The Late Triassic I–Type Granites from the Longmu Co–Shuanghu Suture Zone in the interior of Tibetan Plateau, China: Petrogenesis and Implication for Slab Break–Off

Abstract

The Jiangaidarina granitic mass (JM) is an important part of the magmatic belt in Longmu Co–Shuanghu Suture Zone (LSSZ) in the central Tibetan Plateau. An integrated research involving whole–rock geochemistry, zircon LA–ICP–MS U–Pb ages and Hf isotopic compositions was carried out to define the timing, genesis and tectonic setting of the JM. Zircon LA–ICP–MS U–Pb ages have been obtained ranging from 210 to 215 Ma, rather than the Early Jurassic as previously thought. Fifteen granite samples contain hornblendes and show a negative correlation between P2O5 and SiO2, indicating that the JM is an I –type granite. All the granites are enriched in LREE relative to HREE, with negative Eu anomalies (Eu/Eu*=0.56–0.81), and have similar trace elements patterns, with depletion of Ba, Nb, Sr and P. These suggest that the JM was fractionated, and this is also proved by the characteristic of negative correlations between oxide elements (TiO2, MgO, FeOt, MnO, CaO) and SiO2. Almost all εHf(t) values of the granites are between −10.3 and −5.8, implying that the JM has a crustal source intimately related with the South Qiangtang Block (SQB), except for one (+10.2), showing a minor contribution from mantle source. Moreover, relatively low Na2O/K2O ratios (0.42–0.93) and high A/CNK values (0.91–1.50) reflect that the JM was predominately derived from the medium–high potassium basaltic crust, interacted with greywacke. Our new geochemical data and geochronological results imply that the Late Triassic magmas were generated in a post–collisional tectonic setting, probably caused by slab break–off of the Longmu Co –Shuanghu Tethyan Ocean (LSTO). This mechanism caused the asthenosphere upwelling, formed extension setting, offered an enormous amount of heat, and provided favorable conditions for emplacement of voluminous felsic magmas. Furthermore, the LSTO could be completely closed during the Middle Triassic, succeed by continental collision and later the slab broke off in the Late Triassic.