The 2.03–2.02 Ga arc–back-arc mafic magmatism from the Zhongxiang Complex, northern Yangtze Block: Implications for Columbia supercontinent assembly

Abstract

To better understand Paleoproterozoic tectonic processes in the Yangtze Block during the assembly of the Columbia supercontinent, an integrated geochronological and geochemical study of amphibolite facies mafic rocks exposed in the Zhongxiang Complex in the northern Yangtze Block is presented. Based on trace element and rare earth element (REE) characteristics, these mafic rocks can be divided into two coeval groups. The MORB-like mafic rocks show flat REE patterns and are enriched in large-ion lithophile elements (e.g. Rb, Ba, K, Sr). Their zircon ɛ_Hf(t) values range from + 3.43 to + 4.90. The other mafic rocks are arc-like and have elevated REE contents and display relatively light REE enriched patterns, as well as depletion in high field strength elements (e.g. Nb, Ta, Ti). The zircon ɛ_Hf(t) values of the latter group range from + 0.17 to + 2.68. Zircon U-Pb dating yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2029 ± 15 Ma for the MORB-like mafic rocks, and 2023 ± 14 Ma for the arc-like mafic rocks, respectively. The elemental and isotopic characteristics indicate that the MORB-like mafic rocks may be derived from a depleted asthenospheric mantle source, whereas its counterpart with arc-like signatures originated from a lithospheric mantle source. The low (La/Yb)_CN (1.10–2.84) and Sm/Yb (0.99–1.29) ratios indicate that the mafic rocks are formed by partial melting of spinel lherzolite mantle. Combined with other geological observations, these mafic rocks are inferred to constitute part of a mid-Paleoproterozoic continental arc–back-arc basin system. The 2.03–2.02 Ga mafic rocks from the northern Yangtze Block indicate that the continental margin experienced subduction in response to assembly of the Columbia supercontinent.