Petrology of an arc-oceanic crust contact zone in the Laohushan back-arc basin, the eastern section of the north Qilian Mountains, NW China

A contact zone sandwiched between an are and an oceanic crust was discovered in the Laohushan area in the present study. It consists of a series of north-dipping imbricated thrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 km in an E-W direction and measures about 1-3 km wide. Lithologically, it can be divided into four subzones. Subzone 1 consists of meta-andesite and metasandstone; subzone 2, psammitic schists; subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro, epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: low greenschist facies in subzone 1 --> high greenschist facies in subzone 2 --> low amphibolite facies in subzone 3 --> epidote amphibolite facies in subzone 4. Petrographic and geochemical evidence shows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceanic crustal rocks. The metamorphic mineral assemblages and especially mineral chemistry of the bluish green amphibole from the pelitic schists and epidote amphibolite of subzone 4 suggest that the rocks of the contact zone were metamorphosed at a pressure of up to 0.69 GPa. It is thought that the Late-Mid Ordovician oceanic lithosphere of a back-arc basin was underthrust northerly beneath an arc to a depth of 20-23 km, where the basaltic rocks and gabbro were converted to epidote amphibolite and metagabbro respectively, Then, the root rocks of the arc and these metamorphosed oceanic rocks were brought up to shallower depths by thrust faults to form a contact zone between the are and the oceanic crust in the Laohushan area.