中祁连西段玄武玢岩地球化学特征及其构造意义

Geochemical characteristics and tectonic implications of basaltic porphyry in the western segment of the central Qilian Mountains

中祁连西段玄武玢岩呈浅层侵入于青白口系含磁铁矿的凝灰质板岩和早古生代蛇绿岩套中的玄武岩,后者辉长岩LA-ICP-MS锆石U-Pb年龄为471.6±3.7、469.3±2.8 Ma,结合蛇绿岩构造侵位时代及区域地质背景,推测玄武玢岩应形成于早古生代晚期.玄武玢岩w（SiO2）=48.85%～53.22%,低Al,高Ti、TFe2O3,w（MgO）=4.80%～9.10%,w（Na2O）/w（K2O）〉3.8,为富钠贫钾,富集Nb、Ta、Th、U、LREE,亏损Rb、Ba、Sr、K、P,而Mg、Cr、Ni质量分数变化大.轻稀土略微富集,LREE/HREE为1.12～1.24,（w（La）/w（Yb））N=2.19～2.87,轻微负Eu异常,δδEu=0.69～0.87.研究表明玄武玢岩兼有火山弧和板内岩浆作用的特征,应源于初始的埃达克岩熔体交代的地幔发生部分熔融形成的玄武岩浆,并经历了早期橄榄石的分离结晶作用,形成于北祁连古大洋向南俯冲闭合-碰撞之后的陆缘弧后裂谷环境.

Basaltic porphyry in the western segment of the central Qilian Mountains shallowly intruded into the Qingbaikou system＇s tuffaceous slates containing magnetite and basalts in early Paleozoic ophiolite complex, of which gabbro LA-ICP-MS zircon U-Pb age is 471.6±3.7 Ma and 469.3：t：2.8 Ma. The formation age of basaltic porphyry is presumed to be the late early Paleozoic combined with ophiolite tectonic emplacement and regional geological background. Basaltic porphyry w（SiO2）=48.85% -53.22%, low Al, high Ti and TFe2O3, w（MgO）=4.80%-9.10%, w（Na2O）/w（K2O）〉3.8, sodium-rich and potassium-poor, enriched in Nb, Ta, Th, U, LREE and depleted in Rb, Ba, Sr, K and P. Meanwhile Mg, Cr and Ni concentrations vary greatly. LREE are slightly enriched, LREE/HREE is 1.12-1.24, （w（La）/w（Yb））N is 2.19- 2.87, and δEu is 0.69- 0.87, which shows the characteristic of a slightly negative Eu anomaly. An integrated study indicated that the basaltic porphyry, formed in continental marginal back-arc rift environment after the Northern Qilian Palaeo-Ocean＇ s southward subducted closed-collision, is featured with both volcanic arc and intraplate magmatism. Basaltic porphyry is considered to be derived from basaltic magma originating from partial melting of the mantle metasomatized by primary adakitic melts, and has experienced fractional crystallization of olivine.