1 / 18

Geol 2312 Igneous and Metamorphic Petrology

Geol 2312 Igneous and Metamorphic Petrology. Lecture 2 Classification of Igneous Rock or How many ways can you skin a rock?. Jan. 23, 2009. Goals of a Rock Classification Scheme. Practical – especially for field use Descriptive – minimal genetic terms

byron
Download Presentation

Geol 2312 Igneous and Metamorphic Petrology

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Geol 2312 Igneous and Metamorphic Petrology Lecture 2 Classification of Igneous Rock or How many ways can you skin a rock? Jan. 23, 2009

  2. Goals of a Rock Classification Scheme • Practical – especially for field use • Descriptive – minimal genetic terms • Systematic – allows for easy information retrieval • Hierarchical – allows for greater levels of detail • Natural – defines boundaries that agree with nature Most common schemes based on mode (vol. %) of “essential” primary minerals ... (When observation of primary minerals is not possible (e.g., too fine, altered) chemistry is commonly used) ...with modification by texture and structural features and occurrence of “accessory” and minor minerals

  3. Color Index-CI =% Dark Minerals “Essential” Minerals • DARK MINERALS • Melanocratic • Mafic Minerals • LIGHT MINERALS • Leucocratic • Felsic Minerals Synonymous – Terms Synonymous Terms Spinel Basic Acid

  4. A SIMPLE (but limiting) CLASSIFICATION SCHEME

  5. Classification of Phaneritic Mafic Igneous Rocks Recommended by the IUGS* troctolite gabbro Ol gabbro *International Union of Geological Sciences

  6. Plotting on Ternary Diagrams Rock A Mineral X – 59.5% Mineral Y – 17% Mineral Z – 8.5% Mineral A – 12% Mineral B – 3% Normalization to XYZ only X+Y+Z = 85% Xn = 59.5/85 = 70% Yn = 17/85 = 20% Zn = 8.5/85 =10%

  7. Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Orthopyroxene Clinopyroxene Clinopyroxenite Classification of Ultramafic Igneous Rocks Recommended by IUGS

  8. Mafic Rock Classification Schemes Applied to the Duluth Complex Streckeisen (1976) LeMaitre (1989) Phinney (1972a) Davidson (1969a) Severson and Hauck (1990)

  9. Elements of a Practical Classification Scheme for Duluth Complex Mafic Rocks - uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox) - defines modal boundaries that bracket natural modal populations and cotectic proportions determined from experimental data - uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field. From Miller (1986)

  10. Preferred Modal Classification

  11. Modifiers – For a Complete Rock Description Alteration Foliation/Layering Absolute Grain Size Bulk Rock Texture –based on pyroxene habit Accessory/Minor Minerals For example: Serpentinized, modally layered, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITE

  12. Why PYROXENE-BASED BULK-ROCK TEXTURES? Ophitic-Subophitic Ophitic Subophitic Intergranular Pl Ol Cpx

  13. Classification of Intermediate and Felsic Rocks Intermediate (“ferro-”) Felsic From Streckeisen (1976) Root Rock Name Attach mafic mineral prefix to root rock name for intermediate rocks... or “ferro” if uncertain of mafic mineral phase

  14. The Mafic to Intermediate Transition • Typically based on average An content (> or < An50) • Hyndman (1972) suggests also considering: • rock associations (diorite with more granitoid rocks, gabbro with more mafic rocks) • mafic mineral assemblage (diorite = hornblende or biotite  pyroxene; gabbro = pyroxene  olivine  hornblende) • plagioclase color (diorite whitish or nearly so; gabbro - greenish gray to gray) • Blatt and Tracy (1995) suggest using diorite for rock that contains hornblende over pyroxene and that contain less than 35 modal percent high-temperature mafic minerals (olivine, pyroxene). Problems Can’t “see” AN content in hand samples Even DC felsic rocks are dominated by pyx over hb and bio Suggested Field Criteria for DC IntermediateMafic Dark Mineral Habit prismatic, subprismatic granular to poikilitic Felsic Mesostasis >5 % <5 %

  15. Classification of Volcanic Rocks Recommended by the IUGS Basalt/Andesite Distinctions BasaltAndesite SiO2 < 52% > 52% CI > 35% < 35% Foid Minerals Nepheline Leucite Kalsilite Analcime Sodalite

  16. Chemistry-Based TAS Classification of Aphanitic or Metamorphosed Igneous Rocks

  17. CIPW Normative CalculationsPseudo-mineralogy Calculated from whole rock geochemical analyses; distributes major elements among rock-forming minerals

  18. Classification of Pyroclastic Rocks

More Related