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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
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