Deciphering Caledonian events: Timing and geochemistry of the Caledonian magmatic arc in the Kyrgyz Tien Shan

摘要

In the Kyrgyz Tien Shan (also known as Tian Shan in literature) the Caledonian (Cambro-Silurian) intrusions comprise an extensive magmatic arc stretching from east to west for more than 1000 km. The characteristic feature of the arc is its relatively homogeneous composition of rock types over the whole structure. The Kichy-Naryn and Djetim intrusions presented in this study are slightly elongated in an east-west direction and occupy an area of ca. 100 km~2. The main rock types are diorite, granodiorite and granite. Geological and geochemical features of the Kichy-Naryn and Djetim intrusions demonstrate characteristics of I-type granite series. Rocks of the two intrusions define a continuous high-K calc-alkaline series. Diorite and granite of the Kichy-Naryn intrusion yielded early Silurian crystallisation ages of 436 ± 2 Ma (U-Pb, zircon). Diorite from pebbles in the conglomerate sampled close to the contact of the Kichy-Naryn intrusion yielded a significantly older early Ordovician crystallisation age of 466 ±10 Ma. The obtained ages of 466 and 436 Ma match ages of two major regional magmatic pulses at ca. 435-440 and 460-470 Ma which took place during continuous subduction from the Cambrian to the Silurian. The amount of granites in the Northern Tien Shan, their prolonged history of formation and pronounced I-type geochemical characteristics suggest their formation in an Andean-type active margin environment.