Microblock Amalgamation Research Articles

Paleoproterozoic arc magmatism in the North China Craton: No Siderian global plate tectonic shutdown

Arc magmatism in convergent plate margins has been a major contributor to continental growth. Following arc–arc and arc–continent collisions in the Archean leading to the amalgamation of micro-blocks, the North China Craton (NCC) witnessed major pulses of continental arc magmatism during the Paleoproterozoic. In this study, we present geochemistry, zircon U–Pb geochronology and Lu–Hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major Paleoproterozoic suture zones in the NCC — the Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO). Our zircon U–Pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207Pb/206Pb weighted mean ages of 2410±41Ma for metagranite, 2480±12Ma, 2125±18Ma, 1946±8Ma, 1900±15Ma and 1879±12Ma from metagabbros, 2446±11Ma from charnockite, and 1904±6Ma and 1901±9Ma from metatuffs. The 207Pb/206Pb upper intercept age of zircons in the khondalite shows 2102±76Ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros. The khondalites also carry a group of concordant metamorphic zircons with 207Pb/206Pb mean age of 1881±20Ma. Metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890±14Ma and 1852±19Ma respectively. The age data suggest prolonged arc magmatism in a convergent margin setting during ca. 2.48 to 1.9Ga, followed by metamorphism at ca. 1.89–1.85Ga associated with the final collision. Lu–Hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive εHf values (−1.9 to 6.8; mean 1.8). The εHf and TDMC data suggest that the magmas were mostly derived from Neoarchean and Paleoproterozoic juvenile components. The salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction-derived arc components with minor input from continental crust. The results presented in this study, together with those from previous investigations in different domains of the IMSZ and TNCO suggest major Paleoproterozoic arc magmatic events in the NCC lasting for nearly 600millionyears associated with the final assembly of the crustal blocks into a coherent craton. Construction of the final cratonic architecture of the NCC thus witnessed not only the arc–continent amalgamations at 2.7–2.5Ga, but also major crust building events in the Paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin, followed by intense deformation and metamorphism during the final collision at 1.85–1.80Ga. The prominent Paleoproterozoic magmatic records in the NCC do not support the proposal of global plate tectonic shutdown in the Siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the Paleoproterozoic.

The early Precambrian odyssey of the North China Craton: A synoptic overview

The crustal growth and stabilization of the North China Craton (NCC) relate to three major geological events in the Precambrian: (1) a major phase of continental growth at ca. 2.7 Ga; (2) the amalgamation of micro-blocks and cratonization at ca. 2.5 Ga; and (3) Paleoproterozoic rifting–subduction–accretion–collision tectonics and subsequent high-grade granulite facies metamorphism–granitoid magmatism during ca. 2.0–1.82. The major period of continental growth during 2.9–2.7 Ga in the NCC correlates with the global growth of Earth's crust recognized from other regions. The enormous volume of tonalite–trondhjemite–granodiorite (TTG) rocks and associated komatiite-bearing magmatic suites developed during this period possibly suggest the manifestation of plume tectonics. The cratonization of the NCC at the end of Neoarchean at ca. 2.5 Ga (Archean–Proterozoic boundary) through the amalgamation of micro-blocks was accompanied by granulite facies metamorphism and voluminous intrusion of crustally-derived granitic melts leading to the construction of the basic tectonic framework of the NCC. Several Neoarchean greenstone belts surround the micro-blocks and represent the vestiges of older arc–continent collision. The next major imprint in the NCC is the Paleoproterozoic orogenic events during 2.35 -1.82 Ga which involved rifting followed by subduction -accretion -collision processes, followed by plume-triggered extension and rifting, offering important insights into modern-style plate tectonics operating in the Paleoproterozoic. Extreme crustal metamorphism and formation of high pressure (HP) and ultra-high temperature (UHT) orogens during 1950–1820 Ma accompanied the subduction–collision process and the suturing of continental blocks within the Paleoproterozoic supercontinent Columbia. Multiple subduction zones with opposing subduction polarity promoted the rapid assembly of crustal fragments of the NCC and their incorporation into the Columbia supercontinent. The HP and HT-UHT granulites demonstrate two main stages of metamorphism at ca. 1.95–1.89 Ga and at ca. 1.85–1.82 Ga, exhuming the basement rocks from lowermost crust level to the lower-middle crust level. With the emplacement of extensive mafic dyke swarms associated with continental rifting, and the intrusion of anorogenic magmatic suites, the evolution of the NCC into a stable continental platform was finally accomplished.

Geochronology and petrogenesis of Neoarchean potassic meta-granites from Huai'an Complex: Implications for the evolution of the North China Craton

Neoarchean potassic meta-granites occurring in the Huai'an Complex within the north central part of the North China Craton (NCC) comprise leucocratic syenogranites, biotite monzogranites, and garnet-bearing syenogranites. High-pressure garnet–granulites occur as lenses, sheets or dismembered dikes with the granitoids. Here we present SHRIMP zircon U–Pb data from the leucocratic syenogranites which show an age of 2493 ± 6 Ma (MSWD = 1), and LA-ICP-MS U–Pb data from the biotite-monzogranites and garnet–syenogranites, revealing their formation ages of 2437 ± 10 Ma (MSWD = 2.0) and 1977–2003 Ma, respectively. We also report metamorphic ages of ca. 1800 Ma from zircons in the garnet–syenogranites. Geochemical studies show low MgO (0.5–0.06 wt.%), Cr (6.19–0.53 ppm) and Ni (27.66–2.01 ppm) with varying SiO 2 (62.3 to 76.8 wt.%) contents, indicating their derivation from a crustal source. The leucosyenogranites exhibit elevated positive Eu anomalies with decreasing Sr, Th, U, and bulk REE concentrations, and Th/U ratio, whereas Rb and Ba values increase with decreasing Th/U ratio, indicating fluid–melt interaction. The leucosyenogranites have high Sr and Ba contents and Sr/Y and (La/Yb) N ratios and low Y and HREEs, with zircon εHf(t) values ranging from + 1.7 to + 5.8 and Hf model ages of 2.60–2.73 Ga, reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition, or an eclogitic mineral assemblage in the residue. Geochemical compositions of the biotite–monzogranites, with positive εNd(t) values (+ 2.3 to + 2.5) and model ages < 2.60 Ga, indicate that they are evolved partial melts from a thickened juvenile crust of garnet amphibolite composition, or with garnet and amphiboles in the residue, followed by fractionation dominated by plagioclase precipitation. The garnet–syenogranites display variable contents of SiO 2 (62.3–72.3 wt.%), and relatively high Zr (491–1096 ppm), Ga (23.5–31.2 ppm), Zn (38–124 ppm), Y (34.1–85.6 ppm) and Nb (9.4–90.0 ppm) abundances with 10 4 × Ga/Al ratios of 3.1–4.2. They also possess low Sr (50–161 ppm) concentrations and strong negative Eu anomalies. These features, integrated with Nd model ages of 2.46–2.50 Ga and relatively constant negative εNd(t) values (− 2.5 to − 2.9) with increasing SiO 2 contents, suggest A-type granitic affinities of intraplate extensional or rift setting with evolution of the magma from partial melts derived from Neoarchean tonalitic rocks. The distinct geochemical features and zircon saturation temperatures of the granitoids are indicative of high melting temperatures (900–1000 °C) at relatively low pressures (6–10 kbar). Based on previous works and our new data, we link the ca. 2500–2450 Ma and 2000 Ma magmatic events to a Neoarchean amalgamation of microblocks and Paleoproterozoic mobile belts. The metamorphic ages of 1800–1820 Ma might reflect an amphibolite facies retrograde event.