| Granitoid plutons from the East Junggar, Bogdashan and Tianshan tectonic terrains in Xinjiang, China have been studied for geochronology and Pb, Sr, and Nd radiogenic isotope tracers. The results provide some important information as to the reconstruction of tectonic history and the petrogenesis of the granitoid rocks of the regions.; Middle to Late Devonian volcanism and associated granitoid plutonism in East Junggar and Central Tianshan terrains are inferred to be the products of conventional "arc-type" magmatism, linked to subduction. This magmatic activity occurred during the closing (presumably by subduction) of Devonian seaways, now marked by ophiolitic sutures, and preceded collision. The tectonic setting of Silurian volcanism and granitoid plutonism in Central Tianshan and South Tianshan terrains is not yet clear.; Collision between Central and South Tianshan terrains took place at the latest Devonian and/or earliest Carboniferous, as constrained by stratigraphic relations and reinforced by a 358 {dollar}pm{dollar} 12 Ma Sm-Nd isochron age obtained from a garnet-clinopyroxene amphibolite mylonite, formed by the collision-related dynamic metamorphism. Closing of ancient seaways all across the eastern Tiashan-East Junggar region culminated in terrain collisions, deformation, metamorphism, and suturing, which was complete by the early Middle Carboniferous. East Junggar, Bogdashan, North Tianshan, Central Tianshan, and South Tianshan terrains were amalgamated into a super terrain from the mid-Carboniferous onward.; Magmatic activity continued, however, and perhaps reached its zenith during the ensuing post-orogenic period. This post-orogenic magmatic activity post-dates major collision by up to 60 m.y., as demonstrated by the Late Carboniferous granitoid plutonism in Central and South Tianshan.; Pb, Sr, and Nd isotopic tracers suggest that the granitoid plutons in East Junggar, Bogdashan, and North Tianshan were formed from sources with strong oceanic mantle affinity. The Paleozoic Granitoid plutons in Central and South Tianshan, however, show involvement of both upper and lower continental crust material in their petrogenesis. The Proterozoic gneissic and migmatite granites of South Tianshan show isotopic compositions similar to those of "depleted" (granulite) lower crust.; The post-orogenic granitoid plutons, which invaded East Junggar, Bogdashan, North Tianshan, Central Tianshan, and South Tianshan terrains after collisions had welded these terrains into a continental super terrain, do not show any significant difference in isotopic composition from the pre-collisional granitoid plutons. Petrogenesis of the post-orogenic magmas appears not to be unique, but to represent a continuation of processes that formed their orogenic predecessors in the respective pre-Carboniferous terrains. |
