Geol 2312 Igneous and Metamorphic Petrology Lecture 12 Island Arc Magmatism PPT from Winter 2010 February 29, 2016 +

The initial petrologic model: Igneous activity related to convergent plate situations- subduction of one plate beneath another The initial petrologic model: Subducted oceanic crust is partially melted Partial melts- more silicic than source Melts rise through the overriding plate ® volcanoes just behind leading plate edge Unlimited supply of oceanic crust to melt This simple elegant model fails to explain many aspects of subduction magmatism Partial melts should be less mafic than their parent Ultramafic mantle  mafic basalt Basaltic oceanic crust  intermediate andesites

Active Continental Margin (ACM) Ocean-ocean  Island Arc (IA) Ocean-continent  Continental Arc or 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.

Subduction Products Characteristic igneous associations Distinctive patterns of metamorphism Orogeny and mountain belts Complexly Interrelated Orogenic and Subduction-related synonyms when referring to the common association of basalts, basaltic andesites, andesites, dacites, and rhyolites produced at subduction zones = “orogenic suite”

Structure of an Island Arc Note mantle flow directions (induced drag), isolated wedge, and upwelling to  back-arc basin spreading system Benioff-Wadati seismic zone (x x x x) Volcanic Front h is relatively constant  depth is important Figure 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 10-6 joules/cm2/sec)

Volcanic Rocks of Island Arcs Complex tectonic situation and broad spectrum of volcanic products High proportion of basaltic andesite and andesite Most andesites occur in subduction zone settings Basalts are still very common and important!

Major Elements and Magma Series Tholeiitic (MORB, OIT) Alkaline (OIA) Calc-Alkaline (~ restricted to SZ) All three series occur in SZ setting, yet something about SZ is different that  CALC-ALKALINE Calc-alkaline magma series is used as yet another synonym to orogenic suite by some workers Since other magma series can occur at subduction zones, I recommend that we use the term calc-alkaline strictly to denote a chemical magma series, not a tectonic association

Major Elements and Magma Series a. Alkali vs. silica b. AFM c. FeO*/MgO vs. silica diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics Figure 16.3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.

Figure 16. 4 The three andesite series of Gill (1981) Figure 16.4  The three andesite series of Gill (1981). A fourth very high K shoshonite series is rare. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington).

Figure 16.6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the 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.

Figure 16.6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

Figure 16.6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. The gray arrow near the bottom is the progressive fractional melting trend under hydrous conditions of Grove et al. (2003).

6 sub-series if combine tholeiite and C-A (some are rare) May choose 3 most common: Low-K tholeiitic Med-K C-A Hi-K mixed Figure 16.5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).

Tholeiitic vs. Calc-alkaline differentiation These Harkers represent > 1 volcano from each arc, but give the general idea nonetheless C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment (note Guatemala and PNG in FeO*/MgO) TiO2 decr due to Fe-Ti oxide CaO/Al2O3 should increase with Plag FX, but decreases in all types -> Cpx responsible for CaO SiO2 doesn’t show up in Harker, but it increases progressively in CALC-ALKALINE (although ~ constant in Thol) Figure 16.7. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Calc-alkaline differentiation Early crystallization of Fe-Ti oxide Probably related to the high water content of calc- alkaline magmas in arcs, dissolves  high fO2 High PH2O also depresses plagioclase liquidus  more An-rich As hydrous magma rises, DP  plagioclase liquidus moves to higher T  crystallization of considerable An- rich-SiO2-poor plagioclase The crystallization of anorthitic plagioclase and low- silica, high-Fe hornblende may be an alternative mechanism for the observed calc-alkaline differentiation trend

Other Trends Spatial Temporal “K-h”: low-K tholeiite near trench  C-A  alkaline as depth to seismic zone increases Some along-arc as well Antilles  more alkaline N  S Aleutians is segmented with C-A prevalent in segments and tholeiite prevalent at ends Temporal Early tholeiitic  later C-A and often latest alkaline is common Many exceptions to any trend!

Trace Elements REEs Slope within series is similar, but height varies with FX due to removal of Ol, Plag, and Pyx (+) slope of low-K  DM Some even more depleted than MORB! Others have more normal slopes ® heterogeneous mantle sources HREE flat, so no deep garnet Figure 16.10. REE diagrams for some representative Low-K (tholeiitic), Medium-K (calc-alkaline), and High-K basaltic andesites and andesites. An N-MORB is included for reference (from Sun and McDonough, 1989). After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag.

MORB-normalized Spider diagrams Intraplate OIB has typical hump Figure 14.3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

MORB-normalized Spider diagrams IA: decoupled HFS - LIL (LIL are hydrophilic) What is it about subduction zone setting that causes fluid-assisted enrichment? Figure 14.3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345. Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

Isotopes New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM Figure 16.12. Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures 13-11 and 10-15. After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985). The principal source of island arc magmas is very similar to MORB source (DM) Trace element data still require enriched components The data for the other arcs extend along 2 enrichment trends, one for the Banda arc and the other for the Lesser Antilles extend beyond the OIB field Antilles (Atlantic) and Banda and New Zealand (Pacific) can be explained by partial melting of a MORB-type source + the addition of the type of sediment that exist on the subducting plate (Pacific sediment has 87Sr/86Sr around 0.5123 and 143Nd/144Nd around 0.715) The increasing N-S Antilles enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon

From Peacock (2003) Geophysical Monograph 138 Am. Geophys. Union

Main variables that can affect the isotherms in subduction zone systems: 1) Rate of subduction 2) Age of subduction zone 3) Age of subducting slab 4) Extent to which subducting slab induces flow in the mantle wedge 5) Effects of frictional shear heating along W-B zone Other factors, such as: Dip of slab Endothermic metamorphic reactions Metamorphic fluid flow are now thought to play only a minor role

Typical thermal model for a subduction zone Isotherms will be higher (i.e. the system will be hotter) if a) Convergence rate is slower b) Subducted slab is young and near the ridge (warmer) c) Arc is young (< 50-100 Ma according to Peacock, 1991) yellow curves = mantle flow This is sufficiently representative Figure 16.15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

The principal source components  IA magmas 1. Crustal portion of the subducted slab 1a Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies) 1b Subducted oceanic and forearc sediments 1c Seawater trapped in pore spaces Figure 16.15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

The principal source components  IA magmas 2. Mantle wedge between slab and arc crust 3. Arc crust 4. Lithospheric mantle of subducting plate 5. Asthenosphere beneath slab The last three sources on the list are unlikely to play much of a role Lithospheric mantle of subducting plate (4) is already refractory (extraction of MORB at the ridge) and it heats very little in the upper 200 km of the subduction zone Asthenosphere beneath the slab (5) flows with it, but does not heat much at all, since the isotherms are essentially parallel to the flow lines at these depths Arc crust: isotherms at the base of the (predominantly andesitic) crust  the temperature is too low for melting, even under hydrous conditions (much more important in active continental margins) Figure 16.15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

Left with the subducted crust and mantle wedge Trace element and isotopic data ® both contribute to arc magmatism. How, and to what extent? Dry peridotite solidus too high LIL/HFS ratios of arc magmas  water plays a significant role in arc magmatism Since we know what the general composition of the constituents in Fig. 16-15 are, it is a matter of combining this information with the other information in the figure showing us the pressure-temperature conditions to which the constituents will be subjected as they move through the subduction zone, and considering the consequences

Sequence of pressures and temperatures a rock subjected to during burial, subduction, metamorphism, uplift, etc. is called a pressure-temperature-time (P-T-t) path Oceanic crust, as it subducts, will begin to heat at about 50-70 km depth, and will continue to heat with rising pressure, although slowly due to the depressed isotherms Mantle wedge material will follow the path of drag-induced flow, also illustrated in Fig. 16-15. This is less well known, but should follow a path (like the arrows beginning in the center of the right edge of the figure) of initial cooling from about 1100oC to about 800oC at nearly constant pressure, and then heat up toward 1000oC as pressure increases

P-T-t paths for subducted crust Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma) Subducted Crust Yellow paths = various arc ages Red paths = different ages of subducted slab The yellow P-T-t paths represent various arc ages A newly formed arc will not have subducted sufficient cool slab material to fully depress the initial ocean geotherm that existed prior to the initiation of subduction It will thus follow a path toward higher temperatures at low pressure Only after 30-70 Ma will a steady state equilibrium be attained between subduction and heating of the slab Figure 16.16. Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, 341-353).

Add solidi for dry and water-saturated melting of basalt and dehydration curves of likely hydrous phases Subducted Crust Figure 16.16. Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991). Included are some pertinent reaction curves, including the wet and dry basalt solidi (Figure 7-20), the dehydration of hornblende (Lambert and Wyllie, 1968, 1970, 1972), chlorite + quartz (Delaney and Helgeson, 1978). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. None of the P-T-t paths intersect the dry solidus -> NO partial melting of dry basaltic crust Melting of wet basalt will depend upon the availability of “free” (non-bonded) water Numerous metamorphic reactions involve dehydration of the hydrated oceanic crust and/or subducted sediments Because chlorite and hornblende are the two dominant hydrous minerals in metamorphosed hydrated basalt, the breakdown curves for chlorite + quartz and hornblende are included Biotite and other hydrous minerals will mostly dehydrate between these curves

Mature arcs (lithosphere > 25 Ma): Dehydration D releases water in No slab melting! Slab melting M in arcs subducting young lithosphere. Dehydration of chlorite or amphibole releases water above the wet solidus ® (Mg-rich) andesites directly. Subducted Crust What happens to intermediate cases, such as the curve for the 5 Ma old arc? If dehydration occurs before the subducted material reaches the wet melting point, will it melt? Depends upon the fate of the water released by dehydration If it remains in place, melting will occur when the wet solidus is reached Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus An anhydrous eclogite soon forms Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

Newer models allow for temperature and stress dependence of mantle wedge viscosity. Indicates much higher temperatures in the shallowest part of the subducted slab. Figure 16.17. P-T-t paths at a depth of 7 km into the slab (subscript = 1) and at the slab/mantle-wedge interface (subscript = 2) predicted by several published dynamic models of fairly rapid subduction (9-10 cm/yr). ME= Molnar and England’s (1992) analytical solution with no wedge convection. PW = Peacock and Wang (1999) isoviscous numeric model. vK = van Keken et al. (2002a) isoviscous remodel of PW with improved resolution. vKT = van Keken et al. (2002a) model with non-Newtonian temperature- and stress-dependent wedge viscosity. After van Keken et al. (2002a) © AGU with permission. What happens to intermediate cases, such as the curve for the 5 Ma old arc? If dehydration occurs before the subducted material reaches the wet melting point, will it melt? Depends upon the fate of the water released by dehydration If it remains in place, melting will occur when the wet solidus is reached Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus An anhydrous eclogite soon forms Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

Slab melting in mature arcs no longer precluded by models. Subducted Crust Slab melting in mature arcs no longer precluded by models. Debate renewed. What happens to intermediate cases, such as the curve for the 5 Ma old arc? If dehydration occurs before the subducted material reaches the wet melting point, will it melt? Depends upon the fate of the water released by dehydration If it remains in place, melting will occur when the wet solidus is reached Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus An anhydrous eclogite soon forms Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

Flat HREE pattern argues against a garnet- bearing (eclogite) source LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source Flat HREE pattern argues against a garnet- bearing (eclogite) source Modern opinion has swung toward the non- melted slab for most cases McCulloch (1993) suggests that young, warm subducted crust is more likely to melt before it dehydrates. Thus slab melting may be more common where the a subduction zone is close to a ridge, or during the Archean, when heat production was greater

Mantle Wedge P-T-t Paths Start at X Temperature decreases followed by pressure increase with only slight temperature increase (clockwise P-T-t)

Amphibole-bearing hydrated peridotite should melt at ~ 120 km Phlogopite-bearing hydrated peridotite should melt at ~ 200 km  second arc behind first? Figure 16.19. Calculated P-T-t paths for peridotite in the mantle wedge as it follows paths similar to the flow lines in Fig 16.15. Included are dehydration curves for serpentine, talc, pargasite, and phlogopite + diopside + orthopyroxene. Also the P-T-t path range for the subducted crust in a mature arc, and the wet and dry solidi for peridotite. Subducted crust dehydrates, and water is transferred to the wedge (labeled arrows). Areas in which the dehydration curves are crossed by the P-T-t paths below the wet solidus for peridotite are stippled and labeled D for dehydration. Areas in which the dehydration curves are crossed above the wet solidus are hatched and labeled M for melting. Note that although the slab crust usually dehydrates, the wedge peridotite melts as pargasite dehydrates (Millhollen et al., 1974) above the wet solidus. An alternative model involves dehydration of serpentine  chlorite nearer the wedge tip (lower-case d) with H2O rising into hotter portions of the wedge (gray arrow) until H2O-exess solidus is crossed (lower-case m). A second melting may also occur as phlogopite dehydrates in the presence of two pyroxenes (Sudo, 1988). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Crust and Mantle Wedge Areas in which the dehydration curves are crossed by the P-T-t paths below the wet solidus for peridotite are blue and labeled D for dehydration Areas in which the dehydration curves are crossed above the wet solidus are purple and labeled M for melting. Note that although the slab crust usually dehydrates, the wedge peridotite melts as amphibole dehydrates above the wet solidus A second melting may also occur as phlogopite dehydrates in the presence of two pyroxenes.

Island Arc Petrogenesis Figure 16.18. A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford. 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 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) Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)

Fractional crystallization takes place at a number of levels In the shallower chambers the calc-alkaline fractionation trend takes place in a hydrous magma with the fractionation of magnetite, hornblende, and/or a highly anorthitic plagioclase, as discussed previously The restriction of calc-alkaline magmas to subduction zones may thus result from the uniquely high water content or the thickened arc crust that causes the primary tholeiites to pond and fractionate The SZ environment is a complex one, and the generation of arc magmas reflects a number of sources and stages The wedge may be heterogeneous and variably depleted/enriched, the subducted material has a variety of crustal and sedimentary constituents The thermal and flow patterns are variable, and the nature of the fluid is poorly constrained and probably variable as well