Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault

Abstract

The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32-12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation-inductively coupled plasma-mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic-Jurassic (ca. 215-159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14-5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ~62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian-Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau.

Mountain biodiversity and ecosystem functions: interplay between geology and contemporary environments

“…First, the elevational bands that contain breakpoints surprisingly coincide with the locations of faults. Specifically, the Indus-Yalu suture zone fault [66] and the Xianshui-he fault [67] pass through the elevational bands of 2300-2500 m and 2600-2800 m on the Galongla and Gongga Mountains, respectively. Most of chemical characteristics, including minerals, weathering indices, and soil properties, were significantly (P < 0.05) different across the Indus-Yalu suture zone fault (results not shown).…”

Formation and evolution of Xianshuihe Fault Belt in the eastern margin of the Tibetan Plateau: Constraints from structural deformation and geochronology

“…This indicates that there may have been a small amount of excess Ar, but a flat plateau was formed by stage heating, which shows that the plateau age is reliable within the error range. In other words, the migmatites in the eastern Zheduoshan batholith T A B L E 2 40 Ar/ 39 Ar dating results of the mylonite from the XSF ductile shear belt (Lai & Zhao, 2018;Li et al, 2015Li et al, , 2016Liu et al, 2006;Searle et al, 2016;Tian, 2018;Wu et al, 2016;Zhang et al, 2017), and a large amount of zircon U-Pb geochronology data have been obtained (Figure 9 and Table 3). Combining the previous geochronological results and our dating results, the Zhedushan batholith can be divided into four emplacement stages.…”

“…318 National Highway, finding the age of 18 ± 0.3 Ma for the magma crystallization. Recently, there have been more studies on the petrology, geochemistry, and emplacement geochronology of Zheduoshan batholith (Lai & Zhao, 2018; Li & Zhang, 2013; Li et al, 2015, 2016; Liu et al, 2006; Searle et al, 2016; Tian, 2018; Wu et al, 2016; Zhang et al, 2017), and a large amount of zircon U–Pb geochronology data have been obtained (Figure 9 and Table 3). Combining the previous geochronological results and our dating results, the Zhedushan batholith can be divided into four emplacement stages.…”

“…In addition, in the Yanzigou area, south of the Zheduoshan batholith, there are leucogranite veins interspersed into the granodiorite. Zircon U–Pb dating results show that the leucogranites were formed in the Late Indosinian, and were metamorphozed by the tectonic–thermal events of the Middle Jurassic (Searle et al, 2016). However, Li et al (2015) believed that the leucogranites were formed by Indosinian granites deep melting around 170 Ma, a result of XSF structural deformation.…”

“…This represents onset of the XSF and the eastward extrusion time of the Sichuan‐Yunnan Block on the eastern margin of the Tibetan Plateau. Recently, Li and Zhang (2013) and Li et al (2016) discovered Oligocene‐Miocene migmatization and related tectonothermal events along the eastern part of the Zheduoshan batholith; Li et al (2015) discovered Middle Jurassic syntectonic magmatism in the Gonggashan pluton south of the Zheduoshan batholith for the first time, and Li et al (2015) and Searle et al (2016) classified the Zheduoshan batholith into five lithological units.…”