青藏高原拉萨地块西部雄巴盆地后碰撞钾质-超钾质火山岩年代学与地球化学

The petrogenesis of postcollisional potassic-ultrapotassic rocks in Xungba basin, western Lhasa terrane: Constraints from zircon U-Pb geochronology and geochemistry

青藏高原拉萨地块后碰撞钾质和超钾质岩浆活动广泛分布且已有不少研究成果,但是它们的年龄主要是17-8Ma,而对于拉萨地块西部雄巴地区时代为24-23Ma的岩浆作用则研究较少。本文对雄巴盆地新识别出的三种类型火山岩的锆石LA-ICP-MSU-Pb定年和岩石地球化学研究表明,它们分别是超钾质安粗岩（23·9±0·6Ma）、粗面英安岩（23·3±0·4Ma）和钾质流纹岩（24·1±0·3Ma）,这三种岩石近于同时产出。三类岩石的源区明显不同,其中钾质流纹岩是中、上地壳部分熔融产物;具有埃达克质特征的钾质粗面英安岩可能为加厚下地壳部分熔融;而超钾质安粗岩可能富集地幔部分熔融的产物。雄巴三种火山岩均含数量不等的继承锆石,钾质流纹岩具有-150Ma、-90Ma和-50Ma的继承锆石年龄群;钾质粗面英安岩突出显示了-90Ma左右岩浆活动记录和两个新元古代继承锆石年龄;幔源超钾质安粗岩的继承锆石则绝大多数继承锆石为晚白垩纪以来的岩浆活动记录,突出显示110-80Ma和62-30Ma两个峰值。

The wide-spreading postcollisional potassic and ultrapotassic rocks, with ages ranging from 17 to 8Ma, in Lhasa terrane, Tibetan Plateau have been well studied in the past ten years. But for the 24-23Ma magmatism in Xungba area in western Lhasa terrane are rarely studied. This paper present a series of zircon LA-ICP-MS U-Pb dating and element geochemical studies on the new-recognized potassic and ultrapotassic rocks from Xungba basin. The volcanic rocks are named as latite （23.9±0.6Ma）, trachydacite （23.3±0.4Ma） and rhyolite （24.1±0.3Ma）. The geochemical data of these rocks show that different magma sources should have been responsible for their petrogenesis. The rhyolite is originated from the partial melting of upper-midde crust materials, the adakitic trachydacite possiblely represents partial melting of the thickened lower crust, but the ultrapotassic latite is the product of an enriched upper mantle lower-degree melting. The inherited zircons age populations in the rocks exhibit different patterns. The crustal-derived rhyolite show three groups （-150Ma, -90Ma and -50Ma）, the potassic trachydacite have the major populations of -90Ma and 2 zircon grains with Neoproterozoic ages representing the basement beneath cental Lhasa subterrane. The inherited zircons of the mantle-derived latite mainly record the ages of late Cretaceous （110-80Ma） and Paleogene （62-30Ma） peaks.