Geochemistry, geochronology and petrogenesis of Miocene granites in the Lunang-Mêdog area in the eastern segment of the Gangdise belt, Tibet， China

The major types of Miocene granites in Lunang-Sejila and Mêdog-Bungbung La in the eastern segment of the Gangdise Mountains are monzogranite, biotite granite, granodiorite and quartz diorite. The K-Ar ages of the Mêdog granite are 19-22 Ma and the 40Ar-39Ar ages of the Lunang granite are 14-18 Ma. Geochemical study suggests that the granites in the study area mainly belong to the calc-alkaline and high-K calc-alkaline series and show adakitic affinity, i.e., they have high contents of SiO2 (65.60%-76.40%) and Al2O3 (12.32%-17.23%), high ratios of Sr/Y (2.41-86.46) and (La/Yb)n (6.65-56.14) and a low Y content (4.23×10-6-39.40×10-6). They also display LREE- and LILE-enriched patterns with positive to weak negative Eu anomalies. Miocene granites in this area were mainly derived from partial melting of the mid-lower continental sialic and mafic crust, which is different from Miocene mineralized granite porphyry of adakitic affinity in Gangdise. The initial crustal extension in the syntax area in the eastern Himalayas may have occurred at ~22 Ma, earlier than that in the central segment of the Gangdise (~20 Ma), as indicated by Miocene granite emplacement.