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

构造板块在俯冲带被消耗掉，这是一个广阔的海底区域，在我们的星球表面延伸数千英里。在俯冲过程中，一个板块下沉到另一个板块之下，再循环进入地幔。俯冲带是活火山活动、已知的最大地震和各种各样的矿物存款类型的场所。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、古生物和古气候是必不可少的。阿巴拉契亚-喀里多尼亚山脉是世界上最大的山脉之一，从美国南部和东部延伸到加拿大和格陵兰岛，一直延伸到欧洲。尽管如此，美国东北部的俯冲记录在它留下的岩石记录方面令人惊讶地神秘，特别是在构成山脉深根的变质岩方面。然而，新的发现提供了新的视角，包括认识到康涅狄格州东北部（CT）形成的“超高温”变质岩超过900 °C（约1650 °F）。最重要的是，这个CT位置还保存了地壳俯冲到很深的诱人证据-也许超过60-100英里。该研究项目将集中在这些CT岩石上，因为它们可能是美国第一个明确承认的“超高压”（UHP）变质岩;它们的存在将促使人们重新思考阿巴拉契亚山脉建造期间岩石是如何，在哪里以及为什么俯冲的。更广泛地说，更深入地了解该地区的构造结构可能有助于解决长期存在的问题，包括古生代板块运动以及对山区自然资源的位置和地质形成的相关影响（该地区延伸到北极，并可能向西延伸到阿拉斯加）。该项目将支持和培养耶鲁大学博士。和本科生，并将研究成果纳入耶鲁皮博迪自然历史博物馆的公共科学推广计划。野外区域的片麻岩中含有石榴石，石榴石中含有结晶学定向的层状矿物（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lamantae广泛相关的压力-温度条件，因此，他们拥有相当大的，但尚未开发的潜力，作为成岩指标。例如，层状组合，包括金红石，钛铁矿，石英，角闪石，磷灰石只记录从超高压设置。然而，为了用来诊断变质状态，必须了解薄片的起源。因此，本项目将对CT石榴石中的薄片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来表征薄片组合和化学成分。(2)电子背散射绕射与透射电子显微镜研究，以确定石榴石与层状晶体之间的晶体学关系。(3)利用电子探针对封闭和开放系统石榴石行为进行化学绘图。(4)实地工作，以确定该地区的其他片层轴承石榴石地点。上述研究结果将用于检验层状矿物是石榴石出溶的产物，还是通过其他机制，如耦合渗透和沉淀形成的。如果是前者，那么富含Si、Ti和P的石榴石组合物强烈指向俯冲过程中在~ 5GPa或更高的超高压条件下形成的多晶石榴石前体。无论假设检验结果如何，都必须更好地理解层状构造的形成过程，以帮助指导新英格兰和地球仪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.