An inclusion of culm grit in coarse granite (1909)


Author(s): Hunt. Arthur R. Origin: DA Transactions
Topic(s): geology Year published: 1909
Location(s): Lustleigh Pages: 320-323
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By Arthur R. Hunt. (Read at Launceston, 29th July, 1909.)

ROCK inclusions in granite are well known to geologists, and may not seem to call for much notice; but nevertheless I venture to record the following occurrences :—

On the 13th of last March, while on our way from Lustleigh Station to Foxworthy, by the Cleave, Mrs. Hunt called my attention to an inclusion in a block of granite lying by the roadside for road-making purposes. The place was between Way Farm and Hammerslake, and just about the 569·1 bench mark on the six-inch map. The stone was obtained from the adjoining field on the eastern side of the road. On my return from Foxworthy I brought with me a geological hammer and detached the inclusion, with part of the granite adhering to it, and took a further sample of the same.

The general shape of the inclusion was just that of a rhomboidal lump of culm slate or grit, as it might now break under the hammer. Under a pocket lens the stone itself seemed perfectly crystalline. The two main pieces of the stone as broken weighed together about one and a half pounds, and a little less than an ounce.

Showing a splinter to Mr. A. Somervail, with no information as to locality or environment, he said he should suppose it to be a culm grit in actual contact with the granite, but observed that he had never seen quite so crystalline a specimen. This was an extremely acute diagnosis.

Having had a section prepared, Dr. Flett, F.G.S., of the Geological Survey, very kindly examined it for me. He wrote me that it was “an altered sediment . . . rich in fresh cordierite, with fine yellow pleochroic haloes around zircons enclosed”. He also pointed out that some of the cordierite has weathered to a green pinite, and that other constituents are colourless Andalusite in small grains, biotite, felspars, quartz, and a little blue tourmaline. Dr. Flett mentioned that “enclosures of this kind occur in all the West of England granites”, and he kindly sent me his paper, “First Notes on the Petrography of Western Cornwall”, from the Summary of Progress of the Geological Survey for 1903.

Dr. Flett points out that in the Land’s End the inclusions are typically rounded or ovoid, though sometimes sub-angular, that many are fragments of killas which have suffered considerable resorption, while others appear to be early basic segregations.

Rounded segregations are very common in the Lustleigh granites, and are occasionally found with the enclosing granite weathered away, leaving the segregation like a rounded pebble, on river bottom or roadside. In segregations the union between granite and inclusion is complete, and the two fracture as one stone. The peculiarity of the specimen under discussion is, that the union was very imperfect, and that the angular rhomboidal piece of sedimentary rock projected from the surface of the broken granite; and indeed detached itself with too much completeness for the obtaining a good contact section between the two rocks. I noticed another specimen, a triangular fragment, in another block of broken granite. The outlines of the included fragments seemed quite unaffected by the granite containing them.

The granite itself was of a coarse matrix, and contained very large crystals of orthoclase.

The field from which the road metal was taken is about two-thirds* of a mile from the nearest sedimentary rocks bordering the main granite. On the border, just five-sixths of a mile to the S.E., there is an interesting patch of greenstone in contact with the granite on the north, and apparently in contact with the culm rocks on other sides. In 1900 I found an elvan dyke some six inches wide (and then bifurcating) invading this greenstone at a point almost exactly four inches due south of Lustleigh Station railway bridge, as measured on the six-inch map. To the east, in a small quarry and in the cutting across the railway, the elvan sends small veins into the culm. The greenstone seems entirely altered, and at the time I examined it Dr. Teall informed me that it was an hornfels. Unless perchance the rock was originally a volcanic ash, it seems possible that the culm in places may have been first altered by the volcanic rock, and again long afterwards by the granite. Taken in connection with the included fragments recently observed, I just mention this patch of greenstone (originally made known to me by Mr. Ussher, F.G.S.), as an uncommonly intricate case of contacts and inclusions.

(*) Measured on the six-inch Ordnance Map.

Considering that the culm fragment must have been subjected to the slow cooling process of an extremely coarse granite, it is surprising to find that its external outline seems unaffected by either pressure or solution, and that the stratification laminae are also undistorted. Notwithstanding these facts every particle seems to have been recrystallized, with no remains left of original sand grains, or other sedimentary constituents.

As the planes of a rhomboidal fracture, or jointing, are just what one may often find at present in the culm rocks, it would appear that the planes of fracture must have been superinduced in the culm rocks before they were invaded by the granite. But, per contra, similar rhomboidal fracture is often to be seen in the elvans, which are presumably the most recent of the granites. They certainly invade the main mass occasionally. According to this the pressure must have been of long continuance.

It would be interesting to trace the stages of the crystallization of a culm grit from the unaltered rock to the fragments heated, and then very slowly cooled in the process of crystallizing in very coarse granite. Seeing that the greenstone is also a contact rock, enclosures of that rock might also occur, but at any rate we already have the greenstone completely altered by the granite throughout, yet showing no excess of alteration at the actual contacts which are sharply defined in microsections. I tried for liquid inclusions in the cases both of the elvan dyke contact with the greenstone and the granite contact with the culm-grit inclusion, but the slides cut were very free from quartz and also from liquid inclusions; but the indications of a low temperature in the neighbouring granite are so clear that the inquiry was scarcely worth the cost of additional sections.

I regret that my lack of knowledge of contact minerals has prevented my even attempting to do my subject justice.

On 11 June, I made a further search among the same blocks of granite for inclusions. One, I noticed, was a perfect oval; another was much affected by solution, and was invaded by tourmaline; while a third appeared to have been almost dissolved away and invaded by large orthoclases. Against all these specimens, showing solution, was one, a good equilateral triangle, as exposed on the face of the granite that enclosed it. The variety of the forms of these inclusions invites further inquiry. It is suggested that a fragment with weathered boundaries, when caught up by the granite, might resist its solvent action more effectually than a fragment freshly broken.