Lithological units at the boundary zone between the Jining and Huai'an Complexes (central-northern margin of the North China Craton): A Paleoproterozoic tectonic mélange?

Abstract

In the central-northern margin of the North China Craton, the tectonic boundary between the Jining and Huai'an Complexes is well exposed in the Gushan area. In this boundary zone, exotic blocks of orthopyroxene-bearing monzonitic gneiss, garnet-bearing mafic granulite, marble, and calc-silicate were identified in the meta-psammitic and meta-pelitic matrix. The likely nature of the rock units is a tectonic mélange, which is a key to understand the tectonic evolution of the craton.In this paper, geochronological and geochemical studies have been carried out on representative lithologies from the Gushan area to discuss their petrogenesis and geodynamic settings. The opx-bearing monzonitic gneisses (U–Pb zircon emplacement age of 2.17Ga) are high-K calc-alkaline to shoshonitic I-type rocks with trace-element magmatic arc signature. The trace elements, whole rock Sm–Nd and zircon Lu–Hf isotopes suggest that they were possibly derived from partial melting of Neoarchean lower crust (garnet amphibolite) in a continental arc. The protoliths of the garnet-bearing mafic granulites are inferred to have formed between 2.37 Ga and 1.95Ga, possibly at 2.2Ga. They show N-MORB geochemical affinity and were most likely produced by partial melting of spinel lherzolite in a mid-ocean ridge setting. Detrital zircons from one metapsammite sample (garnet-bearing quartzofeldspathic gneiss) yield nearly concordant 207Pb/206Pb ages ranging from 2.1Ga to 2.0Ga. In situ U–Pb and Hf isotopic data of zircons from the metapsammite sample support that the metasedimentary protoliths were probably derived from a mid-Paleoproterozoic continental arc. Three metamorphic age groups (1.95–1.93Ga, 1.86–1.83Ga, and 1.80Ga) are revealed by the opx-bearing monzonitic gneisses, garnet-bearing mafic granulites, garnet-bearing quartzofeldspathic gneiss, and sillimanite–garnet–K-feldspar gneiss. The predominant group (1.95–1.93Ga) is considered to record the age of near-peak granulite facies metamorphism. The 1.86–1.83Ga group may result from decompression or the thermal imprint of the collision between the Eastern and Western Blocks. The 1.80Ga group is consistent with the age (1.81Ga) of the ductile deformation of the garnet–K-feldspar mylonite, and is interpreted as the timing of a relatively rapid tectonic extension. Thus, the Paleoproterozoic Gushan mélange formed and evolved in a subduction–collision–exhumation process, and finally situated in a likely suture zone between the Eastern and Western blocks or sub-cratons.