There are lots of places where granite forms dikes and sills in other rocks. The ‘other rocks’, as far as I can tell, are typically other igneous rocks, or metamorphic rocks of either sedimentary or igneous origin. I get that the formation environment for granites would typically result in metamorphism of the surrounding rock. But are there any locations where granite intrusions into plain old sedimentary rock are found? Or, lacking that, any spots where the surrounding metamorphic rock is clearly sedimentary in origin, like, I dunno, marble or quartzite or slate or something?
Or is it the case that you can’t get granite and plain old sedimentary rock together, like you can’t get a diamond forming in a bowl of raw egg whites?
The subduction of the Farralon plate stretched the overriding lithosphere allowing magma to rise through cracks in the North American plate. This is readily visible in southern Arizona – south of the Colorado Plateau – which is uplifted former seabottom with intruding basalt and metamorphic mountains.
The Sydney Basin, largely covered by modern Sydney, Australia, is a flattish sandstone plateau, some kilometres deep, and the Blue Mountains area is part of the same thing, uplifted and then canyoned to within an inch of its life. They are criss-crossed by dykes, although these are generally basaltic, if you specifically want granites.
Yeah, I’m specifically looking for granite intrusions. As I understand it, the magma that forms granite is chemically different and thus more viscous than basaltic magma, and therefore less easily injected up along a fault or along a plane. It also requires longer cooling times, I think, in a different thermal environment than basaltic intrusions. In the geological formations I’ve Googled, granite dikes and sills seem to be associated with other igneous rock, or with gneiss or schist where the pre-metamorphic source of the gneiss or schist is somewhat speculative because of the reworking.
That’s why I wondered if you can specifically get granite intrusions forming in rock that stays sedimentary, or, if metamorphic, is just contact metamorphism fading into sedimentary rock with distance from the granite. Or do granite intrusions only form under such high/long-term heat and pressure regimes that they’re always in igneous or thoroughly metamorphosed rock?
Ah, I misread the OP – I was thinking you were looking for igneous intrusions into sedimentary.
Metamorphic rocks are formed deep underground. If you’re finding them on the surface they were pushed up there. The Rocky Mountains, for example are pre-Cambrian rocks over a billion years old raised by the Laramide orogeny a mere 50-million years ago.
Fractured granite reservoirs are a thing, where the basement granite has been tilted and shifted up sideways and all ways and sits right next semi sedimentary rock, and above the source rock.
Have a look here and scroll down to the Rona ridge.
The north sea geology is basically lots of granite with either limestone, salts or devonian red sands on top, then a whe lot of other sediments on top of that then a whole lot of faulting and general geological mish mashing and moving going on . Add to that you can have sedimentary and subsequently metamorphosed rocks laid down on top of the basement, which are subsequently stripped away, deposited somewhere else and replaced by much younger sedimentary rocks directly on top of much older rocks. So there and pretty much most other places , finding wildly different rocks from very different ages right next to each other is not surprising.
It reads to me like what you’re describing is exactly my local geology - the Wicklow mountains are formed from a granite batholith that displaced existing shales and slates and burned some of them into schist at the contact planes.
Bit late to the party, but the answer is a definite “Yes”. One such site is here in Cape Town where granite intrudes what is very clearly sedimentary shale. Darwin found it quite illuminating.
The Serengeti is notable for those interesting looking rocks that poke up and create small “islands” (kopjes) in the plain. However, reading further I find the overlaying soil and rock is mostly volcanic.
However…
West of the line Mugumu - Seronera the underlying rocks are ancient (600 million to 2.5 billion years) and comprises Precambrian volcanic rocks, banded ironstones and mineral-poor granites. Late Precambrian sedimentary rocks cover this shield and form the central and southern hills. East of Seronera, granite and quartzite form the eastern hills and kopjes. The western corridor is of more recent geological history; it is a complex of unconsolidated sediments and alluvial formations, which form the base for more nutrient-rich soils.
