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Subduction zone magmatism

Subduction zone magmatism. Activity along arcuate volcanic island chains along subduction zones Distinctly different from the mainly basaltic provinces thus far Composition more diverse and silicic Basalt generally occurs in subordinate quantities

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Subduction zone magmatism

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  1. Subduction zone magmatism

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

  3. Economic geology • Gold, copper, etc. as hydrothermal deposits around plutons (cf. Andes – Chile) • Submarine alteration of volcanic/volcanoclastic rocks occasionally precipitates (or concentrates) Cu Zn Pb

  4. 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.

  5. Subduction Products • Characteristic igneous associations • Distinctive patterns of metamorphism • Orogeny and mountain belts Complexly Interrelated

  6. Island vs. Continental arc: • Continental arcs have • Thicker lithosphere (deeper melting?/melting of slightly different mantle?) • Thicker crust: possible interactions with preexisting crust/lithosphere • Island arcs are « simpler » as they allow to focus on the primary processes

  7. 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 10-6joules/cm2/sec)

  8. Location of the volcanic arc • Whatever the dip of the Benioff plane, the (main) arc is 100 km above the slab

  9. 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

  10. Major Elements and Magma Series • Tholeiitic (MORB, OIT) • Alkaline (OIA) • Calc-Alkaline (~ restricted to subduction zones)

  11. Arc alkaline series Arc calc-alkaline (B-BA-A-D-R) Arc tholeites

  12. Island-arc subalkaline series

  13. FreshAndesite, note black color,and fracturingOregon

  14. Andesite, note amp -120 cleavage, biotite - brown, augite green, plag zoned

  15. Andesite subhedral phenocryst of plag and pyroxene in fine grained Matrix

  16. Zoned plag in andesite

  17. Dacite, with zoned plag, quartz (untwinned), in fine grained matrix

  18. Perlitic cracks in rhyolite, magnetite, and alkaline feldspar

  19. Rhyolite in glass alkaline phenocrysts with glass inclusions, mag crystals Perlitic cracks.

  20. Flow texture in rhyolite brown color due to devitrification

  21. Welded tuff

  22. Devitrification in rhyolite, spherulites

  23. Island arc alkaline series

  24. Trachyte, alkaline felspar, no twinning, in fine matrix, gas vesicles dark patches

  25. Trachytic texture (aligned feldspars caused flow in a viscose melt)

  26. Trachyte, K-spar untwinned

  27. 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

  28. 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.

  29. Sub-series of Calc-Alkaline • K2O is an important discriminator  3 sub-series Figure 16-4. 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).

  30. Figure 16-6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

  31. Figure 16-6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

  32. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

  33. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

  34. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

  35. May choose 3 most common: • Low-K tholeiitic 6 sub-series if combine tholeiite and C-A (some are rare) • Med-K C-A • Hi-K mixed Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).

  36. Figure 16-9. Major phenocryst mineralogy of the low-K tholeiitic, medium-K calc-alkaline, and high-K calc-alkaline magma series. B = basalt, BA = basaltic andesite, A = andesite, D = dacite, R = rhyolite. Solid lines indicate a dominant phase, whereas dashes indicate only sporadic development. From Wilson (1989) Igneous Petrogenesis, Allen-Unwin/Kluwer.

  37. Trace elements • Decoupling of LIL and HFS (compare OIB) • Nb-Ta « anomaly » • No fractionnation MREE/HREE • Role of fluids (as opposed to unifromally enriched source) • Nb-Ta rich phases in the residuum (Ti-oxides: rutile) • NoGarnet in the residuum

  38. Volatile rich andesite, Oregon

  39. Bombs in Andesite

  40. Isotopes • New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM Figure 16-12. Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures 13-11 and 10-15. After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).

  41. 10Be created by cosmic rays + oxygen and nitrogen in upper atmos. •  Earth by precipitation & readily  clay-rich oceanic seds • 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 • 10Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments • In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14

  42. B 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

  43. 10Be/Betotal vs. B/Betotal diagram (Betotal9Be since 10Be is so rare) Figure 16-14.10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.

  44. In summary • Role of fluids (LIL/HFS) • Role of subducted matter (Be/B) • Multiple sites of melting! (diversity of series) • No garnet but rutile in the residuum

  45. Thermal structure of subduction zones

  46. Possible sources? • Arc crust • Mantle • Subducted crust • Mantle + subducted fluids Unlikely (too thin – in island arcs anyway) Unlikely (solidus too high + role of water) Possible?

  47. Can the subducted slab melt?

  48. P-T path along the subducted slab Subducted Crust Figure 16-16. Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, 341-353).

  49. Subduction zone magmatism(part II)

  50. Island arc magmas: • Arc tholeites (low K, high Fe/Mg) • Calc-alkaline (med K, low Fe/Mg) • « alkaline » (high K)

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