Blueschist Catalina Island, CA. Image: Darrell Henry

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1 Metamorphic Facies and Metamafic rocks (Chapter 25) Blueschist Catalina Island, CA. Image: Darrell Henry

2 Metamorphic Facies Concept Pentii Eskola (1914) Orijärvi region of southern Finland Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi Eskola: difference must reflect differing physical conditions between the regions Concluded that Finnish rocks equilibrated at lower temperatures and dhigher h pressures than Norwegian ones Eskola (1915) developed the concept of metamorphic facies: "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."

3 Metamorphic Facies Concept Dual basis for the facies concept Descriptive: relationship between rock composition and its mineralogy A metamorphic facies is set of repeatedly associated metamorphic mineral assemblages If we find specified assemblage or group of compatible assemblages covering range of compositions in the field, then a certain ti facies may be assigned dto the area Interpretive: range of T and P conditions represented by each facies Eskola was aware of T-P implications of concept and correctly deduced the relative T and P represented by different facies We can now assign relatively accurate T and Pe limits to individual facies

4 Metamorphic Facies Concept Defined on the basis of mineral assemblages that develop in mafic rocks Image: Winter (2001) Eskola (1920) proposed 5 original facies: Greenschist Amphibolite Hornfels Sanidinite Eclogite Eskola (1939) added Granulite Epidote-amphibolite Blueschist Later additions Zeolite Prehnite-pumpellyite Albite-epidote hornfels Hornblende hornfels

5 Metamorphic Facies Concept Defined on the basis of mineral assemblages that develop in mafic rocks IUGS-SCMR SCMR version of facies diagram Image: IUCS-SCMR (2007)

6 Metamorphic Facies Concept Defined on the basis of mineral assemblages that develop in mafic rocks Table Definitive Mineral Assemblages of Metamorphic Facies Facies Definitive Mineral Assemblage in Mafic Rocks Zeolite 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.

7 Metamorphic Facies Concept Metamorphic field gradient - array of peak metamorphic (max T) experienced in a metamorphic terrain Typical Barrovian-type metamorphic field gradient and a series of metamorphic P- T-t paths for rocks found along that gradient in field. Image: Winter (2001)

8 Metamorphic Facies Concept Metamorphic field gradient Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. Image: Winter (2001)

9 Metamorphic Facies Concept Metamorphic series Image: Winter (2001) 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)

10 Mineral changes and associations that develop with increasing metamorphic grade Hydrous minerals are not common in high- T igneous mafic protolith, so hydration is a prerequisite for development of metamorphic mineral assemblages that characterize most facies Unless water is available, mafic igneous rocks will remain largely unaffected in metamorphic terranes, even as associated sediments are completely re-equilibratedequilibrated Coarse-grained intrusives are least permeable, and thus most likely to resist metamorphic changes, while tuffs and graywackes are most susceptible

11 Mineral changes and associations that develop with increasing metamorphic grade Plagioclase As T is lowered, the more Ca-rich plagioclases become progressively unstable There eeis a general e correlation o between bewee T and maximum An- content of stable plagioclase At low metamorphic grades only albite (An0-3) is stable In upper-greenschist facies oligoclase l becomes stable. Andesine(~An40) and more calcic plagioclases are stable in upper amphibolite and granulite facies Excess Ca and Al released may released from calcite, an epidote mineral, titanite, or amphibole, etc., depending on P-T-X Clinopyroxene breaks down to a number of mafic minerals, depending on grade. These include chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc., and the one(s) that form are commonly diagnostic of the grade and facies

12 Mafic Assemblages at Low Grades Zeolite and prehnite-pumpellyite facies Do not always develop - typically require unstable protolith Stilbite on basalt (Poona, India). Image: Darrell Henry (2007) Boles and Coombs (1975) showed that metamorphism of their tuffs in NZ was accompanied by substantial chemical changes due to circulating fluids, and that fluids played important role in metamorphic minerals that were stable i.e. strong component of hydrothermal metamorphism.

13 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) Metamorphism of mafic rocks is first evident in greenschist facies (correlates with chlorite and biotite zones of associated pelitic rocks) Typical minerals include chlorite, albite, actinolite, epidote, quartz, and possibly calcite, biotite, or stilpnomelane Chlorite, actinolite, and epidote ACF diagram - The most characteristic mineral impart the green color from assemblage of the greenschist facies is: chlorite which mafic rocks and facies get + albite + epidote + actinolite ± quartz their name Image: Winter (2001)

14 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) Greenschist to amphibolite facies transition involves 2 major mineralogical changes Image: Winter (2001) 1. Transition from albite to oligoclase (increased Ca-content of stable plagioclase with T) 2. Transition from actinolite to hornblende (amphibole becomes able to accept increasing amounts of aluminum and alkalis at higher T)

15 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) ACF diagram Typically 2-phase Hbl-Plag Most amphibolites are predominantly black rocks with up to 30% white plagioclase l Garnet occurs in more Al- Fe-rich and Ca-poor mafic rocks and clinopyroxene in Al-poor-Ca-rich ones Image: Winter (2001)

16 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) The transition from amphibolite to granulite facies occurs in the range o C Conversion of felsic gneiss (left) to charnockite (right -type of the granulite - opx granitoid). Kabbaldurga quarry, India Image: Darrell Henry In presence of aqueous fluid, associated pelitic and quartzo-feldspathic rocks (including granitoids) begin to melt in this range at low to medium pressures, so that migmatites may form and melts may become mobilized Not all pelites and quartzo-feldspathic rocks reach granulite facies as a result Mafic rocks generally melt at somewhat higher T If H 2 O is removed by earlier melts remaining mafic rocks may become depleted in water Hornblende nd decomposes and orthopyroxene r + clinopyroxene appear This reaction occurs over a T interval of at least 50 o C

17 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) The granulite facies is characterized by the presence of a largely anhydrous mineral assemblage Winter (2001) In metabasites diagnostic i mineral assemblage is orthopyroxene + clinopyroxene + plagioclase + quartz Garnet is also common, and minor hornblende and/or biotite may be present

18 The origin of granulite facies rocks is complex and controversial. There is general agreement, however, on two points 1) Granulites represent unusually hot conditions T > 700 o C, geothermometry has yielded some very high T, i.e. >1000 o C Average geotherm T for granulite facies depths should be ~500 o C, suggesting that granulites are products of crustal thickening and excess heating 2) Granulites are dry These rocks didn t melt due to lack of available H 2 O Granulite facies terranes represent deeply buried and dehydrated roots of continental crust Touret: fluid inclusions in granulite facies rocks of S. Norway are CO 2 -rich, while those in amphibolite facies rocks are more H 2 O-rich ih H 2 O can be removed by H 2 O-undersaturated melts

19 Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies (most common) Winter (2001)

20 Mafic Assemblages of the High P/T Series: Blueschist and Eclogite Facies Mafic rocks (and not pelites) develop conspicuous and definitive mineral assemblages under high P/T conditions Winter (2001) High P/T geothermal gradients characterize subduction zones Mafic blueschists are recognizable by color - useful indicators of ancient subduction zones Great density of eclogites suggests that subducted basaltic oceanic crust becomes more dense than surrounding mantle

21 Blueschist Facies - characterized in metabasites by presence of sodic blue amphibole stable only at high P (e.g. glaucophane) Association of glaucophane + lawsonite is diagnostic. Albite breaks down at high pressure by reaction to jadeitic pyroxene + quartz: NaAlSi 3 O 8 = NaAlSi 2 O 6 + SiO 2 Ab = Jd + Qtz Assemblage jadeite + quartz indicates highpressure blueschist facies Winter (2001)

22 Eclogite Facies - mafic assemblage omphacitic pyroxene + pyrope-grossular garnet (Christmas-tree rocks) Along higher geothermal gradients amphibolite facies, or even the granulite facies, may lead to eclogite facies Winter (2001) Much of blueschist, and all of eclogite facies are marked by high-pressure instability of plagioclase, a common phase in metabasites of any other grade

23 Metamorphic field gradient (and PTt paths) (upper left) crustal thickening (clockwise path) Image: Winter (2001) (left) shallow magmatism heat-flow (above) model for some types of granulite facies metamorphism (counter-clockwise clockwise path)

24 Pressure-Temperature-Time Time (P-T-t) Paths Temporal implication of progressive metamorphism: that rocks pass through series of mineral assemblages upon continuously equilibrate to increasing metamorphic grade Consider complete set of T-P conditions that rock may experience during metamorphic cycle from burial to metamorphism (and orogeny) to uplift and erosion Winter (2001) Such a cycle is called a pressure- temperature-time path, or P-T-t path

25 Pressure-Temperature-Time Time (P-T-t) Paths Metamorphic P-T-t paths may be addressed by: 1) Observing partial overprints of one mineral assemblage upon another - Relict minerals may indicate a portion of either prograde or retrograde path (or both) depending upon when they were created 2) Apply geothermometers and geobarometers to core vs. rim compositions of chemically zoned minerals to document changing P-T conditions experienced by a rock during their growth Winter (2001)

26 Pressure-Temperature-Time Time (P-T-t) Paths Chemical zoning profiles across a garnet from Tauern Window. Conventional P-T diagram (pressure increases upward) showing three modeled "clockwise" P-T-t paths computed from the profiles Winter (2001)

27 Pressure-Temperature-Time Time (P-T-t) Paths Some examples of modeled P-T-t paths representing common types of metamorphism Winter (2001) The paths illustrated are schematic, and numerous variations are possible, depending upon style of deformation and the rates of thickening, heat transfer, magmatism, and erosion, etc.

28 Pressure-Temperature-Time Time (P-T-t) Paths Path (a) is considered typical P-T-t path for orogenic belt with crustal thickening P increases >> T, because of time lag required dfor heat transfer (P equilibrates nearly instantaneously, but heat conducts very slowly through rocks) Thickened crustal block quickly reaches P max while being relatively cool Winter (2001) New geotherm is higher, h but transient, and lasts only as long as the thickened crust and subduction-related heat generation lasts

29 Pressure-Temperature-Time Time (P-T-t) Paths Path (a) is considered typical P-T-t path for orogenic belt with crustal thickening Erosion soon affects thickened crust and dpb begins to decrease before rocks can equilibrate with higher orogenic geotherm Winter (2001) T still increasing due to slow heat transfer so that P-T-t path has a negative slope following Pmax Reach Tmax when cooling effect of uplift and erosion catches up to the increased geotherm, so that thermal perturbation of crustal thickening is dampened and begins to fade

30 Pressure-Temperature-Time Time (P-T-t) Paths Winter (2001) Path (b): rocks heated and cooled at virtually constant pressure by magmatic intrusion at shallow levels P-T-t path for contact metamorphism Depending upon extent of magmatic activity and its contribution to crustal mass, any path transitional between (a) and (b) may occur. represents gradation from high-p (Barrovian) regional metamorphism to regional-contact metamorphism with numerous plutons to local contact metamorphism

31 Pressure-Temperature-Time Time (P-T-t) Paths We may assume that the general form of a path such as (a) represents a typical rock during orogeny and regional metamorphism 1. Contrary to classical treatment of metamorphism, T and P do not both increase in unison as a single unified "metamorphic grade." 2. Pmax and Tmax do not occur at the same time In usual case of "clockwise" P-T-t paths, Pmax occurs much earlier than Tmax. Tmax should represent maximum grade at which chemical equilibrium is "frozen in" and metamorphic mineral assemblage is developed This occurs at P well below Pmax which is This occurs at P well below Pmax, which is uncertain since a mineral geobarometer should record the pressure of Tmax