Continental crustal growth and thickness changes caused by the Paleo- to Neo-Tethys Ocean transition: Evidence from the Early Jurassic magmatism in the Lhasa terrane

Abstract

The Late Triassic–Early Jurassic is a significant period for the evolution of the Paleo-Tethys to Neo-Tethys Ocean on the Tibetan Plateau, which includes complex geological processes of Tethys Ocean plate subductions and continent-arc-continent collisions. It is also an important period for the formation and evolution of the crust of many terranes (such as the Lhasa and South Qiangtang terranes) in the Tibetan Plateau. As a probe to understand the composition and structure of the crust, igneous rocks play an important role in improving our understanding of continental crustal formation and evolution. In this study, zircon U-Pb dating, zircon Hf isotopes, and the whole-rock geochemistry of Early Jurassic magmatic rocks in the Tangjia-Sumdo area of the Lhasa terrane of the Tibetan Plateau were studied. The results indicate that the compositions are granite (187.4 ± 1 and 196.5 ± 1 Ma), granite porphyry (196.0 ± 1 Ma), granodiorite (196.4 ± 1 Ma), quartz monzonite (188.1 ± 1 Ma), and gabbro (182.8 ± 1 Ma). The intermediate-acidic rocks (197–187 Ma) are all I-type granites and the products of the mixture of crust- and mantle-derived magma. The gabbro (∼183 Ma) is derived from the depleted mantle, which is the product of the partial melting of spinel and garnet lherzolites. Based on the geochemical data of intermediate-acidic magmatic rocks from the Triassic to Jurassic in the Lhasa terrane, the changes in the Sr/Y, (La/Yb) _N, Dy/Yb, and Sm/Yb values indicate that the crust of the Lhasa terrane thickened in the Late Triassic (240–200 Ma) and thinned in the Jurassic (200–160 Ma). We suggest that in the Late Triassic (240–200 Ma), the Sumdo Paleo-Tethys Ocean closed and the collisional orogeny between the southern and northern Lhasa terrane led to crustal thickening. The intermediate-acidic rocks (197–187 Ma) are the products of the break-off of the Sumdo Paleo-Tethys Ocean Plate. The gabbro (∼183 Ma) is formed by the intrusion and condensation of magma formed by the partial melting of the mantle wedge metasomatized by subducted sediments in an active continental margin, which is the product of the northward subduction of the Neo-Tethys Ocean. The transition between the Paleo- to Neo-Tethys ocean is an important mechanism for the growth and thickness changes of the crust in the Late Triassic–Early Jurassic of the Lhasa terrane, potentially contributing significantly to the vertical growth and change of the continental crust in the subduction zone.