Transcript Continental Arcs

Continental Arcs
Reading:
Winter Chapter 17
Continental Arc Magmatism *
Potential differences with respect to Island Arcs:
–Thick sialic crust contrasts greatly with mantlederived partial melts may  more pronounced
effects of contamination
–Low density of crust may retard ascent 
stagnation of magmas and more potential for
differentiation
–Low melting point of crust allows for partial
melting and crustally-derived melts
North American
Batholiths
Major plutons of the North American
Cordillera, a principal segment of a
continuous Mesozoic-Tertiary belt from
the Aleutians to Antarctica.
Figure 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)
•Pressure-temperature
phase diagram showing
solidus curves for H2Osaturated and dry
granite.
•An H2O-saturated
granitoid just above the
solidus at A will quickly
intersect the solidus as it
rises and will therefore
solidify.
•A hotter, H2Oundersaturated granitoid
at B will rise further
before solidifying.
Note: because the
pressure axis is inverted,
a negative dP/dT
Clapeyron slope appears
positive.
Winter (2001)
Crustal Melting
a. Simplified P-T phase diagram
b. Quantity of melt generated
during the melting of
muscovite-biotite-bearing
crustal source rocks, after
Clarke (1992) Granitoid Rocks.
Chapman Hall, London; and
Vielzeuf and Holloway (1988)
Contrib. Mineral. Petrol., 98,
257-276.
c. Shaded areas in (a) indicate
zones of melt generation.
Figures from Winter (2001)
Subduction
Section
Schematic diagram to
illustrate how a
shallow dip of the
subducting slab can
pinch out the
asthenosphere from
the overlying mantle
wedge.
Winter (2001)
Thick Crust Model
Schematic cross sections of a
volcanic arc showing an initial state
(a) followed by trench migration
toward the continent (b), resulting in a
destructive boundary and subduction
erosion of the overlying crust.
Alternatively, trench migration away
from the continent (c) results in
extension and a constructive
boundary. In this case the extension
in (c) is accomplished by “roll-back” of
the subducting plate.
An alternative method involves a
jump of the subduction zone away
from the continent, leaving a segment
of oceanic crust (original dashed) on
the left of the new trench.
Winter (2001) .
Continental
Underplating *
Schematic diagram
illustrating (a) the formation
of a gabbroic crustal
underplate at an continental
arc and (b) the remelting of
the underplate to generate
tonalitic plutons.
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 the
dehydration of the subducting slab, hydration and melting of a heterogeneous mantle
wedge (including enriched sub-continental lithospheric mantle), crustal underplating of
mantle-derived melts where MASH processes may occur, as well as crystallization of the
underplates. Winter (2001)
Cascade Arc
System
Map of the Juan de
Fuca plate
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. (after
Hughes, 1990, J. Geophys.
Res., 95, 19623-19638).
Winter (2001)
Instabilities *
• A layer of less dense material overlain
by a denser material is unstable
• The upper layer develops undulations
and bulges (Rayleigh-Taylor
instabilities)
• The spacing of the bulges depends on
the thickness of the light layer and its
density contrast with the heavy layer
Diapirs
Diapir Ascent
• Velocity of ascent depends on diapir
size and shape
• A sphere is the most efficient shape
• Surface area ~ frictional resistance
• Volume ~ buoyant driving force
• Rise velocity proportional to area
squared
Neutral Buoyancy *
• Positively buoyant
– Melt less dense than surrounding rocks
– Primary basalt magma surrounded by
mantle peridotite
• Negatively buoyant
– Melt more dense than surrounding rocks
– Olivine basalt intruded into continental
crust
Density Filter
• Crustal rocks block the
ascent of denser
magmas
• Heat from these
magmas melt the lower
crust
• Residual melts may rise
• Exsolved volatiles also
facilitate rise
How Can Dense Magma
Rise?
• Volumetric expansion on melting?
• Exsolution of bubbles?
• There must be another cause.
Magma Overpressure *
• For a magma lens, pressure is equal to
the lithostatic load
Pm = r g z
• The pressure can be greater in a conduit
connecting a deeper pocket to the surface
• This overpressure can be great enough to
bring denser magma to the surface
Magma Ascent
Dikes
– Sub-vertical cracks in brittle rock
Diapirs
– Bodies of buoyant magma
– They squeeze through ductile material
Dikes *
• Intrusions with very small aspect ratio
• Aspect: width/length = 10-2 to 10-4
• Near vertical orientation
• Generally 1 - 2 meters thick
Dike Swarms
• Hundreds of
contemporaneous
dikes
• May be radial
• Large radial swarms
associated with
mantle plumes
Intrusion into Dikes *
• Stress perpendicular to the fracture is
less than magma pressure
• Pressure must overcome resistance to
viscous flow
• Magma can hydrofracture to rock and
propagate itself
Stress for Dikes
• Dikes are hydraulic tensile fractures
• They lie in the plane of 1 and 2
• They open in the direction of 3
• They are good paleostress
indicators
 vertical
 vertical
Orientation *
• Near-vertical dikes imply
horizontal 3
• Typical in areas of tectonic
extension
• Can be used to interpret
past stress fields
Tectonic Regime *
• Extensional regime
–Basalts common
• Compressional regime
–Andesites common
Extensional Regime
 1 is vertical
 2 and 3 are are
horizontal
• Pm > 3
• Vertical basaltic
dikes rise to surface
Compressional Regime
 3 is vertical
 1 and 2 are are
horizontal
• Pm < 2
• Basalt rise limited
by neutral
buoyancy
Tectonic Room
• Dilatant faults zones
• Bends in a fault zone
• Hinge zones of folds
• Domains of extension
in a compressive
regime
The Intrusion *
Contacts
– Record length and type of effects
Border zone
– May be permeated with changes
due to thermal, chemical, and
deformational effects
Granite Plutons *
• Generally inhomogeneous in composition
• Composite intrusions
– Emplacement of two different magmas
• Zoned intrusions
– Concentric gradations
Composite Intrusions *
• Compositionally or texturally different
• Chilled, fine-grained inner contact
• Variable time intervals (and cooling
histories) between intrusions
Zoned
Intrusions *
• Concentric parts
• Successively less
mafic inward
• Gradational contacts
• Assimilation of
country rock?
Batholiths *
• An example: Sierra Nevada Batholith, CA
• A group or groups of separately intruded
plutons with a composite volume of 106 km3
• Age extends through the entire Mesozoic era
(>130 my)
• Average pluton volume is ~ 30 km3
Emplacement Process
• Stoping
• Brecciation
• Doming
• Ballooning
• Void zones
Rare Earth Element Variation
Rare earth element diagram for mafic platform lavas of the High Cascades.
Data from Hughes (1990, J. Geophys. Res., 95, 19623-19638). Winter (2001)
Spider Diagram
Spider diagram for mafic platform lavas of the High Cascades. Data from Hughes
(1990, J. Geophys. Res., 95, 19623-19638). Winter (2001)
Range and
average
chondritenormalized rare
earth element
patterns for
tonalites from the
three zones of the
Peninsular
Ranges batholith.
Data from Gromet
and Silver (1987)
J. Petrol., 28, 75125.
Winter (2001)