Accretionary wedge #35 – Porphyroblast

I like long words (I might even say I was egregiously polysyllabic in my discourse) and when Accretionary Wedge #35 asked me for my favourite geological word I knew it had to be one of those compound names based on dead languages: something like porphyroblast.

A porphyroblast is a mineral found in a metamorphic rock that has grown larger than the surrounding minerals. As we’ll see later, the growing bit is important.

Porphyroblasts of many minerals can be found, but the classic mineral that forms them is garnet. It’s always a visual treat; in hand-specimen  it forms handsome red lumps and in thin-section it goes dramatically black under crossed-polars. Yum.

Garnet is a keen participant in many metamorphic reactions and the mineral chemistry of garnets is such that the relative amounts of Calcium, Iron, Magnesium and Managnese it contains can vary throughout a single grain.  This means that garnet porphyroblasts can be little time-capsules: the way in which their composition varies shows how the conditions under which they formed changed over time. A typical example is where the garnet core formed under conditions of lower temperature and pressure compared with the rim. As the rock was buried and heated as part of, perhaps, an episode of mountain building, the garnet slowly grew, changing in composition over time. Studying the composition  across the grain now allows us to glimpse a part of the rock’s history that is otherwise lost.

Building mountains involves squashing rocks as well as heating them, which leads to another way in which porphyroblasts can be time-capsules; they can contain fossil fabrics.

Garnet in Irish schist. Note the early 'vertical' fabric in garnet that is now wrapped by later 'horizontal fabric'. White dot is a reminder that I should buy myself a proper scanner

The metamorphic reactions that form garnet typically involve the breakdown of minerals like feldspar and mica. As garnet grows at the expense of these minerals, it fills the space they used to fill. Sometimes the garnet grows around minerals that aren’t involved in the metamorphic reaction, such as quartz. The quartz grains end up as inclusions with the garnet. If the rock matrix the garnet grows in has a fabric then this is preserved within the garnet, even if the matrix outside has been destroyed or rotated. Sometimes this ‘fossil fabric’ is folded, or forms beautiful spiral patterns.

There is a long and sometimes heated scientific controversy about the exact meaning of these inclusion fabrics; some believe that the orientation of these fabrics doesn’t change and that the garnet has stayed fixed while the surrounding fabric has been rotated. Others disagree, but all agree that porphyroblasts provide an important window into the structural history of metamorphic rocks.

Sometimes porphyroblasts include non-tectonic fabrics, such as below:

Big staurolite porphyroblast in South African rock (single grain fills image). Rectangular areas are where staurolite fully replaced another mineral. Surrounding areas are a mixture, where the new mineral grew around the quartz

What a difference a letter makes

A close runner-up in my favourite geological word stakes is porphyroclast. This is also a mineral found in metamorphic rocks that is larger than the surrounding minerals. Unlike a porphyroblast which grew bigger in absolute terms, a porphyroclast is relatively bigger because all the surrounding grains have been made smaller.

The best place to find porphyroclasts is in mylonites, which are rocks that have been intensely deformed in a ductile fashion. Mylonites are found in places, like the bottom of thrust sheets, where huge amounts of deformation (aka strain) have taken place. This strain has been accommodated, not by faulting, but my ‘smearing out’ the rocks.  Smearing rocks means smearing individual grains and this process is associated with reduction in grain-size. The processes that allow smearing (various forms of creep) work at the level of the crystal lattice and so are highly dependent on the type of mineral.

So, if a quartz rich rock is highly deformed the quartz grains may deform relatively easily and end up as a fine-grained matrix. Consider a feldspar grain caught up in a shear-zone. It may be too cold for it to deform internally, in contrast its quartz neighbours. The quartz grains start being smeared out and reduce in size but a feldspar grain doesn’t and so becomes a porphyroclast.

Note the difference with porphyroblasts which grow bigger than their neighbours, instead porphyroclasts stay big while everything around them gets small, (a bit like Gloria Swanson in Sunset Boulevard).

Rotated porphyroclast, borrowed from http://shearsensibility.blogspot.com

Deformation in shear-zones is often asymmetrical (simple rather than pure shear), which means there is an element of rotation to the deformation. Sitting in the middle of this, porphyroclasts sometimes get rotated, which makes them useful shear-sense indicators.

Phenocryst, porphyroblast or porphyroclast? A case-study from my kitchen

In my kitchen I have a ‘granite’ worktop which contains large feldspar grains that are generally bigger than the surrounding minerals (not dramatically so, but bear with me). Ask the man who fitted it and he would tell you that he sold me some granite, which means that these large crystals must be phenocrysts, usually minerals that crystallise out from the magma first, making a porphyritic rock. If the phenocrysts clump together then the rock is glomeroporphyritic which is my second favourite geological word. Try saying glomeroporphyritic three times fast: good test of sobriety I reckon, if your tongue can cope with that you are safe to operate heavy machinery.

'Granite' from my kitchen. Big feldspar grain in centre of picture. Tip of garlic bulb top centre for scale

This may be granite to an architect but to a geologist it is not. It is not even a granitoid as it is a metamorphic rock, an orthogneiss of some sort. Its full of garnet, both as large grains and as inclusions with the feldspar. In places the feldspar shows evidence of multiple phases of growth. So, are these feldspars porphyroblasts perhaps?

Maybe, but rocks are three dimensional and I’ve only shown an image from the top of slab. What about the sides?

Side of slab showing intense metamorphic fabric with wrapped feldspar grains. Anthropomorphic toy train for scale

Well that’s a rather different picture. There is a fairly strong gneissic texture within these rocks; so these are porphyroclasts then?

These feldspars in my work surface, are they phenocrysts, porphyroblasts or porphyroclasts? On balance I think a bit of all three and therefore none of them. It seems likely to me that these were originally large feldspar grains within the original granite and so phenocrysts. There is evidence of recrystallisation which may have made them bigger, but not necessarily much bigger. Looking at the ends of the slab there are feldspar grains wrapped by the fabric so they’ve been deformed like porphyroclasts, but there’s been a some subsequent annealing so there isn’t a particularly dramatic contrast in grain size.

So try as I might, I can’t accurately use any of these lovely words to describe my work-top, but at least it’s still good to make pastry on.

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About Metageologist

Simon Wellings trained as a Geologist but professionally has metamorphosed into something else. He retains a keen interest in Geology, facilitated mainly by the wonders of the Internet. Simon now blogs at Metageologist.
Categories: Rocks & minerals
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