Cambrian bimodal volcanism in the Lhasa Terrane, southern Tibet: Record of an early Paleozoic Andean-type magmatic arc in the Australian proto-Tethyan margin

• Published in: Chemical geology Vol. 328; pp. 290 - 308
• Main Authors: Zhu, Di-Cheng, Zhao, Zhi-Dan, Niu, Yaoling, Dilek, Yildirim, Wang, Qing, Ji, Wen-Hua, Dong, Guo-Chen, Sui, Qing-Lin, Liu, Yong-Sheng, Yuan, Hong-Lin, Mo, Xuan-Xue
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 18.10.2012
• Subjects: Cambrian bimodal metavolcanic rocks; isotopes; Lhasa Terrane; melting; Petrogenesis; rare earth elements; Southern Tibet; trace elements; volcanic activity; volcanic rocks; Whole-rock geochemistry; Zircon U–Pb dating and Hf isotope; zirconium; Myanmar; Andes region; China; Cambrian bimodal metavolcanic rocks; Lhasa Terrane; Zircon U–Pb dating and Hf isotope; Petrogenesis; Southern Tibet; Whole-rock geochemistry
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2011.12.024

This paper reports new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace element, and Sr–Nd isotope data from Cambrian metavolcanic rocks in the central Lhasa subterrane of southern Tibet. These rocks form a bimodal volcanic suite consisting mainly of silicic metavolcanic rocks with subordinate metabasalts. Five silicic metavolcanic samples dated at ca. 492Ma and one metabasalt sample yielding a near-concordant 206Pb/238U age of 492±4Ma indicate that mafic and silicic eruptions were contemporaneous. The metabasalts are mostly high-K calc-alkaline, enriched in Th, U, and light rare earth elements (LREEs), and depleted in Nb, Ta, Ti, Zr, and Hf, geochemically resembling the Andean arc basalts. The silicic metavolcanic rocks are high-K calc-alkaline and low in Nb and Zr. The metabasaltic rocks have negative whole-rock εNd(t) values (−4.7 to −3.5) and varying zircon εHf(t) values (−0.7 to +7.5), differing significantly from those of the silicic metavolcanic rocks, which yield negative whole-rock εNd(t) values of −8.4 to −7.2 and varying zircon εHf(t) values (−13.9 to −4.6). The metabasaltic rocks are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the silicic metavolcanic rocks were derived from basaltic melt-induced anatexis of the ancient Lhasa basement with inherited mantle melt signatures. The Western Qiangtang–Amdo–Tethyan Himalaya situated in the Indian proto-Tethyan margin and the Lhasa and other possible microcontinents or terranes (e.g., Gongshan, Baoshan, Tengchong, Burma, and Sibumasu) paleographically located in the Australian proto-Tethyan margin represent an early Paleozoic Andean-type magmatic arc facing the proto-Tethyan Ocean. The emplacement of the bimodal volcanic rocks and the development of the Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to slab break-off of the proto-Tethyan Ocean lithosphere following the collisional accretion of microcontinents or terranes located outboard of the magmatic arc (possibly Eastern Qiangtang and South China). ► Cambrian bimodal volcanic suite in the Lhasa Terrane was emplaced at ca. 492Ma. ► Metabasaltic rocks exhibit geochemical signatures of continent–arc basalts. ► The Lhasa Terrane represents part of an early Paleozoic Andean-type magmatic arc. ► The generation of the bimodal metavolcanic rocks may be a result of slab break-off.