I. Introduction. Thirty-one years ago, Mr. George Barrow (1) presented to this Society his classic account of the intrusion phenomena of a group of ‘Older Granites’, in the Forfarshire area of the South-Eastern Highlands. That paper is distinguished by reason of the clear enunciation of the two outstanding conclusions to which the author was led. In the one was illustrated for the first time the effect of crustal stresses on the differentiation-course of igneous magma, a process which plays so important a part in present-day theories of petrogenesis; in the other, also for the first time, was indicated the subdivision of a group of highly-metamorphosed sediments into metamorphic zones characterized by the presence of critical index-minerals. So far as I am aware, his paper represents the first attempt in petrological literature to bring precision to the study of regional metamorphism, by laying upon a map zonal lines indicative of varying grades of metamorphism. It is chiefly upon this aspect of Barrow's work that I shall dwell in the matter now set forth. Nineteen years later (2) he extended the area of his zonal metamorphic map, at the same time increasing the number of zones. The map presented to the Geologists' Association thus covers the ground included in the quarter-inch sheet (No. 12) of the Geological Survey map of Scotland, extending to the Highland Border from Stonehaven to Dunkeld. In the meantime, other Scottish investigators by extended surveys of Perthshire and Argyllshire were bringing to light new data on the metamorphism
Summary The address is concerned with some aspects of magmatic evolution in volcanic successions, and includes particularly a discussion of the nature and genetic relations of primary and derivative basic and intermediate magmas. Among the volcanic associations of the oceanic areas, those of the Hawaiian archipelago provide data of critical importance in the study of basaltic evolution. The established chronological succession of magmas in these islands makes it probable that the alkali-poor tholeiites to be recognized in the lavas of the primitive shield volcanoes form the closest approach to the primary magma of the ocean basins, and from them derive by fractional crystallization processes the Hawaiian alkali magma series of alkali olivine-basalt, mugearite (so-called andesite) and trachyte. The course of volcanic variation there exhibited provides a new orientation to the study of the genetic evolution of the Tertiary igneous Brito-Icelandic province, and leads to the conception that the Hebridean alkali magma series derives through tholeiites as an alternative line of descent. The course of fractional crystallization of tholeiite magma in the great layered intrusion with its final product of granophyre is illustrative of the great flexibility in line of descent which fractionation processes permit. Derivation by crystallization differentiation in a convecting system comparable to that demonstrated in layered intrusions seems, indeed, the most likely mechanism for the genesis of the primitive sialic layer of the crust itself. The problem of intermediate magma in the volcanic successions of the orogenic belts is critical in petrogenic theory. Though fractionation processes appear most competent to give a liquid line of descent through andesite to rhyolite, normal andesite magma does not reflect in its chemistry the course of basaltic (tholeiitic) differentiation. Consideration is devoted to the conception that andesites, though ultimately of basaltic parentage, have derived in part by sialic contamination. The testing of the genetic relations of assemblages within the basalt-andesite range of the volcanic suites of the orogens remains one of the most important problems of volcanic petrogenesis. Systematic attack through intimate phase analysis and geochemical study may provide the data which will go far to solve, not only the status of the voluminous andesite, but of intermediate magma in general in igneous rock evolution.
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