1. Volcanic Arcs, Chapters 16 and 17

2. Ocean-ocean convergence  Island Arc (IA)
Ocean-continent convergence  Continental Arc
Figure 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.

3. Arcs are: Arcuate volcanic chains above subduction zones
Distinctly different from mainly basaltic provinces thus far
Compositions more diverse
Basalt generally subordinate
More explosive: viscous, cool, magmas trap gas
Strato-volcanoes most common volcanic landform

4. Chapter 16. Island Arc Magmatism

5. 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 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)

6. Volcanic Rocks of Island Arcs
Complex tectonic situation and broad spectrum of rock types
High proportion of Basaltic - andesite and Andesite
Most Andesites occur in subduction zone settings

7. Recall Major Magma Series
Alkaline series (OIA ocean island alkaline)
Sub-alkaline types:
Tholeiitic series (MORB, OIT)
Calc-Alkaline series (IA island arcs)
C-A ~ restricted to magmas generated near subduction zones, but keep in mind other series occur there too
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

8. Major Magma Series visualized with Major Elements
a. Alkali vs. silica all
b. AFM for subalkaline
c. FeO*/MgO vs. silica
Diagrams for 1,946 analyses from ~ 30 volcanic island arcs and continental arcs
Figure Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90,

9. above a subduction zone are calc-alkaline.
Not all volcanic arcs
above a subduction zone are calc-alkaline.
Figure b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

10. Sub-series Calc-Alkaline
K2O is an important discriminator  Gill (1981) recognized three Andesite sub-series
Figure 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

11. Figure 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 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.

12. If partition on basis of K versus Tholeiitic/calc-alkaline, most common samples are:
Low-K tholeiitic
Med-K C-A
Hi-K mixed
Figure 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).

13. Tholeiitic vs. Calc-alkaline differentiation for our three examples
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 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

14. Tholeiitic vs. Calc-alkaline differentiation seems to depend on K
C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment
High K

15. Calc-alkaline differentiation WHY?
Early (as opposed to late in Tholeiites) crystallization of an Fe-Ti oxide phase.
Probably related to the high water content of calc-alkaline magmas in arcs
Iron is removed early so a middle fractionation high iron composition cannot occur as it does in Tholeiites

16. Other Trends Spatial Temporal Antilles  more alkaline N  S
Aleutians segmented with C-A prevalent in center and tholeiite prevalent at ends
IDEA: source/collection points for high K clays (Illite) near trench?
Temporal
Early Tholeiitic  later C-A and often latest alkaline is common
Many exceptions to any trend!

17. Trace Elements REEs HREE flat in all,
so garnet, which sequesters the HREEs, not in equilibrium with the melt
Garnet last to go in partial melting of Lherzolite. If melted, HREE would be high.
also not from subducted basalt, which becomes eclogite with garnet at 110 km.
Trace Elements
The HREE are flat, implying that garnet, which strongly partitions (holds) the HREE, was not in equilibrium with the melt. Melts derived from eclogite are depleted in HREE (abundant garnet in residue). This causes the characteristic low HREE
Figure 16-10

18. MORB-normalized Spider diagrams
IA: high LIL (LIL are hydrophilic), low HFS
What is it about subduction zone setting that causes fluid-assisted enrichment?
HFS=High Field-strength
Intraplate OIB has similar hump
Most incompatible
Figure 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
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.

19. Isotopes
New Britain, Marianas, Aleutians, and South Sandwich volcanics plot show sediment contamination of DM
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 0.715and 143Nd/144Nd around )
The increasing N-S Antilles Nd enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon
The principal source of island arc magmas is very similar to MORB source
Although the 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 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
Figure Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures and After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).

20. Pb in some arcs overlap with the MORB data; depleted mantle component is a major reservoir for subduction zone magmas
Majority of data enriched in radiogenic lead (207Pb and 206Pb), trending toward the appropriate oceanic marine sedimentary reservoir
Pb in some arcs overlap with the MORB data, again suggesting that a depleted mantle component is a major source reservoir for subduction zone magmas
Majority of data enriched in radiogenic lead (207Pb and 206Pb), trending toward the appropriate oceanic marine sedimentary reservoir
Several arcs could = mixing of DM, PREMA, and sedimentary sources
Sunda data extends to EMII
Aleutians follow nice mixing line between DM or PREMA and Atlantic sediments, perhaps extending beyond toward HIMU (Fig. 14-6)
Pb data clearly indicate a sedimentary component in arc magmas
But When?? Ancient EM component or recent sedimentary additions to the mantle?
Nd-Sr-Pb isotopes cannot distinguish
Figure Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

21. 10Be created by cosmic rays + oxygen and nitrogen in upper atmos.
 Earth by precipitation & readily  clay-rich oceanic sediments
Half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10Be is no longer detectable. 9Be is stable, natural.
10Be/9Be averages about 5000 x  in the uppermost oceanic sediments
In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x atom/g) and 10Be/9Be is <5 x 10-14
9Be is a stable natural isotope & used as a normalization factor

22. Boron B is a stable element
Very brief residence time deep in subduction zones
B in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm
Very brief residence time deep in subduction zone source areas & cycles quickly through to shallow crustal and hydrospheric systems
B concentrations in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm

23. 10Be/Betotal vs. B/Betotal diagram (Betotal  9Be since 10Be is so rare).
This is the smoking gun, the evidence for the fluids (mostly ion-rich water) squeezed out of the sediments.
Figure Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88,
Each arc studied formed linear arrays, each arc having a unique slope
Also shown are other known reservoirs, including typical mantle (virtually no 10Be or B), hydrated and altered oceanic crust (high B, low 10Be), and young pelagic oceanic sediments (low B and 10Be/Be extending off the diagram up to 2000)
The simplest explanation: each arc represents a mixing line between a mantle reservoir (near the origin) and a fluid (or melt) reservoir, that is specific for each arc and itself a mixture of slab crust and sediment
Hypothetical fields for each arc are illustrated, but the exact location along the extrapolated line is unknown

24. The potential 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
2. The mantle wedge between the slab and the arc crust
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

25. Not 1a the subducted basalt fide flat HREEs
The trace element and isotopic data suggest that both 1b and 1c, the subducted sediments and water and 2, the mantle wedge 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 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

26. Freezing Point Depression always occurs in a mixture

27. An upside-down PT diagram
Even small amounts of water (0.5%) and carbon dioxide (0.5%) strongly depress the temperatures of the solidus, moving it below the geotherm at all depths. This effect dominates in subduction environments, where arc magmas are generated. (Modified from B. M. Wilson (1989) Igneous petrogenesis: a global tectonic approach. Chapman and Hall, London.)
An upside-down PT diagram
Effects of the addition of small amounts of volatiles to mantle Iherzolite. A mantle adiabat with potential temperature of 1280 °C is shown for reference.

28. Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first?
Crust and Mantle Wedge
Figure Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 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 The subducted crust dehydrates, and water is transferred to the wedge (arrow). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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.

29. The data from LIL Large Ion Lithophiles and HFS High Field Strength trace elements 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 a non-melting subducted lithosphere slab model for most cases of IA genesis
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

30. Island Arc Petrogenesis Model
Mantle here is too shallow to have Garnet. Subducted slab turns to Eclogite with Garnet at 110 km.
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
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
Figure 16-11b. 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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.

31. Chapter 17: Continental Arc Magmatism
Figure NVZ, CVZ, and SVZ are the northern, central, and southern volcanic zones.

32. Continental Volcanic Arcs
Potential differences with respect to Island Arcs:
Assimilation of thick silica-rich crust versus mantle-derived partial melts ® more pronounced effects of contamination
Low density of crust may slow magma ascent ® more potential for differentiation
Low melting point of crust allows for partial melting and some crust-derived melts

33. A subducting slab with shallow dip can pinch out the asthenosphere from the overlying mantle wedge
Lithospheric Mantle too shallow to have garnet
Figure Schematic diagram to illustrate how a shallow 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.

34. SVZ has a flat HREE which suggests a shallow garnet-free source
NVZ and CVZ have a steep slope with depleted HREE which suggests a deep garnet rich source, (the garnets don’t melt) consistent with a steep slab dip angle and aesthenosphere source.
Figure 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) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

35. LILs are very soluble in aqueous fluids
LILs are very soluble in aqueous fluids. LIL enrichment of the mantle wedge via aqueous fluids from dehydration of the subducting slab and sediments. Similar to Island Arcs
Figure MORB-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, 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) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

36. Assimilation The CVZ exhibits substantial crustal contamination
Recall low 143Nd/144Nd and high 87Sr/86Sr is due to an isotopically enriched source such as continental crust contamination.
The CVZ exhibits substantial crustal contamination
Figure Sr vs. Nd isotopic ratios for the three zones of the Andes. Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

37. Andean Pb enrichments are not much greater than OIBs, and could be derived almost solely from a subducted sediment
Figure Pb/204Pb vs. 206Pb/204Pb and 207Pb/204Pb vs. 206Pb/204Pb for Andean volcanics plotted over the OIB fields from Figures 14-7 and Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

38. Andean chemistry is similar to Island Arcs
Andean chemistry is similar to Island Arcs. They also have as their main source the depleted mantle above the subducted slab.
However, Andean volcanics are more evolved, as they must pass through continental lithosphere, which has a lower melting point than the rising magma.
Figure 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 Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

39. Figure 17-11. Schematic cross sections of a volcanic arc showing
an initial state followed by
trench migration toward the continent resulting in a destructive boundary and subduction erosion of the overlying crust.
Alternatively, trench migration away from the continent 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) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

40. Figure 17-10. Map of the Juan de Fuca plate-Cascade Arc system
Also shown are the approximate locations of the subduction zone as it migrated westward to its present location.

41. Hundreds to thousands of individual intrusions
The range of volcanics from basalts to rhyolites is matched by the plutonics:
Gabbro -> diorite -> tonalite -> granodiorite -> granite Q
Quartzolite 90 90
Quartz-rich
Granitoid 60 60
Granite
Grano-
Tonalite
Alkali Feldspar Granite
diorite 20 20
Quartz
Quartz
Quartz
Syenite
Monzonite
Monzodiorite 5
Syenite
Monzonite
Monzodiorite 10 35 A 65 90 P
Figure 17-15a. 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 line in N. America is after Kistler (1990), Miller and Barton (1990) and Armstrong (1988). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

42. Figure 17-15b. Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After USGS.

43. Granitoid magmas rise to, and freeze at, similar shallow subvolcanic levels of the crust.
Figure Schematic 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,

44. Notice that the great majority of Peruvian samples are calc-alcaline
Consistent with fractional crystallization of plagioclase and pyroxene +/- magnetite, later giving away to hornblende and biotite , from initial gabbroic, tonalitic, or quartz diorite parental material
Notice that the great majority of Peruvian samples are calc-alcaline
Figure 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.

45. Coastal Peru batholiths have the same REE profiles as coastal Peru volcanics
Figure Chondrite-normalized REE abundances for the Linga and Tiybaya 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) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

46. Lima segment intruded into younger, thinner crust so radiogenic 87Sr low, reflecting the mantle derived parent. Arequipa intrudes and assimilated old thick crust so 87Sr high.
Lima segment has high 206Pb reflecting minor assimilation of Pacific sediments
Figure a. Initial 87Sr/86Sr ranges for three principal segments of the Coastal batholith of Peru (after Beckinsale et 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 b. 207Pb/204Pb vs. 206Pb/204Pb data for the plutons (after Mukasa and Tilton, 1984) in R. S. Harmon and B. A. Barreiro (eds.), Andean Magmatism: Chemical and Isotopic Constraints. Shiva. Nantwich, pp ORL = Ocean Regression Line for depleted mantle sources (similar to oceanic crust). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

47. Why are granitoids so abundant?
Experiments show Tonalites
(granitoids with low K-spar) can be formed by the partial fusion remelting of gabbroic magmas under hydrous conditions.
Up-arched mantle results in partial melting and underplate gabbros.
During later compression, heat added by more underplate magmas remelts the underplate gabbros to produce tonalites.
Figure Schematic diagram illustrating (a) the formation of a gabbroic crustal underplate at an continental arc and (b) the remelting of the underplate to generate tonalitic plutons. After Cobbing and Pitcher (1983) in J. A. Roddick (ed.), Circum-Pacific Plutonic Terranes. Geol. Soc. Amer. Memoir, 159. pp

48. Figure 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. Prentice Hall.