Geochemistry and zircon geochronology of a gabbro–granodiorite complex in Tongxunlian, Inner Mongolia: partial melting of enriched lithosphere mantle

Abstract

A complex of gabbro (with metamorphic pyroxenite xenoliths)–gabbroic diorite–granodiorite was recently discovered in Tongxunlian, Xilinhot city, Inner Mongolia. Zircon U–Pb isotopic dating showed that the gabbro and the granodiorite were formed ca. 319 ± 1 Ma and ca. 318 ± 1 Ma respectively, indicating that emplacement of the composite rocks occurred in the late Carboniferous. Positive εHf(t) values of +12.0 to +14.1 and two‐stage model ages (TDM2) of 418 to 537 Ma of these rocks are similar to the age of formation of metamorphic pyroxenite (560 Ma, based on Sm–Nd isochron dating) and suggest that the rocks were derived from depleted lithospheric mantle (metamorphic pyroxenite). Our findings revealed that all of these calc‐alkaline and metaluminous intrusive rocks formed from the fractional crystallization of comagmatic evolution in an island‐arc setting. Moreover, the gabbro–gabbroic diorite in the study region was characterized by a low TiO2 content, a slight deficit of Nb, a surplus of Ta, and relatively low LREE/HREE ratios. Along with a relatively high Zr/Y ratio (4.0 to 5.6), these characteristics indicate that the rocks may have been formed by melting of the mantle wedge via metasomatism. Combination with other features of the rocks indicates a two‐episode tectonic model: we conclude that first, the fluid and Si‐rich melt metasomatism caused partial melting of the enriched lithospheric mantle, and these influences were then stored in the mantle; and second, slab breakoff resulted in upwelling of the upper mantle's soft fluid (stratum), which melted the enriched mantle of the lithosphere and formed the basaltic magma of the gabbro–gabbroic diorite. This study provides new geological evidence to support the Neoproterozoic subduction between the Paleo‐Asian Ocean plate and the Xilinhot microcontinent. Copyright © 2014 John Wiley & Sons, Ltd.