Geochemical and geochronological evolution of the UHT granulites from the Eastern Ghats Mobile Belt, India: implications from F-OH substitution in biotite, thermal events and cooling age

The thin lenses and bands of ultra-high temperature (UHT) granulites with diagnostic sapphirine and quartz in contact are occasionally found on the western flank of the Eastern Ghats Mobile Belt (EGMB), which inherits the suturing and over-thrusting signatures of the EGMB with the Greater Indian Landmass (GIL). The mineral biotite contains an abnormally high fluorine concentration, as revealed by the geochemical examination of these granulites from the research area (Kothuru), which was caused by the F-OH substitution to withstand extreme thermal metamorphic conditions. The geochemical results from the multi-element spider diagram and the Harker’s variation diagram with primitive-mantle normalization also show an enrichment of light rare earth elements and large-ion lithophile elements over depleted heavy rare earth elements and high-field strength elements. An evolutionary history has been established for the high-grade rocks that were subjected to ultra-high temperature metamorphism, based on the geochemical signatures of the examined granulites. The use of isotopic K-Ar cooling ages and electron microprobe monazite chemical ages of the thermal episodes, which act as useful tools to explain the time and span of the polycyclic metamorphic terrane, is another significant component of the current work. Numerous researchers have attempted to estimate the thermal episodes in various areas of the Eastern Ghats Mobile Belt, but their cooling ages are still poorly understood. Consequently, two thermal episodes that happened at 842 Ma and 637 Ma ago, as well as a cooling age of about 448 Ma that marked the quick exhumation of the high-grade rocks under study, have been recorded in the current investigation. The association between the Greater Indian Landmass and Eastern Antarctica during the existence of the Rodinian supercontinent has been established by recorded geochronological events.

在东高止山脉活动带（Eastern Ghats Mobile Belt, EGMB）西翼偶见产出特征性假蓝宝石（sapphirine）与接触态石英的超高温（ultra-high temperature, UHT）麻粒岩薄层与岩带，该构造带继承了其与大印度陆块（Greater Indian Landmass, GIL）之间的缝合与逆冲构造印记。对研究区（科图鲁，Kothuru）产出的该类麻粒岩开展地球化学检测后发现，其所含黑云母（biotite）矿物氟含量异常偏高，该现象由F-OH替代（F-OH substitution）作用引发，以适应极端热变质环境。经原始地幔标准化（primitive-mantle normalization）的多元素蛛网图（multi-element spider diagram）与哈克（Harker）变异图所获地球化学结果同样显示，轻稀土元素（light rare earth elements）与大离子亲石元素（large-ion lithophile elements）相较于亏损重稀土元素（depleted heavy rare earth elements）与高场强元素（high-field strength elements）发生了富集。基于本次研究所检测麻粒岩的地球化学特征，已为经历超高温变质作用（ultra-high temperature metamorphism）的高级变质岩（high-grade rocks）建立了演化历程。本次研究另一重要内容为，采用同位素K-Ar冷却年龄（isotopic K-Ar cooling ages）与电子探针独居石化学年龄（electron microprobe monazite chemical ages）作为阐释多旋回变质地体（polycyclic metamorphic terrane）形成时间与演化跨度的有效工具，以反演相关热事件。此前已有诸多学者尝试估算东高止山脉活动带不同区域的热事件，但相关冷却年龄的认知仍较为匮乏。据此，本次调查记录到两期分别发生于842 Ma与637 Ma前的热事件，以及代表研究区高级变质岩快速折返的约448 Ma冷却年龄。本次记录的地质年代学事件（geochronological events），已佐证了罗迪尼亚超大陆（Rodinian supercontinent）存续时期大印度陆块与东南极洲之间的古板块关联。