Abstract

The age and geochemical characteristics of alkaline rocks of the Dugda massif (Eastern Tuva), attributed to the East Sayan Late Paleozoic rare-metal magmatic zone, have been determined. The massif is composed of nepheline syenites, which dominate over alkaline quartz syenites and alkaline granites. The age of the rocks has been estimated by the ⁴⁰Ar/³⁹Ar method for amphibole and the U–Pb SIMS method for zircon. The age of the rocks was 291 ± 5 Ma for nepheline syenites, 284 ± 2 Ma for alkaline quartz syenites, and 285 ± 4 Ma and 287 ± 1 Ma for alkaline granites. These estimations make it possible to determine the age of the massif with an average value of ~287 Ma. Rocks with different compositions are characterized by close geochemical characteristics. They are enriched in the majority of incompatible elements in comparison with the average composition of the Earth’s crust and are characterized by multi-element spectra similar to A-type granitoids. Variations in the contents of the incompatible elements in the rocks of the massif exceed two orders of magnitude and reach ore concentrations (Zr up to 2.5 wt %, Nb up to 0.2 wt %, Ta up to 250 ppm, and Y up to 0.2 wt %). The geological position of the massif and specific features of the composition of its rocks, allowed us, firstly, to clarify the relationship between the East Sayan Late Paleozoic rare-metal zone and the fault system, which defines the western boundary of the Tuva–Mongolian microcontinent; secondly, to limit the time of alkaline magmatism to the interval of 309–285 Ma; and thirdly, to conclude that the spectrum of rare-metal igneous rocks of the zone is not limited to alkaline granitoids, but also includes alkaline nepheline syenites.

中文翻译：

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Dugda地块（东萨彦岭）碱性岩石的年龄和组成：西伯利亚地台西南褶皱架晚古生代稀有金属岩浆作用的表现形式

摘要

已经确定了属于东萨彦晚期古生代稀有金属岩浆带的杜格达地块（图瓦东部）碱性岩石的年龄和地球化学特征。地块由霞石正长岩组成，主要是碱性石英正长岩和碱性花岗岩。岩石的年龄由⁴⁰ Ar/ ³⁹闪石的 Ar 方法和锆石的 U-Pb SIMS 方法。霞石正长岩的岩石年龄为 291±5 Ma，碱性石英正长岩为 284±2 Ma，碱性花岗岩为 285±4 Ma 和 287±1 Ma。这些估计使得确定地块的年龄成为可能，平均值约为 287 Ma。具有不同成分的岩石具有相近的地球化学特征。与地壳的平均组成相比，它们富含大多数不相容的元素，并具有类似于 A 型花岗岩的多元素光谱。地块岩石中不相容元素含量的变化超过两个数量级并达到矿石浓度（Zr 高达 2.5 wt%，Nb 高达 0.2 wt%，Ta 高达 250 ppm，Y 高达 0.2 wt% %）。该地块的地质位置及其岩石成分的具体特征，使我们首先明确了东萨彦晚期古生代稀有金属带与断层系统之间的关系，断层系统定义了图瓦-蒙古的西部边界。微大陆；其次，将碱性岩浆作用时间限制在309～285 Ma区间；第三，认为该区稀有金属火成岩的光谱不仅限于碱性花岗岩，还包括碱性霞石正长岩。将碱性岩浆作用时间限制在309-285 Ma区间；第三，认为该区稀有金属火成岩的光谱不仅限于碱性花岗岩，还包括碱性霞石正长岩。将碱性岩浆作用时间限制在309-285 Ma区间；第三，认为该区稀有金属火成岩的光谱不仅限于碱性花岗岩，还包括碱性霞石正长岩。