N-MORB and IAT sources in the Proterozoic Miaowan Ophiolite Complex, Yangtze Craton: Evidence for evolving tectonic settings

The Miaowan Ophiolite Complex in the Northern Yangtze Craton consists mainly of layered fine-grained metabasites, pillow lavas, sheeted dikes, gabbros, sepentinized harzburguite, and sepentinized dunite, with rare metasedimentary rocks in the metabasite section. In this study, we divide the Miaowan Ophiolite Complex into two groups including the Miaowan Ophiolite Suite and a Late Magmatic Suite. The Miaowan Ophiolite Suite (MOS) mainly consists of ductily deformed serpentinized harzburgite, sepentinized dunite, gabbro, sheeted dikes, basalt, plagiogranite, and layered metasedimentary rocks. All these units were then intruded by the Late Magmatic Suite (LMS) consisting of pegmatitic-isotropic gabbro and massive diabase. The formation age of the MOS is interpreted to be ca. 1115 Ma. Harzburgites in the MOS are characterized by smooth LREE-depleted and flat MREE-HREE patterns; whereas dunites in the MOS display U-shape REE patterns. Deformed gabbro and basalt in the MOS display flat to slightly LREE enriched patterns, and low Th/Yb ratios and a lack of Nb anomalies, showing N-MORB affinities. The average values of initial ɛNd (t) of rock units in the MOS are +7.0±1.3, indicating that the MOS was derived from a strongly depleted mantle source. Accordingly, the harzburgite, gabbro and basalt in the MOS are interpreted to have formed in an oceanic spreading center and the dunites in the MOS were formed by reaction between the harzburgites and the subduction-related boninitic melts when the MOS was trapped with the harzburgites as a part of the mantle wedge above a subduction zone. The LMS was intruded between ca. 1000 Ma and ca. 970 Ma, consistent with their whole-rock Sm-Nd errochron age (1007±62 Ma). Pegmatitic-isotropic gabbro and diabase in the LMS are characterized by enriched-LREE patterns with high Th/Yb ratios and negative Nb and Zr anomalies, consistent with a subduction-related setting. The average values of initial ɛNd (t) of rock units in the LMS are +6.7±1.6. It is suggested that the LMS formed in a depleted fore-arc setting. Collectively, the Miaowan Ophiolite Complex consists of two magmatic suites including an older MORB-type ophiolitic suite and a younger arc-related magmatic suite, recording the evolution of geodynamic settings in a Proterozoic ocean.