Cretaceous evolution of the Lhasa-Qiangtang Orogen: insights from mafic igneous rocks

The compositional changes of successively emplaced mafic magmatites provide insights into the evolution of orogenic belts, particularly the transformation processes and properties of the orogenic lithospheric mantle. Herein, we present new zircon U‒Pb ages and Lu‒Hf isotopes, as well as whole-rock major, trace element, and Sr – Nd isotope compositions from three mafic intrusions in the Asuo area, central Tibet, to clarify the evolution of the Lhasa – Qiangtang Orogenic Belt (LQOB). These mafic intrusions can be classified into calc-alkaline basalts and are enriched in large ion lithophile elements and depleted in high-field strength elements. Significant differences in the whole-rock trace element compositions, Sr – Nd isotope values, and U – Pb ages stratify them into two groups, namely Groups I and II, of which Group I rocks (ca. 105 Ma) show lower87Sr/88Sri = (0.70694–0.70701), Sm/YbN = (1.51–1.54) and higher εNd (t) = (−0.74 to −0.61) than Group II (ca. 97–95 Ma) (87Sr/88Sri = 0.70759–0.70870; εNd (t) = –4.58 to −3.75; Sm/YbN = 2.43–2.94). These features indicate that the mantle source exhibits a progressive increase in the proportion of garnet, along with significant addition of subduction-related components into the primary melts. One scenario for producing such trace element patterns and gradually enriched radioactive isotope signatures involves the extensive heating of an oceanic slab beneath the LQOB by upwelling asthenosphere, which leads to the extensive melting of sediments within the slab. Identification of ca. 105–95 Ma Asuo mafic rocks suggests their formation in a post-collisional setting with additional heat input from the deep mantle. These results provide new evidence supporting the occurrence of the Lhasa – Qiangtang collision within the framework of north-south bidirectional subduction.