RECONSTRUCTION OF THE PROTOLITHS AND POLYPHASE METAMORPHISM OF THE AMPHIBOLITE SERIES OF THE QINLING GROUP IN SHANGNAN,SHAANXI

The amphibolite series in the middle and lower parts of the Qinling Group in Shangnan, Shaanxi, consists of plagioclase amphiboJite and augen and massive garnet amphibolite. All the rocks exhibit the characteristics of oceanic tholeiite, and the garnet amphibolite resulted from retrogressive metamorphism of eclogite. On the basis of a study of the mineralogy and textures of the rocks, the authors have distinguished three episodes of metamorphism. In the Proterozoic, basalts were metamorphosed into amphibolites. Then during the early Paleozoic the rocks underwent metamorphism of high-pressure epidote amphibolite facies together with eclogites and ultramafic rocks that were emplaced subsequently, with the result that the diagnostic mineral glaucophane formed in large quantities. During the late Paleozoic, the rocks were transformed by superimposition of metamorphism of greenschist facies. It has been ascertained now that there might exist an ophiolite suite in the vicinity of Songshugou, which deserves further study.