Post-collisional W-Mo-Cu-Au Mineralization in the Middle Tien Shan: First Data on U-Pb Isotope Dating (LA-ICP-MS) of Zircon from Intrusive Rocks of the Kensu Pluton (Eastern Kyrgyzstan)

The paper presents first data on isotopic U-Pb study (LA-ICP-MS method) of zircon from intrusive rocks of the Kensu pluton situated in the eastern part of the deep-seated fault system of the "Nikolaev Line." This pluton of high-potassic (shoshonitic series) rocks is accompanied by the Kensu deposit and other occurrences of skarn and porphyry W-Mo-Cu-Au mineralization. Together with the other Au, W, and Cu deposits and occurrences, they are parts of the extended metallogenic belt of Tien Shan. The concordant isotopic U-Pb age data for zircon autocrysts from the rocks of the consecutive intrusive phases span over the interval of approximately 325 to 302 Ma. This interval comprised the crystallization of monzogabbro (321 +/- 4 Ma), monzonite (319 +/- 4 Ma), camptonite (306 +/- 4 Ma), syenite (307 +/- 6 Ma), quartz syenite (305.5 +/- 2 Ma), and quartz monzonite (305 +/- 3 Ma). Zircon antecrysts dated at 316-325 Ma to 335-345 Ma have also been identified. The age data obtained are consistent with the age of subduction processes defined for the western part of the Middle Tien Shan. However, both geochemical characteristics of the rocks from the Kensu pluton and a distinct W-Mo mineralization related to this pluton contradict the subduction-related setting of the pluton emplacement but, instead, point out the post-collisional setting of the intrusion emplacement. This discrepancy can be explained by a "scissor-like" (from east to west) closure of the Turkestan paleoocean that has resulted in the post-collisional regime in the eastern part of the "Nikolaev Line," whereas subduction continued in its western part. The rocks also contain zircon xenocrysts with a much older age (in to order of 1.9 Gb) that probably represents the age of the Tarim craton basement rocks; this indicates an involvement of the ancient material in the magma generation.