Abstract Ten nepheline syenite intrusions have been concordantly emplaced within or close to a polydeformed, regionally metamorphosed sedimentary sequence in the Rengali Province, eastern India during the Mesoproterozoic. The intrusions and the country rocks share a common deformation history and record three phases of folding. The nepheline syenites preserve deformation structures and textures indicative of continuous deformation that started in the presence of a melt and ended under sub-solidus conditions. This deformation in nepheline syenites is divided on the basis of mechanical behavior into three stages: an early one with the development of varied types of magmatic layering and flattened magmatic enclaves, a middle one with the development of asymmetrical magmatic folds and intramagmatic shear zones, and a late stage with melt-enhanced embrittlement. Igneous zircons from two of the intrusions, namely Chhatabar and Gunjighara are dated at 1356 ± 5 Ma and 1358 ± 4 Ma (U–Pb concordant age, LA–ICP–MS), respectively. Electron microprobe U–Th–total Pb geochronology of monazites from two samples of sillimanite grade mica schists in the host metasedimentary sequence of the Chhatabar nepheline syenite yields metamorphic ages of 1362 ± 12 Ma and 1376 ± 23 Ma, which indicate broad synchronism of regional metamorphism of the country rocks and the nepheline syenite magmatism. Contrary to the common proposition that nepheline syenite intrusions in eastern India and in some other regions of Precambrian crust were emplaced anorogenically in intracontinental rifts and subsequently deformed by continental collision, we develop a structural interpretation in which the nepheline syenites were emplaced syntectonically in the sense that magma transport and cooling occurred during regional metamorphism and deformation. The study shows that an integration of structural, textural and geochronological data from both nepheline syenite and country rock is crucial for an understanding of the spatial and temporal relationships between contractional deformation and alkaline magmatism. Our results imply that deformed nepheline syenites may not always be useful in constraining the initiation of a Wilson Cycle heralded by intracontinental rifting.

中文翻译：

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来自印度东部 Rengali 省的霞石正长岩侵入体：综合地质背景、微观结构和年代学观察对其同构造侵位

摘要 10 个霞石正长岩侵入体在中元古代印度东部 Rengali 省的多变形、区域变质沉积层序内或附近一致分布。侵入体和围岩具有共同的变形历史并记录了三个折叠阶段。霞石正长岩保留了变形结构和纹理，表明在熔体存在下开始并在亚固相线条件下结束的连续变形。霞石正长岩的这种变形根据力学行为分为三个阶段：早期发育不同类型的岩浆层理和扁平岩浆包体，中期发育不对称岩浆褶皱和岩浆内剪切带，以及具有熔融增强脆化的晚期阶段。来自两个侵入体，即 Chhatabar 和 Gunjighara 的火成岩锆石的年龄分别为 1356 ± 5 Ma 和 1358 ± 4 Ma（U-Pb 一致年龄，LA-ICP-MS）。来自恰塔巴尔霞石正长岩寄主变沉积序列中的两个硅线石级云母片岩样品的独居石的电子探针 U-Th-总 Pb 年代学产生的变质年龄分别为 1362±12Ma 和 1376±23Ma，表明区域变质作用具有广泛的同步性乡村岩石和霞石正长岩岩浆作用。与印度东部和前寒武纪地壳其他一些地区的霞石正长岩侵入体在陆内裂谷中以非造山方式侵位并随后因大陆碰撞而变形的普遍观点相反，我们开发了一种构造解释，其中霞石正长岩在同构造上就位，因为岩浆运输和冷却发生在区域变质和变形过程中。研究表明，综合来自霞石正长岩和围岩的结构、构造和地质年代学数据对于理解收缩变形和碱性岩浆作用之间的空间和时间关系至关重要。我们的结果意味着变形的霞石正长岩可能并不总是有助于限制由陆内裂谷预示的威尔逊旋回的开始。霞石正长岩和围岩的结构和地质年代学数据对于理解收缩变形和碱性岩浆作用之间的空间和时间关系至关重要。我们的结果意味着变形的霞石正长岩可能并不总是有助于限制由陆内裂谷预示的威尔逊旋回的开始。霞石正长岩和围岩的结构和地质年代学数据对于理解收缩变形和碱性岩浆作用之间的空间和时间关系至关重要。我们的结果意味着变形的霞石正长岩可能并不总是有助于限制由陆内裂谷预示的威尔逊旋回的开始。