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首页> 外文期刊>Current Science: A Fortnightly Journal of Research >Stable isotope evidence for ca. 2.7-Ga-old Archean cap carbonates from the Dharwar Supergroup, southern India
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Stable isotope evidence for ca. 2.7-Ga-old Archean cap carbonates from the Dharwar Supergroup, southern India

机译：约有稳定的同位素证据。来自印度南部Dharwar Supergroup的2.7-Ga古老的太古代碳酸盐岩

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摘要

Carbon isotope compositions of carbonate rocks from similar to 2.7-Ga-old Neoarchean Vanivilas Formation of the Dharwar Supergroup presented earlier by us are re-evaluated in this study, besides oxygen isotope compositions of a few silica dolomite pairs. The purpose of such a revisit assumes significance in view of recent field evidences that suggest a glaciomarine origin for the matrix-supported conglomerate member, the Talya conglomerate, which underlies the carbonate rocks of the Vanivilas Formation. An in-depth analysis of carbon isotope data reveals preservation of their pristine character despite the rocks having been subjected to metamorphism to different degrees (from lower greenschist to lower amphibolite facies). The dolomitic member of Vanivilas Formation of Marikanive area is characterized by highly depleted delta C-13 value (up to -5 parts per thousand VPDB) and merits as the Indian example of ca. 2.7-Ga-old cap carbonate. This inference is further supported by estimated low temperature of equilibration documented by a few silica dolomite pairs from the Vanivilas Formation collected near Kalche area. These pairs show evidence for oxygen isotopic equilibrium at low temperatures (similar to 0-20 degrees C) with depleted water (delta O-18 = -21 parts per thousand to -15 parts per thousand VSMOW) of glacial origin. We propose that the mineral pairs were deposited during the deglaciation period when the ocean temperature was in its gradual restoration phase. The dolomite of Marikanive area is the first record of cap carbonates from the Indian subcontinent with Neoarchean antiquity.
机译：除了一些二氧化硅白云岩对的氧同位素组成外，我们还重新评估了我们早先提出的与达哈尔超群的2.7-Ga年代的新阿尔巴尼亚Vanivilas组相似的碳酸盐岩的碳同位素组成。鉴于最近的现场证据表明基质支撑的砾岩成员塔利亚（Talya）砾岩是冰川海洋的起源，该目的具有重要意义，塔利雅砾岩位于Vanivilas组的碳酸盐岩之下。对碳同位素数据的深入分析显示，尽管岩石经历了不同程度的变质作用（从较低的绿片岩到较低的角闪岩相），但仍保留了其原始特征。 Marikanive地区Vanivilas组的白云岩层的特征是δC-13值高度枯竭（每千份VPDB高达-5份），是印度约旦的一个例子。 2.7-Ga-老碳酸酯。来自卡尔切地区附近的瓦尼维拉斯地层的几对硅白云岩对记录的估计低温平衡进一步支持了这一推论。这些对显示了在冰冷来源的贫水（δO-18 =千分之-21到千分之十五的VSMOW）下，在低温（类似于0-20摄氏度）下氧同位素平衡的证据。我们认为，矿物对是在海洋温度处于逐渐恢复阶段的冰消期期间沉积的。 Marikanive地区的白云岩是印度次大陆新古纪古代的帽状碳酸盐的第一个记录。

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