Talk:Metamorphic zone
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Unification of Related Terms
editA metamorphic zone is a tool, like a hand lens. It is built from metamorphic isograds. All these tools lead to the most important one, the metamorphic facies. Eskola's theory, which created metamorphic facies, allows all the other related theoretical tools to be related by the physical theory of thermodynamics.
To create a unified theory that allows one to study the metamorphic grade, the intensity of metamorphism from features seen on the ground, one should probably build the theory from pieces. Metamorphism is not a change in one property, but complex changes in many properties. There are, therefore, many isograds passing through one point on the ground: there is no unique metamorphic grade; the grade of each property can change independently.
Minerals, related by thermodynamics to temperature & pressure, form only one class of metamorphic property. (People who study such things sometimes call themselves 'chemical petrologists'.) Texture, fabric, and other features can be used to map metamorphism. Francis Turner, in the 1930s, used general lithology to distinguish at least four sub-zones in his chlorite zone in New Zealand.
It's my opinion that isograds should be fundamental objects. These are traces on the ground of isogradic surfaces. ('Trace' is perhaps better than 'secant'.) The index mineral is well-described by Barrow, which is still good reading. These minerals need be visible in the field, mapable, and they identify just one class of isograd: they are chemical, and vary with the composition of the rock (which is why Barrow chose pelitic rocks).
The surface of the ground between isograds is a metamorphic zone. When zones have enough minerals to have a thermodynamic variance of 2, and both temperature & pressure were independently variable, such zones are independent of the chemical compositions of the rocks and are termed metamorphic facies. (See the Talk section of Metamorphic facies.)
The above requires one to abandon Tilley's definition of the metamorphic isograd, and modern researchers have likely done this. (I've been isolated from the literature.) In my opinion, isograds are curves on the ground that connect points where one property of metamorphism has the same grade. They describe the orientation of the source of metamorphism that created them: when you step across an isograd, you walk either toward or away from the source of the actions that formed it.
Zones described here are ribbon-shaped surfaces bounded by isograds. When mineral zones progress toward their source, each reduces the number of independently variable actions imposed by the source (its chemical variance), and facies are more equant-shaped surfaces whose minerals preserve a specific range of temperatures and pressures at which the facies equilibrated.
If contributors should choose this organization (which appears natural to me), the related entries in the Wikipedia could each emphasize their place in this scheme. This might simplify them, and allow each to be more easily remembered. Just a suggestion.
Facies Series
editThe sequence of metamorphic zones is called a metamorphic facies series, and the most common of these is Barrovian (called after George Barrow who first mentioned it in 1912). In this series of zones, both pressure and temperature increase gradually along the metamorphic gradient.
- Yes, but each zone must itself be a facies. In my experience, the variance of many metamorphic rocks is six or more: it must be two to be a facies. Zones can decrease in their variance until they become facies. Facies can be contiguous, or they can be separated by zones that are not facies.