西藏班公湖中特提斯洋盆的打开时间:镁铁质蛇绿岩地球化学与锆石U-Pb LAICPMS定年结果

Opening time of Bangong Lake Middle Tethys oceanic basin of the Tibet Plateau:Constraints from petro-geochemistry and zircon U-Pb LAICPMS dating of mafic ophiolites

对青藏高原西北部班公湖缝合带开展了路线剖面地质调查,在北面日土蛇绿岩带上采得2处镁铁质岩墙(辉长岩和辉绿岩),岩石地球化学分析表明这两处镁铁质岩墙具有平缓的球粒陨石标准化稀土元素分布型式,显示出洋中脊玄武岩特征,说明它们形成于洋中脊环境,其锆石U-PbLAICPMS年龄测定结果分别为(181.9±2.6)Ma(MSWD=11.5)和(184.4±4.4)Ma(MSWD=2.2),表明班公湖中特提斯洋是在早侏罗世晚期打开的。另外,在狮泉河—改则蛇绿岩带上采得橄榄辉绿岩,岩石以K2O(质量分数为1.49%～1.56%)、Al2O3(质量分数为14.76%～15.21%)含量高,Fe2O3/FeO比值高(1.73～2.49),富集大离子不相容元素(Rb、Ba、Th、Sr)和轻稀土元素(LREE)为特点,在基性岩构造环境判别图上显示出洋岛玄武岩的亲合性。该样品给出的(176.2±9.0)Ma(MSWD=3.1)的锆石U-PbLAICPMS年龄说明,班公湖中特提斯洋从早侏罗世晚期打开,经过近10Ma的扩张至中侏罗世早期已变成一个成热的洋盆。

Field geological investigation along two routes of Rutog-Shiquanhe and Duobuza-Gertse across the Bangong Lake suture was carried out.Among the samples collected,two mafic dykes of gabbro and diabase from the Rutog ophiolite belt display flatten chondrite-normalized rare earth element patterns,typical of oceanic-ridge basalt.Their zircon U-Pb ages of 181.9±2.6 Ma （MSWD=11.5） and 184.4±4.4 Ma （MSWD=2.2） dated by LAICPMS technique indicate that the Bangong Lake middle Tethys oceanic basin was opened during the Late Early-Jurassic Epoch.Meanwhile,olivine diabase samples collected from the Shiquianhe-Gertse ophiolite belt have high K2O （1.49%-1.56%） and Al2O3 （14.76%-15.21%） contents with high Fe2O3/FeO ratios （1.73-2.49）,and are rich in large ion lithophile elements （Rb,Ba,Th,Sr） and light rare earth elements （LREE）.They display oceanic island basalt （OIB） affinity in tectonic discrimination diagrams for mafic magmatic rock.Formation age of 176.2±9.0 Ma （MSWD=3.1） yielded by zircon U-Pb LAICPMS technique shows that the Middle Tethys oceanic basin opened during the Late Early-Jurassic Epoch had become a relatively matured oceanic basin after an opening period of about 10 Ma.