tectonic petrology - robust tests of paleotectonic environments
DESCRIPTION
Tectonic petrology - robust tests of paleotectonic environments. GEOS408/508- Lec 7. Magmatism at ocean ridges. Magmatism @ spreading ridges MORB. Most voluminous, decompression melting of peridotite 5-7 km oceanic crust; makes the harzburgite-gabbro-basalt “trinity” - PowerPoint PPT PresentationTRANSCRIPT
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Tectonic petrology - robust tests of paleotectonic
environments
GEOS408/508- Lec 7
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- tholeiites
- isotopically depleted, especially the Nd isotopes are robust
to later modification (eN ~ + 8-12)
- Depleted LREE
- show low pressure fractionation trends without silica
enrichment
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Magmatism @ spreading ridges MORB
• Most voluminous, decompression melting of peridotite
• 5-7 km oceanic crust; makes the harzburgite-gabbro-basalt “trinity”
• Tholeiites, low pressure fractionation Fe enrichment, no silica enrichment
• Depleted source - trace and isotopes• End ~ 10, O and sr isotopes are variable• Slow spreading ridges are different• Most gets subducted, some survives as ophiolites
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- isotopes span the mantle array
- trace elements are enriched relative to MORB, close to 1
on a chondrite-normalized diagram
- the only rocks that can have high 3/4 ratios (15 Ra or
more)- that is a definitive plume indicator;
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generate bimodal associations basalts -rhyolites. Magmatic
volume is relatively small and the exposures are over a broad
area – not linear belts. Basalts are both tholeiitic and alkaline.
Other features:
- rhyolites are crustally derived and have radiogenic
isotopes that show that distinctively (high 87/86, low
143/144 – depending on the exact nature of the
basement);
oxygen isotopes are elevated in rhyolites also consistent with crust origin;
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Continental extension
• Same as MORB, small degree melts
• Alkalic, and tholeiitic
• Bimodal magmatism - rhyolites are lower crustal melts - reflect the isotopic character of the host crust
• Different viscosity - no mixing, lead to “Daly gap”
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- calc-alkaline compositions
- depletion of HFSE
- fractionation trends toward higher silica
+ much more…..
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HFSE anomalies
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Cordilleran arcs
• Calc-alkaline tonalites, granodiorites• Water -rich• Higher silica than island arcs• Crustal recycling significant• MASH zones• Systematic geographic distribution of isotopes• Depletion in HFSE
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- true granites and not tonalities or granodiorites
- radiogenic and stable isotopes should unambiguously
show crustal signatures
- reflect the isotopic composition of the local crust
- REEs always show Eu anomalies
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Collisional magmatism
• Occurs during “hard collision” of continents (Himalayan);
• Accompanied by high grade metamorphism and migmatization;
• Mimimum granitic melts mostly through dehydration melting of muscovite and biotite;
• Caused by radiogenic heating (self ignition) or underplating from the mantle; also possibly by decompression during uplift
• This process makes true granites