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Continental Arcs Continental Arc Magmatism Potential differences with respect to Island Arcs: Continental Arcs – Thick sialic crust contrasts greatly with mantle- derived partial melts may → more pronounced effects of contamination Reading: – Low density of crust may retard ascent → stagnation Winter Chapter 17 of magmas and more potential for differentiation – Low melting point of crust allows for partial melting and crustally-derived melts South American Subduction Plate Section Map of western South America showing the plate tectonic framework, and the distribution of volcanics and crustal types. NVZ, CVZ, and SVZ are the northern, central, and southern volcanic zones. Schematic diagram to illustrate After Thorpe and Francis (1979) Tectonophys., 57, 53-70; Thorpe et how a shallow dip of the al. (1982) In R. S. Thorpe (ed.), subducting slab can pinch out (1982). Andesites. Orogenic Andesites the asthenosphere from the and Related Rocks. John Wiley & overlying mantle wedge. Winter Sons. New York, pp. 188-205; and (2001) Harmon et al. (1984) J. Geol. Soc. London, 141, 803-822. Winter (2001) Chemical Variation AFM and K2O vs. SiO2 diagrams (including Hi-K, Med.-K and Low-K types of Gill, 1981; see Figs. 16-4 and 16-6) for volcanics from the (a) northern, (b) central and (c) southern volcanic zones of the Andes. Open circles in the NVZ and SVZ are alkaline rocks. Data from Thorpe et al. (1982,1984), Geist (personal communication), Deruelle (1982), Davidson (personal communication), Hickey et al. Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average SiO = 60.7, K O = 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). CVZ (1986), López-Escobar et al. 2 2 (10 samples, ave. SiO = 54.8, K O = 2.77, data from Deruelle, 1982; Davidson, pers. (1981), Hörmann and Pichler 2 2 comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO2 = 52.1, K2O = 1.07, data from (1982). Winter (2001) Hickey et al. 1986; Deruelle, 1982; López-Escobar et al. 1981). Winter (2001) 1 MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 samples, average SiO = 60.7, K O = 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). 2 2 Andean and 1484 SW Pacific analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. CVZ (10 samples, average SiO = 54.8, K O = 2.77, data from Deruelle, 1982; Davidson, 2 2 Wiley. New York, pp. 25-95. Winter (2001) 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) Thick Crust Cascade Model Volcanism Schematic cross sections of a volcanic arc showing an initial state (a) followed by trench migration toward the Map of the Juan de Fuca continent (b), resulting in a destructive plate-Cascade Arc system, boundary and subduction erosion of after McBirney and White, the overlying crust. Alternatively, (1982) The Cascade Province. trench migration away from the In R. S. Thorpe (ed.), continent (c) results in extension and a Andesites. Orogenic Andesites constructive boundary. In this case the and Related Rocks. John extension in (c) is accomplished by Wiley & Sons. New York. pp. “roll-back” of the subducting plate. An 115-136. (after Hughes, 1990, alternative method involves a jump of J. Geophys. Res., 95, 19623- the subduction zone away from the 19638). Winter (2001) continent, leaving a segment of oceanic crust (original dashed) on the left of the new trench. Winter (2001) . Extrusion Rates Rare Earth Element Variation Time-averaged rates of extrusion of mafic (basalt and basaltic andesite), andesitic, and silicic (dacite and rhyolite) volcanics (Priest, 1990, J. Geophys. Res., 95, 19583-19599) and Juan de Fuca-North American plate convergence rates (Verplanck and Duncan, 1987 Tectonics, 6, 197-209) for the past 35 Ma. The volcanics are poorly exposed and sampled, so the timing should be considered tentative. Winter (2001) Rare earth element diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res., 95, 19623-19638). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 2 Spider Diagram North American Batholiths Major plutons of the North American Cordillera, a principal segment of a continuous Mesozoic- Tertiary belt from the Aleutians to Antarctica. 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) Spider diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res., 95, 19623-19638). Winter (2001) South American Central Peru Cross Section Batholiths 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. From Myers (1975) Geol. Soc. Amer. Bull., 86, 1209-1220. REE Abundances in Peru 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) Figure 17-17. Harker-type and AFM variation diagrams for the Coastal batholith of Peru. 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) 3 Peruvian Isotope Data Continental Underplating Initial 87Sr/86Sr ranges for three principal segments of the Coastal batholith of Peru (after Beckinsale et al., 1985) in W. S Pitcher, M. P. Schematic diagram Atherton, E. J. Cobbing, and R. D. illustrating (a) the formation Beckensale (eds.), Magmatism at a of a gabbroic crustal Plate Edge. The Peruvian Andes. underplate at an continental Blackie. Glasgow, pp. 177-202. b. 207Pb/204Pb vs. 206Pb/204Pb data for arc and (b) the remelting of the plutons (after Mukasa and the underplate to generate Tilton, 1984) in R. S. Harmon and tonalitic plutons. After B. A. Barreiro (eds.), Andean Cobbing and Pitcher (1983) Magmatism: Chemical and Isotopic Constraints. Shiva. Nantwich, pp. in J. A. Roddick (ed.), 235-238. ORL = Ocean Circum-Pacific Plutonic Regression Line for depleted Terranes. Geol. Soc. Amer. mantle sources (similar to oceanic Memoir, 159. pp. 277-291. crust). Winter (2001) Isotopic age vs. distance across (a) the Western Cordillera of Peru (Cobbing and Pitcher, 1983 in J. A. Roddick (ed.), Circum-Pacific Plutonic Terranes. Geol. Soc. Amer. Range and average chondrite-normalized rare earth element patterns for tonalites Memoir, 159. pp. 277-291) and (b) the Peninsular Ranges batholith of S. California/Baja from the three zones of the Peninsular Ranges batholith. Data from Gromet and Mexico (Walawander et al. 1990 In J. L. Anderson (ed.), The Nature and Origin of Cordilleran Magmatism. Geol. Soc. Amer. Memoir, 174. pp. 1-8). Silver (1987) J. Petrol., 28, 75-125. Winter (2001) 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 Pressure-temperature phase diagram showing solidus curves for H2O-saturated and dry (including enriched sub-continental lithospheric mantle), crustal underplating of mantle- granite. An H2O-saturated granitoid just above the solidus at A will quickly intersect the derived melts where MASH processes may occur, as well as crystallization of the solidus as it rises and will therefore solidify. A hotter, H2O-undersaturated granitoid at underplates. Remelting of the underplate to produce tonalitic magmas and a possible zone B will rise further before solidifying. Note: because the pressure axis is inverted, a of crustal anatexis is also shown. As magmas pass through the continental crust they may negative dP/dT Clapeyron slope will appear positive. Winter (2001) differentiate further and/or assimilate continental crust. Winter (2001) 4.
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