Compositional diversity of ca. 110 Ma magmatism in the northern Lhasa Terrane, Tibet: Implications for the magmatic origin and crustal growth in a continent–continent collision zone

Abstract

The Yanhu area in the northern Lhasa subterrane exposes diverse rock types including basalts, rhyolites, quartz dioritic porphyries, and associated dioritic enclaves. The basalts and rhyolites occur as a bimodal volcanic suite, and the quartz dioritic porphyries intrude into the older basalts as a small apophysis. In this paper, we report for the first time the zircon LA-ICP-MS U–Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr–Nd isotopic data from the diverse Early Cretaceous magmatic rocks from Yanhu. The three basalt samples that we dated yielded zircon U–Pb ages of 110±0.7Ma, 108.9±1.1Ma, and 111.8±3.2Ma. The zircons from one quartz diorite porphyry yielded an age of 109.7±0.8Ma, which is coeval with the dioritic enclave (110.4±1.4Ma). The basalts show a high-K calc-alkaline signature, enriched in Rb, Th, U, and light rare earth elements (REEs) and depleted in Nb, Ta, Ti, Zr, and Hf. These rocks possess varying whole-rock εNd(t) (−0.6 to +2.1) and zircon εHf(t) (+0.6 to +8.9) values. The rhyolite samples are high-K calc-alkaline and are metaluminous to slightly peraluminous. These rocks yielded whole-rock εNd(t) of +0.1 to +0.9 and zircon εHf(t) of +5.1 to +12.4. The quartz dioritic porphyries are characterized by high Al2O3 content (15.9–16.1wt.%), high Sr (466–556ppm), low Yb (1.36–1.41ppm), and low Y (13.8ppm) abundances. Similar geochemical signatures are also present in the dioritic enclaves, revealing that both the quartz dioritic porphyries and the dioritic enclaves have an affinity with adakitic rocks. Moreover, the host rocks and the enclaves display homogeneous εNd(t) (+3.5 to +3.6 and +3.4 to +4.0, respectively) and zircon εHf(t) values (+12.4 to +16.9 and +11.5 to +15.7, respectively). Our geochemical data indicate that the magmatic rocks from Yanhu were derived from the partial melting of distinct source regions, i.e., a heterogeneous metasomatized mantle wedge source for basalts, a juvenile crust source for rhyolites, and a thickened mafic lower crust source that mixed with basaltic magmas for the adakitic rocks (including quartz dioritic porphyries and dioritic enclaves). Compared to typical arc basalts, the basalts from Yanhu are relatively enriched with high field strength elements (HFSEs) (e.g., Zr, Nb), resembling those of within-plate basalts elsewhere. In combination with the presence of a coeval bimodal volcanic rock suite, the ca. 110Ma magmatism in Yanhu is inferred to have occurred in an extensional setting. Our new data, together with recently published data, enable us to correlate the generation of the compositional diversity of the ca. 110Ma Yanhu magmatic rocks that formed in an extensional setting to the slab break-off during the southward subduction of the Bangong–Nujiang Ocean lithosphere. The presence of basaltic magmatism and coeval silicic magmatic rocks with positive zircon εHf(t) indicate that the extensive magmatism at ca. 110Ma have contributed significantly to the crustal growth of the northern Lhasa subterrane.