THE DISTRIBUTION OF IGNEOUS ROCKS IN SPACE AND TIME

 

CONSANGUINITY -  The term consanguinity (Iddings) is used to indicate the fact that certain groups of igneous rocks, the members of which are associated in space and time, possess a community of character or family likeness which is expressed in their chemical, mineralogical, textural, and geological features. While in chemical composition consanguineous series or suites may range from acid to ultrabasic types, some mineral and chemical characters are constant, i.e. are common to practically all members; while other characters are serial, that is to say, they show regular variation throughout the series. Thus, in some suites, a constant character is oversaturation with silica, which causes free silica to appear in quite basic members. A serial character may be afforded by the regular variation of the alkalis, or of ferrous iron oxide and magnesia throughout the suite. Some series may be characterised throughout by a peculiar mineralogical feature, such as the occurrence of anorthoclase, as in certain Norwegian, East Mrican, and Antarctic suites. Consanguinity in an igneous series leads to the hypothesis that the assemblage has been derived by some process of differentiation from a common initial magma, from a number of closely related magmas.

THB DIAGRAMMATIC REPRESENTATION OF IGNEOUS ROCK SERIES

The chemical and mineral relationships obtaining in a consanguineous series can be exhibited by suitable graphs and may thus be rendered in forms which permit rapid visualisation of their characters. Diagrams of this kind are called Variation·diagrams. The diagram which is most frequently used is one in which silica is plotted against the other oxides in such a way that an analysis is represented by a series of points on a vertical line. By joining up the points for each oxide in a series of analyses, curves are obtained which show graphically the variation of each constituent with regard to silica. As the amount of silica constitutes a rough index of the stage of differentiation a rock has reached, the curves collectively present a picture of the course of differentiation which has produced the rock series under examination, as well as a succinct statement oits chemical characters.

                     Fig 1 – Silica variation diagram of Granite – Diorite series

Above Fig. illustrates a typical variation diagram, obtained by plotting analyses 2, 3, 4. and 5 in following Table (A) and 2 in Table (B). The chemical composition of rocks dealt with in this section is illustrated by the selected analyses given in Table

Table (B)

Together these analyses represent a very common type of igneous rock series which ranges from granite to gabbro through the intermediate stages of tonalite, quartz-diorite, and diorite. The curves are fairly regular and require very little smoothing, indicating that the analyses plotted have real serial relations. The crosses indicate the points for the analysis of quartz-gabbro (Table B, I). The aberrant position of these points shows that this rock is alien to the series; and if included it would seriously disturb the regularity of the curves. Nepheline-syenite the analysis of which is represented by circles, is clearly an even more discordant rock in this series than quartz gabbro.

The curves for Al2O3 Cao, FeO, and MgO, all bend downwards to the right of the diagram, indicating a decrease in these constituents as the acid end of the series is approached. These constituents are said to vary sympathetically, the soda and potash curves, however, bend upward to the right. indicating an increase in these constituents in the acid members of the series. They vary sympathetically with each other, but are in antipathetic relation to Al2O3, CaO, FeO, and MgO.

The curves for Al2O3 Cao, FeO, and MgO, all bend downwards to the right of the diagram, indicating a decrease in these constituents as the acid end of the series is approached. These constituents are said to vary sympathetically, the soda and potash curves, however, bend upward to the right. indicating an increase in these constituents in the acid members of the series. They vary sympathetically with each other, but are in antipathetic relation to Al2O3, CaO, FeO, and MgO. diagram of the average granite-gabbro series, with the percentage of Salic minerals substituted for the silica percentage is given in Fig.2. The curves follow much the same courses as in Fig.1,but are more regular, although the improvement is not so apparent in this series as in many others.

In this diagram is also shown a curve of the silica number, which represents the excess or defect of silica in molecular values, in respect to the amount which is required just to saturate the rock. It is obtained from the calculation of the norm. Free quartz computed in molecules gives the excess silica, while deficits are calculated from the amounts of olivine and feldspathoids in the norm. In the series illustrated by Fig (2), only the gabbro gives a deficit of silica. The curve cuts the base-line, i.e. the silica number is zero, at the point representing 65 per cent. of felsic minerals. A perpendicular erected at
this point may be called the saturation line. The points at which it cuts the curves gives the approximate composition of the justsaturated rock of the series under consideration, which may be called the saturation composition. The position of the saturation line, and the saturation composition, vary in different (Figs. 45,46), and are thus of diagnostic value. For a discussion of other type of variation diagram the work of Harker and Holmes cited above should be consulted.

KINDREDS OF IGNEOUS ROCKS - Groups, series, or suites of igneous rocks which show consanguineous chemical and mineral characters, and which appear to be genetically related, may be called kindreds. Several terms have been used in closely similar senses, as series, suite, tribe, clan, branch, stem (German stamm). The relationship implied by consanguinity may be of different degrees of closeness. The term kindred may be used for the widest kind of grouping in which the consanguinity between the rocks is of the most tenuous kind. Within kindred may be found tribes, the rocks of which show a closer relationship; and within tribes, clans, with a very high degree of consanguinity. Kindreds, tribes, and clans, cut across formal classifications; and certain abundant rock types, such as basalt, andesite, and granite, may recur in several different groupings. Although it may be composed of a wide diversity of rock types, kindred are marked first of all by a certain community of chemical and mineral characters, which may be exhibited in constant or serial relations. It may befurther characterised by the different bulk-development of its component rock types. Thus, in certain kindreds andesites are the most abundant rock types, and basalt is a subordinate member i whilst in others, basalts predominate greatly over andesites. The degree of differentiation is another factor whereby kindreds may be distinguished. Some kindreds consist of huge masses of uniform rocks (e.g. the basalt floods) i others contain a great diversity of types within a small compass. Furthermore, there are differences among kindreds in regard tothe relative development of plutonic, hypabyssal, and volcanic facies, at least in the accessible crust. This may, of course, be due to accidents of erosion and exposure, but it is believed that there are also intrinsic differencesinthis respect. Finally, kindreds are distinguishable by their extension in space and time (petrographic provinces  and periods) and by their varying relations to geotectonic processes. These topics will be dealt with in later sections.

A very wide and vague, but nevertheless valid, grouping of igneous rocks, is that into alkalic and calcic (=subalkalic, calc-alkalic), which has been much exploited in the past. It is probable that both terms cover several distinct kindreds, and that some kindreds pass over the gradational boundary between the two groups. The terms alkalic and calcic, nevertheless express a real tendency in igneous rocks for the separation of two broadlycontrasted assemblages, which have different chemical, mineral, geographical, and geotectonicrelations. The mineralogical contrast may be expressed as below 

Chemically, alkalic rocks are characterised by high percentages of alkalis in relation to silica and alumina. In calcic rocks the ratio of alkali to silica and alumina is not so high, and constituents such as lime and the ferromagnesian oxides are relatively more abundant. In alkalic kindreds saturation occurs at a relatively high silica percentage and felsic/mafic ratio; whereas in calcic kindreds the saturation composition falls at a low silica percentage, and a comparatively small felsic/mafic ratio.

PETROGRAHIC PROVINCS AND PERIODS – Province – Area Period - Time

A petrographic province is the geographical extension of a kindred, and a petrographic period is likewise its extension in time_ The first of these terms was proposed by J. W. Judd in the following words: There are distinct petrographical provinces within which the rocks erupted during any particular geological period present certain well-marked peculiarities in mineralogical composition and microscopical structure, serving at once to distinguish them from the rocks belonging to the same general group, which were simultaneously erupted in other petrographical provinces.

A. Harker's definition is that a petrographic province is a more or less clearly defined tract within which the igneous rocks belonging to a given period of igneous activity, present a certain community of petrographical character traceable throughout all their diversity, or at least obscured only in some of the more extreme members of the assemblage. In both these definitions the phrase italicised insists on a time limitation. Many objections to the idea of petrographical provinces have been based on the mistaken view that the term applied to all the igneous rocks within a given region regardless of their ages.

The boundaries of petrographically provinces and periods are vague and ill-defined because the true spatial and temporal associations of a kindred are with a certain geological environment. Hence the modern tendency is to place much more emphasis on the nature of the kindred itself, and on its connection with a definite tectonic process, than on its mere geographical extension. For this reason, the geographical names which have been applied alike to petrographic provinces and to kindreds, such as Atlantic for the alkalic kindreds, Pacific for the calcalkalic kindreds, and the later terms, such as Arctic and Mediterranean, are to be deprecated as useless and misleading. It is nevertheless true that, for igneous rocks erupted within a given period of magmatic activity, large areas of the earth's surface can be mapped out into more or less definite provinces in which different kindreds hold away. The continued use of the term petrographic province it therefore justified. Similarly. The term petrographic period may be used to indicate that petrographic provinces have a more or less definite extension in time as well as in space.