GLY 155 Introduction to Physical Geology, W. Altermann

Transcription

1 Earth Materials Systematic subdivision of magmatic rocks Subdivision of magmatic rocks according to their mineral components: Content of quartz SiO 2 ( free quartz presence) Quartz with conchoidal breakage pattern

2 Subdivision of magmatic rocks according to their mineral content: Feldspars constitute a mixing row of different minerals Potassium feldspar K[AlSi 3 O 8 ] - (Orthoclase) Anorthite Ca[Al 2 Si 2 O 8 ] Na[AlSi 3 O 8 ] Albite Alkali feldspar mixing row with K-feldspar K[AlSi 3 O 8 ] Albite Na[AlSi 3 O 8 ] Albite Anorthite

3 Mixing row: K-feldspar (Orthoclase) Alkali feldspar Plagioclase (Albite Anorthite) Alkali feldspars Gap in the mixing raw with two co-existing feldspars

4 Depending on the Al and Si order in lattice (building temperature and cooling velocity): Sanidine: High temperature-alkali feldspar. Abundant in fast cooled volcanic rocks, often clear, translucent. Mostly K > Na; Al and Si not well ordered Orthoclase: Crystallises at intermediate temperatures around sanidine, during slow cooling. K > Na; Al and Si not dominant. Microcline: Low temperature - alkali feldspar K > Na; Al and Si well ordered in the crystal lattice. Perthite: Orthoclase slow cooling, de-mixing of homogenous, K-rich, alkali feldspars (lamellar patterns of de-mixing) Matrix (host) = orthoclase or microcline with lamellae of albite Na[AlSi 3 O 8 ]

5 Subdivision of magmatic rocks according to their mineral content: layered silicates (phyllosilicate), mica group minerals dark Mg-Fe-micas (biotite group) pale Al-rich micas (muscovite, phengite, paragonite...) = white micas Biotite: K(Mg,Fe 2+,Mn) 3 [(OH,F) 2 (Al,Fe 3+ )Si 3 O 10 ] mixing row phlogopite: pale, Fe-free end member Muscovite: KAl 2 AlSi 3 O 10 (OH) 2 ]

6 Subdivision of magmatic rocks according to their mineral content: Chain- and band silicate (Inosilikates) Pyroxene AB[(Si,Al) 2 O 6 ] A= Na, Ca, Mg, Fe 2+, Mn; B= Mg, Fe 2+ Mn, Fe 3+, Al, Ti, Cr Orthopyroxene [Opx] (e.g. Enstatite, Hypersthene, Bronzite) Clinopyroxene [Cpx] (e.g. Diopside, Augite, Aegirin) Typical cleavage angles of pyroxenes

7 Subdivision of magmatic rocks according to their mineral content: Amphiboles: A 0-1 X 2 Y 5 [(Si,Al) 8 O 22 (OH,F,Cl) 2 ] A=K, Na; X=Na, Ca, Mg, Fe 2+, Mn; Y=Mg, Fe 2+, Mn, Fe 3+, Al, Ti Clino amphibole: e.g. hornblende, riebeckite

8 Subdivision of magmatic rocks according to their mineral content: Island silicates: Olivine: mixing row of forsterite (Mg 2 [SiO 4 ]) and fayalite (Fe 2 [SiO 4 ]) Accessory minerals: Zircone ZrSiO 4 Apatite Ca 5 [(PO 4 ) 3 /OH,F)] Ilmenite FeTiO 3 Magnetite Fe 3 O 4 Haematite Fe 2 O 3 Sulphides e.g. Pyrite FeS 2

9 Subdivision of magmatic rocks according to their texture Mostly massive, no layering and no cleavage, nor schistosity are recognisable equant (equal) grain sizes (crystalline)/porphyritic (with larger idiomorphic crystals in the fine matrix) at visible crystallite size (>0,06mm) mostly plutonic: fine crystalline: mm middle crystalline: mm coarse crystalline: mm Fluidal texture

10 Magmatic rocks can be classified accordingly to the crystallisation texture (e.g.size of the minerals). Phaneritic or phenocrystalline rocks are rocks where the minerals can be recognised with the naked eye. Aphanitic rocks are fine crystalline and minerals can not be recognised without a lens. Porphyritic or porphyric are rocks where large minerals like feldspar (feldspar porphyry) or quartz (quartz porphyry) float in fine crystalline matrix in which the minerals are too small to be recognised with the naked eye. Important!

11 Subdivision of magmatic rocks according to their content of mafic (dark) components Volume percentage of dark (mafic) minerals (e.g. olivine, pyroxene, amphibole, biotite, ore minerls) in the rock. With rising SiO 2 -content in the rock, the content of mafic minerals decreases and quartz and feldspar content increases. Colour Number M = Total content of mafic components Vol.-% High colour number = melanokratic [Greek, melanos = black], dark. Dark rocks with high content of mafic minerals (e.g. gabbros). Melanokratic are rocks with >60% of dark minerals, at contents of >90% the rock is dubbed holo-melanokratic or ultramafic. Low colour number = leukokratic [Greek, leukos = white], pale. Pale rocks with low content of mafic minerals (e.g. granite, syenite). Leukokratic are rocks with less than 30 % of mafic minerals. At total absence of mafic components the rock is holo-leukokratic.

12 Plutonic or intrusive rocks with M < 90 are classified accordingly to the mineralogy and relative content of felsic components. In intrusive rocks quartz and foides (Nepheline, Leucite, Sodalithe, etc.) never occur together Foides replace the feldspars in rocks with very low SiO 2 content. They crystallise from silicate melts with very low SiO 2 and high alkali content. The most common feldspar replacing minerals are: Leucite (K[AlSi 2 O 6 ]) Nepheline ((Na,K)[AlSiO 4 ]), Sodalithe (Na 8 [(AlSiO 4 ) 6 /Cl]), Noseane (Na 8 [(AlSiO 4 ) 6 /(SO) 4 ]), Hauyn ((Na,Ca) 8-4 [(AlSiO 4 ) 6 /(SO 4 )] 2-1 ), Lapislazuli ((Na,Ca) 8 [(Si,SO 4,Cl)/AlSiO 4 ) 6 ]) Analcime (Na[AlSi 2 O 6 ] H 2 O). SiO 2 -oversaturated rocks Quartz + Feldspars SiO 2 -undersaturated rocks Feldspars + Foides

13 Classification of rocks Streckeisen: The Streckeisen Diagram constitutes of two combined triangular diagrams, where each of the four apices contains 100% of a different leukokratic component: Quartz (Q), Alkalifeldspar (A), Plagioclase (P) and Feldspar-replacing, Foides (F = Foides). The volume percentage for each component is recalculated to 100% for Q +A+P in the upper triangle and F, A, P, in the lower triangle. Both triangles have the common base A P. 1. Q: Quartz 2. A: Alkalifeldspar (K-feldspar incl. Perthite and Albite with <5% An) 3. P: Plagioclase with >5% An 4. F: Foides (Feldspar- substitutes)

14 Upper part of the Streckeisen Diagram = the Q-A-P triangular diagram. Each corner of the triangle contains 100vol. % of the component Q, A or P, respectively. The components Q, A and P usually do not make up the total 100 vol. % of the rock, therefore the percentage is recalculated to 100% in total. A rock with e.g. 60% Quartz, 10% Alkalifeldspar, 10% Plagioclase and 20% of other (e.g. mafic) minerals will be recalculated = (80) to = 100, and thus have 12.5% Plagioclase, 12.5% Alkalifeldspar and 75% Quartz. The rock in the triangle has 40% Quartz, 49% Plagioclase and 20% Alkalifeldspar

15 In the lower part of the Streckeisen diagram, Q is substituted by F, because in melts where there is not enough SiO 2 to make more feldspars, foids are crystallising instead from the magma, Quartz (Q) has no chance to be crystallised at all. Thus, rocks with foid minerals never have quartz and always plot in the lower triangle

16 Rock groups in Streckeisen diagram according to their Q content: 1. Plutonites with % Q (very rare to not existent) Plutonites with 20-60% Q (granitic rocks and granodioritic rocks including Alkali-feldspar granites, Trondhjemites and Tonalites, depending on the P/A ratios) Plutonites with 5-20% Q (Quartzalkali-feldspar syenite to quartz diorite & gabbros, depending on their P/A ratios). Plutonites with 0-5% Q are the namegiving rocks to this group (Syenites, Monzonites, Diorites, Gabbros, depending on P/A ratios) Plutonites with 0% Q and 0-10% F (= foidal rocks, without quartz, but still subdivided into Syenites, Monzonites, Diorites, Gabbros, depending on P/A) Plutonites with % F 15. Plutonites with >60 % F = Foidolites

17 Plutonic rocks that contain mainly Plagioclase (Pl), Olivine (Ol), Pyroxenes (Px) and Hornblende (Hbl), are classified in triangular diagrames with following corners: Px-Ol-Pl, Opx-Cpx-Pl, Px-Hbl-Pl. Important are at this stage of your studies: Anorthosites (>90% Plagioclase) Gabbros (rich in Clinopyroxenes) Norites (rich in Orthopyroxenes) Ultra-mafic rocks (M > 90) can be subdivided into: Pyroxenites (Ol < 40) {Olivine:(Forsterite (Mg 2 [SiO 4 ]) Fayalite (Fe 2 [SiO 4 ])} Peridotite (Ol > 40)

18 Important! IUGS - International Union of Geological Sciences (IUGS) classification, of plutonic (left) and volcanic (right) rocks, after A. Streckeisen, Q= Quartz; A=Alkalifeldspar; P= Plagioclase

19 Fine crystalline & porphyric Volcanite Rhyolithe Dacite Andesite Basalt Picrite Komatiite Coarse crystalline Plutonite Granite Granodiorite Diorite Gabbro Peridotite Mineral component in % volume Quartz K-feldspar Ca-rich Plagioclase Na-rich Rocks become progressively darker Olivine Increasing SiO 2 content Volcanic and plutonic rocks classified accordingly to their mineralogy

20 TAS Diagram (Total Alkali -Silica) Volcanic rocks in the TAS diagram

21 felsic intermediate mafic coarse crystalline Granite Granodiorite Diorite Gabbro fine crystalline Rhyolite Dacite Andesite Basalt Increasing SiO 2 content Increasing Na content Increasing K content Increasing Ca content Increasing Mg content Increasing Fe content Increasing viscosity Increasing melting temperature

22 Associations of magmatic rocks: Magmatic rocks often occur in characteristic associations and provinces: Rock associations or magmatic suites, magmatic provinces. Such occurrences evidence syn-genetic or co-genetic, or syn-tectonic provenance of these rocks and a common Parental-Magma. (e.g. alkali magmatites, calcalkalimagmatites, komatiites, etc.). Parental magma, is a magma from which the crystallisation starts followed by progressing differentiation. E.g. a basaltic magma is a parental magma for andesites, dacites and rhyolithes. Primary magma, is a magma originated through partial melting of other rocks and which has not been changed through differentiation or assimilation of other rocks. Primary magma that builds a beginning of differentiation processes becomes a Parental magma. Rock associations are correlated with plate tectonic scenarios and often named after their geographical positioning.

23 GLY 155 Introduction to Physical Geology, W. Altermann

24 GLY 155 Introduction to Physical Geology, W. Altermann