Petrogenesis and tectonic implication of the Late Mesozoic volcanic rocks in East Mongolia

Abstract

This paper presents new geochemical and geochronological data of the Late Mesozoic volcanic rocks in East Mongolia. These volcanic rocks belong to high‐K calc‐alkaline and shoshonitic series and exhibit features of bimodal rocks. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb, Ta, and Ti anomalies, with 87Sr/86Sr(i) and ƐNd(t) values of 0.70502–0.70572 and −1.72459 to +1.720736, respectively, which suggest that the mafic magma was derived from a lithospheric mantle source that might have been metasomatized by subduction‐derived fluids, experienced fractional crystallization, and was contaminated by crustal materials. The felsic rocks have similar rare earth element patterns to the mafic rocks but show much more significantly negative Eu and Sr anomalies. The felsic rocks have higher 87Sr/86Sr(i) values of 0.706496–0.71104 than the mafic rocks but similar ƐNd(t) values (−0.28003 to +2.928506) to the mafic ones. These data indicate that the felsic rocks originated from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K‐Ar dating, together with previous data, shows that the Late Mesozoic volcanism in East Mongolia took place during the Late Jurassic‐Early Cretaceous between ca. 155 and 99 Ma. The model of back‐arc extension, possibly induced by slab rollback of the westward subducted Pacific Plate and the subduction zone retreat, can explain the geodynamic setting and the eastward younging trend of the Late Mesozoic volcanism in East Mongolia and in adjacent NE China.