Petrogenesis and tectonic setting of Carboniferous hornblende gabbros of the northern Great Xing'an Range, NE China: Constraints from geochronology, geochemistry, mineral chemistry, and zircon Hf isotopes

Abstract

This study presents new information on the petrogenesis and tectonic setting of a hornblende gabbro in the northern Great Xing'an Range of northeastern China using new whole‐rock geochemical, mineral geochemical, and in situ zircon U–Pb and Hf isotopic data obtained for samples taken from near the town of Tayuan. Zircon U–Pb dating indicates that the hornblende gabbro was emplaced during the late Carboniferous (~311 Ma). The hornblende gabbros are alkaline with high K2O + Na2O (4.25–6.42 wt.%), low SiO2 (41.69–50.22 wt.%), and variable MgO (4.49–7.16 wt.%) and TiO2 (1.23–2.77 wt.%) contents. The pressure and temperature conditions of gabbro crystallization were determined using amphibole–plagioclase and amphibole thermobarometry, which indicate that the Tayuan hornblende gabbro formed at pressures of 4.1–6.9 kbar and temperatures of 686–762 °C, respectively. The hornblende gabbros are enriched in light rare earth elements and large ion lithophile elements, are depleted in the heavy rare earth elements and high field strength elements, and have positive zircon εHf(t) values (+3.9 to +9.9), all of which are indicative of formation from magmas generated by the partial melting of a depleted region of the lithospheric mantle that was previously metasomatized by subducted slab‐derived fluids and/or melts just before the generation of the hornblende gabbro magmas. The field observational, geochronological, geochemical, mineral geochemical, and zircon Hf isotopic data presented here are indicative of a complex petrogenetic history that involved crystal fractionation and magma mixing. These hornblende gabbros were emplaced in an extensional setting associated with the collision of the Xing'an and Songnen massifs.