Continental outbuilding along the margin of an Archean cratonic nucleus in the North China Craton

Abstract

The history of crustal growth in the Early Earth, formation of cratonic nuclei and the building of large continents have important implications in understanding the secular evolution of our planet. The North China Craton (NCC) is envisaged in recent models as a collage of several Archean microblocks which were amalgamated along multiple zones of ocean closure during the late Neoarchean, and offers window to gain insights on the origin of cratonic nuclei and their subsequent outbuilding. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Qianhuai Block. We present petrological, geochemical and zircon U–Pb geochronological and Lu-Hf isotopic data on a suite of rocks ranging from hornblendite through gabbro and dioritic gabbro to TTG (tonalite-trondhjemite-granodiorite) and granitoids exposed in the Miyun area, and representing the cross section of the lower to mid segments of a continental magmatic arc. Geochemically, the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity, magnesian composition and arc-related features. The metagranites and TTG gneisses are characterized by negative Nb-Ta anomalies and positive K and Pb anomalies. The diorite, gabbros and hornblendite display negative anomalies of Th-U, Nb-Ta and Zr-Hf and positive anomalies at Ba, Pb and Sm with negative Eu anomalies and minor positive Ce anomalies, attesting to arc-related origin. In the tectonic discrimination diagrams, the rocks plot in the volcanic arc field, indicating formation within a subduction-related setting.

The oldest magmatic population with ages of ca. 2560 and 2529 Ma from the hornblendite and the TTG gneiss represent the magma emplacement ages with fractional crystallization of underplated basalts derived from slab melts, leaving hornblendite as the cumulate. Magmatic zircon grains from the metagranite and granodiorite show upper intercept ages of 2457 ± 21 Ma and 2475 ± 12 Ma whereas those from the TTG gneisses yield ages of 2529 ± 19 Ma and 2522 ± 38 Ma. Younger age groups of 2444.4 ± 8.3 Ma, 2418 ± 16 Ma and 2161 ± 31 Ma also occur in these rocks indicating multiple thermal events. The zircon grains from diorite and gabbros show 207Pb/206Pb weighted mean ages of 2411 ± 11 Ma–2486 ± 13 Ma with younger ages of 2.0–2.37 Ga. These rocks also carry older xenocrystic grains of ca. 2.51–2.65 Ga. The zircons in hornblendite show an upper intercept age of 2560 ± 33 Ma and a lower intercept age of 1947 ± 54 Ma. Young groups of ca. 2.2 Ga and 1.8 Ga are also found in this rock. The ca. 1.9 to 1.8 Ga ages correspond to the timing of metamorphism. The Hf isotopic data on zircons from the various rock types of the Miyun magmatic suite show both negative and positive εHf(t) values ranging from −9.64 to +7.04 with Hf model ages in the range of 2606–2844 Ma (TDM1) and 2635–3147 Ma (TDM2), suggesting crustal growth through both juvenile magmas from depleted mantle sources and reworked Meso- to Neoarchean crustal components. Our study reveals that the ca. 2.7–3.0 Ga core of the Qianhuai microcontinent represents one of the ancient cratonic nuclei in the NCC that underwent extensive continental outbuilding during late Neoarchean – early Paleoproterozoic through multiple magmatic events mostly related to convergent margin arc-type processes. It is possible that similar processes operated around the other Archean microcontinents in the NCC in the process of building the major crustal blocks which were finally amalgamated at the end of the Paleoproterozoic.