TAS classification
TAS classification – Total Alkali Silica – is employed to classify common volcanic rocks based upon alkali and silica content (recalculated to exclude CO2 and H2O). The TAS classification employs the fact that relative proportions of alkalis and silica play an important role in determining mineral assemblages. The typical silica content of rocks: The TAS classification is used only for volcanics rocks for which the mineral mode cannot be determined. Where mineralogy is known, systems such as the QAPF diagram are applied. The TAS classification is not applicable to all volcanic rocks. Some volcanic rocks cannot be named using the TAS diagram, while additional chemical, mineralogic, or textural criteria are necessary for the classification of rocks with unusual mineral assemblages, such as the lamprophyres. |
Sodic (as used in the TAS classification) above means that Na2O - 2 is greater than K2O, and potassic that Na2O - 2 is less than K2O (ultrapotassic igneous rocks) Rocks with low alkali content are typically ferromagnesian, mafic volcanics. more : Flow Charts for Classification of Igneous Rocks carbonatite classification charnockite classification Flow chart for melilitic, kalsilitic, leucitic rocks... and lamprophyres, kalsilitic rock classification, lamproite classification, melilitic rock classification high-Mg classification norm ne v. norm ab classification plutonic QAPF pyroclastic rock classification TAS ultramafic classification volcanic QAPF |
tectonites
Stress is defined in terms of a force and an area across which it acts to strain a rock mass – compression and extension are simple stresses. Shear is a stress that results from the opposition of forces that are not aligned. Strain is a measure of deformation and is the deflection divided by the original dimension. The modulus of elasticity is stress divided by strain. Tectonites form by constrictional deformation (strain ellipsoids). When mineral grains are platy (oblate) the fabric belongs to an S-tectonite. The planar fabric is typically defined by platy minerals such as micas or distorted grains such as quartz, so flattened as to develop schistosity. Schistosity results from a combination of platy minerals and more equant phases such as feldspar and quartz. The term is most commonly applied to medium-grade rocks such as schists of nearly any bulk composition. Gneissic foliation results from equant minerals such as feldspars and quartz. When the minerals are aligned and cigar shaped (prolate), the fabric belongs to an L-tectonite that has deformed under a prolate strain ellipsoid. Linearity is typically defined by either linear minerals, such as hornblende, or by stretched grains, such as quartz. Fabrics in which minerals are distorted into both linear alignment and planar foliation are found in L-S tectonites. Most metamorphic rocks are L-S tectonites that form from a more general strain history in which stresses summate from several directions (shown as an oblique in image above). A porphyroclast is a relict grain larger than the other grains surviving in a deformed rock. Whereas porphyroblasts are larger than surrounding grains because they grew within the rock, porphyroclasts are larger because they have survived the size reduction suffered by surrounding grains. Deformed porphyroclasts act as shear sense indicators – grain tail complexes align with shear, sigma clasts have wedge shaped tails that do not cross the reference plane of shear, and delta clasts have curved tails that cross the reference plane of shear. Mica fish are large isolated mica crystals within a fine-grained recrystallized mylonitic matrix; they are named for their appearance of a lozenge shape and monoclinic shape symmetry with one curved and one planar side, and they can also show stair-stepping. Mica fish lie with their long axis in the extensional quadrant of the deformation, showing steeper inclination to the fabric attractor than mylonitic foliation. This, together with their asymmetry and stair-stepping of trails can be used as a shear sense indicator. The evolution of mica fish probably result from a combination of slip on the basal plane, rigid body rotation, boudinage, and recrystallization at the edges. Slickensides are polished, parallel striations on rock surfaces that were produced by relative motion across opposite sides of fault planes. Although slickensides may appear similar to glacial striations, they pass into the body of the rock. Slickensides can be very helpful in determining the direction of movement. | |
| S-tectonites | S-tectonites are rocks in which all the grains are pancake shaped (oblate). The "S" indicates schistosity, or planar rock fabric. A penetrative planar foliation develops throughout the rock mass at the initiation of shearing. Incipient shear foliation typically orients normal (perpendicular) to the direction of principal shortening as minerals flatten, and the shear foliation is diagnostic of the direction of shortening. |
| L-tectonites | L-tectonites show lineations due to grains that are aligned and cigar shaped (prolate). In rock masses that undergo large degrees of lateral movement the strain ellipse will lengthen into a cigar shaped volume, and shear foliations distort into a rodding lineation or a stretch lineation. |
| L-S tectonites | L-S-tectonites have both a linear alignment and a foliation. Within assymmetric shear zones, minerals are flattened and skewed. Pronounced displacements within shear zones may cause shear foliation at a shallow angle to the gross plane of the shear zone. Such foliation at a shallow angle to the main shear foliation can manifest as a sinusoidal set of foliations that curve into the main shear foliation. S/C fabrics result from deformations sufficiently strong for S-surfaces to rotate into parallelism with C-surfaces (discrete planes of concentrated simple shear). |
links: images: animation: formation of a delta clast; formations: linear fabric in mylonitic granite from Norway, with clearly stretched K-feldspar grains in a fine-grained matrix of quartz, plagioclase, and biotite, and close-up; linear fabric in mylonitic gneiss; strongly lineated massive gneiss; shear zone; distorted xenoliths in shear zone; slickensides, 2, 3; slickenlines; slickenside lineations on serpentinized ultramafic rocks, Pembine-Wausau volcanic terrane, Wisconsin; contact between massive gneiss (bottom) and megacrystic gniess (top) with parallel lineation, in which the megacrystic gneiss cuts the lineation developed in the massive gneiss; foliation with planar cleavage in slates; slate outcrop; pencil slate; schist; wavy cleavage; C and S foliation; S/C fabric; Rapikivi granite metamorphosing into gneiss (Norway); gneissic banding, Siroua, Morocco; crenulation cleavage in phyllite, Baraboo, Wisconsin; crenulated metamorphic fabric, 2; contact between Tertiary fanglomerates and retrograde mylonitized tectonites, Clara Peak; cleavage refraction between phyllite (left) and quartzite (right), Van Hise Rock, Baraboo, Wisconsin; metaquartzite outcrop; stretched conglomerate; metaconglomerate outcrop; mylonite; pseudotachylite (item); serpentinite; large boudin; highly sheared Carboniferous gypsum layer (even at relatively shallow 'brittle' depths, gypsum undergoes ductile deformation); shear, 2; phyllonite with well developed composite surfaces (yellow dots, NSC; green dots DSF), Late Alleghanian dextral shear zones, Piedmont region; tiny feldspar porphyroclasts in ultramylonite; garnet schist with quartz porphyroblast, Wisconsin; distorted porphyroclasts: assymetric K-spar porphyroclast, 2; sigma clast showing sinistral (left-lateral) shear; sigma clast; pyrite sigma clast; delta clast; contact mylonite-ultramylonite in quartzofeldspathic rock, the ultramylonite matrix is very fine grained and composed of mixed quartz, mica, and K-feldspar with a large albitic content; porphyroclasts in garnet gneiss; thin-sections: micas that form foliations and cleavages in foliated schist; crenulation cleavage; crenulation cleavage that has formed as a result of shortening at a low angle to a pre-existing cleavage; dextral shear in mylonitic quartzofeldspathic rock, blue line indicates discrete C-zone and the green line shows orientation of S-surface in recystallized quartz; distorted porphyroclast; peridotite ultramylonite with extreme grain size reduction associated with localization into 5 mm to 30 m wide shear zones after relatively large and uniform strain, North Pyrenees; mica fish, 2; asymmetric muscovite porphyroclast (mica fish), dextral shear, Goat Rock Fault Zone mylonite in the Georgia Piedmont; sheared amphibolite with assymetric feldspar porphyroclast (blue) and an adjacent NSC surface that has possibly separated the two feldspar grains - brown flaky material is biotite, and green material is hornblende. | |
theolitic
| Tholeiitic basalts are those low in sodium and silica. Tholeiitic basalts included most ocean-floor basalts, most oceanic island basalts, and continental flood basalts, such as the Columbia River Plateau. Theolitic basalts are those without olivine, though some may contain olivine inclusions derived from the mantle.
Above intraplate mantle plumes, oceanic island basalts (OIBs) comprise mostly sub-alkaline, tholeitic (theolitic) basalts (OIT) and some alkaline basalts (OIA), whereas sub-continental mantle plumes generate either continental flood basalts (CFB), which are huge basaltic outpourings of tholeiitic basalts, or large mafic intrusions, such as the Bushveld. (Tholeitic series form only basalts; saturated alkaline series can be bimodal, forminig basalts and some rhyolites; and, undersaturated alkaline series display the entire range, from basalts or basanites to phonolites.) The Columbia River Basalts comprise a thick succession of theolitic basalts that erupted from fissures between about 17-6 Ma (northwestern US). The Deccan traps are flood basalts that spread over an area of more than 5,00,000 sq. kms in five Indian states (Maharastra, Gujarat, Madhya Pradesh, Andhra Pradesh and Mysore). The Deccan area is covered by theolitic basalt, alkaline and mafic rock types. The lower unit of the traps, which is exposed in the eastern and southern part of the Deccan country, represents quiet eruptions and comprises uniform horizontal theolitic flows. |
| links: images: CFB: Columbia River Flood Basalts, 2; Deccan Traps; Serra Geral/Etendeka; Karoo, 2; Newark; Siberian Traps, 2: |
tonalites
Tonalite feldspars are plagioclases (typically oligoclase or andesine), and less that 10% are alkali feldspars (K-spars). Common accessory minerals include amphiboles and pyroxenes. Tonalite is an old synonym for quartz diorite, which contains only 5-20% quartz. Trondhjemite (named for Trondheim, Norway) is a light-colored (leucocratic) intrusive igneous rock common in Archaean terranes, which is a tonalite variety with high albite and low anorthite and low orthoclase content. Trondjemite occurs in conjunction with tonalite and granodiorite as the TTG (tonalite-trondhjemite-granodiorite) ortho-gneiss suite. Trondhjemite is an orthoclase deficient variety of tonalite with minor biotite as the sole mafic mineral. |
links: Tonalite: images: hand-specimens: (image above left) coarse-grained tonalite with albite plagioclase (57%), quartz (40%), muscovite (2%), K-feldspar (1%), and trace oxides, tonalite; formations: biotite-hornblende tonalite of the Goergetown Intrusive Suite, partial melting of eclogite produces tonalitic melts with plag + qtz + hbl, diorite-tonalite layered rock, magmatic fabric in tonalite, mafic dike cut through the tonalite of the Moruya Batholith, leucocratic halo around the mafic enclave in tonalite, mafic inclusions occur as angular blocks floating in the tonalite, tonalitic melt isolating angular clast of diorite, mafic dike intrusive in the tonalite; close-ups: tonalite with phaneritic texture; webpages: USGS photogallery, Igneous Processes in the Moruya Batholith, NSW; Trondhjemite: hand-specimens: Chalcopyrite and bornite disseminated and in fractures cutting chloritic trondhjemite of the Powell pluton; close-up: interlayering between mafic and trondhjemitic compositions in the upper part of the Tromsdalstind eclogite body, trondhjemite, partially assimilated (crenulate margins) xenoliths of hydrothermally altered dolerite hosted by trondhjemite; webpages: Northern Scandinavian Caledonides |
tourmalines
mineral / chemical formula |
properties / significance / occurrence |
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tourmalines AX3Y6(BO3)3 Si6O18(O, OH, F)4 Na(Al,Fe,Li,Mg,Mn,Cu,V,Cr)3(Al,Mg,Fe,V,Cr)6[(Si,Al,B)6O18](BO3)3(OH,F,O)4 |
images - click to enalarge - tourmaline crystals: top, schorl; middle, close-up of segment of bicolor tourmaline crystal (l) superimposed on pink Elbaite in quartz; bottom, tri-color Elbaite in quartz.
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Trigonal aluminum boron silicate crystals with variable compositions involving isomorphous replacement (solid solution) with Na, Ca, Fe, Mg, Li and other elements. Tourmalines are piezoelectric and pleochroic. Tourmaline forms long, variably colored, columnar, three-sided prismatic crystals, typically triangular in cross-section and asymmetrical, hemimorphic terminations. Tourmaline is found in igneous rocks (granite, pegmatites) and metamorphic rocks (schist, marble). Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble. As a durable mineral, minor amounts of tourmaline occur in sedimentary rocks (sandstones, conglomerates) |
| [images: crystals: schorl, dravite, elbaite, uvite; weblinks: Mineral galleries, Gemstone.org, Tourmaline classification, Mindat tourmaline group; structure: Tourmaline Group, (crystals UC) | ||
trace elements
Proportions of the commonest mineral forming elements in mantle-derived magmas appear to be quite uniform and independent of pressure, producing basaltic melts. However, concentrations of the 80 trace elements within the mantle are quite variable, and when combined with isotope ratios, provide a chemical fingerprint of different mantle reservoirs. Trace elements obey Henry’s Law and also exhibit a large range in behavior, so they are collectively sensitive to processes to which major elements are insensitive. The high ionic charge to ionic radius ratio of the HFSE renders them particularly insoluble and very immobile during weathering and metamorphism. This behavior proves particularly valuable in the study of ancient igneous rock suites because HFSE can preserve clues to the environment in which rocks formed. Thus, rare earth patterns can provide information on the premetamorphic or presedimentary history (provenance) of a rock. Victor Goldschmidt classified the geochemical behaviour of the elements according to their affinites for the gaseous state (atmophiles), for the sulfide liquid phase (chalcophiles), for silicate phases (lithophiles), and for a metallic liquid phase (siderophiles). However, a more useful geochemical classification groups the elements based on how they behave in the silicate portion of the Earth, the mantle and crust. The presence of siderophile trace elements in the mantle and mantle-derived rocks, suggests that segregation of the Earth’s iron-nickel core must have been almost complete before the Earth had entirely accreted from the cloud of gas and dust surrounding the early Sun (the solar nebula). Much of the upper mantle has undergone partial melting at some point in geological history, creating the continental crust. The abundances of trace gases in the mantle, together with their isotopic composition, indicate that the solid portion of the Earth underwent extensive volcanic outgassing during the first few hundred million years of Earth history. |
actinides : alkali metals : alkali earth metals : halogens : inert elements : lanthanides : metals : non metals : transition metals : Chemical Periodic Table : Elemental Composition of Crust : Geochemical Periodic Table : Goldschmidt's classification : HFSE : LILE : incompatible elements : rare earth elements : REE : trace elements (top) : links: webpages: online textbook of geochemistry - Trace Elements in Igneous Processes |
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