EOSC 321Labs Instructor: M. Kopylova - 5 -
Igneous petrology EOSC 321
Laboratory 6:
Felsic plutonic rocks
Material Needed: a) Microscope, b) Classification triangles and instructions on determination of plagioclase composition included with lab handout; d) 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-Fsp 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 (P1375A) 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 (diagram approved by the International Union of the Geological Sciencesas an international standard). 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
Reference collection: Felsic Plutonic rocks
Thin Section: P 1375A
Sample: P 1375A
Rock Type: Alkali Feldspar Granite
Location: ?
Thin Section Description:
Texture: Granophyric, with intergrown Qz and K Fsp
25% Biotite, pleochroic from dark brown to green, aftern found around opaque minerals
5% Opaque mineral, euhedral isometric or elongate. The mineral that occurs in long grains may be ilmenite.
30% Quartz is in small grains intergrown with KFsp. Quzrtz is recognized by the absence of any secondary alteration and slightly yellow interference colours because of the thicker thin section. The grains are subhedral and often show angles typical of hexagonal crystals.
30% K feldspar, subhedral, with alteration products - grey-yellow dust of sericite and clay minerals. Intergrown with quartz. Distinguished by the negative relief
Secondary Minerals: Long needles of unknown colourless mineral (Apatite? Secondary amphibole?)
1% of carbonate
Grey dust of sericite or clay on K Fsp
Thin Section: 1351-1375 (all in Box 1)
Sample: P 2672
Rock Type: Granite
Location: Boulder batholith
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: 1862
Sample: P 3174
Rock Type: Quartz Monzonite
Location: Feather Falls Town, California
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: 1497
Sample: P 1357
Rock Type: Micrographic alkali feldspar granite. Granite with micrographic texture is also known as granophyre.
Location: Cathedral Mountain, Atlin
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: 1659
Sample: P1684
Rock Type: Sheared Tonalite
Location: Bethlehem Copper Valley
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. Plagioclase adjacent to epidote shear zone has n < nQz 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.
Sample: P3191
Thin Section: 1794
Rock Type: Granodiorite
Location:
Thin Section Description:
Texture: Hypidiomorphic, with subhedral KFsp and Plag and anhedral Qz
40% Plagioclase, euhedral, in large grains, zoned, An28. Some growth zones are accentuated by secondary sericite (black dust). Recognized by polysynthetic twinning
40% Quartz, subhedral in large grains and anhedral in interstitial grains. Recognized by the absence of alteration.
15% K-Fsp subhedral grains, sericitized. Recognized by low relief, occasional presence of perthities and tartan twinning of microcline.
2% Biotite, completely altered. Common around an opaque mineral.
1% Opaque mineral, subhedral
0.5% Sphene, yellow anhedral grains with high-relief and high-birefringence, common around opaques.
Several grains of apatite, always close to an opaque mineral. Present in high-relief prismatic long euhedral grains.
Secondary Minerals: Sericite after Plag and K-Fsp,
Chlorite and magnetite after biotite
Thin Section: 1479
Sample: P 1252
Rock Type: Mela- tonalite
Location: KCMCT
Thin Section Description:
Texture: Hypidiomorphic, with subhedral Kfsp, Plag and mafic minerals, and anhedral Qz
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: 1447
Sample: P 1253
Rock Type: Granite
Location:
Thin section description:
Texture: hypidiomorphic with granophyric intergrowths of K-Fsp and Qz
40% Quartz, anhedral, absolutely unaltered (in contrast to feldspars), with undulose extinction sometimes
39% K-Fsp, anhedral, some grains are altered to sericite and clay minerals, often perthitic. Recognized by the absence of twinning and by the low relief. Many grains are present in granophyric intergrowths of K-Fsp and quartz.
15% Plagioclase, euhedral large laths with polysynthetic twinning and zoning. Often sericitized in grain cores.
5% Biotite, in small subhedral grains pleochroic in green shades. Altered to opaque fine-grained mineral.
1% Opaque mineral in euhedral to anhedral grains
Thin Section: 1437
Sample: P1236
Rock Type: Pyroxene-bearing Quartz syenite
Location: Central Rock, Ardnamurchan
Thin Section Description:
Texture: Hypidiomorphic, with euhedral Plag and Aug and anhedral Kfsp and Qz
37% K-Fsp (orthoclase), with or without perthites, in large anhedral crystals, altered to sericite and grey powder of clays.
25% Plagioclase, euhedral, with polysynthetic twins, zoned with An50 in cores and An30 in rims, altered to sericite
10% Quartz, with numerous inclusions of tiny unidentifyable mineral
10% Biotite
3% Clinopyroxene, Augite. Colourless or slightly greenish, in small euhedral grains.
3% Opaque mineral
2% Apatite in large elongate grains
Hornblende - few grains, pleochroic in khaki colours
Secondary Minerals: Sericite and clay minerals after Fsp and Plag. The degree of alteration increases in felsic minerals from quartz to plagioclase to K-Fsp
Chlorite in anhedral green grains
Thin Section: 1438
Sample: P 1237
Rock Type: Augite Quartz monzonite
Location: Centre 3, Ardnamurchan
Thin Section Description:
Texture: Hypidiomorphic, 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: 675
Sample: P 2109
Rock Type: Hypersthene Diorite
Location: Arendol
Thin Section Description:
Texture: Hypidiomorphic with subhedral Hy and Hb, and anhedral Plag
82% Plagioclase An35, rarely twinned, most grains are clear. Distinguished from quartz by cleavage, and from K-Fsp - by N> N Balsam
10% Hornblende, pleochroic from dark to light green, subhedral to anhedral
5% Hypersthene, grains with high relief, pleochroic from pink to greenish, subhedral to anhedral, with low birefrengence.
3% Biotite, pleochroic from yellow to brown
1% Augite, grains with high relief, pleochroic from pink to greenish, subhedral to anhedral, with high birefrengence.
Thin Section: 1375 A
Sample: P946
Rock Type: Sheared Leuco- tonalite
Location: Roadcut north of Hope
Thin Section Description:
Texture: Concertal, i.e. where the boundary between two 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 Balsam
50% Quartz, anhedral, in small grains with interpenetrating irregular boundaries.
0.5% Biotite, partly altered.
Thin Section: 1424
Sample: P989
Rock Type: Aplitic Quartz-rich granitoid.
Location: Caulfield, KcMcT
Thin Section Description:
Texture: Hypidiomorphic, 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
Thin section: 1226
Sample: P328
Rock Type: Monzonite
Location: Buckingham, P.Q.
Thin section description:
Texture: Hypidiomorphic
50% Plagioclase, subhedral to anhedral, N > N Bals, An23
40% K-Fsp, perthite, patchy type, subhedral to anhedral
3% Amphibole, in euhedral to subhedral grains, zoned, in association with Bi, sphene, Apat, and an opaque mineral. Amph does not look like a common Hb; it has darker khaki colours of pleochroism, and almost parallel extinction. The only type of amphibole with a parallel extinction that can occur in granitoids is kaersutite, a Na-Ca amphibole.