Earth Mat Flashcards - Ch 18
Subdivisions on metamorphic pressures
low pressure (0 – 2 kbar ≈ 0 – 6 km depth),
moderate pressure (2 – 6 kbar) or high
pressure ( > 6 kbar ≈ > 20 km depth).
provide critical information because they effectively indicate the temperature/pressure conditions of metamorphism.
index minerals,
are lines drawn on geological maps that mark the first appearance of a particular index mineral.
Isograds
consists of the regionbounded by two isograd lines.
A metamorphic zone
bounded by the chlorite and biotite isograds
Chlorite Zone
The six metamorphic zones based on the six index mineral isograds are called
Barrovian zones.
occurs between the biotite isograd—marking the first appearance of biotite—and the almandine (garnet) isograd.
The biotite zone
Almandine forms through the chemical transformation of
chlorite and magnetite.
is bounded by the almandine garnet isograd—marking the first appearance of almandine garnet—and the staurolite isograd.
The almandine zone
lies between the staurolite isograd—marking the first appearance of the higher temperature mineral staurolite—and the kyanite isograd.
The staurolite zone
Kyanite forms through
dehydration reactions of
staurolite [Fe2+2Al9Si4O23(OH)] or
pyrophyllite [Al2Si4O10(OH)2]
occurs inside the sillimanite isograd and marks the highest temperature zone defined by Barrow and Tilley
The sillimanite zone
Limitations of barrovian zones and isograds
not useful for non-pelitic rocks, subduction zones or contact metamorphism
Sillimanite and potassium feldspar can also develop by
dehydration of muscovite in the presence of quartz,
introduced the concept of metamorphic facies—a more comprehensive approach to assessing the conditions recorded by metamorphic rocks.
Eskola
are distinctive mineral assemblages in metamorphic rocks that form in response to a particular range of temperature and/or pressure conditions.
Metamorphic facies
Introduced the zeolite facies; the prenite-pumpellyite facies
Turner; Coombs
- include non-foliated, fine-grained hornfels rocks and coarser-grained rocks with granoblastic textures.
- form by heat-induced metamorphism in aureoles surrounding igneous intrusions.
Hornfels facies
is the low-temperature hornfels facies, with temperatures generally < 450 °C and pressures < 2 kbar (depth < 6 km).
The albite-epidote hornfels facies
The albite-epidote hornfels facies is roughly the low-pressure equivalent
greenschist facies
compose the bulk of many metamorphic aureoles, forming at temperatures generally between 450 and 600 °C and at pressures. <2.5 kbar (<8km)
Hornblende hornfels facies rocks
The Hb-Hornfels is the low pressure equivalent of
amphibolite
- facies develop at temperatures of 600–800 °C and at pressures < 2.5 kbar (< 8 km)
Px-Hornfels
are very rare, forming in very high temperature (> 800 °C) and low pressure (< 2.5 kbar ≈ < 8 km) conditions in association with basic and ultrabasic intrusions
Sanidinite hornfels facies
the dehydration rxn of phlogophite to sanidinte and enstatite is associated with what facies
Sanidinite hornfels facies
a low-grade metamorphic facies produced by temperatures between ∼150 and 300 °C and pressures less than 5 kbar (∼15 km depth).
Zeolite
are a hydrous sodium and calcium aluminum tectosilicate mineral group formed by diagenetic or low-temperature metamorphic reactions.
Zeolites
Critical zeolite facies minerals, which commonly coexist with quartz, include
analcime, laumontite, heulandite, and wairakite.
Zeolite facies minerals originate from
the hydrothermal alteration of volcanic protoliths
the devitrification of basaltic glass and tuff,
reaction of pelites and graywackes with saline waters.
Transition of zeolite minerals as PT conditions increase
Stillbte -> Heulandite -> Laumontite -> Waikarite
facies minerals are produced by hydrothermal alteration and burial metamorphism at temperatures and pressures that exceed zeolite facies conditions.
Prehnite-pumpellyite
(250–350 °C) (< 6 kbar, ∼20 km depth)
minerals found in the prehnite pumpellyite series
albite, chlorite, muscovite, illite, phengite, smectite
Higher temperature alteration of prehnite and pumpellyite results in _______________, two minerals that mark the transition to the higher grade albite-epidote hornfels facies and the greenschist facies
the neocrystallization of actinolite and epidote
The higher temperature assemblage containing pumpellyite and actinolite has been called the
transitional pumpellyite-actinolite facies
generally form under medium temperature (350–550 °C) and pressure (3–10 kbar ≈ 10–30 km depth) conditions associated with dynamothermal metamorphism at convergent plate boundaries.
Greenschist facies rocks
Where metapelites occur, the greenschist facies can be subdivided into three Barrovian zones
- The chlorite zone corresponds to lower greenschist facies conditions with minerals such as chlorite, dolomite, stilpnomelane, and calcite.
- The biotite zone corresponds to upper greenschist facies conditions and contains biotite and tremolite.
- The lower part of the almandine garnet zone corresponds to the uppermost greenschist to epidote-amphibolite facies.
form at high temperatures (∼550–750 °C) and moderate to high pressures (4–12 kbar ≈ 12–40 km depth) in regional orogenic belts at convergent margins.
Amphibolite facies rocks
The transition from greenschist to amphibolite facies is marked by
an increase in hornblende, garnet, and anthophyllite
Plagioclase minerals become less sodic and more calcic during this transition.
appearance of staurolite in pelitic rocks
transformation from kyanite to sillimanite in pelitic rocks,
The amphibolite facies encompasses several different Barrovian zones (see Figure 18.3) that include:
The upper part of the almandine zone
All of the staurolite zone
The lower part of the sillimanite zone
The low-temperature part of the amphibolite facies corresponding with the almandine zone is also known as the
epidote-amphibolite facies
consists of high-temperature (∼700–900 °C) and moderate to high-pressure (3–15 kbar ≈ 10–50 km depth) mineral assemblages.
The granulite facies
Difference betweeen the lower granulite (I) and upper granulite (II) facies
Hydrous minerals like hornblende and biotite can occur in the lower part of the granulite facies (granulite I), while the upper part (granulite II) is characterized entirely by anhydrous minerals.
Common minerals in granulite facies include ________in pelitic and quartz-feldspathic rocks. Calcareous rocks are characterized by the
quartz and orthoclase;
appearance of wollastonite and the absence of hydrous minerals like phlogopite.
are hypersthene-bearing granitic gneisses.
found in granulite facies
Charnockites
Granulite facies metamorphism occurs in the highest temperature dynamothermal metamorphism regions at:
Convergent plate boundaries
The base of thick continental crust
The uppermost part of the mantle
The granulite facies corresponds with which zones
the upper parts of the Barrovian sillimanite zone and, at even higher temperatures, the cordierite-garnet zone.
consists of moderate to high pressure (4–20 kbar ≈ 13–66 km depth) and low temperature (150–500 °C) mineral assemblages.
The blueschist facies
The blue amphibole mineral glaucophane gives this facies its distinctive color. Other common minerals in the blueschist facies include:
Magnesio-riebeckite
Lawsonite
Jadeite pyroxene
Aegirine
Crossite
Kyanite
The eclogite facies (see Figure 18.2) typically develops at
high temperatures (400 – 900 °C) and very high pressures (12 – 25 kbar ≈ 40 – 82 km).
Eclogite facies rocks occur in three major environments:
In the lower continental crust and mantle (> 40 km depth) and later exposed on Earth’s surface in deeply eroded fold and thrust belts.
At convergent margins in ophiolite complexes and subduction zone mélanges.
As xenoliths in diamond-bearing kimberlite pipes.
occur within the eclogite facies (Figure 18.5) at pressures > 25 kbar (> 80 km depth) and temperatures > 600 °C.
Ultra-high pressure (UHP) minerals
UHP conditions are indicated by critical minerals such as:
Coesite, a high pressure polymorph of silica.
Diamond, the high pressure polymorph of carbon.
Majorite, a high pressure mineral (Mg₂Al₃Si₂O₁₂ – MgSiO₃).
Coesite was first observed in a laboratory, where it was synthetically created by
Loring Coes, Jr., in 1953.
Naturally occurring coesite was first noted at
Meteor (Barringer) Crater, Arizona
discovered the first dynamothermally produced coesite in Alpine rocks.
Christian Chopin (1984)
is a sequence of facies that occurs across a metamorphic terrane due to differences in pressure and temperature (P/T) conditions.
A metamorphic facies series
Low P/T series group: two low pressure and high temperature facies series are recognized:
(1) the very low P/T contact facies series, and (2) the somewhat higher P/T Buchan facies
Moderate P/T series group:
Barrovian facies series,
High P/T series group
Sanbagawa facies series and Franciscan facies series,
The geothermal gradient for the contact facies series is_implying a significantly higher than average heat input due to magmatic activity.
> 80 °/km,
Low pressure contact metamorphism produces what textures
hornfelsic and/or granoblastic
With increasing temperature, the contact facies series progresses through the sequence:
Zeo - AbEp - HbHf - PxHf - SnHf
The Buchan facies series records high geothermal gradients ranging
from 40 to 80 °C/km.
The Buchan facies series is also known as the
Abukuma facies series,
The Buchan facies series progresses, with increasing temperature and pressure, through:
Zeo - PP - Grn - Amp - Gra
Buchan facies series metamorphism reflects
higher temperatures, but only moderate increases in pressure.
Buchan facies series develop by
How about non foliated?
regional metamorphism and magmatic arc activity at convergent margins;
crustal thinning and heating
The Barrovian facies series develops in response to geothermal gradients of
∼ 20 – 40 °C/km,
With increasing temperature, the Barrovian facies series progresses through the same facies sequence as the Buchan facies series, from:
Zeo - PP - Grn - Amp - Gra
Hows does buchan differ from barrovian series
Higher P/T ratio
Kya in Buchan
Tectonic affinity of Barrovian facies
thickening orogenic belts at convergent plate boundaries, especially collisional orogens.
The Sanbagawa facies series are produced under geothermal gradients in the range of
10 to 20 °C/km.
The Sanbagawa facies series progression includes:
Zeolite,
Prehnite-pumpellyite,
Blueschist facies, followed in some cases by
Greenschist, and/or
Amphibolite facies.
Sanbagawa facies series is characterized by slightly higher temperatures.
This may result from
Slower subduction giving the rocks more time to heat up as pressures increase, or
Higher geothermal gradients during subduction
The Franciscan facies series develop where geothermal gradients are
< 10 °C/km.
The Franciscan facies series progresses from:
Zeolite,
Prehnite-pumpellyite,
Blueschist, possibly to
The eclogite facies
Franciscan series metamorphism reflects
the progressive rapid increase in pressure relative to slow increases in temperature during regional metamorphism as rocks are rapidly dragged downward in subduction zones.
Occur when chemical reactions have reached completion, resulting in no further net changes.
Equilibrium Condition
Disequilibrium Indicators:
Reaction rims on minerals.
Coexisting minerals that cannot exist in equilibrium.
Incomplete replacement of minerals.
Equilibrium Indicators:
Absence of disequilibrium features.
Planar grain contacts between mineral crystals.
Equilibrium mineral reaction and assemblage diagrams are known as
petrogenetic or paragenetic grids.
Petrogenetic vs Paragenetic
Petrogenetic refers to the conditions under which the rock originated, while
Paragenesis refers to the formation sequence of equilibrium minerals.
Components of ACF ternary diagram
A = (Al2O3 + Fe2O3) − (Na2O + K2O)
C = (CaO – 3.33 P2O5)
F = FeO + MgO + MnO.
particularly useful for displaying common equilibrium mineral assemblages that occur in rocks derived from the quartzo-feldspathic, basic, calcareous, and pelitic protoliths
ACF diagram
proposed by Eskola (1915), is used to discriminate equilibrium mineral assemblages derived from pelitic and quartzo-feldspathic protoliths with excess Al2O3 and SiO2
The A′KF diagram
particularly useful in discriminating mineral compositions in ferromagnesian-rich basic and ultrabasic rocks as well as many pelitic rocks.
The AFM diagram
diagrams have also been developed for calcareous rocks as illustrated in which the three components are CaO, MgO, and SiO2.
CMS ternary
Regional metamorphism, which produces the vast majority of metamorphic rocks, occurs primarily at:
Divergent plate boundaries, associated with continental and oceanic rifts and sea floor spreading, where hydrothermal processes dominate.
Convergent plate boundaries, associated with subduction, magmatic arcs, and continental collisions, where dynamothermal metamorphic processes dominate.
Continental rift basin metamorphism can occur by a number of processes:
Contact metamorphism from the intrusion of shallow dikes, sills, and flood basalts.
Hydrothermal alteration associated with hot magmatic and wall rock volatile fluids.
Dynamic metamorphism due to brittle extensional faulting in the upper crust and ductile shearing in the lower crust.
Burial metamorphism producing zeolite and prehnite-pumpellyite facies assemblages due to the deposition of thick, non-marine detrital sediment sequences in rift basins.
extensive hydrothermal metamorphism of basalt, gabbro, and peridotite occurs at ocean ridges, resulting
albite-epidote hornfels, zeolite, and prehnite-pumpellyite
At deeper levels within the oceanic crust and upper mantle, gabbro and peridotite are altered to higher temperature
hornblende hornfels facies assemblages
The paired metamorphic belts encircling the Pacific Ocean were recognized as
subduction zone (outer metamorphic belt) and magmatic arc (inner metamorphic belt) assemblages,
The outer metamorphic belt consists of what series
Sanbagawa or Franciscan
The inner metamorphic belt consists of what series
Buchan or Barrovian
characteristic of Phanerozoic subduction zones,
Blueschist facies mineral assemblages
Forearc basins consists of what facies
zeolite to prehnite-pumpellyite
the metamorphic grade of accretionary prism, metamorphic belts
High; Low
the forearc basement can have what kind of metamorphism
Dynamothermal metamorphism can also produce either high P/T facies associated with the underlying subduction zone or moderate P/T due to the adjacent magmatic arc.
facies associated with magmatic arc complexes
Contact, Buchan, and Barrovian
The metamorphic facies commonly found in pull-apart basins include
zeolite and prehnite-pumpellyite facies.
(sub-greenschist conditions)
are among the most distal tectonic features created in the overlying plates of many convergent margins.
Fold and thrust belts and foreland basins
The horizontal shortening is accommodated by folds and thrust faults that result in the
telescoping or “piggybacking” of thrust slices.
facies assemblages in fold and thrust belts.
assemblages in fold and thrust belts.
In the adjoining foreland basin, an initially deep basin fills with marine deposits producing alternating shale, sandstone, chert, and carbonate layers producing what is referred to as
flysch deposits.
With continued thrusting and infilling in foreland basins, fine-grained marine rocks are succeeded by sandstones and conglomerates in what are referred to as
molasse deposits.
facies to be expected in a Continent – continent collision
Barrovian facies series
