Quiz:
Geo 406 Lecture 12 – Arcs
Announcements
Returned assignments:
Goals for today
Understand MORB & OIB
Convergent Arc Magmatism
Melt generation
At subduction zones
Due to release of fluids from slab:
hydrates mantle, which
flows down with slab, and
melts when minerals like amphibole and biotite react to liberate H2O.
amphibole liberates the most H2O, so produces the most melt
biotite-based melting can produce a secondary front.
Why doesn’t the slab melt?
Too cold
Relationship between slab dip and trench-arc distance
Describe mantle flow and back-arc basins
Rocks produced
Wider range than previous environments
Basalts through to Rhyolites
Phenocrysts:
Basalt: Olivine + Augite + Plag ± ilm/mag
Andesite: Plag + augite ± opx ± hbl ± ilm/mag
Dacite: Plag + aug/hbl + qtz + bt
Age of arc:
Young island arcs: basalt
Mature island arcs: andesite
Continental arcs: andesite to rhyolite
WHY?
thicker crust = more fractionation, assimilation
Tholeiite and Calc-alkaline trends (C-A most common)
C-A requires a Fe-rich fractionating mineral
Early crystallization of ilm/mgt or possibly Fe-rich hbl
promoted by high fO2 caused by H2O
Lots of An-rich plagioclase formed
Can also classify melts based on K content
K-h variation:
low-K tholeiitic trend near trench
C-A further
Alkaline further, as depth of melting increases
Along arc-trends as well
Temporal trends: early tholeiitic  C-A  Alkaline
Explosive volcanism is common
Both pyroclastic material and lavas
Plinian eruptions
WHY?
Lots of gases
Plutonism (esp at Continental Volc. Arcs)
From gabbro to granite
But tonalite is dominant
Granitoids
Source of granitic melts
Fractionation of basalts
but think about M&M exercise: how much of the initial basalt ended up as granitic melt?
would require huge cumulate reisuduum someplace
Melting of crust (partial melting)
I-type = igneous source is melting
metaluminous
example: Sierra Nevada
S-type = sedimentary source rock
peraluminous
Occurrence
Subduction zones / batholiths
qtz diorite, tonalite, granodiorite, granite
Collision
Rifting
melting crust
A-type: anorogenic (not part of mountain building event)
peralkaline
Ocean Islands
late, very evolved
M-type: from mantle
Mid-ocean ridge
very evolved
M-type: from mantle
Mineralogy
fsp (ksp + albitic plag), qtz, hbl, micas, but some depend on alumina saturation:
Recall:
peraluminous = Al2O3 > K2O + Na2O + CaO
metaluminous= K2O + Na2O + CaO > Al2O3 > Na2O + K2O
peralkaline= Na2O + K2O > Al2O3
peraluminous:
bt + ms; little hbl; no pxn
metaluminous:
rare pxn, but cpx & opx possible
hbl more common
biotite present
peralkaline:
cpx (aegerine-augite, bright green)
riebeckite amphibole (deep blue in thin section)
no mica (Al goes into fsp)
Texture
hypidiomorphic granular
sometimes porphyritic
emplacement
recall field trip:
catazone: strong foliation, concordant
mesozone: roof pendants common
epizone: associated with volcanism; contact metamorphic zones; economic mineral
deposits
Often zoned plutons
successive intrusion of later magmas from deeper chamber
crystallization from walls inward
contamination from wall rock
debate as to the existence of classic diapers
possibly only inflation occurs
Rhyolites
Material
~90% pyroclastic material
~10% lavas
ignimbrites = pyroclastic flow deposit, made up of welded and non-welded tuffs
hot flows of large volume
from collapse of Plinian column (containing gas, ash, crystals, lithics, pumice)
Often erupted from caldera
roof caves in after eruption
Big eruption but still order of magnitude smaller than flood basalts
Example: Long Valley Caldera
eastern Sierra Nevada
700 Ka, Bishop Tuff erupted (600 km3)
magma chamber still present / active