Igneous petrology EOSC 321

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1 Igneous petrology EOSC 321 Laboratory 5: Conoscopy. Felsic plutonic rocks Learning Goals. After this Lab, you should be able: Use conoscopic obervations to determine the uniaxial or biaxial character of a mineral Determine optic sign of anisotropic minerals Identify the key rock-forming minerals in felsic plutonic rocks Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke) Test. You will receive a thin section of an unknown mafic-intermediate volcanic rock for your independent 45 minute examination. Draw a sketch of the key features of the rock, write its petrographic description and give it a rock name. You may use your previous lab notes, textbooks etc., but NO ELECTRONIC DEVICES! Your petrographic report should be completed and handed to the TA by the end of the first 45 minutes of your Lab. 1

2 TEST 3 Mafic Volcanic Rocks 45 mins Thin section Magnification Field of view width Your Name: Student ID: Texture: Description 1 : Rock name: 1 Should include composition of plagioclase if it is in your thin section 2

3 Activity 1. The method of conoscopy and observing interference figures of minerals. Today we will learn the last unknown to us advanced technique of petrographic observation the conoscopy. It will become for you a powerful method of mineral identification. Read pages of Perkins, Elmer to understand the principle of conoscopy and to see photographs of interference figures. Anisotropic minerals under the microscope can be characterized by assigning them into two classes-- biaxial and uniaxial minerals. These characteristics depend on the symmetry of the minerals and are usually listed in mineral identification flow charts, like the one on page 40 of your lab textbook. For uniaxial minerals, we need two indices of refraction to describe the mineral s refractive index. For biaxial minerals, we need three. An optic axis is a direction through a crystal along which no double refraction occurs. One can determine whether a mineral has 1 or 2 optic axes by observing an interference figure of the mineral. Furthermore, conoscopy classifies minerals into optically positive and optically negative. Optic sign describes the relationship between indices of refraction with respect to the symmetry of the crystal. To observe interference figures, you should: 1. Make sure your microscope is centered and the cross hairs are in focus. 2. Find a grain oriented with the optic axis vertical. Such grains will appear isotropic (black or almost black in XPL) because optical properties in all directions in this cross-section are similar. These grains will also have the lowest order interference colours. Initially, grains in this orientation will be provided for you in an oriented thin section, but eventually you will have to find these in thin sections yourself. 3. Focus on the grain in low power (crossed polars), move up to medium power and check focus, then move up to high power and double check focus (make sure you are not focused on a crack or impurity in the grain). Be EXTRA careful when focusing in high power! Always use the fine adjustors! Go slow and watch what you re doing! It is very easy to shatter thin sections this way! 3. Insert the Bertrand lens. A centered optic axis figure should appear. 4. Insert the accessory plate to determine the optic sign of the mineral. If the NW-SE corners turn yellow, the mineral has a positive optic sign, if the NW-SE corners turn blue, the mineral has a negative optic sign. Activity II. a. Quartz c or Scapolite c. Look at the mineral in orthoscopic mode, low power, crossed polars. How can you tell that these mineral cross-sections are good for conoscopic observations? 3

4 b. Examine Quartz c or Scapolite c using conoscopic illumination to find an interference figure. Make two coloured drawings showing (1) the interference figure and (2) the interference figure with the accessory plate inserted. Compare your drawings with figures on page 21, 22 and 26 of the textbook to determine if quartz is uniaxial or biaxial and what its optic sign is. c. Barite or Muscovite or Topaz T/s. Find an interference figure using the techniques described above. Determine the following: biaxial or uniaxial? positive or negative? Sketch the (1) interference figure and (2) the interference figure with the accessory plate inserted. NOTE: In the real world (i.e. as a geologist) you will not be given oriented thin sections of minerals such that you will easily find perfect interference figures. You will *mostly* find that interference figures may not appear centered. A useful tip from a trained petrologist: When viewing an un-centered interference figure, if you see parallel movement of the isogyre, the mineral is uniaxial, if you see a change in curvature of the isogyre, the mineral is biaxial. 4

5 Activity III 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 minerals, 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 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 plutonic rocks reflects the increased stability of amphibole at depth in the crust. In contrast, in volcanic suites hornblende occurs infrequently and often in a highly resorbed state. Hornblende crystals change in colour from brown through green-brown to green with progressively increasing Fe and Ti contents in the melt. 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. The degree of ordering in K-Fsp and therefore formation of orthoclase or microcline 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 rocks 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 biotite, amphibole 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 Sciences as 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. See the following important rock characteristics in the Reference Collection in preparation for the next week quiz: q Uniaxial character of quartz that distinguishes it from other felsic minerals 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 q 5

6 Reference collection: Felsic Plutonic rocks Thin Section: P 1375A Sample: P 1375A Rock Type: Alkali Feldspar Granite Location:? Texture: Granophyric, with intergrown Qz and KFsp 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 KFsp Thin Section: (all in Box 1) Sample: P 2672 Rock Type: Granite Location: Boulder batholith 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 60 o 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 6

7 Comment: Good thin section to practice recognition of Plag, Qz and KFsp. KFsp 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 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 60 o, 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 Texture: Micrographic 69% Perthitic K-Fsp, in intergrowth with Qz, always covered with dust-like secondary alteration products 25% Quartz, anhedral, in intergrowth with KFsp 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 KFsp 7

8 Sample: P3191 Thin Section: 1794 Rock Type: Granodiorite Location: Texture: Hypidiomorphic, with subhedral KFsp and Plag and anhedral Qz 40% Plagioclase, euhedral, in large grains, zoned, An 28. 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% KFsp 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 KFsp, Chlorite and magnetite after biotite Thin Section: 1479 Sample: P 1252 Rock Type: Mela- tonalite Location: KCMCT Texture: Hypidiomorphic, with subhedral KFsp, Plag and mafic minerals, and anhedral Qz 43% Plagioclase, euhedral, slightly zoned, An 20, 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% KFsp - 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 8

9 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 KFsp and Qz 40% Quartz, anhedral, absolutely unaltered (in contrast to feldspars), with undulose extinction sometimes 39% KFsp, 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 finegrained mineral. 1% Opaque mineral in euhedral to anhedral grains Thin Section: 1437 Sample: P1236 Rock Type: Pyroxene-bearing Quartz syenite Location: Central Rock, Ardnamurchan Texture: Hypidiomorphic, with euhedral Plag and Aug and anhedral Kfsp and Qz 37% KFsp (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 An 50 in cores and An 30 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 KFsp Chlorite in anhedral green grains 9

10 Thin Section: 1438 Sample: P 1237 Rock Type: Augite Quartz monzonite Location: Centre 3, Ardnamurchan Texture: Hypidiomorphic, with euhedral Plag and Aug and anhedral KFsp and Qz 48% Plagioclase, subhedral, with polysynthetic twins, zoned with An 55 in cores and An in rims, altered to sericite 14% KFsp (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 KFsp and Plag. Thin Section: 675 Sample: P 2109 Rock Type: Hypersthene Diorite Location: Arendol 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. 10

11 Thin Section: 1375 A Sample: P946 Rock Type: Sheared Leuco- tonalite Location: Roadcut north of Hope 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 KFsp 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 Texture: Hypidiomorphic, fine-grained. 20% Plagioclase, antiperthitic, subhedral, n Balsam < n < n Quartz, => An 15, 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 11

12 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, An 23 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. 2% Biotite 1% Sphene, in euhedral yellow-brown grains with high relief 0.5% Apatite, common around Bi and as inclusion in it. Opaque mineral Secondary Alteration: Larger grains of Muscovite and smaller grains of sericite after Plag and K-Fsp Comment: An increased alkalinity of this rock can be inferred by petrographic observations. The indicators of an increased alkali character are: 1). The sodic amphibole; 2) The complete absence of quartz. 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% KFsp-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 KFsp Comment: Note that KFsp in this rock is represented by two varieties: orthoclase and microcline. Secondary alteration affected only orthoclase. 12

13 Thin section: 893 Sample: P 394 Rock Type: Micrographic granite Location: Sudbury Thin section description: Texture: micrographic or granophyric, with regular intergrowths of KFsp and Qz 70% granophyric intergrowths of KFsp and quartz. KFsp is perthitic ortoclase, and it is present in granophyric textures and around plagioclase crystals mantling them. Altered to sericite and a powder of clay minerals 18% Plagioclase, euhedral laths with polysynthetic twinning, sometimes overgrown by K- Fsp rims. Altered to sericite and clay minerals 5% Quartz 2% Augite, in small relic colourless, high relief grains in cores of euhedral prismatic laths of hornblende 3% Hornblende, pleochroic from yellow to greenish, partly replaced by chlorite. Always in association with Aug and perhaps is a late magmatic phase after Aug or has a deuteric origin. 2% Opaque mineral in euhedral to anhedral grains, in association with sphene. Sphene Apatite Comment: Intergrowths of an opaque mineral with sphene could have formed after biotite. 13

Igneous petrology EOSC 321

Igneous petrology EOSC 321 Laboratory 2: Determination of plagioclase composition. Mafic and intermediate plutonic rocks Learning Goals. After this Lab, you should be able: Determine plagioclase composition