Multi-method chronometric constraints on the evolution of the Northern Kyrgyz Tien Shan granitoids (Central Asian Orogenic Belt): From emplacement to exhumation

摘要

Multichronological data reveal the thermotectonic history of the northern Kyrgyz Tien Shan granitoids (Kyrgyzstan) from emplacement to exhumation. Zircon SHRIMP and LA-ICP-MS U/Pb concordia ages suggest a Middle to Late Ordovician crystallization age (440-470 Ma) for the most voluminous; Caledonian intrusion phase, which is associated with the evolution and closure of the Early Palaeozoic Terskey Ocean. The presence of some additional Early Ordovician - Cambrian U/Pb ages point towards a prolonged production of granitoids during the entire Early Palaeozoic. A sampled younger granitoid (292 ± 5 Ma) was formed during the final closure of the Turkestan Ocean when Tarim eventually collided with the Kazakhstan plate during Hercynian orogeny. ~(40)Ar/~(39)Ar step-wise heating plateau-ages (biotite Ar/Ar: 400-440 Ma; K-feldspar Ar/Ar: 235-375 Ma) bear witness to rapid Silurian - Early Devonian post-magmatic cooling of the Caledonian intrusives, followed by a more modest rate of cooling during the Late Devonian until the Late Triassic. Low-temperature techniques such as apatite fission track (AFT) and Apatite (U-Th-Sm)/He (AHe) thermochronology, give Late Jurassic - Cretaceous ages (90-160 Ma) with some Cenozoic outliers. Thermal history modelling allows us to distinguish two marked cooling phases: (1) Mesozoic cooling occurred as the result of denudation and exhumation of the Tien Shan basement during a pulse of tectonic reactivation, associated with the Cimmerian orogeny. (2) Late Cenozoic cooling (～10-3 Ma) reflects a far-field effect of the India-Eurasia collision. Some samples also experienced a Late Oligocene - Miocene reheating event, which could be the result of burial due to sediment load stripped from the adjacent, eroding mountain ranges.