Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another Ocean-ocean Island Arc

Transcription

1 Igneous activity related to subduction (Chapters 16, 17)

2 Subduction-related activity Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another Ocean-ocean Island Arc Ocean-continent Continental Arc

3 Island arc activity Activity along arcuate volcanic island chains along subduction zones i.e. island arcs Distinctly different from the mainly basaltic provinces thus far Augustine Volcano, Alaska Image source: Wikipedia: 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

4 Island arc activity Cross-section of an island arc subduction zone Subduction Products Characteristic igneous associations Distinctive patterns of metamorphism Orogeny and mountain belts

5 Island arc activity Cross-section of an island arc subduction zone Subduction Features 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 Variable heat flow

6 Island arc activity Cross-section of an island arc subduction zone Subduction Features subduction dip angles = (45 ave) depth to plate below arc is generally constant at 110 km no matter the dip angle i.e. horizontal distance of arc from trench dependent on dip Volcanism accounts for ~10% of heat at arc

7 Island arc activity Volcanic rocks of island arcs Table Relative proportions of Quaternary volcanic island arc rock types. Locality B B-A A D R Talasea, Papua Little Sitkin, Aleutians Mt. Misery, Antilles (lavas) Ave. Antilles Ave. Japan (lava, ash falls) after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite A = andesite, D = dacite, R = rhyolite Complex tectonic situation and broad spectrum of volcanic compositions i.e. basalts to rhyolites = orogenic suite High proportion of basaltic andesite and andesite Most andesites occur in subduction zone settings

8 Island arc activity Major elements and magma series a. Alkali vs. silica - minor alkaline magmas bafm b. - both tholeiites and calc-alkaline lk li magmas exist c. FeO*/MgO vs. silica - both tholeiites (more in this diagram) and calc-alkaline magmas exist Tholeiitic (MORB, OIT) Alkaline (OIA) Cl Calc-Alkaline lin (~ restricted t to subduction zones) note subduction zones contain all three magma types diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics

9 Island arc activity Sub-series of Calc-Alkaline magmas K 2 O is an important discriminator for the 3 sub- series The three andesite series of Gill (1981)

10 Island arc activity Major element chemistry of individual island arc systems Calc-alkaline basalts are commonly high-alumina basalts (17-21% Al 2 O 3 ) Examples of actual island arc magma series Tonga-Kemedic: Low-K tholeiitic Guatemala: Medium-K calcalkaline Papu New Guinea: High-K calc-alkaline Variations are controlled by fractional crystallization and possible magma mixing i

11 Island arc activity Major element chemistry of individual island arc systems AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series. Generally, the higher the K 2 O, the less the Fe- enrichment nt

12 Island arc activity Major element chemistry of individual island arc systems Fractionation features Calc-alkaline trend shows that with increasing SiO 2 there is no dramatic Fe enrichment TiO 2 decreases due to Fe- Ti oxide The CaO/Al 2 O 3 ratio decreases due to fractionation by Cpx

13 Island arc activity Reasons for calc-alkalinealkaline differentiation Early crystallization of an Fe-Ti oxide phase Probably related to the high water content of calc-alkaline magmas in arcs, dissolves high f O2 High water pressure also depresses the plagioclase liquidus and more An-rich As hydrous magma rises, ΔP plagioclase liquidus moves to higher T crystallization of considerable An-rich-SiO 2 -poor plagioclase The crystallization of anorthitic plagioclase and low-silica, high-fe hornblende is an alternative mechanism for the observed calc-alkaline differentiation trend

14 Island arc activity Petrography of island arc volcanics Major phenocryst mineralogy (Changes with ihk K-type) Plagioclase is the most common phenocryst (An ) - likely related to high H 2 O depolymerizing the magma and favors An plag. Mafic minerals (cpx, opx or ol) are generally Mg-rich, and cpx is Al-rich Hornblende is common in med-high K andesites - stable only at elevated H 2O contents in the melt (may undergo later dehydration due to sudden loss of H 2 O or magma mixing) Also, biotite in more evolved magma. Disequilibrium textures are common

15 Island arc activity Trace elements Even the most primitive arc basalts have low Ni ( ppm), Cr and V ( ppm) - too low to be primary mantle melts REEs Slope within series is similar, but height varies with degree of fractionation due to removal of Ol, Plag, and Px (+) slope of low-k similar to DM HREE flat, so no deep garnet peridotite or eclogite source

16 Island arc activity Trace elements spider diagrams Trace elements normalized to MORB show two distinctions between rocks from ocean island basalts (OIB) and island arcs (IA). OIB shows enrichment of both LIL (Sr to Ba) and some HFS elements (Th to Yb) IA basalts show enrichment in LIL but not HFS elements (decoupling of LIL and HFS) Because LIL is hydrophylic, it implies that H 2 O in subduction zone systems helps concentrate the LIL elements.

17 Island arc activity Be and B data 10 Be created by cosmic rays + oxygen and nitrogen in upper atmosphere falls to Earth by precipitation & incorporates into clay-rich oceanic sediments Half-life of only 1.5 Ma (long enough to be subducted, but quickly ikl lost to mantle systems). After about 10 Ma 10 Be is no longer detectable 10 Be/ 9 Be averages about 5000 x in the uppermost oceanic sediments In mantle-derived MORB and OIB magmas & continental crust 10 Be is below In mantle-derived MORB and OIB magmas, & continental crust, Be is below detection limits (<1 x 10 6 atom/g) and 10 Be/ 9 Be is <5 x 10-14

18 Island arc activity Be and B data 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 Conclusions based on Be and B data: there is participation of young sediments and altered oceanic crust in the subductionbased production of igneous rocks

19 Island arc activity Petrogenesis of Island Arc Magmas Other factors, now considered minor, include: 1) dip of the slab 2) frictional heating 3) endothermic metamorphic reactions 4) metamorphic fluid flow Of many variables that can affect isotherms in subduction zone systems, the main ones are: 1) rate of subduction 2) age of subduction zone 3) age of subducting slab 4) extent to which subducting slab induces flow in mantle wedge

20 Island arc activity Petrogenesis of Island Arc Magmas For typical thermal models of 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 (< Ma)

21 Island arc activity Petrogenesis of Island Arc Magmas 2. Mantle wedge between slab and arc crust 3. Arc crust 4. Lithospheric mantle of subducting plate 5. Asthenosphere beneath slab Principal source components that may contribute to island arc magmas 1. Crustal portion of 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

22 Island arc activity Petrogenesis of Island Arc Magmas P-T-t paths for peridotite in mantle wedge as it follows a typical path Included are some P-T-t path range for subducted crust in mature arc, and wet and dry solidi for peridotite. Subducted crust dehydrates, and water is transferred to wedge (arrow). Amphibole-bearing hydrated peridotite should melt at ~ 120 km Phlogopite-bearing hydrated peridotite should melt at ~ 200 km

23 Island arc activity Petrogenesis of Island Arc Magmas Trace element and isotopic data suggest that both subducted crust and mantle wedge contribute to arc magmatism. Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in thermal regime H 2O must be involved in process LIL/HFS ratios of arc magmas imply water plays significant role in arc magmatism 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 garnet-bearing (eclogite or garnet peridotite) source Thus modern opinion has swung toward non-melted slab for most cases

24 Island arc activity Petrogenesis of Island Arc Magmas Proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dhd dehydrate.

25 Island arc activity Petrogenesis of Island Arc Magmas A multi-stage, multisource process 1. Dehydration of slab provides LIL, 10 Be, B, etc. enrichments. ih These components are transferred to wedge in fluid phase (or melt?) 2. Mantle wedge provides HFS and other depleted and compatible element characteristics

26 Island arc activity Petrogenesis of Island Arc Magmas A multi-stage, multisource process 3. Phlogopite is stable in ultramafic rocks beyond conditions at which amphibole breaks down 4. P-T-t paths for wedge reach phlogopite-2-pyroxene dehydration reaction at about 200 km depth 5. Parent magma for calc- alkaline series is a high alumina basalt, a type of basalt that is largely restricted to subduction zone environments

27 Continental arc activity Potential differences with respect to Island Arcs: 1. Thick sialic crust contrasts greatly with mantle-derived partial melts - and is likely source of more pronounced effects of contamination Arenal Volcano, Costa Rica. Image source: arenal.net 2. Low density of crust may retard ascent, and produce stagnation of magmas and more potential for differentiation i.e. more evolved magmas

28 Continental arc activity Potential differences with respect to Island Arcs: 3. Low melting point of crust allows for partial melting and crustally derived melts Arenal Volcano, Costa Rica. Image source: arenal.net 4. Subcontinental lithospheric mantle is likely to be locally enriched, and this will be reflected in magmas that are derived from this mantle

29 Continental arc activity Volcanism in South America Map of western South America NVZ - Accreted Mesozoic and Cenozoic oceanic crust and island arcs, km thick CVZ - Precambrian metamorphic crust, km thick SVZ - Accreted Mesozoic and Cenozoic oceanic crust and island arcs, km thick These are separated by inactive gaps. This belt of igneous rocks developed over last 500 Ma and is commonly termed an Andeantype margin.

30 Continental arc activity Volcanism in South America Schematic diagram to illustrate how shallow dip of subducting slab can pinch out the asthenosphere from overlying mantle wedge. Active zones of Andes correspond to more steeply-dipping (>25 ) slab i.e. mantle wedge must be involved in melting process. Shallow dips are considered to be results of thicker and less dense oceanic crust e.g. Nazca Ridge.

31 Continental arc activity Volcanism in South America AFM and K 2 O vs. SiO 2 diagrams for Andes volcanics Orange circles in the NVZ and SVZ are alkaline rocks. All rocks exhibit typical calcalkaline trends NVZ and SVZ are mostly high Al basaltic andesites and andesites, but with some dacites and rhyolites CVZ is generally more Si-rich with a range of basalt to rhyolite but most commonly andesite/dacite

32 Continental arc activity Volcanism in South America Chondrite-normalized REE diagram for selected Andean volcanics. NVZ -lower HREE (suggests residual garnet in a deeper melt source). CVZ - enriched LREE likely due to continental crustal involvement. SVZ - low REE slope and high HREE (suggests no garnet in melt source due to shallower slab dip)

33 Continental arc activity Volcanism in South America MORB-normalized spider diagram for selected Andean volcanics. Note decoupled LIL/HFS patterns typical of subduction zone magmas i.e. there must be dehydration of ocean crust. NVZ - pattern similar to island arc magmas. CVZ - pattern shows more enrichment due to continental crustal involvement. SVZ - pattern similar to island arc magmas.

34 Continental arc activity Ptr Petrogenesis of fthe Andes 1. Major and trace element data consistent with origin from fluid-fluxed and LILenriched mantle wedge above subducting and dehydrating Nazca plate. 2. Each of volcanic zones indicate magmas interact with local continental crust as they pass through crust. However, initial melts must come from mantle wedge.

35 Continental arc activity Cascade Magmatic Arc extends 1000 km and corresponds to oblique subduction of Juan de Fuca plate that ends in strike-slip faulting to the N and ds Current chain of Quaternary andesitic stratovolcanoes, but volcanism goes back to Phanerozoic

36 Continental arc activity Cascade Magmatic Arc Bimodal mafic-silicic volcanism due to mantle melts and crustal melts, however more mafic melts than in Andes Basin and Range extension likely related to back-arc extension and/or Yellowstone hot-spot interactions

37 Continental arc activity Cascade Magmatic Arc Time-averaged rates of extrusion of mafic (basalt and basaltic andesite), andesitic, and silicic (dacite and rhyolite) volcanics and djuan de Fuca North American plate convergence rates for the past 35 Ma. Increase in mafic magmas at 7.5 Ma due to extension

38 Continental arc activity Cascade Magmatic Arc REE diagram for mafic platform lavas of High Cascades. Mafic magmas range from LREE-depleted MORB-like magmas to variably enriched OIB-like magmas to subduction zone-type magmas indicative of fluid alteration

39 Continental arc activity Plutonism in Continental Arcs Major plutons of North American Cordillera, a principal p segment of a continuous Mesozoic-Tertiary belt from Aleutians to Antarctica. Cordilleran-type batholiths - these are generally composite bodies with 100s-1000s of individual intrusions over millions of years. Compositional range: gabbro-diorite- tonalitegranodiorite-granite -- this is similar to the volcanic equivalent compositional range The transition is commonly from mafic magmas during extensional regimes (back-arc extension) to silicic magmas during compressional phase. This oscillation may be related to variable spreading rates

40 Continental arc activity Plutonism in Continental Arcs Major plutons of North American Cordillera,,principal p segment of continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. Cordilleran-type batholiths - these are generally composite bodies with 100s- 1000s of individual intrusions over millions of years. Compositional range: gabbro-dioritetonalite-granodiorite-granite -- this is similar to volcanic equivalent compositional range

41 Continental arc activity Plutonism in Continental Arcs Major plutons of South American Cordillera, a principal p segment of a continuous Mesozoic-Tertiary belt from Aleutians to Antarctica. Transition is commonly from mafic magmas during extensional regimes (back-arc arc extension) to silicic magmas during compressional phase. This oscillation i may be related to variable spreading rates.

42 Continental arc activity Plutonism in Continental Arcs Schematic cross section of Coastal batholith of Peru. Shallow flat-topped and steep-sided "bell-jar"-shaped plutons are stoped into place. Successive pulses may be nested at a single locality Flat sides and tops are consistent with cauldron subsidence after volcanic eruptions

43 Continental arc activity Plutonism in Continental Arcs Harker-type and AFM variation diagrams for the Coastal batholith of Peru. Calc-alkaline trends consistent with fractional crystallization of plagioclase and pyroxene +/- magnetite, and later hornblende and biotite.

44 Continental arc activity Plutonism in Continental Arcs Chondrite-normalized REE abundances for the Linga and Tiybaya super-units of the Coastal batholith of Peru and associated itdvolcanics. Note similarity between volcanics and plutonics.

45 Continental arc activity Plutonism in Continental Arcs Schematic diagram of (a) formation of gabbroic crustal underplate (higher density) at continental arc (b) magma conduits get shut off, basaltic magma accumulates (magmatic underplating) and remelting of underplate to generate tonalitic plutons (not from differentiation i i of basaltic melt).

46 Continental arc activity Plutonism in Continental Arcs Schematic cross section (1) ()dehydration of subducting slab (2) hydration and melting of heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle) (3) crustal underplating of mantle-derived melts where MASH (melting, assimilation, storage and homogenization) processes may occur.

47 Continental arc activity Plutonism in Continental Arcs Schematic cross section (4) Remelting of underplate to produce tonalitic magmas and possible zone of crustal anatexis. (5) As magmas pass through continental crust they may differentiate further and/or assimilate continental crust.