Quiz:
Geo 406 Lecture 14 – Granites
Announcements
Returned assignments:
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