U–Pb geochronology and geochemistry of dikes in the Changbaishan Tianchi volcanic field (NE China) and their relations with the coeval Jingbohu and Longgang monogenetic volcanic fields

Abstract

ABSTRACT The plain around the intraplate Changbaishan Tianchi volcano in NE China includes monogenetic cones whose shallowest plumbing system consists of dikes, and three main phases of activity are recognized since 2.8 Ma. The monogenetic volcanism mainly developed in the shield-forming phase (2.8–0.3 Ma), but the age and geochemical features of many of these vents remain poorly known. Here, we present geochemical, Sr‒Nd‒Pb isotopic and zircon trace element data from two dikes feeding monogenetic vents located north and northwest of the Changbaishan Tianchi volcano. Our aim is to constrain the emplacement age, the magma evolution processes, and the mantle source. We compare our results with those from the Jingbohu and Longgang monogenetic vents located north and west of the Changbaishan Tianchi volcano to investigate possible mantle heterogeneities in the recent NE China volcanism. The two Changbaishan dikes have 230Th disequilibrium-corrected ages of 1.27 and 1.00 Ma and show a basaltic composition with a potassic affinity. The Mg# values are 35.48–34.27 and 66.09–65.79. The whole-rock compositions, trace element compositions, and Sr‒Nd variations are consistent with fractional crystallization and minor contamination with lower crustal material, indicating an OIB-like mantle source with an EMI component possibly related to fluids released from the underlying Pacific flat slab. The melting depth determined from Na/Ti and Sm/Yb is between 140 and 180 km, i.e. the asthenospheric mantle. The dikes are characterized by (K2O+Na2O)/δEu, La/Ba, Ba/Nb, La/Sm, and La/Nb values similar to those of the Jingbohu and Longgang basaltic rocks. We conclude that the NE China volcanism at approximately 1 Ma was related to a single, relatively homogeneous OIB-like mantle source modified by ascending slab-derived fluids. According to the available geophysical data, the ascending magmas stopped at the mantle/lower crust interface and experienced limited crustal contamination.