混合岩研究及地球动力学意义

Migmatite and its geodynamic implications

混合岩化作用(陆壳深熔)是大陆地壳演化的一个重要过程,可以在不同区域岩石圈演化和相伴构造热事件背景下发生。混合岩化温度往往可以维持在岩石固相线之上达30Ma,且整个深熔过程中岩石通常由初期的半深熔向高度深熔演化。这些特点使得混合岩中深熔锆石的Th/U比值随年龄越年轻而逐渐变大,并且警示混合岩锆石U-Pb年龄往往给出混合岩化的持续时间而不是单一时间点。深熔过程中,不同深熔反应类型对深熔熔体的地球化学特征影响较大,富水熔融可以降低斜长石在源区的稳定域,因此可能导致富水熔融形成的深熔熔体具有高Sr/Y和低Y的特征,从而提示在利用单一高Sr/Y和低Y特征来判别岩石是否具有高压成因需格外小心。此外,深熔过程中熔体提取速率可能大于矿物溶解速率和同位素扩散速率,因此可能发生不平衡熔融导致不同反应类型形成的熔体具有不同的初始同位素比值。熔体产生之后,由于混合岩地体具有缓慢的冷却速率,熔体有充分时间发生矿物结晶分离,残余熔体则在构造应力的作用下,被抽离源区,上升侵位至上部地壳。因此,混合岩地体中保留的大量浅色脉体只有少部分记录初始深熔熔体地球化学特征,绝大部分代表熔体结晶分离过程中的早期结晶产物,其地球化学特征与侵入浅部地壳的深熔花岗岩呈互补关系。陆壳深熔可以大大降低岩石的流变学性质。因此,造山带深熔物质在重力和高原-盆地压力差作用下,可能发生垂向和侧向挤出。下地壳流是深熔物质侧向挤出的重要形式,以混合岩以及相关淡色花岗岩的地球化学性质入手,为识别古老造山带的地壳流提供了一个新的思路。最后,文章以华北克拉通新太古代25亿年混合岩事件和大别—苏鲁造山带中生代混合岩为例,对中国东部混合岩研究进行了展望。

Migmatization （anatexis） plays a key role in the evolution of continental crust, and may occur under a variety of backgrounds related to the local lithospheric evolution and tectonic thermal events. The anatexis temperature of migmatite terranes may remain above the solidus for a time of as long as 30 million years, during which period, rocks were progressively transformed from metatexites to diatexites. The Th/U ratios of the anatexis zircons also increase with the decreasing zircon U-Pb ages. These characteristics make the zircon U-Pb ages determined from the migmatite terranes usually representing a time duration rather than a single time point. During the anatexis process, different melting reactions may have large impacts on the geochemical signature of the anatexis melts. For example, water-saturated melting may reduce the stability field of plagioclase in the source region to induce the plagioclase-limited melting reaction, which may give rise to the high Sr/Y and low Y characteristic of the anatexis melt. This, on the other hand, indicates that the high Sr/Y-low Y is not the unique signature for high-pressure melts. In addition, due to the possible higher melt extraction rate than the mineral dissolution and isotope diffusion rates, it is likely that the anatexis melts formed by fractional melting may have distinct initial isotopic signatures. After the melts formed, the initial melts might have undergone significant crystallization differentiation because of the low cooling rate of migmatite terranes. This makes only a small part of leucosomes in the migmatites having the record of the initial melts characteristics, whereas most parts may represent the cumulate products from crystallization differentiation, with the residual melts ascending and intruding into the upper crustal level. Crustal anatexis may also reduce significantly the rock strength, which resulted in the vertical and lateral extrusion of the middle-lower crustal material under gravity and lateral pressure difference between plateau and basin. Channel flow is an important form of lateral extrusion. Geochemical study of the migmatites and related leueogranites may provide a novel way to study the once-existing channel flow in ancient orogenic belts. Finally, this paper proposes that further migmatite study should focus on the 2.5 Ga migmatization event developed in the North China Craton and on the Mesozoic mimnatite distributed in Dabie-Sulu orogen.