Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China

Petrographic observations. and mineralogical and geochemical analyses, have revealed that the Hegenshan ophiolite is of mid-ocean ridge origin and has been subjected to dynamothermal metamorphism at medium P/T conditions. The metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote-amphibolite facies, with peak temperature conditions of 570-640degreesC at pressures of 4-10 kbar. The amphiboles formed by this metamorphism show K-Ar ages of 110-130 Ma. The metamorphic conditions and K-Ar ages suggest that the Hegenshan ophiolite is located at the suture between the Siberian and North China continental blocks, where the continental collision in this area took place in middle Mesozoic time. Given the temporal and spatial distribution of the igneous activity around the Da Hinggan Ling Mountains, it is suggested that the extensive Yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision. Alternatively, it may possibly have resulted from the collision itself, at the final stage. (C) 2002 Elsevier Science B.V. All rights reserved.