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Bulletin of the Geological Society of America Vol. 66 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 66. PP. 405-426 APRIL 1956 DISTRIBUTION OF IGNEOUS ROCKS IN SPACE AND TIME By G. W. TYRRELL ABSTRACT Consanguinity in an assemblage of rocks, associated in time and place, is held to indi- cate that it has been derived by a more or less uniform geological process from a common initial magma. The most fundamental grouping of igneous rocks is thus based on distribu- tional relations, which are in turn dependent on geological process and event. The close association of igneous activity with earth movement is well known. The repetition of petrographic "provinces" or "kindreds" in time and place is clearly a mani- festation of the cyclic character of tectonic events in geological history. The tectono- igneous cycle is elaborated and discussed with special reference to Scotland and North- west Europe, and comparisons are made with eastern North America and Indonesia. CONTENTS TEXT Page Page Plutonic and volcanic associations 417 Introduction 405 Nature and origin of igneous cycles 420 Consanguinity 406 References cited 424 Kindreds of igneous rocks 407 Petrographical provinces and periods 408 TABLES Igneous action and tectonics 409 Table Page Geological cycle 409 1. Correlation of the .igneous and tectonic Diastrophic subdivision of the crust 409 phases of the Tertiary and Quaternary of Diastrophic (tectonic) cycle 410 Indonesia with those of the Paleozoic of Igneous cycle, with special reference to Scot- Scotland 418 land, Northwest Europe, and eastern North 2. Tectono-igneous cycle 420 America 412 3. Comparison of the igneous phases of the Quartz dolerite dikes and sills of North Britain. 415 Caledonian, Hercynian, and Alpine oro- Tectono-igneous cycle in Indonesia 416 genic cycles 421 INTRODUCTION think were characteristic igneous rocks have been relegated to the category of metamorphism My assignment today is to introduce the (plutonism in Read's sense) and with, I think, subject of your deliberations—The distribution some justification. With some reservations I of igneous rocks in space and time,—-a subject am disposed to accept Read's (1948, p. 3) which, while it concerns high-temperature classification of rocks as neptunic (mainly materials, will probably not generate so much marine sediments), volcanic (magmatic, igneous, heat as the symposium 4 years ago introduced dominantly effusive and basic in composition), by my friend Professor H. H. Read. It is, how- and plutonic (metamorphic, migmatitic and ever, a subject which has attracted much at- granitic, of deep-seated origin). The last- tention in the past 60 years and has given rise named group also includes, beside granite, to a voluminous literature. Nevertheless it many formerly described as plutonic igneous has not yet advanced far beyond the stage of rocks. fact collection, and the important generaliza- Because of the doubts about the origin of tions which I feel are inherent in this field on many plutonic rocks I am now disposed to rest the relations of igneous activity to tectonics generalizations on the distribution of igneous and its correlation with the geological cycle are rocks in space and time mainly on the un- still largely to be made. doubted igneous rocks which have clearly been In recent years many of what we used to derived from liquid or quasi-liquid magmas, 405 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/4/405/3426860/i0016-7606-66-4-405.pdf by guest on 27 September 2021 406 G. W. TYRRELL—IGNEOUS ROCKS IN SPACE AND TIME together with the intrusive rocks, sometimes of This view is confirmed from the etymological quite coarse textures ("phitonic"), which are point of view by my colleague Professor A. W. associated with them—that is, on what W. Q. Gomme1, to whom I applied for information Kennedy (Kennedy and Anderson, 1938, p. 25) as to what the term magma meant to the ancient has called the volcanic association. But I still Greeks. think that consideration of many of those types It is no answer to say that salt, clay, and now assigned to Read's plutonic group can certain metamorphic rocks must therefore be help us toward the above-desired end. regarded as magmatic because, under some By a curious coincidence some groups of circumstances, they seem to conform to the lavas have a very close chemical resemblance above definition. The writer maintains that to plutonic groups, as rhyolites with granites; rock-flowage materials can readily be dis- dacites, latites, and andesites with members tinguished from magmatic when the associated of the granodiorite - tonalite - quartz - diorite phenomena are taken into consideration. A group; trachytes with alkali syenites; phono- perfect transition must exist, and many Trans- lites with nepheline syenites; and basalts with formists are willing to admit it, between just- gabbros. I cannot help thinking that in at mobilized granitic material and granitic magma least some cases there is a genetic connection which might consolidate as a granitic vein or as between lavas and the corresponding plutonic a flow of rhyolite; but we are not called upon rocks. And this leads me to question the com- to fix an arbitrary boundary between mobilized plete divorce in geology, petrology, and genesis material and magmatic material any more than of the volcanic from the plutonic rocks ad- we are bound to fix a point at which white ends vocated by Kennedy (Kennedy and Anderson, and black begins in the perfect transition of 1938, p. 31) and Read (1948, pp. 3-4). It is shades of gray which intervenes between white true that both authors concede that certain and black. Common sense applied to the ob- intrusive dolerites, gabbros, and other "plu- servation of relevant facts must decide such tonic" rocks may sometimes be included in issues, and there is room for different opinions. volcanic associations. Discussion as to what are and what are not CONSANGUINITY igneous rocks has been hampered by an un- necessarily restricted definition of magma as It is a generally accepted fact that certain "completely fluid rock substance" (Read, 1948, groups of igneous rocks show various degrees of p. 5), and F. F. Grout (1948, p. 46) has rightly family likeness (Judd) or consanguinity (Id- animadverted upon it. If this definition is ac- dings). This community of character is ex- cepted literally, practically the only truly pressed in their chemical, mineral, textural, magmatic rocks are the pure glasses like ob- and geological features. While consanguineous sidian. When examined after sudden quenching suites may range in chemical composition from even the basaltic liquids of the lava lakes acid to ultrabasic, may exhibit many kinds of (e.g., Halemaumau) are seen to have had a textures, and may consist of extrusions or in- few crystals in them, and there are plenty of trusions or both, some mineral and chemical instances of both effusive and intrusive action characters, which may be described as con- in which the magmas concerned were rich in stant, are common to practically all members. crystals. The writer knows of instances of Other characters are serial and show a regular the Lower Carboniferous plagioclase basalts variation throughout the group which may be (Markle type) of the Midland Valley of Scot- illustrated by suitable graphs. Thus, in some land in which the feldspar crystals exceed 50 1 Professor of Greek, University of Glasgow. per cent by volume of the rocks, and are some- Professor Gomme says tie word comes from a root meaning to knead, especially to knead in a mould times so clotted together as to simulate an as, for example, a kind of barley cake which was anorthosite, but which yet form lavas and small not baked but eaten in the soft state. The past tense (kneaded) was also used of a poultice. The intrusions. It would be more accurate to say term magma itself is actually found only in some that a magma is a mush of liquid and crystals Greek medical literature for a thick unguent; and that is capable of effective intrusion or ex- Webster's New International Dictionary, 1st. Ed. (1932 issue, p. 1295) confirms that it is still trusion,—a material something like porridge in used in this sense, but only rarely, by pharmacolo- which crystals correspond to the oat grains. gists. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/4/405/3426860/i0016-7606-66-4-405.pdf by guest on 27 September 2021 CONSANGUINITY 407 suites, a constant chemical character is over- kindred is here used for the widest and most saturation with silica; highly silica ted feld- tenuous kind of family likeness as that, for spars and quartz appear in small quantity even example, between the members of the alkalic in some of the basic members. Regular varia- or calcic kindreds. The writer further suggested tions of silica, alkalis, ferrous iron, magnesia, that the progress of research might reveal that and lime may serve as examples of serial char- kindreds included tribes, the rocks of which acters. My late friend and colleague, M. A. would show a closer relationship than those of Peacock, indeed founded a very successful kindreds; and that tribes might be found to be criterion for the discrimination and comparison composed of clans, the members of which would of igneous series on the "lime index"-—a rela- show the highest degree of consanguinity of tion between silica, alkalis, and lime. Some all. Both the alkalic and calcic kindreds, if in- suites or series may be characterized through- deed they are true kindreds, probably contain out by a peculiar mineralogical feature, such several tribes and clans. as the occurrence of anorthoclase in Norwegian, By their nature kindreds, tribes, and clans East African, and Antarctic suites, or the oc- cut across the more formal groupings and classi- currence of orthorhombic pyroxenes in even fications, and certain abundant rock types such the acid members of the so-called charnockite as basalt, andesite, and "granite" (if I may series.
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