Petrogenesis of late Early Oligocene trachytes in central Qiangtang Block, Tibetan Plateau: crustal melting during lithospheric delamination?
Late Early Oligocene mantle-derived potassium (K)-rich lavas occur widely in the Qiangtang Block, central Tibet. However, it remains ambiguous whether partial melting of crust beneath central Tibet occurred in the Oligocene. Here, we report the newly identified trachytes in the Gemuchaka area of central Qiangtang Block. Zircon U-Pb and biotite 40Ar-39Ar analyses suggest that they were erupted at ca. 30 Ma, coeval with other K-rich lavas in the central Qiangtang Block. They have relatively constant and high SiO2 (62.4–62.9 wt.%), Al2O3 (17.2–17.4 wt.%), K2O (6.0–6.1 wt. %), and Na2O (4.6–4.7 wt.%) contents. They are characterized by enrichments of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and by depletions of heavy REEs (HREEs) and high field strength elements (HFSEs), with negligible Eu but clear negative Ta-Nb-Ti anomalies. They have low εNd(t) (−5.5 to −4.6) but high (87Sr/86Sr)i (0.7076 to 0.7077) values, with variable zircon εHf(t) (−5.78 to 0.92) and δ18O (6.51 to 8.54) values. Except for obviously higher SiO2 contents, the trachytes are geochemically similar to the K-rich rocks in Gemuchaka area. We suggest that the trachytes were probably generated by remelting of the K-rich lower crust, which was possibly formed by the underplating of enriched mantle-derived K-rich lavas during the delamination of the lower part of thickened lithospheric mantle. After the delamination, the upwelling asthenosphere provided heat for partial melting of the lithospheric mantle and newly-formed lower crust. Our study provides the first petrological evidence for the existence of late Early Oligocene crust-derived rocks in the Qiangtang Block.