1Continental ArcsContinental ArcsReading:Winter Chapter 17Continental Arc MagmatismContinental Arc MagmatismPotential 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 meltsSouth American South American PlatePlateMap 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. 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)Subduction Subduction SectionSectionSchematic diagram to illustrate how a shallow dip of the subducting slab can pinch out the asthenosphere from the overlying mantle wedge. Winter (2001)Chemical Chemical VariationVariationAFM and K2O vs. SiO2diagrams (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. (1986), López-Escobar et al. (1981), Hörmann and Pichler(1982). Winter (2001)Chondrite-normalized REE diagram for selected Andean volcanics. 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)2MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average SiO2 = 60.7, K2O = 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, average 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)Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001)Cascade Cascade VolcanismVolcanismMap 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. (after Hughes, 1990, J. Geophys. Res., 95, 19623-19638). Winter (2001)Thick Crust Thick Crust ModelModelSchematic 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) .Extrusion RatesExtrusion RatesTime-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 VariationRare Earth Element VariationRare 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.3Spider DiagramSpider DiagramSpider diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res., 95, 19623-19638). Winter (2001)North American North American BatholithsBatholithsMajor 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 afterKistler (1990), Miller and Barton (1990) and Armstrong (1988). Winter (2001)South American South American BatholithsBatholithsMajor plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After USGS.Central Peru Cross SectionCentral Peru Cross SectionSchematic 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.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)REE REE Abundances in Abundances in PeruPeruChondrite-normalized REE abundances for the Linga andTiybaya 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)4Peruvian Peruvian Isotope DataIsotope DataInitial 87Sr/86Sr ranges for three principal segments of the Coastal batholith of Peru (after Beckinsaleet al., 1985) in W. S Pitcher, M. P. Atherton, E. J. Cobbing, and R. D.Beckensale (eds.), Magmatism at a Plate Edge. The Peruvian Andes.Blackie. Glasgow, pp. 177-202. . b. 207Pb/204Pb vs. 206Pb/204Pb