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Arc Magmatism (Volcanic/Plutonic) •Arcuate/Linear Chains of Volcanoes And/Or Plutons Developed Above Active Subduction Zones

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Arc Magmatism (Volcanic/Plutonic) •Arcuate/Linear Chains of Volcanoes And/Or Plutons Developed Above Active Subduction Zones Arc magmatism (volcanic/plutonic) •Arcuate/linear chains of volcanoes and/or plutons developed above active subduction zones. Why are the chains arcuate? •The arc magma paradox. Why is there magmatic activity in regions where cold lithospheric slabs are being subducted into the mantle? •Three main arc types: Continental Arcs (oceanic lithosphere subducted beneath mature continental lithosphere) Island Arcs I (oceanic lithosphere subducted beneath oceanic lithosphere): Island Arcs II (oceanic crust subducted beneath thinner, commonly immature, continental fragments or peninsulas) •Examples: Active Continental Arcs: Cascades, Andes, Eastern Aleutiams Active island Arcs I: Marianas, Tonga-Kermadec, Scotia, Western Aleutians… Active island arcs II: Japan, Kamchatka, New Zealand… Inactive Plutonic Arcs: Sierra Nevada, Peninsular Ranges, Idaho batholith, Coast Range (BC) batholiths, Peruvian batholith… •Compositionally more diverse, more silicic than MORB, OIB or CFB, more explosive Tholeiitic series (aka low K series): mostly island arcs Calc-alkaline series (aka medium- to high-K series): mostly continental arcs Both lava series show complete spectrum of rock types from basalt through rhyolite. Most abundant rock type is ANDESITE Global perspective—regions where new continental crust is generated Ocean crust Ocean crust → Island Arc (IA) Ocean crust Continental crust → Continental Arc Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer. Schematic cross sectional view of an Island Arc After Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec) Summary of tectonic features of magmatic arcs Spacing: Inactive zone 60-70 km Volcanic front Arc width: 25-250 km Trench-arc gap: 160 ± 60 km Volcanic arc Alkalic, e.g., Granada trench trench—trench transform Depth from arc volcanoes to Benioff-Wadati zone: •Volcanism may migrate inward from trench with time • Main arc: 100-120 km •Volcano spacing averages ~60 - 70 km, as low as 20 km • Back arc: up to 200 km •Low dip and/or low convergence angle: amagmatic arc • Forearc (rare): <100 km Volcanoes behind the main arc: usually more K-rich Cessation of volcanism ~ 5 Ma after cessation of subduction Mariana Island Arc Island arc volcanoes: largely submarine Extension axis Trench Magmatic arc Remnant arc Martinez & Taylor 2003 Development of remnant arcs and back arc basins Mt. Mayon stratocone Examples of active arc volcanoes Strombolian eruption at Cerro Negro, Nicaragua, 1968 Andesite lava flow on Bagana, Papua New Guinea Block-and-ash flow, Montserrat Continental Arc Magmatism The dip of the subducting slab has an important role in controlling the amount of magmatic activity (if any) in a continental arc. A shallow dipping slab pinches out the asthenosphere in the overlying mantle wedge and also restricts most of the slab dehydration processes to the lithospheric mantle Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Continental Arc Magmatism Map 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. Also shown is the Columbia Embayment (the western margin of pre-Tertiary continental rocks) and approximate locations of the subduction zone as it migrated westward to its present location (after Hughes, 1990, J. Geophys. Res., 95, 19623-19638). Due to sparse age constraints and extensive later volcanic cover, the location of the Columbia Embayment is only approximate (particularly along the southern half). Reproduced from Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Continental Arc Magmatism 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. 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. Reproduced from Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Continental arc magmatism 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). Reproduced from Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall Continental Arc Magmatism Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After USGS. Reproduced from Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Volcanic Rocks of Island Arcs Table 16-1. Relative Proportions of Quaternary Volcanic Island Arc Rock Types Locality B B-A A DR Talasea, Papua 9 23 55 9 4 Little Sitkin, Aleutians 0 78 4 18 0 Mt. Misery, Antilles (lavas) 17 22 49 12 0 Ave. Antilles 17 42 39 2 Ave. Japan (lava, ash falls) 14 85 2 0 After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite A = andesite, D = dacite, R = rhyolite Note the preponderance of basaltic andesite and andesite in island arcs Arc Magmatism 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) From: R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95 and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Classification of arc volcanics The most useful single discriminator is K2O which Gill used to define 3 sub-series as shown The three andesite series. Contours represent the concentration of 2500 analyses of andesites. Andesite: grey, porphyritic rocks with abundant plagioclase + pyroxene phenocrysts (no quartz, sanidine of feldspathoids) 4 e nit ho os 3 sh High K R 2 D A B Med K 1 Low K 48 53 57 63 68 Island arc tholeiite After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K island arc series. Large symbols = averages. Differentiation within a series (most likely due to fractional crystallization) is indicated by arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag. Distinction between tholeiitic and calc-alkalic series in island arc lavas This is also known as a Miyashiro diagram. From: Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. AFM diagram is also used to distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series. Note the FeO enrichment trend in the tholeiitic series relative to the calc-alkalic series. From: Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Six andesite sub-series Low K, medium K and high K series exist in both the tholeiitic and calc-alkalic trends giving a total of six sub-series. The low-K series is almost exclusively tholeiitic and largely restricted to island arcs, the medium-K series is primarily calc-alkalic while the high-K series is mixed. Note that the points plotted are based on a fit to the trends in a K2O vs SiO2 plot and a FeO/MgO vs. SiO2 plot. The value on the trends at 57.5 SiO2 is then plotted in this diagram. Note also that there is a continuum of compositions and the pigeon holes are largely as an aid to communication. It is not uncommon for a series at a single volcano to pass from tholeiitic to calc-alkalic or vice versa. FromGill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. Mineral assemblages in arc rocks Volcanic: Basalt Andesite Dacite Rhyolite Plutonic: Gabbro Diorite Granodiorite Granite SiO2 <53 wt% 53-63 63-68 >68% An80-90 An70-80 An20-30 ~30 Plagioclase Commonly zoned Orthoclase 0 ~30 0~30 Quartz ~50% ~30% <2% Pyroxene Hornblende ~20% <5% <10% <5% Biotite Olivine ~50% <10% Total mafic Total felsics ~50% >90%.
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