Arcs

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L22-23-Arcs.html

-After MORB, arcs are second most voluminous igneous province

-Arc rocks are part of or accreted to continents

B. Subduction zones

-Oceanic lithosphere bends and descends into mantle

-EQ occur in top of cold slab

-Igneous activity doesn''t occur right at convergent boundary but behind it: forms an island arc if overriding slab is oceanic; continental volcanic arc if overriding slab is continental

-Volcanoes are located above where slab is at ~100 km depth: volcanism in Andes occurs only where angle is steeper; melting in mantle wedge is probably triggered by release of fluids from metamorphic reactions in subducting slab

C. Arc rocks

-Andesites are most common, but entire range exists

-Factors influencing composition: oceanic (mafic) vs. continental (silicic) plates; angle

Most rocks are sub-alkaline: small % are alkaline; sub-alkaline can be divded into tholeiitic and calc-alkaline

-Rocks can be subdivided based on K content: low K = island arc tholeiite; intermediate K = calc-alkali suite; high K series; ultrahigh K = shoshonites

II. Composition of volcanic rocks

A. At a single volcano, composition changes through time > mafic to silicic

-Note: range of rock types is completely continuous from basalt to rhyolite

-Major elements show trends of low-P F.C.

B. Mafic rocks

-Tholeiite (K<0.5%) : sparsely porphyritic with plag + pyr; REE => undepleted source

-Calc-alkali basalt (K ~1%): aAlso called high-alumina basalts (~18%); strongly porphyritic with plag +/+ pyr

-Shoshonite (K > 2%): phenocrysts of olivine + plag + pyr ; may have leucite in groundmass

-Difference also shows up in REE

-Unusual high-Mg rocks in some arcs called boninites: no olivine; low incompatibles

-Since mafics form early, often covered by younger lavas => may be more common than currently accounted

C. Andesites

-Most common composition

-Typically explosive eruptions => tuffs and agglomerates

-Most are porphyritic: complexly zoned plag, opx, cpx, occasional olivine (not with opx); C-A rocks may have magnetite phenocrysts; high-K andesites " sanidine "

-Phenocrysts often not in equilibrium with glass : magma mixing, or; rapid rise through lithosphere

-Groundmass usually vitric - crystal tuffs = phenocrysts + glass shards

D. Silicic rocks

-Generally more vitric, even obsidian

-High Si => more explosive - almost exclusively tuffs

-May show flow-banding

-Moderately porphyritic with wide variety

-Dacite and rhyolite (+ andesite) contain mafic xenoliths or schlieren

E. Incompatible elements

-K content of volcanic arc rocks strongly correlated with depth to Benioff zone: for Si = 55%:

-Similar correlations for other incompatibles. Why?Depth or % melting, or thickness of lithosphere

-Sr-Nd systematics => crustal component

-Pb isotopes also clearly show influence of crust

III. Plutonic rocks

A. General

-Harder to study, because modern ones are inaccessible. Only old eroded ones available.

-Full range of compositions can be found

-3 series based on K can be recognized

B. Composition

-Andesite in volcanics => diorite plutons, but this is mostly minor

-Most common is granite to granodiorite, forming batholiths

-No evidence for large magma chamber

C. Possible origin

-Basaltic magma ponds at base of crust and differentiates: it rises through the crust; such rocks would retain evidence of mantle origin.

-Or, basaltic magma rising into crust could melt surrounding rocks to form silicic magma with a crustal character.

-Granite types : I-type (Rich in Na and Ca, hornblende is major mafic mineral/ Initial 87Sr/86Sr < 0.708 => mantle origin/Associated porphyry copper deposits ); S-type (Na-poor, Al-rich, biotite is major mafic mineral/Initial 87Sr/86Sr > 0.710 => crustal contamination/Associated tin deposits); I-type occur closer to convergent boundary; S-type is water-rich, so decrease in P leads to rapid crystallization

IV. Processes

A. Caveat

-Arc rocks form under widely varying conditions

-No single process can explain them.

-Only commonality is that they are generated by subduction

B. Island arcs

-Both plates are oceanic => magmas must originate in oceanic crust or in mantle

-May be generated: In the mantle wedge above Benioff zone; by melting of eclogite; by induced melting at base of crust

-Subducting slab brings water to wedge.

-Only basaltic magmas are produced => silicic rocks must be differentiation products.

C. Continental arcs

-More diverse rock types, greater amount of silicic rocks

-Compositional range from mafic to silicic may be the result of: differentiation or magma mixing

-Two-stage model for silicic roc : first, basaltic magma ponds at base of crust and solidifies; later, more magma supplies heat to melt low-T fraction of gabbros; these silicic melts rise into crust & differentiate to "true granites"

D. Model of a subduction zone

-During subduction, does descending plate shear past overriding plate, or are they coupled? Shear => mantle wedge is stationary; coupled => mantle wedge is dragged down with subducting plate

-Geotherm for subduction zone looks different from ones calculated early in the semester: note that it''s always below the lherzolite anhydrous solidus; but geotherm intersects wet solidus at ~100 km depth

-Recall most volcanoes are located ~120 km above Benioff zone: depth beneath arc doesn''t vary with subduction angle or rate, but, if angle <25o, no igneous activity occurs; melt generated at 100 km rises and cools until it intersects solidus again, where it solidifies; clearly, magmas reach the surface!