Igneous petrology EOSC 321

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1 Igneous petrology EOSC 321 Laboratory 6: Intermediate and Felsic Volcanic Rocks Learning Goals. After this Lab, you should be able: Identify the key rock-forming minerals in intermediate and felsic volcanic rocks Name intermediate and felsic volcanic rocks based on their textures and mineral modes using triangle diagrams 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); e) Hand lens This lab introduces intermediate and felsic volcanic rocks ranging in composition from andesite to rhyolite. For classification of intermediate and felsic volcanics you should plot quartz, plagioclase and alkali feldspar modes in the "Volcanic" triangle. Classification of these rocks is difficult because viscous silicic magmas tend to solidify into glasses rather than crystallize. You can examine such glassy rocks in T/s's 302 (pumice) and in T/s 765 (vitrophyric andesite). Later devitrification of these glasses produces fine-grained aggregates of felsic minerals (T/s 1416, 866). In the rocks where it is impossible to determine mineral abundances of phases in the groundmass, the rock classification must be based on phenocrysts. The IUGS recommends that rocks identified in such a manner be called PHENOTYPES (like pheno-latite). If based on phenocrysts, the position of a rock on classification triangles will be biased to more mafic and basic rocks and usually erroneous for the rock as a whole. In intermediate-silicic volcanics, plagioclase is usually the most abundant phenocryst phase and generally displays complex oscillatory zoning. When you examine thin sections with zoned plagioclase, you need to determine the magnitude of the zoning by determining the composition of the plagioclase in the core and in the rime of grains. Quartz is the next most common felsic mineral, whereas alkali feldspar is not common in calc-alkaline volcanic rocks and is restricted only to the alkaline series. Clear (not zoned or twinned) low birefringent phenocrysts could be quartz or K feldspar. To identify them, you need to use conoscopy. In the conoscopic view, see the movement of the interference figure for the mineral. If isogyres move vertically or horizontally, the mineral is uniaxial and therefore is quartz. If isogyres rotate and change the curvatures, the mineral is biaxial and therefore is K feldspar or plagioclase. Observe Quartz phenocrysts in T/s 765, 321 and P400 In volcanic felsic rocks you can come across 2 varieties of K-Fsp: sanidine and anorthoclase. The latter is very rare. Sanidine may have simple Carlsbad twinning or may 1

2 have no twinning at all. As a simple rule of thumb, you should call any volcanic K-Fsp sanidine. Amphiboles in andesites are strongly pleochroic hornblendes frequently showing intense opaque reaction rims due to low-pressure instability. When reddish-brown, the amphiboles are termed oxyhornblende. They are also distinguished from hornblende by their higher birefringence and lower extinction angles. Oxyhornblende is not a distinct mineral species, but a hornblende in which a substantial amount of iron has been oxidized to the Fe 3+ state and (OH) - is replaces by O 2 _ to balance the charge. There is a continuous range of compositions and properties between members of the hornblende and oxyhornblende. Conventional hornblende can be converted to oxyhornblende by heating in an oxidizing environment, and the process may be reversed by heating in a reduced environment. Similar processes form oxybiotite (T/s P1577, 321) Make sure you see rhyolite with spherulitic textures (T/s 1412, 1419, 699) characteristic of felsic volcanics. Spherulites are spheroidal bodies in a rock composed of an aggregate of fibrous crystals of one or more minerals radiating from a nucleus, with glass or crystals in between. The most common occurrence of spherulitic texture is a radiating aggregate of acicular alkali feldspars with glass between them, though quartz or other mineral may also be present, resulting in the intergrowth texture. The colour variation in the spherulites is caused by variations in the density of fibres. Make sure that you see the following minerals and textures in the Reference Collection for Intermediate-Felsic Volcanics: q Oxyhornblende and oxybiotite q Sanidine, with or without simple twinning q Conoscopic evidence for quartz q Resorbed, rounded shapes of many felsic phenocrysts q Trachytic, vitrophiric, spherulitic, perlitic amd pumiceous (in the order of importance) 2

3 TEST 1 Granitoids 45 minutes Thin section Your Name: Magnification Student ID: Field of view width Texture: Description * : Rock name: * Should include determination of the plagioclase composition 3

4 Activity I Work in groups of 2. Eight hand specimen stations are prepared for you. Please move from one station to another, spending ~10 min on each short assignment and filling in the answers below. The TA will check the completion of all stations after ~ 1 h. Leave yourself enough time (at least one hour) to look at the intermediate-felsic thin sections. 1. Samples P1350, P1345, P1347 were called keratophyre by geologists who put together the EOAS teaching collection in the s. This rock name is no longer in use. Find the name for these rocks that meet the International Union of Geological Sciences (IUGS) recommendations and explain how you have arrived at this name. 2. Samples P61, P683 are amygdaloidal, i.e. containing former vesicles filled by secondary minerals. What are the minerals in the amygdales? 3. Sample P400 were called vitrophyre by geologists who put together the EOAS teaching collection in the s. Give this rock a different name that reflects its composition rather than the texture. If you cannot do so, explain why. 4. Samples P254, P1487 are rhyolites. Identify phenocrysts in these rocks and draw a conclusion on what minerals can be found as phenocrysts in rhyolites. 4

5 5.Samples P60, P2441 have spherulitic textures. Below provide full descriptions of these two macrospecimens. 6.Sample P347 and Walcott Tuff are fine-grained pyroclastic rocks, i.e. tuffs. However, hand specimens of this tuff look like cogerent non-fragmental aphanitic volcanic rocks. Only the absence of euhedral phenocrysts can be a warning sign for an experienced geologist that it may be a tuff. The evidence for its pyroclastic origin can be found only in thin section. Make sure you check it out! 7. Samples P570, P568, P565, P571, P564, P561, P562, P1254 are andesites. Identify phenocrysts in all of these rocks and draw a conclusion on what minerals are the most common phenocrysts in andesites in general 5

6 Activity II Now we will examine the intermediate and felsic volcanic rocks under the microscope. Reference collection: Intermediate and Felsic Volcanic rocks Thin Section: thin sections Sample: P 631 Rock Type: Andesite Location: Mt. Shasta, Fort Ebbutt Texture: Porphyritic. Texture of the groundmass is hypohyalline, i.e. with a considerable amount of glass 20% Phenocrysts: 14% Plagioclase, euhedral, strongly zoned, N> N Balsam, cores An 55-68, rims An % Oxyhornblende, strongly pleochroic from yellow-brown to almost colourless, zoned, euhedral, extinction angle 4-6 o, high 3-4 order interference colours, with reaction rims of an opaque mineral. Oxyhornblende is distinguished from hornblende by higher birefringence, lower extinction angles and darker brown colour. 0.5% Felsic mineral with N< N Balsam, in euhedral grains with circular cracks around spherulitic aggregates. Most likely is a secondary zeolite after plagioclase. However, it may also be a primary K-Fsp. 80% Groundmass: 35% Plagioclase, euhedral laths, zoned, An45. 35% Glass, colourless, N< N Balsam, with incipient crystallization and therefore not completely isotropic. 10% An opaque dark-brown-reddish mineral in euhedral long grains Ilmenite? Secondary Minerals: Zeolite after Plagioclase 6

7 Thin Section: thin sections Sample: P 581 Rock Type: Dacite Location:? Texture: Porphyritic. The groundmass has a segregational texture, with rock patches of different mineralogy. 25% Phenocrysts: 21% Plagioclase, euhedral, severely zoned, An 33 in core, An 42 in rims. N core > N rim. Partly altered to sericite. Forms glomeroporphyric intergrowths and is present in fragments of broken larger grains. 2% Biotite in euhedral crystals partly replaced by chlorite and epidote. 2% Quartz, in smaller euhedral grains, sometime with uneven domain extinction. The groundmass has segregational texture with patches enriched in biotite and quartz. These areas are comprised of larger euhedral biotite (50%) and larger grains of quartz (50%). 75% groundmass: 50% Quartz in equi-dimensional, anhedral interlocking grains 12% Plagioclase in subhedral twinned grains of larger microphenocrysts. 12% Biotite in euhedral grains. Opaque mineral Secondary Minerals: Sericite after Plagioclase Chlorite after Bi, green Epidote after Bi, yellow grains with strong pleochroism from yellow to green. Comment: Bi-rich segregations in the groundmass may have resulted from the crystallization of volatile-enriched residual pockets of melt. The volatile enrichment of the magma in these areas is suggested by larger grains sizes of Bi and Qz (like in pegmatites) and by more abundant biotite the only hydrous primary mineral of the rock. Note also that plagioclase rims in the rock are more calcic than cores. Such reverse zoning is rather unusual for a magmatic rock, 7

8 Thin Section: 765 Sample: P 1580 Rock Type: Vitrophyric Andesite Location: Mt. Lassen Texture: Vitrophyric vesicular. Phenocrysts are set in a glassy groundmass. 20% Phenocrysts. 12% Plagioclase, euhedral, severely zoned, N > N Balsam, An 29. Several grains show an unusual patchy zoning. Inclusions of light-brown and grey glass are very common and give the plagioclase a "sieved" texture. 5% Biotite, euhedral 3% Quartz, clear, unzoned, un twinned grains with low birefringence. Conoscopy confirms its uniaxial character 1 grain of oxyhornblende with high birefringence 1 grain of euhedral pyroxene (Opx? Cpx?) 10% Glomeroporhyric intergrowths The integrowths found in the thin section are: Plag+ Plag; Bi +smaller Plag crystals nucleating on their rims; Ol with Hb rims and smaller Plag crystals; Opaques + Plag + Bi + Hb 80% groundmass: 60% Colourless to grey glass with N< N Balsam, N~ % Round vesicles 4% Light brown glass associated with Plag phenocrysts 3% Hornblende in larger euhedral microphenocrysts 3% Plagioclase in euhedral grains. 0.5% Biotite, euhedral 0.5% Pyroxene, euhedral Apatite 8

9 Thin Section 1006 Sample Number P242 Rock Name: Silicitized Trachyte Location: Chaffee, Colorado Texture: Vitrophyric. Phenocrysts are set in the glassy groundmass Amygdaloidal, i.e. with vesicles filled with secondary minerals 9% Phenocrysts: 8% Sanidine in subhedral resorbed crystals. (-), 2V~ 20o. Distinguished from quartz by elongate grain habits, imperfect cleavage and N< N Balsam. Distinguished from albite by the lack of polysynthetic twinning. Some crystals are entirely replaced by an aggregate of secondary quartz. 1% Biotite, euhedral few grains of a euhedral opaque mineral 91% Groundmass: 75% Glass, clear, with tiny unidentifiable crystallites marking flow textures. The glass has darker, almost black colour next to vesicles where it is more oxidized. Perlitic fractures. 10% Vesicles of elongate irregular shapes marking flow directions. The vesicles are filled with a fine-grained aggregate of secondary quartz. 5% Quartz of secondary origin filling out fractures and veins Secondary Minerals: Fine-grained Quartz after Sanidine, glass and vesicles. Thin Section: 302 Sample: P 2067 Rock Type: Pumiceous Rhyolite Location: Askja eruption 1880, Iceland Texture: Pumiceous - Frothy vesicular Glassy (holohyaline) Groundmass: 50% oval and irregularly shaped vesicles marking flow structures and flow turbulence 50% glass with N << N Balsam, clear in T/s 302 and grey-brown in T/s 581, contain tiny crystallites of opaque and light minerals. Comment: The rock is classified as "rhyolite" based on the very low refractive index of the glass ( << N Balsam ) typical of acid and silicic glass. 9

10 Thin Section: 722 (2 thin sections) Sample: P 514 Rock Type: Andesite Location: Mt. Shasta Texture: Porphyritic with hypocrystalline vesicular groundmass with trachytic texture. 7% Phenocrysts: 5% Augite in euhedral grains. Often in glomeroporphyric intergrwoth 2% Plagioclase, euhedral, zoned, An 47 Few grains of hypersthene Opx with the parallel extinction 93% groundmass: 55% Plagioclase microlites aligned in the direction of flow. 10% Vesicles, round 25% Light brown glass with N < N Balsam 3% Augite, euhedral microphenocrysts 2% Opaque mineral Comment: The rock should be classified as basalt based on the composition of the plagioclase (labradorite) and as andesite based on colour index (less than 35% mafic minerals). Small proportion of dark minerals ( ~ 7%) and silicic glass ( N< N Balsam ) show it is most likely andesite. Thin Section: Sample: P 1577 Rock Type: Porphyritic andesite Location: Texture: Porphyritic. 31% Phenocrysts: 25% Plagioclase, euhedral and slightly resorbed, with polysynthetic or simple twinning, with melt inclusions. Some grains are surrounded by a Plag corona without melt inclusions. 5% Oxybiotite, euhedral, dark brown-red colour, surrounded by black rims of opaque mineral, recognized by bird s extinction 1% Oxyhornblende. Euhedral, rhombic, pleochroic from yellow-green to brown-red, surrounded by black rims of opaque mineral 10% Microphenocrysts, with sizes 10 times smaller than those of phenocrysts 59% groundmass: Cryptocrystalline mixture of felsic minerals (as suggested by grey interference colour) and mafic minerals (as suggested by high relief) 10

11 Thin Section: 1412 Sample: P1343 Rock Type: Spherulitic rhyolite Location: Texture: Porphyritic, with spherulitic texture of the groundmass 13% Phenocrysts: 10% Subhedral, resorbed Quartz phenocrysts 2% Altered feldspar, euhedral, rectangular, partly replaced by sericite. Those with polysynthetic twinning are likely plagiclases, those with simple twins are alkaline Fsp 0.5% Black euhedral grains of mafic mineral, completely altered to opaques 87% groundmass: 85% Spherulites - spheroidal bodies composed of an aggregate of fibrous crystals of felsic minerals 1% Long laths of feldspars 1% Quartz Thin Section: 1419 Sample: P1350 Rock Type: Spherulitic rhyolite Location: Texture: Porphyritic, with spherulitic texture of the groundmass 13% Phenocrysts: 10% Subhedral, resorbed Quartz phenocrysts 2% Altered feldspar, euhedral, rectangular, partly replaced by sericite. Those with polysynthetic twinning are likely plagiclases, those with simple twins are alkaline Fsp 0.5% Black euhedral grains of mafic mineral, completely altered to opaques 87% groundmass: 85% Spherulites - spheroidal bodies composed of an aggregate of fibrous crystals of felsic minerals 1% Long laths of feldspars 1% Quartz 11

12 Thin Section: 1416 Sample: P1347 Rock Type: Rhyolite with perlitic fractures Location: Texture: Porphyritic, with former perlitic texture of the glassy groundmass 7% Phenocrysts: 5% Plag phenocrysts, euhedral with polysynthetic twinning 2% KFsp, euhedral, not twinned 93% groundmass: Fine-grained aggregate of long laths of feldspar ( ~ 80%) and quartz ( ~ 20%), and fine powder of opaque mineral Comment: The most interesting thing is this thin section is round perlitic cracks that can be observed in PPL. This concentric fracture pattern result from contraction of some volcanic glasses upon cooling. The presence of perlitic cracks means that the rock was initially vitrophyric, and the existing groundmass minerals are the result of devitrification rather than the direct crystallization from melt. Thin Section: 1384 Sample: P972 Rock Type: Rhyolite Texture: Porphyritic, with trachitic texture of the groundmass 8% Phenocrysts: 5% KFsp, zoned, euhedral, not twinned, some grains are resorbed and then rimmed by the second generation of the KFsp growth 1% Plag phenocrysts, euhedral with polysynthetic twinning 1% Quartz 0.5% Biotite, long laths 0.5% Hornblende, with high relief, pleochroic in green 92% groundmass of trachitic texture 60% Fine-grained aggregate of long laths of feldspar, likely both Fsp and Plag are present 20% Bi 12% Hb 12

13 Thin Section: 1138 (2 thin sections) Sample: P 683 Rock Type: Andesite (??) Location: Texture: Amygdaloidal, aphyric, with intersertal texture of the groundmass 40% Amygdales. Can be infilled either with KFsp, or with Qz, or has a concentric texture, with the outer KFsp layer and the inner Chl-epidote layer. 60% Intersertal groundmass, with 50% Plag laths and secondary epidote and hematite in the interstities Secondary: 20% Epidote, yellow, high-relief mineral with bright interference colours of the second order. Occurs as large crystals in some amygdales and is abundant as large poikilitic grains in the altered groundmass. 8% Chlorite. Green, relatively low relief, occupies centers of amygdales. Has anomalous yellow interference colour. XPL shows that large green patches of chlorite are aggregates of spherulites. 8% KFsp. Grows in the outer parts, selveges of amygdales. Has typical grey powder of secondary clay mineral and tartan twinning (rarely). 20% Hematite. Dark red, high-relief mineral 8% Quartz. In amygdales and more rarely replacing the groundmass. Comment: hematite is identified by the peculiar red colour of the altered groundmass in macrospecimen Thin Section: 699 Sample: P2693 Rock Type: Spherulitic rhyolite Location: Texture: Glassy - holohyaline 95% groundmass of colourless glass with small euhedral felsic unidentifiable grains, isometric or long 5% Spherulites - spheroidal bodies composed of an aggregate of fibrous crystals of felsic minerals, yellow to light brown in colour 13

14 Thin Section: 1468 Sample: P 400 Rock Type: Rhyolite Texture: Vitrophyritic, with perlitic texture of the glassy groundmass 33% Phenocrysts: 30% Resorbed and euhedral Quartz 1% Biotite 1% Garnet, euhedral, subround, isotropic, with high relief 1 crystal of large plagioclase with a blue high relief mineral 66% groundmass of glassy texture with round perlitic cracks Comment: Round perlitic cracks can be observed in PPL. This concentric fracture pattern result from contraction of some volcanic glasses upon cooling. Thin Section: 321 Sample: Rock Type: Rhyolite Texture: Vitrophyritic, with perlitic texture of the glassy groundmass 32% Phenocrysts: 15% Plag phenocrysts, twinned 15% Quartz, longer crystals, not twinned, with cleavage fractures perpendicular to elongation 2% Biotite, oxidized on the surface into black Fe oxide minerals. Some grains completely turned to black cryptocrystalline Fe oxides 66% groundmass of brown glass with white patches. The patches are recrystallized and contain fine felsic minerals. Comment: Uneven crystallization of glass can relate to uneven distribution of volatiles, which enhance faster diffusion and crystallization. Thin Section: WT 85-3, 85-4, 85-5, 85-6A, 85-7 Sample: Walcott Tuff Rock Type: Felsic Tuff Location: Texture: Pyroclastic and Glassy The rocks contain shards of volcanic glass welded together. In rocks with more intense welding the rocks shows oriented (planar or linear) texture. 14