Igneous Review/Overview:
Felsic Plutonic Rocks: includes granite, granodiorite, tonalite, syenite, quartz monzonite, monzonite,
quartz diorite, diorite.
Textural varieties: fine-, medium-, or coarse-grained, pegmatitic, porphyritic, graphic.
Minerals: quartz, K-feldspar (microcline-orthoclase), plagioclase, muscovite, biotite,
hornblende, titanite, zircon, sulfides, and other accessory minerals. Pegmatites noted for
unusual minerals including lepidolite, tourmaline, beryl, spodumene, and many more.
Tectonic significance: the continental crust, recycled versus juvenile crust, batholiths and
orogenesis, causes of melting in the continental crust.
Felsic Volcanic Rocks: includes rhyolite, trachyte, latite, dacite, andesite.
Textural varieties: pyroclastic tuff, vitrophyre, tephra (ash, lapilli, bombs), obsidian, pumice,
and porphyries (incl. porphyry copper and molybdenite deposits).
Minerals: quartz, K-feldspar (sanidine-orthoclase), plagioclase, biotite, hornblende, titanite,
zircon, topaz, sulfides, accessory minerals.
Tectonic significance: Explosive volcanism, calderas, continental rifts, hotspots.
Mafic Volcanic Rocks: includes basalt, porphyritic basalt, diabase, quartz tholeiite, olivine tholeiite,
alkali olivine basalt.
Textural varieties: pahoehoe, aa, scoria, amygdular basalt, culumnar jointing, pillow lavas,
mafic tuff.
Minerals: plagioclase, olivine, orthopyroxene, clinopyroxene, magnetite, ilmenite, sulfides.
Tectonic significance: MORB (mid-ocean ridge basalt), continental flood basalts, hot spots
and ocean islands, rift valleys, causes of melting in the mantle.
Mafic Plutonic Rocks: includes gabbro, norite, anorthosite, troctolite, peridotite, pyroxenite.
Minerals: plagioclase, olivine, orthopyroxene, clinopyroxene, magnetite, ilmenite chromite,
sulfides, and other accessory minerals.
Textural varieties: fine-, medium-, or coarse-grained, pegmatitic, porphyritic, cumulate,
poikilitic.
Tectonic significance: Ophiolites (obducted oceanic crust), layered mafic intrusions,
differentiation suites and associations, magma series.
Alkaline Igneous Rocks: (silica undersaturated) includes nepheline syenite, phonolite, kimberlites.
Minerals: K-feldspar, plagioclase (esp. albite-oligoclase), nepheline, sodalite, corundum,
leucite, melilite, perovskite, accessory minerals.
Tectonic significance: unusual mantle, some hot spots, continental rifts.
Study of Igneous Rocks:
Observations: large-scale features (map-scale), field characteristics (outcrop scale),
petrography (thin section-scale, textures and mineralogy), chemistry of whole-rocks and
individual minerals, application of discrimination diagrams (e.g., AFM diagram, Pearce
diagrams, etc.), application of phase diagrams (e.g., binary systems (Ab-Qtz, Ab-Kfs,
Ab-An, An-Di), ternary systems (felsic: Kfs-Ab-Qtz; mafic: An-Ab-Di, Fo-Di-An), also
wet vs. dry melting, and subsolidus relations).
Interpretations: crystallization sequences, melting source(s), percent of melting, P-T of
melting, role of volatiles, melt migration, emplacement, post-emplacement modification,
tectonic significance.
Metamorphic Review/Overview:
Pelitic Rocks: includes slate, phyllite, muscovite schist, and some gneiss.
Protolith: shale/mudstone.
Minerals: quartz, muscovite, chlorite, biotite, chloritoid, garnet, staurolite, kyanite,
andalusite, sillimanite, K-feldspar (microcline), plagioclase, cordierite, sapphirine,
ilmenite, pyrite and other accessory minerals.
Textural varieties: slaty cleavage, may be foliated, lineated, crenulated, or porphyroblastic.
Significance: often the best rocks for thermobarometry and P-T paths (especially highly
aluminous pelites), Barrovian (or Buchan) series: chlorite-, biotite-, garnet-, staurolite-,
kyanite- (or andalusite-), sillimanite-grade.
Metabasic Rocks: includes greenschist, greenstone, amphibolite, mafic granulite, blueschist,
eclogite.
Protolith: mafic intrusive and extrusive rocks (basaltic rocks, gabbroic rocks, etc.)
Minerals: plagioclase, actinolite, glaucophane, gedrite, epidote, omphacite, garnet, ilmenite,
rutile, quartz, accessory minerals.
Textural features: may be foliated or non-foliated, banded, layered or massive, can be
lineated or porphyroblastic.
Significance: often useful for thermobarometry and P-T paths, metamorphic facies named
for mafic rock types (greenschist, blueschist, amphibolite, eclogite).
Marbles, calc-silicates and skarns:
Protolith: limestone, dolostone, marl.
Minerals: calcite, dolomite, quartz, phlogopite, graphite, talc, tremolite, diopside,
wollastonite, garnet (grossular), vesuvianite, spinel, humite, accessory minerals.
Textural varieties: generally non-foliated or weakly foliated, often layered or banded.
Significance: mineral equilibria controlled by fluid composition as well as P-T conditions,
good monitor of metamorphic fluid flow, skarns result from metasomatism.
Quartzite:
Protolith: quartz-rich sandstone.
Minerals: quartz (abundant), plagioclase, K-feldspar, muscovite, accessory minerals.
Textural varieties: non-foliated or weakly foliated.
Significance: Not useful for thermobarometry, often useful in mapping.
Serpentinite:
Protolith: ultramafic rocks.
Minerals: serpentine (after olivine and/or orthopyroxene), chromite, magnetite, ilmenite,
chlorite, talc, tremolite/actinolite, anthophyllite, accessory minerals.
Textural varieties: often massive, may be foliated.
Significance: often marks location of crustal suture zone, base of ophiolite, sliver of upper
mantle, regional strain may be concentrated into serpentinites, at very high grades will
become chlorite peridotite or garnet peridotite.
Quartzofeldspathic gneiss:
Protolith: felsic igneous rocks, esp. granite/rhyolite to tonalite/quartz dacite.
Textural varieties: banded gneiss, augen gneiss, foliated, lineated, granulitic.
Minerals: quartz, K-feldspar, plagioclase, muscovite, biotite, hornblende, garnet, accessory
minerals.
Significance: deformed crustal meta-igneous rocks.
Metamorphic Review/Overview Continued:
Hydrothermal mineralization:
Textural features: commonly vein systems (fracture flow system), but may include
disseminated mineralization (pervasive flow system) or whole-scale replacement
(including skarn).
Minerals: quartz, calcite, pyrite, chalcopyrite, galena, sphalerite, molybdenite, fluorite,
barite, cinnabar, sulfur, graphite, copper, silver, gold, zeolites, opal, petrified wood.
Significance: Hydrothermal ore deposits (economically important), modern analogs are hot
spring/geyser systems and “black smokers” near oceanic spreading centers, often
associated with igneous activity.
Deformed and Cataclastic rocks:
Protoliths: various.
Textural varieties: breccia, protomylonite, mylonite, ultramylonite, pseudotachylite, augen
gneiss, and includes virtually all foliated and lineated rocks. Also, sigmoidal inclusion
trails (rolled porphyroblasts or overgrown earlier fabrics).
Significance: connection to structural geology; deformation mechanisms, strain fabrics,
petrofabrics, large-scale structures, kinematic indicators, brittle and ductile behavior.
Study of Metamorphic Rocks:
Observations: large-scale features (map-scale), field characteristics (outcrop scale),
petrography (thin section-scale, textures and mineralogy), chemistry of whole-rocks and
individual minerals, determination of protolith characteristics, application of phase
diagrams (e.g., petrogenetic grids, P-T diagrams, T-X diagrams, composition of fluid
phase), application of thermobarometry.
Interpretations: reaction sequences, diffusion and mass transport processes, prograde
metamorphism, retrograde metamorphism, local and regional fluid flow, thermal
gradients, pressure gradients, P-T paths, tectonic significance.
Sedimentary Review/Overview (briefly):
Clastic sedimentary rocks: includes conglomerate, sandstone, siltstone, shale, mudstone.
Textural features: clast sizes, sorting, and shape, depositional structures, lithification
processes.
Minerals: quartz, feldspar, clay minerals, other silicates, and also carbonates.
Significance: weathering, transport, depositional environments, facies, basin development
and evolution, trangression/regression sequences.
Chemical and organic sedimentary rocks: includes limestone, dolostone, bedded chert, evaporites,
phosphates, banded iron formations, coal.
Minerals: calcite, dolomite, quartz, gypsum, anhydrite, halite, sylvite, hematite, magnetite.
Significance: marine paleoenvironments, physio-biological sedimentation processes,
diagenetic processes.
© Steven R. Dunn, Mount Holyoke College, 2004