Late Neoarchean crustal evolution of the eastern North China Craton: A study on the provenance and metamorphism of paragneiss from the Western Shandong Province

Abstract

Metamorphosed clastic rocks provide important constrains on the composition of upper continental crust. Here we report and characterize a sequence of Neoarchean paragneiss in association with migmatite from the Qixingtai area in Western Shandong Province (WSP) in the North China Craton (NCC), that represent metamorphosed moderate- to fine-grained clayey clastic rocks.The garnet from the paragneiss is characterized by nil or weakly compositional zoning. The biotite shows low TiO2 content (<2wt.%) and high Fe/[Fe+Mg] molar ratios (0.45–0.48). The P–T estimates suggest that the paragneiss sequence was metamorphosed at 5.5–5.7kbar and 678–679°C. The detrital zircons from the sillimanite garnet gneiss yield concordant 207Pb/206Pb ages from 2.78 to 2.53Ga (discordance ≤15%), suggesting that the protolith sediments were deposited after 2.53Ga. The overgrowth rims on the detrital zircons of the paragneiss and those from the leucosome of the migmatite show similar low Th/U values, moderate Th and high U and common lead contents, indicating that the metamorphic and anatectic zircons formed under fluid-rich conditions. The intrusive contact between the massive monzogranite and the banded migmatite suggests that the regional metamorphic/anatectic event occurred before the emplacement of the monzogranite. However, the upper intercept ages, defined by the overgrowth rims of the pre-existing zircons from the leucosome, are broadly similar to those of the crystallization of the magmatic zircons from the monzogranite within errors at ∼2.50Ga.The paragneiss exhibits SiO2 contents in the range of ∼58wt.% (garnet gneiss)–∼70wt.% (sillimanite gneiss), and negative correlations with Al2O3 (23.4–15.4wt.%), TiO2 (0.90–0.35wt.%), MgO (4.5–1.1wt.%), FeOt (8.3–2.6wt.%) and Cr (382–131ppm). The REE patterns also show a systematic change from garnet gneiss to sillimanite gneiss. In trace element discrimination diagrams, the paragneiss plots between the end-members of komatiite/basalt and TTG/granite, suggesting that the sediments sourced from a mixture of ultramafic–mafic and felsic protoliths. Combined with the geochronological data, we suggest that the 2.75–2.71Ga komatiite–tholeiite sequence of the Taishan association and TTGs in the central terrane and the 2.56–2.53Ga TTGs in the southwestern part of the WSP as the potential provenance for the clastics.The protolith of the paragneiss in the Qixingtai area belong to the sedimentary sequence of the Taishan association. Spatially associated coeval arc-like assemblage and other geological evidence suggest that, (1) the metasediments were likely deposited in a back arc basin, (2) the upper amphibolites-facies metamorphism, constrained by the mineral assemblages in the paragneiss, might be related to the closure of the basin, and (3) the generation of large volumes of matured K-rick continental crust occurred not earlier than 2.53Ga in the WSP granite–greenstone belt.