Crystallographically-oriented Lamellae Phases in Garnet and Their Potential Use as Petrogenetic Indicators, Central Maine Terrane, Connecticut

地壳板块在俯冲带被消耗殆尽，这是横跨地球表面绵延数千英里的广阔海底区域。在俯冲过程中，一个板块下沉到另一个板块之下，并重新进入地幔。俯冲带是活火山活动、已知最大地震和各种矿床类型的所在地。追踪记录在山带中的俯冲历史对于重建整个地质时期的板块运动、古地理、古生物学和古气候是必不可少的。阿巴拉契亚-喀里多尼德山脉是世界上最大的山脉之一，从美国南部和东部一直延伸到加拿大和格陵兰岛，一直延伸到欧洲。尽管如此，就其留下的岩石记录而言，美国东北部的俯冲作用记录却令人惊讶地神秘莫测，特别是就构成山带深层根部的变质岩而言。然而，新的发现提供了新的视角，包括在康涅狄格州东北部（CT）发现的“超高温”变质岩，形成温度超过900°C（约1650°F）。重要的是，这个CT位置还保留了地壳俯冲到很深的地方的诱人证据——也许超过60-100英里。研究项目将集中在这些CT岩石上，因为它们可能是美国首次确定的“超高压”（UHP）变质岩；它们的存在促使人们重新思考，在阿巴拉契亚造山过程中，岩石是如何、在哪里以及为什么被俯冲的。更广泛地说，对该地区构造结构的深入了解可能有助于解决长期存在的问题，这些问题涉及古生代板块运动，以及与山带（延伸到北极，可能远至阿拉斯加西部）自然资源的位置和地质形成的相关含义。该项目将支持和培训耶鲁大学的博士和本科生，并将研究成果整合到耶鲁大学皮博迪自然历史博物馆的公共科学推广项目中。野外地区的片麻岩中含有石榴石，其中含有晶体取向的片状矿物（金红石、钛铁矿、磷灰石、角闪石、辉石、石英和其他相）。与世界其他地区的比较表明，这种薄片与压力-温度条件广泛相关，因此，它们作为岩石成因指标具有相当大的潜力，但尚未开发。例如，包括金红石、钛铁矿、石英、角闪洞和磷灰石在内的片层组合仅在UHP环境中被记录。然而，为了用于诊断变质制度，必须了解片层的起源。因此，本项目将对CT石榴石中的片层进行矿物学和岩石学研究，包括：(1)通过电子成像和微探针技术以及拉曼光谱表征片层组合和化学成分。(2)电子背散射衍射和透射电镜研究确定石榴石和片层之间的晶体学关系。(3)利用电子探针成像封闭和开放体系石榴石行为的化学作图。(4)野外工作，确定该地区其他含片层石榴石的地点。上述研究结果将用于测试片层是石榴石析出的产物，还是由渗透和降水耦合等其他机制形成的。如果是前者，那么富Si、Ti和p的石榴石组成强烈指向在俯冲过程中形成的UHP条件下（5gpa或更高）的多数石榴石前体。无论假设检验的结果如何，必须更好地理解片层形成过程，以帮助指导新英格兰和全球其他造山带的岩石构造解释。

Tectonic plates are consumed at subduction zones, vast undersea regions which stretch thousands of miles across the face of our planet. During subduction, one plate sinks beneath another to be recycled into Earth's mantle. Subduction zones are the sites of active volcanism, the largest known earthquakes, and a wide array of mineral deposit types. Tracing the history of subduction as recorded in mountain belts is essential for reconstructing plate movements, paleogeography, paleobiology, and paleoclimate throughout geologic time. The Appalachian-Caledonide mountain chain is one of the largest in the world, stretching from the southern and eastern United States up through Canada and Greenland and across to Europe. Nonetheless, the record of subduction in the northeastern United States is surprisingly cryptic in terms of the rock record it left behind, particularly in terms of the metamorphic rocks comprising the deep roots of the mountain belt. New discoveries, however, are providing fresh perspectives, including the recognition of "ultrahigh-temperature" metamorphic rocks in northeastern Connecticut (CT) formed in excess of 900 °C (about 1650 °F). Of major significance is that this CT locality also preserves tantalizing evidence for subduction of crust to great depths-perhaps in excess of 60-100 miles. The research project will focus on these CT rocks as they may be the first "ultrahigh-pressure" (UHP) metamorphic rocks definitively recognized in the United States; their presence would catalyze a rethinking of how, where, and why rocks were subducted during Appalachian mountain building. More broadly, a deeper understanding of the tectonic architecture of the region may help resolve longstanding questions involving Paleozoic plate movement and associated implications for the location and geologic formation of natural resources in the mountain belt (which extends into the Arctic and potentially as far west as Alaska). The project will support and train Yale Ph.D. and undergraduate students, and integrate research results into public science outreach programs at the Yale Peabody Museum of Natural History. Gneisses in the field area host garnets that contain spectacular examples of crystallographically-oriented lamellae minerals (rutile, ilmenite, apatite, amphibole, pyroxene, quartz, and other phases). Comparisons with other localities worldwide indicate that such lamellae correlate broadly with pressure-temperature conditions and, thus, they hold considerable but as yet mostly untapped potential as petrogenetic indicators. For example, lamellae assemblages including rutile, ilmenite, quartz, amphibole, and apatite are documented only from UHP settings. In order to be used to diagnose metamorphic regimes, however, lamellae origins must be understood. Consequently, this project will undertake mineralogical and petrological study of the lamellae in the CT garnets involving: (1) Characterization of lamellae assemblages and chemical compositions via electron imaging and microprobe techniques together with Raman spectroscopy. (2) Electron-backscatter diffraction and transmission electron microscopy studies to determine crystallographic relationships between garnet and lamellae. (3) Chemical mapping using the electron microprobe to image closed and open-system garnet behavior. (4) Field work to identify other lamellae-bearing garnet localities in the region. The results of the above research will be used to test whether or not the lamellae are the products of exsolution from garnet, or if they formed by some other mechanism such as coupled infiltration and precipitation. If the former, then the Si-, Ti-, and P-rich garnet compositions point strongly toward majoritic garnet precursors formed at UHP conditions ~5 GPa or higher during subduction. Regardless of hypothesis test outcomes, lamellae formation processes must be better understood to help guide petrotectonic interpretations in New England and other orogenic belts across the globe.