Geochemistry of the Paleoproterozonic Nanying granitic gneisses in the Fuping complex: Implications for the tectonic evolution of the Central Zone, North China Craton

Abstract

The Nanying granitic gneisses represent a major ∼2.0 Ga magmatic event in the Fuping Complex of the Central Zone of the North China Craton. They occur in the Chengnanzhuang shear zone, which is chiefly developed along the contact between the Neoarchean Fuping tonalite-trondhjemite-granodiorite (TTG) gneisses and the Wanzi paragneisses. The Nanying granitic gneisses were derived from monzogranite and syenogranite, with minor amounts of quartz diorite and granodiorite, and belong to the metaluminous to peraluminous, high-potassium calc-alkaline series, correlations between major and trace elements, and similarities in chondrite-normalized rare-earth-element (REE) patterns, support field relationships (e.g. gradational contacts) indicating that the Nanying granitic gneisses originated from similar magmas. The granodioritic, monzogranitic and syenogranitic gneisses have similar ranges of initial 87Sr/86Sr values (0.7030-0.7093, 0.7021-0.7193 and 0.7049-0.7111, respectively), although these variations are partly attributable to local resetting of the isotopic system during late deformation/hydrothermal alteration. The whole-rock 208Pb/204Pb, 206Pb/204Pb and 207Pb/204Pb values of the Nanying granitic gneisses are 37.32-76.8, 15.24-23.82 and 14.92-17.92, respectively. These Sr-Pb isotope systematics, together with previously published Nd isotope compositions, suggest that the Nanying granitic gneisses were derived from partal melting of the ∼2.50 Ga Fuping TTG gneisses, with local assimilation involving the Wanzi paragneisses. The Nanying granitic gneisses are geochemically similar to the CCA and CCL groups of granitoids, except for a few samples, which plot in the HLO group. The Nanying granitic gneisses exhibit depletion in Rb, Sr, Ba, Ti and U, and enrichment in Th, Nb, Zr, Hf and REE, similar to those of syn-shear, high-potassium calc-alkaline granitoids and late-shear alkaline granitoids; they therefore represent the product of a syn-collisional to post- collisional magmatic event. This collisional event, which is significantly older than the ∼1.84-1.80 Ga event that has been proposed for the juxtaposition of the Eastern and Western Archean continental blocks in the final stabilization of the North China Craton, supports a tectonic model involving multiple collisional events for the amalgamation of the internal lithotectonic domains within the Central Zone and the assembly of the North China Craton as a whole. © 2004 Elsevier Ltd. All rights reserved.