High Sr/Y two-mica granite from Quedang area, southern Tibet, China: Formation mechanism and tectonic implications

Recent studies indicate the Himalayan orogen has commonly experienced >30 Ma episodes of high temperature metamorphism and partial melting that resulted in the formation of migmatite and granites. Characterizing the geochemical nature of and mechanisms for these anatectic events is critical to understand geochemical and tectonophysical effects of magmatic processes in large collision belts. A series of similar two-mica granites occur in the core and to the south of the Yardoi gneiss dome. The Quedang pluton intruded into the Permian flysch formation. Similar to Yardoi and Dala two-mica granites, whole rock geochemical data of the Quedang granites show the following characteristics: (1) high SiO2 (68.2~69.3 wt%) and Al2O3 (>15.1 wt%), low TFeO(<2.0 wt%) and MgO(<1.5 wt%); (2) enriched in LREE, depleted in HREE, flat HREE patterns for elements from Ho to Yb with (Ho/Yb)N=1.2~1.4, and weak or no Eu anomalies with Eu/Eu?=0.9~1.0; (3) relatively low contents of Y(<8.1×10-6) and Yb (<0.7×10-6), high Sr/Y(>37.5) and La/Yb(>29.3) ratios. These data indicate that the Quedang granites were derived from partial melting of a source region dominated by amphibolite with subordinate metapelite.