Final amalgamation of the Tianshan and Junggar orogenic collage in the southwestern Central Asian Orogenic Belt: Constraints on the closure of the Paleo-Asian Ocean

Abstract

The Tianshan and Junggar orogenic collage occupies the southwestern part of the Central Asian Orogenic Belt and was assembled by collision/accretion of several continental blocks and island arcs during late Paleozoic-early Mesozoic time, following the consumption of the Junggar and South Tianshan oceanic basins in the western segment of the Paleo-Asian Ocean. Considerable and continuing controversy has surrounded when, where, and how this orogenic collage was eventually amalgamated, which plays a crucial role in understanding the formation of the Central Asian Orogenic Belt. This review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains. A comprehensive data compilation indicates that the formation of ophiolites along major sutures in the Tianshan and Junggar region lasted to the early Carboniferous for the Kalamaili suture zone, to the mid-Carboniferous for the West Junggar region and the North Tianshan suture zone, and to the late Carboniferous for the South Tianshan suture zone. Ca. 325-310 Ma (ultra-)high pressure metamorphism along the western part of the South Tianshan suture occurred nearly synchronously with the collision between the Tarim Craton and the Central Tianshan-Yili Block. The oceanic closure and collision events were sequentially followed by widespread and intensive A-type granitic and bimodal magmatism, which commenced in the mid-Carboniferous along the Kalamaili belt and in the latest Carboniferous to earliest Permian in other regions. Contemporaneously, large-scale strike-slip shearing took place along major faults subparallel to main sutures, with a dominant dextral sense and deformation time since the Carboniferous-Permian transition. Available data and geological evidence support an eastward propagating, scissor-like closure of the western segment of the Paleo-Asian Ocean along the South Tianshan and North Tianshan suture zones over a period from the mid- to end Carboniferous, with the South Tianshan belt representing the final suturing site. In the early Permian, the Tarim mantle plume likely merely affected the southwestern part of the assembled Tarim and Tianshan region, as indicated by regional discrepancy in type and geochemistry of early Permian A-type granitoids. The amalgamation of the Tianshan and Junggar orogenic collage was associated with the tectonic bending of the Kazakhstan orocline, leaving a piece of trapped oceanic crust in its middle part, beneath the present Junggar Basin.