Geochemistry of metasedimentary units of the Cambro-Ordovician Ross Orogen in North Victoria Land and Oates Land, Antarctica

Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of >=95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/ 86Sr t=490 Ma ratios of 0.7120 - 0.7174, epsilonNd, t=490 Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (epsilonNd, t=500 Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (epsilonNd, t=510 Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (epsilonNd, t=510 Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.

位于南极维多利亚地北部与奥茨地的三个早古生代罗斯造山地体中的变沉积岩，经地球化学特征归类为未成熟岩屑砂岩至杂砂岩，以及中等成熟度页岩。化学风化指数（Chemical Index of Weathering）值≥95的高成熟岩类通常缺失。地球化学数据以及铷-锶（Rb-Sr）、钐-钕（Sm-Nd）同位素测试结果显示，东部罗伯逊湾地体中寒武-奥陶纪罗伯逊湾群的浊积型变沉积岩属于一套高度均一的序列，无显著成分差异。仅在反映原岩化学风化程度与碎屑机械分选程度的化学参数上存在明显差异。地球化学数据、4.9亿年时87Sr/86Sr比值为0.7120~0.7174、εNd（t=490 Ma）值为-7.6~-10.3，以及单阶段钕模式年龄为17~19亿年，均指示其物源为化学演化程度较高的地壳，形成时代总体为古元古代晚期。未发现来自原始“蛇绿岩”物源的显著沉积充填证据。中寒武统摩尔组（鲍尔斯地体）的变沉积岩在成分上具有显著非均质性。其常量与微量元素数据、钐-钕同位素数据（εNd（t=500 Ma）值为-14.3~-1.2，单阶段钕模式年龄为17~21亿年）可通过两种沉积输入端元的混合得到解释：一是形成时代至少为古元古代早期的演化陆壳物源，二是原始玄武质物源。威尔逊地体的变沉积岩化学非均质性主要继承自其原岩的成分差异，这一点可由镍-二氧化钛（Ni vs TiO₂）图解得到证实。威尔逊地体更靠西侧的陆内区域样品的单阶段钕模式年龄为16~22亿年（εNd（t=510 Ma）为-7.0~-14.3），指示其物源中来自形成时代总体为古元古代早期的地壳物质占比相对较高。而位于东部更靠陆外区域的变沉积岩序列（εNd（t=510 Ma）为-5.4~-10.0；单阶段钕模式年龄为14~19亿年）则相反，其物源受形成时代更年轻的原始物源的影响更强。鲍尔斯与威尔逊地体的变沉积岩的化学与同位素特征，可通过两种截然不同的物源端元的可变贡献得到解释：一是类似于研究区以西数百公里出露于乔治五世地与阿德利地的南极克拉通陆壳，二是可能以鲍尔斯地体寒武纪岛弧为代表的原始玄武质物源。尽管罗伯逊湾地体的变沉积岩数据也与南极克拉通型物源的成因相符，但从其地球化学与同位素地球化学特征中，未发现该序列存在岛弧组分的直接证据。