Geochemistry on rocks from Shackleton Range

During the GEISHA expedition (Geologische Expedition in die Shackleton Range 1987/88), the Pioneers Escarpment was visited and sampled extensively for the first time. Most of the rock types encountered represent amphibolite facies metamorphics, but evidence for granulite facies conditions was found in cores of garnet. These conditions must have been at least partly reached during the peak of metamorphism. For the Pioneers Escarpment a varicolored succession of sedimentary and bimodal volcanic origin is typical. It comprises: quartzites muscovite quartzite, sericite quartzite, fuchsite quartzite, garnet-quartz schists etc.; pelites: mica schists and plagioclase or plagioclase-microcline gneisses, aluminous schists; marls and carbonates: grey meta-limestones, carbonaceous quartzites, but also pure white, often fine-grained, saccharoidal marble, or a variety of tremolite marble, olivine (forsterite) marble, diopside-clinopyroxene-tremolite marble, etc.; basic volcanic rocks: amphibole fels, amphibolite schist, garnet amphibolite, and acidic to intermediate volcanic rocks: garnet-biotite schist, epidote-biotite-plagioclase gneiss, microcline gneiss. These rocks are considered to be a supracrustal unit, called the Pioneers Group. In the easternmost parts of the Pioneers Escarpment, e.g. at Vindberget, nonmetamorphic shales, sandstones and greywackes crop out, which are cover rocks of possibly Jurassic age. These metasediments, which represent a quartz-pelite-carbonate (QPC) association, indicate that deposition took place on a stable shelf, i.e. on the submerged rim of a craton. Marine shallow-water sedimentation including marls and aluminous clays form the protoliths. The volcanics may be part of a bimodal volcanics-arkose-conglomerate (BVAC) association. Geochemical analyses support the assumption of volcanic protoliths. This is demonstrated especially by the elevated amounts of the immobile, incompatible high-field-strength elements (HFSE) Nb, Ta, Ti, Y, and Zr encountered in some of the gneisses. Microscopic investigation suggests the existence of ortho-amphibolites. This is confirmed by the geochemistry. A bimodal volcanic association is evident. The amphibolites plot in both the tholeiite and calc-alkaline fields. The acidic volcanics are mainly rhyolitic. The sediments and volcanics were subjected to conditions of 10-11 kbar and 600°C during the peak of metamorphism, i.e. granulite facies metamorphism, which can be deduced from the Fe mole ratios of 0.71-0.73 in the garnet cores. Due to the relatively low temperatures, no anatectic melting took placc. The rims of the garnets show a Fe mole ratio of 0.84-0.86, and the coexisting mineral association garnet-biotite-staurolite-kyanite indicate amphibolite facies. The thermobarometry shows P-T conditions of 5-6 kbar and 570-580°C for this stage. The metamorphic history indicates deep burial at depths down to 35 km (subduction?) i.e. high pressure metamorphism, followed by pressure release due to uplift associated with retrograde metamorphism. This may have happened during a pre-Ross metamorphic event or orogeny. The Ross Orogeny at about 500 Ma probably just led to the weak greenschist facies overprint that is evident in the rocks of the Pioneers Group. Finally, sedimentation resumed in the area of the present Shackleton Range, or at least in the eastern part of the Pioneers Escarpment, probably when detritus from erosion of the basement (Read Group and Pioneers Group) was deposited, forming sandstones and greywackes of possibly Jurassic age. There is no indication that these sediments belong to the former Turnpike Bluff Group.在GEISHA科考队（全称：Geologische Expedition in die Shackleton Range 1987/88）科考期间，研究人员首次对先锋陡崖（Pioneers Escarpment）开展了全面考察与系统采样。本次采集的岩石类型以角闪岩相变质岩为主，但在石榴子石核部发现了麻粒岩相形成条件的证据，该变质条件至少在变质作用峰期曾局部达到。 先锋陡崖（Pioneers Escarpment）以一套色彩丰富的沉积岩与双峰式火山岩系为典型特征，其岩性组合包括：石英岩类：白云母石英岩、绢云母石英岩、铬云母石英岩、石榴子石-石英片岩等；泥质岩类：云母片岩、斜长片麻岩或斜长-微斜长片麻岩、富铝片岩；泥灰岩与碳酸盐岩类：灰色变质灰岩、炭质石英岩，以及纯净洁白、常呈细粒结构的砂糖状大理岩，还包括各类透闪石大理岩、橄榄石（镁橄榄石）大理岩、透辉石-单斜辉石-透闪石大理岩等；基性火山岩类：角闪石岩、角闪片岩、石榴子石角闪岩；中酸性火山岩类：石榴子石-黑云母片岩、绿帘石-黑云母-斜长片麻岩、微斜长片麻岩。 上述岩石被归为一套表壳岩系，命名为先锋群（Pioneers Group）。在先锋陡崖最东部区域，例如文德贝格（Vindberget），出露有未变质的页岩、砂岩与杂砂岩，其为可能形成于侏罗纪的盖层岩石。这套变质沉积岩属于石英-泥质岩-碳酸盐岩（QPC）组合，指示其沉积环境为稳定陆棚，即克拉通淹没边缘。原岩为包括泥灰岩与富铝黏土在内的海相浅水沉积岩系。该套火山岩可能属于双峰式火山岩-长石砂岩-砾岩（BVAC）组合的一部分。 地球化学分析佐证了火山岩原岩的推测，这一点在部分片麻岩中富集的惰性不相容高场强元素（HFSE）铌（Nb）、钽（Ta）、钛（Ti）、钇（Y）与锆（Zr）中得到了充分体现。镜下观察显示存在正角闪岩，该结论也得到了地球化学数据的支持。研究明确了双峰式火山岩组合的存在：角闪岩同时落入拉斑玄武岩与钙碱性岩套的地球化学范畴，中酸性火山岩则以流纹质为主。 从石榴子石核部的铁摩尔比（0.71~0.73）可推断，沉积物与火山岩在变质作用峰期经历了10~11千巴、600℃的变质条件，即麻粒岩相变质作用。由于温度相对较低，未发生深熔作用。石榴子石边部的铁摩尔比为0.84~0.86，其与黑云母、十字石、蓝晶石共生的矿物组合指示其形成于角闪岩相环境。温压测算显示，该阶段的温压条件为5~6千巴、570~580℃。 变质演化历史表明，岩石曾经历了深埋藏（埋藏深度可达35km，可能与俯冲作用有关），即高压变质作用，随后伴随抬升发生压力释放，并伴随退变质作用。该过程可能发生于前罗斯变质事件或前罗斯造山运动时期。约5亿年前的罗斯造山运动仅对先锋群岩石造成了微弱的绿片岩相叠加改造。 最终，在现今沙克尔顿山脉（Shackleton Range）区域，至少在先锋陡崖东部，沉积作用再次发生：基底岩系（里德群Read Group）与先锋群侵蚀产生的碎屑物质得以沉积，形成了可能形成于侏罗纪的砂岩与杂砂岩。目前并无证据表明这些沉积物属于原特恩派克布拉夫群（Turnpike Bluff Group）。