Igneous Petrology Lecture 7 Continental Flood Basalts Island

Igneous Petrology Lecture 7: Continental Flood Basalts, Island Arc Magmatism and Continental Arc Magmatism Tamer Abu-Alam University of Tromsø – The Arctic University of Norway Tamer. abu-alam@uit. no

Chapter 15: Continental Flood Basalts Large Igneous Provinces (LIPs) l Oceanic plateaus l Some rifts l Continental flood basalts (CFBs) Figure 15. 2. Columbia River Basalts at Hat Point, Snake River area. Cover of Geol. Soc. Amer Special Paper 239. Photo courtesy Steve Reidel.

Large Igneous Provinces (LIPs) Figure 15. 1. Map of the major large igneous provinces (LIPs) on Earth, including continental flood basalt provinces, volcanic passive margins, oceanic plateaus, aseismic submarine ridges, ocean basin flood basalts, and seamount groups. After Saunders et al. (1992) and Saunders (pers. comm. ).

Tectonic Setting of CFBs • Continental hot spots • Continental rifting may be associated with hot spots • Successful rifts • Failed rifts (aulacogens)

Figure 15. 3. Flood basalt provinces of Gondwanaland prior to break-up and separation. After Cox (1978) Nature, 274, 47 -49.

Figure 15 -4. Relationship of the Etendeka and Paraná plateau provinces to the Tristan hot spot. After Wilson (1989), Igneous Petrogenesis. Kluwer.

Figure 15. 5 Setting of the Columbia River Basalt Group in the Northwestern United States. Pink star is the location proposed by Camp and Ross (2004) of the 16. 6 Ma outbreak of the plume and plume-related basaltic volcanism. Yellow star is the location of the deep plume conduit proposed by Jordan et al. (2004). Blue areas are Quaternary basalts and pink areas are rhyolite centers. Heavy dashed curves represent the progressive younging of rhyolitic centers (with ages in Ma). Those on the east represent the proposed Yellowstone hotspot track (heavy arrow). Those on the west are the opposing westward track leading to Newberry Volcano (N) with ages reported by Jordan et al. (2004). After Camp and Ross (2004).

Figure 15. 5 (continued). The cross-section is diagrammatic, generally across southern Oregon and Idaho (south of the main CRBG) and illustrates the westward deflection of the plume head by the deep keel of the North American craton to beneath the thinner accreted terranes and the migration of the hotspot tracks both east and west. After Jordan et al. (2004) © AGU with permission.

Figure 15. 6. Time-averaged extrusion rate of CRBG basalts as a function of time, showing cumulative volume. After Hooper (1988 a) The Columbia River Basalt. In J. D. Macdougall (ed. ), Continental Flood Basalts. Kluwer. 1 -34.

Figure 15. 7 Variation in wt. % of selected major element oxides vs. Mg# for units of the Columbia River Basalt Group. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from BVTP (Table 1. 2. 3. 3), Hooper (1988 a), Hooper and Hawkesworth (1993).

Figure 15. 8. Condrite-normalized rare earth element patterns of some typical CRBG samples. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol. , 34, 1203 -1246.

Figure 15. 9. N-MORB-normalized spider diagram for some representative analyses from the CRBG. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol. , 34, 1203 -1246. Picture Gorge from Bailey (1989) Geol. Soc. Amer. Special Paper, 239, 67 -84.

Figure 15. 10 OIB-normalized spider diagram for some representative CRBG analyses. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. (data as in Figure 15 -8).

Figure 15. 11. Ce/Zr vs. Ce/Nb (un-normalized) for the basalts of the Columbia River Basalt Group. After Hooper and Hawkesworth (1993) J. Petrol. , 34, 1203 -1246.

Figure 15. 12. 87 Sr/86 Sr vs. 143 Nd/144 Nd for the CRBG. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper (1988 a), Carlson et al. (1981), Carlson (1984), Mc. Dougall (1976), Brandon et al. (1993), Hooper and Hawkesworth (1993).

Figure 15. 13. 208 Pb/204 Pb vs. 206 Pb/204 Pb for the basalts of the CRBG. Included for reference are EMI, EMII, the DUPAL group, the MORB array, and the NRHL (northern hemisphere reference line) connecting DM and HIMU mantle reservoirs from Figure 14 -6. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper (1988 a), Carlson et al. (1981), Carlson (1984), Mc. Dougall (1976), Brandon et al. (1993), Hooper and Hawkesworth (1993).

Figure 15. 14. A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett. , 162, 63 -80.

Figure 15. Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir. Winter (20010 An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 15. 6 Dewey and Burke model for the evolution of a continental rift by the concatenation of a series of 3 -rift triple junctions, each centered on a hotspot. Two arms of each hotspot link up to adjacent hotspots, although generally not perfectly. The third arm fails and becomes a rift valley (aulacogen). The hotspots need not be coeval and different segments can form sequentially. From Dewey and Burke (1974)

Chapter 16. Island Arc Magmatism l Arcuate volcanic island chains along subduction zones l Distinctly different from mainly basaltic provinces thus far § § Composition more diverse and silicic Basalt generally subordinate More explosive Strato-volcanoes most common volcanic landform

• 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

The Subduction Factory From Tatsumi, Y. (2005) The subduction factory: How it operates in the evolving Earth. GSA Today, 15, 4 -10.

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

Structure of an Island Arc 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/cm 2/sec)

Volcanic Rocks of Island Arcs • Complex tectonic situation and broad spectrum of volcanic products • High proportion of basaltic 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)

Major Elements and Magma Series a. Alkali vs. silica b. AFM c. Fe. O*/Mg. O vs. silica diagrams for 1946 analyses from ~ 30 island 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). A fourth very high K shoshonite series is rare. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC 76 (Carnegie Institute of Washington).

Figure 16. 6. a. K 2 O-Si. O 2 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 K 2 O at low Si. O 2. 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. Fe. O*/Mg. O vs. Si. O 2 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 K 2 O - Fe. O*/Mg. O diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calcalkaline 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 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 f. O 2 • High PH 2 O also depresses plagioclase liquidus more An-rich • As hydrous magma rises, DP plagioclase liquidus moves to higher T crystallization of considerable An-rich-Si. O 2 -poor plagioclase • The crystallization of anorthitic plagioclase and lowsilica, high-Fe hornblende may be an alternative mechanism for the observed calc-alkaline differentiation trend

Other Trends Spatial • “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

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. An N-MORB is included for reference (from Sun and Mc. Donough, 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 Mc. Donough (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 Mc. Donough (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 -11 a. 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 Mc. Culloch et al. (1994). Atlantic sediment data from White et al. (1985).

Pb is quite scarce in the mantle • Low-Pb mantle-derived melts susceptible to Pb contamination • U, Pb, and Th are concentrated in continental crust (high radiogenic daughter Pb isotopes) • 204 Pb non-radiogenic: 208 Pb/204 Pb, 207 Pb/204 Pb, and 206 Pb/204 Pb increase as U and Th decay • Oceanic crust also has elevated U and Th content (compared to the mantle) • Sediments derived from oceanic and continental crust • Pb is a sensitive measure of crustal (including sediment) components in mantle isotopic systems • 93. 7% of natural U is 238 U, so 206 Pb/204 Pb will be most sensitive to a crustal-enriched component 9 -20 9 -21 9 -22 234 U 206 Pb 235 U 207 Pb 232 Th 208 Pb 238 U

H I M U Figure 16. 13. Variation in 207 Pb/204 Pb vs. 206 Pb/204 Pb 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 207 Pb/204 Pb and 206 Pb/204 Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

10 Be 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 § After about 10 Ma 10 Be is no longer detectable § Not a part of main mantle systems 10 Be/9 Be averages about 5000 x 10 -11 in the uppermost oceanic sediments • In mantle-derived MORB and OIB magmas, & continental crust, 10 Be is below detection limits (<1 x 106 atom/g) and 10 Be/9 Be is <5 x 10 -14

Boron is a stable element • Very brief residence time deep in subduction zones • B in recent sediments is high (50 -150 ppm), but has a greater affinity for altered oceanic crust (10 -300 ppm) • In MORB and OIB it rarely exceeds 2 -3 ppm

10 Be/Be total vs. B/Betotal diagram (Betotal Figure 16. 14. 10 Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb. , 88, 111 -123. 9 Be because 10 Be so rare)

Petrogenesis of Island Arc Magmas Why is subduction zone magmatism a paradox?

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 Other factors, such as: • Dip of slab • Endothermic metamorphic reactions • Metamorphic fluid flow are now thought to play only a minor role

l l 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 Figure 16. 15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res. , 98, 83098319) 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 1 a Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies) 1 b Subducted oceanic and forearc sediments 1 c Seawater trapped in pore spaces Figure 16. 15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res. , 98, 83098319) 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 Figure 16. 15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res. , 98, 83098319) 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

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

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 Figure 16. Subducted crust pressure-temperature-time (P-Tt) 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. Subducted crust pressure-temperature-time (P-Tt) 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.

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

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. v. K = van Keken et al. (2002 a) isoviscous remodel of PW with improved resolution. v. KT = van Keken et al. (2002 a) model with non-Newtonian temperature- and stress-dependent wedge viscosity. After van Keken et al. (2002 a) © AGU with permission.

Subducted Crust Slab melting in mature arcs no longer precluded by models. Debate renewed.

• 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 garnetbearing (eclogite) source • Modern opinion has swung toward the nonmelted slab for most cases

Mantle Wedge P-T-t Paths

l l 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 H 2 O rising into hotter portions of the wedge (gray arrow) until H 2 O-exess solidus is crossed (lowercase 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

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.

A multi-stage, multi-source process • Mantle wedge HFS and other depleted and compatible element characteristics • Slab dehydration (and perhaps melting) LIL, 10 Be, B, etc. 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 in some cases? )

l l Phlogopite is stable beyond amphibole breakdown Wedge P-T-t paths reach phlogopite dehydration at ~ 200 km depth

Fractional crystallization takes place at a number of levels

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

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

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, 5370; 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.

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.

Chapter 17: Continental Arc Magmatism Figure 17. 3. AFM and K 2 O vs. Si. O 2 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.

Chapter 17: Continental Arc Magmatism Figure 17. 4. Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average Si. O 2 = 60. 7, K 2 O = 0. 66, data from Thorpe et al. 1984; Geist, pers. comm. ). CVZ (10 samples, ave. Si. O 2 = 54. 8, K 2 O = 2. 77, data from Deruelle, 1982; Davidson, pers. comm. ; Thorpe et al. , 1984). SVZ (49 samples, average Si. O 2 = 52. 1, K 2 O = 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.

Chapter 17: Continental Arc Magmatism Figure 17. 5. MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average Si. O 2 = 60. 7, K 2 O = 0. 66, data from Thorpe et al. 1984; Geist, pers. comm. ). CVZ (10 samples, ave. Si. O 2 = 54. 8, K 2 O = 2. 77, data from Deruelle, 1982; Davidson, pers. comm. ; Thorpe et al. , 1984). SVZ (49 samples, average Si. O 2 = 52. 1, K 2 O = 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.

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), de. Silva (1991), Kay et al. (1991), Davidson and de. Silva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 17: Continental Arc Magmatism Figure 17. 7. 208 Pb/204 Pb vs. 206 Pb/204 Pb and 207 Pb/204 Pb vs. 206 Pb/204 Pb 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), de. Silva (1991), Kay et al. (1991), Davidson and de. Silva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 17: Continental Arc Magmatism Figure 17. 8. 87 Sr/86 Sr, D 7/4, D 8/4, and d 18 O vs. Latitude for the Andean volcanics. D 7/4 and D 8/4 are indices of 207 Pb and 208 Pb 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), de. Silva (1991), Kay et al. (1991), Davidson and de. Silva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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.

Chapter 17: Continental Arc Magmatism Figure 17. 10. Map of the Juan de Fuca plate. Cascade Arc system, after Mc. Birney 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.

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.

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.

Chapter 17: Continental Arc Magmatism Figure 17. 13 a. 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.

Chapter 17: Continental Arc Magmatism Figure 17. 13 b. Spider diagram for mafic platform lavas of the High Cascades. Data from Hughes (1990, J. Geophys. Res. , 95, 1962319638). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 17: Continental Arc Magmatism Figure 17. 14. Summary of 206 Pb/204 Pb 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.

Chapter 17: Continental Arc Magmatism Figure 17. 15 a. 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.

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

Chapter 17: Continental Arc Magmatism Figure 17. 16. Schematic cross section of the Coastal batholith of Peru. The shallow flat-topped and steepsided “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.

Chapter 17: Continental Arc Magmatism Figure 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.

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.

Chapter 17: Continental Arc Magmatism Figure 17. 19. a. Initial 87 Sr/86 Sr 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. 207 Pb/204 Pb vs. 206 Pb/204 Pb 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.

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.

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).

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.

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.

Chapter 17: Continental Arc Magmatism Figure 17 -24. Pressure-temperature phase diagram showing the solidus curves for H 2 O-saturated and dry granite. An H 2 O-saturated granitoid just above the solidus at A will quickly intersect the solidus as it rises and will therefore solidify. A hotter, H 2 O-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 d. P/d. T Clapeyron slope will appear positive. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.