#migmatite

Latent Heat and the Lower Crust

When continents collide, sedimentary rocks are shoved deep into the lower crust, where they are squeezed and heated and turned into metamorphic rocks. This rock shows basically the upper limit of metamorphism; the rock is no longer solid and I don’t know whether to call this metamorphic or igneous. This rock type is called a migmatite: it has crossed its melting temperature and part of the rock has melted to form light colored bands that separated from the still-solid dark colored bands. This rock type is common in metamorphic terranes and it occurs when you take some of the most common minerals on Earth, including Quartz, Plagioclase, and K-Feldspar, up to a temperature of about 650℃, a temperature we’d consider part of the Amphibolite Facies. Those minerals are found all the time in sedimentary rocks, including shales and sandstones, so when rocks are thrust to the bottom of continents in mountain ranges, migmatites should be common. In fact, new research suggests that this rock type imposes a fundamental constraint on metamorphism, limiting the upper temperature of the crust.

If you had a high school chemistry class, you probably heard of the concept of Latent Heat. If you take a phase, such as water or ice and add heat to it until it undergoes a phase transition, that phase stops heating up and stays at a constant temperature until the phase transition has finished. Ice water (at 1 atmosphere), for example, is fixed at 0℃ until the ice melts, while boiling water is fixed at 100℃. Rocks are more complicated because they’re not just a single mineral, but the rule is the same; when the rocks start melting, they get stuck very close to a single temperature, because any extra heat that comes into the rock goes into latent heat rather than raising the temperature.

On the other hand, granitic rocks may not melt at the same temperature as a sedimentary rock, because the melting temperature of a dry granite is higher than the melting temperature of fine-grained sedimentary rocks. Even though the melting temperature of these rocks is a fairly narrow range, they are common globally, because once the rocks start melting, generating that melt locks the temperature where it starts until the whole rock has melted away.

If Facebook doesn’t reduce the image size too much, you can zoom in on this photo and see that there are some nice garnets forming in the darker layers. If it’s too reduced, click through to the original image at the link below.

-JBB

Image credit: https://flic.kr/p/8Rm94D

Palisades

This photo shows some of the highest mountains in the Sierra Nevada of California – the Palisades. The rocks you see at the heart of these peaks are metamorphic; most of the Sierra Nevada, including the surrounding peaks are often granite but these are not. They started their lives as sedimentary rocks formed off the western coast of Laurentia, the continent that we call California today. These sediments were caught in-between the granite plutons as the granite that forms the heart of the Sierra Nevada. As the big blobs of magma intruded, these sediments were heated, stretched, and even sometimes melted on their own. If you zoom in on this photo, you can see strands of igneous rock called dikes, some of them probably 10 meters wide, that cut across these metamorphic rocks. Those are often formed by the metamorphic rocks themselves melting from the surrounding heat.

These metamorphic rocks weather in different patterns from the surrounding granite. Here it has created a steep wall popular with mountain climbers

The white in the foreground is actually ice, from a rapidly melting glacier on the slopes of the palisades. A few decades ago it covered the entire bowl at the center of this view; today due to climate warming it has broken into smaller pieces, and those are unlikely to last long.

-JBB

Image credit: https://flic.kr/p/29c3dex

A granite is born

At first glance these rocks snapped while wandering around some old mountain roots in the Sierras de Cordoba in Argentina don't seem like much, but they record the moment in high grade metamorphism when old rocks turn into new, with nothing but a few zircons left behind to testify to their previous history. The first photo shows a mass of fine-ish grained 'baby' granite, and floating within it a large rounded chunk (in the rough centre and a few attendant blebs) of the grey and white banded gneiss from which it formed, that was still floating unmelted in the magma when it froze (and by then probably some distance from its source rock). In the second photo the act of birth is even more intimately caught. The chunks of gneiss are picked out with a white outline of fresh quartz rich melt, also frozen in place. These rocks are called migmatites and mark a generational turnover in the rock cycle, when deep crust melts (usually during a continental collision and mountain building moment), rises and freezes, becoming shallower, younger crust, potentially susceptible to uncovering and erosion into sediments, whose burial unto melting depth will start the whole cycle anew...

The first photo is of a boulder in a river bed, the area in the photo is maybe a metre squared, the second boulder is about a metre in the longest dimension.

Scandinavian Crust

This is an absolutely spectacular shot of several stages of crust formation from Kosterhavet National Park in Sweden.

The oldest rocks in this area are highly metamorphosed sedimentary rocks. They are the dark and white, wavy-interfingered rocks. Those rocks started off as sediments and then were caught up in one of several orogenies, or mountain building events, that occurred between 1 and 2 billion years ago.

The sediments are 1.7 billion years old. They were deposited in a basin and then rapidly incorporated in a growing mountain range where they were buried and heated. The heat at the bottom of the mountain range was enough that the rocks began melting. The dark part is metamorphosed gneiss that reached the upper part of the amphibolite facies; the light part is felsic minerals that melted out of the sediments and basically formed a granite. This type of high grade metamorphic rock, heated to such a high grade that they begin melting, is called a migmatite.

This migmatite is cross-cut by a dolerite dike. Dolerite is a term for a mafic, basaltic igneous rock that has slightly crystallized. Basalts are low viscosity and high temperature, so if basalt is trapped underground in a dike it starts to form tiny crystals. This slightly crystalline rock is called a dolerite, or a diabase elsewhere in the world.

Sweden represents at least 2 characterized orogenies, the Gothian orogeny starting about 1.7 billion years ago and the Sveconorwegian Orogeny from about 1.3 to 0.9 billion years ago. These orogenies probably represent continued subduction along the coast of Baltica. This long-lived subduction built mountain ranges, sent magmatic rocks like the dolerite from the mantle up into the crust, and eventually culminated in the assembly of the supercontinent Rodinia.

-JBB

Image credit: Thomas Eliasson of Geological Survey of Sweden https://flic.kr/p/8PadbZ

Hammer of the gods, will drive our ships to new land

This amazing golden view was captured in Valhalla Provincial Park, British Columbia, Canada, last fall.

Take a close look at the golden rocks and you’ll see a neat geologic pattern. These rocks are metamorphic – literally gneisses and migmatites – and you can see the texture of light and dark foliation even in this shot.

The rocks of the Valhalla Gneiss Complex first formed during the Cretaceous period as the Farallon Plate subducted beneath North America. That plate released water into the mantle that reduced the melting temperature of the mantle, triggering volcanism from what is today Mexico to Canada. These rocks began their life deep in the crust as a magma body, a mush zone of crystals that may have supplied volcanoes many kilometers above this.

That process also helped build the Rocky Mountains. Two large mountain building events accompanied this volcanism, creating areas where faults ran through the older sedimentary rocks and built them into highground. These rocks first formed about 70 million years ago and they were at there greatest depth of burial around 60 million years ago. At that time, they were so deeply buried that they began remelting, creating the interlocking, tiny channels of melt you can see in the metamorphic rock that was mostly solid – a rock type called a migmatite. Similar rocks may be found in the Himalayas today – even as part of Everest itself.

At some point, the processes along the Pacific Coastline of North America shifted. Instead of mountains growing, the Rockies began to collapse. Large areas of the mountain range pulled apart, exposing the once deeply buried magma chambers. This process pulled on and strained the rocks, bringing them up to the surface while they were still hot. Faults that move deep into the crust will find rocks that can flow rather than fracture. Flowing rocks will develop a final metamorphic fabric – a “foliation” defined by alignment of minerals in planes. The shear zone that brought these rocks up to the surface is found right in this creek today.

The shear granitic gneisses here today are now a popular site for rock climbing.

-JBB

Image credit: Dru! https://flic.kr/p/RGemr8