冈底斯带东段鲁朗—墨脱地区中新世花岗岩 的地球化学、年代学及成因

冈底斯东段的鲁朗-色季拉和墨脱-崩崩拉一带花岗岩的岩石类型主要为二长花岗岩、黑云母花岗岩、花岗闪长岩、石英闪长岩等.墨脱花岗岩的K-Ar年龄为19～22Ma;鲁朗花岗岩的40Ar-39Ar年龄为14～18Ma.岩石地球化学研究结果表明,本区花岗岩主要属于高钾钙碱性系列和钙碱性系列,同时具有某些埃达克岩的特征,表现为高SiO2(65.60%～76.40%)、Al2O3(12.32%～17.23%)、Sr/Y(2.41～86.46)、(La/Yb)n(6.65～56.14)比值,低Y(4.23×10-6～39.40×10-6)等特点.呈典型的LREE和LILE富集型分配模式,Eu为正到弱负异常.本区中新世花岗岩主要来源于中下陆壳的硅铝质成分和镁铁质成分的重熔,不同于具埃达克岩成分的冈底斯中新世含矿花岗斑岩.以中新世花岗岩侵位为标志,东喜马拉雅构造结地区的初始伸展可能在22Ma左右,早于冈底斯中段(20Ma左右).

Geochemistry, geochronology and petrogenesis of Miocene granites in the Lunang-Medog 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.