Igneous petrology EOSC 321

Laboratory 6:

Felsic plutonic rocks

Duration: This lab is to be completed within the lab period. The results will be evaluated and returned to you in the following lab period.

Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke)

Introduction: During this lab we shall survey a diverse assortment of silicic quartzfeldspathic plutonic rocks, i.e. granitoids. Granitoids are the most abundant plutonic rocks in the upper continental crust. The major rock-forming minerals of granitoids are plagioclase, alkali feldspar, quartz, pyroxene, amphibole, biotite and magnetite. Sphene and apatite are common accessory mineral, even in the more basic rocks, while allanite (REE-bearing radioactive epidote) occurs quite frequently in the highly differentiated granites (T/s1236). The dominant pyroxene phase is an augite, joined by hypersthene in the intermediate composition range (see T/s 675). Hornblende is one of the major mafic minerals crystallizing from magmas ranging from basic to acid in composition. The abundance of hornblende in the plutonic rocks reflects the increased stability of amphibole at depth in the crust. In contrast, in volcanic suites Hb occurs infrequently and often in a highly resorbed state. Hb crystals change in colour from brown through green-brown to green with increasing differentiation of the magma, i.e. with progressively increasing Fe and Ti contents. Na enters Amph in granitoids of increased alkalinity, and kaersutite (Na-Ca Amph) occurs in such rocks instead of Hb. Biotite may be commonly altered to chlorite as a consequence of interaction with late-stage hydrothermal fluids. Orthoclase is by far the most common type of K-Fsp in the granitoids, while microcline occurs only in the most differentiated rocks. The degree of ordering in K-Sp seems to be mainly controlled by the concentration of volatile components in the melt, with microcline crystallization being favored by the most volatile-rich conditions. Granophyric intergrowths (T/s 1447, 1497, 893) are characteristic of the most highly differentiated rocks which formed from the most volatile-rich magmas.

The mineralogy and texture of granitoids reflects the history of magmatic crystallization in subvolcanic magma chambers. However, as with all slowly cooled plutonic rocks there is abundant evidence for the growth of subsolidus minerals such as Bi, Amph and chlorite due to interaction of the solid rock with high-T residual fluids exsolved from the magma. This interaction and the resulting minerals are called "deuteric". However, it is not always possible to distinguish between hydrous minerals of late magmatic origin crystallizing from water-saturated residual granitic melts, and deuteric hydrous minerals.

Note that we cannot use rock names such as "aplite" or "granophyre". These are textural terms, and should be used as modifiers to a rock name from the proper IUGS diagram. Aplite is a loose term and sometimes is defined as “a leucocratic microgranite occurring in dykes or veins” (Atlas of Igneous rocks and their textures) or as “a granite with a sugary fine-grained texture” (Winter, 2001). We also added prefixes "leuco-" or "mela" to identify rock with unusually high modes of light or mafic minerals for  a rock category. These prefixes impart some descriptive information, and their use is flexible and voluntary.

Make sure that you can distinguish K-Fsp, Plag and Qz in thin sections and see the following important rock characteristics in the Reference Collection:

q       Optical difference between Ca-amphibole (Hb) and Na-Ca amphibole

q       Typical accessory minerals - sphene, apatite and allanite

q       Characteristic alteration of K-Fsp to sericite and clay mineral

q       Perthite, antiperthite and granophyric (micrographic) texture

In the third hour you will receive a thin section of an unknown granitoid for your independent examination. Write its petrographic description and give a rock name. Determination of the plagioclase composition is a necessary part of the petrographic description. Your petrographic report should be completed and handed to the TA by the end of the Lab.       

Reference collection:  Felsic Plutonic rocks

Thin Section Description:

Texture: Hypidiomorphic, with subhedral Kfsp, Plag and mafic minerals, and   anhedral Qz

36%     Plagioclase, subhedral, strongly zoned from An 44 in the core to An 27 in the rim

25%     Quartz, anhedral

23%     K feldspar, orthoclase, perthite, subhedral, with alteration products - grey-yellow dust of sericite and clay minerals

10%     Biotite, pleochroic from yellow to dark brown, replaced by green- blue chlorite

6%       Hornblende, pleochroic from yellow-green to green, often twinned, easily distinguished by cleavage at 60o

1%       Opaque mineral, euhedral, associated with mafic minerals.

0.5%    Apatite, euhedral, in hexagonal or rod-like grains, often in inclusions in HB

Sphene             Few grains, brown, very high relief, very high pearl interference colours, present in fractures in Hb

Zircon              Few grains, colourless, parallel extinction, with high relief and interference colours of the 2-3 order.

Secondary Minerals: Chlorite after Biotite and Hornblende, green- blue, may be isotropic

Comment: Good thin section to practice recognition of Plag, Qz and K-Fsp. F-sp always show secondary alteration and therefore in plain polarized light has a "dusty" appearance.

Qz is recognized by the lack of alteration and by non-uniform domain extinction. Plagioclase is recognized by a characteristic polysynthetic twinning.

Thin Section Description:

Texture: Hypidiomorphic, with subhedral Kfsp, Plag and mafic minerals, and   anhedral Qz

40%     Plagioclase, subhedral, strongly zoned from An 38 in the core to An 20 in the rim. Myrmekyte (intergrowth of Plag and Qz) occurs on rims of bigger Plag grains.

20%     K feldspar - microcline with a characteristic tartan twinning, subhedral.

10%     Quartz, anhedral

5%       Biotite, subhedral, pleochroic from yellow to green

3%       Hornblende, subhedral, pleochroic from yellow-green to green, often twinned, easily distinguished by cleavage at 60o, rimmed by secondary epidote

1%       opaque mineral, euhedral,

Apatite, Sphene

Secondary Minerals: Epidote after Hornblende, with high relief and bright 2nd order interference colours.

Thin Section Description:

Texture: micrographic

69%     Perthitic K-Fsp, in intergrowth with Qz, always covered with dust-like secondary          alteration products

25%     Quartz, anhedral, in intergrowth with K-Fsp

3%       Plagioclase, zoned, with polysynthetic twinning, n < n balsam, => very sodic.

3%       Biotite, euhedral, pleochroic from light yellow to dark green, often rims opaque mineral

1%       opaque mineral, euhedral

Secondary Minerals: Sericite and clay after K-Fsp

Thin Section Description:

Texture: Hypidiomorphic. The rock shows mylonitization and shearing along fractures, and veins of secondary alteration.

60%     Plagioclase, euhedral. An40, zoned with more sodic rims. Sericitized, especially             on rims. Plag. adjacent to epidote shear zone has n < n Qz and is albite.

40%     Quartz, anhedral to euhedral

0.5%    Chlorite replacing a mafic mineral

Opaque mineral            few grains

Secondary minerals: Clinozoizite (colourless non-pleochroic epidote), calcic amphibole (hornblende or actinolite) and quartz comprise late veins. Clinozoizite comprize 80-90% of veins and is recognizable by abnormal interference colours: very bright, 2nd order blue colours. Amphibole has clevage at~ 60o, very low extinction angles, and lower birerefringence.

Thin Section Description:

Texture: Porphyritic

67%     Plagioclase, euhedral, zoned with An30 cores and An13 rims. Altered to sericite

25%     Quartz in large subhedral phenocrysts and in anhedral grains in the groundmass.             Easily recognized by uneven “domain” extinction

5%       Orthoclase, recognized by N<N Balsam

0.5%    Microcline, with a characteristic tartan twinning, perthitic, n < n Balsam

3%       Biotite is altered to chlorite and an opaque mineral

Secondary Minerals: Zoizite after an opaque mineral

43%     Plagioclase, euhedral, slightly zoned, An20, sometime with antiperthitix textures, replaced by secondary sericite

35%     Quartz, anhedral

15%     Biotite, euhedral to subhedral. Half of Bi is replaced by green chlorite.

10%     Hornblende, pleochroic from dark to light green, euhedral.

5%       K-Fsp - perthitic orthoclase

1%       Sphene in large euhedral and subhedral crystals and in fine-grained mass replacing         biotite. Has brown colour, a very high relief, high 3rd order and pearl interference     colours

0.5%    Apatite in small euhedral grains in hornblende and biotite

Opaque mineral

Secondary Minerals:  Sericite after Plag,

                                    Chlorite and sphene after biotite,

                                    Carbonate

Comment: Note a change in relief in carbonate as you rotate the stage! This effect can be used to identify carbonate.

Thin Section Description:

Texture: Porphyritic with micrographic groundmass

15%     Phenocrysts:

Plagioclase (zoned with cores at An37 and rims at An20), Bi and K-Fsp

85%     Groundmass

            35%     Quartz, anhedral, with numerous inclusions of tiny unidentifyable mineral

            35%     K-Fsp, anhedral, in granophytic intergrowths with quartz, perthitic, altered

            10%     Plagioclase with low N (albite)

            5%       Biotite replaced by chlorite and a fine-grained sphene with high relief.

            Opaque mineral, euhedral

Apatite

Secondary Minerals: Sericite after Plag and K-Fsp

                                    Chlorite and sphene after biotite,

Comment: The difference in grain sizes between phenocrysts and groundmass minerals are not very pronounced. Grains of intermediate sizes are arbitrarily assigned to the grounmass or to a phenocrystal assemblage. The texture can alternatively be called "inequigranular"; in this case the division of rock into grounmass and phenocrysts is abandoned.

Thin Section: 1438                                                                                         15

Sample: P 1237

Rock Type: Augite Quartz monzonite

Location: Centre 3, Ardnamurchan

Thin Section Description:

Texture: Hipidiomorphic, with euhedral Plag and Aug and anhedral Kfsp and Qz

48%     Plagioclase, subhedral, with polysynthetic twins, zoned with An55 in cores and An37-43 in rims, altered to sericite

14%     K-Fsp (orthoclase), perthitic, strongly zoned, in large subhedral             crystals, altered to sericite and grey powder of clays.

6%       Quartz, anhedral, with numerous inclusions of tiny unidentifiable mineral,

2%       Biotite, partly replaced by chlorite

3%       Clinopyroxene, Augite. Colourless or slightly greenish, in large euhedral grains, often twinned

3%       Hornblende, pleochroic in khaki colours

2%       Opaque mineral

1%       Apatite in large elongate grains

Secondary Minerals:

6%       Chlorite in anhedral green grains

            Sericite and clay minerals after Fsp and Plag.

Thin Section Description:

Texture: Concertal,i.e. where the boundary between 2 crystals involve interdigitations and hence appears to be notched or serrated in section. The rock is also deformed and acquired porphyroclastic texture. Small grains of quartz are porphyroclasts, i.e. fragments of broken, deformed larger grains.

50%     Plagioclase, An20, in subhedral large grains. The composition is determined entirely by N, since n Bals <n Plag <n Qz. Cannot be mistaken for K-Fsp with n < n Bal

50%     Quartz, anhedral, in small grains with interpenetrating irregular boundaries.

0.5%    Biotite, partly altered.

Thin Section Description:

Texture: Hipidiomorphic, fine-grained.

20%     Plagioclase, antiperthitic, subhedral, n Balsam < n < n Quartz,  => An15, with or without polysynthetic twinning

78%     Quartz, in anhedral fine grains

1%       Biotite. partly replaced by chlorite

0.5%    Opaque mineral in anhedral to subhedral grains

1%       Microcline

Comment: An example of hypersolvus granite, i.e. a granite with only one feldspar, a single-feldspar granite. It crystallize in systems with low H2O pressure at shallow depths and a single Fsp field above the solvus. The hypabyssal affinity of this rock is therefore suggested by its fine-grained texture and the absence of the second feldspar.

Thin section: 1076                                                                                         

Sample: P 643

Rock Type: Muskovite granite                                                                       

Location: Arrowhead, BC                                                                              

Thin section description:                                                                              

Texture: Allotriomorphic, composed of mostly anhedral grains

38%     Quartz 

32%     K-Fsp-Orthoclase, altered to sericite and clay minerals

10%     Plagioclase, N~ N Balsam, => oligoclase. Altered to sericite and clay mineral

14%     Microcline, with a characteristic tartan twinning. No alteration

4%       Muscovite (colourless mica), in big anhedral or smaller long euhedral laths

2%       Biotite, partly chloritized

Secondary Alteration: Chlorite after Bi,

sericite and a powder of clay mineral after Plag and K-Fsp

Comment: Note that K-Fsp in this rock is represented by 2 varieties: orthoclase and microcline. Secondary alteration affected only orthoclase.

Comment: Intergrowths of an opaque mineral with sphene could have formed after biotite.

            Thin section description:                                                                              

            Texture: Hipidiomorphic

82%     Plagioclase, euhedral to anhedral, strongly zoned, An47 cores to albite-oligoclase in       rims. Cores are commonly altered to saussurite and sericite

8%       Quartz, anhedral, interstitial                                                                  

2%       K-Fsp, perthitic orthoclase, (-) relief, has “dusty” appearance due to a strong alteration to fine-grained clay minerals. Can be easily distinguished from Plag under one polar due to this alteration

2%       Hornblende, pleochroic from light yellow to light green, subhedral

3%       Biotite, partly altered to chlorite

1%       Opaque mineral, anhedral to subhedral                                                             

Few grains        Allanite, zoned with dark brown core and yellow rim, poor cleavage, very high relief, biaxial

Comment: The presence of allanite (REE-bearing epidote) in this rock indicates the highly evolved nature of the magma and its enrichment in incompatible elements.