The story so far...

Some 200 million years later, the subduction zone had jumped further southwest, cooking the sediments in Bergslagen and producing the other ore district in Sweden, that helped us become a superpower in the 17th century.

In mid-to-late Proterozoic time (1.7-1.5Ga) came the last stage of crust formation in Sweden, as the southwest Swedish domain bubbled out of yet another subduction zone. This is the granitoid rocks of the Gothenburg area, the late-stage products of volcanic arc-magmatism, much like the Sierra Nevada in the USA.

Finally, the continent now known as North America once again bumped into Baltica and crinkled the Gothenburg rocks in the Sveconorwegian (or Grenvillian) orogeny, one billion years ago. After this, not much has happened to Sweden geologically. 

The Chalmers Polytechnic campus sits on two kinds of rock. On the geologic map above, the brown stuff indicates 1.6Ga (billion years) old gneisses. They are mostly granitic and have been folded and refolded in at least two deformation events. The green stuff on the map is the mafic intrusion that I have studied.

Geological map of Chalmers intrusion

Mafic rock is dark in colour and is made up of minerals such as pyroxene, hornblende, plagioclase, olivine and biotite. Granitic rock consists mostly of quartz, plagioclase and potassic feldspars, and some mica. Gneiss is a rock that has been under stress so that minerals are orientated along axes of deformation. It looks banded, in other words.

The mafic rock of the Chalmers intrusion is a gabbro. That is, it consists of mostly pyroxene and plagioclase and it was emplaced deep in the Earth's crust (that's why it's called an intrusion). If it had been buoyant enough to get to the Earth's surface and erupted as a lava, it would have been called basalt. The pyroxenes have been altered to hornblende in most cases, and the plagioclase laths look somewhat dissolved. However, there are no signs of these alterations having taken place in a low-temperature environment.

Looking at the field relationships between the gneiss and the gabbro, two things are worth mentioning. First, the edge of the intrusion is surrounded by mafic rock where the minerals are orientated. It looks like the gabbro was stressed during the second deformation event, but that only the outer parts of it were affected. Or, other mafic material was injected along the cleavage planes of the surrounding rock as the intrusion was emplaced.

Second, at the E-building one can observe the reason for why this intrusion was studied at all. Here, the heat from the intrusion has caused the surrounding gneiss to melt. This granitic melt has then re-intruded the gabbro. One can see that both kinds of rock were in a molten, plastic state from the round appearance of the contacts. There are mushroom-like structures of granitic rock in the gabbro. So these structures were formed at the time when the gabbro was emplaced. This is crucial for the rest of the study.