U-Pb zircon geochronology of major lithologic units in the eastern Himalaya: Implications for the origin and assembly of Himalayan rocks

Abstract

Models for the origin and deformation of Himalayan rocks are dependent upon geometric and age relationships between major units. We present field mapping and U-Pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern Himalaya, revealing that: (1) the South Tibet detachment along the Bhutan-China border is a top-to-the-north ductile shear zone; (2) Late Triassic and Early Cretaceous sedimentary samples from the northern Indian margin show a similar age range of detrital zircons from ca. 3500 Ma to ca. 200 Ma, but the Late Triassic rocks are distinguished by a significant age cluster between ca. 280 and ca. 220 Ma and a well-defined age peak at ca. 570 Ma, (3) an augen gneiss in the South Tibet detachment shear zone in southeast Tibet has a Cambrian–Ordovician crystallization age, (4) Main Central thrust hanging-wall paragneiss and footwall quartzites from the far western Arunachal Himalaya share similar provenance and Late Proterozoic maximum depositional ages, and (5) Main Central thrust footwall metagraywacke from the central western Arunachal Himalaya has a Paleoproterozoic maximum depositional age, indicated by a single prominent age peak of ca. 1780 Ma. Recent work in the eastern Himalaya demonstrates that in the early-middle Miocene, the Himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south. A proposed subsequent (middle Miocene) brittle low-angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here. The ca. 280–220 Ma detrital zircons of the Late Triassic strata are derived from an arc developed along the northern margin of the Lhasa terrane. Detritus from this arc was deposited on the northern margin of India during India-Lhasa rifting. Along-strike heterogeneity in Main Central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western Arunachal. Nonetheless, the Late Proterozoic rocks in the Main Central thrust hanging wall and footwall in far western Arunachal can be correlated to each other, and to previously analyzed rocks in the South Tibet detachment hanging wall to the west and in the Indian craton to the south. These findings are synthesized in a reconstruction showing Late Triassic India-Lhasa rifting and Cenozoic eastern Himalayan construction via in situ thrusting of basement and cover sequences along the north Indian margin.