Post-collisional Hercynian magmatism in the Tien Shan: lithospheric shearing vs. mantle plume origin
1 St. Petersburg State University, Russia, email@example.com
2 Centre for Russian and Central EurAsian Mineral Studies (CERCAMS), Department of Mineralogy, Natural History Museum, London, UK
3 Center of Isotopic Research, A.P. Karpinsky All Russia Geological Research Institute (VSEGEI), St. Petersburg, Russia
The Hercynian Tien Shan orogen formed during Late Palaeozoic collision between the Karakum-Tarim and the Paleo-Kazakhstan continent. The western part of the Tien Shan is composed of three structural units or terranes: (1) the Northern Tien Shan, the deformed margin of the Palaeo-Kazakhstan; (2) the Middle Tien Shan, a Late Paleozoic volcano-plutonic arc; and (3) the Southern Tien Shan, an intensely deformed fold and thrust belt. The Middle and Southern Tien Shan terranes are separated by the Southern Tien Shan Suture (STSS) formed after the closure of the Paleo-Turkestan ocean.
The post-collisional intrusions are spread out in all the terranes of the Tien Shan regardless of the STSS, and comprise diverse calc-alkaline, alkali-calcic granitoid and alkaline s.s. complexes closely associated in time and space. Most of the intrusions are tectonically controlled and associate with major tectonic lineaments. Twenty intrusions in the Northern, Middle and Southern Tien Shan along a ca. 2000 km √ long profile from Aral Sea in Uzbekistan to the Pobeda Peak in Kyrgyzstan were dated using U-Pb zircon method by SHRIMP-II facility in VSEGEI St. Petersburg. To distinguish between subduction-related and post-collisional magmatism, 10 intrusions from the Middle Tien Shan volcanic arc were included in the data set. The results show that all the post-collisional intrusions formed within a very narrow time span from 295 Ma to 280 Ma. The intrusions of the Middle Tien Shan arc in Uzbekistan formed within a 320 √ 305 Ma time span in subduction-related environment.
Two stages of gold mineralisation are associated with subduction-related and post-collisional granitoids, making the Tien Shan the richest gold province in Eurasia. However, the two stages are represented by different genetic types of Au mineralization: porphyry and hydrothermal systems in volcanic arc rocks and orogenic gold deposits in post-collisional rocks. The new data show that subduction-related calc-alkaline magmatism is restricted in space while the post-collisional granitoids are spread out in all the terranes of the Tien Shan regardless of the STSS and comprise compositionally diverse magmatic series.
Several authors suggested that diverse post-collisional magamatism originated from a mantle plume which developed under Central Asia immediately after Hercynian collision. However based on tectonic analysis and timing of post-collisional magmatic event we propose different model for a post-collisional evolution of the Tien Shan. From our point of view the emplacement of the post-collisional intrusions and orogenic gold deposits was controlled by the trans-crustal scale shear zones which formed as a result of the east-west strike-slip motions that affected the Tien Shan subsequent to collisional events. Our data show that these motions took place in a relatively narrow time span from 295 Ma to 280 Ma. We believe that plate-scale shear zones provided suitable conduits for ascending asthenospheric material and heat influx in the crust. The compositional diversity of post-collisional magmatism in various terranes of the Tien Shan probably reflects different compositions of the crust.