Deciphering the Tectonic History of Slowly Cooled Precambrian Polymetamorphic Terranes in the American Southwest

破译美国西南部缓慢冷却的前寒武纪多变质地体的构造历史

• 批准号: 0126148
• 负责人: Christopher Andronicos
• 金额: $ 2.66万
• 依托单位: University of Texas at El Paso
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-03-01 至 2003-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0126148&HistoricalAwards=false
• 关键词: Deciphering; Tectonic; History; Slowly; Cooled

The Proterozoic tectonic history of the American southwest is the subject of considerable controversy. Controversy revolves around the nature of ~1.4 Ga plutonism and tectonism and its relationship to Paleoproterozoic (1.8-1.6 Ga) crustal accretion. To help resolve this question The P.I. will investigate the Cimarron Mountains of northern New Mexico and the Needle Mountains of southwestern Colorado. These two regions preserve deformation and plutonic histories that record both Paleoproterozoic accretion and subsequent Mesoproterozoic reactivation. 1.4 Ga metamorphism has erased the Paleoproterozoic thermal signature in most of northern New Mexico and southern Colorado. Therefore, these two proposed study areas may provide the only opportunity to decipher the Paleoproterozoic metamorphic and cooling history in northern New Mexico and southern Colorado.The Cimarron Mountains contain an upper amphibolite grade block and a greenschist grade block separated by the Fowler Pass Shear Zone (FPSZ). Metamorphic mineral assemblages across the shear zone suggest up to 10 km of vertical separation. Cooling ages from the high-grade block suggest high-grade metamorphism occurred at ~1.4 Ga. In contrast, rocks in the low-grade block record a 1678 Ar 40 /Ar 39 cooling age: this is the oldest cooling age reported in northern New Mexico. Detailed structural analysis of the two blocks and the F.P.S.Z. will be undertaken to clarify the deformation history and tectonic significance of this structure and its relationship to the metamorphic evolution of the region.The Needle Mountains record a much different history. Paleoproterozoic supracrustal rocks in this region are divided into two packages. The Uncompahgre Formation, consisting of quartzites and schists, is regionally metamorphosed to the greenschist facies. These rocks rest on a sheared unconformity above the second package, which is composed of amphibolite facies gneiss, migmatite, and schist. The 1.46 Ga Eolus granite intrudes both packages and has a well-developed contact aureole with peak metamorphic assemblages including sillimanite and K-feldspar. However, many of the rocks far from the granite have assemblages that appear to be unaffected by 1.4 Ga metamorphism. This situation may provide the opportunity to determine the thermal structure of the crust during Paleoproterozic orogenesis as well as during the 1.4 Ga overprint. In order to better understand the role of 1.4 Ga tectonics in the evolution of southwest North America, the P.I. will undertake a structural, metamorphic, and geochronologic study of the two ranges. Both of these areas are similar in having 1.4 Ga metamorphic features and Paleoproterozoic thermal and metamorphic signatures. These regions are therefore well suited to determine which deformational and metamorphic features belong to each of the two events. The preservation of Paleoproterozic metamorphic features makes these areas ideal for testing of models for slow cooling and long-term residence in the middle crust in newly formed lithosphere.

美国西南部的元古代构造史是一个有相当大争议的主题。关于1.4Ga深成岩作用和构造作用的性质及其与古元古代（1.8- 1.6Ga）地壳增生的关系，一直存在争议。为了帮助解决这个问题。将考察新墨西哥州北方的西马龙山脉和科罗拉多西南部的尼德尔山脉。这两个地区保存的变形和深成的历史，既记录了古元古代加积和随后的中元古代复活。1.4 Ga变质作用消除了北方新墨西哥州和科罗拉多南部大部分地区的古元古代热特征。因此，这两个拟议的研究领域可能提供唯一的机会，以破译古元古代变质和冷却的历史，在北方新墨西哥州和科罗拉多南部。西马龙山脉包含一个上角闪岩级块和绿片岩级块分离的福勒通行证剪切带（FPSZ）。整个剪切带的变质矿物组合表明垂直分离可达10公里。高级块体的冷却年龄表明高级变质作用发生在~ 1.4Ga。相比之下，低品位地块的岩石记录了1678 Ar 40 /Ar 39冷却年龄：这是北方新墨西哥州报告的最古老的冷却年龄。两个区块和F.P.S.Z.的详细结构分析。本文将阐明该构造的变形历史、构造意义及其与本区变质演化的关系。本区古元古代表壳岩可分为两大类。Uncompahgre地层由石英岩和片岩组成，区域变质为绿片岩相。这些岩石位于第二个包裹体上方的剪切不整合面上，该不整合面由角闪岩相片麻岩、混合岩和片岩组成。1.46 Ga的Eolus花岗岩侵入两个包，并有一个发育良好的接触晕与峰变质组合，包括硅线石和钾长石。然而，许多远离花岗岩的岩石组合似乎不受1.4 Ga变质作用的影响。这种情况可能提供了机会，以确定在古元古代造山作用以及在1.4 Ga的叠加地壳的热结构。为了更好地理解1.4 Ga构造在北美西南部演化中的作用，P.I.将对这两个山脉进行构造、变质和年代学研究。这两个地区具有相似的1.4 Ga变质特征和古元古代的热变质特征。因此，这些地区非常适合于确定哪些变形和变质特征属于这两个事件。古元古代变质特征的保存使这些地区成为测试新形成岩石圈中地壳缓慢冷却和长期居住模型的理想地区。