Earth Mat Flashcards - Ch 10
are suites of rocks that form in response to similar geological conditions.
Petrotectonic associations
% magma generated at moden divergent, convergent, and hotspots
62, 26, and 12%
Layer 3 of the oceanic crust contains
Istoropic gabbro (upper)
Cumulate gabbro (mid)
Cumulate peridotite (lower)
are tholeiites with low SiO2, low potassium, high MgO, Al2O3 and compatible elements concentrations
MORB
what does depleted source mean in the context of igneous petrology
mantle lherzolite undergone previous melt cycles that has largely removed imcompatible elements
N-MORB vs E-MORB
N-MORB depleted in imcompatible elements (LREE, HFS, LIL)
E-MORB has La/Sm >1 ratio,
What does the presence of E-MORB represent
small degrees of partial melting (10-15~)
tapping aa deep mantle source
enriched from magmaa mixing, assimilation, or partial melts from subducted oceanic lithosphere
Basats where Al2O3 concentrations exceed 16%
Low K2O
High Aluminum Basalts or
ARC THOLEIITES
Describe the rocks produced by oceanic ridges
High alumina basalts
Higher TiO2 (>13%) concentration in andesitic and rhyolitic basalts
thinner ocean lithosphere in the overlying plate generally produces
metaluminous, mafi c to intermediate rocks
Thicker continental lithosphere overlying the subduction zones commonly yields
peraluminous, potassic, intermediate to silicic rocks.
Phanerozoic convergent margins are dominated by what magma series
Calc - alkaline
Signature calc-alkaline association of convegent margin
Basalt Andesite Dacite and Rhyolite (BADR)
How does Calc-Alkaline basalts differ from tholeiites
higher alkali, no Fe enrichment
SiO2 range of Andesites
> 52-63%
Common in youthful Island arc systems
>52-57% SiO2
Basaltic Andesite
Common in mature Island arc systems
>57-63% SiO2
Silicic Andesites
Conditions for voluminous andesite production
> 25° subduction angles
25 km continental crust anatexis
70-200 km deep subduction
are quartz – phyric volcanic rocks, intermediate between andesite and rhyolite
Dacite
SiO2 range of Dacite
63-68% (most dacites)
extends to 77%^ in TAS
Plutonic equivalent of dacites
granodiorites
common dacite phenocryst
Olioclase to labradorite
Sanidine
are generally composed of ∼ 66 – 69% SiO 2 , although the lower TAS limit begins at 57% SiO
Trachyandesites
Trachyandesites commonly contain phenocrysts of ___________plagioclase feldspar amidst a groundmass of ____________
andesine to oligoclase
orthoclase and augite.
Variations of Arc Rocks in terms of K2O concentratio
High - Calc Alkaline to Shoshonite (>40 km thick overlying slab)
Mid - Calc Alkaline (20-40 km)
Low - Tholeiite (0 - 20km)
________ which represent the plutonic equivalent of dacites and rhyodacites, contain > 20% quartz and more plagioclase than potassium feldspar
Granodiorites
Island arc granodiorites are generally
metaluminous, containing hornblende, biotite and minor amounts of muscovite.
Plagiogranite types
Tonalites and Trondhkemites
In Island arc pluton first described from Monte Adamello near Tonale in the eastern Alps, contain calcium plagioclase and quartz with minor amounts of potassium feldspar, biotite and hornblende.
Tonalites
are granodioritic rocks in which sodium plagioclase represents half to two - thirds of the total feldspar component.
depleted Bt and Pl
Trondhjemites
Low poratssium arc tholeiites occur on the
ocean side of the volcanic arc
How are Is. Arc Tholeiite basalts distinguised from MORB
greater concentrations of potassium and other LIL elements and lower concentrations of HFS elements
are high magnesium (MgO/MgO + total FeO > 0.7) intermediate volcanic rocks that contain a SiO 2 - saturated (52 – 68% SiO 2 ) groundmass.
Boninites
Other properties of Boninites include:
- Opx phenocrysts (with a lack of plagioclase)
- HREE, HFS Depletion
- Enrichment in Xr, Ni, Zn, Ba, Sr
- Magma derived found trench proximal
What does this imply?
boninites are aproduct of subduction related melting in forearc of youthful is. arc sys.
are silica - saturated ( > 56% SiO 2 ) rocks with high Sr/Y and La/Yb ratios (LREE enriched relative to HREE) and low HFS, (such as Nb and Ta) concentrations
Are also the volcanic equivalent of TTGs
Adakites
formational conditions of adakite
slab melting of eclogite / garnet amphibolte from desecding ocean lithosphere
continent-continent collision sites: shallow subduction
the TTG association is called
Tonalite, Trondhjemite, Granodiorite associateions
this magma assosication is generated by fractional crystallization of basaltic magma derived by partial melting of overlying mantle peridotite fluxed by fluids from the subducting slab
Calc - alkaline
TTG are associated with what environment
Achean (more dominant) to Recent Subduction zones
An opx bearing suite of rocks of generally granitic composition, occurs with TTG
Charnockites
How are archean subductions described
Higher geothermal gradients
shallow subduction and melting of ocean lithosphere
minimal input from overlying lithosphere wedge
abundance of tonalite plutons and adakite volcanics
These basalts are enriched with H20, Alkalis and LIL elements
REE 5-20x that of chrondrites
Relative enrichment in volatile elements, Th and LREE (subduction related fluid in magma genesis)
found in Back arc basins
Back Arc Basin Basalts (BAB)
Why does BAB produce a wide array of rock types
Mutiple processes
- partial melt of mantle perodotite (MORBS)
- Hydrated mantle wedge, recycled subducted lithosphere, subducted marine sediment sources (Calc-Alkaline)
- Adiabatic melt of mantle peridotites at BAB ridges
Imagine the TiO2, MnO x 10, P2o5 x 10 ternary diagram
Ti02 at top, MnO at left, P2O5 at right
TiO2 rich : MORB (L), OIT (R)
MnO : CAB (L + Bot), IAT (L + Up)
P2O5 : OIA (R)
Search sa net ang ternary
Imagine the Ti/100, Zr, Y x 3 ternary diagram
Ti/100 top, Zr left, Y x 3 right
Ti/100 : WBP (L), IAT (R)
Zr : CAB
Mid : MORB IAT
are dark - colored, potassium - rich trachyandesites, commonly containing olivine and augite phenocrysts with a groundmass of labradorite plagioclase, alkali feldspar, olivine, augite and leucite.
Shoshonites
Petrotectonic characteristics of Shoshonites
late stage ocean-continent subduction
occurs in thickened lithosphere farthest from trench region, in continent-continent collisions, and some back arc basins
Conditions for the formation of rhyolites, rhyodacites and shoshonite and granitic plutons
-flat subduction (<25°)
-thick continental lithospehre (>25)
the kind of basalts seen in continent-continent collisions
alkaline basalts, from upwelling mantle melts
Strictly speaking, the term “ granite ” is restricted to plutonic rocks containing
20 – 60% quartz and
35 – 90% alkali to plagioclase feldspars
Orogenic granites
Anorogenic granites
M, I, S types
A type
mantle derived parental magmas (87Sr/86Sr < 0.704)
associated with calc-alkaline tonalites, qtz diorite and gabbroics
develop in Is. Arc settings
What granite
M type
melting of igneous protolith from subduction (slab or wedge)
Na2O and Ca2O enrichment, Al2O3 depletion
Mantle Source (87Sr/86Sr < 0.704)
What granite
I type
Melting of sedimentary crustal rocks in collision zones
Na2O depletion, Al2O3 enrichment (peraluminous)
Ealier sedimentary cycle (87Sr/86Sr < 0.704)
Also known as Two-Mica Granites, Hb absent
What granite
S-Type
Anorogenic; does not involve subduction and collision
Alkali and LIL enriched, depleted in refractory elements
peralakline
low water contents
A type
Mineralizations associated with granites
Au-Cu mineralization : M type
Cu, Mo, W deposits : I type
Sn deposits : S Type
are fault - bounded, deformed rock sequences that mark the site of present or former convergent margins
Alpine orogenic complexes
constitute one type of Alpine deposit in which the oceanic or backarc basin lithosphere or volcanic arc basement rocks are preserved in orogenic belts
Ophiolites
A trinity associated wuth ophiolites, consist of pelagic chert, serpentinite, and spilites
Steinmann trinity
these ophiolites develop due to extensional tectonics that result in backarc spreading or forearc spreading producing oceanic lithosphere.
SSZ Ophiolites
consist of concentrically layered (zoned) plutons formed in convergent margin settings.
Alaska - type intrusions
Zones in an alaska type intrusio
Dunite core, Pyroxenite shell, surrounded by massive gabbro, occasional granitic zones in its perimeter
Alaska - type intrusions are economically important as sources of metals, particularly
PGE
Intraplate magmatism produces a wide range of igneous rock types including:
-Tholeiitic to alkalic basalt and related gabbros of hotspots and LIP.
-Siliceous anorogenic granite and rhyolite.
-Silica - undersaturated rocks.
-Basic – ultrabasic suites including komatiites and kimberlites.
-Carbonatites.
greatest manifestation of intraplate magmatism
LIPs (>10^6 km^3)
What kind of LIP is Yellowstone
Silicic LIP (SLIP)
How are OIB different from MORB
moaare alkalic, less depleted
from partial melts of deeper and undepleted mantle source
Hypothesis proposed for OIB chemistry
- Small degrees of melting of a primitive mantle source.
- Melting of a mantle source enriched in alkali elements.
- Incorporation of subducted oceanic crust in the source region.
- Entrainment of subducted sedimentary rocks in the source region
Nepheline normartive OIB magmas are derived from
Parial melting of gar px,
gar px came from MORB and mantle peridotite mixing
Fractionization sequence at OIB
Increasing SiO2 (x) and Alkalis (y)
-Basalts -> Hawaiites -> Mugeraites -> Benmorites -> Trachyte -> Phonolites (Silica undersat) or Rhyolites (Silica Oversat)
Largest flood basalt events
Siberian Traps : Permo-Triassic
CAMP : Triassic-E. Jurrasic
Deccan Trap : Cret-Ternary
also corresponds with the largest extinctions in history
Continental rifts produce a wide array of rocks that include
alkalic basalt, alkaline and silicic rocks
The widespread occurrence of basalt and rhyolite without significant andesite is referred to as
bimodal volcanism
Bimodal Volcanim occurs in
Continental rifts and hotspots
are anorogenic bodies injected into stable continental cratons at moderate depths
Layered basic – ultrabasic intrusions
largest layed intrusio
Stillwater complex, Montana
Bushveld Complex, S. Africa
Skaergaard Intrusion, Greenland
are ultrabasic volcanic rocks found almost exclusively in Archean (>2.5 Ga) greenstone belts
Komatiites
consists of needle - like, acicular olivine, pyroxene (augite and/or pigeonite) and chromite phenocrysts in a glassy groundmass
Spinifex texture
Spinifex texture can be found on
Upper parts of komatiite flows; chilled margins of silla and dikes
Only known phanerozoic (younger) komatiites
Gorgona Is., Colombia
Komatiite origin
Melting in hydrated mantle wedge
deep mantle plume hotspot leading to large partial melting
Gar Peridotite partial melting (10-30%) at 8-10 Gpa
are brecciated, magnesium - rich, ultrabasic rocks that rapidly rise to Earth ’ s surface via cylindrical diatremes from deep within the mantle.
Kimberlites
important CO2 energy sources propelling kimberlites
Carbonate minerals
are shallow intrusive to volcanic rocks that contain > 20% CO3 minerals
Carbonatite
are magnesium - rich, volatle - rich, porphyritic rocks containing mafic phenocrysts such as biotite, phlogopite, amphibole, clinopyroxene and melilite.
Lamprophyres
are potassium - rich, peralkaline rocks containing minerals such as leucite,sanidine, phlogopite, richterite, diopside and olivine.
Lamproites
this texture refers to sodium plagioclase overgrowths on pre -existing orthoclase crystals
Rapakivi
