Pre-Cenozoic Igneous Rocks of the Tibetan Plateau

Magmatism of multiple types records the histories of origin, drift, and accretion of various terranes comprising of the present-day Tibetan Plateau. This chapter provides a synoptic overview of the pre-Cenozoic igneous rocks in the Tibetan Plateau in terms of spatial and temporal distributions, rock associations, geochemical characteristics, petrogenesis, and geodynamic interpretations. The Early Paleozoic (Cambrian-Ordovician) igneous rocks are poorly exposed in the Southern Qiangtang subterrane, Central Lhasa subterrane, and Tethyan Himalaya. These rocks are mostly high-K calc-alkalic to shoshonitic, as well as minor peraluminous, resulting from the southward subduction, roll-back, and break-off of the proto-Tethys slab beneath the northern Gondwana margin. The Late Paleozoic (Permian) igneous rocks commonly occur in the Northern Qiangtang subterrane, Southern Qiangtang subterrane, and Tethyan Himalaya. These rocks are composed mainly of basalts and ultramafic-mafic dykes, showing geochemical affinity to ocean island basalts (OIB), alkaline, and tholeiitic basalts. These features indicate an extensional tectonic setting most likely related to a mantle plume represented by the coeval Panjal Traps in NW India. The Mesozoic igneous rocks are widespread in the entire Tibetan Plateau. The Triassic peraluminous granites widespread in the Northern Qiantang subterrane are attributed to the collision between southern and northern Qiangtang. The Jurassic to Early Cretaceous igneous rocks in the Southern Qiangtang subterrane are medium-K to high-K calc-alkalic and show arc signatures, which form an E-W belt that was attributed to the northward subduction of the Bangong-Nujiang Tethys. The Late Triassic to Early Cretaceous are widespread in the Northern and Central Lhasa subterranes, representing a magmatic arc related to either the northward subduction of the Neo-Tethys or the southward subduction of the Bangong-Nujiang Tethys. The Late Cretaceous igneous rocks in the Southern Qiangtang, Northern, and Central Lhasa subterranes show geochemical features of high-Mg andesites or adakites and are generally interpreted to originate from thickened or delaminated lower crust following the Lhasa-Qiangtang collision. The igneous rocks in the Southern Lhasa subterrane were emplaced from the Late Triassic to Late Cretaceous and were dominated by medium-K calc-alkalic gabbros, diorites, and granodiorite. These rocks show arc signatures related to the northward subduction of the Neo-Tethys. The Mesozoic igneous rocks in the Tethyan Himalaya consist mainly of basalts and show OIB compositions, in which the Triassic and Jurassic rocks likely indicate a rift setting at passive continental margin, whereas the Early Cretaceous rocks that have been named the Comei large igneous province, were the products of the Kerguelen mantle plume.