Geological Faculty Saint Petersburg State University, Russia
Tuesday 11 February - 4.00 pm
EARTH SCIENCES SEMINAR ROOM
The presented results are part of a bigger project on Hercynian post-collisional magmatism in the Tien Shan carried out during the last two decades. Results are presented for two vast terranes of the Tien Shan which previously were inaccessible due to their remoteness or political reasons: the whole Tajik part of Tien Shan and the Alai ridge in Kyrgyzstan. In both areas the main types of granites and alkaline rocks including carbonatites were sampled. In Tajikistan special attention was paid to subduction-related granitoids that were studied for comparison with arc magmatism elsewhere in Tien Shan.
Four samples of subduction-related granites from the Gissar ridge in Tajikistan yield ages showing active subduction under Gissar block that continues from 321 to 300 Ma. This is similar to ages of subduction-related granites in the Middle Tien Shan terrane in Uzbekistan (315-300 Ma). The ages of 17 post-collisional intrusions including alkaline rocks and carbonatites in both terranes are in the range from 300 to 274 Ma. Some of the alkaline complexes have slightly younger ages but none formed in post-Permian time as shown on some regional geological maps. Post-collisional rocks of the Alai ridge have crustal isotopic Pb-Sr-Nd compositions supporting suggestion that the basement of the Alai segment comprises a Precambrian micro-continent. Similar crustal signatures were previously reported for other terranes of the Tien Shan in Kyrgyzstan and Uzbekistan.
The features of the post-collisional intrusions in Tajik Tien Shan and Kyrgyz Alai ridge match well the general characteristics of the post-collisional magmatism in Tien Shan: (1) Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries, (2) The majority of post-collisional intrusions were emplaced within a relatively short time span between 295 and 280 Ma, (3) Ages of intrusions emplaced syn-kinematically into the regional shear zones, and ages of alkaline intrusions indicating regional extension also match the 295-280 time span, (4) Similar ages were reported for the major orogenic gold deposits in the Tien Shan, (5) The post-collisional intrusions are geochemically diverse and their volume varies from one terrane to the other. This suggests different scenarios of post-collisional development in various terranes of the Tien Shan.
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