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-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

- 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

• 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 anomalies

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