Transcript Lecture 33 - Metamorphic MInerals II

• What if we had staurolite and andalusite
together? What conditions would that indicate?
Epidote Group
• Sorosilicates (paired silicate tetrahedra)
• Include the mineral Epidote Ca2FeAl2Si3O12(OH),
Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
(polymorph)
Garnets
Garnet: A2+3 B3+2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
Garnet (001) view blue = Si purple = A turquoise = B
Staurolite
• Aluminosilicate - Fe2Al9Si4O22(OH)2
• Similar structure to kyanite with tetrahedrally
coordinated Fe2+ easily replaced by Zn2+ and
Mg2+
• Medium-grade metamorphic mineral,
typically forms around 400-500 C
– chloritoid + quartz = staurolite + garnet
– chloritoid + chlorite + muscovite = staurolite +
biotite + quartz + water
• Degrades to almandine (garnet at higher T)
– staurolite + muscovite + quartz = almandine +
aluminosilicate + biotite + water
Metamorphic chain silicates
• Actinolite and tremolite are chain silicates
derived from dolomite and quartz and
common in low-mid grade metamorphic
rocks
• Riebeckite and Glaucophane are also
chain silicates – higher grade minerals,
often a blue color
• These minerals usually lower P, higher T
conditions
Metamorphic Facies
• Where do we find
these regimes of
P-T ‘off’ of the
typical continental
isotherms??
• How is the
environment that
forms a blueschist
facies rock different
from one forming a
hornfels?
Metamorphic Facies
• Table 25-1. The definitive mineral assemblages
thatTable
characterize
eachAssemblages
facies (for
mafic Facies
rocks).
25-1. Definitive Mineral
of Metamorphic
Facies
Zeolite
Definitive Mineral Assemblage in Mafic Rocks
zeolites: especially laumontite, wairakite, analcime
Prehnite-Pumpellyite
prehnite + pumpellyite (+ chlorite + albite)
Greenschist
chlorite + albite + epidote (or zoisite) + quartz ± actinolite
Amphibolite
hornblende + plagioclase (oligoclase-andesine) ± garnet
Granulite
orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±
hornblende)
Blueschist
glaucophane + lawsonite or epidote (+albite ± chlorite)
Eclogite
pyrope garnet + omphacitic pyroxene (± kyanite)
Contact Facies
After Spear (1993)
Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
Facies Series
• Miyashiro (1961) initially proposed five facies series,
most of them named for a specific representative
“type locality” The series were:
1. Contact Facies Series (very low-P)
2. Buchan or Abukuma Facies Series (low-P
regional)
3. Barrovian Facies Series (medium-P
regional)
4. Sanbagawa Facies Series (high-P,
moderate-T)
5. Franciscan Facies Series (high-P, low T)
Fig. 25-3.
Temperaturepressure diagram
showing the three
major types of
metamorphic
facies series
proposed by
Miyashiro (1973,
1994). Winter
(2001) An
Introduction to
Igneous and
Metamorphic
Petrology.
Prentice Hall.
Isograds
• Lines (on a map) or Surfaces (in the 3D
world) marking the appearance or
disappearance of the Index minerals in
rocks of appropriate composition
e.g. the ‘garnet-in isograd’; the ‘stauroliteout isograd’
Complicated by the fact that most of these
minerals are solid solutions
• Isograds for a
single shale unit
in southern
Vermont
• Which side
reflects a higher
grade, or higher
P/T environment?
Meteorite/ Comet Impacts
• P/T space??
Energy of an impact
• Kinetic energy – going from
very fast to stopping is a BIG
change in energy
• What happens to that
energy??
• Impactite – any mineral formed
as a result of this impact
Shatter cone
Materials indicating Meteorite
Impacts
• Tektites
• Glass formed from impact
Diaplectic Glass
• Glass formed through fusion of different
minerals – not melted, but fused…
What Happens to minerals that
are there but not melted?
• Shock Quartz
• Lamellae retaining evidence of impact
Diamonds
• Very small diamonds can form from
impacts and are found in meteorite impact
craters around the world
Quartz Polymorphs
• Coesite and Stishovite found associated with
impact craters