GROWTH AND UNROOFING OF THE HIMALAYA FROM DETRITAL MONAZITE PETROCHRONOLOGY (Invited Presentation)

Monazite petrochronology has been a useful tool for documenting cryptic shear zones and for dating the motion of large structures within the Himalaya, yet it remains underutilized in the foreland basin successions despite its potential to record both the metamorphic and igneous history of the orogen. We present detrital monazite U-Th-Pb geochronology with trace element chemistry and zircon U-Th-Pb geochronology from the Miocene-Pliocene Siwalik Group in western Nepal to explore: (1) the timing of prograde/retrograde conditions in the Himalaya and (2) the unroofing history of the fold-thrust belt.

The Eocene marked a transition to thick crust in the Himalayan belt, capable of crustal melting, and garnet-stable pressures. Trace element chemistry in detrital monazite demonstrates the establishment of garnet-stable prograde conditions and the appearance of igneous monazite by ca. 37 Ma. The disappearance of garnet-stable monazite after ca. 23 Ma marked a transition to rapid exhumation and retrograde conditions in the fold-thrust belt.

The middle Miocene-Pliocene Siwalik Group was derived from the Tethyan Himalayan Sequence, Lhasa terrane, Greater Himalayan Sequence protolith, and metamorphosed Greater Himalayan Sequence, with the appearance of Lesser Himalayan Sequence detritus at 10 Ma. Notably, monazite Th-Pb ages show initial contributions from Cenozoic Himalayan metamorphic rocks to the foreland basin at ca. 13 Ma, derived from the Greater Himalayan Sequence. The relatively late arrival of metamorphosed Greater Himalayan Sequence detritus to the foreland is inconsistent with the classic channel flow model, which requires contemporaneous mid-crustal flow and exhumation of metamorphosed Greater Himalayan Sequence from ca. 23–16 Ma.