Subduction, accretion, and collision during the Neoproterozoic-Cambrian orogeny in the Great Xing'an Range, NE China: Insights from geochemistry and geochronology of the Ali River ophiolitic mélange and arc-type granodiorites

In this study, we report three Neoproterozoic ophiolitic mélanges in the Ali River (Jifeng-Gaxian-Huanerku) area from the northern Great Xing'an Range, NE China. Detailed field mapping reveal that all three mélanges disperse in a strongly sheared metasedimentary matrix composed mainly of mica-schist, quartz-schist and marble. These mélanges, which have undergone polyphase deformation with northwest-verging nappe structures, are composed of a suite of amphibolite-facies mafic to ultramafic blocks including serpentinite, olivine pyroxenite, pyroxenite, diabase, gabbro and basalt. Lithological and geochemical characteristics of these ultramafic-mafic rocks suggest that their protoliths were from an ophiolitic mélange formed in a Neoproterozoic supra-subduction zone. The protolith ages were constrained by LA-ICP-MS/SHRIMP U-Pb dating of zircons from the Jifeng gabbro, the Gaxian pyroxenite, and the Huanerku gabbro, which yielded ages of 647 ± 5, 628 ± 10 and 697 ± 3 Ma, respectively. The zircons from the gabbros have obvious positive εHf (t) values that range from +8.3 to +17.2 (average = +13.5) with corresponding TDM1 ages of ca. 680 Ma, indicating that these rocks are the remnants of juvenile crust derived from depleted mantle in the Neoproterozoic. All rock types in the mélange underwent amphibolite facies metamorphism. The tectonic emplacement of these ophiolitic mélanges was no later than 557 ± 2 Ma, which is the age of continental arc-type granodiorites (negative εHf (t) values from -7.4 to -17.8) that intruded these rocks. The mafic-ultramafic rocks in the Ali River mélanges are interpreted to be fragments of a ca. 690-620 Ma supra-subduction zone ophiolite that was incorporated into the Xinlin-Xiguitu suture zone during subduction-accretion. Together with the ca. 500 Ma Toudaoqiao blueschists, the Ali River ophiolitic mélange in the northern Great Xing'an Range provides important evidence for the Neoproterozoic subduction-accretion-collision processes associated with the amalgamation of the Erguna and Xing'an blocks.