Transcript Tectonic petrology - robust tests of paleotectonic

Tectonic petrology - robust
tests of paleotectonic
environments
GEOS408/508- Lec 7
- 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
Magmatism @ spreading
ridges MORB
• Most voluminous, decompression melting of
peridotite
• 5-7 km oceanic crust; makes the harzburgite-gabbrobasalt “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
- 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;
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;
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”
- calc-alkaline compositions
- depletion of HFSE
- fractionation trends toward higher silica
+ much more…..
HFSE anomalies
Cordilleran arcs
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Calc-alkaline tonalites, granodiorites
Water -rich
Higher silica than island arcs
Crustal recycling significant
MASH zones
Systematic geographic distribution of isotopes
Depletion in HFSE
- 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 a nomalies
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