Geochemical and geochronological constraints on the tectonic setting of the Xinlin ophiolite, northern Great Xing’an Range, NE China

Abstract

The Xinlin ophiolite, located in the Great Xing’an Range domain of the Central Asian Orogenic Belt (CAOB), is considered to represent a fragment of an oceanic plate. The ophiolite consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, basalts, and diabase dikes, which are intruded by younger leucogranite dikes. Here we characterize the geochemical features and formation age of the ophiolite. The ultramafic rocks are dominated by harzburgite with minor dunite, and are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), consistent with remnants of oceanic mantle. The mafic rocks show depletion in some high-field-strength elements (HFSEs) and have relatively low TiO2 and MgO contents, and likely originated from an E-MORB-like source in a back-arc setting. This suggests that the ophiolite is of supra-subduction zone (SSZ) origin. The cross-cutting leucogranite dikes, most of which are trondhjemitic, have high Al2O3 contents, low K2O/Na2O ratios, and negative zircon εHf(t) values (-8.5 to -3.8; average = -5.8), which are features distinct to other granites (e.g., fractionation-, shear-, and subduction-types) within the ophiolite and to coeval granitoids in the northern Great Xing’an Range. Zircon U-Pb dating on samples of gabbro and a leucogranite dike yielded ages of ca. 480 and 324 ± 1 Ma, respectively, suggesting that the ophiolite formed in the Early Ordovician and was emplaced at ca. 320 Ma. Together with the ages of ca. 480 Ma for the Duobaoshan arc and ca. 340 Ma for subduction-related high-Mg diorites, we suggest that ophiolite formation was probably related to the opening of an Ordovician back-arc basin, whereas formation of the younger leucogranite dikes was probably related to closure of the back-arc basin.