1. Reading: Winter Chapter 17
Continental Arcs
Reading:
Winter Chapter 17

2. Continental Arc Magmatism *
Potential differences with respect to Island Arcs:
Thick sialic crust contrasts greatly with mantle- derived 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

3. 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 line in N. America is after Kistler (1990), Miller and Barton (1990) and Armstrong (1988).
Winter (2001)

4. Pressure-temperature phase diagram showing solidus curves for H2O- saturated 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, H2O- undersaturated 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)

5. Crustal Melting Simplified P-T phase diagram
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,
Shaded areas in (a) indicate zones of melt generation.
Figures from Winter (2001)

6. 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)

7. 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) .

8. 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

9. 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)

10. 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 (after Hughes, 1990, J. Geophys. Res., 95, ). Winter (2001)

11. 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

12. Diapirs

13. 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

14. 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

16. 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

17. How Can Dense Magma Rise?
Volumetric expansion on melting?
Exsolution of bubbles?
There must be another cause.

18. 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

19. Magma Ascent Dikes Sub-vertical cracks in brittle rock Diapirs
Bodies of buoyant magma
They squeeze through ductile material

20. Dikes * Intrusions with very small aspect ratio
Aspect: width/length = 10-2 to 10-4
Near vertical orientation
Generally meters thick

21. Dike Swarms Hundreds of contemporaneous dikes May be radial
Large radial swarms associated with mantle plumes

22. 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

23. 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

24. vertical
vertical

25. Orientation * Near-vertical dikes imply horizontal 3
Typical in areas of tectonic extension
Can be used to interpret past stress fields

26. Tectonic Regime * Extensional regime Basalts common
Compressional regime
Andesites common

27. Extensional Regime 1 is vertical 2 and 3 are are horizontal
Pm > 3
Vertical basaltic dikes rise to surface

28. Compressional Regime 3 is vertical 1 and 2 are are horizontal
Pm < 2
Basalt rise limited by neutral buoyancy

30. Tectonic Room Dilatant faults zones Bends in a fault zone
Hinge zones of folds
Domains of extension in a compressive regime

31. The Intrusion * Contacts Record length and type of effects Border zone
May be permeated with changes due to thermal, chemical, and deformational effects

32. Granite Plutons * Generally inhomogeneous in composition
Composite intrusions
Emplacement of two different magmas
Zoned intrusions
Concentric gradations

33. Composite Intrusions *
Compositionally or texturally different
Chilled, fine-grained inner contact
Variable time intervals (and cooling histories) between intrusions

34. Zoned Intrusions * Concentric parts Successively less mafic inward
Gradational contacts
Assimilation of country rock?

35. 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

37. Emplacement Process
Stoping
Brecciation
Doming
Ballooning
Void zones

38. Rare Earth Element Variation
Rare earth element diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res., 95, ). Winter (2001)

39. Spider Diagram
Spider diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res., 95, ). Winter (2001)

40. Range and average chondrite-normalized rare earth element patterns for tonalites from the three zones of the Peninsular Ranges batholith.
Data from Gromet and Silver (1987) J. Petrol., 28,
Winter (2001)