喀喇昆仑河尾滩地区龙山组火山岩年代、地球化学特征及其构造环境

FORMATION AGE,GEOCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF THE BASALTS FROM LONGSHAN FORMATION IN HEWEITAN AREA,KARAKORUM

喀喇昆仑地区中侏罗统龙山组灰岩中夹层状基性火山岩,岩性主要为块状玄武岩、杏仁状玄武岩和枕状玄武岩。通过LA-ICP-MS锆石U-Pb定年获得加权平均年龄为174.4Ma±2.7Ma,属中侏罗世早期,该年龄与龙山组灰岩中获取的化石时代一致。玄武岩具有较低的w(SiO2)(42.54%~46.4%)和w(TiO2)(0.8%~1.07%),较高的w(Al2O3)(13.11~16.86%),w(CaO)(8.66~13.4%)及较高的Mg#(52.6~66)。岩石稀土元素总量较低(ΣREE=38.4×10-6~49.7×10-6),轻、重稀土元素相对分馏不明显(LREE/HREE=1.85~2.34),轻稀土元素略富集,表现出弱的正铕异常(δEu=1.29~1.7),整体具有与富集型洋脊玄武岩(E-MORB)相似的特征。微量元素蛛网图中岩石相对富集大离子亲石元素(Rb,Ba,K,Sr)及Pb正异常明显,相对亏损高场强元素(Th,U,Nb,Ti)。微量元素地球化学特征表明,玄武岩源于富集地幔,为尖晶石二辉橄榄岩部分熔融的产物,且上升过程中受一定程度的地壳混染。综合野外地质特征和地球化学特征认为,龙山组玄武岩形成于具有陆壳基底的初始洋盆环境。

The Longshan Formation in Karakoram area is mainly composed of limestone,with some interbedded massive,amygdaloidal and pillow basalts.LA-ICP-MS zircon U-Pb dating of the basalt yielded an age of 174.4Ma±2.7Ma(middle Jurassic period),which coincides with the fossil age obtained from the limestone of Longshan Formation.Geochemical analysis indicates that the basalts are low in w(SiO2)(42.54%~46.4%)and w(TiO2)(0.8%~1.07%),high in w(Al2O3)(13.11%~16.86%),CaO(8.66%~13.4%)and Mg#.REE (ΣREE=38.4×10^-6~ 49.7×10^-6)is low and fractionation of light and heavy rare earth elements is not obvious (LREE/HREE=1.85~2.34).Light rare earth elements are slightly enriched with mild Eu anomalies(δEu=1.29~1.7),and with characteristics similar to E-MORB type.Those rocks with Pb positive anomaly are enriched in LILE and depleted in HFSE.Trace elements analysis indicates that the basalts are originated from the enriched mantle and resulted from partial melting of spinel lherzolite,with significant crustal contamination during ascending process.Based on the field geological features and geochemical characteristics,it is considered that the basalts of Longshan Formation were formed in the initial oceanic basin environment with continental a crust basement.