Metamorphic Zones, Index Minerals, Isograds, Facies and Facies Series the onslaught of terminology to understand how we categorize metamorphic rocks and their conditions of formation! Chemistry of the protolith is the most important clue toward deducing the parent rock 1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates 2. Mafic rocks. Basalts or gabbros, some graywackes 3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges. 4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components 5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2. 6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al Categories are often gradational, and cannot include the full range of possible parental rocks One common gradational rock type is a sand-shale mixture:psammite Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies

What textures do you see?

Review: Types of Protolith 1.Pelitic/mudrocks - high Al, K, Si 2. Quartzo-feldspathic - high Si, Na, K, Al 3. Calcareous- high Ca, Mg, CO2 4. Mafic- high Ca, Mg, Fe 5. Ultramafic- very high Mg, Fe, low Si, Al Chemistry of the protolith is the most important clue toward deducing the parent rock 1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates 2. Mafic rocks. Basalts or gabbros, some graywackes 3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges. 4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components 5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2. 6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al Categories are often gradational, and cannot include the full range of possible parental rocks One common gradational rock type is a sand-shale mixture:psammite Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies WHY IS CHEMICAL COMPOSITION OF PROTOLITH IMPORTANT?

Metamorphic Grade Refers to maximum T or P of metamorphism What grades have we talked about? The idea of grade is general. We can better express the maximum P, T constraints using the concepts of metamorphic zone and facies. More on this! Chemistry of the protolith is the most important clue toward deducing the parent rock 1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates 2. Mafic rocks. Basalts or gabbros, some graywackes 3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges. 4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components 5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2. 6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al Categories are often gradational, and cannot include the full range of possible parental rocks One common gradational rock type is a sand-shale mixture:psammite Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies

Orogenic Regional Metamorphism of the Scottish Highlands: Development of the Index Mineral Concept George Barrow (1893, 1912) SE Highlands of Scotland - Caledonian Orogeny ~ 500 Ma Lots of folding Granites George Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism Metamorphism and deformation during the Caledonian orogeny, which reached its maximum intensity about 500 Ma ago Deformation in the Highlands was intense, and the rocks were folded into a series of nappes Numerous large granites were also intruded toward the end of the orogeny, after the main episode of regional metamorphism

Barrow’s Area Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

Orogenic Regional Metamorphism of the Scottish Highlands Barrow studied pelitic rocks Could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased Barrow noted significant and systematic mineralogical changes in the pelitic rocks He found that he could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased (which he could correlate to increased grain size) The new mineral that characterizes a zone is termed an index mineral

The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are: Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)

Each of these minerals is an INDEX mineral. Chlorite zone Biotite zone Garnet zone Staurolite zone Kyanite zone Sillimanite zone WHAT IS AN INDEX MINERAL

Sequence = “Barrovian zones” The P-T conditions referred to as “Barrovian-type” metamorphism (fairly typical of many belts) Now extended to a much larger area of the Highlands ANOTHER DEFINTION: Isograd line that separates the zones (a line in the field of constant metamorphic grade). Also reflects the FIRST APPEARANCE of the index mineral. This sequence of zones now recognized in other orogenic belts, and is now so well established in the literature that the zones are often referred to as the Barrovian zones Tilley, Kennedy, etc. confirmed Barrow’s zones, and extended them over a much larger area of the Highlands Tilley coined the term isograd for the line that separates the zones An isograd, then, is meant to indicate a line in the field of constant metamorphic grade Really = the intersection of the isogradic surface with the Earth’s surface

Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

To summarize: An isograd represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade When one crosses an isograd, such as the biotite isograd, one enters the biotite zone Zones thus have the same name as the isograd that forms the low-grade boundary of that zone Because classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones Later we shall see broader categories: metamorphic facies Barrovian zones have become the norm to which we compare all other areas of regional metamorphism OK practice, but we shouldn’t let these zones constrain our thinking or our observations Other zones may be important and useful locally A chloritoid zone is prevalent in the Appalachians (X)

A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district Pelitic compositions are similar, but the sequence of isograds is: chlorite biotite garnet andalusite sillimanite

The stability field of andalusite occurs at pressures less than 0 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 1 GPa = 10kbars The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type Both terms are common in the literature, and mean essentially the same thing Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

Metamorphic Facies Eskola (1915) developed the concept of metamorphic facies: What is a metamorphic facies? On the basis of the relationship between rock composition and mineral assemblage, and the worldwide occurrence of virtually identical mineral assemblages, Eskola (1915) developed the concept of metamorphic facies: At the time Grubenmann’s epizone, mesozone, and catazone were very broad and poorly defined and the isograd-zones were restricted to pelitic compositions, and were too narrow for easy correlation from one locality to another Eskola’s facies were initially based on metamorphosed mafic rocks Basaltic rocks occur in ~ all orogenic belts, and the mineral changes in them define broader T-P ranges than those in pelites, so facies provided a convenient way to compare metamorphic areas around the world

Metamorphic Facies Fig. 25-2. Temperature- pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed They are thus separated by mineral reaction isograds The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

Metamorphic Facies The range of temperature and pressure conditions represented by each facies Eskola aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed Can now assign relatively accurate temperature and pressure limits to individual facies 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 proposed Advances in experimental techniques and the accumulation of experimental and thermodynamic data have allowed us to assign relatively accurate temperature and pressure limits to individual facies

Metamorphic Facies Eskola (1920) proposed 5 original facies: Greenschist Amphibolite Hornfels Sanidinite Eclogite Easily defined on the basis of mineral assemblages that develop in mafic rocks More facies have been added since original designations Each was easily defined on the basis of distinctive mineral assemblages that develop in mafic rocks (clearly reflected in the facies names, most of which correspond to common metamorphic mafic rock types)

Metamorphic Facies Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.

Metamorphic Facies Fig. 25-2. Temperature- pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed They are thus separated by mineral reaction isograds The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

Metamorphic Facies defined for mafic protolith The definitive mineral assemblages that characterize each facies (for mafic rocks). The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table 25-1 (details and variations later)

1) Facies of high pressure It is convenient to consider metamorphic facies in 4 groups: 1) Facies of high pressure The blueschist and eclogite facies: low molar volume phases under conditions of high pressure Blueschist facies occurs in areas of low T/P gradients, characteristically developed in subduction zones Eclogites are stable under normal geothermal conditions May develop wherever mafic magmas solidify in the deep crust or mantle: crustal chambers or dikes, sub-crustal magmatic underplates, subducted crust that is redistributed into the mantle The name is due to the blue-gray color of most mafic blueschists, imparted by sodic amphiboles

Metamorphic Facies 2) Facies of medium pressure Most metamorphic rocks now exposed belong to the greenschist, amphibolite, or granulite facies The greenschist and amphibolite facies conform to the “typical” geothermal gradient

Fig. 25-9. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic Facies 3) Facies of low pressure Albite-epidote hornfels, hornblende hornfels, and pyroxene hornfels facies: contact metamorphic terranes and regional terranes with very high geothermal gradient. Sanidinite facies is rare- limited to xenoliths in basic magmas and the innermost portions of some contact aureoles adjacent to hot basic intrusives Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2

Metamorphic Facies 4) Facies of low grades Rocks often fail to recrystallize thoroughly at very low grades, and equilibrium is not always attained Zeolite and prehnite- pumpellyite facies are thus not always represented, and the greenschist facies is the lowest grade developed in many regional terranes Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2

Metamorphic Facies Review Metamorphic zone (e.g., chlorite zone) Index Mineral Isograd Metamorphic Facies

Facies Series/Field Gradient A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients, and, if extensive enough, cross through a sequence of facies Miyashiro extended the facies concept to encompass broader progressive sequences: facies series

Field gradient Fig. 25-3. Temperature- pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The 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 thermally The 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 metamorphism The 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 facies Sanidinite facies rocks are rare, requiring the transport of great heat to shallow levels

Pressure-Temperature Time Paths Facies concept leads to idea that metamorphic petrologists try to reconstruct CONDITIONS of metamorphism. Also important is TIME. Time tells us about the RATES of processes.

Regional Metamorphism 3 stages: Burial/crustal thickening--why does trajectory have steep slope? Heating stage Uplift stage

Regional Metamorphism What are prograde vs. retrograde metamorphic paths or reactions?

Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

Example of Contact Metamorphism What does this diagram show? ------------------> Explain how the metamorphic grade and assemblages MIGHT change with distance from this dike.