柴北缘大陆深俯冲板片折返过程中的深熔作用研究

Partial melting of subducted continental plate during exhumation from the North Qaidam UHP terrane,western China

柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据：（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英＋钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（string of beads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的206Pb/238U年龄约为～910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的206Pb/238U年龄结果约为～450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其206Pb/238U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。

Thin layer and veinlet of granitic leucosomes are widely distributed within felsic gneiss in Xitieshan area of the North Qaidam ultrahigh-pressure （UHP） metamorphic terrane, western China. Anatexis of the felsic gneiss is evidenced by petrological study in field and thin section： （ 1 ） mineral assemblage of Kfs ＋ Qz ＋ P1 within garnet; （2） cuspate wedge-shaped pockets consisting of Kfs ＋ Qz ＋ P1 ± Mus along boundary of plagioclases; （3） microcline recognized along grain boundary of irregular muscovite; （4） string of beads along boundary of quartz, plagioclase or K-feldspar. A combined study of cathoduluminescence （CL） images and U-Pb dating results indicate that the zircons from granitic leucosome show obvious core, mantle and rim with distinct ages. High-luminescent inherited magmatic cores record a Neoproterozoic age of approximately 910Ma, with high Th/U ratios. The low-luminescent unzoning metamorphic mantle record eclogite-facies metamorphic age of approximately 450Ma. The overgrowth rims are similar to typical anatectic zircon in terms of pronounced oscillatory zoning and low Th/U ratios, which gave a U-Pb age of 432 ± 3Ma. A combined study of field observation, microscopic textures and zircon U-Pb dating indicates that the granitic leucosome could be drived from parital melting of the host felsic gneiss at HP granulite-facies conditions, probably triggered by breakdown of muscovite. These data together with previous research imply that the deeply subducted continental crust could have experienced multiply anatexis during subduction/ collision and exhumation.