IGNEOUS ROCK ASSOCIATIONS
I. Basaltic associations of the ocean crust.
A. Midocean ridge basalts (MORB, abyssal tholeiites)
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represent new crust generated by volcanic eruptions concentrated along ocean ridges of magma
formed in the mantle.
olivine to quartz normative, exceptionally low in K2O (<0.3 wt.%) and TiO2.
also serpentinized peridotites and gabbros (ophiolites) forming the crust.
B. Ocean Island Basalts (OIB; mostly alkali olivine basalts)
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mostly formed by hotspot volcanism deep in the mantle
usually silica undersaturated, olivine basalts; can include phonolites and nephelinites; can have a
wide range of chemical compositions due to fractional crystallization.
richer in alkalis and TiO2 compared to MORB’s
II. Continental tholeiitic provinces
A. Flood (plateau) basalts
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slightly oversaturated in silica
occur in large provinces:
- Columbia River basalts (141,000 km2, 180,000 km3) [miocene]
- Parana Basin, Brazil [K]
- Karro Province, S. Africa [J]
- Deccan Traps, India [C-Tr boundary]
B. Layered Basic Intrusions (stratiform)
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generally found in stable cratonic areas.
differentiated into peridotite, gabbro, "granophyre"
often sources for Pt, Cr, V.
some examples:
- Bushweld, S. Africa (100,000 km3)
- Dufek, Antarctica (>100,100 km3)
- Stillwater, Montana
- Skaergaard, Greenland (500 km3)
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III. Continental alkalic mafic magmas from deep mantle sources
A. Alkali basalt and nepheline magmas
examples: East African Rift; Rhine Province, W. Germany; Basin and Range province, U.S.
B. Carbonatites
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composed chiefly of carbonates (calcite, dolomite, ankerite)
< 2 km2, probably form by miscibility from nephelinites.
C. Kimberlites
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serpetinized, porphyritic, phlogopite peridotites
non-orogenic parts of continental platforms
vertical pipes <0.5 Km2 scattered over a given large area.
originate >100 km in the mantle
often contain diamonds
examples: S. Africa; Siberia; Canada, Australia, U.S. (Arkansas, Kentucky, Tennessee, Kansas,
some in Missouri)
IV. Andesites and associated calc-alkaline series of island arcs and continental margins
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alkaline earth and Al-rich
dominantly volcanic
principally andesites with associated basalts and rhyolites
generated above Benioff zones (above subducted oceanic lithosphere) in the mantle wedge
examples: Andes, Cascades of Oregon and Washington, Japan, Phillipines and other circum-Pacific
island arcs
V. Continental Granitic Provinces
A. Suites of isolated granite plutons (e.g., Basin and Range)
B. Granitic batholiths
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usually composed of many individual plutons ranging from gabbro to granite but granodiorite
and quartz monzonite predominate.
examples: Sierra Nevada Batholith, Idaho Batholith, Lachlan Fold Belt (Australia), Hercinian belt of
Europe
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VI. Archean Cratonic Provinces
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every continent has them
dominated by calc-alkaline tonalite and granodiorite plutons
greenstone belts (mafic volcanics; perhaps equivalent to MORB’s)
N. American examples: Most of central Canada, Ontario, Quebec, Labrador, Greenland, Wyoming
VII. Anorthosite Provinces
massive anorthosite provinces
may represent tops of very large, deep, sub-continental magma chambers
example: Grenville-age anorthosites of Adirondacks, Quebec, Labrador
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