Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt

The Hercynian Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum–Tarim and the Kazakhstan paleo-continents. In order to constrain timing of Hercynian post-collisional magmatism, 27 intrusions were sampled for U–Pb zircon dating along a ca. 2000 km – long profile in Uzbekistan and Kyrgyzstan. The samples were dated utilizing sensitive high resolution ion microprobe (SHRIMP-II). The obtained ages, together with previously published age data, allowed the timing of Hercynian post-collisional magmatism to be constrained and interpreted in the context of the Paleozoic magmatic evolution of the region. Apart from Hercynian post-collisional magmatism, two older magmatic episodes have been recognized, and the following sequence of events has been established: (1) approximately 10 Ma after cessation of continuous Caledonian magmatism a number of Late Silurian–Early Devonian intrusions were emplaced in the Middle and Northern Tien Shan terranes between 420 and 390 Ma. The intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of Caledonian Paleo-Kazakhstan continent; (2) the next relatively short Late Carboniferous episode of subduction under Paleo-Kazakhstan was registered in the Kurama range of the Middle Tien Shan. Calc-alkaline volcanics and granitoids with ages 315–300 Ma have distinct metallogenic affinities typical for subduction-related rocks and are not found anywhere outside the Middle Tien Shan terrane west of the Talas–Farghona fault; (3) the Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries. The post-collisional intrusions formed within a relatively short time span between 295 and 280 Ma. The model for Hercynian post-collisional evolution suggests that after collision the Tien Shan was affected by trans-crustal strike-slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust. This produced both granitoid magmas and hydrothermal fluid flow. As a result post-collisional intrusions and orogenic Au deposits, known in the region, formed coevally and were tectonically controlled; (4) between 240 and 220 Ma a Triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale. Probably the influx of heat into the crust during the Triassic was tectonically focused and varied significantly in different terranes. In the region under investigation the Triassic thermal event was not accompanied by any significant magmatic activity. Thus, after cessation of Hercynian post-collisional magmatism ca. 280 Ma ago there was a long magmatically quiet period in the Tien Shan.

Research highlights
► Back arc extension-related intrusions formed 420–390 Ma ago.
► Subduction-related intrusions have ages 315–300 Ma.
► Post-collisional intrusions formed within a relatively short time span 295–280 Ma.
► Post-collisional magmatism affected the whole region across terrane boundaries.