F. H. (Frederick Henry) Hatch.

Text-book of petrology, containing a summary of the modern theories of petrogenesis, a description of the rock-forming minerals, and a synopsis of the chief types of the igneous rocks and their distribution as illustrated by the British Isles online

. (page 10 of 22)
Online LibraryF. H. (Frederick Henry) HatchText-book of petrology, containing a summary of the modern theories of petrogenesis, a description of the rock-forming minerals, and a synopsis of the chief types of the igneous rocks and their distribution as illustrated by the British Isles → online text (page 10 of 22)
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and soda-granites of the alkali series. It includes
most of the rocks known as biotite-granite and
hornblende-biotite-granite, which grade on the one
side into the granodiorites, and on the other into the




FIG. 75. Biotite-granite Shap (adamellite) composed of orthoclase
;Or.), plagioclase (PL), quartz (Q.), and hiotite (the dark mineral).
(From a photograph by R. H. RastalL)

1 Die Triadische Eruptionsfolge bei Predazzo, Vidensk. Skrifter
I. Math.-natur. Klasse, 1895, p. 61. The name adamellite was
used many years ago by Cuthrein for the rocks now known
as tonalite ; but the name never came into general use, and
there is, therefore, no valid objection to its being used in
Brogger's sense.



CHAP, i] THE PLUTONIC ROCKS 169

alkali-granites, according as lime-bearing plagioclase
or alkali-felspar predominates. For practical purposes
Lindgren's * useful suggestion may with a small
modification be adopted, namely, that a rock in
which less than one-third of the felspar is orthoclase
would then be classed with the granodiorites (or
with the diorites when the silica is less than 66 per
cent.), and one with more than two-thirds of ortho-
clase, with the granites (or syenites when SiO 2 is
less than 66 per cent.). Only rocks of which between
one-third and two-thirds of the total felspar is
orthoclase, would be referred to the adamellite (or
monzonite) family.

THE GRANODIORITE SUB -FAMILY.

This division of the granite family is characterized
by the great predominance of a lime-bearing plagio-
clase (basic oligoclase or andesine) and the subordin-
ation of alkali-silicates. Orthoclase is always present
in small quantity : it must not exceed one- third of
the total felspar content.

The name " granodiorite " was first proposed in
1892 by G. F. Becker in conjunction with H. W.
Turner 2 and W. Lindgren. 3 These authors have
defined the type as a light-grey granitic rock in which
soda-lime felspars (basic oligoclase or andesine) largely

1 W. Lindgren : Amer. Jonrn. of Sci., 1900, p. 269.

2 H. W. Turner : " The Rocks of the Sierra Nevada," 14th
Ann. Eep. U.S. Geol. Surv., 1894, p. 478 and 482.

3 W. Lindgren : Amer. Journ. of Sci. , Series 4, vol. iii., 1897,
p. 308, and 1900, Series 4, vol. ix., p. 269.



170 CLASSIFICATION [FART in

predominate over orthoclase, the remaining essential
constituents being quartz, biotite, and usually horn-
blende, together with sphene, apatite and magnetite.
It occurs in association with tonalite, diorito and
gabbro, and no doubt represents the extreme acid end
of the gabbro-diorite series. According to Lindgren,
the percentage of silica, which rises as high as 73 per
cent., may fall as low as 59 per cent., and he makes the
distinction from the more acid members of the diorite
family depend on the orthoclase. But most, if not all,
diorites contain some small quantity of orthoclase, and
it is better for practical purposes to make the percent-
age of silica the means of dividing the granodiorites
from the diorites, and to call no rock with less tha,n
66 per cent, of silica a granodiorite. The ferromag-
nesian minerals are bioiite, hornblende, and sometimes
augite, with sphene, apatite and magnetite as acces-
sory constituents. Some hornblende-biotite-granites
belong here. By decreasing silica the granodiorites
pass into the tonalites and quartz-diorites. Rocks of
this type occur abundantly in the Andes (Andes
granite} 1 and in the Sierras of California, but are
otherwise not of very common occurrence. They
occur possibly among the plutonic complexes of the
Central Highlands in association with tonalite, diorite
and gabbro, and also perhaps among the old plutonic
gneisses of the North- West Highlands. The chemical
composition of some typical granites of the adamellite
and granodiorite types is given on page 171.

1 O. Nordenskjold, Die krystallinische Gesteine der Magel-
lanslander, Svenska Exped. till MageUanslandern, Stockholm,
1901, vol. i., p. 175.



CHAP. l]



THE PLUTONIC ROCKS



171



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172



CLASSIFICATION



[PART in




FIG. 76.

Hornblende-granite (granodiorite).
f. Felspar (chiefly plagioclase).
q. Quartz. m. Mica.

h. Hornblende, with granules of Epidote.

THE SYENITE 1 FAMILY.

This large family of intermediate rocks is char-
acterized by the presence of one or more of the
alkali-felspars (orthoclase, perthite, soda-orthoclase,
anorthoclase, microcline), or of the felspathoids (neph-
eline, leucite, and sodalite), together with a subordi-

1 The term syenite was used by Pliny to designate a rock
occurring at Syene (Assouan), in Egypt. This rock is a horn-
blende-biotite-graiiite. Werner applied it to all granular
igneous rocks containing dominant felspars, with subordinate
amphibole, pyroxene and other minerals, and without noticeable
quartz.



CHAP, i] THE PLUTONIC ROCKS 173

nate soda-lime-felspar, and one or more of the following
ferro-magnesian constituents: amphibole (hornblende,
arfvedsonite, barkevikite), pyroxene (augite, segirine),
and mica (biotite, lepidomelane, etc.). Rarer types
are characterized by the presence of cancrinite, zircon,
eudialyte, katapleiite, or melanite. Among accessory
minerals, sphene, apatite, zircon, and the usual iron-
ores (magnetite, ilmenite, and haematite), deserve
especial mention. A rarer accessory is perovskite.

The typical structure of the syenites is the granitic.
When quartz is present it occasionally occurs in micro-
graphic intergrowth with orthoclase. The nepheline-
syenites show a tendency to the development of the
porphyritic structure, with large phenocrysts of
nepheline.

THE POTASH-SYENITE SUB-FAMILY.

The original type selected by Werner for the
syenites is the orthoclase-hornblende-rock of Plauen,
near Dresden. Werner's object was to establish a type
of felspathic igneous rock having a granitic structure,
but distinguished from granite by the absence of
noticeable quartz. The rock of Syene (Assouan), in
Egypt, from which Pliny derived the word syenite,
contains quartz, and is in fact a hornblende-biotite-
granite. The Plauen rock (plauenite of Brogger), on
account of its dominant hornblende, may be con-
veniently termed a hornblende-syenite, and the terms
augite- or biotite-syenite may be used for syenites
characterized by the combination orthoclase-augite
and orthoclase-biotite.

In these rocks there is always present, in sub-



174



CLASSIFICATION



[PART in



ordination to orthoclase, a member of the soda-lime
series (oligoclase or andesine). Quartz is not entirely
absent, as the microscopic examination of thin sections
shows. By the increase in this mineral the rocks pass
through quartz-syenite into potash-granite. The dark-
coloured minerals are often present in considerable
quantity, and then impart a dark tint to the rock.
The hornblende, which, with rare exceptions, is a
green variety, may be recognized by its pronounced
pleochroism, its well-marked cleavage, its prism angle

ANALYSES OF POTASH- SYENITES.





i.

Eeichenstein.


ii.

Plauen.


in.

Biella.


Average.


Si0 2


62-51


59-83


59-37


60-57


A1 2 3


12-78


16-85


17-92


15-85


Fe 2 3


2-56





6-77 )


8-23


FeO


4-70


7.01


2-02 1





MgO


3-3:3


2.61


1-83


2-59


CaO


4-76


4.43


4-16


4-44


Na 2


2-71


2-44


1-24


2-13


K 2


4-81


6-57


6-68


6-02


H 2


1-53


1-2D


0-38


1-06


Ti0 2


0-81





0-26


0-53


P 2 5








0-58


)-58)




100-56


101-03


101-21





I. Augite-syenite, Reichenstein, Silesia. Traube : Neues
Jahrbuch, 1890., I, p. 206.

II. Hornblende-syenite, Plauen, near Dresden. F. Zirkel :
Fogg. Ann. B., 122, 1864, p. 622.

III. Hornblende-syenite, Biella, Piedmont. Cossa : Mem.
Acad. d. Sc. di Torino. (2) B. VIII., p. 28.



CHAP, i] THE PLUTONIC ROCKS 175

of 124, its small extinction angle, etc. The augite is
more variable in character : sometimes it is almost
colourless or pale-green ; at other times it partakes
more of the nature of diallage, having then a violet-
brown tint. The mica is always a brown biotite. It
is very liable to chloritization, turning green in the
first stage of the process. Muscovite is never present
except as a secondary constituent.

The foregoing analyses of typical potash -syenites
were selected by Brogger l for the calculation of the
average composition of potash-syenites.

THE SODA-SYENITE SUB-FAMILY.

In the soda-syenites the dominant felspar is either
soda-orthoclase or anorthoclase, although albite and
orthoclase may also be present without being inter-
grown to form perthite. The pyroxenes and am phi-
boles are represented by the soda-silicates segirine,
arfvedsonite, barkevikite, riebeckite, etc. In conse-
quence of the subordination of dark-coloured mineral
the soda-syenites have in general a lighter colour than
the potash-syenites. They grade by increasing quartz
into the soda-granites, and by the appearance and in-
crease of nepheline into nepheline-syenites.

The following are the chief types :

Pulaskite. This type was first described by J. F.
Williams 2 from the Fourche Mountains in Arkansas
as consisting essentially of a soda-orthoclase (micro-

1 Die Triadische Eruptionsfolge bei Predazzo. Vidensk.
Skrifter I. Math.-natur. Klasse, 1895, No. 7, p. 31.

2 J. F. Williams : The Igneous Rocks of Arkansas. Ann.
Rep. Geol. Survey of Arkansas, 1890, p. 74.



176



CLASSIFICATION



[PART in



perthite) together with biotite and a brown or' green
hornblende (barkevikite or arfvedsonite). Plagioclase
and quartz are, as a rule, absent. The occasional
presence of nepheline and sodalite indicates a passage
to the nepheline-syenites.



MAP OF THE

CHRISTIANA REGION.



Quartz- Syenite.
( Nordma rkite).



\ Granitite.



Rhomben-porphyrie
\&-A itgite-porphyrite



{Basic Rocks.



\ Devonian &
\Siliirian.



I A rcluean.



Scale. 1:1,250,000

= 05068 inch to the

mile, or about 19-73

niles to the inch.




Typo; Etching a



FIG. 77.

Map of the Christiania District, showing the distribution of the
Nordmarkite and other members of that petrographical province.
(In reproducing this map from the Quart. Joiirn. of the Geol. Soc. (vol.
I., 1894, p. 16), I have at Prof. Brogger's request interchanged the
references to the Nordmarkite and the Granitite, which had been
incorrectly given.)

Closely related to pulaskite is the nordraarkite
type, described by Brogger 1 as covering a large area

1 Brogger : Die post-silurischen Eruptivgesteine des Chris-
tiania-gebietes. Zeitsch. fitr Kryst., vol. xvi., 1890, p. 55.



CHAP, i] TliE PLUTONIC ROCKS 177

in the Christiania district, except that it contains
quartz, and thus constitutes a passage to the soda-
granites. The felspar is a red micropertbitic ortho-
clase, veined with albite. Some oligoclase is also
present; and the coloured minerals, which are present
in subordinate quantity, consist of segirine and arfved-
sonite.

The Laurvikite l type of Brogger 2 is characterized
chiefly by its rhomb-shaped felspars, which consist
of eryptoperthite or anorthoclase, or of both. Plagio-
clase is absent. The coloured minerals comprise green
segirine, hornblende (barkevikite) and biotite (lepido-
melane). Accessory constituents are olivine, apatite
and zircon, and occasionally a little nepheline and
sodalite.

Rosenbusch 3 proposes to embrace the alkali-
arnphibole syenites under the name Umptekite,
which was given by Ramsay* to a type occurring
on the margin of the nepheline-syenite massif of the
Kola Peninsula.

The Akerite type, described 'by Brogger 5 from
Aker in /the Christiania district, is a quartz-bearing
augite-syenite which, besides alkali-felspar (a perthitic
orthoclase), contains much plagioclase, and is therefore
closely allied to the monzonite family (see p. 183). The
augite of this rock is a green idiomorphic diopsied.

1 Name derived from Laurvik in Southern Norway.

2 Brogger: Zeitsch. fur Kryst. , vol. xvi., 1890, p. 30.

3 MikrosTcopische Physiographic, 1907, p. 151.

4 Ramsay : Fennia II. No. 2. Helsingfors, 1894.
6 Brogger : loc. cit., p. 45.

M



178



CLASSIFICATION



[PART HI



The soda-syenites may also be classified according
to their pyroxenes and amphiboles into riebeckite-
syenite, arfvedsonite-syenite, cegirine-syenite, etc.

The following are analyses of the chief types :

ANALYSES OF SODA-SYENITES.

















I.


n.


IIL


IV.


v.




Nordmarkite


Umptekite.


Pulaskite.


Laurvikite,


Akerite.


























Si0 2


64-04


6371


60-03


5685


5i'79


A1 2 3


17-92


16-59


20-76


21-56


16-64


Fe 2 3
FeO


0-96

2-08


2-92
0-66


4-01
0-75


3-44
1-14


i 9-58


MgO


059


0-90


0-80


0-85


2-63


CaO


1-00


3-11


2 62


5-26


5 12


Na 2


6-67


8-26


5-96


6-07


630


K 2


6-08


2-79


5-48


3-66


2-55


H 2


1-18


0-19


0-59


0-52


1-27


Ti0 2


0-62


0-86











Other












constits.


23


0-20


0-07










101-37


100-19


101-07


99-35


99-88



I. Nordmarkite, Tonsenas, Christiania. P. Janasch, quoted
by Brogger : Zeitsch. fur Kryst., vol. xvi., 1890, p. 54.

II. Umptekite, Umpjaur, Kola Peninsula, quoted from
Rosenbusch : Gesteindehre, p. 112.

III. Pulaskite, Fourche Mts., Arkansas, -ibid.

IV. Laurvikite, red variety, Nottero, Christiania. G.
Forsberg in Brogger : Zeitsch. fur Kryst., vol. xvi., 1890, p. 30.

V. Akerite, Vettakollen, near Christiania. Th. Kjerulf,
quoted by Brogger : Die Triadische Eruptionsfolge bei Predazzo,
Vidensk. Skrifter I., 1895, No. 7, p. 33.

THE FELSPATHOID-SYENITE SUB-FAMILY.

We come now to the consideration of the important

division of the syenite family which in addition to an






CHAP, i] THE PLUTONIC ROCKS 170

alkali-felspar (orthoclase, rnicrocline, micro-perthitc,
soda-orthoclase, or albite) contains one or more of
the felspathoid minerals (nephe'ine, leucite, sodalitc,
etc).

The pyroxenes and amphiboles are of great variety,
and occur either independently or intergrown. Among
the former, diopside, segirine-augite and aegirino
(acmite), and among the latter, brownish-green horn-
blende, barkevikite, arfvedsonite, and hastingsite are
known ; while the biotite is usually the variety known
as lepidomelane. The m.embers of this sub-family
are usually classified according to the nature of the
felspathoid constituent into nepheline-syenite, leucite-
syenite, sodalite-syenite, etc.

Nepheline-Syenite. A great number of types of
nepheline-syenite have been distinguished. Of these
the most widely distributed is the Foya type (foyaite), 1
which was first described by Blum from the Foya
Hills, in the Serra de Monchique of Southern Portugal.
In this type potash-felspar (orthoclase) and nepheline
are present in approximately equal proportions, while
the coloured minerals are subordinate. According
as the latter are represented by hornblende, augite,
or biotite, the varieties hornblende- foya/ite, augite-
foyaite and biotite- foyaite can be distinguished.

In some nepheline-syenites, a soda-felspar accom-
panies the potash mineral. Thus litchfieldite is the

1 It should be noted that Brogger (Zeitsch. fur Kryst., vol. xvi.,
1890, p. 39) has adopted the term foyaite for nepheline-syenites
having a trachytic structure (i.e. with tabular felspars in
parallel arrangement) in contradistinction to ditroite, which
he uses for those having a granitic structure.



180 CLASSIFICATION [PART in

name given by Bay ley 1 to a type in which orthoclase
is subordinate to albite ; and in the mariupolite of
Morozewicz 2 albite is alone present.

Ditroite (from Ditr6 in Transylvania) is a type
characterized by abundant sodalite and subordinate
coloured minerals (chiefly biotite, seldom augite).
In miaskite (so named by G. Rose from Miask
in the Ilmen Mountains of Russia) both sodalite and
cancrinite are present, and biotite is abundant.

The laurdalite of Brogger 3 is a characteristic type
developed typically at Laurdal in Southern Norway.
It is a coarse-grained rock containing both nepheline
and sodalite, the former often in large phenocrysts.
The felspar, which may be either a soda-orthoclase
(cryptoperthite) or a soda-microcline (anorthoclase),
occurs in rhomb-shaped crystals, a character which
allies laurdalite to laurvikite. Olivine is an accessory
constituent.

Eudialyte-syenite, a rock consisting of alkali-
felspars, nepheline, aegirine and eudialyte, has been
described by W. Ramsay from the Kola Peninsula.
The kadapleiite-syenite of Tornebohm 4 contains,
besides alkali-felspars, nepheline and segirine, eudia-
lyte and katapleiite. Gancrinite-syenite, also described
by Tornebohm, consists of albite, a little orthoclase,
nepheline, segirine and cancrinite.

All the types of nepheline-syenite described above
contain felspar in greater or less quantity. There

1 Bull Geol. Soc. America, 1882, III., p. 231.

2 Tscherm. Min. u. Pet. Mitt., vol. xxi. (1902), p. 238.

3 Zeitsch. fur Kryst., vol. xvi. 1890, p. 32.

4 Sveriges Geol. Under sokningar, 1906.



CHAP, i] THE PLUTONIC ROCKS 181

exists, however, a type in which the felspathic con-
stituent is absent, the rock then consisting solely
of nepbeline with a ferromagnesian constituent.
Thus Ramsay l has described as urtite a type which
consists solely of nepheline and segirine, and Adams 2
has described as monmouthite a similar rock, con-
sisting essentially of nepheline and black hornblende,
which occurs in the township of Monmouth in Ontario.

Leucite-syenite. The leucite-syenites are far
less abundant than the nepheline-syenites. The first
example of this rock was described by J. F. Williams 3
from Magnet Cove in Arkansas. The rock consists
of porphyritic crystals of pseudo-leucite in a ground-
mass of nepheline, orthoclase and melanite. A similar
rock was described by Hussak 4 from Serra dos Pogos
de Caldas in Brazil.

Sodalite-syenite. A type of syenite in which
sodalite predominates has been described by Ramsay
from the Kola Peninsula as tawite. It is composed
of sodalite and segirine. A type consisting solely of
sodalite appears to exist in Greenland. It may be
described as sodalite-rock. 5

The average chemical composition of the nepheline-
syenites has been calculated by Brogger 6 from thirteen



1 Geol. Foren. i. Stockh. Forh., vol. xviii. (1896), p. 459.

2 Amer. Journ. of Sci., vol. xvii. (1904), p. 269.

3 Ann. Eep. Geol Surv. Arkansas, for 1890 and 1891.

4 Neues Jahrb., 1892, ii. p. 146.

5 Rosen busch : Mikroskopische Physiographic, 1907, p. 240.

6 Brogger : Die Eruptionsfolge der Triadischen Eruptivgesteine
bei Predazzo in Siidtyrol. Vidensk. Skrifter L, 1895, No. 7,
p. 29.



182



CLASSIFICATION



[PART in



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CHAP, i] THE PLUTONIC ROCKS 183

analyses of typical occurrences to be Si0 2 = 55'78,
Ti0 2 = 0'55, A1 2 O 3 = 21-34, Fe 2 O 3 = 5\35, MgO = 0'78,
CaO - 1-93, Na a O = 7-82, K 2 O = 5-83, H 2 O = 1-29. Ratio
of the bases CaO : Na 2 O : K 2 O = 1:4:2.

The analyses given in the table on p. 182 are of well-
known types.

THE MONZONITE FAMILY.
Occupying a middle position in the Intermediate
Group between the syenites and the diorites are the
monzonites ; 1 that is to say they are rocks in which




* *

FIG. 78.

Diagram showing the relation of rocks of the Monzonite Series.

1 This name was given by de Lapparent in 1864 to the rocks
of Monzoni in the Tyrol (Ann. d. Mines. (6), vi., 1864, p. 259),
and has been used with varying significance by different authors.
It is used here in the sense defined by Brogger : " Das wirklich
charakterische bei diesen Gesteinen ist class sie in der Regel
Orthoklas und Plagioklas fiihren ungefahr gleich reichlich, oder
jedenfalls beide reichlich fiihren." Die Triadische Eruptions-
folge bei Predazzo, loc. cit. p. 21.



1F4 CLASSIFICATION [PART m

the soda-lime felspars (plagioclase) play a greater role
than in the syenites, but not so great as in the
diorites. Since the two kinds of felspar may be
associated in every proportion, it is necessary to
adopt some arbitrary dividing lines between the
monzonites and the syenites on the one side, and
between the monzonites and the diorites on the
other. The suggestion made by Lindgrcn l is adopted
here, namely, that rocks having alkali-felspar to the
extent of more than two-thirds of the total felspar
are classed with the. syenites, and those with less than
one-third with the diorites. Consequent!} 7 the mon-
zonites embrace all those intermediate plutonic rocks
in which the ratio of alkali-felspar to soda-lime
felspar is less j than two-thirds and more than
one-third.

With regard to silica the monzonites have a
percentage range between 52 and 66. With more
than 66 per cent, they pass into the adamellite
division of the granites, and with less than 52 per
cent., into olivine rocks of the kentallenite type. 2
The most acid division of the family, corre-
sponding to the tonalites in the diorite family and
to the quartz-syenites in the syenite faniily, are the
banatites. The range of silica in this sub-division
is fixed by Brogger at between 63 and 66 per cent.

The monzonites are characterized by a fairly high
percentage of alumina, which averages about 17 per



1 Amer. Journ. of ~8ci. , vol. ix., 1900, p. 269.
2 Brogger: Die Triadische Eruptionsfolge bei Predazzo, loc.
p. 61.



CHAP. l]



THE PLUTONIC ROCKS



185



cent. ; lime is from 6 to 8 per cent. ; while potash and
soda are present in roughly equal percentages. The
soda-lime felspar may vary from andesine to labra-
dorite. Quartz is absent, or present only in small
quantities in the most acid members (the banatites).
The dark minerals pyroxene (both augite and hy-




FIG. 79. _V :

Mcmzonite, Monzoni, Tyrol.

(Between crossed nicols.)
Or. Orthoclase. PL Plagioclase.

P. Pyroxene. H. Hornblende.

O. Quartz. B. Biotite.

(After Brogger.)

persthene), hornblende and biotite, are variable in
quantity, but always sufficiently developed to deter-
mine a dark colouration of the rock. Nepheline and
leucite occur occasionally, thus affording a passage to
the nepheline-syenites. Oiivine only appears in the



186



CLASSIFICATION



[PART in



more basic members of the family. Accessory
minerals are sphene, zircon, apatite, and the usual
iron-ores. The mean chemical composition, as cal-
culated by Brogger from a number of analyses of
typical monzonites, is given in the following table:

ANALYSES OF MONZONITES.


1 2 3 4 5 6 7 8 10 12 13 14 15 16 17 18 19 20 21 22

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