Petrology and geochemistry of postcollisional volcanic rocks from the Tibetan Plateau; implications for lithosphere heterogeneity and collision-induced asthenospheric mantle flow

This article summarizes the geological setting, spatial-temporal distribution, and major-element, trace-element, and Nd-Sr-Pb isotopic compositional variation of rocks representative of Tibetan postcollisional magmatic activity. The implications of petrogenesis and spatial-temporal distribution are discussed in relation to lithospheric mantle heterogeneity and a possible role for collision-induced asthenospheric mantle flow. Rocks indicative of postcollisional volcanism are widely distributed across the terranes making up the Tibetan plateau. Three stages of activity are recognized (ca. 45-25, 25-5, and 5-0 Ma), mostly conforming to potassic to ultrapotassic shoshonitic and high-potassium calc-alkaline types. These show strong relative enrichments in large-ion lithophile elements (LILE), U, Th, and light rare earth elements (LREE); depletions in high field strength elements (HFSE) and heavy rare earth elements (HREE)--with (La/Yb) (sub N) ratios ranging from 4.3 to 699, mainly 40-50; and Sigma REE abundances of 50-2560 ppm, mainly 300-500 ppm--in most cases lacking significant negative Eu anomalies. However, the element distributions for kamafugite and carbonatite show ocean island basalt-like nondepleted or even slightly enriched HFSE patterns. The plots of epsilon Nd versus (super 87) Sr/ (super 86) Sr define a mixing array between Neo-Tethyan mid-ocean ridge basalts (MORB) and High Himalayan crustal compositions, with epsilon Nd(t) varying from +5.95 to-17.42 and (super 87) Sr/ (super 86) Sr (i) 0.702059 to 0.746320. The range of Nd and Sr isotopic compositions in the northern parts of the plateau, Sanjiang, and west Qinling is relatively small compared to that from Gangdese to the south, where (super 87) Sr/ (super 86) Sr ratios range from 0.703785 to 0.746320 and (super 143) Nd/ (super 144) Nd from 0.511737 to 512710. The variation of Pb isotopic ratios is somewhat less, with (super 206) Pb/ (super 204) Pb ranging from 18.149 to 19.345, (super 207) Pb/ (super 204) Pb from 15.476 to 15.803, and (super 208) Pb/ (super 204) Pb from 37.613 to 40.168. In general, magmatic isotopic compositions indicate the regional-scale presence of DUPAL-like mantle, reflecting additions of the "enriched mantle" components (EM1, EM2) to an ambient MORB-HIMU (high mu , i.e., high U/Pb mantle) asthenospheric hybrid. The observed geochemical, isotopic, and mineral phase compositional variations of primitive magmatic products and their entrained mantle xenoliths clearly suggest LILE-enriched and HFSE-depleted phlogopite/amphibole-bearing mantle wedge sources contaminated by (presumably subduction-related) hydrous fluids or small-fraction H (sub 2) O-CO (sub 2) -rich melts. Tibetan lithospheric mantle appears to reflect the presence of and interaction between at least three compositional end-members. The overall spatial-temporal pattern of Tibetan collisional and postcollisional activity is consistent with the hypothesis that the Neo-Tethyan asthenospheric mantle was laterally displaced along discrete northeast- and southwestward flow channels in response to the India-Asia collision.