Continental Arcs Reading Winter Chapter 17 Continental Arc

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 H 2 Osaturated and dry granite. • An H 2 O-saturated granitoid just above the solidus at A will quickly intersect the solidus as it rises and will therefore solidify. • A hotter, H 2 Oundersaturated granitoid at B will rise further before solidifying. Note: because the pressure axis is inverted, a negative d. P/d. T Clapeyron slope appears positive. Winter (2001)

Crustal Melting a. Simplified P-T phase diagram b. Quantity of melt generated during the melting of muscovitebiotite-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 Mc. Birney 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, 1962319638). 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

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 horizontal • P m > 3 • Vertical basaltic dikes rise to surface

Compressional Regime 3 is vertical 1 and 2 are horizontal • P m < 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 km 3 • Age extends through the entire Mesozoic era (>130 my) • Average pluton volume is ~ 30 km 3

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)