Quote:
Originally Posted by renormalised
Looks like a very heavily weathered quartz/biotite porphyry. It's not a conglomerate....at least not one I've seen like that. The grains are too angular to have been transported by water and worn. Plus the presence of biotite mica would preclude that anyway (it quickly breaks up and dissolves away when transported). The red colouration is due to the weathering of the biotite and the feldspars in the matrix of the rock (both contain Fe...feldspars, especially plagioclase, have small amounts as inclusions, biotite is Fe mica and has quite a lot). The feldspars are the red material you see, the Fe is a patina on the surface of the crystals.
That you found it in clay suggests that this rock was weathered in situ, with little or no transportation. It's probably what's left of a rather large lump of granitic material that has weathered to clays, which can be quite common in many weathering regimes where you get alternating periods of wet and dry episodes.
I'm curious...do the clays you find this rock in follow any geological boundary or are comprised of linear features within the surrounding rocks?? The reason why I asked is because this could also be a rock they call a fault breccia (a cataclasite). The weathering to clays of these rocks commonly occurs along faults.
|
Thanks for that Carl.
The area is elevated above a lava plain but there are no definite geological boundaries such as vertical faces. I'm not sure how far the faultline extends. Very obvious faultline features occur about 3 km away. Whether the faultline extends into the area I don't know.
The area is heavily forrested so it's difficult to access the geology of the region. There are some isolated bare patches where a metal detector search could be conducted.
It's very unusual for a metal detector to go off under these conditions. I live in the same area where the lava plains occur where metal detectors don't react to Fe rich basaltic rock.
That puzzles me. I wonder if a cross section to the rock will provide some answers.
Regards
Steven