Zircon U–Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…4d). In addition, the relatively low ε Nd (t) and ε Hf (t) isotopic values of the Napuri adakitic rocks compared to Gangdese Jurassic-Eocene granitoids (e.g., Ji et al, 2009Ji et al, , 2012Wen et al, 2008a) further rules out this possibility. The Group I samples are also characterized by positive Eu and Sr anomalies, which contradicts a model of partial melting of arc-type tonalites with amphibole and plagioclase as residual phases (Kamei et al, 2009).…”

“…Magma mixing generally requires mantle-derived basaltic and crust-derived felsic endmembers (e.g., Streck et al, 2007). In southern Lhasa, the candidates for mantle-derived basaltic and crust-derived felsic end-members are most plausibly represented by the 65-40 Ma Linzizong mafic magmas (e.g., Lee et al, 2012;Mo et al, 2007) and the synchronous (45-53 Ma) Gangdese granites (e.g., Ji et al, 2009Ji et al, , 2012Wen et al, 2008a), respectively. However, the ε Nd (t) and ε Hf (t) values of the Napuri adakitic rocks are lower than those of both the felsic and basaltic end-members, Defant and Drummond (1990) and Petford and Atherton (1996).…”

“…Although the Napuri adakitic magmas are contemporary with the Gangdese peak activities at ca. 50 Ma (e.g., Ji et al, 2009;Lee et al, 2009Lee et al, , 2012Wen et al, 2008a), their higher SiO 2 (66.4-73.9 wt.%) and lower MgO (0.4-1.0 wt.%) and compatible element (Cr = 2.9-7.4 ppm; 5 55 60 65 70 75 80 45 0 5 55 60 65 70 75 80 45 0 5 55 60 65 70 75 80 45 0 5 55 60 65 70 75 80 45 0 5 55 60 65 70 75 Ni = 1.6-4.5 ppm) contents also do not accord with a mantle origin (Martin et al, 2005). Their relatively enriched Sr-Nd isotopic compositions also argue against such a source (Fig.…”

“…The Gangdese batholith, consisting of the Latest Triassic-Late Miocene intermediate-felsic intrusive rocks, is one of the most important geologic units of southern Lhasa (e.g., Chu et al, 2006;Chung et al, 2003Chung et al, , 2009Ji et al, 2009Ji et al, , 2012Wen et al, 2008a,b). The adakitic intrusive rocks within the Gangdese batholith are considered to have been generated in two main stages, the Jurassic-Cretaceous (160-77 Ma) and : (1) Jurassic-Cretaceous adakitic rocks generated in an arc setting were related to Neo-Tethyan subduction processes, by melting of either subducted oceanic slabs (e.g., Jiang et al, 2012;Ma et al, 2013a;Wei et al, 2007;Zhang et al, 2010b;Zhu et al, 2009) or thickened mafic continental lower crust (e.g., Guan et al, 2010;Wen et al, 2008b); (2) Cenozoic adakitic rocks occurring as small-volume plugs or dikes/sills, which intrude or crosscut the Gangdese batholith, the Linzizong volcanic successions and associated sedimentary formations, and extend~1300 km across nearly the entire southern Lhasa (Fig.…”

“…Here we use the slab break-off model to account for both the formation of the Napuri adakitic rocks and the associated magmatic "flare-up" event (e.g., Chung et al, 2005;Ji et al, 2009Ji et al, , 2012Lee et al, 2009Lee et al, , 2012Wen et al, 2008a;Zhu et al, 2011). When two continents collide, the tensile stresses between the buoyant continental lithosphere and previously subducted oceanic lithosphere lead to the separation of the oceanic slab from the continental lithosphere (e.g., Davies and von Blanckenburg, 1995;van Hunen and Allen, 2011;von Blanckenburg and Davis, 1995).…”