Abstract:

Porphyry granites were genetically related to regional thrusting in the Fenghuoshan Mts., northern Tibetan Plateau. Both the granite emplacement and thrusting occurred in Oligocene, a key period of regional tectonic evolution according to field geological data. The porphyry granite and dark xenolith within the granite were sampled in northern Fenghuoshan Mts., and minerals of the samples were identified in detail by using microscope. SHRIMP U-Pb isotopic dating of zoned zircons from magmatic crystallization of the samples gives two groups of ages. Mean age of one group of U-Pb isotopic ages is (34.5±1.4)Ma, representing time of partial melting of the crust and formation of granitic magma. Mean age of another group of U-Pb isotopic ages is (27.6±0.5)Ma, representing time of granitic magma emplacement into the Yaxicuo group. Porphyry granite intruded in 27.6 Ma in northern Fenghuoshan Mts. belongs to the youngest granite complex outcropped in north Tibetan Plateau. U-Pb isotopic dating of the porphyry granite and its xenolith provides important time constrains for Cenozoic stratigraphic system, regional thrusting and plateau uplift.

Key words: porphyry granite of the Fenghuoshan Mts., U-Pb isotopic dating of zircon, SHRIMP, thrust system, northern Tibetan Plateau