Field-Based Analysis of Subduction Petrofabrics

该项目由明尼苏达大学的一组研究人员进行，确定了一个发掘出来的俯冲复合体的高压岩石组构，其特征是在变质沉积岩（大理石、石英岩）和变质玄武岩（榴辉岩、蓝片岩）中保存了普遍的、同步的高压组合。在土耳其Sivrihisar地块进行野外测绘，结合微结构、岩石学和稳定同位素分析，确定了俯冲带（40-75 km）深度范围内的大规模流动、变形机制、流动规律和流体-岩石相互作用。显微结构技术包括电子背散射衍射（EBSD），用于确定榴辉岩中的斜辉石、大理石中的（纤维）方解石和石英岩中的石英的流变控制相的晶体取向；场和岩石学的运动学分析，包括利用lawsonite的形状和方向来确定运动涡度；以及剪切感的测定。岩石学研究包括矿物组成和分带的显微分析，以及对地块内不同现场高压组合保存程度的影响因素的评价。通过氧同位素分析，包括矿物-全岩分析和原位离子探针分析，对蓝片岩-榴辉岩过渡带中分带颗粒的成分和结构进行了分析，评估了流体-岩石相互作用的程度、条件和途径。俯冲地壳的变形行为强烈影响汇聚板块边界的地球物理和地球化学演化，包括俯冲过程中变形、变质作用、流体-岩石反应的相互作用和深埋岩石返回地球表面，以及俯冲过程的速率。本文通过对高压矿物从地图尺度到颗粒尺度的变形特征进行表征，解决了俯冲带高压岩石大尺度输运机制问题。Sivrihisar地块包含世界上保存最完好和最广泛的榴辉岩之一，这种岩石类型预计在俯冲带的深处很常见，但很少暴露在地表，因为在挖掘过程中，榴辉岩很容易被破坏。这种重要的岩石类型，可能是45-300 km俯冲洋壳的主要岩性，以前没有对其进行过微结构研究。

This project, carried out by a team of researchers from the University of Minnesota, determines the high-pressure petrofabrics of an exhumed subduction complex characterized by pervasive, syn-kinematic high-pressure assemblages preserved in metasedimentary (marble, quartzite) and metabasaltic (eclogite, blueschist) rocks. Field mapping in the Sivrihisar Massif, Turkey, coupled with microstructural, petrologic, and stable isotope analyses are used to determine large-scale flow, deformation mechanisms, flow laws, and fluid-rock interaction at a range of depths in a subduction zone (40-75 km). Microstructural techniques include electron back-scattered diffraction (EBSD) to determine crystallographic orientation of the rheology-controlling phases clinopyroxene in eclogite, (fibrous) calcite in marble, and quartz in quartzite; field and petrographic analysis of kinematics, including determination of kinematic vorticity using lawsonite shape and orientation; and determination of sense of shear. Petrologic investigation includes microprobe analysis of mineral compositions and zoning, and evaluation of factors influencing the degree of preservation of high-pressure assemblages at different field sites in the massif. The extent, conditions, and pathways of fluid-rock interaction is assessed via oxygen isotope analyses, including mineral-whole rock analyses and in situ ion probe analysis of compositionally and texturally zoned grains from blueschist-eclogite transition zones.The deformation behavior of subducted crust strongly influences the geophysical and geochemical evolution of convergent plate boundaries, including the interaction of deformation, metamorphism, and fluid-rock reaction during subduction and return of deeply buried rocks to the Earth's surface, and the rates of subduction processes. This research addresses questions of the mechanisms of large-magnitude transport of high-pressure rocks in subduction zones by characterizing deformation characteristics from the map scale to the grain-scale using high-pressure minerals. The Sivrihisar Massif contains the one of the world's best-preserved and most extensive outcropping of lawsonite eclogite, a rock type predicted to be common at depth in subduction zones, but rarely exposed at the surface owing to the ease with which lawsonite is destroyed during exhumation. There have been no previous microstructural studies of this important rock type, which may be the dominant lithology in subducted oceanic crust between 45-300 km.