Transcript Basalts and Ultramafic Volcanic Rocks

Basalts and Ultramafic
Volcanic Rocks
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Basalt: pl, augite, hypersthene, olivine,
spinel (45-52 SiO2)
Ultramafic volcanics :
komatiite : olivine, high Mg, low Ti
kimberlite : olivine, phlogopite, matrix
(diatreme, brecciated)
Classification of Igneous Rocks
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
lite
cto
Tro
Ga
bb
ro
90
Olivine
gabbro
Olivine
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Lherzolite
Olivine
(b)
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Komatiite
olivine
rare ultramafic volcanic rocks(>18 wt.% MgO ).
Komatiite sample displays "spinifex texture" defined by extremely
acicular olivine phenocrysts(blue colored )--probably a sign of rapid
crystallization from a significantly-undercooled magma.
Occurrences
• Rift volcanism: tensional forces (MORB)
tholeiitic, tholeiitic-rhyolitic, andesite (rare)
• Subduction zone volcanism: compressional
setting (andesite, dacite, rhyolite) composite
volcano. diverse basalt (calc-alkaline ~
tholeiitic)
• Intraplate volcanism: hot spot environmentOIB, LIPs(Large Igneous Proveince)
Ophiolite development
Alkali vs. Silica diagram for Hawaiian volcanics:
Seems to be two distinct groupings: alkaline and subalkaline
12
10
Alkaline
8
6
4
2
Subalkaline
35
40
45
50
%SiO
55
60
65
AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction
between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii
(solid circles) plus the calc-alkaline rocks of the Cascade
volcanics (open circles). From Irving and Baragar (1971).
After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
Ocean Intraplate Volcanism
Ocean islands and seamounts
Commonly associated with hot spots
Figure 14-1. After Crough (1983) Ann.
Rev. Earth Planet. Sci., 11, 165-193.
Currently there are 3 Hawaiian volcanoes
that we can easily classify as active:
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Kilauea, actively erupting since 1983
Mauna Loa, which last erupted in 1984 and
is building for a new eruption in the next
few years
Loihi, which erupted in 1996
All three of these active Hawaiian
volcanoes share the Hawaiian hot spot, but
retain unique volcanic histories and
compositions.
Mauna Loa, or "Long Mountain" in Hawaiian, is located
on the island of Hawaii. It is pictured above rising 13,680
ft. (4,170 m) above sea level (this photo was taken from
over Loihi seamount, some 30 km or so to the south).
Since 1832, Mauna Loa has erupted 39 times; its last
eruption was in 1984.
Hawaiian Scenario
Cyclic, pattern to the eruptive history
1. Pre-shield-building stage somewhat
alkaline and variable (alkali olivine
basalt)
2. Shield-building stage begins with
tremendous outpourings of tholeiitic
basalts
Hawaiian Scenario
3. Postshield Stage Waning activity more
alkaline, episodic, and violent (Mauna Kea,
Hualalai, and Kohala). Lavas are also
more diverse, with a larger proportion of
differentiated liquids
4. Rejuvenated Stage A long period of
dormancy, followed by a late, posterosional stage. Characterized by highly
alkaline and silica-undersaturated
magmas, including alkali basalts,
nephelinites, melilite basalts, and
basanites
Mantle origin
• Generation depth >40km (Seismic data)
• Phase equilibria > 80km
• Mantle xenolith
These pictures show an example
of an olivine-clinopyroxene
bearing mantle xenolith from the
1800-1801 lava flow of
Hualalai.
Mantle plume
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Age progression of volcanism
Trends of volcanic chain
Plume hypothesis
Geochemistry (He-isotope, eNd,…)
Hotspots
• But plate tectonics can not easily explain volcanism in
the interiors of plates
• Because of the presumed excess heat responsible for
volcanism, such features are called hotspots
Iceland
Yellowstone
Azores
Hawaii
Afar
Galapagos
Reunion
Easter
Tristan
Hotspots
• Where these hotspots occur in the ocean basins they
generally occur at the tips of “aseismic ridges” or island
and seamount chains
• Tracks on the same plate are generally parallel
Intraplate Volcanism
• Columbia River Basalt (Flood Basalt)
• No petrographic, chemical variation
• Qtz-tholeiite, olivine tholeiite, tholeiitic
andesite
LIPs(Large Igneous Proveince)
2,000,000 km3 of lava
Global distribution of flood basalt provinces.
There are no "volcanoes" as such found
in these provinces !!
Map showing the present extent
of the Columbia River flood
basalts (gray area on the map).
A sequence of about 20 Columbia River basalt lava flows in the canyon of the
Grande Ronde River, Washington state. Each flow is 15 to 20 meters thick.
Magmatic history
• Matle melting, ascent of magma
• Pl, ol, px fractionation
• Magma mixing, assimilation of crust
Enriched mantle-plume component (Ba, Th, Nb)
Fractionation(Eu-anomaly)
Nd, Sr
Figure 15-4. Present setting of the Columbia River Basalt Group in the Northwestern United States. Winter (2001). An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Also shown is the Snake River Plain (SRP) basalt-rhyolite
province and proposed trace of the Snake River-Yellowstone hot spot by Geist and Richards (1993) Geology, 21, 789-792.
Model for the CRFB Province
•Melting within a plume head (initial stages of the Yellowstone hot spot).
•The plume head contains stringers of recycled oceanic crust that melts before the peridotite,
yielding silica-rich basaltic magma equivalent to the main Grande Ronde basalts.
•The large plume head stalls and spreads out at the base of the lithosphere and the basaltic
magma underplates the base of the crust, where it melts some crust to create rhyolite.
•Basalt escapes along a northward trending rift system to feed the CRBG.
Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM
is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or
enriched OIB source reservoir. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
The Muskox Intrusion
Mechanisms of
differentiation
Layering in igneous rocks
Large Igneous Provinces
Large Igneous Provinces
• Many hotspots can be traced back to massive volcanic
eruptions of flood basalts, creating LIPs
Columbia River Basalts16.5 Ma
Washington, USA
Deccan Traps 66 Ma India
Mantle Plumes
• Morgan deduced that these thermal plumes must rise
from a thermal boundary layer and proposed that they
originate at the core-mantle boundary (~2900 km)
Plume
Plume
Mantle Plumes
• These plumes can also explain the LIPs
• Models show that plume heads should be created as a
plume initiates and rises