秦祁结合部位天水地区磨石沟变基性火山岩锆石U-Pb年代学、地球化学及其构造意义

Moshigou Meta-Basalt in the Tianshui Area in the Conjunction of the Qinling-Qilian Orogens:Zircon U-Pb Geochronology,Geochemistry and Tectonic Implications

秦祁结合部位天水地区新阳镇磨石沟新发现一套绿片岩相基性火山岩系,原岩为玄武岩,伴随有少量碳硅质岩和辉绿岩脉产出。磨石沟变基性火山岩SiO2含量为47.32%～50.18%,MgO含量为4.71%～7.33%,Al2O3含量(12.33%～14.97%)较低,TiO2含量为0.98%～1.98%(平均为1.44%),与洋脊玄武岩的Al2O3、TiO2含量相近,类似于E-MORB。∑REE为46.52×10-6～113.21×10-6,LREE/HREE为2.00～4.01,δEu为0.86～1.09(平均为0.98),(La/Yb)N为1.18～3.59。磨石沟变基性火山岩属于拉斑玄武岩系列,高场强元素及相关比值表明具有E-MORB特征,其岩浆源区为弱富集—富集地幔,岩石成分主要受岩浆熔融区控制,玄武质岩浆上升过程中遭受到不同程度的地壳物质或俯冲组分的混入,但地壳和俯冲带物质对岩浆源区和成岩过程的贡献较少。结合其地球化学性状与构造环境判别图解判断,磨石沟变基性火山形成于弧后盆地后期演化阶段。LA-ICP-MS锆石U-Pb测年结果表明其形成时代为中志留世Wenlock期(432.6±3.8Ma)。磨石沟变基性火山岩代表的弧后盆地的形成与中志留世"天水-武山"古洋盆向北的俯冲作用密切相关,说明秦祁结合部位早古生代构造格局是由一个较完整的洋-弧-盆体系组成。

The newly discovered Moshigou lower greenschist facies metamorphism basalts crop out at Xinyang town of Tianshui area,the conjuction of the Qinling-Qilian orogens.Some bedded siliceous rocks and veined diabases also expose together with the meta-basalts.SiO3 and MgO contents of the meta-basalts vary from 47.32% to 50.18% and from 4.71% to 7.33%,respectively.The low Al3O3 content（12.33%～14.97%）and high TiO3 content（0.98%～1.98%,with an average of 1.44%）are close to those of MORB,and similar to that of E-MORB.The ΣREE of the Moshigou meta-basalts are 46.52 × 10-6 ～113.21 × 10-6.LREE/HREE range from 2.00 to 4.01,δEu range from 0.86 to 1.09 and （La/Yb）N range from 1.18 to 3.59.The Moshigou meta-basalts belong to the tholeiitic series according to their geochemical charateristics.The abundance and variation of high field strength elements and ratios of associated elements suggest that Moshigou meta-basalts are E-type MORB which is derived from weakly to strongly enriched mantle.The composition of the meta-basalts are mainly controlled by magma source,but hybridized by crust or subduction materials in a small degree during the upwelling of basaltic magma.The geochemistry characteristics indicate that the generation of Moshigou meta-basalt results from the late stage of the backarc basin.LA-ICP-MS zircon U-Pb dating yields a 306Pb/238 U weighted average age of 432.6 ＋ 3.8 Ma,indicating that the meta-basalt should be formed in the Wenlock of the Middle Silurian.The formation of the back-arc basin represented by Moshigou meta-basalt is closely related to the northward subduction of Tianshui-Wushan Ocean in the Middle Silurian,which suggest a tectonic framework with the relatively integrated ocean-arc-basin system in the Early Paleozoic at the conjunction of the Qinling-Qilian orogens.