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?
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
Rock and Mineral Composition
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
Generalized Mineral Associations for SubAlkaline 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
Whole Rock Major and Trace Element
Composition of Basalt
Chemical Composition of Magmatic Rocks
• Variation Diagrams
– Cartesian graphs (x vs. y), usually some oxide
vs. SiO2 or MgO
– Triangular diagrams
– Normalized diagrams
Variation and Triangular Diagrams
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
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?
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%?
End-Member Mineral Compositions
Worldwide magmatic rock
<- compositions
Transition Group Metals
Platinum Group Metals
Compatible Trace Elements
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.
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
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.
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%
Late Triassic Plutonic Modes
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
Visual Estimation of Modal Abundance
Volcaniclastic Rock Classification
Classification based
only on clast size.
Scheme has NO
genetic significance.
IUGS Classification
International Union of Geological Sciences
Q=quartz; A=alkali fsp; P=plagioclase fsp; F=feldspathoids