喀喇昆仑地体甜水海地区102Ma辉长岩的发现及其对区域中生代构造演化的约束

Discovery of 102Ma gabbro in the Tianshuihai area of Karakoram terrane,and its constraints on regional Mesozoic tectonic evolution

喀喇昆仑地体是青藏高原的重要构造单元。近来,我们对其中的林济塘中生代盆地地层进行了调查,并在喀喇昆仑地体甜水海地区褶皱的中侏罗统龙山组灰岩中发现了辉长岩侵入体。辉长岩具有高Al2O3(15.07%~15.32%)与TiO2(1.31%~1.34%)、贫P2O5(0.19%~0.21%)和低碱(Na2O+K2O=3.49%~3.72%)的特征,明显富集轻稀土(La/Yb=6.17~7.39),富集Th、La、Nd与Tb,亏损Rb、Sr、P与Y,辉长岩Sr-Nd同位素表现出富集的特征(εNd(t)值分别为-5.8与-5.9)。LA-ICP-MS锆石U-Pb测年结果显示辉长岩岩浆锆石206Pb/238U年龄为102±1Ma(MSWD=0.92;n=14),指示岩体形成于早白垩世晚期。辉长岩中含有大量继承锆石,38颗继承锆石年龄分布在2450~232Ma之间,其中16颗为前寒武纪年龄锆石,意味着该地区可能存在前寒武纪基底。这些特征意味着辉长岩在形成的过程中经历壳源物质的强烈混染(Nb/U、Ta/U、Ce/Pb分别为26.6~29.1、1.69~1.76、9.72~11.3)。这种经历壳源物质的强烈混染岩浆的形成可能与软流圈上涌有关,即102Ma辉长岩形成时甜水海地区处于伸展背景。结合区域地质特征,指出102Ma时研究区进入新的伸展与海盆发育阶段,并建立了喀喇昆仑地体中生代三阶段构造演化模型,即侏罗纪-早白垩世早期的正常俯冲与弧盆演化、早白垩世的平板俯冲与造山阶段及晚白垩世的正常俯冲与弧盆演化。

The Karakoram terrane is an important unit of the Tibet Plateau.Recently,we carried on geological survey on the Linjitang Mesozoic basin,southeastern Karakoram,and found a gabbro intrusion in the Tianshuihai area.The gabbro has some distinct geochemical signatures,including:1)high Al2O3(15.07%~15.32%)and TiO2(1.31%~1.34%),low P2O5(0.19%~0.21%)and alkali(Na2O+K2O=3.49%~3.72%);2)enriched in LREE(La/Yb=6.17~7.39)and Th,La,Nd and Tb,and depleted in Rb,Sr,P and Y;and 3)enriched Sm-Nd isotope withεNd values between-5.80 and-5.90.The gabbro has LA-ICP-MS zircon 206Pb/238U age of 102±1Ma(MSWD=0.92;n=14),indicating that the pluton was emplaced in the late Early Cretaceous.The gabbro contains a great number of inherited zircons,38 of which have ages ranging from 2450 to 232Ma,16 of them are Precambrian zircons,suggesting existence of Precambrian basement.These geochemical evidence and occurrence of inherited zircons indicate that the gabbro with extensive crustal contamination in extensional setting related to asthenosphere upwelling,this means that the Tianshuihai area was in an extensional setting at 102Ma.Combining with regional geological evidence,it is suggested that tectonic framework of the Tianshuihai area was changed into extensional at an initial stage of the Late Cretaceous Linjitang basin,and proposed that the Karakoram terrane had gone through three stage tectonic evolution in Mesozoic period,i.e.the Jurassic-Early Cretaceous normal subduction and arc-basin system,plate subduction and orogeny at Early Cretaceous,and the Late Cretaceous normal subduction and arc-basin system.