Pre-Middle Jurassic Accretion-Related Metamorphism in the South-Central Mountains, Northern California

本研究的目的是探讨加州北部克拉马斯山脉中南部增生地体中前中侏罗世与会聚有关的变质作用的发生、性质和时间。组成西部古生代和三叠纪带(WPzTr)的许多细长的构造变质吸积地体(WPzTr)包含大量与区域缩短和岩浆加热有关的低P/T变质作用的证据，这些变质作用发生在北美西部边缘真正广泛的大陆边缘岩浆弧的中晚侏罗世演化期间。WpzTr地体之一的Stuart Fork也包含了与沿收敛边缘俯冲有关的早期中侏罗世前变质作用的证据，保存为独特的高P/T蓝片岩变质组合，部分被较年轻的深成晕所掩盖。WPzTr南部的其他地体含有记录较少的低级变质矿物组合，这些矿物组合不易归因于特定的变质条件、地温梯度或构造环境。这项研究的目的是对这些南部WPzTr地体中的前中侏罗世变质和变形进行详细的岩石学和热年代学研究。综合野外、岩石学和地质年代学资料，将有助于约束这些地体在中侏罗世组装之前的成因、演化和构造环境。这些数据还将被用来模拟它们对大陆边缘的吸积，并解决这样一个悬而未决的问题：科迪勒拉中部早中生代边缘是作为一个单独的历时俯冲边界演化，还是作为一个平行的、可能是同时代的俯冲带系统演化。这项综合研究将采用标准光学显微镜、详细的电子探针分析(包括背散射电子成像)和X射线衍射分析。这些方法特别适合低级变质岩的研究。此外，40Ar/39Ar年龄测定将与普林斯顿大学的保罗·R·雷恩博士合作，提供年代学和热历史限制。我们计划在两年内进行这项调查，包括两个夏季田野季节。

The purpose of this study is to investigate the occurrence, nature and timing of pre-Middle Jurassic convergence-related metamorphism in accreted terranes of the south-central Klamath Mountains of northern California. A number of elongate, tectonometamorphic accreted terranes comprising the western Paleozoic and Triassic belt (WPzTr) contain abundant evidence of low-P/T metamorphism related to combined regional shortening and magmatic heating which occurred during Middle and Late Jurassic evolution of an really widespread continental-margin magmatic arc at the western edge of North America. One of the WpzTr terranes, the Stuart Fork, also contains evidence of an earlier pre-mid-Jurassic metamorphism related to subduction along a convergent margin, preserved as distinctive high-P/T blueschist metamorphic assemblages that are partly obscured within younger plutonic aureoles. Other terranes in the southern WPzTr contain poorly documented low-grade metamorphic mineral assemblages which are not easily attributed to particular metamorphic conditions, geothermal gradients or tectonic environments. The goal of this study is to conduct a detailed petrologic and thermochronlogic investigation of pre-Middle Jurassic metamorphism and deformation within these southern WPzTr terranes. Integrated field, petrologic and geochronologic data will help to constrain the origin, evolution and tectonic environment of these terranes prior to their assembly in mid- Jurassic time. These data will also be employed to model their accretion to the continental margin, and to address and unresolved problem of whether the early Mesozoic margin of the central Cordillera evolved as a single, diachronous subduction boundary, or as a system of parallel, perhaps, coeval, subduction zones. This integrated study will employ standard optical microscopy, detailed electron microprobe analysis (including back-scattered electron imagining), and XRD analysis. These methods are particularly well-suited to the study of low-grade metamorphic rocks. In addition, chronological and thermal- history constraints will be provided by 40Ar/39Ar age-dating in collaboration with Dr. Paul R. Renne of Princeton University. We plan to conduct this investigation over a period of two years, including two summer field seasons.