Late Cretaceous tectono-magmatic event in Songliao Basin, NE China: New insights from mafic dyke geochronology and geochemistry analysis

The ages and source of Late Cretaceous mafic dolerite dykes of the Songliao Basin were determined using geochronological, geochemical, zircon Hf, whole-rock Sr-Nd isotopic, and zircon fission tract. The results show that (a) petrographic data reveal doleritic, poikiloblastic textures, and amygdale structures of the dolerite dykes. (b) LA-MC-ICP-MS U-Pb analysis of zircons yielded an age of 70.0 +/- 3.0 Ma of one of these mafic dykes in agreement with the zircon fission track age (69-68 Ma) of host rocks within uncertainty, placing them within the Late Cretaceous. (c) Geochemical researches indicate that the dykes are classified as the alkaline and tholeiitic series according to their major elements and characterized by high Na2O (2.79-4.89 wt.%), TiO2 (1.63-2.71 wt.%), and low K2O (0.46-2.45 wt.%). And they have enrichment of incompatible elements (e.g., K, Ti, Rb, Ba, Th, U, Nb, Ta, and REEs), high Rb/Sr (0.026-0.066), La-N/Yb-N (3.83-15.93), near-chondritic HREEs ([Gd/Yb](N) = 1.78-3.37), low Sm/Nd (0.202-0.282) ratios, and with negligible Eu anomalies (Eu/Eu-* = 0.98-1.06). The dykes contain relatively low amounts of radiogenic Sr, as shown by (Sr-87/Sr-86)(i) values (0.7037-0.7045), and have positive values of epsilon(Nd)(t) (3.66-5.40) and epsilon(Hf) (t) (2.66-7.45), suggesting that the mafic dykes have been derived from partial melting of the depleted lithospheric mantle and contaminated by crust in the source region and formed in back-arc rift settings caused by the subduction of the Pacific Oceanic Plate. All the above indicate large-scale extensional event in Songliao Basin NE China at about 70 Ma.