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

本研究的目的是探讨北加州克拉马斯山脉中南部的前中侏罗统与辐合有关的变质作用的产状、性质和时间。由西古生代和三叠纪带（WPzTr）组成的许多长条形、构造变质的增生地体含有丰富的低p /T变质证据，这些低p /T变质与区域缩短和岩浆加热的联合作用有关，这种作用发生在北美西部边缘一个真正广泛的大陆边缘岩浆弧的中晚侏罗世演化过程中。WpzTr地体之一Stuart Fork也包含了早期中侏罗世前变质作用的证据，这些变质作用与汇聚边缘的俯冲有关，被保存为独特的高p /T蓝片岩变质组合，部分被较年轻的深成光环所掩盖。西pztr南部的其他地体含有文献记载较少的低品位变质矿物组合，这些矿物组合不容易归因于特定的变质条件、地热梯度或构造环境。本研究的目的是对WPzTr南部地体的前中侏罗统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合前的起源、演化和构造环境。这些数据还将用于模拟它们向大陆边缘的增加，并解决一个尚未解决的问题，即中央科迪勒拉的早中生代边缘是作为一个单一的、跨时的俯冲边界演变，还是作为一个平行的、可能是同时期的俯冲带系统。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XRD分析。这些方法特别适合于研究低品位变质岩。此外，与普林斯顿大学的Paul R. Renne博士合作，将通过40Ar/39Ar年龄测年提供时间和热历史约束。我们计划进行为期两年的调查，其中包括两个夏季野外季节。

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.