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Metasomatism and Intrusion in Jamaica

Published online by Cambridge University Press:  01 May 2009

C. T. Trechmann
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Extract

The nearest known pre-Cretaceous rocks to Jamaica are Jurassic in Western Cuba, and Carboniferous in Colombia, Venezuela, and Honduras, all about 500 miles away. The evolution of Jamaica seems to have been somewhat as follows. Andine (?) orogenesis on or near the site took place, accompanied by magmatic invasion; the rocks are nowhere seen “in situ”, but they have supplied pebbles of granite, granodiorite, porphyry, hornfels, vein stuff, and limestones, more or less foliated or marmorized, but not gneisses nor schists. There is no evidence that any of them are older than Cretaceous, but they go to form the oldest beds of the island, the massive Blue Mountain purple conglomerates whose composition varies from place to place and of which no natural base is seen. Interbedded with these conglomerates are fossiliferous shales and Rudist limestones of Upper Senonian to Maestrichtian age. Low down, however, there is in Eastern Jamaica a bed of algal and foraminiferal limestone, without Rudistae, which was partly torn up and incorporated as rolled pebbles in the conglomerate. Some of these pebbles contain Camerina dickersoni Palmer, a foraminifer of Upper Cretaceous age, so the pebbles are but little older than the conglomerate containing them. Laramide orogenesis took place and this series was invaded by granodiorite and other intrusions. The uplifted area was eroded and some of the igneous and associated Cretaceous rocks exposed to denudation. Depression occurred and Lower or Lower Middle Eocene Carbonaceous shale or Richmond Beds, a Flysch-like series of limestones, shales, and conglomerates, were deposited.

Type
Articles
Information
Geological Magazine , Volume 79 , Issue 3 , June 1942 , pp. 161 - 178
Copyright
Copyright © Cambridge University Press 1942

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References

1 Kindly identified by Professor Rutten, L. M..Google Scholar

1 Abstr. Proc. Geol Soc., No. 1373, 20th 07, 1940, p. 100.Google Scholar

2 Report of the Government Geol Department, Supplement to the Jamaica Gazette, vol. xlviii, No. 5, 27th 03, 1925, p. 33.Google Scholar

3 Geohgie en Mijnbouw, Jaarg. No. 5, Mei, 1939, pp. 128–133.Google Scholar

1 If these observations, and mine in Jamaica, are valid, one may ask whether such a rock as that forming the spine of Mt. Pelée was ever hotter than at the moment of extrusion or whether it need ever have been a molten magna. The ash of the nuée ardente of 1902 when it falls against glass objects merely melts its grains into the surface causing a local devitrification and a perlitic-like cracking of the glass below. On the other hand on falling against argillaceous or porcelain objects it forms a molten glaze which tends to cover the felspar crystals and to spread over the adjacent surface. This points to some active gaseous agency in addition merely to the heated ashesGoogle Scholar

1 Quart. Journ. Geol., Soc., lxxxv, 1929, 458.Google Scholar

2 Geol. Mag., lxxiii, 1936, 261.Google Scholar

1 Dr. F. Raw postulates a long and complicated dynamic history for a specimen, of this rock, calling it originally a porphyritic-diorite-aplite extensively mylonized and recrystallized under plastic deformation, etc. Abstracts Proc. Geol. Soc. 20th 07, 1940, p. 106.Google Scholar

1 This mineral was kindly identified by the British Museum, Natural History.Google Scholar

2 It now appears that none of these rocks contain any glass. I am very sceptical about the assumption that micro- or cryptocrystalline structure in the ground-mass represents devitrified glass. Geol. Mag., lxxiv, 1937, 502Google Scholar

1 Dissolved out with acid, these were kindly determined by the British Museum, Natural History.Google Scholar

1 Dr. Matley's statement that these limestone inclusions are well down in the granodiorite is incorrect as the photograph will show. Abst. Proc. Geol. Soc., 20th 07, 1940, p. 105.Google Scholar

1 Abstracts Proc. Geol. Soc., No. 1379, 14th 06, 1941, p. 64.Google Scholar

1 Suess tells us that green rocks “never form the axis of a range ” Face of the Earth, vol. iv, p. 561. Their mode of formation in Jamaica may help to explain this fact.Google Scholar