Jurassic Volcanic Rocks in the Transantarctic Mountains: A Test of a Model for Continental Flood Basalt Magmatism in Antarctica

该奖项支持一个研究南极洲二叠纪至三叠纪冈瓦纳序列上的侏罗纪火山岩的项目。这些岩石构成了冈瓦纳超大陆分裂时岩浆记录的南极部分。这些侏罗纪岩石沿着横贯南极山脉的长度分布，包括硅质和拉斑岩成分。从晚三叠世毕肯前陆盆地沉积终止到中侏罗世费拉尔群洪水玄武岩喷发的时间间隔，最完整的地层记录位于横贯南极山脉中部。该地区上法拉组以喷流凝灰岩和呼吸岩浆沉积为主。上覆的Prebble组以细粒至粗粒的呼吸岩浆矿床为主，包括增生的罗钻凝灰岩和角砾岩。岩石的组成是双峰的，硅质活动开始于火山活动，并持续到洪水玄武岩喷发时期，而玄武岩活动开始于普雷布尔的爆炸性喷发，然后转变为熔岩的平静渗出。玄武质火山碎屑岩与熔岩岩浆相融，具有相同的地球化学特征。硅质岩石可能是无水岩。含大角度碎屑的粗石英砂岩和岩浆活动晚期活动的单斜构造的存在表明同生基底块体断裂。地层学、构造学、岩石学和古火山学资料表明，这些火成岩形成于构造-岩浆裂谷环境。在南维多利亚陆，相关地层包括洪水熔岩、玄武岩火山碎屑沉积和一些新近报道的硅质火山碎屑岩。在一个地方，一个主要的不整合将玄武岩火山碎屑岩与下伏的信标岩分开。硅质凝灰岩与玄武岩火山碎屑岩及不整合的关系尚不清楚。据报道，这些硅质凝灰岩中含有一种褐煤。该项目的目的是收集资料，以检验提出的南极洲侏罗纪岩浆活动的构造-岩浆模型。具体而言，目标是：1)建立侏罗纪火山碎屑岩的详细地层格架，以评估同沉积伸展正断层作用；2)研究岩石的古火山学，特别是由呼吸岩浆作用形成的岩石，以了解其物理环境；3)进一步表征岩石的地球化学特征，以更好地定义岩浆组成的时间变化、硅质和玄武岩的关系；确定了硅质岩浆的源区特征；4)确定了硅质火山活动的起生时代。这些数据将用于验证或修改所提出的模型，并评估该模型对大陆洪水玄武岩岩浆作用的适用性。

Abstract This award supports a project to study the Jurassic volcanic rocks which overlie the Permian to Triassic Gondwana sequence in Antarctica. These rocks constitute the Antarctic part of the magmatic record of the break-up of the supercontinent Gondwana. These Jurassic rocks, which occur along the length of the Transantarctic Mountains, include both silicic and tholeiitic compositions. The most complete stratigraphic record of the time interval from the Late Triassic termination of deposition in the Beacon foreland basin to the Middle Jurassic eruption of the flood basalts of the Ferrar Group is located in the central Transantarctic Mountains. In that region, the upper Falla Formation is dominated by volcanogenic rocks including plinian airfall tuffs and phreatomagmatic deposits. The overlying Prebble Formation is dominated by fine to coarse grained phreatomagmatic deposits, including accretionary lapilli tuffs and breccias. Compositionally the rocks are bimodal with silicic activity initiating volcanism and continuing into the flood basalt eruptive episode, whereas the basaltic activity begins with explosive eruptions in the Prebble and changes to quiet effusion of lava. The basaltic pyroclastic rocks are comagmatic with the lavas, having the same geochemical characteristics. The silicic rocks are probably anatectic. Coarse quartzose sandstones with large angular clasts and the presence of monoclinal structures active late in the magmatic activity, indicate contemporaneous basement block faulting. Stratigraphic, structural, petrologic and paleovolcanologic data indicate that these igneous rocks accumulated in a tectono-magmatic rift setting. In south Victoria Land, the correlative strata include flood lavas, basaltic pyroclastic deposits and some recently reported silicic pyroclastic rocks. At one locality a major disconformity separates the basaltic pyroclastic rocks from the underlying Beacon. The relations of the silicic tuffs to the basalt ic pyroclastic rocks and to the disconformity are unclear. The silicic tuffs are reported to include an ignimbrite. The objectives of this project are to gather data to test the tectono- magmatic model proposed for the Jurassic magmatism in Antarctica. Specifically, the objectives are to: 1) establish a detailed stratigraphic framework for the Jurassic pyroclastic rocks in order to assess syndepositional extensional normal faulting, 2) investigate the paleovolcanology of the rocks, in particular the rocks formed by phreatomagmatic processes, in order to understand the physical environment, 3) further characterize the geochemistry of the rocks in order to better define time dependent changes in magma compositions, relations between silicic and basaltic rocks, and source region characteristics of the silicic magmas, and 4) establish the age of onset of the silicic volcanism. These data will be used to validate or modify the proposed model and to assess the model's applicability to continental flood basalt magmatism in general.