Melting of the mantleMelting of the mantleIncreasing temperature:Intraplate igneousIncreasing temperature: Intraplate igneous activity (OIB, continental basalts etc.)DiDlDecreasing pressure: Divergent plate boundaries (MORBs, rifts, back-arc basins )etc.) Adding volatiles: Convergent plate boundaries (arc lavas, continental margins, etc.)Plate Tectonic - Igneous Genesis g1. Mid-Ocean Ridges5. Back-Arc Basinsg2. Intracontinental Rifts3. Island Arcs6. Ocean Island Basalts7. Miscellaneous Intra-C ti t l A ti it4. Active Continental Margins Continental Activitykimberlites, carbonatites, anorthosites...gOcean-ocean → Island Arc (IA)Ocean-continent → Continental Arc orAi C i lM i(ACM)Active Continental Margin (ACM) Figure 16.1. 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.Structure of an Island ArcFigure 16.2. Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6joules/cm2/sec)Volcanic Rocks of Island ArcsCl iii dbd f•Complex tectonic situation and broad spectrum of volcanic productsHih ti fb lti dit d ditTable 16-1. Relative Proportions of Analyzed•High proportion of basaltic andesite and andesite– Most andesites occur in subduction zone settingspyLocality B B-AADRMt. Misery, Antilles (lavas)217 22 49 12 0Ave. Antilles217 ( 42 ) 39 2L A till171225Island Arc Volcanic Rock Types(3)Basalts are still very Lesser Antilles171225Nicaragua/NW Costa Rica16433310W Panama/SE Costa Rica134 49 16 0 0Aleutians E of Adak15536900Aleutians, Adak & W118 27 41 14 0( 3 )common and important!Little Sitkin Island, Aleutians20784180Ave. Japan (lava, ash falls)214 ( 85 ) 2 0Isu-Bonin/Mariana147 36 15 1 < 1Kuriles134 38 25 3 < 1Talasea Papua29235594Talasea, Papua9235594Scotia165333001 from Kelemen (2003a and personal comunication).2 after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesiteA = andesite, D = dacite, R = rhyoliteClassification of Igneous RocksFigure 2.3. A classification and nomenclature of volcanic rocks After IUGSof volcanic rocks. After IUGS. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Island Arc PetrogenesisFigure 16.18. A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the tl (ilt) hihmantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707Geophys. Res., 94, 46974707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.Island Arc PetrogenesisAltered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose.Further dehydration takes place at greater depths as other hydrousFurther dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa. The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about 80-100 km depth.In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause ,melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)amphibole and probably phlogopite (yellowish area)Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, dl(k)and melts at A(120 km)Fractional crystallization happens in the arc crustContinental Arc MagmatismFigure 17.2. Schematic diagram to illustrate how a shallow dip of the subducting slab can pinch out theshallow dip of the subducting slab can pinch out the asthenosphere from the overlying mantle wedge. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Continental Arc MagmatismPotential differences with respect to Island Arcs:Potential differences with respect to Island Arcs:• Thick sialic crust contrasts greatly with mantle-derived partial meltsÆmore pronounced effects ofderived partial melts Æmore pronounced effects of contamination•Low density of crust may retard ascentÆstagnationLow density of crust may retard ascentÆstagnation of magmas and more potential for differentiation• Low melting point of crust allows for partial melting gp p gand crust-derived meltsContinental Arc PetrogenesisFigure 17.23. 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. Remelting of the underplate to produce tonalitic magmas and a possible zone of crustal anatexis is also shown. As magmas pass through the continental crust they may differentiate further and/or assimilate continental crust. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.Chapter 17: Continental Arc MagmatismFigure 17-24. Pressure-temperature phase diagram showing the 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: the pressure axis is inverted to strengthen the analogy with the Earth, so a negative dP/dT Clapeyron slope will appear positive. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. PrenticeOverall Conclusions and Andesite Petrogenetic Model: IgStrong link between convergent plate boundaries and Calc-Strong link between convergent plate boundaries and Calcalkaline volcanism (and associated intrusives).Andesite is the dominant rock type found in most island arcs; more common in older, more mature arcs, such as Japan; while basalts and basaltic-andesite are more common on younger less mature arcs such as the Marianason younger, less mature arcs,