SHRIMP U-Pb Geochronology of Transantarctic Mountains Basement

该奖项由极地项目办公室的南极地质和地球物理项目提供，为与南极东部大陆盾的构造发育有关的地质年代学研究提供资金。东南极地盾代表了地球上最古老和最大的克拉通省之一，有着漫长的太古宙到元古代的历史。它是冈瓦纳大陆东部马赛克的中心块，可能在罗迪尼亚超大陆的新元古代合并中发挥了重要作用。由于大部分地盾被极地冰盖覆盖，我们对早期太古宙到新元古代地质事件的了解主要来自恩德比地和莫德女王地等地区的沿海露头。然而，地壳厚度和地球物理模式表明，横贯南极山脉造山带形成了东南极盾的太平洋边缘。横贯南极山脉中部的下伏是盾体本身的结晶基底岩（Nimrod群），与新元古代和下古生代边缘盆地沉积组合（Beardmore群和Byrd群）并列。这些岩石单元共同记录了太古宙地壳生长、元古代地壳改造、新元古代裂谷边缘沉积物沉积和早古生代板块边缘收敛。因此，横贯南极中部山脉是东南极洲太平洋边缘研究前寒武纪结晶盾及其变形的表壳覆盖的关键区域。最近利用超高分辨率离子探针（SHRIMP）对Nimrod和Beardmore群进行了U-Pb年龄测定，极大地扩展了我们对新元古代晚期和早古生代东南极盾演化和裂谷构造向活动边缘构造过渡的认识。从迄今为止相对较少的样品分析中，SHRIMP分析显示了Nimrod群的多阶段太古宙和元古代历史，并对Beardmore群的沉积时代和沉积物源提出了新的限制。这项新项目将大大扩展横贯南极山脉基底和表壳组合的年代学框架，其目标如下：详细研究了宁录群的变质岩和火成岩，以全面表征前寒武纪火成岩、变质岩和碎屑锆石组分的太古代和元古代历史，以及罗斯时代的前、同、后构造岩浆层序。以碎屑锆石年龄分析为基础，扩大了对Beardmore群及相关下古生代层序硅质碎屑岩的研究，以约束沉积年龄和沉积物源。利用现有的样品材料和横贯南极山脉其他地区的其他研究人员提供的一些样品材料，将有可能解决这些过渡新元古代-下古生代序列之间的区域物源变化。为了尝试对不同岩石单元的沉积年龄进行直接测年，还将分析成岩xenotime。为了完成这项研究，SHRIMP将与堪培拉澳大利亚国立大学（ANU）的研究人员合作完成U-Pb锆石和xenotime分析。分析将于2000年7月至12月6个月期间在虾设施完成。澳大利亚国立大学将免费提供样品处理，并以合作费率提供仪器访问。因此，这项工作的全部费用将由NSF（虾机时间和附带支持费用）、南卫理公会大学（工资和基本办公支持）和澳大利亚国立大学（降低内部仪器费用，不收取样品处理费用，以及办公空间、图书馆访问和计算机支持的休假支持）共同承担。

Goodge OPP 9912081This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for a geochronological investigation relating to the tectonic development of the East Antarctic continental shield. The East Antarctic shield represents one of Earth's oldest and largest cratonic provinces, with a long-lived Archean to Proterozoic history. It is the central piece in the eastern Gondwanaland mosaic, and it likely played an important role in the Neoproterozoic amalgamation of the Rodinia supercontinent. Because much of the shield is covered by the polar ice cap, our knowledge of early Archean to Neoproterozoic geologic events comes mostly from coastal outcrops in areas such as Enderby Land and Queen Maud Land. However, crustal thicknesses and geophysical patterns show that the Transantarctic Mountains orogenic belt forms the Pacific edge of the East Antarctic shield. Underlying the central Transantarctic Mountains are crystalline basement rocks of the shield itself (Nimrod Group), juxtaposed against Neoproterozoic and lower Paleozoic marginal-basin sedimentary assemblages (Beardmore and Byrd groups). Together, these rock units record Archean crustal growth, Proterozoic crustal modifications, sediment deposition across a Neoproterozoic rift margin, and early Paleozoic plate-margin convergence. The central Transantarctic Mountains is therefore a critical area along the Pacific margin of East Antarctica to study both the Precambrian crystalline shield and its deformed supracrustal cover.Recent U-Pb age dating of the Nimrod and Beardmore groups by use of the Super High Resolution Ion MicroProbe (SHRIMP) has greatly expanded our understanding of East Antarctic shield evolution and the transition from rift- to active-margin tectonics during the late Neoproterozoic and early Paleozoic. From a relatively small number of samples analyzed to date, SHRIMP analysis has demonstrated a multi-stage Archean and Proterozoic history of the Nimrod Group, and placed new constraints on the depositional age and sedimentary provenance of the Beardmore Group. This new project will significantly expand the geochronological framework of basement and supracrustal assemblages in the Transantarctic Mountains, with goals as follows:1. Detailed study of metamorphic and igneous rocks from the Nimrod Group, in order to fully characterize the Archean and Proterozoic history of Precambrian igneous, metamorphic and detrital zircon components, as well as the Ross-age pre-, syn- and post-tectonic magmatic sequence.2. Expanded study of siliciclastic rocks from the Beardmore Group and related lower Paleozoic sequences, primarily by age analysis of detrital zircons in order to constrain depositional ages and sedimentary provenance. With existing sample material and some provided by other researchers from other parts of the Transantarctic Mountains, it will be possible to address regional provenance variations among these transitional Neoproterozoic-lower Paleozoic sequences. Diagenetic xenotime will also be analyzed in order to attempt direct dating of the depositional age of the different rock units.To complete this study, SHRIMP U-Pb zircon and xenotime analyses will be completed in collaboration with researchers at the Australian National University (ANU) in Canberra. Analyses will be completed in the SHRIMP facility during a 6-month period from July to December, 2000. ANU will provide sample processing at no charge and instrument access at collaborative rates. Therefore, the full costs of this work are to be shared between NSF (SHRIMP machine time and incidental support costs), Southern Methodist University (salary and basic office support), and ANU (reduced in-house instrument rates, no charge for sample processing, and sabbatical support with office space, library access, and computer support).