40Ar/39Ar Thermochronological Study of Xianshuihe Fault Zone, Eastern Tibet
Date Issued
2010
Date
2010
Author(s)
Hsu, Jin-wei
Abstract
In the eastern margin of the Tibet plateau, the Xianshuihe fault zone transacts Paleozoic-early Mesozoic sedimentary sequences which were intruded by a number of the Early Mesozoic or Cenozoic granites. Granite, pegmatite and gneiss samples were collected along the Xianshuihe fault zone for the present U-Pb and 40Ar/39Ar geochronological study, aiming to reveal the movement history for the fault zone and to explore its implications to regional tectonics.
The results show mica 40Ar/39Ar ages clustering in two groups: 5.5-4.2 Ma and 153-141 Ma. The younger ages appear in micas obtained from gneiss, granite and pegmatite in Kongashan, whereas the older ages were derived from granites in the Songpan-Garze fold belt. Two K-feldspars from granites in Kongashan display ages of 4.4+/-0.2 and 4.2+/-0.3 Ma, while other K-feldspars from the granites collected from outcrops in the Songpan-Garze fold belt show much older ages as 142+/-3 Ma, 70.6+/-1.8 Ma and 44.1+/-1.8 Ma. However, these 40Ar/39Ar ages are still much younger than their respective zircon U-Pb ages (213+/-4 Ma, 211+/-3 Ma) except one with 9.2+/-1.7 Ma.
Considering their respective closure temperatures, these radiometric ages would indicate a cooling rate of 100C/Myr for the Kongashan area since ca. 9 Ma. The granites collected from the outcrops north of Xianshuihe fault zone in Songpan-Garze fold belt show two stages of rapid cooling, with a rate of 10C/Myr prior to ca. 150-140 Ma and 10C/Myr after 30-20 Ma, respectively. Other granite from the outcrop south of Xianshuihe fault only displays one rapid cooling stage prior ca. 140 Ma. This would suggest that an erosion and exhumation event in the Songpan-Garze fold belt may have occurred in Early Cretaceous (ca. 150-140 Ma). The collision between India and Eurasia caused further uplift in the eastern Songpan-Garze fold belt during Late Oligocene-Miocene.
The present thermochronological study suggests that the granitic rocks cropping out along the Xianshuihe fault zone may have emplaced in the Early Mesozoic and experienced a prolonged cooling history until the Late Tertiary. While, the young radiometric ages from granite, gneiss and pegmatite samples indicate that the shearing activity of the Xianshuihe fault zone most likely occurred in the Miocene and Pliocene, in response to the active India-Eurasia collision starting since Tertiary.
Subjects
40Ar/39Ar dating
Xianshuihe fault zone
Tibet
Type
thesis
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