西藏青龙乡地区早白垩世侵入岩年代学和地球化学:对班公湖-怒江洋盆闭合时限的制约

Geochronology and Geochemistry of the Early Cretaceous Intrusive Rocks in Qinglong Area,Tibet:Constraints on Closure Time of Bangonghu-Nujiang Ocean Basin

以西藏冈底斯带北缘青龙乡出露的侵入岩为研究对象,进行了岩石学、锆石U-Pb测年和岩石地球化学分析,据此对其岩石成因和大地构造意义进行探讨。侵入岩岩性主要为二云母花岗岩和黑云母石英二长岩。LA-ICP-MS锆石U-Pb测年结果显示二云母花岗岩年龄为123.4±2.2 Ma,属早白垩世中期;黑云母石英二长岩年龄为112.2±1.5 Ma,属早白垩世晚期。岩石地球化学特征显示,二云母花岗岩为过铝质钙碱性系列岩石,明显富集大离子亲石元素Rb、Th、K、Pb,亏损高场强元素Nb、Ta、Ti,为岛弧花岗岩;黑云母石英二长岩为准铝质钾玄系列岩石,明显富集大离子亲石元素及高场强元素Rb、K、Th、Ce、Zr、Hf,而Ba、Nb、Sr、P、Ti亏损较明显,为同碰撞花岗岩。两者球粒陨石标准化稀土元素配分模式均表现为轻稀土富集的右倾型,反映了源区地壳成熟度较高,二云母花岗岩总体具极不明显的负Eu异常,黑云母石英二长岩则具明显的负Eu异常。综合地球化学特征及区域构造演化,得出该地区出露的二云母花岗岩形成于班公湖-怒江洋洋壳俯冲于拉萨地体之下的消减环境,是下地壳基性物质部分熔融产物,同时有上地壳沉积物的参与;黑云母石英二长岩是拉萨地块与羌塘地块在碰撞期间,受到挤压增厚的上部地壳(变)沉积岩发生部分熔融的产物。结合年代学研究,将班公湖-怒江洋盆闭合时限约束在123~112 Ma之间,为早白垩世晚期。

This paper reports the petrology,zircon U-Pb age and geochemical results of the intrusive rocks exposed in the Qinglong area located in the northern margin of Gangdise belt,Tibet,and discusses their petrogenesis and tectonic significance.The lithology of the intrusive rocks includes two mica granite and biotite quartz monzonite.LA-ICP-MS zircon U-Pb dating showed that the two mica granite and biotite quartz monzonite were crystallized at 123.4±2.2 Ma and 112.2±1.5 Ma,respectively,and accordingly belong to the Middle of Early Cretaceous and the late Early Cretaceous.The rocks are characterized by high silicon,high aluminum,low calcium,low magnesium,low iron and low titanium contents.The two mica granite is peraluminous calc-alkaline,while the biotite quartz monzonite is quasi-aluminous shoshonite.The two mica granite is obviously enriched in LILE(Rb,Th,U,K,Pb),and depleted in HFSE(Nb,Ta,Ti),similar to those of the island arc granites.The biotite quartz monzonite is enriched in LILE and HFSE such as Rb,K,Th,Ce,Zr,Hf and obviously depleted in Ba Nb,Sr,P,Ti,similar to those of collisional granites.The both of REE diagrams show rightward incline and the HREE were enriched,which reflects the characteristics of the crust in the source area.In general,the two mica granite has negligible negative Eu anomaly,while the biotite quartz monzonite has obvious negative Eu anomaly.It is concluded that the two mica granite was formed under subduction environment,resulting from partially melting of the mafic lower crust with involvement of the upper crustal sediments.The biotite quartz monzonite was formed in a compressive environment due to collision between the Qiangtang subterrane and the Lhasa subterrane,and was the product of partial melting of upper crust(metamorphic)sedimentary rocks.According to the geochemical characteristics and formation ages of the intrusive rocks in the Qinglong area,the Bangonghu-Nujiang oceanic basin was closed around 123 to 112 Ma,which is the late Early Cretaceous.