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

美国西南部的元古界构造史一直备受争议。围绕~1.4Ga深成作用和构造作用的性质及其与古元古代(1.8~1.6Ga)地壳增生的关系存在争议。为了帮助解决这个问题，私家侦探将调查新墨西哥州北部的西马龙山脉和科罗拉多州西南部的针叶山脉。这两个区域保存了记录古元古代增生和随后的中元古代再活动的变形和深成岩历史。1.4 Ga变质作用抹去了新墨西哥州北部和科罗拉多州南部大部分地区的古元古代热特征。因此，这两个研究区可能是破译新墨西哥州北部和科罗拉多州南部古元古代变质和冷却历史的唯一机会。Cimarron山脉包含一个被Fowler Pass剪切带(FPSZ)隔开的上角闪岩级块体和一个绿片岩级块体。横跨剪切带的变质矿物组合表明，垂直距离可达10公里。高级地块的冷却年龄表明，在~1.4Ga发生了高级变质作用。相比之下，低品位区块中的岩石记录了1678 Ar 40/Ar 39的冷却年龄：这是新墨西哥州北部报道的最古老的冷却年龄。将对这两个区块和F.P.S.Z进行详细的构造分析，以阐明该构造的变形历史和构造意义，以及它与该地区变质演化的关系。针状山脉记录了一段截然不同的历史。本区古元古代表壳岩系分为两套。不压实组由石英岩和片岩组成，区域性变质为绿片岩相。这些岩石位于第二套岩石上方的剪切不整合上，由角闪岩相片麻岩、混合岩和片岩组成。1.46Ga Eolus花岗岩侵入这两个包体，并具有发育的接触光环，峰期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具有似乎不受1.4Ga变质作用影响的组合。这种情况可能为确定古元古代造山作用和1.4Ga叠加期地壳的热结构提供了机会。为了更好地了解1.4Ga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。这两个地区在1.4Ga变质特征和古元古代热变质特征上是相似的。因此，这些区域非常适合确定哪些变形和变质特征属于这两个事件中的每一个。古元古代变质特征的保存使这些地区成为检验新形成的岩石圈中地壳缓慢冷却和长期驻留模型的理想地点。

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.