When did crustal melting weaken the heart of the Himalaya?

主要的山带是地壳的变形，受到重力的破坏。埋藏在这些区域的岩石软化、拉伸和融化，对它们的机械强度产生了剧烈的影响。仅仅百分之几的部分融化就能极大地削弱大陆地壳，并迅速改变山带的演变。在喜马拉雅地区，对花岗岩的研究主要集中在明显的、苍白的中新世花岗岩（浅花岗岩）上。这些岩浆形成于肥沃的岩石迅速从地壳中被挖掘出来，减压融化。然而，这些融化是那次戏剧性挖掘的征兆，而不是原因。在喜马拉雅岩芯中挖掘的线索一定存在于更早的事件中。在从巴基斯坦到不丹的整个喜马拉雅山脉上，已经发现了早期融化的零星证据2（图1）。这些隐蔽的、变形的含蓝晶石的亮花岗岩和部分熔融的片麻岩（混辉岩）在古近纪的渐深埋藏和加热过程中结晶。然而，这些证据在新近纪造山过程中被大量改造的岩石中经常被忽视。因此，了解这些年轻山脉的古近纪地壳融化是确定地壳增厚被发掘所取代的临界点的关键。此外，这种熔融的空间分布将有助于识别驱动构造挤压的潜在构造机制（临界锥度、楔形构造或河道流动）。该项目旨在探究领域关系和矿物组合，以定义喜马拉雅结晶核心在加热过程中的熔融反应。该项目的结果将对喜马拉雅古近系和更古老的碰撞造山带的粘度变化有深入的了解，为试图解释所有山带如何演变的热力学模型提供关键的约束。研究方法：本研究的初始阶段将检查OU收集的古近系花岗岩样本，以确定最适合进行详细研究的野外区域。在印度喜马拉雅的野外工作将使这些花岗岩体的空间关系及其变形和变质历史得以评估。混合岩、白花岗岩和花岗质熔体的潜在烃源岩将进行全岩微量元素和同位素（Nd和Sr）研究，而熔体附属相（单氮石、锆石）将通过U-Pb系统测定年代，并分析Hf同位素以追踪熔体的历史。利用变质矿物的伪截面模型，将独居石和锆石与结晶反应联系起来

Major mountain belts are contortions of the Earth's crust, ravaged by gravity. Rocks buried in these zones soften, stretch and melt, with drastic consequences for their mechanical strength. Just a few percent of partial melt can dramatically weaken the continental crust1 and rapidly change the evolution of the mountain belt.In the Himalaya, research on granites has mainly focused on conspicuous, pale bodies of Miocene-aged granites (leucogranites). These magmas formed when fertile rocks were rapidly exhumed from the mid-crust, decompressed and melted. However, these melts were a symptom of that dramatic exhumation, not its cause. Clues to what triggered that exhumation in the Himalayan core must lie in earlier events.Sporadic evidence for earlier melting has been recognised along the entire Himalayan chain from Pakistan to Bhutan2 (Figure 1). These cryptic, deformed kyanite-bearing leucogranites and partly-molten gneisses (migmatites) crystallized during Paleogene prograde burial and heating. However, such evidence is commonly overlooked among rocks with textures heavily reworked during Neogene mountain-building.Understanding Paleogene crustal melting in these youthful mountains is therefore key for establishing the tipping point at which crustal thickening was overtaken by exhumation3. Moreover the spatial distribution of such melting will help fingerprint the underlying tectonic mechanism that drove the tectonic extrusion (critical taper, wedge tectonics or channel flow).This project aims to interrogate field relations and mineral assemblages to define melt reactions during heating in the crystalline core of the Himalaya. Results from the project will yield insights into viscosity changes in both the Paleogene Himalaya and older collisional orogens, providing critical constraints on thermomechanical models that attempt to explain how all mountain belts evolve.Methodology: The initial phase of this study will examine samples of Paleogene granites from the OU collection to identify the most appropriate field area for detailed study. Fieldwork in the Indian Himalaya will allow the spatial relationships of these granitic bodies and their deformational and metamorphic histories to be assessed. Migmatites, leucogranites and potential source rocks for granitic melt will be subjected to trace element and isotopic (Nd and Sr) whole-rock study, while melt accessory phases (monazite, zircon) will be dated via the U-Pb system and analysed for Hf isotopes to trace the history of the melts. Monazite and zircon ages will be linked to crystallization reactions by employing pseudosection modeling of metamorphic minerals4