Jingen Dai

Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology

To test previously suggested exhumation histories of the Gangdese Batholith in the central part of the Transhimalayan plutonic belt, we conducted paired apatite and zircon (U-Th)/He thermochronological investigations of the Yarlung Zangbo gorge in the central part of the batholith. Age-elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation (∼46–48, ∼22–18, and ∼11–8 Ma). Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history. The Cretaceous–Early Paleogene exhumation of the Gangdese Batholith was probably caused by both the northward subduction of the Neo-Tethys and the collision between the Lhasa and Qiangtang blocks. The Early Miocene rapid exhumation might be a response to shortening caused by the Gangdese Thrust or erosion driven by dynamic uplift following lithospheric delamination. In contrast, the Late Miocene exhumation is coincident with both the proposed capture of the Yarlung Zangbo gorge by a foreland draining catchment and the intensification of the Asian monsoon, as well as normal faulting. Hence, the latest stage of exhumation might be attributed to the incision of the Yarlung Zangbo gorge, the activity of a north-south fault, or both.

Insights into the early Tibetan Plateau from (U–Th)/He thermochronology

The central Songpan–Ganzi belt, located on the eastern margin of the Tibetan Plateau, has a similar high elevation and low relief to parts of central Tibet. Thermochronological studies from the central Tibetan Plateau reveal a history of slow exhumation (rates <0.05kmMa−1) since 45Ma; however, the exhumation history of the central Songpan–Ganzi belt is unknown. To address this, we conducted an apatite and zircon (U–Th)/He thermochronology study of bedrock samples collected across the central Songpan–Ganzi belt and into central parts of the Tibetan Plateau. Zircon (U–Th)/He ages range from 54.2±7.5 to 146.5±10.0Ma and the majority of apatite (U–Th)/He ages fall between 74.7±19.0 and 35.7±9.4Ma. Thermal history models of these data show rapid cooling in the late Mesozoic and much slower cooling diagnostic of low rates of erosion throughout most of the Cenozoic. The late Mesozoic rapid cooling is consistent with the existence of significant topography and relief at least in some parts of the Songpan–Ganzi belt at that time. We find no evidence for a regional Miocene acceleration in erosion, although three samples from the headwaters of the Salween and Mekong rivers gave younger AHe ages between 15 and 16Ma that reflect an acceleration in river incision. Supplementary material: Laser ablation–inductively coupled plasma mass spectrometry zircon U–Pb data from central and eastern Tibet are available at www.geolsoc.org.uk/SUP18647.

Deep carbon cycle recorded by calcium‐silicate rocks (rodingites) in a subduction‐related ophiolite

Carbon cycling in subduction zones remains poorly constrained due to the lack of relevant geological records. Here we report magnesium isotope data (δ26MgDSM3) from calcium-silicate rocks (rodingites) from the Xigaze ophiolite, southern Tibet, which is thought to represent remnants of Neo-Tethyan oceanic lithosphere. Behaviors of immobile trace elements in rodingites resemble those of their mafic dike protoliths, showing subduction-related signatures. The majority of rodingites exhibits low δ26Mg values of −0.72‰ to −0.33‰ with a weighted average of −0.47 ± 0.11‰ (2 SD), significantly lighter than that of their protoliths (−0.31 ± 0.03‰). This difference likely reflects the interaction of the protolith with isotopically light carbonate fluids. Modeling indicates that this hypothesis requires the input of 5 to 15 wt % carbonates during rodingitization. Our study suggests that rodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates.