Chapter 17: Continental Arc Magmatism

1
Chapter 17 Continental Arc Magmatism

• Potential differences with respect to Island
  Arcs
• Thick sialic crust contrasts greatly with
  mantle-derived partial melts may more
  pronounced effects of contamination
• Low density of crust may retard ascent
  stagnation of magmas and more potential for
  differentiation
• Low melting point of crust allows for partial
  melting and crustally-derived melts

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Chapter 17 Continental Arc Magmatism
Figure 17.1. 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. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
3
Chapter 17 Continental Arc Magmatism
Figure 17.2. 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.
4
Chapter 17 Continental Arc Magmatism
Figure 17.3. AFM and K2O vs. SiO2 diagrams
(including Hi-K, Med.-K and Low-K types of Gill,
1981 see Figs. 16-4 and 16-6) for volcanics from
the (a) northern, (b) central and (c) southern
volcanic zones of the Andes. Open circles in the
NVZ and SVZ are alkaline rocks. Data from Thorpe
et al. (1982,1984), Geist (personal
communication), Deruelle (1982), Davidson
(personal communication), Hickey et al. (1986),
López-Escobar et al. (1981), Hörmann and Pichler
(1982). Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
5
Chapter 17 Continental Arc Magmatism
Figure 17.4. 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.
6
Chapter 17 Continental Arc Magmatism
Figure 17.5. 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.
7
Chapter 17 Continental Arc Magmatism
Figure 17.6. 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.
8
Chapter 17 Continental Arc Magmatism
Figure 17.7. 208Pb/204Pb vs. 206Pb/204Pb and
207Pb/204Pb vs. 206Pb/204Pb for Andean volcanics
plotted over the OIB fields from Figures 14-7 and
14-8. 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.
9
Chapter 17 Continental Arc Magmatism
Figure 17.8. 87Sr/86Sr, D7/4, D8/4, and d18O vs.
Latitude for the Andean volcanics. D7/4 and D8/4
are indices of 207Pb and 208Pb enrichment over
the NHRL values of Figure 17-7 (see Rollinson,
1993, p. 240). Shaded areas are estimates for
mantle and MORB isotopic ranges from Chapter 10.
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.
10
Chapter 17 Continental Arc Magmatism
Figure 17.9. 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. 25-95. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
11
Chapter 17 Continental Arc Magmatism
Figure 17.10. 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). Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
12
Chapter 17 Continental Arc Magmatism
Figure 17.11. Schematic cross sections of a
volcanic arc showing an initial state (a)
followed by trench migration toward the continent
(b), resulting in a destructive boundary and
subduction erosion of the overlying crust.
Alternatively, trench migration away from the
continent (c) 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.
13
Chapter 17 Continental Arc Magmatism
Figure 17.12. Time-averaged rates of extrusion of
mafic (basalt and basaltic andesite), andesitic,
and silicic (dacite and rhyolite) volcanics
(Priest, 1990, J. Geophys. Res., 95, 19583-19599)
and Juan de Fuca-North American plate convergence
rates (Verplanck and Duncan, 1987 Tectonics, 6,
197-209) for the past 35 Ma. The volcanics are
poorly exposed and sampled, so the timing should
be considered tentative. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
14
Chapter 17 Continental Arc Magmatism
Figure 17.13a. Rare earth element diagram for
mafic platform lavas of the High Cascades. Data
from Hughes (1990, J. Geophys. Res., 95,
19623-19638). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
15
Chapter 17 Continental Arc Magmatism
Figure 17.13b. Spider diagram for mafic platform
lavas of the High Cascades. Data from Hughes
(1990, J. Geophys. Res., 95, 19623-19638).
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
16
Chapter 17 Continental Arc Magmatism
Figure 17.14. Summary of 206Pb/204Pb from
sulfides in Tertiary Cascade intrusives as a
function of latitude. After Church et al. (1986),
Geochim. Cosmochim. Acta, 50, 317-328. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
17
Chapter 17 Continental Arc Magmatism
Figure 17.15a. Major plutons of the North
American Cordillera, a principal segment of a
continuous Mesozoic-Tertiary belt from the
Aleutians to Antarctica. From The Geologic Map of
North America, GSA and USGS. The Sr 0.706 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.
18
Chapter 17 Continental Arc Magmatism
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.
19
Chapter 17 Continental Arc Magmatism
Figure 17.16. 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, 1209-1220.
20
Chapter 17 Continental Arc Magmatism
Figure 17.17. 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. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
21
Chapter 17 Continental Arc Magmatism
Figure 17.18. 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.
22
Chapter 17 Continental Arc Magmatism
Figure 17.19. 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. 177-202. .
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. 235-238. ORL Ocean
Regression Line for depleted mantle sources
(similar to oceanic crust). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
23
Chapter 17 Continental Arc Magmatism
Figure 17.20. 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. 277-291.
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Chapter 17 Continental Arc Magmatism
Figure 17.21. Isotopic age vs. distance across
(a) the Western Cordillera of Peru (Cobbing and
Pitcher, 1983 in J. A. Roddick (ed.),
Circum-Pacific Plutonic Terranes. Geol. Soc.
Amer. Memoir, 159. pp. 277-291) and (b) the
Peninsular Ranges batholith of S. California/Baja
Mexico (Walawander et al. 1990 In J. L. Anderson
(ed.), The Nature and Origin of Cordilleran
Magmatism. Geol. Soc. Amer. Memoir, 174. pp. 1-8).
25
Chapter 17 Continental Arc Magmatism
Figure 17-22. Range and average
chondrite-normalized rare earth element patterns
for tonalites from the three zones of the
Peninsular Ranges batholith. Data from Gromet and
Silver (1987) J. Petrol., 28, 75-125. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
26
Chapter 17 Continental Arc Magmatism
Figure 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. Prentice
Hall.
27
Chapter 17 Continental Arc Magmatism
Figure 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. Prentice Hall.