5. Metamorpic Facies

5. Metamorpic Facies - Metamorphic Facies q q q q Concept...

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Unformatted text preview: Metamorphic Facies q q q q Concept introduced by Eskola (1915) to describe sets of repeatedly associated metamorphic mineral assemblages that reflect metamorphism of different rock types under the same, restricted range of P-T conditions. Background pioneered by Goldschmidt (1911, 1912) from study of contact metamorphosed pelitic, calcareous, and psammitic hornfelses near Oslo, Norway Discovered relatively simple mineral assemblages (< 6 major minerals) in the inner zones of the aureoles around granitoid intrusives Recognised these to be quilibrium mineral assemblage related to bulk composition (Xbulk) Metamorphic Facies q q q Goldschmidt found that certain mineral pairs (e.g. anorthite + hypersthene) were always present in rocks of the appropriate composition, whereas the compositionally equivalent pair (e.g. diopside + andalusite) was not If two alternative assemblages are X-equivalent, we must be able to relate them by a reaction In this case the reaction is simple: MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 Enstatite Anorthite Diopside Andalusite MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 Enstatite Anorthite Diopside Andalusite Goldschmidt concluded that the left side of the reaction was always stable under the metamorphic conditions existing near Oslo Metamorphic Facies q q Eskola worked in similar honfelses in S. Finland But found rocks with K-feldspar + cordierite near Oslo contained the compositionally equivalent pair biotite + muscovite in S. Finland Reaction: 2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 8 H2O Biotite Muscovite + 15 SiO2 = 3 Mg2Al4Si5O18 + 8 KAlSi3O8 Quartz Cordierite K-Feldspar q q q Concluded that this must reflect differing physical conditions of metamorphism Finnish rocks equilibrated at lower temperatures and higher pressures than the Norwegian ones 2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2 Biotite Muscovite Quartz = 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O Cordierite K-Feldspar Escola concluded that the left side of the reaction was stable in Finland (high P/low T), whereas right side was stable near Oslo (high T/low P) Metamorphic Facies q From this Eskola (1915) developed the concept of metamorphic facies. In his words: "In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition. We are led to a general conception which the writer proposes to call metamorphic facies." Metamorphic Facies Today there is a dual basis for the facies concept: q Descriptive: The relationship between mineralogy and bulk composition (Xbulk) A fundamental feature of Eskola's concept A metamorphic facies is then a set of repeatedly associated metamorphic mineral assemblages So if we find a specific assemblage (or better yet, a group of compatible assemblages representing a range of bulk compositions) in the field, then a certain metamorphic facies may be assigned to the area Metamorphic Facies q Interpretive: The range of temperature and pressure conditions represented by each facies Eskola was aware of the P-T implications and correctly deduced the relative temperatures and pressures of the facies he proposed But we can now assign relatively accurate temperature and pressure limits to individual metamorphic facies based on experimental petrology Metamorphic Facies q Eskola (1920) originally proposed five facies: Greenschist Amphibolite Hornfels Sanidinite Eclogite q Each facies was easily defined on the basis of the mineral assemblages that develop in mafic rocks Metamorphic Facies q Eskola (1939) later added three more facies: Granulite Epidote-amphibolite Glaucophane-schist (now called Blueschist) q and changed the name of the hornfels facies to the pyroxene hornfels facies Metamorphic Facies Temperature Formation of Zeolites EpidoteAmphibolite Facies Sanadinite Facies Amphibolite Facies PyroxeneHornfels Facies Granulite Facies GlaucophaneSchist Facies Greenschist Facies Pressure Eclogite Facies Metamorphic facies proposed by Eskola and their relative temperature-pressure relationships Metamorphic Facies q Several additional facies have been proposed since, the most notable of which are: Zeolite Prehnite-pumpellyite q q These are the result of the work of Coombs in the "burial metamorphic" terranes of New Zealand Fyfe et al. (1958) also proposed the following contact metamorphic facies: Albite-epidote hornfels Hornblende hornfels Metamorphic Facies Temperature-pressure diagram showing the generally accepted limits of the various metamorphic facies. Boundaries between facies are approximate and gradational Metamorphic Facies Definitive mineral assemblages for metamorphic facies in mafic rocks Metamorphic Facies Definitive mineral assemblages for metamorphic facies in mafic rocks Metamorphic Facies Convenient to consider metamorphic facies in four groups: 1) Facies of High Pressure q Blueshist and eclogite facies Greenschist, amphibolite and granulite facies Albite-epidote, hornblende and pyroxene hornfels facies Zeolite and prehnite-pumpellyite facies 2) Facies of Medium Pressure q 3) Facies of Low Pressure q 4) Facies of Very Low Grade q Metamorphic Facies 1) Facies of High Pressure q q Blueschist facies occurs in areas of low T/P gradients, characteristically developed in subduction zones Eclogites stable under normal geothermal conditions and so occur in mafic rocks in deep crust and mantle: Deep crustal magma chambers or dikes Sub-crustal magmatic underplates Subducted crust into the mantle Metamorphic Facies 2) Facies of Medium Pressure Most metamorphic rocks now exposed belong to the greenschist, amphibolite, or granulite facies qThe greenschist and amphibolite facies conform to "typical" geothermal gradient q Metamorphic Facies 3) Facies of Low Pressure q q Albite-epidote, hornblende, and pyroxene hornfels facies: contact metamorphic terranes and regional terranes with very high geothermal gradient. Sanidinite facies is rare and limited to xenoliths in mafic magmas and innermost portions of some contact aureoles adjacent to hot mafic intrusives. Metamorphic Facies 4) Facies of Very Low Grade q q Best developed during burial and hydrotheral (MOR) metamorphism but rocks often fail to fully recrystallize at very low grades and equilibrium not always attained So zeolite and prehnitepumpellyite facies not always represented and greenschist facies is the lowest grade found in many regional terranes Metamorphic Facies Combined Concepts of Isograds, Zones and Facies q Used when mapping metamorphic rocks: q q q Chlorite zone of the greenschist facies Staurolite zone of the amphibolite facies Cordierite zone of the hornblende hornfels facies q q Metamorphic maps typically include isograds that define zones and ones that define facies boundaries Determining a facies or zone is most reliably done when several rocks of varying composition and mineralogy are available Typical mineral changes in metabasic rocks during progressive metamorphism. Approximate location of the Barrovian metamorphic zones in peiltes are included for comparison. Metamorphic Facies Series q q q Traverse up-grade through a metamorphic terrane should cross through a sequence of zones and facies that reflect the P-T gradient of metamorphism within the terrane Led Miyashiro (1961, 1973) to introduce concept of metamorphic facies series for progressive sequences Myashiro initially proposed five facies series named after representative type areas: 3 3 3 3 3 Contact Facies Series (very low P) Buchan or Abukuma Facies Series (low P, regional) Barrovian Facies Series (moderate P, regional) Sanbagawa Facies Series (high P, moderate T) Franciscan Facies Series (high P, low T) Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. Metamorphic Facies Series q Since metamorphic field gradients are highly variable, Miyashiro (1973) later grouped his metamorphic facies series into three broad pressure categories or baric types: 3 High P/T Series v v (Zeolite)-(prehnite/pumpellyite)-blueschist-eclogite Typical of subduction zones (Zeolite)-(prehnite/pumpellyite)-greenschist-amphibolite-(granulite) Typical of common (Barrovian-type) orogenic belts (Zeolite)-albite-epidote hornfels-hornblende hornfels-pyroxene hornfels Typical of high-heat flow (Buchan/Abukuma-type) orogenic belts 3 Medium P/T Series v v 3 Low P/T Series v v P-T diagram showing three major types of metamorphic facies series proposed by Miyashiro ...
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This note was uploaded on 04/07/2008 for the course GEOL 320 taught by Professor Milam during the Winter '08 term at Ohio University- Athens.

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