Possible source dichotomy of contemporaneous post-collisional barren I-type versus tin-bearing A-type granites, lying on opposite sides of the South Tien Shan suture

Two granitoid complexes in the eastern Kyrgyz Tien Shan, situated north and south of the Southern Tien Shan Suture, were studied. The suture formed as a result of the closure of the Turkestan Ocean and collision of the Tarim microcontinent in the south with the Middle Tien Shan in the north. The timing of collision is still disputed. The deformed calc-alkaline Terektinsky complex, situated immediately north of the suture, represents one of the largest shear-zone related intrusions in the Tien Shan (130 × 5–15 km in size). Small stocks of evolved A-type granites of the Inylchek complex, hosting economic tin mineralization, were emplaced immediately south of the suture opposite the Terektinsky complex. Two samples from the Terektinsky complex and 3 samples from three A-type stocks were collected for U–Pb zircon SHRIMP-II geochronology. The ages at 2σ level obtained for the Terektinsky complex north of the suture (294 + 5 Ma and 291 + 5 Ma) and ages of the small granite bodies south of the suture (299 + 4 Ma, 295 + 4 Ma, 289 + 6 Ma; Tashkoro, Inylchek and Maida'adir intrusions, respectively) are nearly identical, within error limits. They show that the Southern Tien Shan Suture in the eastern Kyrgyz Tien Shan had already formed by ~ 295 Ma, and had evolved into a transcrustal mega-shear zone controlling emplacement of granitoids. Geochemical distinction between the two magmatic systems is based on 10 original bulk and trace analyses of rocks from this study and on a large dataset extracted from previously published research and unpublished reports. Geochemically, the rocks of the Terektinsky complex comprise calc-alkaline (high potassium I-type) series while the granites of the Inylchek complex are typical A-type granites with an elevated alumina saturation index and higher boron contents compared to a “standard” A-type rapakivi granite. Contrasting metallogenic features of the two granitoid complexes south and north of the Southern Tien Shan Suture are defined by their sources: a fertile fore-arc complex, and/or passive margin sediments of Tarim to the south, and barren metamorphic Precambrian basement of the Middle Tien Shan to the north.