Geochemical and Sr–Nd–Pb–O isotopic compositions of the post-collisional ultrapotassic magmatism in SW Tibet: Petrogenesis and implications for India intra-continental subduction beneath southern Tibet

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…Previous studies have shown that the adakitic rocks are K-rich, forming a 1000-km-long belt oriented sub-parallel to the IYS. The ages of the adakitic rocks range from 26.2 to 10 Ma (Cai et al, 2005;Chung et al, 2003Chung et al, , 2009Gao et al, 2007a;Guo et al, 2007;Hou et al, 2004;Xu et al, 2010;this study), within the range of the 22.5-12 Ma ages obtained for potassic-ultrapotassic igneous rocks in the Lhasa terrane (e.g., Ding et al, 2003;Gao et al, 2007b;Miller et al, 1999;Nomade et al, 2004;Williams et al, 2001Williams et al, , 2004Zhao et al, 2009), except for the 26.2 Ma age of adakitic rocks in Linzhi (Chung et al, 2003). Recently, Miocene adakitic dykes have been found on the southern side of the IYS, including the Mayum and Kuday dykes King et al, 2007) within the northern belt of the Tethyan Himalayan tectonic zone.…”

“…10b). Xu et al (2010) proposed that adakitic magmas in the Lhasa terrane were produced by mixing between melt derived from subducted Indian mafic lower crust in the Himalaya terrane and ultrapotassic lava in the Lhasa terrane, which is similar to the concept proposed by Zhao et al (2009). However, this idea is inconsistent with some of the geochemical characteristics of the adakitic rocks in the Lhasa terrane.…”

“…These results suggest that the crust beneath the Himalayan area was not the source of the adakitic rocks in the Lhasa terrane. The geochemical differences between the adakitic rocks in the western and eastern Lhasa terranes are unlikely to have been caused by mixing between ultrapotassic magma and melt from the lower crust below the Lhasa terrane, or with mafic lower crust of the subducted Indian Plate Zhao et al, 2009). On the other hand, these distinctive geochemical characteristics of the adakitic rocks in the western and eastern Lhasa terranes probably indicate compositional heterogeneities in the lower crust within this region.…”

“…However, this idea is inconsistent with some of the geochemical characteristics of the adakitic rocks in the Lhasa terrane. First, while the adakitic lavas in the Lhasa terrane were erupted alone or with ultrapotassic and potassic lavas, they have different Sr-Nd isotope compositions to the ultrapotassic and potassic rocks (Chung et al, 2003;Guo et al, 2007;Hou et al, 2004;Miller et al, 1999;Zhao et al, 2009), which implies that they have different sources. Second, as discussed above, some adakitic rocks, such as those in the western Lhasa terrane, do not show these mixing characteristics.…”

“…To identify spatial differences in geochemistry, adakitic rocks of the Lhasa terrane are divided into eastern and western groups, either side of the longitude of approximately 87°E (Fig. 1) (Guo et al, 2007;Hou et al, 2006;Xiao et al, 2007;Zhao et al, 2006Zhao et al, , 2009Zhou and Murphy, 2005).…”