40 Ar/ 39 Ar and K/Ar dating on the volcanic rocks in the Songliao basin, NE China: constraints on stratigraphy and basin dynamics

Abstract

Precise 40Ar/39Ar and K/Ar ages were determined on sanidine and whole-rock separates of volcanic rocks in the Songliao basin (SB). The volcanism is characterized by a series of continual and episodic eruptions in the late Jurassic (147–157 Ma) and early Cretaceous (113–136 Ma). A fairly accurate chronostratigraphic column was first established based on those data. The volcanic activities happened synchronously with closure of the Mongolia–Okhotsk ocean north to the SB. Structures of the volcanogenic successions are S-dipping or SE-dipping fault blocks coupled with N-dipping or NW-dipping major detachment systems. Distribution of the volcanics, revealed by high-resolution seismic reflections, is neither restricted within nor in agreement with the framework of the overlying sedimentary sequence striking NNE. This new evidence fits the overlap model, proposed in the present paper, that formation of the volcanic successions in the SB is related to the subduction of the Mongolia–Okhotsk plate under the Mongolia–North China block during the Jurassic and early Cretaceous, whereas the overlying sedimentary sequence unconformably on the volcanics is tectonically controlled by the oblique subduction of the Pacific plate under the Eurasian plate since late Aptian. In addition, the coal-bearing epiclastic sediments intercalating the volcanics were deposited at the subduction–quiescence episodes when extensional collapse occurred.