Gangdese batholith

Gangdese batholith
Tectonic units of the Himalaya. Green is the Indus-Yarlung suture zone. Red is the Transhimalaya, including the Gangdese batholith. Lhasa to the east.
Location	Tibet Autonomous Region, China
Coordinates	30°N 91°E / 30°N 91°E / 30; 91Coordinates: 30°N 91°E / 30°N 91°E / 30; 91
Geology	Batholith

The Gangdese batholith or Gangdese volcanic arc is a major geological structure in the south of the Lhasa terrane in Tibet, to the north of the Himalayas. The batholith formed around 100 million years ago, and was volcanically active for about 20 million years. It reactivated around 65 million years ago as the Indian plate approached Eurasia, and was active for another 20 million years.

The Lhasa terrane moved northward and collided with the Qiangtang terrane along the Bangong-Nujiang suture towards the end of the late Jurassic (c. 163–145). Collision activity continued until the early Late Cretaceous (c. 100–66) Ma. The collision caused a peripheral foreland basin to form in the north part of the Lhasa terrane, which persisted into the Early Cretaceous. In some parts of the foreland basin the north-dipping subduction of the Neotethyan oceanic crust below the Lhasa terrane caused volcanism. The Gangdese volcanic arc was formed as this subduction continued along the southern margin of the Lhasa terrane. The batholith intrudes the southern half of the Lhasa terrain. It is the largest Transhimalayan plutonic complex.

U–Pb zircon dating suggests that there were two separate stages of plutonism in the Gangdese batholith, one in the Late Cretaceous (c. 103–80 Ma) and the other in the early Paleogene (c. 65–46 Ma), peaking around 50 Ma. Between these two stages the Gangdese was quiescent during the period c. 80–70 Ma, possibly due to flattening of the northward Neotethyan subduction. Contact with India began along the Yarlung-Zangbo suture around 50 Ma during the Eocene, and the two continents continue to converge. The second stage of activity may be due to the approach of India, preceded by rollback of the subducted slab and peaking at the time of collision. North-dipping seismic reflections deep in the crust below the Gangdese batholith at a depth of 40 to 60 kilometres (25 to 37 mi) may mark the downdip of the Yarlung-Zangbo suture, or may mark a more recent reverse fault.