Microblock amalgamation in the North China Craton: Evidence from Neoarchaean magmatic suite in the western margin of the Jiaoliao Block

Abstract

The Archaean Earth is considered to have been characterized by microcontinents that formed, dominantly, through the accretion of oceanic arcs and plateaus. The North China Craton (NCC) provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the Archaean. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block that forms part of the composite Eastern Block of the NCC. We present petrological, geochemical and zircon U–Pb geochronological data from the Taipingzhai charnockite suite, and associated amphibolites, metagabbros and orthogneisses from the Qianxi Complex. Geochemically the rocks show a wide range of SiO2 (charnockite suite: 52.57–75.50wt.%; metagabbro: 43.71wt.%; amphibolite: 50.24wt.%; garnet-bearing biotite: 63.73wt.%), and MgO (charnockite suite: 0.89–5.01wt.%; metagabbro: 3.99wt.%; amphibolite: 6.23wt.%; garnet-bearing biotite: 2.08wt.%). The composition of the felsic units straddle from diorite through syeno-diorite to granite with both alkalic and subalkalic affinity, with dominantly magnesian composition and arc-related features. Their immobile trace element relationships suggest calc-alkaline affinity. They show positive Pb, Ba, La, Nd, and Gd and negative Nb, Ta, Sr, Th and Ti anomalies with slightly negative anomalies of Ce and Y, attesting to arc-related features. In tectonic classification diagrams, the rocks plot in the VAG+syn-COLG field or the VAG area suggesting subduction-related origin.The dominant population of zircons in all these rocks displays magmatic crystallization features including high Th/U values with core-rims textures indicating subsequent thermal events. The zircon U–Pb data yield upper intercept ages of 2587±10Ma to 2543±17Ma and 207Pb/206Pb mean ages of 2578±7.3Ma to 2536±8Ma for the charnockite suite, marking the timing of emplacement of the arc magmas. The overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207Pb/206Pb ages between 2533Ma to 2490Ma. Zircons in the metagabbro preserve upper intercept ages of 2556±20Ma representing the crystallization age of this rock. The younger ages of 2449±58Ma (upper intercept age) and 1845±25Ma (207Pb/206Pb spot age) are interpreted to represent subsequent multiple thermal events in this area. Zircons in the amphibolite preserve the 207Pb/206Pb mean age of 2539±9Ma, representing the crystallization age of this rock. The garnet-bearing biotite gneiss shows an upper intercept age of 2562±10Ma (MSWD=0.66; N=36) and the 207Pb/206Pb mean age of 2561±9Ma (MSWD=0.63; N=33) which is taken to represent the crystallization age of this rock. Some inherited zircons are also identified with 207Pb/206Pb ages of 2664±26Ma and 2628±26Ma. Zircon Lu–Hf data show dominantly positive εHf(t) values and combined with crustal residence ages, the results suggest Mesoarchean to Neoarchean juvenile crust formation in the NCC. We interpret the data presented here to represent a phase of major late Neoarchaean arc magmatism along the western margin of the Jiaoliao Block related to the birth of microcontinental nuclei within the NCC. Our data suggest that the Western and Eastern Blocks might not have existed as discrete crustal blocks, and that the construction of the NCC is a result of the assembly of several microblocks or terranes at the end of Archaean. Similar Archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early Earth.