变质岩岩石学-5 变质相变质带和变质相系2010

The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure
Zones thus have the same name as the isograd that forms the low-grade boundary of that zone Because classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones
Ky
Sil
Barrow’s Area
Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.
5.1 变质带的概念


变质带是英国地质学家George Barrow (1893) 研究苏格兰高地加里 东期泥质片岩时提出的: 不同变质程度的岩石在区内呈带状有规律 地分布，可划分出若干个变质强度带，通常称为变质带。 George Barrow was one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism
Grt Bi Chl
Note：



Barrow noted significant and systematic mineralogical changes in the pelitic rocks He subdivided the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased The new mineral that characterizes a zone is termed an index mineral
第五章 变质带、变质相和 变质相系
Metamorphic zones, facies, and facies series
本章要点：
1、何为变质带（递增变质带），有几种类型？ 2、何为变质相？ 3、接触变质作用有哪些变质相？其主要变质反应和矿物组合如何？ 4、区域变质作用有哪些变质相？其主要变质反应和矿物组合如何？ 5、何为变质相系？主要有几种类型？与大地构造的关系？
Metamorphic Zones Barrovian metamorphic zones are defined by reactions that result in the appearance or disappearance of minerals and can be mapped as isograds: chl —> bi —> gt —> st —> ky —> sill —> sill + or


A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district
Pelitic
compositions are similar, but the sequence of isograds is: – chlorite – biotite – cordierite – andalusite – sillimanite
总结:

An isograd (等变度) represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade

When one crosses an isograd, such as 来自百度文库he biotite isograd, one enters the biotite zone

在世界造山带也发育有类似的变质带系列，这 些变质带也称之为巴洛带Barrovian zones The P-T conditions referred to as ―Barrovian-type‖ metamorphism (fairly typical of many belts) = line that separates the zones (An isograd is meant to indicate a line in the field of constant metamorphic grade)



George Barrow (1893) 研究苏格兰高地加里东期的Dalradian 片岩时， 以泥质岩石中新矿物的首次出现为标志划分了三个变质带，即十字 石带、蓝晶石带和夕线石带。 后来Barrow (1912)、Tilley (1925) 和 Harker (1932)进一步扩展为6个 变质带。 每一个特征矿物首次出现的点的连线称为等变度（Isograd）。并且 较高级变质带都是在前一个变质带组合基础上发育形成，因此，这 一变质带系列称为递增变质带(Progressive metamorphic zones)，也 称为巴洛型变质带 (Barrovian metamorphic zones)

矿物平衡共生组合的定义 封闭体系的吉布斯相律和Goldschmidt矿物相律 开发体系下的Korzhinskii 矿物相律---在一定的温 度压力范围内，在开放体系下同时共存的平衡共生 矿物相的最大数目等于独立有效惰性组分数，而与 活动组分和杂质组分等无关。

共生图解：
– ACF、A‘KF和AFM图解有哪些特点？分别适用于何种类型 （或成分）的岩石？ – 如何把矿物成分和原岩成分投影在这些图解上？ – 如何用这些图解进行变质矿物共生分析？
“Imagination is more important than knowledge.”
--Einstein
子曰：学而不思则罔，思而不学则殆。 With some good knowledge the imagination will become more realistic
回顾：共生组合&共生图解
Barrovian zone chlorite zone biotite zone garnet zone staurolite zone kyanite zone sillimanite zone 2nd sillimanite zone or sillimanite + orthoclase zone mineral assemblage chlorite + mus + qtz + H2O + relict minerals chlorite + biotite + mus + qtz + H2O chlorite + biotite + garnet + mus + qtz + H2O staurolite + 2 AKFM phases + mus + qtz + H2O kyanite + 2 AKFM phases + mus + qtz + H2O garnet + biotite + sillimanite + mus + qtz + H2O sill + or + qtz + H2O + melt and no mus
Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).
The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are:






Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)

Isograd
Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.