东昆仑万保沟群火山岩(Pt_2w)岩石地球化学特征及其构造背景

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF VOLCANIC ROCKS FROM THE WANBAOGOU GROUP IN EAST KUNLUN OROGENIC BELT

东昆仑万保沟群火山岩(Pt2w)为一套溢流相的基性火山岩夹少量的碳酸盐岩-硅质岩、泥质岩,形成时代为中元古代(Pt2),其玄武岩的锆石SHRIMP U-Pb谐和年龄为1343 Ma±30 Ma。火山岩的爆发指数Ep=0.18,岩石类型以变质玄武岩为主体,变质玄武质火山角砾岩和变质玄武质凝灰岩少量。万保沟群火山岩(Pt2w)在主量元素成分上表现为低SiO2(42.95%～52.68%,平均为47.52%)、低K2O(0.06%～1.62%,平均0.53%)和高Na2O(1.53%～4.23%,平均2.76%),火山岩中碱性系列与拉斑系列并存。稀土元素含量高(ΣREE=83.71×10-6～232.59×10-6),稀土元素配分模式呈轻稀土强烈富集的右倾斜型,其(La/Yb)N=1.92～10.72,无铈异常(Ce/Ce*=0.92～1.06),铕异常也不明显(Eu/Eu*=0.83～1.10)。普遍高U(0.42×10-6～4.2×10-6)、Th(0.93×10-6～7.0×10-6)、Nb(9.9×10-6～44×10-6)、Ta(0.83×10-6～2.7×10-6)、Zr(141×10-6～357×10-6)和Hf(4.6×10-6～9.6×10-6),微量元素比值蛛网图呈现为一个“大隆起”的分布型式。火山岩以典型的洋岛玄武岩为绝对主体,洋脊型玄武岩少量。Pt2w洋岛玄武岩与Pt2q碳酸盐岩共同构成了洋岛(或海山)的“双层型”结构,洋岛(或海山)的下部为Pt2w玄武岩,上部沉积的则为Pt2q的碳酸盐岩。火山岩的形成环境为东昆仑地区在中元古代时期的一有限洋盆。

Volcanic rocks from Wanbaogou group in east Kunlun orogenic belt are suite of basic volcanic rocks belonging to overflow facies, along with a little of carbonate rocks-sillcalite and pelite. They formed in Mid-Proterozoic period with zircon SHRIMP U-Pb age of 1 343 Ma ± 30 Ma. Volcano erupt index is 0.18. Metamorphic basalts are predominant in volcanic rocks,with minor metamorphic basaltic volcanic breccia and volcanic tuff. The major elements of volcanic rocks geochemistry are characterized by lower in SiO2 （42.95 % - 52.68 %, average 47. 52 %）, lower in K20 （0.06%-1.62% ,average 0.53%）,and highe in Na20 （1.53%-4.23% ,average 2.76%）. Rock series of volcanic rocks include alkaline and tholeiite series. REE contents of volcanic rocks are highe,and EREE ranges from 83.71 × 10^-6 to 232.59 × 10^-6. Chondrite-normalized REE patterns show oblique to the HREE side with intensive enrichment in LREE,and （La/Yb）N values ranges from 1.92 to 10.72. There is no anomaly in Ce and Eu,and Ce/Ce values ranges from 0.92 to 1.06,and Eu/Eu from 0.83 to 1.10,respectively. The contents of Th,U,Nb,Ta,Zr,and Hi are generally highe ranging from 0.42×10^-6 to 4.2×10^-6,0.93×10^-6 to 7.0×10^-6,9.9× 10^-6 to 44X10-6,0.83×10^-6 to 2.7×10^-6,141×10^-6 to 357×10^-6,and 4.6×10^-6 to 9.6×10^-6 ,respectively. MORB-normalized spidergrams of trace elements show “large upwarp” distribution type. Therefore,volcanic rocks from Wanbaogou group in east Kunlun orogenic belt mainly belong to oceanic island type basalt （OIB）, and there is also a little of mid-ocean ridge basalt （MORB）. These oceanic island type basalts （Pt2w） with carbonate rocks of Qingbanshisuzhan rock suite （Pt2q） are made up of “double-layer type” structure of oceanic island or seamount. The lower part of oceanic island or seamount consist of oceanic island type basalts （Pt2w） ,and the upper part of oceanic island type basalts （Pt2w） is carbonate rocks of Qingbanshisuzhan rock suite （Pt2q）. these volcanic rocks formed in the micro-ocean of Mid-Proterozoic in east Kunlun area.