Download pdf 1.5Mb

Continental margin magmatism
Wilson 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,
•
–
•
–
intermediate composition, granulite (high-T mm rock
w/plagioclase + pyroxene), amphibolite, pockets of
high-P eclogite
Gross compositions
•
•
•
–
granodiorite
Lower crust, 10-70 km,
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 margins
Collisional zones
Extensional regions (rifts)
1
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 nonvolcanic zones
Uplift, exposure of batholiths in non volcanic
zones may reflect shallow slab dip
2
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?
3
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
Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies
Facies
Zeolite
Definitive Mineral Assemblage in Mafic Rocks
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
Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
After Spear (1993)
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 highP 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 facies
metamorphic rocks
forms low-P metamorphic belt near the surface
4
The Andes
Lava compositions
Chemical composition of magmas
–
Magma series
•
low K (rare); calccalc-alkaline;
alkaline high-K; calcalkaline; shoshonitic
•
Segmentation of the Andes
–
–
•
NVZ & SVZ mainly low to med. K
CVZ high K to shoshonitic
Volcanic and plutonic rocks are
chemically similar
Pluton compositions (Peru)
The Andes
Chemical composition of magmas
–
Trace elements
•
LREE enriched
•
•
Volcanic rocks similar to plutons
Basalts similar to OIB
–
–
•
•
Eu anomaly = plagioclase fractionation
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.
5
The Andes
Chemical composition of magmas
–
Trace elements
•
LREE enriched
•
•
Volcanic rocks similar to plutons
Basalts similar to OIB
–
–
•
•
–
Eu anomaly = plagioclase fractionation
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 NOT
MORB
Requires involvement of several
sources in varying proportions
•
Mantle wedge
–
–
–
Asthenosphere ± enriched lithosphere
Subducted crustal components
Assimilated crust of overriding plate
6
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 magmas
e.g., compare CVZ to SVZ or NVZ
Difficult to detemine how crust gets
into the magmas
•
Source contamination vs. crustal
assimilation
7
Central Volcanic Zone,
22º S, Chile
Andesite-Dacite
Stratovolcanoes on 65 km
thick continental crust
Licancabur, 5921 masl
Chilques, 5778 masl
Lascar, 5641 masl
Tatara-San
Pedro Volcanic
Complex, 36º S
Chile
122
Cezanne:Desktop Folder:Petrology
302:TSP
SpCollectionImageNameFld
1
SpCollectionImageSize
false
Descabezado Grande
firstOpen
Azul
Placeta San Pedro lavas
110
8
Continent-Continent collision zones
Granitoid plutons form in several settings
Mount
Everest
29,028’ asl
8,848 masl
9
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
•
Five separate belts of granitoids parallel
the Himalaya
–
–
–
650-700 oC wet; 800 oC dry
Very high LILE
Very radiogenic Sr and Pb
10