1) Island arcs form as a result of subduction between two oceanic tectonic plates, resulting in a chain of volcanic islands. 2) Volcanic activity in island arcs produces both tholeiitic and calc-alkaline magmas. The proportions of these magma types can vary spatially along the arc and temporally as the arc evolves. 3) Island arc magmas are generated by melting of the subducting oceanic crust and mantle due to heat and fluids released by dehydration reactions during subduction at depths between 50-150km.

1. Subduction related igneous activity : Island Arcs
Prepared By:
Roshan paudel
Tribhuwan University
mail2roshanji@gmail.co
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2. Introduction
• Subduction is a complex process and
produces not only some characteristic
igneous associations, but also
distinctive patterns of metamorphism.
• it is also responsible for the creation of
mountain belts (orogeny)
• if the overriding plate is oceanic, the
resulting magmatism forms an island
arc
• if the overriding plate is of continental
character, the resulting igneous activity
is referred to as a continental arc or an
active continental margin.

3. Island and volcanic arcs
• Intra-oceanic Subduction - results in an arcuate chain of volcanic islands.
• Island arcs are generally 200 to 300 km wide and can be several thousand kilometers long
• The oceanic plate on the left, comprising oceanic crust and rigid lithospheric
• upper mantle, is shown subducting beneath the oceanic plate on the right.
• A trench, commonly deeper than 11 km, is the surface expression of the plate boundary.
• Wadati-Benioff zone represents the upper boundary of the cool, relatively brittle subducting
slab where it slips against the overriding plate and mantle.
• Volcanic front is the point of volcanic activity closest to the trench.
• The fore arc is the portion of the arc between the volcanic front and the trench. It is composed
of flows and pyroclastic material from the volcanic arc, immature sediments eroded from the
growing arc, and oceanic sediments scraped from the subducting plate
• The accretionary prism or accretionary wedge is the accumulation of slivers of oceanic crust
and mantle (ophiolite) that are caught up in the thrusting and are later incorporated into the
pile.
• A back-arc basin is typically developed behind the arc. This is a site of MORB-like volcanism
that creates thin ocean-type crust in an extensional tectonic environment behind the volcanic
arc

4. Island arc volcanic rocks and magma series
• The abundance calc-alkaline
rock in the volcanic arc is shown
in the figure
A.Alkali versus silica
It can be seen alkaline magmas are
only a minor constituent
oforogenic zones because they are
more common above plumes in
intraplate and ridge settings
Figure : Alkali verses silica

5. B. AFM
-Both tholeiites and calk-alkaline
magmas are well represented in
volcanic arcs but more evolved
rocks toward the alkali corner
- The diagram indicates that
tholeiites and calc-alkaline
magmas occur in subequal
amounts
Figure :AFM in island arcs

6. C.FeO/MgO versus silica
• The field boundary for this diagram has a
positive slope, indicating
that FeO/MgO tends to increase with silica
during differentiation within a series, but the
distinction is made not on a differentiation
trend but on the magnitude of the FeO/MgO
ratio for any single analysis (although
factored for the silica content)
• The diagram indicates that tholeiites are
more common Figure :FeO/MgO versus silica

7. • This figure suggests that the
low-K type is tholeiitic, the
medium-K series is more
calc-alkaline, and the high-K
series is mixed.
• Most orogenic suites might
most simply be described by
three principle series:
-low-K-tholeiitic
- medium-K-calc-alkaline
- high-K (mixed)

8. Spatial and temporal variation in island arc
A. Spatial
low-K tholeiite near trench C-A alkaline as depth to seismic zone increases
• Some along-arc as well as Antilles more alkaline N- S
• Aleutians is segmented with C-A prevalent in segments and tholeiite prevalent at
ends
B. Temporal
• Early tholeiitic later C-A and often latest alkaline is common.

9. Petrography of island-arc volcanics
• Island-arc volcanic rocks are generally phyric , particularly the calc-alkaline ones.
• Andesites and basaltic andesites are typically much more phyric than basalts.
• Surprisingly, dacites, and especially rhyolites, are less phyric than andesites,
perhaps because of restricted diffusion in the highly polymerized liquid, which
inhibits migration of constituents to, and hence the growth of, large crystals
• Different composition
a) Plagioclase
b) Albite
c) Augite
d) Orthopyroxene
e) Black hornblende
f) Biotite

10. Petrogenesis of island-arc magmas
a.Thermal constraints
• Subduction zone magmatism is a paradox in the sense that great quantities of
magma are generated in regions where cool lithosphere is being subducted into the
mantle and isotherms are depressed, not elevated.
• Variables affecting the distribution of isotherms in subduction zone systems:
1. The rate of subduction
2. The age of the subduction zone
3. The age of the subducting slab
In addition to these “first-order” plate tectonic variables, but also believed to be
important (although less easy to assess), are:
1. The extent to which the subducting slab induces flow in the mantle wedge and
the vigor and geometry of that flow.
2. The effects of frictional or shear heating along the Wadati-Benioff zone

11. • Other factors, such as the dip of the
slab, endothermic metamorphic
reactions, and metamorphic fluid flow,
are now thought to play only a minor
role (Peacock, 1991; Furukawa,
1993).
• Isotherms will be higher (i.e., the
system will be hotter) if:
1. The convergence rate is slower
2. The subducted slab is young and
near the ridge (hence warmer)
3. The arc is young (650 to 100 Ma,
according to Peacock,1991)
Figure : Cross section of subduction zone showing
isotherms

12. • The principal ones, with numbers corresponding to circled numbers in the figure,
are:
1. The crustal portion of the subducted slab, which includes three components:
a. The altered oceanic crust itself, which is hydrated by circulating seawater, and
partly metamorphosed to greenschist facies (including chlorite, actinolite, and
albite)
b. Subducted oceanic and fore-arc sediments
c. Seawater trapped in pore spaces
2.The mantle wedge between the subducting slab and the arc crust
3. The arc crust
4. The lithospheric mantle of the subducting plate
5. The asthenosphere beneath the slab

13. b.Dehydration and melting in the subducted slab

14. Possible model
Figure : A proposed model for subduction zone
magmatism

15. Mantle re-enrichment
• Re-enrichment of Earth's mantle can
occur when continental crustal materials
begin to founder into the mantle by
either subduction or, to a lesser degree,
by delamination processes, profoundly
affecting the mantle's trace element and
volatile compositions
Figure : Magma re-enrichment

16. Thank you