Transcript GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 28 Metamorphism of Calcareous and Ultramafic Rocks April 13, 2009

GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 28
Metamorphism of Calcareous
and Ultramafic Rocks
April 13, 2009
METAMORPHISM OF CALCAREOUS ROCKS
TYPES OF META-CALCAREOUS ROCKS
Metacarbonates are metamorphosed calcareous rocks in
which the carbonate component is predominant
Marbles are nearly pure carbonate
Calc-silicate rocks: carbonate is subordinate and may be
composed of Ca-Mg-Fe-Al silicate minerals, such as
diopside, grossular, Ca-amphiboles, vesuvianite, epidote,
wollastonite, etc.
Skarn: calc-silicate rock formed by metasomatism between
carbonates and silicate-rich rocks or fluids
 Contact between sedimentary layers
 Contact between carbonate country rocks and a hot,
hydrous, silicate intrusion, such as a granite
METAMORPHISM OF CALCAREOUS ROCKS
LIMESTONE AND DOLOSTONE
Because calcite has a broad P-T stability
range, the metamorphism of pure
limestone to marble results in coarsening,
but little change in mineralogy.
Metamorphism of impure limestones
and dolostones (containing minor
amounts of other minerals like quartz,
feldspar and heavy minerals) create a
far more diverse assemblage of Ca-MgSi-bearing minerals contained in
metacarbonates
METAMORPHISM OF CALCAREOUS ROCKS
THE CMS-HC CHEMOGRAPHIC SYSTEM
Cal – Calcite
Wo – Wollastonite
Di – Diopside
Tr – Tremolite
Tlc – Talc
Fo – Mg-olivine
Bru – Brucite
Per – Periclase
Dol - Dolomite
Impure
Limestone
Dolostone
Winter (2001) Figure 29-1.
Chemographics in the CaOMgO-SiO2 -CO2 -H2O system,
projected from CO2 and H2O.
The green shaded areas
represent the common
composition range of
limestones and dolostones. Due
to the solvus between calcite
and dolomite, both minerals can
coexist in carbonate rocks. The
dark red left half of the triangle
is the area of interest for
metacarbonates. Carbonated
ultramafics occupy the right half
of the triangle.
METAMORPHISM OF CALCAREOUS ROCKS
MAJOR LOW PRESSURE ISOGRADS
T-XCO2 phase diagram for siliceous
carbonates at P = 0.1 GPa
Per-in
Di-in
Fo-in
Tr-in
Winter (2001) Figure 29-2. A portion of the Alta aureole in Little
Cottonwood Canyon, SE of Salt Lake City, UT, where talc, tremolite,
forsterite, and periclase isograds were mapped in metacarbonates by
Moore and Kerrick (1976) Amer. J. Sci., 276, 502-524.
Tlc-in
METAMORPHISM OF
CALCAREOUS ROCKS
LOW PRESSURE ~ 0.1 GPa
OPEN FLUID CONDITIONS*
XCO2< 0.67
3) Tr + Dol + H2O + CO2
Tlc + Cal
*XCO2 of fluid is externally buffered at some fixed value; here = 0.41)
2) Tr + H2O + CO2
Tlc + Qtz + Cal
Only if Qtz
remains after
reaction #1
(unlikely in calc rx)
1) Tlc + Cal + CO2
Dol + Qtz + H2O
Remember, T cannot increase at a reaction
until one of the phases is consumed
METAMORPHISM OF
CALCAREOUS ROCKS
LOW PRESSURE ~ 0.1 GPa
OPEN FLUID CONDITIONS*
XCO2< 0.67
6) Tr + Fo + H2O + CO2
Tr + Dol
5) Tr + Fo + H2O + CO2
Tlc + Dol
Only if Tlc
remains after
reaction #3
(rare in calc rx)
4) Di + H2O + CO2
Tr + Cal + Qtz
Only if Qtz
remains after
reaction #2
(only in calc-sil rx)
METAMORPHISM OF
CALCAREOUS ROCKS
LOW PRESSURE ~ 0.1 GPa
OPEN FLUID CONDITIONS*
XCO2< 0.67
8) Per + Cal + CO2
Dol
7) Wo + CO2
Cal + Qtz
If Qtz remains after
prev. reactions
(e.g. calc-sil rx
or Si-metasomatized
calcareous rx)
6x) Tr + Cal = Di + Fo (Tr-out)
METAMORPHISM OF CALCAREOUS ROCKS
PROGRESSIVE METAMORPHISM UNDER CLOSED CONDITIONS
RED LINE - FLUID COMPOSITION
CONTROLLED BY REACTIONS
Five-phase
Invariant Points
Reaction produces
1:1 moles of H2O and
CO2 , therefore fluid
composition is fixed
at XCO2=0.5 until a
phase is lost
A. Dol + Qtz + Tlc
+ Cal + Tr
B. Dol + Qtz + Tr
+ Di + Cal
C. Tr + Cal + Di +
Fo + Dol
Phase in Red are
may be consumed
at invariant points,
which then allows
reactions to
proceed along
univariant curves
METAMORPHISM OF CALCAREOUS ROCKS
METAMORPHISM AT HIGHER PRESSURES
• Reactions at
higher T
• Diopside larger
stability field
•Talc stable only
at low XCO2
METAMORPHISM OF CALCAREOUS ROCKS
CALC-SILICATES
Calc-silicate rocks present
challenges in simply defining
their complex compositions
Necessary components include
Si, Ca, Mg, Fe, Na, and K
Resultant metamorphic mineral
suites are hybrids of calcareous
and pelitic systems
- Pelitic rocks
- Calc-silicate rocks
Winter (2001) Figure 29-9. Map of
isograds in the pelitic Waterville and
calcareous Vassalboro formations of
south-central Maine. After Ferry (1983) J.
Petrol., 24, 343-376.
METAMORPHISM OF ULTRAMAFIC ROCKS
Occurences of Ultramafic Rocks in
the Crust
Alpine peridotites: uppermost mantle
attached to the base of oceanic
lithosphere slabs (ophiolites) that
become incorporated into the continental
crust along subduction zones; commonly
get trapped between two terranes
during an accretion event
Originally composed of olivine +
orthopyroxene + clinopyroxene (6:3:1) =
Lherzolite
Due to hydrothermal alteration at ocean
ridges, the ultramafic rock comes into
orogenic belts being strongly altered
(serpentinized). Regional
metamorphism then overprints this.
Chain of ultramafic bodies
in Vermont indicating a
suture zone of the
Ordovician Taconic
Orogeny. The ultramafics
mark a closed oceanic
basin between North
American rocks and an
accreted island arc
terrane. From Chidester,
(1968) in Zen et al.,
Studies in Appalachian
Geology, Northern and
Maritime. Wiley
Interscience.
METAMORPHISM OF ULTRAMAFIC ROCKS
COMMON METAMORPHIC MINERALS
- Typical
Mantle
Lherzolite
Ath
En
Atg, Crs
Qtz - Quartz
Di – Diopside
Tr – Tremolite
Tlc – Talc
Ath – Anthophyllite
En – Enstatite
Atg – Antigorite (Serpentine)
Crs – Chrysotile (Serpentine)
Fo – Mg-olivine
Bru – Brucite
Per – Periclase
METAMORPHISM OF ULTRAMAFIC ROCKS
Figure 29-11. Chemographics of ultramafic rocks in the CMS-H system (projected from H2O) showing the stable mineral assemblages (in the presence of
excess H2O) and changes in topology due to reactions along the medium P/T metamorphic field gradient illustrated in Figure 29-10. The star represents the
composition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO2-rich or
SiO2-poor varieties. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.
METAMORPHISM OF ULTRAMAFIC ROCKS
Winter (2001) Figure 29-10. Petrogenetic grid for water-saturated ultramafic rocks in the system CaO-MgO-SiO2-H2O produced using the
TWQ software of Berman (1988, 1990, 1991). The green arrow represents a typical medium P/T metamorphic field gradient. The dark
blue area represents the stability range of anthophyllite in “normal” ultramafic compositions. The lighter blue area represents the overall
stability range of anthophyllite, including more siliceous ultramafic rocks. After Spear (1993) Metamorphic Phase Equilibria and PressureTemperature-Time Paths. Mineral. Soc. Amer. Monograph 1.