苏鲁仰口地区辉长岩深俯冲过程中的矿物相转变及变质演化P.doc

苏鲁仰口地区辉长岩深俯冲过程中的矿物相转变及变质演化P 【中文摘要】青岛仰口地区发育完整的辉长岩向榴辉岩过渡转变的变质岩石组合，依次为变辉长岩→过渡型榴辉岩→含柯石英榴辉岩。岩相学的研究结果表明，变辉长岩保存了原岩的辉长结构，发现斜长石早期分解形成AbMsZoGrtQtz等，在残晶斜长石和单斜辉石接触部位发育大量细粒石榴子石和绿辉石冠状体。在过渡型榴辉岩中仍可识别出变余辉长结构，石榴子石和绿辉石也呈冠状体，但晶体颗粒变粗，呈现向典型榴辉岩转变的显微结构特征，典型的矿物组合为GrtOmpPheQtz等。含柯石英榴辉岩中已不保存原岩的岩浆结晶结构和矿物组合，岩石呈典型的粒状变晶结构，块状构造，矿物组合为GrtOmpPheRtCoe等，在含柯石英榴辉岩中部分石榴子石和绿辉石四周还见有少量细小角闪石和斜长石的后成合晶。上述特征表明，青岛仰口地区含柯石英榴辉岩的原岩为辉长岩，在扬子板块和华北板块碰撞过程中，辉长岩发生深俯冲，并经历了石英榴辉岩相进变质作用形成了过渡型榴辉岩；然后这些过渡型榴辉岩进一步深俯冲至地幔深度，经历了超高压变质作用形成了含柯石英榴辉岩，最后这些超高压岩石发生构造构造折返并经历了角闪岩相退变质作用的改造。成因矿物学的研究表明，变辉长岩和过渡型榴辉岩中的石榴子石、单斜辉石和多硅白云母等的化学成分与含柯石英榴辉岩中的同种矿物化学成分存在明显差异。1石榴子石变辉长岩和过渡型榴辉岩中石榴子石相对富含而贫MgO，相应的端元组分含量为Alm45-64％、Spes1-4％、Pyr4-14％、Gro26-44％;含柯石英榴辉岩中石榴子石的含量明显偏低而MgO含量明显偏高，相应的端元组分含量为Alm44-49％、Pyr17-30％、Gro23-35％、Spes1％。片麻岩中石榴子石以低MgO和高MnO为特点，且具有明显的扩散环带，端元组分含量变化于Alm40-49％、Pyr1-2％、Gro29-43％、Spes13-25％之间。由核部到边部，Gro含量逐渐减少，而Spes含量明显增加，指示了折返过程中降温降压的P-T演化过程。2单斜辉石变辉长岩中普通辉石残晶具有明显的成分发育环带，自核部到边部Na_2O和Al_2O_3含量明显增加，相应的硬玉Jd端元组分自核部的0．2％逐渐增加到边部的10．5％。与残晶普通辉石对比，呈冠状体的绿辉石的Na_2O和Al_2O_3含量进一步升高，相应的硬玉Jd端元组分达到了14-23％。含柯石英榴辉岩中的绿辉石的Na_2O和Al_2O_3含量最高，硬玉Jd端员组分端元组分含量高达42-65％。3多硅白云母硅含量从变辉长岩到榴辉岩明显增加，而片麻岩中的多硅白云母发育很好的环带，由核部到边部，Si和Mg含量逐渐降低，Al和Fe含量逐渐升高，Mg和Fe含量呈明显负相关性。4退变角闪石均为钙质角闪石，退变的长石均为钠长石。苏鲁仰口地区超高压岩石的变质演化可划分三个阶段深俯冲石英榴辉岩相进变质阶段、超高压峰期变质阶段和构造折返角闪岩相退变质阶段。早期深俯冲石英榴辉岩相的矿物组合GrtOmpPheQtzRt，记录的进变质温压条件为T540-630℃和P1．8-2．3 GPa；超高压变质阶段的共生矿物组合CoeGrtOmpRt，记录的超高压峰期阶段的温压条件为T660-794℃和P3．12-4．09 GPa；构造折返角闪岩相退变质阶段的矿物组合HblPlQtz，记录的退变质温压条件为T528-666℃和P0．80-1．10GPa。多种类型岩石不同性质锆石微区定年结果表明，苏鲁仰口地区古-新元古代巨量的陆壳物质在早三叠纪246-244 Ma发生深俯冲至65 km的深处，并伴随着石英榴辉岩相变质作用；接着这些高压石英榴辉岩相岩石继续深俯冲，在新三叠纪227-225 Ma俯冲到100-120 km的地幔深处，发生了柯石英榴辉岩相超高压变质作用。大约16Ma后，这些超高压岩石构造抬升到20-30km的中-下地壳深处，并叠加了角闪岩相退变质作用，相应的构造抬升速率为0．50-0．60 cm／y，表明苏鲁地体超高压岩石经历了一个相对快速折返的动力学过程。; 【Abstract】 A gradational sequence of main metamorphic rocks transation crop out at Yangkou Beach completely, which are metagabbro, transitional eclogite and coesite-bearing eclogite from the core to the rim. Metagabbro preserves primary gabbroic texture and primary plagioclase broke down to pesudomophy of Ab Ms Zo Grt Qtz and so on. Aboundent of fine-grained Grt and Omp coronas mainly develop along the boundaries between plagioclase and clinopyroxene. Transitional eclogite still preserve primary gabbroic texture, fine-grained Grt and Omp coronas are Coarser and microstructure characteristics are very similar with eclogite, the typical mineral assemblage are Grt Omp Phe Qtz. Coesite-bearing eclogite contains Grt Omp Phe Rt Coe and without gabbroic texture, whereas a few fine-grained symplectite of hornblende and plagioclase develop along the boundary or the rim of garnet and omphcite. Above shows the protolith of coesite-bearing eclogite is gabbro. Gabbro transed to transitional eclogite during the collision of Sino-krean plate and Yangtz plate, with the deeper subduction to mantle depth, the transitional eclogite transed to coesite-bearing eclogite. After that, all these UHP rock exhumed to low crust depth.Based on the mineral genetic analysis, the composition of garnet, clinopyroxene and phengite in the metagabbro or transitional eclogite are different with the coesite-bearing eclogite obviously. 1 Garnet The garnet in metagabbro is high in and low in MgO, and Alm 45-64, Spes 1-4, Pyr 4-14, Gro 26-44; While coesite-bearing eclogite is low in and high in MgO, and Alm 44-49, Pyr 17-30, Gro 23-35, Spes 1; The garnets in gnesis show the diffusion zoning and are characteristic with low MgO and high MnO, Alm 40-49, Pyr 1-2, Gro 29-43, Spes 13-25, from core to rim, Gro decreases and Spes increases, which indicates a exhumation with P and T both down. 2 Clinopyroxene The relic augites develop growth zoning in metagabbro, the content of Na_2O and Al_2O_3 increase from core to rim obviously, the Jd content increases from 0.2 to 10.5 accordingly. The coronas omphacite has higher content of Na_2O and Al_2O_3, and Jd content varies 14-23; While the omphacite in coesite-bearing eclogite has the highest content of Na_2O and Al_2O_3, and Jd varies 42-65. 3 Phengite The silicon content of phengile increases from metagabbro to coesite-bearing eclogite, while the phengite in the gneiss shows well diffusion zoning, from core to rim, Si and Mg content decrease while Al and Fe increase, respectively. Mg and Fe show negative linear relation. 4 Amphibole and plagioclase in retrogressive phase are Ca-Homblende and albite, respectively.The metamorphic evolution of UHP rocks in Sulu Yangkou area can be divided into 3 phases Quartz eclogite-facies progressive metamorphism, UHP coesite-bearing eclogite-facies metamorphism and amphibolite-facies retrogressive metamorphism. The mineral assemblage of Quartz eclogite facies progressive metamorphism are Grt Omp Phe Qtz Rt, yielded T 540℃- 630℃at P 1.8 - 2.3 Gpa; The mineral assemblage of UHP coesite-bearing eclogite-facies metamorphism are Grt Omp Phe Coe Rt, yielded T 660℃- 800℃at P 3.12 - 4.09 Gpa; The mineral assemblage of amphibolite facies retrogressive metamorphism are Hb1 P1 Qtz, yielded T 528℃- 666℃at P 0.8 - 1.1 Gpa;The Zircon U-Pb age dating from metagabbro, eclogite and ultramafic rocks yielded that the protolith were ed in paleoproterozoic - Neoproterozoic. The protolith was subducted to mantle depth about 65 km to Quartz eclogite-facies progressive metamorphism at early Triassic 246 - 244 Ma, and then subducted to deeper mantle depth about 100-120 km to UHP coesite-bearing eclogite-facies metamorphism at Late Triassic 227 - 225 Ma. About 16 Ma later, these rocks were exhumed to mid-low crust depth about 20-30 km and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from U-Pb age and metamorphic P-T conditions is about 0.5 - 0.6 cm/y, which indicates a rapid exhumation.