Adakite-like geochemical signature produced by amphibole-dominated fractionation of arc magmas: An example from the Late Cretaceous magmatism in Gangdese belt, south Tibet

• Published in: Lithos Vol. 232; pp. 197 - 210
• Main Authors: Xu, Wang-Chun, Zhang, Hong-Fei, Luo, Bi-ji, Guo, Liang, Yang, He
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2015
• Subjects: Adakite-like rock; Gangdese belt; Geodynamic process; Normal arc rock; Petrogenesis; South Tibet; South Tibet; Geodynamic process; Gangdese belt; Petrogenesis; Adakite-like rock; Normal arc rock
• DOI: 10.1016/j.lithos.2015.07.001
• ISSN: 0024-4937

Late Cretaceous (~106–76Ma) adakite-like intrusive rocks in the middle-eastern Gangdese belt occur in an E–W trending belt paralleling the Indus–Yarlung suture, south Tibet. Their petrogenesis and geodynamic processes have been a subject of debate. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for adakite-like intrusive rocks as well as the normal arc rocks (gabbros and gabbroic diorites) in the middle Gangdese belt. LA-ICPMS U–Pb zircon analyses yielded an identical age of ~88Ma for two adakite-like rocks, which are slightly younger than the gabbro and gabbroic diorite (ca. 94–90Ma). Both the adakite-like rocks and the normal arc rocks have similar whole-rock Sr–Nd and zircon Hf isotope compositions, indicating that they have been derived from a common source. Similarly, the adakite-like and normal arc intrusive rocks in the eastern Gangdese belt also show similar Sr–Nd–Hf isotope compositions. In the middle-eastern Gangdese belt, the >85Ma Late Cretaceous intrusive rocks consist of a magma series from gabbro to granodiorite, including both normal arc rocks and adakite-like rocks. These rocks overlap in space and time that conform to a normal arc differentiation trend. In terms of major and trace elements, they also show a clear evolution from the normal arc magmatic into adakitic field. Thus, we suggest that these >85Ma Late Cretaceous intrusive rocks were ultimately derived from melting of the hydrated mantle wedge and the adakite-like rocks can be generated in normal arc magmas by amphibole-dominated fractionation. Taking into accounting for the spatial and temporal distribution of the Cretaceous magmatic rocks in the Lhasa terrane, we prefer a model of early Late Cretaceous rollback following Early Cretaceous low-angle oceanic slab subduction. At intermediate pressure and H2O-rich conditions, fractionation of amphibole changes the major and trace element compositions of arc magmas, and will efficiently drives basaltic composition to andesitic composition in arc magmas. •Zircon U–Pb dating, geochemistry and Hf isotope composition for Late Cretaceous intrusions.•Both normal arc rocks and adakite-like rocks overlap in space and time.•Adakite-like and normal arc intrusive rocks show similar Sr–Nd–Hf isotope compositions.•They also show a clear evolution in terms of major and trace elements.•The adakite-like rocks produced in normal arc magmas by amphibole-dominated fractionation.