Eocene adakitic quartz monzonites and granite porphyries from the northern Qiangtang Block, central Tibet: Partial melting of sediment-rich melange?

The timing and mechanism of crustal thickening and initial surface uplift of the Tibetan Plateau remain disputed. Here, we report zircon U-Pb geochronological and O isotopic and whole-rock geochemical and Sr-Nd isotopic data for Eocene (41-37 Ma) granite porphyries and quartz monzonites from the Qoima Co area of the northern Qiangtang Block, central Tibet. The granite porphyries are characterized by high silica content (72 wt%), adakitic trace-element compositions, enriched Sr-Nd isotope signatures [(Sr-87/Sr-86)(i) = 0.7074, epsilon Nd (t) = -4.5], and a mean zircon delta O-18 value of +6.28 & PLUSMN; 0.85 & PTSTHOUSND;. The quartz monzonites exhibit high K2O content (5.1-6.8 wt%) and K2O/Na2O ratios (1.3-2.3). They also display adakitic geochemical characteristics, such as low Y (12-25 ppm) and Yb (1.1-1.8 ppm) content, high Sr/Y (98-187) and La/Yb (59-134) ratios, negligible Eu and positive Sr anomalies, and depletion in Nb and Ta. The quartz monzonites have (Sr-87/Sr-86)(i) ratios ranging from 0.7069 to 0.7078, epsilon Nd (t) values of -5.1 to -2.9, and mean delta O-18 values ranging from +6.27 & PLUSMN; 0.64 & PTSTHOUSND; to +7.91 & PLUSMN; 0.32 & PTSTHOUSND;. We suggest that the granite porphyries were most probably derived by the partial melting of thickened, sediment-bearing lower crust and that the quartz monzonites were most likely generated by the partial melting of sediment-rich melange. Combining these results with the existing tectonic and geophysical data, we conclude that continental subduction and melange underplating may have been responsible for crustal shortening and tectonic thickening.