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Rock Types - Perils of Classification

Rock Types - Perils of Classification

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Rock Types - Perils of Classification

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  1. Rock Types - Perils of Classification • In principle, a Rock Type has a narrowly defined composition and particular fabric. • In practice, only a few major names are unambiguous and used uniformly by petrologists. Option 1: Adopt a flexible strategy for naming and classification because of the continuous chemical spectrum observed for igneous rocks on Earth. Option 2: Use IUGS approach of fixed, well-defined limits and well established and agreed upon names. This method results in several different classification schemes and diagrams for broadly different rock suites.

  2. Granitic Rocks Quartz-rich felsic rocks collectively referred to as granitoids 3 special fabric categories: PORPHYRY: Phorphyritic aphanitic to finely phaneritic w/ abundant phenocrysts and occurring in a pluton APLITE: Fine grained phaneritic, leucocratic (all fsp and qtz), typically found in thin dikes PEGMATITE: Phaneritic rocks w/ highly variable grain size. Individual xtals range in size from cm’s to m’s. Barker, 1979

  3. Gabbros and Ultramafic Rocks GABBROS:Phaneritic rocks composed of plagioclase, pyroxene, and olivine - compositionally similar to basalts ULTRAMAFICS: Phaneritic rocks w/ <10 modal % felsic minerals Le Maitre, 1989

  4. Whole Rock Chemistry Classification • Aphanitic and Glassy rocks - very old classification system developed prior to the advent of modern chemical analyses. • Example: Overlap in chemical compositions of Dacite and Andesite, but global average composition of each is distinct.

  5. Global Averages for Felsic Rocks Shaded areas correspond to those of the IUGS diamond Asterisks represent global average. 2864 analyses for andesite and 727 analyses for dacite Le Bas et al., 1992

  6. Mafic Rock Types • Diabase or Dolorite: rock of basaltic composition with a transitional grain size between phaneritic and aphanitic. Commonly occurs as dikes and sills. • Picrite: olivine-rich basalt or picrobasalt with MgO >18 wt.% and Na2O+K2O between 1 to 3 wt.% • Komatiite: similar to picrite, but low total alkalies (Na2O+K2O) and TiO2. Both are less than 1 wt.%

  7. CIPW Norm Calculations • Developed by Cross, Iddings, Pirsson, and Washington to determine a hypothetical mineral assemblage from whole-rock chemical analyses. • Useful to facilitate comparisons between basaltic rocks in which complex solid solutions in mineral phases tend to conceal whole-rock chemical variations. • Allows easy comparison between aphanitic and glassy rocks. • Allows comparison between mica and amphibole bearing rocks and those that do not contain hydrous phases, but are similar chemically. NB that “norms” or normative abundance refers to the calculated wt.% of a specific mineral

  8. IUGS Classification of Aphanitic and Glassy Rocks Distinction between Trachyte (Q <20%) and Trachydacite (Q > 20%) based on normative qtz from a recalculation Q+An+Ab+Or=100% The amount of normative olivine distinguishes Tephrite (<10%) from Basanite (>10%) Dotted line encloses 53% of all rocks from the global database Le Maitre, 1989

  9. Silica Saturation I • CIPW norm emphasizes the concentration of silica in relation to other oxides -> assign SiO2 first to feldspars, then, pyroxenes, and finally to quartz. • Calculations done based on moles not weight percentages. Related to variations in the the SiO2 to MgO+FeO ratio and the SiO2 to Na2O ratios as shown below. This serves as a model for a crystallizing magma and illustrates the degree of silica saturation. (Mg,Fe)2SiO4 + SiO2 = 2(Mg,Fe)SiO3 olivine orthopyroxene 2:1 1:1 NaAlSiO4 + 2SiO2 = NaAlSi3O8 nepheline albite 2:1 6:1

  10. Silica Saturation II Silica-oversaturated: rocks contain Q (quartz or its polymorphs- cristobalite and tridymite), such as granite Silica-saturated: rocks contain Hy, but no Q, Ne, or Ol (no quartz, feldspathoids, or olivine), such as diorite and andesite Silica-undersaturated: rocks contain Ol and possibly Ne (Mg- olivine and possibly feldspathoids, analcime, perovskite, melanite garnet, and melilite), such as nepheline syenite

  11. Alumina Saturation I Index based on Al2O3/(K2O + Na2O + CaO) Ratio equals 1 for feldspars and feldspathoids

  12. Alumina Saturation II • Inherent weakness of either silica or alumina saturation classifications is the mobility of Na and K. These elements are easily mobilized and transferred out of a magma by a separate fluid phase. Preferential alkali loss may be inferred from the presence of metaluminous minerals as phenocryts (formed prior to extrusion) in a glassy matrix. • Si can also be mobilized in escaping steam. • Al tends to be less mobile. • Peralkaline rhyolites can be subdivided into: • Comendites: Al2O3 > 1.33 FeO + 4.4 (wt. %) • Pantellerites: Al2O3 < 1.33 FeO + 4.4 (wt. %)

  13. Alkaline and Subalkaline Rock Suites 15,164 samples • NaAlSiO4 + 2SiO2 = NaAlSi3O8 Irregular solid line defines the boundary between Ne-norm rocks Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988

  14. Tholeiitic vs. Calc-alkaline Trends Terms emerged from tangled history spanning many decades. CA label proposed by Peacock in 1931. Tholeiite originated in mid-1800’s from Tholey, western Germany. Rocks show stronger Fe/Mg enrichment than CA trend. Tholeiites are commonly found island arcs, while CA rocks are more commonly found in continental arcs. Cole, 1982

  15. K2O content of subalkaline rocks K2O content may broadly correlate with crustal thickness. Low-K 12 km Med-K 35 km High-K 45 km Ewart, 1982

  16. Classification of Basalts • Three basalt types recognized based on their degree of silica saturation: • Quartz-hypersthene normative (Q + Hy) quartz tholeiite • Olivine-hypersthene normative (Ol + Hy) olivine tholeiite • Nepheline normative (Ne) alkaline basalt • Tholeiitic basalts make up the oceanic crust, continental flood basalt provinces, and some large intrusions. • Alkaline basalts are found in oceanic islands and some continental rift environments.

  17. Yoder & Tilley Basalt Tetrahedron Yoder & Tilley, 1962; Le Maitre