天水细尾子沟镁铁—超镁铁质岩锆石LA-ICP-MS U-Pb年龄、岩石地球化学特征及其构造意义

LA-ICP- MS Zircon U-Pb Age and Petrogeochemical Characteristics of Xiweizigou Mafic—Ultramafic Rocks in Tianshui Area,and Its Tectonic Implications

秦祁交接部位天水新阳镇细尾子沟新发现一套镁铁—超镁铁质岩,超镁铁质岩主要由蛇纹石化橄辉岩和透辉石岩组成,岩石具有低Si O2含量、高Mg O含量、稀土元素和微量元素与原始地幔含量相当的特点。镁铁质岩主要由变辉长岩和变辉长闪长岩组成,岩石具有堆晶岩的特点,具有低Si O2和Ti O2含量、高Mg O含量和Mg#值,具有较低的Ti/V比值(20±)、较高的稀土元素丰度和HREE较平坦的稀土元素配分模式;镁铁质岩属于拉斑玄武岩系列,其岩浆源区是受地壳物质交代改造过的富集型岩石圈地幔,具有E-MORB的特征。综合构造环境和区域大地构造背景,认为细尾子沟镁铁—超镁铁质岩为较典型的岛弧SSZ型蛇绿岩。LA-ICP-MS锆石U-Pb测年结果表明细尾子沟岛弧SSZ型蛇绿岩形成于晚奥陶世晚期(443.6±1.8Ma)。细尾子沟岛弧SSZ型蛇绿岩的形成与"天水—武山"古洋盆自南向北的俯冲作用密切相关,表明秦祁交接部位早古生代构造格局是一个完整的洋—弧—盆体系。

The newly discovered Xiweizigou mafic—ultramafic rocks are cropped out in Xinyang town of Tianshui area at the conjunction of the Qinling—Qilian orogenic belts. The ultramafic rocks are composed of serpentinization olivine—pyroxenite and diopsidite,and characterized by low SiO2 and high MgO contents,and similar to that of primitive mantle in rare earth element and trace element. The mafic rocks are composed of metagabbro and meta-gabbrodiorite and have the characteristics of cumulates,and are characterized by low SiO2 and TiO2 contents,high MgO content and Mg^#,low Ti/V ratio（ 20 ±）,high abundances of rare earth elements and relatively flat in HREE. The mafic rocks belong to the tholeiitic series according to their geochemical characteristics. The abundance and variation of high field strength elements and ratios of associated elements suggest that the mafic rocks show characteristics of E-type MORB,and are derived from enriched lithosphere mantle which had been hybridized by the crust. Taking the tectonic setting and regional tectonic background into account,the geochemistry characteristics indicate that the Xiweizigou mafic—ultramafic rocks are relatively typical SSZ-type ophiolite related to arc—island setting. LA-ICP-MS zircon U-Pb dating yields a206 Pb /238 U weighted average age of 443. 6 ± 1. 8 Ma,indicating that the SSZ-type ophiolite should be formed in the Late Ordovician. The formation of the SSZ-type ophiolite is closely related to the northward subduction of Tianshui—Wushan Ocean,and suggest a tectonic framework with an integrated ocean—arc—basin system in the Early Paleozoic at the conjunction of the Qinling—Qilian orogenic belts.