rocas metamorficas

GEOS 444 Lecture Topics 3/28/05Read Chapter 21, 22, 25 (496-500)Bear Mtn Field Trip, Sat. 4/2, Lot 28 8:00am, bring lunch + H2OIgneous Mid-term due Thursday, -5% per day late! Basic Metamorphic Processes Metamorphic Rock Classification and Facies Index Minerals, Isograds, Facies SeriesIntroduction to Metamorphism

ImportanceMineral ResourcesMountain Building Events History of Continental CrustUncut Ruby and Sapphire

Metamorphic PetrologyMetamorphic rocks usually record peak metamorphic conditionsUse specific mineral(s) to determine P-T Index Minerals(Example: Garnet)

Metamorphism Changes in rocks due to increasing P-T conditions and/or interaction with fluids.

Metamorphism usually involves changes in: mineralogy formation of metamorphic minerals texture development of metamorphic fabrics

Metamorphic Conditions All changes occur in the SOLID state between ~100C and 800 CSolid State Recrystallization = Metamorphism Metamorphic Grade refers to general P-T conditions

High-temperature limit grades into melting Migmatites (mixed rocks) are gradational

Agents of MetamorphismTemperature

Pressure: lithostatic - uniform P, due to weight of overlying rock; 1 kb (0.1 GPa) = 3.3 km depth.differential (deviatoric) unequal P in different directions; produces metamorphic rock structures.

Fluids: H2O-dominated CO2. Derived from metamorphic reactions (internal) or magmatic fluids (external).

Types of MetamorphismTwo main types at tectonically active regions:(1) Contact Metamorphism (2) Regional Metamorphism

Contact Metamorphismthermal metamorphism due to heat of igneous intrusions narrow zones (

Contact Metamorphismthermal metamorphism at high T low P conditions

Types of MetamorphismTwo main types at tectonically active regions:(1) Contact Metamorphism (2) Regional Metamorphism

Regional Metamorphism Large, regional areas of crust affected (thousands of km2); one or more episodes of orogeny with combined elevated geothermal gradients and deformation Associated with mountain building processes at convergent plate boundaries (subduction zones; collision zones)Examples: Andes, Himalayas, Appalachians Full range of P-T metamorphic conditions; foliated rocks are a characteristic product

Regional Metamorphismlow to high P-T conditions

Non-foliatedFoliated

Slaty CleavageCommon Metamorphic FabricsSchistocityGneissic Banding

Origin of Metamorphic FoliationProduced by differential stressCompressiveShearing

GraniteGranitic Gniess

aligned micas (muscovite)

Protolith = rock type prior to metamorphismBroad Compositional Categories based on mineralogy and textures ultimately inherited from the protolith.

Quartz Sandstone

(a) Limestone (fiossiliferous)

ShaleSchist

3 Most Important Compositional Categories Pelites derived from Al-rich, fine-grained clastic sediments (shales, siltstones). Classic slate-phyllite-schist-gneiss sequence.

2. Calcareous derived from carbonate rocks (limestones, dolostones, shaly ls). Marbles, calc-silicate rocks.

3. Mafic and Ultramafic ultramafic to mafic igneous rocks. Greenstones, amphibolites, granulites.

P-T ClassificationMetamorphic Facies - is a set of compatible mineral assemblages in metamorphic rocks that occur over a relatively restricted range of P-T conditions for a variety of protolith compositions.

If we find rocks with these minerals in the field, then a certain facies (P-T conditions) may be assigned to the area.

Facies are simply broad characterizations of the P-T conditions experienced in an area. They are represented by boxes on a P-T diagram

Facies are defined by specific mineral assemblages, dependent on P-T and protolith composition.

Facies boundaries are defined by important mineral reactions and the appearance of distinctive minerals.

IMPORTANT CONCEPT:

Metamorphic assemblages are a function of P-T and protolith chemistry

Different protoliths will yield different mineral assemblages at = P-T conditions

Facies boundaries are defined by important mineral reactions and the appearance of distinctive minerals.

Interpretation of Metamorphic P-T Conditions???

Interpretation of Metamorphic P-T Conditions???Metamorphic rocks usually record peak metamorphic conditionsUse specific mineral(s) to determine P-T Index Minerals(Example: Garnet)

Concept of Index MineralsIndex MineralsChlorite, biotite, garnet, kyanite, sillimaniteNeed to exist over narrow P-T range

Geologic Mapping of Metamorphic Terranes Index minerals are mapped into zones with equivalent P-T conditions Boundaries between zones are called isograds (lines of equal P-T)

Metamorphic zones are higher resolution P-T estimates based on a single protolith (e.g. pelites)

Geothermobarometry Precise P-T from composition of co-existing minerals pairsExample: Garnet-biotite thermometerUse instrument called electron microprobe (x-rays analysis)garnetbiotite

Plate Tectonics and Metamorphism

Facies SeriesA traverse up grade through a metamorphic terrane should follow one of several possible P-T trajectories and, if extensive enough, cross through a sequence of facies

Variable P-T Conditions in a Convergent Plate Settinghigh P, low Thigh P and TLow P, high T

High P/TMed P/TLow P/T

Facies SeriesMiyashiro (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)

Barrovian Facies Series (Medium P)Chlorite zoneBiotite zone. Garnet zone.Staurolite zone.Kyanite zone.Sillimanite zone.

Buchan Facies Series (Low P)Chlorite zoneBiotite zone. Cordierite zone.Andalusite zone.Sillimanite zone.

The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure

BarrovianBuchan

FranciscanBarrovianBuchan

Paired Metamorphic BeltsLow P/T SeriesHigh P/T Series

= High P, Low T= Low P, High T

High P, Low TLow P, High T

We may all recognize a melt, but we may not be so good at recognizing the solid products crystallized from oneSmall, elongate, fairly coarse-grained and cross-cutting segregations of granitoid material in gneisses:Thin dikes of melt or precipitates from fluids, or fluid-enhanced recrystallization along fluid-filled fractures?The distinction between a silicate-saturated aqueous fluid and a fluid-saturated silicate meltDescriptive: the relationship between the composition of a rock and its mineralogyEskola based his proposed facies on the mineral assemblages developed in metamorphosed mafic rocks, and this descriptive aspect was a fundamental feature of the conceptEskola was aware of the temperature-pressure implications of the concept, and he correctly deduced the relative temperatures and pressures represented by the different facies that he proposedAdvances in experimental techniques and the accumulation of experimental and thermodynamic data have allowed us to assign relatively accurate temperature and pressure limits to individual faciesMiyashiro extended the facies concept to encompass broader progressive sequences: facies series

Eskola was aware of the temperature-pressure implications of the concept, and he correctly deduced the relative temperatures and pressures represented by the different facies that he proposedAdvances in experimental techniques and the accumulation of experimental and thermodynamic data have allowed us to assign relatively accurate temperature and pressure limits to individual faciesThe high P/T series, for example, typically occurs in subduction zones where normal isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermallyThe facies sequence here is (zeolite facies) - (prehnite-pumpellyite facies) - blueschist facies - eclogite facies. The medium P/T series is characteristic of common orogenic belts (Barrovian type)The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies)Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2)The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid

The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan or Ryoke-Abukuma type), rift areas, or contact metamorphismThe sequence of facies may be a low-pressure version of the medium P/T series described above (but with cordierite and/or andalusite), or the sequence (zeolite facies) - albite-epidote hornfels facies - hornblende hornfels facies - pyroxene hornfels faciesSanidinite facies rocks are rare, requiring the transport of great heat to shallow levelsThe contrast between Miyashiros higher-T-lower-P Ryoke-Abukuma belt (Section 21.6.3) and the classical Barrovian sequence also led him to suggest that more than one type of facies series is probable

Metamorphic field gradients are highly variable (even in the same orogenic belt) and transitional series aboundMiyashiro later chose to limit the number instead to the three broad major types of facies series (also called baric types) illustrated in Fig. 25-3Although the boundaries and gaps between them are somewhat artificial, the three series represent common types of tectonic environments

The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineralThe geothermal gradient in this northern district was higher than in Barrows area, and rocks at any equivalent temperature must have been at a lower pressureThis lower P/T variation has been called Buchan-type metamorphism. It too is relatively commonMiyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-typeBoth terms are common in the literature, and mean essentially the same thing

Shikoku and Honshu in Japan: a pair of parallel metamorphic belts are exposed along a NE-SW axis parallel to the active subduction zoneThese belts are of the same age, suggesting that they developed together

rocas metamorficas

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