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” 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 Sr isotopes are variable due to interactions with seawater. Nd will not be modified by seawater interactions so this system is used in MORB studies.

What we can think of happening in a MOR environment, seawater interaction and alteration. About 10% average partial melting beneath MOR.

Mantle plumes, LIPS and flood basalts

MORB lies in DM corner of this plot MORB lies in DM corner of this plot. Plumes lie between MORB and BSE, indication that they mix and drag up parts of mantle.

Magmas in continental extension Magmas in continental extention 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” Daly gap refers to bimodal distribution of SiO2 content of rocks.

Subduction zone magmas A whole different story. Two main types of subduction zones (Island and continental arcs) but both a fundamentally calc-alkaline, depleted in HFSEs, and lie on the fractionation trend towards higher Si, as distinct from tholeiites. + much more…..

AFM diagrams for Andean arc rocks, separated into the northern, central and southern volcanic zones. No Daly gap. Higher Si content for CVZ compared to NVZ is probably related to thicker crust in the central Andes. Implication of high-K rocks in NVZ and CVZ?

HFSE anomalies Arc volcanics show a distinct depletion in some HFSEs, mostly Ta. Nb, Zr. Overall trend is consistent with crustal fractionation. This pattern is often attributed to wet melting but it is still debated.

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 Continental arcs are also calc-alkaline but generally more Si-rich.

Magmatism in collisional orogens The final tectonic setting we will consider are collision zones.

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