A Neoarchean arc-backarc pair in the Linshan Massif, southern North China Craton

Abstract

The Linshan Massif is located in the southern North China Craton (NCC) and consists mainly of tonalitic (TTG) and dioritic gneisses, and a volcano-sedimentary-dominated assemblage, all of which were intruded by post-collisional potassic granites. The volcano-sedimentary-dominated assemblage is composed mostly of deformed and foliated basaltic amphibolite and meta-sedimentary rocks, with minor interlaying meta-gabbros, forming a stack of tectonically imbricated sedimentary and volcanic rocks. The TTG gneisses formed at 2.6–2.52 Ga, and the meta-gabbros, basaltic amphibolites and diorites formed at 2.51–2.50 Ga. They underwent subsequent metamorphism at 2.49–2.48 Ga. Furthermore, all their igneous zircons possess large positive εHf (t) values (+2.9 to +7.4). The basaltic amphibolites can be divided into two groups: depleted basalts and enriched basalts. Depleted basalts are characterized by flat LREE patterns and low Th/Nb ratios with positive Nb anomalies, similar to N-MORB-type patterns. Enriched basalts display LREE-enriched patterns with high Th/Nb ratios and pronounced negative Nb anomalies, showing an arc-related affinity. The meta-gabbros have LREE-enriched patterns with negative Nb and Zr anomalies, consistent with subduction-related signatures. Considering mixed arc- and MORB- signatures with positive zircon εHf(t) values, we propose that the meta-gabbros and basaltic amphibolites may have formed in a back-arc setting. The tonalites and the diorites showing LREE-enriched patterns and depleted in HFSEs are interpreted to represent arc-related magmatism. Combined with the documented 2.55 Ga Dengfeng fore-arc accretionary complex, we propose the following geodynamic evolution model for the Southern NCC: (1) at 2.6–2.54 Ga, a TTG-dominated oceanic arc developed above a west-dipping subduction zone, forming the 2.55 Ga Dengfeng accretionary complex and related tectonic units; (2) at 2.52–2.50 Ga, slab rollback subsequently drove back-arc rifting of the mature TTG-dominated intra-oceanic arc, resulting in the formation of two arc segments with the intervening Linshan back-arc basin; and (3) at 2.50–2.48 Ga, the TTG-dominated arc collided with the Eastern Block resulting in the closure of the back-arc basin and formation of the accreted arc-back-arc package in the Linshan Massif. We suggest that the Neoarchean Central Orogenic Belt in the southern NCC contains at least two amalgamated oceanic arc terranes, separated by a belt of back-arc basin magmatic rocks caught within an intra-oceanic mélange/suture zone.