Some fundamental questions about IG and MET rocks

1. Some fundamental questions about IG and MET rocks • How are rocks sampled in the field and analyzed in the lab to determine their chemical, modal, and mineralogical composition? • What do these analyses tell us about the composition of magmatic rocks? • How can the data be presented to elucidate compositional patterns and contrasts? • How do we classify magmatic rocks to convey meaningful petrogenetic information on the origin and evolution of the magma from which they solidified? • What special information can be obtained from trace elements and isotopic compositions?

2. Rock Properties and Their Significance • Composition • Whole Rock chemical composition (XRF, DCP, ICP-MS) • Mineralogical (Major and Trace elements) (XRD; electron microprobe (EDS and WDS) • Modal (calculated or by point counting) • Field Relations • Contact relations • Stratigraphy • Dimensions of rock bodies • Fabric • Texture • Structure

3. Rock and Mineral Composition

4. Analytical Procedures • Sampling • Weathering and Alteration • Analyses • Accuracy and Precision • Modal Analyses (visual, point-counting, X-ray) • Chemical Analyses • Major elements (oxides > 0.1 wt.%), trace elements (<0.1 wt.% or < 1000 ppm) • Volatiles (H2O and CO2) -> LOI

5. Generalized Mineral Associations for Sub-Alkaline Rocks Aphanitic Rocks => fined grained and generally volcanic-extrusive Phaneritic Rocks => coarse grained and generally intrusive-plutonic Ca/Na and K/Na ratios increase in Feldspars (plagioclase and alkali) and Mg/Fe ratios increase in mafic solid solutions (olivine, pyroxenes, amphiboles, and micas) with increasing temperature

6. Whole Rock Major and Trace Element Composition of Basalt

7. Chemical Composition of Magmatic Rocks • Variation Diagrams • Cartesian graphs (x vs. y), usually some oxide vs. SiO2 or MgO • Triangular diagrams • Normalized diagrams

8. Variation and Triangular Diagrams

9. Alkaline vs. Sub-alkaline Rocks 46.7% widely scattered <- Basalts 53.3% tightly clustered in a central band Analyses of a global sample of 41,000 igneous rocks of all ages

10. Attributes of Total Alkalies Diagram I • Magmatic rocks constitute a continuous chemical spectrum, i.e. no breaks or discontinuities. Other elemental combinations show similar trends • Questions? • How is such a chemical spectrum created? • Is there a similar range in liquid (magma) compositions? • What processes of magma generation from solid rocks can give rise to the observed range? • Could this spectrum be generated from a much narrower source range and the derived liquids modified to yield the observed diversity?

11. Attributes of Total Alkalies Diagram II • Variations in the elements (SiO2 and Na2O + K2O) only show a part of the possible full compositional range (30-80% and 0-20%, respectively). • What petrologic factors produce this relatively restricted range for these major elements? • Why are there no magmatic rocks whose SiO2 content is 95 wt% or those with T.A. of 50 wt%?

12. End-Member Mineral Compositions Worldwide magmatic rock <- compositions

13. Transition Group Metals Platinum Group Metals

14. Compatible Trace Elements

15. Classification Schemes I • Based on Fabric • Phaneritic: rocks with mineral grains that are large enough to be identified by eye. Texture is typical of slowly cooled intrusive rocks. • Aphanitic: rocks with grain too small to be identified by eye. Texture is most common in rapidly solidified extruded magma and marginal facies of shallow intrusions.

16. More on Fabric Classification • Porphyritic texture: magmatic rocks with bimodal grain size distributions. • Larger grains are called phenocrysts • Smaller grains constitute the groundmass or matrix • Porphyritic aphanitic rocks are more common than porphyritic phaneritic rocks • Glassy or vitric texture: rocks that contain variable proportions of glass. • Holocrystalline rocks: wholly composed of crystals • Vitrophyric rocks: porphyritic rock with phenocrysts in a glassy matrix

17. Classification based on Field Relations • Extrusive or volcanic rocks: typically aphanitic or glassy. Many varieties are porphyritic and some have fragmental (volcaniclastic) fabric. High-T disordered fsp is common (e.g. sanadine). Also see leucite, tridymite, and cristobalite. • Intrusive or plutonic rocks: typically phaneritic. Monomineralic rocks of plagioclase, olivine, or pyroxene are well known but rare. Amphiboles and biotites are commonly altered to chlorite. Muscovite found in some granites, but rarely in volcanic rocks. Perthitic fsp, reflecting slow cooling and exsolution is widespread.

18. Classification based on Modal Mineralogy • Felsic rocks: mnemonic based on feldspar and silica. Also applies to rocks containing abundant feldspathoids, such as nepheline. GRANITE • Mafic rocks: mnemonic based on magnesium and ferrous/ferric. Synonymous with ferromagnesian, which refers to biotite, amphibole, pyroxene, olivine, and Fe-Ti oxides. BASALT • Ultramafic rocks: very rich in Mg and Fe. Generally have little feldspar. PERIDOTITE • Silicic rocks: dominated by quartz and alkali fsp. Sometimes refered to as sialic (Si + Al). NB that the mineral modes are expressed as vol% or wt%

19. Late Triassic Plutonic Modes

20. Color Index • Another way to estimate the mode of some minerals within the rock. • Defined as the modal proportion of dark-colored minerals in a rock. Should really be based on the proportion of ferromagnesian minerals as feldspars may range in color. • Leucocratic: 0-30% mafics • Mesocratic: 30-60% mafics • Melanocratic: 60-100% mafics

21. Visual Estimation of Modal Abundance

22. Volcaniclastic Rock Classification Classification based only on clast size. Scheme has NO genetic significance.

23. IUGS ClassificationInternational Union of Geological Sciences Q=quartz; A=alkali fsp; P=plagioclase fsp; F=feldspathoids