When did crustal melting weaken the heart of the Himalaya?

主要的山脉带是地壳的扭曲，受到重力的破坏。埋藏在这些带中的岩石会软化、拉伸和融化，从而对其机械强度产生严重的影响。在喜马拉雅山脉，对花岗岩的研究主要集中在显眼的、苍白的中新世花岗岩(淡花岗岩)上。这些岩浆是当富饶的岩石从中地壳快速剥离、解压和熔融时形成的。然而，这些融化是那次戏剧性挖掘的症状，而不是其原因。从巴基斯坦到不丹2的整个喜马拉雅山脉链条上都发现了早期融化的散布证据(图1)。这些神秘的、变形的含蓝晶石的淡色花岗岩和部分熔融的片麻岩(混合岩)在古近纪进行性埋藏和加热过程中结晶。然而，在新近纪造山过程中结构被大量改造的岩石中，这种证据通常被忽视。因此，了解这些年轻山脉中的古近纪地壳熔融是确定地壳增厚被折返所取代的转折点的关键3。此外，这种熔融的空间分布将有助于确定驱动构造挤压的潜在构造机制(临界锥形、楔形构造或沟道流)。该项目旨在询问场关系和矿物组合，以确定喜马拉雅结晶核心加热期间的熔体反应。该项目的结果将深入了解古近纪喜马拉雅造山带和更老的碰撞造山带的粘度变化，为试图解释所有山脉带如何演化的热力学模型提供关键约束。方法：本研究的初始阶段将检查来自OU收集的古近纪花岗岩样本，以确定最适合进行详细研究的野外区域。在印度喜马拉雅的实地考察将能够评估这些花岗岩体的空间关系及其变形和变质历史。将对混合岩类、浅色花岗岩类和潜在的花岗岩熔体源岩进行微量元素和同位素(Ndsr)全岩研究，而熔体副相(独居石、锆石)将通过U-Pb系统测年，并进行Hf同位素分析，以追踪熔体的历史。独居石和锆石的年龄将通过变质矿物的假截面模拟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