秦岭造山带若干岩石高温高压脱水熔融的特征及其意义

Dehydration Melting at High Pressure and High Temperature in Block Rocks of Qinling Orogen and Its Implications

对秦岭造山带和邻区块状岩石样品进行了高温(1100～1300℃)和高压(1.2～1.8GPa)脱水熔融实验。通过对实验产物综合分析发现许多样品中出现了熔融玻璃和雏晶。玻璃代表的熔体成分为基性和中性,部分接近于超基性范围,熔体与原来岩石全岩成分比较,更偏基性。熔体出现的空间和成分都显示了局部熔融体系的特征,即含水矿物(角闪石或黑云母)和浅色矿物(石英或斜长石)控制了熔融的发生并且决定了熔体的成分。脱水熔融产生了比原岩更偏基性的熔体,这意味着熔融后残留部分将愈偏酸性。如果这种机制存在于大陆中—下地壳,将对探讨大陆地壳的结构和物质组成,解释某些地区的长英质中下地壳的成因等具有重要的意义。

Contrasting with powder melting in rock samples, the natural block rock melting may supply much more information on the kinetic processes of partial melting in the natural state of rocks in the deep lithosphere. P-wave velocities of the rocks collected from the Qinling Orogenic belt and its adjacent regions were measured under high pressure and high temperature simultaneously. After checking the products by optical microscope and EPMA in details, we found partial melting occurred in some of the samples. Some pieces of melting glasses and crystallites, which in the color of black, brown, light yellow or colorlessness, distribute between the hornblende （or biotite）and plagioclase（ or quartz）. Most of the melting glasses are ranging between basic and intermediate rocks in composition, some close to in ultrabasic composition. An extraordinary feature is that nearly all the melts in the samples have less content of SiO2 than the starting bulk rock samples; this means that the melts are more basic than their parent bulk rocks. This seems contradict with the traditional law of magmatic evolution that we have accepted without doubt. The second feature is that the melts are strict controlled spatially and chemically by hornblende（ or biotite）and plagioclase（ or quartz）. The composition of the melting glasses and crystallites are in the range between hornblende and plagioclase, in which the hornblende is abundant in TiO2, MnO,∑FeO, MgO and CaO, and the plagioclase is abundant in SiO2, Al2O3, and Na2O. The partial melting was caused by the dehydration of hornblende under the P-T condition of middle-lower continental crust （750 -920℃ , 0. 63 -0. 90 GPa, corresponding to 21 -30 km）. The melt appears more basic, and in the mean time leaves a more acidic remnant. If this occurred in the middle-lower continental crust, it would give an answer to the origin of the felsic lower continental crust.