Early Cretaceous intrusive rocks in the Gangdese arc of southern Tibet do not support shallow subduction of the Neo-Tethyan oceanic lithosphere

Abstract

The subduction geometry of the Neo-Tethyan oceanic lithosphere during the Early Cretaceous (i.e., shallow vs. steep subduction) remains in dispute due to the limited occurrence of the Early Cretaceous igneous rocks in the Gangdese arc. This study reports petrographical, zircon UPb geochronological, zircon Hf isotopic, whole-rock geochemical, and mineral geochemical data for the ultramafic-mafic and intermediate-felsic rocks in the eastern Gangdese arc, southern Tibet. All these rocks were emplaced at ca. 112–105 Ma (i.e. ca. 110 Ma). The ultramafic-mafic rocks (including hornblendite and hornblende gabbro) display field, petrographic, and geochemical features of cumulates and can be explained by the accumulation of hornblende + plagioclase from an andesitic parental magma. The intermediate-felsic rocks (including quartz diorites and granodiorites) show adakitic features and are most likely related to melts that experienced high-pressure hornblende fractionation followed by plagioclase accumulation. Hornblende thermobarometer reveals that these ultramafic-mafic and intermediate-felsic rocks formed at 837–907 °C and 6.6–11.1 kbar, indicating a rapid decompression condition that is most likely related to an extension setting. The data reported in this study and the literature reveal that the Early Cretaceous magmatism was active in the southern margin of the Gangdese arc, suggesting that the Neo-Tethyan oceanic lithosphere most likely experienced a steep, rather than a shallow subduction beneath the Lhasa Terrane during the Early Cretaceous.