The Carnmenellis Granite: Its Petrology, Metamorphism and Tectonics

Abstract

I. Introduction The Carnmenellis granite and its metamorphic aureole were mapped, in 1839, by de la Beche. The area has since been re-surveyed by the Geological Survey and the results recorded in the one-inch maps, nos. 352 and 359 (New Series) and in the Survey Memoir on the Falmouth and Camborne district (Hill & MacAlister, 1906). The granite is an important unit in the chain of post-Carboniferous granite intrusions forming the backbone of the peninsula of Cornwall and Devon. The consanguinity of these intrusions has long been known, but the recognition of the composite character of individual intrusions is comparatively recent. Intrusive relationship between major composite masses was observed on Dartmoor by the members of the Geological Survey (Reid & others, 1912), Brammall (1923), and Osman (1928), and on Bodmin Moor by myself (1927). Intrusion sequences are discussed by Brammall (1923 & 1932) and Osman (1928). Important petrological contrasts between the Devonian and the Cornish granites were pointed out by Flett and Dewey (Reid & others, 1912), and problems of variation within individual complexes have since been made the basis of detailed study by Osman with special reference to the Dartmoor and Scilly Isles masses, by Richardson (1923) dealing with the St. Austell granite, and by myself in a paper on the Bodmin Moor granite (1927), and the Carnmenellis granite, the subject of the present contribution. I would add that, subsequent to the presentation of the paper to the Society, I have had the advantage of re-examining my analytical data

The distribution of the rare earth elements within the Carnmenellis pluton, Cornwall

“…The Carnmenellis pluton was originally mapped in detail by Ghosh (1934) and is largely composed of porphyritic biotite granites into which two bosses of finegrained granite have been intruded. The biotite granites are cut by microgranite sheets, typically less than 50 cm in width, which have been emplaced into fracture planes within the pluton.…”

“…The biotite granites are cut by microgranite sheets, typically less than 50 cm in width, which have been emplaced into fracture planes within the pluton. Ghosh (1934) identified three phases of porphyritic biotite granites which he labelled Types I, II, and II1 ( fig. 1).…”

“…Ghosh distinguished two phases of coarse-grained granite in the field on textural criteria (Types I and II) but more recent authors (Chayes, 1955;A1 Turki and Stone, 1978) conclude that they are modally and chemically identical, and should not be regarded as separate units. However, the nomenclature of Ghosh (1934) will be used in this paper as it is based upon regional mapping of the pluton. group.…”

Petrological, geochemical and geochronological characterisation of heat-producing granites

“…2.6a) were described by Ghosh (1934), but work by Al-Turki and Stone (1978) documented only minor differences (Rb concentration and biotite abundance) between Type 1 and Type II, subsequently known as the outer granite. The inner granite (Type III) contains more K, Rb and Zn and less Ti, Fe, Mg, Ca and Zr than the outer granite and is viewed as a separate, later intrusive phase (Al-Turki and Stone, 1978).…”

“…Early work on the Carnmenellis pluton classified three granite types (Ghosh, 1934), but later work concluded the only differences between Type 1 and Type II were slight variations in Rb concentration and biotite abundance (Al-Turki and Stone, 1978). Work on composite granites has informed the idea that pluton emplacement occurs through a series of magma pulses Glazner et al, 2004;Miller, 2008), and in Cornwall samples were collected from the Type I (CBM1, Carnsew Quarry) and Type II granites (RQ1, Rosemanowes Quarry; Fig.…”