Continental Arcs

Subduction-related Igneous activity - Continental Arcs

(Chapter 17)

last update:10/11/06

Continental Island Arc Volcanism

Potential differences with respect to Island Arcs:

Thick sialic crust contrasts greatly with mantle-derived partial melts may - and is likely a consequence of more pronounced effects of contamination

Low density of crust may retard ascent, and produce stagnation of magmas and more potential for differentiation i.e. more evolved magmas

Low melting point of crust allows for partial melting and crustally-derived melts

Subcontinental lithospheric mantle is likely to be locally enriched, and this will be reflected in magmas that are derived from this mantle

Map of western South America showing plate tectonic framework, and distribution of volcanics and crustal types.

NVZ - Accreted Mesozoic and Cenozoic oceanic crust and island arcs, 30-45 km thick

CVZ - Precambrian metamorphic crust, 50-75 km thick

SVZ - Accreted Mesozoic and Cenozoic oceanic crust and island arcs, 30-45 km thick

These are separated by inactive gaps.

This belt of igneous rocks developed over the last 500 Ma and is commonly termed an Andean-type margin.

After Thorpe and Francis (1979) Tectonophys., 57, 53-70; Thorpe et al. (1982) In R. S. Thorpe (ed.), (1982). Andesites. Orogenic Andesites and Related Rocks. John Wiley & Sons. New York, pp. 188-205; and Harmon et al. (1984) J. Geol. Soc. London, 141, 803-822. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Schematic diagram to illustrate how a shallow dip of the subducting slab can pinch out the asthenosphere from the overlying mantle wedge.

The active zones of the Andes correspond to the more steeply-dipping (>25 ) slab i.e. the mantle wedge must be involved in the melting process.

The shallow dips are considered to be the results of thicker and less dense oceanic crust e.g. Nazca Ridge.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

AFM and K₂O vs. SiO₂ diagrams (including Hi-K, Med.-K and Low-K types of Gill, 1981) for Andes volcanics

Orange circles in the NVZ and SVZ are alkaline rocks.

All rocks exhibit typical calc-alkaline trends

NVZ and SVZ are mostly high Al basaltic andesites and andesites, but with some dacites and rhyolites

CVZ is generally more Si-rich with a range of basalt to rhyolite but most commonly andesite/dacite

Data from Thorpe et al. (1982,1984), Geist (personal communication), Deruelle (1982), Davidson (personal communication), Hickey et al. (1986), López-Escobar et al. (1981), Hörmann and Pichler (1982). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chondrite-normalized REE diagram for selected Andean volcanics.

NVZ -lower HREE (suggests residual garnet in a deeper melt source).

CVZ - enriched LREE likely due to continental crustal involvement.

SVZ - low REE slope and high HREE (suggests no garnet in the melt source due to shallower slab dip)

NVZ (6 samples, average SiO₂ = 60.7, K₂O = 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, ave. SiO₂ = 54.8, K₂O = 2.77, data from Deruelle, 1982; Davidson, pers. comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO₂ = 52.1, K₂O = 1.07, data from Hickey et al. 1986; Deruelle, 1982; López-Escobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics.

Note the decoupled LIL/HFS patterns typical of subduction zone magmas i.e. there must be dehydration of ocean crust.

NVZ - pattern similar to island arc magmas.

CVZ - pattern shows more enrichment due to continental crustal involvement.

SVZ - pattern similar to island arc magmas.

Petrogenesis of the Andes

1. Major and trace element data consistent with origin from a fluid-fluxed and LIL-enriched mantle wedge above the subducting and dehydrating Nazca plate

2. Each of the volcanic zones indicate magmas interact with the local continental crust as they pass through this crust. However, the initial melts must come from the mantle wedge.

Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific analyses. Note the general Si-richer (more fractionated?) nature of the Andes magmas in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Cascade Magmatic Arc

extends 1000 km and corresponds to the oblique subduction of the Juan de Fuca plate that ends in strike-slip faulting to the N and S

Current chain of Quaternary andesitic stratovolcanoes, but volcanism goes back to the Phanerozoic

Bimodal mafic-silicic volcanism due to mantle melts and crustal melts, however more mafic melts than in the Andes

Basin and Range extension likely related to back-arc extension and/or Yellowstone hot-spot interactions

Map of the Juan de Fuca plate-Cascade Arc system

after McBirney and White, (1982) The Cascade Province. In R. S. Thorpe (ed.), Andesites. Orogenic Andesites and Related Rocks. John Wiley & Sons. New York. pp. 115-136.

Also shown is the Columbia Embayment (the western margin of pre-Tertiary continental rocks) and approximate locations of the subduction zone as it migrated westward to its present location i.e. a constructive boundary

after Hughes, 1990, J. Geophys. Res., 95, 19623-19638). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Time-averaged rates of extrusion of mafic (basalt and basaltic andesite), andesitic, and silicic (dacite and rhyolite) volcanics and Juan de Fuca-North American plate convergence rates for the past 35 Ma.

Increase in mafic magmas at 7.5 Ma due to extension

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Rare earth element diagram for mafic platform lavas of the High Cascades.

Mafic magmas range from LREE-depleted MORB-like magmas to variably enriched OIB-like magmas to subduction zone-type magmas indicative of fluid alteration

Data from Hughes (1990, J. Geophys. Res., 95, 19623-19638). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Plutonism in Continental Arcs

Major plutons of the North American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica.

Cordilleran-type batholiths - these are generally composite bodies with 100s-1000s of individual intrusions over millions of years.

Compositional range: gabbro-diorite- tonalite-granodiorite-granite -- this is similar to the volcanic equivalent compositional range

The transition is commonly from mafic magmas during extensional regimes (back-arc extension) to silicic magmas during compressional phase. This oscillation may be related to variable spreading rates.

After Anderson (1990, preface to The Nature and Origin of Cordilleran Magmatism. Geol. Soc. Amer. Memoir, 174. The Sr 0.706 line in N. America is after Kistler (1990), Miller and Barton (1990) and Armstrong (1988). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica.

After USGS.

Schematic cross section of the Coastal batholith of Peru. The shallow flat-topped and steep-sided "bell-jar"-shaped plutons are stoped into place. Successive pulses may be nested at a single locality. The heavy line is the present erosion surface.

1000 individual plutons with separate surges

The flat sides and tops are consistent with cauldron subsidence after volcanic eruptions

From Myers (1975) Geol. Soc. Amer. Bull., 86, 1209-1220.

Data span several suites from W. S. Pitcher, M. P. Atherton, E. J. Cobbing, and R. D. Beckensale (eds.), Magmatism at a Plate Edge. The Peruvian Andes. Blackie. Glasgow. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Harker-type and AFM variation diagrams for the Coastal batholith of Peru.

Calc-alkaline trends consistent with fractional crystallization of plagioclase and pyroxene +/- magnetite, and later hornblende and biotite.

Chondrite-normalized REE abundances for the Linga and Tiybaya super-units of the Coastal batholith of Peru and associated volcanics.

Note similarity between volcanics and plutonics.

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.

Schematic diagram of

(a) the formation of a gabbroic crustal underplate (higher density) at an continental arc

(b) magma conduits get shut off, basaltic magma accumulates (magmatic underplating) and the remelting of the underplate to generate tonalitic plutons (not from differentiation of a basaltic melt).

After Cobbing and Pitcher (1983) in J. A. Roddick (ed.), Circum-Pacific Plutonic Terranes. Geol. Soc. Amer. Memoir, 159. pp. 277-291.

Schematic cross section of an active continental margin subduction zone showing

(1) the dehydration of the subducting slab

(2) hydration and melting of a heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle)

(3) crustal underplating of mantle-derived melts where MASH (melting, assimilation, storage and homogenization) processes may occur, as well as crystallization of the underplates

(4) Remelting of the underplate to produce tonalitic magmas and a possible zone of crustal anatexis.

(5) As magmas pass through the continental crust they may differentiate further and/or assimilate continental crust.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

	Thick sialic crust contrasts greatly with mantle-derived partial melts may - and is likely a consequence of more pronounced effects of contamination
	Low density of crust may retard ascent, and produce stagnation of magmas and more potential for differentiation i.e. more evolved magmas
	Low melting point of crust allows for partial melting and crustally-derived melts
	Subcontinental lithospheric mantle is likely to be locally enriched, and this will be reflected in magmas that are derived from this mantle