88–1.82 Ga. It is represented by the Barrovian metamorphic zones in the east WRB, staurolite-overprints in the north HRB, metabasic dykes up to HP granulite facies in the north LB, and HP granulites in the south YB, registering another collisional orogeny in the NNCC. Based on these four phases of Orosirian metamorphism, and combined with other geological data in the NNCC, we present an alternative tectonic model which involves the following tectonic scenarios: (i) The Siderian–Rhyacian (2.46–2.00 Ga) extension occurred in back-arc or within-plate settings, resulted in the YB separated from LB, and the Paleoproterozoic volcano-sedimentary sequences, granite intrusions and basic dykes formed in the WRB, JMB, HRB. (ii) The early Orosirian (2.00–1.95 Ga) crust thickening orogeny occurred on account of the collision between the NCC and an unknown continent, as indicated by M-I metamorphism, which gave rise to an orogen trended mainly in north-east between the YB and LB, nominated as the west North China Orogen (WNCO). (iii) The mid-Orosirian (1.95–1.88 Ga) oceanic subduction and back-arc extension occurred as suggested by M-II and M-III metamorphism. And (iv) the late Orosirian (1.88–1.82 Ga) collisional orogeny resulted from a collision between the NCC and the south Siberian Craton, which gave rise to an orogen trended in east–west, named the north North China Orogen (NNCO).