Carboniferous magmatic records in the eastern Gangdese batholith, southern Tibet

The Gangdese batholith consists dominantly of magmatic rocks with ages from Mesozoic to Cenozoic (ca. 230 similar to 10Ma), however, increasing number of studies have documented that magmatic suites of Late Paleozoic age could provide critical constraints on the tectonic processes in the Lhasa terrane prior to the subduction of Neo-Tethyan oceanic lithosphere. Zircon U-Pb dating yields that the protoliths of the granitic gneisses from Jiacha and Lang formed at similar to 344. 8Ma and ca. 362. 0 similar to 344. 0Ma, respectively, which bracket the range of timing reported in the literature. Combined those literature data, new geochemical data show that these Carboniferous granites are characterized by high SiO2 (67. 46% similar to 75. 33%), but low Al2O3 (12. 66% similar to 15. 82%), CaO (0. 79% similar to 4. 32%), FeO (0.48% similar to 3. 00%) and MgO (0. 28% similar to 1. 64%). Based on their K2O/Na2O ratios, they can be divided into two series : potassium-rich and sodium-rich, respectively. These rocks are characterized by (1) enrichment in light rare earth elements (LREE); (2) substantial negative Eu anomalies; and (3) depletion in Nb, Ta, and Ti, but no Zr and Hf negative anomalies. Geochemical and geochronological data suggest that: (1) the Carboniferous granites in the eastern Gangdese were formed in a back-arc extension setting, possibly related to the subduction of the Paleo-Tethys Ocean to the northern margin of Gondwana continent; (2) sodium-rich granites represent the differentiation products of the mafic parental magmas, whereas the potassium-rich ones from mixing of mantle-derived magma with middle-lower crustal; (3) this phase of magmatism lasted at least similar to 30Myr.