Thermal and exhumation history of the central Tianshan (NW China): Constraints by U–Pb geochronology and Ar-Ar and (U–Th)/He thermochronology

Abstract:

Geochronology and thermochronology using multiple mineral-isotopic chronometers reveals the thermo-tectonic history of the central Tianshan (NW China) from emplacement to exhumation. Granites from the central Tianshan, which are associated with the southward subduction of the northern Tianshan Ocean, have been dated at 362–354 Ma using the LA–ICP–MS Zircon U–Pb method. A younger diorite sample (282 ± 1 Ma, Zircon U–Pb method by LA–ICP–MS) from northern Tianshan formed during the final closure of the Northern Tianshan Ocean when the Junggar Block collided with the Yili–Central Tianshan Block. ⁴⁰Ar/³⁹Ar step–wise heating plateau dates (biotite Ar/Ar: 312–293 Ma; Plagioclase Ar/Ar: 270–229 Ma) from the Central Tianshan show rapid post-magmatic cooling during the Late Carboniferous–Early Permian followed by a more modest rate of cooling from the middle Permian to the middle Jurassic. The northern Tianshan diorite (biotite Ar/Ar: 240 ± 1 Ma) also reveals a middle Jurassic cooling. Apatite (U–Th )/He dates from the central Tianshan samples range from ca. 130 Ma to ca. 116 Ma. The Apatite (U–Th )/He date for the northern Tianshan sample is ca. 27 Ma. Previous studies also reported Apatite (U–Th)/He ages of ca. 44 Ma–11 Ma in the Baluntai area of the southern Central Tianshan^[1]. Two episodes of cooling are distinguished by thermal history modelling: (1) Mesozoic cooling occurred as the result of the exhumation and tectonic reactivation of the central Tianshan; and (2) The Tianshan orogenic belt has been rapidly exhumed since the Middle Cenozoic.

References

[1] Lü, H.H., Chang, Y., Wang, W., Zhou, Z.Y., 2013. Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology. Science China: Earth Sciences, 43(12): 1964-1974 (in Chinese).