Reading: Winter, Chapter 16 Arc Magmatism Reading: Winter, Chapter 16

Island Arc Magmatism Activity along arcuate volcanic island chains along subduction zones Distinctly different from the mainly basaltic provinces Composition more diverse and silicic Basalt generally occurs in subordinate quantities More explosive than the quiescent basalts Strato-volcanoes are the most common volcanic landform

The initial petrologic model: Oceanic crust is partially melted Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another The initial petrologic model: Oceanic crust is partially melted Melts rise through the overriding plate to form volcanoes just behind the leading plate edge Unlimited supply of oceanic crust to melt Partial melts should be less mafic than their parent Ultramafic mantle  mafic basalt Basaltic oceanic crust  intermediate andesites

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. SAfter Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.

Subduction Products Characteristic igneous associations Distinctive patterns of metamorphism Orogeny and mountain belts 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 High proportion of basaltic andesite and andesite Most andesites occur in subduction zone settings

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.

Sub-series of Calc-Alkaline K2O is an important discriminator  3 sub-series The three andesite series of Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington). The three andesite series of Gill (1981) A fourth very high K shoshonite series is rare

K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series 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.

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

FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

From Winter (2001) Note: Tonga-Kermadec is both low-K and tholeiitic PNG is high-K and calc-alkaline Central America is medium-K and calc-alkaline by the criteria of (b), but borderline by that of (c). From Winter (2001)

Can determine C-A vs. Tholeiite for a particular value of SiO2 (Gill chose 57.5) -> FeO/MgO = ~ 2.3 FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

Similarly, 57. 5 wt. % SiO2 -> K2O at Low-K vs. Med-K = ~ 0 Similarly, 57.5 wt. % SiO2 -> K2O at Low-K vs. Med-K = ~ 0.75 and Med-K vs. Hi-K = ~ 1.8 FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

Tholeiitic vs. Calc-alkaline 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) From Winter (2001)

Tholeiitic vs. Calc-alkaline C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment Tholeiitic silica in the Skaergård Intrusion No change

Calc-alkaline differentiation Early crystallization of an Fe-Ti oxide phase Probably related to the high water content of calc-alkaline magmas in arcs, dissolves  high fO2 High water pressure also depresses the plagioclase liquidus and  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 is an alternative mechanism for the observed calc-alkaline differentiation trend

E Most arcs, when considered as a whole show more variation than previous Harkers and AFMs indicated Fig 16-8 shows the considerable variation in the Sunda-Banda Arc K2O-SiO2 diagram of nearly 700 analyses for Quaternary island arc volcanics from the Sunda-Banda arc. From Wheller et al. (1987) J. Volcan. Geotherm. Res., 32, 137-160.

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 Thus heterogeneous mantle sources HREE flat, so no deep garnet 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 Winter (2001) 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.

Petrogenesis of Island Arc Magmas Why is subduction zone magmatism a paradox? Paradox: Great quantities of magma are generated in regions where cool lithosphere is being subducted into the mantle and isotherms are depressed, not elevated No adequate petrogenetic model can be derived without considering the thermal regime in subduction zones

Of the many variables that can affect the isotherms in subduction zone systems, the main ones are: 1) the rate of subduction 2) the age of the subduction zone 3) the age of the subducting slab 4) the extent to which the subducting slab induces flow in the mantle wedge Other factors, such as: dip of the slab frictional heating 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) the convergence rate is slower b) the subducted slab is young and near the ridge (warmer) c) the arc is young (<50-100 Ma according to Peacock, 1991) yellow curves = mantle flow 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). This is sufficiently representative

The principal source components  IA magmas 1. The 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 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 The mantle wedge between the slab and the arc crust The arc crust The lithospheric mantle of the subducting plate The asthenosphere beneath the slab 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 last three sources on the list are unlikely to play much of a role Lithospheric mantle of subducting plate (4) is already refractory, due to the 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 The overriding crustal component is considered to be minor in island arcs, but is much more important in active continental margins

Left with the subducted crust and mantle wedge The trace element and isotopic data suggest that both contribute to arc magmatism. How, and to what extent? Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in the thermal regime shown 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

The sequence of pressures and temperatures that a rock is subjected to during an interval such as burial, subduction, metamorphism, uplift, etc. is called a pressure-temperature-time or 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

The LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source The flat HREE pattern argues against a garnet-bearing (eclogite) source Thus 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? Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 16-15. Included are some P-T-t path range for the subducted crust in a mature arc, and the wet and dry solidi for peridotite from Figures 10-5 and 10-6. The subducted crust dehydrates, and water is transferred to the wedge (arrow). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). 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 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)

Multi-stage, Multi-source Process Dehydration of the slab provides the LIL enrichments + enriched Nd, Sr, and Pb isotopic signatures These components, plus other dissolved silicate materials, are transferred to the wedge in a fluid phase (or melt?) The mantle wedge provides the HFS and other depleted and compatible element characteristics Mass-balance calculations suggest that the contribution of the subducted sediments is only a few percent in most arc systems, but the result is the high LIL/HFS patterns The nearly closed-cell induced flow in the wedge may result in progressive depletion of the wedge as arc magmas are extracted. This provides an explanation of some HREE and more compatible trace element data, which in many cases is more depleted than MORB

Phlogopite is stable in ultramafic rocks beyond the conditions at which amphibole breaks down P-T-t paths for the wedge reach the phlogopite-2-pyroxene dehydration reaction at about 200 km depth 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. If this occurs above the wet peridotite solidus, a second phase of melting will occur at B a position appropriate for the secondary volcanic chain that exists behind the primary chain in several island arcs The P-T-t paths are nearly parallel to the solidus, and may be above it or below it. Thus dehydration may or may not be accompanied by melting, so that the development of a second arc will depend critically upon the thermal and flow regime of a particular arc Melting initiated by the breakdown of the potassium-rich mica will probably be more potassic, as is true in most secondary arc occurrences. The K-h relationship probably more complex, reflecting the decreasing quantity of H2O with depth and thus the degree of partial melting, as well as the depth of melting (which becomes more alkaline with depth), and perhaps the vertical length of the rising melt diapir column within the mantle wedge

The parent magma for the calc-alkaline series is a high alumina basalt, a type of basalt that is largely restricted to the subduction zone environment, and the origin of which is controversial Some high-Mg (>8wt% MgO) high alumina basalts may be primary, as may some andesites, but most surface lavas have compositions too evolved to be primary Perhaps the more common low-Mg (< 6 wt. % MgO), high-Al (>17wt% Al2O3) types are the result of somewhat deeper fractionation of the primary tholeiitic magma which ponds at a density equilibrium position at the base of the arc crust in more mature arcs Here fractional crystallization of olivine and augite in the presence of water can produce the low-Mg high-alumina basalts and basaltic andesites observed Further rise of these hydrous basalts  volatile loss & partial crystallization Magmas that do reach the surface are therefore highly phyric basaltic andesites and andesites. More evolved liquids are probably the products of fractional crystallization in shallow chambers

Fractional crystallization occurs at various levels 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. 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