Late Mesoproterozoic to early Neoproterozoic ridge subduction along southern margin of the Jiangnan Orogen: New evidence from the Northeastern Jiangxi Ophiolite (NJO), South China

Abstract

The Precambrian ophiolites in South China include the Northeastern Jiangxi Ophiolite (NJO) and the Fuchuan Ophiolite in southern Anhui. These two ophiolites contain important information on the Precambrian tectonics of South China. The NJO occurs along the southern margin of the Jiangnan Orogen, considered as the suture between Yangtze and Cathaysia Blocks in South China. Here we report new results for NJO from our field observations, U–Pb ages and Hf isotopic compositions of zircons from the cumulate and diabase units, chemical compositions of the clinopyroxene from the cumulates, and whole-rock geochemistry of the dismembered ophiolite units in an attempt to evaluate the late Mesoproterozoic to early Neoproterozoic evolution of the South China. Zircon U–Pb age data indicate that NJO formed at ∼1020Ma. Whole-rock geochemistry reveals that the tholeiitic cumulate and diabase members from the NJO have elemental compositions deviating from typical MORB- to arc-signatures. Clinopyroxene from the cumulates exhibits non-alkaline rifting-related features. Whole-rock and mineral chemistry, in combination with depleted MORB mantle (DMM)-like zircon Hf isotope composition (ɛHf(t)zircon=12–16) and whole-rock Nd isotope compositions (ɛNd(t)=4.5–7.8), argues that the cumulates and diabases share a common depleted mantle source with variable contamination by continental crust during magma ascent and/or emplacement. Elemental geochemistry and Nd isotope compositions of the serpentinites also reveal their DMM signature.Based on our new geochronological and geochemical data for NJO, as well as previous studies on the slightly younger adakites (970Ma) intruded into NJO and the Nb-enriched basalts (∼930Ma) in the northeastern section of the suture zone between Yangtze and Cathysia Blocks, we propose that NJO was a ridge-subduction type ophiolite. This new model may shed new light on the late Mesoproterozoic to early Neoproterozoic tectonic evolution of the South China.