Post-collisional Hercynian
magmatism in the Tien Shan:
lithospheric shearing vs. mantle plume origin
1 St. Petersburg State
University, Russia, konopelko@inbox.ru
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.