班-怒结合带西段革吉地区帕阿岩基的成因:元素地球化学、锆石U-Pb年代学及Hf同位素约束

Petrogenesis of the Pa'a batholiths in the Geji area,western segment of the Bangong Co-Nujiang River suture zone: Constraints by geochemistry,zircon U-Pb geochronology and Hf isotope

班公湖-怒江结合带是藏北一条重要的特提斯主洋残余,在洋盆消减过程中发生了强烈岩浆作用,在其南缘形成了沿狮泉河-永珠缝合带两侧广泛分布的白垩纪大规模岩浆岩和火山岩,为了解班公湖-怒江特提斯洋俯冲和闭合过程提供了良好窗口。本文对出露于革吉地区帕阿花岗闪长岩岩基进行了详细的野外地质填图、全岩元素地球化学、锆石U-Pb年代学和Hf同位素组成研究。研究结果表明,花岗闪长岩和其闪长质包体的锆石206Pb/238U年龄分别为155. 7±1. 0Ma(谐和年龄,MSWD=0. 26,N=18)和156. 5±2. 3Ma(年龄加权平均值,MSWD=2. 6,N=16),岩浆侵位于晚侏罗世,不是以往所认为的白垩纪岩浆产物。花岗闪长岩主要属于高钾钙碱性系列,高铝饱和指数(A/CNK=0. 91～1. 04),显示了过铝质特征。花岗闪长岩富集轻稀土元素(LREE/HREE)=5. 51～8. 54,(La/Yb)N=6. 27～20. 2)和大离子亲石元素,亏损Nb、Ta、Sr、Ti等高场强元素,具有中等至弱Eu负异常(δEu=0. 61～0. 85)。锆石Hf同位素分布集中,花岗闪长岩和闪长质包体的锆石εHf(t)均为负值,分别为(-9. 4～-6. 3)和(-10. 9～-8. 6),Hf地壳模式年龄分别为(1. 32～1. 48Ga)和(1. 44～1. 56Ga),指示了帕阿花岗闪长岩岩基主体系古老地壳基底部分熔融产物,存在少量地幔物质的加入,形成于弧环境,可能属于班公湖-怒江洋盆南向俯冲到拉萨地块古老结晶基底重熔形成的一套具弧属性的未分异I型花岗岩。

The Bangong Co-Nujiang River suture zone, locating in the boundary zone of South Qiangtang Terrane and Lhasa Terrane, has widely been accepted as a vital remnant of the main Meso-Tethys. Abundant magmatic rocks were induced by the oceanic subduction and closure. A large-scale Cretaceous material-thermal activity product widely distributed along the southern margin of the Shiquanhe-Yongzhu suture zone, which provided us good opportunity for understanding the tectonic evolution of the Bangong Co-Nujiang River Tethys ocean. In this paper, the whole rock element geochemistry, zircon U-Pb chronology and Hf isotopic composition were studied in the Pa'a batholiths that were exposed in the north region of Geji County in the western part of the Bangong Co-Nujiang River suture zone. The zircon 206Pb/238U age of the experimental samples granodiorite and its dirorite inclusions were 155.7±1.0Ma (concordia age, MSWD=0.26, N=18) and 156.5±2.3Ma (weighted average age, MSWD=2.6, N=16) respectively, reflecting the magma emplacement occurred in the Late Jurassic, which was not the Cretaceous magma products previously thought. Granodiorites belong to the high-K calc-alkaline series with aluminum feature. Samples of granodiorite were enriched in light rare earth elements (LREE/HREE=5.51~8.54,(La/Yb)N=6.27~20.2), large ion lithophile and depleted Nb, Ta, Sr, Ti and other high field strength elements, and a weak Eu negative anomaly (δEu=0.61~0.85). The Hf isotopes of zircons from granodiorite and diorite inclusion are concentrated, the values of εHf(t) are both negative (-9.4^-6.3 and -10.9^-8.6), Hf crustal age are older (1.32~1.48Ga and 1.44~1.56Ga), which indicated the most of the Pa'a batholiths were the product of partial melting of the ancient crust basement with a small amount of mantle material added. Combined with the chronological dating results in the study area and regional tectonic evolution, the Pa'a batholith was formed in the Late Jurassic. The Pa'a batholith probably belong to undifferentiated I-type granites with arc feature and formed by the south-direction subduction of the Bangong Co-Nujiang River oceanic crust to the ancient crystalline basement of the Lhasa block.