Metamorphic evolution of sapphirine-and sodicgedrite-anorthite-bearing granulites, Rampur domain, Eastern Ghats Province, India

The Eastern Ghats Belt (EGB) have been extensively explored by the geoscientific community since few decades, however, there still prevail some gaps regarding the unique mineral assemblage reported through this communication. This study documents the occurrence of sapphirine, spinel, orthopyroxene, sodic-gedrite, calcic-amphibole, biotite, and plagioclase assemblage resulting in UHT metamorphic conditions. The significance of this study lies in the peculiarity of sapphirine being present within anorthite matrix which has been reported for the first time from the Indian subcontinent. The studied assemblage has also been correlated with the more or less similar assemblage of rock called ‘Sakenites’ reported from southern Madagascar to unravel the most probable source rock anorthosites which led to such rare UHT mineral assemblage. Also, the gedrite present in the assemblage is Na-rich representing high-grade phase sodic-gedrite which had traversed across the amphibolite facies before attaining conditions of UHT granulites. The P-T path estimated also reveals decompression path with almost uniformly decreasing P-T conditions in contrast to the commonly reported isothermal decompression path from various other domains and provinces of the EGB. Another important aspect that needs to be highlighted is the UHT conditions of metamorphism at low to moderate pressures of 5.5 kbar. The corresponding retrograde assemblage has been recalibrated by the sequential removal of sapphirine and spinel while T-X (H2O) diagram is also constrained that suggests low water contents in the assemblage during the peak metamorphic condition. Conversely, high-water contents during retrograde history of the studied granulites have been reported through T-X (H2O) constraints. The introduced source of fluid is thought to be external during the later stages of exhumation and subsequent upliftment.
The analysed EMP U-Th-total Pb monazite chemical age constraints also suggest Mesoproterozoic to Neoproterozoic episodes corresponding to ~846 Ma to ~959 Ma pair of thermal events. These metamorphic events have also been correlated to reconstruct the Rodinian supercontinent at ~959 Ma and the initiation of its subsequent break-up at ~846 Ma.

东高止造山带（Eastern Ghats Belt, EGB）近数十年来已受到地学界的广泛勘探，但本通讯所报道的独特矿物组合仍存在部分研究空白。本研究记录了假蓝宝石（sapphirine）、尖晶石（spinel）、斜方辉石（orthopyroxene）、钠韭闪石（sodic-gedrite）、钙闪石（calcic-amphibole）、黑云母（biotite）与斜长石（plagioclase）组合的产出，该组合指示超高温（Ultrahigh-Temperature, UHT）变质条件。本研究的意义在于，首次在印度次大陆（Indian subcontinent）报道了赋存于钙长石基质（anorthite matrix）中的假蓝宝石这一特殊产出形式。本次研究的矿物组合还与马达加斯加南部（southern Madagascar）产出的“萨肯岩（Sakenites）”相似组合进行对比，以厘清形成该罕见超高温矿物组合的最可能源岩——钙长岩。此外，组合中的韭闪石富钠，属于高级变质相的钠韭闪石，其在达到超高温麻粒岩（UHT granulites）相条件之前曾经历角闪岩相（amphibolite facies）演化。本次研究估算的温压路径（P-T path）显示为减压路径（decompression path），其温压条件近乎均匀降低，这与东高止造山带其他众多区域及地块普遍报道的等温减压路径（isothermal decompression path）截然不同。另一需重点说明的重要特征是，该变质作用发生于5.5千巴（kbar）的低-中等压力条件下的超高温环境。对应的退变质组合（retrograde assemblage）通过依次移除假蓝宝石与尖晶石进行了重新校准，同时T-X(H₂O)图解（T-X (H2O) diagram）也限定了峰期变质条件（peak metamorphic condition）下矿物组合的低水含量。反之，通过T-X(H₂O)约束可知，该麻粒岩在退变质历史（retrograde history）中具有较高的水含量。研究认为，在折返（exhumation）及后续抬升（upliftment）的后期阶段，流体来源为外源流体。
本次分析的电子探针（Electron Microprobe, EMP）U-Th-总铅独居石（monazite）化学定年约束结果显示，其热事件年龄区间为约846 Ma至约959 Ma，对应中元古代（Mesoproterozoic）至新元古代（Neoproterozoic）时期。这些变质事件还被用于重建约959 Ma的罗迪尼亚超大陆（Rodinian supercontinent），以及约846 Ma罗迪尼亚超大陆后续裂解的起始过程。