SOME PROBLEMS ON GRANITES AND VERTICAL GROWTH OF THE CONTINENTAL CRUST IN THE EASTERN TIANSHAN MOUNTAINS, NW CHINA

Abstract

Granite formation after major continental collision in the eastern Tianshan Mountains can be divided into three stages: the compression-extension transition stage (310 ~ 285Ma), the post-collision extension stage (285 ~ 250Ma) and the intra-plate stage (250 ~ 208Ma). Distinguishable granite features of each stage in pluton morphology, rock association, petrography and geochemistry depend upon lithosphere thickness, isotherm shapes and geodynamics of these stages. Apart from underplating, intraplating, which is capable to induce partial melting at various crustal levels, is another essential mechanism for generation of granite magmas and crustal growth. Some of the gneissic granites in eastern Tianshan were formed by metamorphism-metasomatism with ultimate genetic affiliation to post-collisional underplating and intraplating of mantle magmas. Post-metamorphic peak ductile shear zones with highly reduced grain sizes are most favorable sites for the formation of gneissic granites. Despite that most of the gneissic granites are emplaced at deep crustal levels, they can also be formed at relatively shallower levels depending upon the assistance of heat from underplating. Apart from tectonic forces, formation of the Kanguer ductile shear zone is closely connected to continental vertical growth, particularly intraplating, and includes complicated feedback between tectonism, magmatism, metamorphism, and fluid migration. Both the precollisional and post-collisional magmatic rocks from the Bogda orogen are characterized by positive εNd (t) values, indicating that the mantle of this orogen were already depleted before collision and that post-collisional depletion could have been inherited from pre-collisional mantle. An Indosinian (Triassic) event of vertical crustal growth is discerned in eastern Tianshan. Magmatism and ore-formation in accompany with this event was related to the subduction and subsequent collision of the Palaeo-Tethys, and hence represent the tectonic transition of eastern Tianshan from the Central Asia regime to the Tethys regime.