中祁连西段石板墩堆晶岩的地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of Shibandun Mafic-ultramafic Cumulates in Western Part of the Central Qilian

石板墩堆晶岩位于中祁连地块西段党河断裂带北侧,主要由橄榄岩、蛇纹石化橄辉岩和辉长岩组成,具有多旋回、多韵律层的产出特征。辉长岩LA-ICP-MS锆石U-Pb年龄为(486.5±3.3)Ma。岩石地球化学结果显示,蛇纹石化橄辉岩和辉长岩配分型式十分相似,具有富集大离子亲石元素、亏损高场强元素、LREE相对富集、HREE平坦型分布以及正Eu异常(Eu/Eu*=0.97~3.04)的特点。研究结果表明,蛇纹石化橄辉岩、辉长岩为同源岩浆作用的产物,源区为被俯冲流体交代过的软流圈地幔,形成于火山弧环境,是在岩浆作用过程中不断发生堆晶作用,并在堆晶之后再次泵入混合大量新的玄武岩浆反复进行所形成。结合区域大地构造背景,认为中祁连西段是早古生代早期在残留的微陆块基础上形成的一个火山弧增生杂岩地体。

The Shibandun mafic-ultramafic cumulates are widely distributed in the western part of the Central Qilian,north of the Danghenanshan fault zone. The mafic-ultramafic cumulates comprise multiple sequences of（ from bottom to top） peridotite,olivine pyroxenite,gabbro,magnetite and diorite. LA-ICP-MS zircon U-Pb dating of the gabbro yielded a mean206 Pb/238 U age of（ 486. 5 ± 3. 3） Ma. Geochemical analysis indicates that serpentinized olivine pyroxenite has similar multi-element patterns to the gabbros,and they both show large ion lithophile element（ LILE） enrichments and high field strength element（ HFSE） depletions. They are also relatively enriched in LREE,and with flat HREE patterns and positive Eu anomalies（ 0. 97-3. 64）. The study shows that the Shibandun mafic-ultramafic cumulates were likely co-magmatic,and the magma was likely derived from the subduction fluid-metasomatized asthenospheric mantle. The magmatic evolution process may haveinvolved repeated cycles of magma fractionation,cumulate formation and basaltic magma replenishment. The Shibandunmafic-ultramafic cumulates were likely formed in volcanic arc setting. Integrated with the regional tectonic background,we propose that the western part of the Central Qilian microcontinent was likely a volcanic arc accretionary complex that was formed on the residual microcontinental margin.