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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia
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Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

机译：1.2 Ga亚马逊克拉通的古地理：格伦维利亚早期与劳伦蒂亚（Lantaria）拉诺（Llano）段碰撞

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A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondonia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New ~(40)Ar/~(39)Ar age dating of biotite and plagioclase yield ages of ～1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062 ± 3 Ma, similar to the ～1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n = 16 sites, P_(lat) = 24.6°N, P_(long) = 164.6°E, A_(95) = 5.5°, Q = 5), the first reported pole for the Amazon craton for the 1200-600 Ma Rodinia time period, constrains the paleogeographic position of Amazonina at ～1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia's Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have caused the Llano deformation in early Grenvillian times.
机译：为了限制亚马逊克拉通的中古生代古地理，对巴西西部朗多尼亚州的新弗洛雷斯塔组平坦的辉长岩和玄武岩进行了古地磁，年代学和岩石学研究。测量辉长岩样品上磁化率的各向异性，发现平坦的叶面具有放射状的纹样，这为野外证据表明辉长岩是未变形的大基石的一部分提供了证据。对辉长岩中氧化物的岩石学观察发现，在门槛的原始冷却过程中产生了两种磁铁矿晶粒：大的，氧溶解的钛磁铁矿晶粒和火成反应边缘的细粒磁铁矿。黑云母的新的〜（40）Ar /〜（39）Ar年龄和斜长石的屈服年龄为〜1.2 Ga，这代表了镁铁质岩浆侵位后的快速冷却。玄武岩样品的整个岩石测年产生的总气年龄为1062±3 Ma，与先前工作人员报告的〜1.0 Ga K / Ar年龄相似。但是，年龄谱的强烈成分依赖性使得这个年轻的整个岩石年龄不可靠，除非作为最小约束。在玄武岩中发现了一个单一的磁性成分，这与定向在WNW且陡峭向上的辉长岩中的特征剩磁没有区别。该磁化被认为是主要的，是在门槛和相关熔岩冷却期间获得的。根据新弗洛雷斯塔地层计算的古磁极（n = 16个位置，P_（纬度）= 24.6°N，P_（长）= 164.6°E，A_（95）= 5.5°，Q = 5），是第一个报告的磁极在1200-600 Ma Rodinia时期的亚马逊克拉通中，限制了Amazonina的古地理位置（约1.2 Ga）。西部亚马逊克拉通与Laurentia Grenville边缘的Llano段的并置导致NF极位于1.2 Ga上Laurentia和Greenland组合APWP的一部分，这表明与亚马逊克拉通的碰撞可能导致了格林维尔早期的Llano变形。

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