This post is second in a series of posts about metamorphism.
Metamorphic grade, zones and index minerals
Science is based on a solid understanding of underlying physical principles. Although I have chickened-out of the detail, everything I mention in the previous post is based on a solid understanding of chemistry and physics. The knowledge we have today of the chemistry of rock-forming minerals wasn’t available to earliest scientists studying metamorphic rocks. Their study of Geology was driven by fieldwork and observation.
Some useful metamorphic concepts came from the study of rocks in the Highlands of Scotland in the early Twentieth Century. George Barrow identified patterns in the minerals found in these rocks, based on the appearance of characteristic index minerals (chlorite, biotite, garnet, staurolite, kyanite, sillimanite). His insight was that these index minerals were distributed in distinctive ways.
The rocks of the Scottish Highlands are mostly metamorphosed sediments. What were sandstones, muddy sandstones and mudstones are now (meta)-quartzites, schists, slates and so on. The main influence on the proportions of minerals within them is what the original sediment (the ‘protolith’) was, a quartzite has more quartz than a slate for example. Barrow’s insight was to focus on the index minerals and be less worried about the many other minerals present in the rock (e.g. quartz, plagioclase, ilmenite, magentite, titanite, tourmaline, muscovite, etc). This is something field Geology often does, identifies important features that allow us to filter down the sheer mass of information available from rocks in the field.
Using the concept of index minerals allowed Barrow to map metamorphic zones, each one an area on the map defined by the first appearance of its particular index mineral. He interpreted these zones in terms of progressive metamorphism, from low grade to high grade.
Using this concept grade and index minerals, a walk across Scotland becomes a walk through different metamorphic conditions. Starting in the chlorite zone (near the Laphroaig whisky distillery on Islay perhaps, bottle marked A), rocks are slatey, with well preserved sedimentary features. Since they were muds and sandstones, there have been some metamorphic reactions producing chlorite, but they are still low grade, greenschist facies rocks. As you move Northwest into higher grade rocks (biotite, garnet zones), new minerals appear in a regular sequence and the texture of the rocks change, becoming more schist-like. By the time you reach the Macallan whisky distillery further north (bottle B), you are in high-grade rocks (sillimanite zone); like the whisky, the rocks are dramatically different from where we started. They are now gneisses, often migmatitic, containing pods and lenses of granite: they are so hot that they have started to melt.
By the end of your journey you’ve undergone some changes too, most likely you’ve become damper in the rain and become covered in midge-bites, but also fallen in love with the scenery (and the whisky). The sillimanite grade rocks you’re standing on at the end have been on a journey too. They started as low-grade rocks, then were the same as rocks in the garnet zone but they carried on, reaching higher and higher temperatures.
The patterns are based on observation in the field, but can linked to the theory. The index minerals appear when the rock has passed across a metamorphic reaction (a line in P-T space). The reaction creates a new mineral that isn’t seen at a lower grade. High grade rocks have passed through multiple metamorphic reactions as grade increases: we see evidence of this in thin-section, where we sometimes see ‘frozen reactions’, with some minerals growing at the expense of others.
The sequence of metamorphic zones that George Barrow found in Scotland is found in many places, such the eastern US and the Himalayas, where major mountain belts have been created. It is seen as characteristic of regional or Barrovian metamorphism, where the metamorphism is driven by mountain-building. Rocks caught up between converging plates are buried (increasing the pressure). Temperatures increase as rocks are further from the surface, meaning the heat naturally produced by radioactive decay builds up. Also heat may be produced by friction associated with the deformation of the rocks.
The Buchan series of metamorphic zones, which includes andalusite and cordierite was also defined in Scotland and is associated with lower pressure conditions, which in Scotland are associated with intrusions of gabbro and perhaps extension.
This post is second in a series of posts about metamorphism which attempts to cover pretty much all aspects of metamorphism and how we study it.
A random note on the spelling of whisky
As a snooty English type, I persist in spelling things differently to many other native English speakers. My spelling of the word whisky as whisky (instead of whiskey) is such an example and the reason for the difference is sort-of interesting. Whisk(e)y is a Celtic invention, from around a thousand years ago. The name comes from ‘water of life’ (uisce beatha in Irish). As English spread into these gaelic speaking countries, this name was anglicised into whisk(e)y. In Scotland, the spelling of whisky stuck, in Ireland whiskey was preferred. In Britain now, Scottish whisky is most popular so that spelling is used. In America, the Irish spelling of whiskey is better known. Many settlers entered the US from both Scotland and Ireland, but my guess is that many of the Scottish ones were from Protestant sects that regard alcohol as ‘the devil’s own buttermilk’ and so less likely to start distilleries.
Irish whiskey (Bushmills is especially nice) is also drunk in England, by real enthusiasts. We try to use the Irish spelling for this whiskey, but are often too drunk to remember.