2 Progressive Metamorphism Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditionsPrograde metamorphism: changes in a rock that accompany increasing metamorphic gradeRetrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous eventRetrograde metamorphism: any accompanying changes
3 Progressive Metamorphism A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find todayMetamorphic rocks usually maintain equilibrium as grade increasesA rock at a high metamorphic grade thus probably progressed through a sequence of mineral assemblages as it passed through all of the mineral changes necessary to maintain equilibrium with increasing temperature and pressure, rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today
4 P-T-t PathThe preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature)All rocks that we now find must also have cooled to surface conditionsAt what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate?If a metamorphosed sedimentary rock experienced a cycle of increasing metamorphic grade, followed by decreasing grade, at what point on this cyclic P-T-t path did its present mineral assemblage last equilibrate?The zonal distribution of metamorphic rock types preserved in a geographic sequence of increased metamorphic grade suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) experienced by that rock during metamorphism
5 Prograde ReactionsRetrograde metamorphism is of only minor significancePrograde reactions are endothermic and easily driven by increasing TDevolatilization reactions are easier than reintroducing the volatilesGeothermometry indicates that the mineral compositions commonly preserve the maximum temperatureRetrograde metamorphism is of only minor significance, and is usually detectable by observing textures, such as the incipient replacement of high-grade minerals by low-grade ones at their rims
6 Types of ProtolithThe common types of sedimentary and igneous rocks fall into six chemically based-groups1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si4. Carbonates- high Ca, Mg, CO25. Quartz - nearly pure SiO2.6. Quartzo-feldspathic - high Si, Na, K, AlChemistry of the protolith is the most important clue toward deducing the parent rock1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates2. Mafic rocks. Basalts or gabbros, some graywackes3. 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 components5. 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, AlCategories are often gradational, and cannot include the full range of possible parental rocksOne common gradational rock type is a sand-shale mixture:psammiteOther rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies
7 Examples of Metamorphism Interpretation of the conditions and evolution of metamorphic bodies, mountain belts, and ultimately the evolution of the Earth's crustMetamorphic rocks may retain enough inherited information from their protolith to allow us to interpret much of the pre-metamorphic history as well
8 Orogenic Regional Metamorphism of the Scottish Highlands George Barrow (1893, 1912)SE Highlands of ScotlandCaledonian orogeny ~ 500 MaNappesGranitesGeorge Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphismMetamorphism and deformation in the SE Highlands of Scotland occurred during the Caledonian orogeny, which reached its maximum intensity about 500 Ma agoDeformation in the Highlands was intense, and the rocks were folded into a series of nappesNumerous large granites were also intruded toward the end of the orogeny, after the main episode of regional metamorphism
9 Barrow’s AreaRegional 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.
10 The Scottish Highlands Barrow studied the pelitic rocksCould subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increasedBarrow noted significant and systematic mineralogical changes in the pelitic rocksHe 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
11 Low Grade Barrovian Zones Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albiteBiotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albiteGarnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase
12 High Grade Barrovian Zones Staurolite zone. Schists with staurolite, biotite, muscovite, quart, garnet, and plagioclase. Some chlorite may persistKyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and stauroliteSillimanite 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)
13 Barrovian zonesThe P-T conditions referred to as Barrovian-type metamorphism (fairly typical of many belts)Now extended to a much larger area of the HighlandsIsograd = line that separates the zones (a line in the field of constant metamorphic grade)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 zonesTilley, Kennedy, etc. confirmed Barrow’s zones, and extended them over a much larger area of the HighlandsTilley coined the term isograd for the line that separates the zonesAn isograd, then, is meant to indicate a line in the field of constant metamorphic gradeReally = the intersection of the isogradic surface with the Earth’s surface
14 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.
15 SummaryAn isograd (in this classical sense) represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic gradeWhen one crosses an isograd, such as the biotite isograd, one enters the biotite zoneZones thus have the same name as the isograd that forms the low-grade boundary of that zoneSince 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 zonesLater we shall see broader categories: metamorphic faciesBarrovian zones have become the norm to which we compare all other areas of regional metamorphismOK practice, but we shouldn’t let these zones constrain our thinking or our observationsOther zones may be important and useful locallyA chloritoid zone is prevalent in the Appalachians (X)
16 Banff and Buchan Districts The pelitic compositions are similarBut the sequence of isograds is:chloritebiotitecordieriteandalusitesillimanite
17 The stability field of andalusite occurs at pressures less than 0 The stability field of andalusite occurs at pressures less than GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressureThe 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 Barrow’s 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 thingThe 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).
18 Regional Burial Metamorphism Otago, New Zealand exampleJurassic graywackes, tuffs, and volcanics in a deep trough metamorphosed in the CretaceousThe fine grain size and immature nature of the material is highly susceptible to alteration, even at low gradesVoluminous Permian through Jurassic sedimentation and volcanism graywackes, tuffs, and some volcanics in a deep trough that was metamorphosed in the CretaceousThe fine grain size and immature nature of the material makes it highly susceptible to metamorphic alteration, even at low gradesThe “type locality” of burial metamorphism
19 Otago, New Zealand Section X-Y shows more detail Geologic sketch map of the South Island of New Zealand showing the Mesozoic metamorphic rocks east of the older Tasman Belt and the Alpine Fault. The Torlese Group is metamorphosed predominantly in the prehnite-pumpellyite zone, and the Otago Schist in higher grade zones. X-Y is the Haast River Section of Figure From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.
20 Otago, New Zealand Isograds mapped at the lower grades: 1) Zeolite 2) Prehnite-Pumpellyite3) Pumpellyite (-actinolite)4) Chlorite (-clinozoisite)5) Biotite6) Almandine (garnet)7) Oligoclase (albite at lower grades is replaced by a more calcic plagioclase)The isograds mapped at the lower grades are listed below and are well represented in the Haast River section (Fig next frame)
21 Regional Burial Metamorphism Metamorphic zones of the Haast Group (along section X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib. Mineral. Petrol., 27,
22 Otago, New ZealandOrogenic belts typically proceed directly from diagenesis to chlorite or biotite zonesThe development of low-grade zones in New Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot waterWhereas pelitic sediments may not react until higher grades
23 Paired Metamorphic Belts of Japan The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.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
24 Paired Metamorphic Belts of Japan The NW belt (“inner” belt, inward, or away from the trench) is the Ryoke (or Abukuma) BeltLow P/T Buchan-type of regional orogenic metamorphismDominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mappedA high-temperature-low-pressure belt, and granitic plutons are common
25 Paired Metamorphic Belts of Japan Outer belt, called the Sanbagawa BeltIt is of a high-pressure-low-temperature natureOnly reaches the garnet zone in the pelitic rocksBasic rocks are more common than in the Ryoke belt, however, and in these glaucophane is developed (giving way to hornblende at higher grades)Rocks are commonly called blueschists
26 Paired Metamorphic Belts of Japan Two belts are in contact along their whole length across a major fault zone (the Median Line)Ryoke-Abukuma lithologies are similar to seds derived from a relatively mature volcanic arcSanbagawa lithologies more akin to the oceanward accretionary wedge where distal arc-derived sediments and volcanics mix with oceanic crust and marine sediment
27 Paired Metamorphic Belts of Japan The 600oC isotherm could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc
28 Miyashiro (1961, 1973) suggested that the occurrence of coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common occurrence in a number of subduction zones, either modern or ancientSome of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.Miyashiro (1961, 1973) noted the paired nature of the Ryoke-Sanbagawa belts, and suggested …Miyashiro called these paired metamorphic beltsMay be separated by km of less metamorphosed and deformed material (“arc-trench gap”) or closely juxtaposed (Ryoke-Sanbagawa)In the latter cases the contact is commonly a major faultMost of these belts are quite complex, and are not always coeval
29 Contact Metamorphism of Pelitic Rocks Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodiesIntrusions are shallow, and contact effects overprinted on an earlier low-grade regional orogenic metamorphism
30 Skiddaw Aureole, UKThe aureole around the Skiddaw granite was sub- divided into three zones, principally on the basis of textures:Unaltered slatesOuter zone of spotted slatesMiddle zone of andalusite slatesInner zone of hornfelsSkiddaw graniteIncreasing Metamorphic Grade
31 Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck.First effects (1-2 km from contact) = 0.2 - 2.0 mm sized black ovoid “spots” in the slatesAt the same time, recrystallization -> slight coarsening of the grains and degradation of the slaty cleavageSpots were probably cordierite or andalusite, since re-hydrated and retrograded back to fine aggregates of mostly muscoviteBoth cordierite and andalusite occur at higher grades, where they are often partly retrograded, but not farther outSpots that we now see in most of the spotted slates are probably pseudomorphs
32 The Skiddaw Aureole, UKMiddle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartzCordierite-andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.Cordierite forms ovoid xls with irregular outlines and numerous inclusions, in this case of biotite, muscovite, and opaquesThe biotite and muscovite inclusions often retain the orientation of the slaty cleavage outside the cordieritesThis indicates that the growing cordierite crystals enveloped aligned micas that grew during the regional eventExcellent textural evidence for the overprint of contact metamorphism on an earlier regional oneMicas outside the cordierites are larger and more randomly oriented, suggesting that they formed or recrystallized during the later thermal eventAndalusites have fewer inclusions than cordierite, and many show the cruciform pattern of fine opaque inclusions known as chiastolite1 mm
33 The Skiddaw Aureole, UK Inner zone: Thoroughly recrystallized Lose foliation1 mmAndalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 22-49). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.Both andalusite and cordierite are minerals characteristic of low-pressure metamorphism, which is certainly the case in the Skiddaw aureole, where heat is carried up into the shallow crust by the granitesThe rocks of the inner zone at Skiddaw are characterized by coarser and more thoroughly recrystallized texturesSame mineral assemblage as the middle zoneSome rocks are schistose, but in the innermost portions the rock fabric loses the foliation, and the rocks are typical hornfelses
34 The Skiddaw Aureole, UK The zones determined on a textural basis Better to use the sequential appearance of minerals and isograds to define the zonesBut low-P isograds converge in P-TSkiddaw sequence of mineral development with grade is difficult to determine accuratelyThe zones at Skiddaw were determined by Rastall (1910) on a textural basisA more modern and appropriate approach would be to conform to the practice used in the regional example above, and use the sequential appearance of minerals and isograds to define the zonesThis is now the common approach for all types of regional and contact metamorphismThe first new mineral in most slates is biotite, followed by the approximately simultaneous development of cordierite and andalusitePerhaps the textural zonation is more useful in some cases
35 Contact Metamorphism of Pelitic Rocks Inner aureole at Comrie (a diorite intruded into the Dalradian schists back up north in Scotland), the intrusion was hotter and the rocks were metamorphosed to higher grades than at SkiddawTilley describes coarse-grained non-foliated granofelses containing very high-temperature minerals such as orthopyroxene and K-feldspar that have formed due to the dehydration of biotite and muscovite in the country rocksOrthopyroxene occurs in pelitic and quartzo-feldspathic rocks only at the very highest grades of contact and regional metamorphism, grades that may not be reached prior to melting in many instancesTypical mineral assemblages = hypersthene + cordierite + orthoclase + biotite + opaquesSome very interesting silica-undersaturated rocks also occur in the inner aureoleContain such non-silicate high-temperature phases as corundum and Fe-Mg spinelTilley noted that the low-silica rocks occur only in the inner aureole, and attributed their origin to loss of SiO2 into the dioriteBetter explanation is that SiO2 (and H2O) were scavenged by granitic partial melts formed in the sediments adjacent to the contact with the hot diorite
36 Skarn Formation at Crestmore, CA, USA Crestmore quarry in the Los Angeles basinQuartz monzonite porphry of unknown age intrudes Mg- bearing carbonates (either late Paleozoic or Triassic)Brunham (1959) mapped the following zones and the mineral assemblages in each (listed in order of increasing grade):
37 Forsterite Zone: Monticellite Zone: Vesuvianite Zone: Garnet Zone: calcite + brucite + clinohumite + spinelcalcite + clinohumite + forsterite + spinelcalcite + forsterite + spinel + clintoniteMonticellite Zone:calcite + forsterite + monticellite + clintonitecalcite + monticellite + melilite + clintonitecalcite + monticellite + spurrite (or tilleyite) + clintonitemonticellite + spurrite + merwinite + meliliteVesuvianite Zone:vesuvianite + monticellite + spurrite + merwinite + melilitevesuvianite + monticellite + diopside + wollastoniteGarnet Zone:grossular + diopside + wollastoniteIn this progression we can see the sequential development of index minerals, such as clinohumite, followed by forsterite, clintonite, monticellite, melilite, spurrite/tillyite, merwinite, vesuvianite, diopside, wollastonite, and finally grossular garnet
38 Skarn Formation at Crestmore, CA, USA An idealized cross-section through the aureoleIdealized N-S cross section (not to scale) through the quartz monzonite and the aureole at Crestmore, CA. From Burnham (1959) Geol. Soc. Amer. Bull., 70,The list of zones is at first quite confusing, and again serves to illustrate a common problem faced by petrologists (and probably all scientists)We can collect quality data, but can become overwhelmed by the quantity at times, and it is often difficult to recognize meaningful patternsTwo approaches are helpful in this case:
39 Skarn Formation at Crestmore, CA, USA The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new minerals as grade increasesThis can only occur by a chemical reaction in which some minerals are consumed and others producedIf this is so, one ought to be able to relate minerals in the lower and next higher zone by a balanced chemical reaction
40 Skarn Formation at Crestmore, CA, USA a) Calcite + brucite + clinohumite + spinel zone to the Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction:2 Clinohumite + SiO2 9 Forsterite + 2 H2Ob) Formation of the vesuvianite zone involves the reaction:Monticellite Spurrite Merwinite Melilite+ 15 SiO H2O 6 Vesuvianite + 2 CO2For example, the step from the first zone to the second zone is (a)When we address isograds as reactions we can then turn to what variables are involvedIn the present case, we discover than the majority of these prograde reactions consume SiO2Since quartz is not found in the Crestmore aureole, the SiO2 must be added in the form or dissolved silica in hydrothermal fluidsWe can thus conclude that diffusion of silica from the monzonite into the country rocks must play a critical role in the aureole development
41 Skarn Formation at Crestmore, CA, USA 2) Find a way to display data in simple, yet useful waysIf we think of the aureole as a chemical system, we note that most of the minerals consist of the components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)Addressing the list of mineral assemblages in the zones at Crestmore can be bewilderingFind a way to display…What if we graphically plot the minerals on a triangular CaO-MgO-SiO2 diagram?
42 Zones are numbered (from outside inward) CaO-MgO-SiO2 diagram at a fixed pressure and temperature showing the compositional relationships among the minerals and zones at Crestmore. Numbers correspond to zones listed in the text. After Burnham (1959) Geol. Soc. Amer. Bull., 70, ; and Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.Zones are numbered (from outside inward)Silica-saturated water escaping from the porphry permeates the silica-free marbles and a gradient in silica content results due to the diffusionSilica reacts with the carbonates to produce skarns consisting of Ca-Mg silicates, while CO2 is liberated by the reactionsThe porphyritic nature of the pluton thus supports the idea of fluid releaseThe zones at Crestmore could have formed at constant temperature, and reflect a diffusion gradient in SiO2 onlyThis is probably not the case, however, since temperature should also increase toward the plutonOnly by knowing the pressure-temperature stability ranges of the minerals, and the pressure- temperature dependence of the reactions relating them, could we fully understand the processes at Crestmore