Geochemical constraints on depositional environment of metacarbonate rocks from the Neoproterozoic Cauvery Suture Zone, Southern Granulite Terrane, India

Metacarbonates serve as proxies for understanding the chemical oceanography of the present, as well as the past-day ocean’s evolution history and these are extensively described in the Neoproterozoic-Cambrian collisional belts of East Gondwana. The Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India, is one such prominent example of their occurrence, along with several Precambrian dismembered ophiolites, arc-related intrusions and other metamorphic rocks. These are dominantly exposed in the western, southern and south-central parts of the suture, around the Neoproterozoic Kadavur Gabbro-Anorthosite and Manamedu Ophiolite Complexes. Their whole-rock geochemistry reveals the protoliths of metacarbonates have been derived from different sources of continental inputs and deposited under open to passive margin settings. The δ¹³C_(V-PDB)) - δ¹⁸O_(V-SMOW) data (−0.01 to + 3.03‰; +15.81 to + 24.84‰) reveal as high-temperature carbonates of typical limestone variety, deposited under warmer palaeoclimatic conditions of the marine environment and later subjected to metamorphism. The presence of higher ⁸⁷Sr/⁸⁶Sr ratios (0.707514 to 0.71511) and negative εNd values (−17.3 to −10) suggests that they originated from large crustal inputs with more interactions of seawater with the sediments, evolved during Ediacaran to early Cambrian periods. The above results reveal that these are possible remnants of the Neoproterozoic Mozambique Ocean and coeval with the metacarbonates of Sør Rondane Mountains-East Antarctica, Highland Complex-Sri Lanka, Madagascar and Mozambique Belts of Neoproterozoic Gondwana.

变碳酸盐岩（Metacarbonates）可作为解析现代海洋化学以及古海洋演化历史的代用指标，这类岩石在东冈瓦纳（East Gondwana）的新元古代-寒武纪碰撞带中已有广泛研究与报道。印度南部麻粒岩地体（Southern Granulite Terrane, SGT）内的高韦里缝合带（Cauvery Suture Zone, CSZ）便是这类岩石产出的典型实例之一，其伴生地质体包括多套前寒武纪解体蛇绿岩套、弧相关侵入岩及其他变质岩。这类变碳酸盐岩主要出露于该缝合带的西部、南部及中南部区域，紧邻新元古代Kadavur辉长岩-斜长岩（Kadavur Gabbro-Anorthosite）和Manamedu蛇绿岩杂岩（Manamedu Ophiolite Complexes）。全岩地球化学分析显示，变碳酸盐岩的原岩源自多种陆源输入物，沉积于开放至被动大陆边缘环境。其碳同位素δ¹³C_(V-PDB)和氧同位素δ¹⁸O_(V-SMOW)数据（范围为-0.01~+3.03‰；+15.81~+24.84‰）表明，该类岩石为典型灰岩类高温碳酸盐岩，形成于温暖的海洋古气候环境，后续经历了变质作用。较高的⁸⁷Sr/⁸⁶Sr比值（0.707514~0.71511）以及负εNd值（-17.3~-10）则指示，其起源于大量陆壳输入物，且海水与沉积物发生了较强的相互作用，演化时限为埃迪卡拉纪至寒武纪早期。上述研究结果表明，这些变碳酸盐岩可能是新元古代莫桑比克洋的残余体，与新元古代冈瓦纳的南极东部Sør Rondane山脉、斯里兰卡高地杂岩、马达加斯加及莫桑比克带的变碳酸盐岩形成时代一致。