A Note on the Origin of Basic Xenoliths in Plutonic Rocks, with special reference to their Grain-size

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Abstract
SUMMARY AND CONCLUSIONS: It has been suggested that the grain-size of coarse basic plutonic rocks may be reduced in one or all of three ways:— (1) If the temperature of the invading magma be sufficient, the reduction may be effected by the formation of a granoblastic structure. It is pointed out that recrystallized inclusions are found more often in basic plutonic masses, where the earlier crystalloblastic structures are not obliterated by superimposed hybridization. In 1933 (p. 156) the writer suggested that relict felspars with granular inclusions (or clouding) may be useful criteria of a recrystallization antedating hybridization. (2) The grain-size may be reduced by the development of highly poikilitic crystals of hornblende and/or biotite, and by the final disruption of these. Such crystals may be formed as poikiloblasts in the initial recrystallization; but more often they may be attributed to hybridization, and in this process hornblende plays an important rôle. (3) The grain-size may be reduced by the formation of granular masses of pyroxene or by criss-cross flakes of biotite during hybridization. Thomas and Campbell Smith (1932) and Nockolds (1932) have discussed the development of large felspars within the fine grained xenolith and its final incorporation in the acid magma, but it is not the purpose of the present paper to deal with this stage. It is hoped that this discussion may have indicated a means by which coarse basic plutonic rocks may become fine grained xenoliths, and it is finally urged that it is not always necessary to postulate a fine grained origin for these inclusions.

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