花岗质体系中钾长石的实验早期结晶:对花岗质岩石岩浆组构成因的启示

IF 1.3 4区 地球科学 Q2 GEOLOGY
J. Díaz-Alvarado
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引用次数: 1

摘要

某些花岗闪长岩至花岗质岩石最显著的特征之一是钾长石巨晶的存在。例如,西班牙中央体系岩基的花岗闪长岩和二长花岗岩呈现出不同数量的大(长度可达10cm)自面体钾长石晶体。斑岩结构、自面体形状、斜长石和黑云母包裹体的排列以及岩浆组构表明这些巨晶的岩浆起源。本文介绍了高k2o花岗闪长岩体系的相平衡研究。采用花岗闪长岩组成(GEMbiot)进行了一系列实验,研究了Gredos地块侵位条件(4 h2o wt.%, 0.4GPa)下的结晶序列。实验结果表明:1010oC时,正辉石为液相相,与富h2o熔体在980 ~ 940oC时发生反应,使黑云母稳定。斜长石在910oC左右结晶,k长石在750 ~ 700oC之间结晶,晶体分数在0.5左右。而在850℃时,掺入褐铁矿的实验中,在与堇青石反应的捕虏体域和花岗闪长岩域均发现自面体K-长石(≈5vol%),其k2o wt.%从正常实验的4.5上升到掺入实验的5.9。这些结果表明,体同化作用促进了巨大花岗闪长岩岩浆体中k2o的体非均质富集,从而引发了钾长石巨晶的早期结晶。由于巨晶的早期结晶作用,以钾长石巨晶为定义的岩浆岩片理形成于侵位过程中和侵位过程之后,并受到构造运动学的高度影响。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Experimental early crystallization of K-feldspar in granitic systems : implications on the origin of magmatic fabrics in granitic rocks
One of the most outstanding characteristics of some granodioritic to granitic rocks is the presence of K-feldspar megacrysts. For instance, granodiorites and monzogranites of the Spanish Central System batholith present variable amounts of large (up to 10cm in length) euhedral K-feldspar crystals. The porphyritic textures, the euhedral shape, the alignment of plagioclase and biotite inclusions and the magmatic fabrics point to a magmatic origin for these megacrysts. This work presents a phase equilibria study in a high-K 2 O granodioritic system. A series of experiments were conducted with a granodioritic composition (GEMbiot) to study the crystallization sequence at the emplacement conditions in the Gredos massif, i.e. 4 H 2 O wt.% and 0.4GPa. Experimental results show that orthopiroxene is the liquidus phase at 1010oC, which reacts with the H 2 O-rich melt to stabilize biotite between 980 and 940oC. Plagioclase crystallizes at around 910oC, and K-feldspar crystallizes in the matrix between 750 and 700oC when the crystal fraction is around 0.5. However, at 850 oC, a pelite-doped experiment shows euhedral K-feldspar (≈5vol%) in both the reactive xenolith domain together with cordierite and the granodioritic domain, where the K 2 O wt.% rise from 4.5 in the normal experiment to 5.9 in the doped experiment. These results suggest that the bulk-assimilation process promotes the bulk and heterogeneous K 2 O enrichment in a huge granodioritic magma volume, which triggers an early crystallization of K-feldspar megacrysts. Because of this early crystallization of the megacrysts, the magmatic foliations defined by K-feldspar megacrysts are formed during and after the emplacement processes and are highly influenced by tectonic kinematics.
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来源期刊
Geologica Acta
Geologica Acta 地学-地质学
CiteScore
2.50
自引率
6.70%
发文量
13
审稿时长
>12 weeks
期刊介绍: - Relevant conceptual developments in any area of the Earth Sciences. - Studies presenting regional synthesis. - Thematic issues or monographic volumes presenting the results from one or more research groups. - Short papers reflecting interesting results or works in progress. - Contributions and results from Research Projects, Workshops, Symposiums, Congresses and any relevant scientific activity related to Earth Sciences. - Geologica Acta aims to stimulate rapid diffusion of results and efficient exchange of ideas between the widespread communities of Earth Science researchers (with special emphasis on Latinamerica, the Caribbean, Europe, the Mediterranean
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