Xenoliths of Igneous Origin in the Trégastel-Ploumanac'h Granite, Côtes du Nord, France

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Abstract
I. I ntroduction I n the northern coastal region of Brittany, in the neighbourhood of Trégastel and Ploumanac'h (PL XVIII), to the north of Lannion, a red, mainly porphyritic, granite is admirably exposed in shore and inland sections, and is remarkable for the number of xenoliths which it contains, of a kind usually referred to as basic segregations or basic patches. Especially clear evidence is available to prove that these inclusions have had their origin in hybrid rocks formed from an earlier intrusion of olivine-norite. In view of the similarity of the xenoliths to basic patches that occur in other granites where evidence of their manner of formation is not so clear, it has been thought desirable to place our observations upon record. The red granite of Trégastel and Ploumanac'h forms the outer and oldest member of a complex that consists of several distinct intrusions of granite and occupies the country between Ploumanac'h and Trébeurden, an area of some 40 square miles. It was regarded by Prof. C. Barrois (1903) as being of Devonian age since it has escaped the movements and foliation which have affected the older rocks with which it is in contact, but the more recent explanation of the Geological Survey map places its intrusion at the beginning of the Carboniferous. The Geological Survey map shows this granite-complex asconsisting of three granitic masses. A centrally situated ‘granulite ’ in the He Grande and neighbouring mainland; an outer ‘granite porphyroïde rose ’ at Ploumanac'h; and, between these two,

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