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Continental margin magmatismWilson p. 191-225

• In this lecture:

– Overview of crustal structure and properties

– Where and how continental crust forms

– Continental margin rocks and processes

– Classic example: the Andes• Structure• Partial melting• Magma sources and differentiation• Metamorphism

– Chemical composition of magmas

– Isotopic composition of magmas

– Petrogenesis of Andean arcs and batholiths

– Continent-continent collision

Continental margins

Major surface features– Ocean basins & continental land masses – Continents isostatically compensated– Oceans 0- 200 Ma– Continents 0 - 4.4 Ga

• Precambrian shields > 543 Ma• Continental platforms, sediments on pC basement• Younger, Cenozoic folded mountain belts

Continental crust– Vertical structure = complex layering– Upper crust, 0-10 km,

• granodiorite– Lower crust, 10-70 km,

• intermediate composition, granulite (high-T mm rock w/plagioclase + pyroxene), amphibolite, pockets of high-P eclogite

– Gross compositions• Granodiorite-diorite, ca. 60% SiO2• Enriched in incompatibel LILE • Depleted in compatble siderophile/chalcophile elements

– Conclude that crust formed by repeated partial melting of upper mantle over Gyrs

Crust Formation –– Continental marginsContinental margins– Collisional zones– Extensional regions (rifts)

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The Andes

Classic example of ocean-continent subduction zone– 3 main volcanic arcs active today:

• NVZ, CVZ, SVZ• Summarize tectonic, geologic, and

geochemical characteristics:

The Andes

Melting process similar to island arcs– H2O fluxed melting of asthenosphere

Slab-dip controls on mantle wedge melting– Explains 3 volcanic arcs separated by non-

volcanic zones– Uplift, exposure of batholiths in non volcanic

zones may reflect shallow slab dip

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The Andes

Crustal thickness– Velocity-density variations– CVZ is extraordinarily thick crust (70 km)– SVZ more like Cascades (30-40 km thick)

General structure– PreCambrian crystalline basement surrounds

CVZ batholiths• CVZ granites carry larger chemical signature of

old pC rocks

The Andes

Calc-alkaline plutons/batholiths– roots of former volcanic arcs– linear chains of plutons– batholiths are uplifted/eroded when

subduction angle shallows to < 10o

– Pluton volumes exceed volcanoes 10:1

How does crust grow?– accretion of island arcs– vertical addition of new magma

• Granitoid plutons• Mafic “underplating”

– major question: • How much of new crust is from mantle

vs. re-melted lower crust?

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Metamorphism in continental arcs

Metamorphic facies (Eskola, 1915)– Descriptive:

• relationship between composition of a rock and its mineralogy

• A facies is a set of repeatedly associated metamorphic mineral assemblages found in an area

– Interpretive: • range of temperature and pressure

conditions represented by each facies

• Experimental studies constrain relatively accurate temperature and pressure limits to individual facies

Facies Definitive Mineral Assemblage in Mafic Rocks Zeolite zeolites: especially laumontite, wairakite, analcime

Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite)

Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite

Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet

Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± hornblende)

Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite)

Eclogite pyrope garnet + omphacitic pyroxene (± kyanite)

Contact Facies

After Spear (1993)

Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies

Mineral assemblages in mafic rocks of the facies of contact meta-morphism do not differ substantially from that of the corresponding regional facies at higher pressure.

Metamorphism in continental arcs

Paired metamorphic belts– Downgoing slab crust:

• greenshcist > blueschist > amphibolite > eclogite

• dehydration, densification of slab• liberated H2O fluxes melting of mantle• If obducted in accretionarty prism, forms high-

P metamorphic belt – Overriding plate:

• Heating of lower crust by magma forms granulite- amphibolite facies metamorphic rocks

• Heating of upper crust by shallow plutons or subvolcanic plumbing systems forms amphibolite – greenschcist -phrenite faciesmetamorphic rocks

• forms low-P metamorphic belt near the surface

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The Andes

Chemical composition of magmas– Magma series

• low K (rare); calccalc--alkalinealkaline; high-K; calc-alkaline; shoshonitic

• Segmentation of the Andes– NVZ & SVZ mainly low to med. K– CVZ high K to shoshonitic

• Volcanic and plutonic rocks are chemically similar

Lava compositions

Pluton compositions (Peru)

The Andes

Chemical composition of magmas– Trace elements

• LREE enriched– Eu anomaly = plagioclase fractionation

• Volcanic rocks similar to plutons• Basalts similar to OIB

– Could refelct melting of LILE enriched lithospheric mantle?

• LILE enriched• HFSE (Nb, Ta) depleted

Figure 17-18. Chondrite-normalized REE abundances for the Linga and Tiybaya super-units of the Coastal batholith of Peru and associated volcanics. From Atherton et al. (1979) In M. P. Atherton and J. Tarney(eds.), Origin of Granite Batholiths: Geochemical Evidence. Shiva. Kent. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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The Andes

Chemical composition of magmas– Trace elements

• LREE enriched– Eu anomaly = plagioclase fractionation

• Volcanic rocks similar to plutons• Basalts similar to OIB

– Could refelct melting of LILE enriched lithospheric mantle?

• LILE enriched• HFSE (Nb, Ta) depleted

– Compare NVZ, CVZ, SVZ• CVZ magmas contain higher amounts of

incompatible trace elements• Reflects assimilation of crust in

thickened setting

The Andes

Isotopic composition of magmas– Sr, Nd, Pb isotope ratios vary in a

segmented pattern• CVZ is most “crustal”

– Assimilation of small amounts of preCambrian crust?

• NVZ, SVZ less crustal, but clearly NOTMORB

– Requires involvement of several sources in varying proportions• Mantle wedge

– Asthenosphere ± enriched lithosphere– Subducted crustal components– Assimilated crust of overriding plate

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The Andes

Isotopic composition of magmas– Sr, Nd, Pb, Oxygen isotope ratios vary in a segmented pattern

NVZ

CVZ

SVZ

Petrogenesis

H2O fluxed melting of asthenosphere

Basalt initiates melting in lower crust

MASH zones develop in lower crust“Melting+Assimilation+Storage+

Homogenization”

Subequal proportions of mantle and melted continental crust comprise batholiths

Thicker, older crust promotes larger amounts of assimilation by ascending basaltic magmase.g., compare CVZ to SVZ or NVZ

Difficult to detemine how crust gets into the magmas• Source contamination vs. crustal

assimilation

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Central Volcanic Zone, 22º S, ChileAndesite-DaciteStratovolcanoes on 65 km thick continental crust

Licancabur, 5921 masl

Chilques, 5778 masl

Lascar, 5641 masl

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Descabezado Grande Azul

Placeta San Pedro lavas

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Tatara-San Pedro Volcanic Complex, 36º S Chile

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Continent-Continent collision zones

Granitoid plutons form in several settings

MountEverest29,028’ asl8,848 masl

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Continent-Continent collision zones

Continent, micro-continent or arc collisions• Growth of crust• Associated with ophiolites

Type example: Collision of India and Asia• Stacking of continental crust• Thickening to 70 km• Inversion of isotherms• Partial melting

– 650-700 oC wet; 800 oC dry• Five separate belts of granitoids parallel

the Himalaya– Very high LILE– Very radiogenic Sr and Pb