Volcanism at the end of continental rifting: The Cretaceous syn-rift to post-rift transition in the Songliao Basin (NE China)

Abstract

The evolution of volcanism at the latest stages of continental rifting and its transition to the post-rift stage are poorly known. A Cretaceous rifting episode characterized the Songliao Basin (NE China) between 150 and 105–100 Ma. The transition to the post-rifting stage was marked by an increase in the subsidence rate due to thermal contraction. Here we present new major, trace element and Sr-Nd isotopic data on the volcanic rocks sampled in wells located in the Songliao Basin. These rocks are representative of two major eruptive cycles: K1yc1 (115–106 Ma, syn-rift stage) and K1yc3 (105–102 Ma, transition to the post-rift stage). The K1yc1 and K1yc3 rocks were erupted during the late phases of the Songliao rifting and have been found in the two members of the Yingcheng Formation. The K1yc1 rocks consist of trachybasalts, basaltic trachyandesites, and rhyolites. The K1yc3 rocks show a homogeneous rhyolitic composition. Both the K1yc1 and K1yc3 rocks display an alkaline, Within-Plate affinity. The trace element patterns and the isotopic composition of the K1yc1 mafic rocks indicate that they are the product of the fractional crystallization from a parent magma with an OIB signature. The felsic rocks from K1yc1 and K1yc3 have similar major and trace element patterns and originate from fractional crystallization of the K1yc1 trachybasalt and assimilation of upper crust material. The bimodal K1yc1 volcanism is related to fractionation processes producing low amounts of magmas with intermediate compositions. The lack of mafic eruption in K1yc3 is due to the reduction of porosity of the crustal rocks and a transition from ‘hot’ to ‘cold’ crust related to the thermal subsidence from syn-rift to post-rift stage. The late stage of the Songliao rifting volcanism coincides with a shift from a roll-backing phase of the Paleo-Pacific slab to its passive sinking in the asthenosphere.