Early Jurassic Granitoids in the Pairigang Area, Tibet: Implications for the Evolution of the Neo-Tethyan Ocean and the Accretion of Juvenile Crust in the Southeastern Lhasa Block

Abstract

The Jurassic represents one of the most significant periods within the widespread Mesozoic magmatism of the Lhasa Block; however, its formation age and petrogenesis have led to controversy over the deep genetic mechanisms responsible for magmatism. To address this, we present new zircon geochronological, Hf isotopic, and geochemical data for the Pairigang granite porphyries and granodiorites in the Tangjia area, southeastern Lhasa Block. The analysis results demonstrate that the Pairigang granites that intruded in 195-192 Ma (Early Jurassic) are characterized by marked light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) enrichment, display marked depletion in Nb and Ta, reflecting an affinity with arc magmatic rocks. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf (t) values (−7.33 to +1.47) and model ages indicate involvement of both ancient crustal material and mantle-derived components. Based on the geochronological and geochemical analyses, this study suggests that the Early Jurassic Pairigang intrusive rocks are I-type granites, formed by partial melting of mafic lower crustal material and subsequent magma mixing with mantle-derived components. Combined with the previous studies, we propose that the Early Jurassic Pairigang granitic rocks resulted from the northward subduction of the Yarlung–Zangbo Neo-Tethyan ocean. Variations in zircon Hf isotopes from Jurassic magmatic rocks across the Lhasa Block further indicate that the southeastern Lhasa Block represents an accretionary wedge system comprising juvenile crust formed during the evolution of the Tangjia–Sumdo Paleo-Tethyan Ocean and the Yarlung–Zangbo Neo-Tethyan Ocean.