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首页> 外文期刊>Contributions to Mineralogy and Petrology >Regional-scale fluid flow and element mobility in Barrow’s metamorphic zones, Stonehaven, Scotland
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Regional-scale fluid flow and element mobility in Barrow’s metamorphic zones, Stonehaven, Scotland

机译：苏格兰斯通黑文巴罗变质带的区域尺度流体流动和元素迁移

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摘要

The geochemistry of metamorphic quartz vein formation in Barrow’s index mineral zones north of Stonehaven, Scotland, was investigated in order to assess regional fluid flow and mass transfer. Metamorphic grade in the Dalradian metasediments increases to the north–northwest away from the Highland Boundary Fault (HBF) and associated ophiolitic rocks of the Highland Border Complex (HBC), passing through the Chlorite (Chl), Biotite (Bt), Garnet (Grt), Chloritoid (Cld), and Staurolite (St) zones. Syn-metamorphic fluid infiltration at 462±8.8 Ma (Breeding et al. in Am Mineral 89:1067, 2004) produced considerable quartz veining. Vein abundance varies from about 5 to 15 volume percent of the outcrops; veins tend to be more abundant in metapelitic layers than in metapsammitic ones. Metamorphic veins are surrounded by centimeter- to decimeter-wide zones of chemical and mineralogical alteration (selvages). Porphyroblasts, particularly Bt, Grt, Cld, and St, are typically larger in selvages than in wallrocks distal to veins. The altered selvages underwent fluid-driven addition of Na, Ca, and Sr, and loss of K, Rb, and Ba. Alteration is most intense within ∼750 m of the HBF, but is still very significant at the northern end of the field area some 2 km away. Mg/FeT (FeT=total iron) was either unchanged or increased due to alteration. Silica was added at some Chl and Bt zone localities near the HBF. Pb mass transfer was variable although Pb was added at a number of locations. Rare Earth elements (REE) were generally immobile, but light REE and possibly heavy REE were lost at one field site. The gain of Na and Ca and loss of K promoted the growth of plagioclase at the expense of micas (particularly muscovite) in selvages and wallrock inclusions throughout the field area and, probably, some calcite and/or dolomite growth directly adjacent to the HBF. The Ca gains were also critical for epidote production in the Bt zone. Gains of Ca and increases in Mg/FeT helped to stabilize Grt at the expense of Cld and St in some selvages. Hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one Cld zone locality. The metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow. In the first model, fluid flow in a direction of increasing temperature downward along the HBF added Na and Ca, and removed K from the Dalradian. In the second model, fluid flow upward from the HBC transported Na and Ca into the overlying Dalradian and, at the same time, stripped out K. The latter model is favored because it can most readily account for silica addition near the HBF, but neither model can be ruled out at present. In either case, the veins represent fractures that transmitted very large time-integrated fluid fluxes of at least ∼104 m3 (fluid)/m2 (rock). Consequently, the veins were conduits for regional fluid flow that caused considerable open-system chemical and mineralogical alteration during metamorphism.
机译：为了评估区域流体流动和传质，研究了苏格兰斯通黑文以北的巴罗指数矿区的变质石英脉形成的地球化学。 Dalradian沉积物中的变质坡度从高地边界断裂（HBF）和高地边界综合体（HBC）的相关脂石岩中穿过绿泥石（Chl），黑云母（Bt），石榴石（Grt） ），类绿素（Cld）和星形石（St）区。 462±8.8 Ma的同变质流体渗透（Breeding等人，Am Mineral 89：1067，2004）产生了大量的石英脉。静脉的丰度在露头的5％到15％之间变化。在化生层中，静脉往往比在间质层中的静脉更丰富。变质脉被化学和矿物学改变（保留）的厘米至分米宽的区域包围。结缘的成岩细胞，特别是Bt，Grt，Cld和St，通常比边缘远处的围岩大。更改后的织边经历了流体驱动的Na，Ca和Sr的添加，以及K，Rb和Ba的损失。在HBF的约750 m范围内，变化最为剧烈，但在距地面约2 km的田野北端仍然非常明显。 Mg / FeT （FeT =总铁）不变或由于改变而增加。在HBF附近的一些Chl和Bt区位置添加了二氧化硅。尽管在许多位置都添加了铅，但铅的质量传递是可变的。稀土元素（REE）通常是固定的，但轻稀土和可能重稀土在一个野外地点丢失了。 Na和Ca的增加和K的损失促进了斜长石的生长，但以整个田间地区的边际和围岩包裹体中的云母（尤其是白云母）为代价，并且可能有一些方解石和/或白云石直接与HBF相邻。钙的增加对于Bt区域的附生子产生也至关重要。 Ca的增加和Mg / FeT 的增加有助于稳定Grt，而在某些布边上以Cld和St为代价。在一个Cld区域位置的强烈变质岩中发现了角闪石和卡明岩。交代学对传质过程施加了重要的约束，并提出了两种区域流体流动的模型。在第一个模型中，流体沿HBF沿温度升高的方向向下流动，添加了Na和Ca，并从Dalradian中去除了K。在第二种模型中，从HBC向上的流体流将Na和Ca输送到上覆的Dalradian，同时去除了K。后一种模型受到青睐，因为它可以最容易地解释HBF附近的二氧化硅添加量，但是两者目前可以排除模式。在任何一种情况下，静脉代表的裂缝都传递了非常大的时间积分流体通量，至少为〜104 m3 （流体）/ m2 （岩石）。因此，静脉是区域流体流动的管道，在变质过程中引起相当大的开放系统化学和矿物学变化。

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