Low-pressure Granulite Facies Research Articles

Importance of fluid immiscibility in the H2O-NaCl-CO2 system and selective CO2 entrapment in granulites: experimental phase diagram at 5-7 kbar, 900°C and wetting textures

New experimental data on fluid immiscibility in the H 2 O-NaCl-CO 2 system at 900 degrees C and 5-7 kbar have been obtained using the synthetic fluid-inclusion technique. The main result is a significant enlargement of the immiscibility field as pressure decreases from 7 to 5 kbar. Combined with previous data, our experiments show that immiscibility is probably a widespread phenomenon in low-pressure granulite-facies rocks. Because CO 2 -rich fluids and NaCl-rich aqueous fluids have very contrasting wetting behaviour, fluid unmixing could result in a selective entrapment of the CO 2 -rich component in granulites (Watson & Brenan, 1987). To check this hypothesis, we performed an experiment in which polycrystalline quartz was heat-treated in the presence of small volume percentages of the two immiscible fluids. The observed pore geometry is characterized by a combination of large, isolated CO 2 -rich bubbles, and an interconnected network of NaCl-H 2 O-filled channels along quartz edges. A model combining unmixing and the subsequent escape of the aqueous fluid by porous flow could therefore explain the CO 2 -rich fluid inclusions in low-pressure granulites.

Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of metamorphic minerals

Dating of relic metamorphic assemblages can provide important information about the timing and character (metamorphic grade and/or PT-evolution) of early high grade episodes in polymetamorphic provinces. Using Pb stepwise leaching of metamorphic silicates, we have dated multiple granulite facies metamorphic episodes in the Central Zone (CZ) of the Limpopo Belt. Ages of 2.52 Ga were obtained from sillimanite and cogenetic garnet and ages of about 2.01 Ga from titanite, garnet and clinopyroxene. Together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data, these results lead us to a reinterpretation of the tectono-metamorphic history of the CZ. Three distinct high grade events at about 3.2−3.1 Ga, 2.65−2.52 Ga and 2.0−0.05 Ga are recognized. Each of these is suggested to correspond to a tectonic episode of distinct character: (a) for the 3.2 Ga event magmatic activity can mainly be identified (best represented, for example, by the Sand River Gneisses or the Messina Layered Intrusion). The field relationships concerning the tectonometamorphic history of this Early-Archean event are largely erased by at least two high grade metamorphic overprints. (b) Late-Archean (∼2.6-2.52 Ga) low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism. The anticlockwise P-T evolution of these granulites probably reflects deep crustal processes, associated with magmatic underplating (or in-plating), contemporaneous with vertical crustal growth of the Zimbabwe craton around 2.6 Ga. (c) During the Proterozoic event (∼2.05-1.95 Ga) tectonic thickening was caused by the collision of the Kaapvaal and Zimbabwe cratons. The CZ was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise P-T evolution. The structural, metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny.

Palaeoproterozoic granulite-facies metamorphism and granitoid intrusions in the Ubendian-Usagaran Orogen of northern Malawi, east-central Africa

The Paleoproterozoic basement of northern Malawi shows evidence for granulite-facies metamorphism older than the 1930±30 Ma-old Nyika Granite. Low-pressure granulite-facies metamorphism in regionally coherent and extensively exposed cordierite-garnet granulite reached 750–850°C and 5-5.5 kbar. The Chelinda Granite intruded during this event and has been dated by single zircons at 1995±0.4 Ma (207Pb206Pb age; 2σ-mean errors). Anatectic melt which formed concurrently with cordierite growth in the cordierite-garnet granulite yielded a 207Pb206Pb zircon age of 1988±0.6 Ma. These ages are interpreted to date the peak of regional low-pressure granulite-facies metamorphism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulitefacies metamorphism. Their zircon ages range from 1930±30 Ma to 1969±0.4 Ma and constrain the end of regional high-temperature conditions. Enderbitic gneiss, which is tectonically intercalated with the cordierite-garnet granulite, records peak-metamorphic conditions of ca 850–880°C and 9–11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a 207Pb206Pb age of 2002±0.3 Ma. This age for high-pressure granulite-facies metamorphism in northern Malawi is similar to the recently reported age for eclogite-facies metamorphism in the Usagaran Belt of central Tanzania and indicates that orogenic activity during the Ubendian-Usagaran Orogeny culminated at ca 2000 Ma.Pre-2000 Ma magmatic events encompass the intrusion of the enderbitic gneiss precursor at 2093±0.6 Ma. Furthermore, a late granite intrusion of the Rumphi Igneous Complex (RIC) yielded a single-zircon age of ca 2048±0.7 Ma. The chemistry of those late-stage granites of the RIC displays an affinity to volcanic-arc granites. Therefore, the age of 2048 Ma is interpreted to date a mature stage of volcanic-arc magmatism. The nature of the pre-2000 Ma magmatic events remains open, however; the granitoids might have been emplaced in a long-lived Andean-type subduction zone. Tectonic juxtaposition of both granulite-facies rocks and of the latter two with the RIC occurred after Paleoproterozoic granulite-facies metamorphism in northern Malawi.

Early Paleozoic (Pan African) thermal event of the Larsemann Hills and its neighbours, Prydz Bay, East Antarctica

The early Paleozoic (Pan African) thermal event of the Larsemann Hills and its adjacent areas, East Antarctica is discussed based upon the isotope ages we obtained. An Sm-Nd internal isochron for a representative mafic granulite yields an age of 540 Ma±75 Ma. Another Sm-Nd internal isochron, which is made up of the assemblage of the peak metamorphism and its whole rock as well, gives an age of 497 Ma ± 7 Ma The isotopic chronological data of single zircon stepwise evaporation dating and 40Ar-39Ar analysis provide further evidence for the early Paleozoic event of high-grade metamorphism in the region. The data from the field geological investigation in the Larsemann Hills also show that there is not only strong regional partial melting but also low-pressure granulite-facies metamorphism accompanied by it in the region. The early Paleozoic (Pan African) thermal event of the region may be related to the final formation of the East Antarctica craton, even of Gondwanaland.

Geology and structure of Depot Peak, MacRobertson Land. More evidence for the continuous extent of the 1000 Ma event of East Antarctica

The 1000 Ma event of East Antarctica is a granulite facies metamorphic event that affected rocks underlying more than 1000 km of the Antarctic coastline between 50°E and 70°E. The limits of the belt are defined by isotopic dating but attempts to define the belt on metamorphic grounds are sparse. Depot Peak provides an important link for determining the history of this belt as it is the only outcrop linking the Prince Charles Mountains to the Framnes Mountains and the western parts of the Rayner Complex. Much of the Prince Charles Mountains are also 1000 Ma old but much of their metamorphic history is yet to be established. The rocks at Depot Peak are garnet and cordierite‐bearing gneisses of low pressure granulite facies that were intruded by a large sheet of garnet granite prior to metamorphism and deformation. They experienced two deformation phases, D1 and D2, and they record two different metamorphic events around 560 MPa, 700°C (M1) and 510 MPa, 600°C (M2). It is difficult to demonstrate to what exte...

Microfabrics indicating granulite-facies metamorphism in the low-pressure central Damara Orogen, Namibia

The reconstruction of the fluid evolution during progressive metamorphism in the central Damara orogen by Hoernes and Hoffer indicated that fluid-present conditions were only periodically attained among these rocks and that the highgrade metamorphism along the west coast of Namibia occurred under essentially water-absent conditions. The characteristic mineral assemblage in the metapelites is: plagioclase, quartz, alkali-feldspar, biotite, cordierite, garnet ± sillimanite, apatite, zircon, ore. Orthopyroxene-bearing parageneses in the metapelites are missing and granulite-facies metamorphism may not be detected on the basis of mineral assemblages. A detailed study along a 100 km traverse across the migmatite area in the central Damara Orogen (Namibia) reveals several microfabric features which indicate, in analogy to many other well known granulite-facies terrains where similar microfabrics have been described, that the Pan-African regional metamorphism in this area culminated at conditions of low-pressure granulite facies. The observed features are: 1. (a) The activation of basal 〈 a〉- and prism 〈 c〉-glide systems in quartz under conditions of plastic deformation, and the development of equivalent subgrain boundaries, parallel prism faces, and parallel basal planes. 2. (b) The orientated exsolution of rutile needles in quartz. 3. (c) The exsolution of string-perthites, mesoperthitic string-perthites and antiperthites in feldspars. 4. (d) In addition, the homogeneous distribution of major elements in garnet crystals and garnet-biotite thermometry suggest granulite-facies conditions.

40Ar/39Ar thermochronology and thermobarometry of metamorphism, plutonism, and tectonic denudation in the Old Woman Mountains area, California

Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of 40 Ar/ 39 Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, south-eastern California. Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and ∼700 °C. 40 Ar/ 39 Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. 40 Ar/ 39 Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Late Cretaceous plutons of the 73 Ma Old Woman-Piute batholith. Metamorphic temperatures from garnet-biotite thermometry increase from 650 °C, and 40 Ar/ 39 Ar ages of hornblende from Proterozoic rocks decrease from 1600 to 73 Ma toward these plutons. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. Similar pressures are obtained from hornblende barometry of Late Cretaceous granodiorite. 40 Ar/ 39 Ar and apatite fission-track data from the 73 Ma batholith indicate that the rocks exposed in the Old Woman Mountains cooled rapidly to below 350 to 300 °C by 70 ± 1 Ma and that moderately rapid cooling at rates of 10 to 30 C°/m.y. continued until after 60 Ma. The presence of maximum Mesozoic temperatures on the borders of plutons, hornblende ages of ca. 73 Ma, and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite-facies metamorphism were related to heating from the intrusion of the 73 Ma Old Woman-Piute batholith. Amphibolite-facies metamorphism along pluton contacts was superimposed on the regional greenschist-facies metamorphism related to thrusting and crustal thickening. The extremely rapid cooling in the Old Woman Mountains between 73 and 71 Ma was due largely to conduction of heat into the country rocks, whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km/m.y. until about 68 to 65 Ma.

A barometric response to late compression in the Strangways Metamorphic Complex, Arunta Block, central Australia

Evidence for four major deformation events ( D 1– D 4) and two metamorphic events ( M 1, M 2) is shown by granulite- and amphibolite-facies rocks of the Strangways Metamorphic Complex, central Australia, which is divided into two domains: the Ongeva Granulites to the east, and the Anamarra Granite domain to the west. D 1 and D 2 produced layer-parallel foliations, with many of the effects of D 1 overprinted by intense non-coaxial shear during D 2. Low-pressure granulite-facies metamorphism, M 1, accompanied D 1–2, at conditions of P = 5.3 ± 1.2 kbar and T > 750°C. The effects of M 1 are characterized by S 1 spinel + quartz-bearing assemblages, with overprinting S 2b sillimanite + biotite + magnetite assemblages, implying approximately isobaric cooling. Effects of M 2 are characterized by S 1–2 cordierite pseudomorphed by orthopyroxene + sillimanite + biotite + magnetite symplectites, at conditions of P = 7.5 ± 0.8 kbar and T ≊ 800° C . This retrograde increase in pressure was probably the result of crustal thickening during D 3, which produced NE-plunging F 3 and F 4 folds in the Ongeva Granulites, and a widespread shear foliation in the Anamarra Granite domain. The increase in pressure with M 2 may reflect the beginning of a clockwise P- T- t path typical of collisional-style tectonics. Sillimanite and orthopyroxene-bearing ultramylonites were produced during D 4, an extension event parallel to the D 3 transport axis and probably related to crustal collapse after thickening during D 3. Wide N-dipping shear zones ( D 5) containing sillimanite, kyanite, staurolite and overprinting greenschist-facies assemblages bound the Strangways Metamorphic Complex. These zones indicate a S-directed sense of shear and were probably responsible for bringing the Strangways Metamorphic Complex to higher crustal levels, during both a probable late Proterozoic event and the Mid Carboniferous Alice Springs Orogeny.

The geological evolution of the Reynolds Range, central Australia: evidence for three distinct structural-metamorphic cycles

The Reynolds Range is affected by three distinct structural-metamorphic cycles, DI- MI, DII- MII and DIII- MIII, which represent the local expressions of a series of orogenic events that affected large sections of the Arunta Block, central Australia. The tectonic cycles recognized in the Reynolds Range correlate extremely well with similar events in the nearby Anmatjira Range. DI- MI took place around 1820 Ma and only affected the NW Reynolds Range. DII- MII occurred between 1760 and 1650 Ma and involved NE-SW shortening with identical timing relationships between the deformations and metamorphism along the entire Reynolds Range. DII commenced with the formation of a fabric, SII 1, preserved as inclusion trails in porphyroblasts. SII 1 formed early during a progressive event, DII 1−2, which involved the formation and tightening of upright NW-SE-trending isoclinal folds. A penetrative subvertical NW-SE-trending fabric, SII 2, developed axial planar to these folds. The general symmetry of DII 2 microstructures suggests that FII 2 folds were formed during progressive coaxial deformation under plane strain conditions. DII 3 structures are associated with conjugate sets of crenulation bands. Within the macroscopic crenulation bands, large sections of earlier folds and fabrics are rotated to a near recumbent position. The dominant set of crenulation bands is upright and trends ENE-WSW while the conjugate to this set trends NW-SE. No penetrative fabric formed during DII 3. DII 4 is associated with a system of amphibolite-grade NE-dipping thrusts interconnected by numerous smaller shear zones. The sense of movement is NE-over-SW. During DII essentially penetrative coaxial deformation ( DII 2) evolved into essentially localized non-coaxial deformation ( DII 3- DII 4). DII deformation occurred concomitant with a low-pressure granulite-facies metamorphic event ( MII, 1730 Ma). Peak metamorphic sillimanite + cordierite + orthopyroxene + ilmenite + spinel + garnet + K-feldspar + quartz assemblages define SII 2. Oriented sillimanite + biotite assemblages that formed early during DII 3 and unoriented andalusite + chlorite + muscovite assemblages that formed late during DII 3 indicate that cooling of MII was initially isobaric. During DII 4 thrusting staurolite + garnet + biotite + quartz and biotite + kyanite + quartz assemblages, defining the mylonitic fabric, overprint the late DII 3 andalusite assemblages. DII 4 shear movement is therefore accompanied by a slight rise in pressure. DII 4 shear zones gradually increase in grade eastward along the Reynolds Range. This trend coincides with the variation in peak metamorphic grade attained during MII, indicating that the rocks had not yet fully cooled when DII 4 thrusting commenced. DIII- MIII is late Devonian to Carboniferous in age and caused minor differential movement (<100 m) and retrogression along DII 4 shear zones.

Geological history of Adélie Land and King George V Land, Antarctica: Evidence for a polycyclic metamorphic evolution

Abstract Metamorphic and structural studies in Adelie Land and King George V Land, East Antarctica, show that this part of the East Antarctic Shield can be divided into four different sections. The areas east of Commonwealth Bay show evidence for a polycyclic metamorphic evolution in which low-pressure granulite facies M 1 fabrics (4–4.5 kbar; ∼ 750°C) are overprinted by retrograde hydrated decompression textures of amphibolite facies during M 2 and by late, high-pressure, low-temperature coronas during M 3 (5.7–7.0 kbar, 500–600°C). The regional structure and relative timing of events in the other three areas is consistent with the recognition of these three metamorphic events. The Cape Denison orthogneiss in Commonwealth Bay was intruded after the pervasive low-pressure M 1 event but before the major ductile deformation phases and M 2 . The Cape Denison orthogneiss therefore only suffered the M 2 amphibolite facies metamorphic event and subsequent overprinting by M 3 mid-greenschist facies assemblages involving lawsonite. The Cape Hunter phyllites were deposited after M 2 but before M 3 and consequently were only affected by the mid-greenschist metamorphic event, which also overprinted the amphibolite facies fabrics at Cape Denison. The areas west of Commonwealth Bay underwent the low-pressure granulite facies metamorphism during M 1 , decompression during M 2 but no late M 3 overprint. The variation of the grade of M 3 in the different areas from granulite facies pressures east of Commonwealth Bay to no effect west of Cape Hunter is interpreted as a consequence of differential uplift along shear zones with a north-south strike, which exposed successively deeper levels towards North Victoria Land to the east. This spatial association and the age of M 3 may suggest that there is a connection between M 3 in King George V Land and Adelie Land and the Ross orogeny in North Victoria Land. The recognition of three metamorphic events can be correlated with three radiometric ages previously established for the Cape Denison orthogneiss.

Comparative geochemistry of migmatized, interlayered quartzofeldspathic and pelitic gneisses: A contribution from rocks of southern Finland and northeastern Saskatchewan

Migmatites in southern Finland and northeastern Saskatchewan are exposed on a regional scale and have similar geologic and metamorphic settings. At least casually similar cordierite-bearing metamorphic assemblages in other regions (e.g., Black Forest, Germany; Lapland, Finland; Vijayan Series, Ceylon; Westport Area, Ontario, Canada; East Kimberley, Cape Naturaliste, and Eyre Peninsula areas, West Australia) fit the description of low-pressure granulite facies. These low-pressure gneisses and migmatites contrast chemically with granulite facies rocks that are thought to have formed in a medium- or high-pressure environment; for example, they are not depleted in rubidium. Details of this study indicate that the transformation to migmatite in the interlayered gneisses is accompanied by an increase in grain size and sharpened segregation or layering between aluminum-rich mafic and quartzofeldspathic minerals. As a result, pelitic layers become richer in iron and magnesium and depleted in potassium. Reciprocal, complementary changes are found in the quartzofeldspathic layers. Variation in the abundance of Rb, Sr, Zr, Y and Nb correlates well with model mineralogic and major element compositions.

Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa, and their bearing on granite genesis and the composition of the lower crust

The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue. Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr 87/Sr 86 ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition. The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.

The structure of south Renland, Scoresby Sund. With special reference to the tectono-metamorphic evolution of a southern internal part of the Caledonides of East Greenland

Renland occupies an internal position within the southern extreme of the outcrop of the Caledonian mobile belt of East Greenland exposed between latitudes 70° and 82° N. In south-west Renland migmatised paragneisses derived from sediments comparable to the late Precambrian Lower Eleonore Bay Group form a multilayered sequence with a minimum thickness of 1500 m. The migmatites are interleaved with thick concordant sheets of garnetiferous augen granite, the formation of which may be linked with the low-pressure granulite or transitional amphibolite-granulite facies conditions attained during migmatisation of the paragneisses. These conditions persisted during the folding together of paragneisses and granites into regional structures of nappe dimensions which had a north or north-west direction of transport. Refolding of the nappes under continued high-grade conditions gave rise to structures locally coaxial with nappe axes. Reversals of facing of nappes occur in backfolds. Linear fabrics of sillimanite and biotite and prolate ellipsoidal augen of feldspar are parallel to fold axes and show that constrictional deformation dominated the later stages of the nappe phase and the refolding event. The constriction is attributed to compressing of rocks in south-west Renland between nappes advancing from the south and a rising mass of granite and basement gneisses in the north. Intrusion of concordant sheets of biotite-rich hypersthene monzonite (mangerite) followed the nappe deformation in south-east Renland. The principal sheet, which is 500 m thick, forms the rim to part of a lopolithic basin. Thinner sheets of monzonite injected into migmatites within the basin have been disrupted by further migmatisation and granitisation. Stable assemblages in pyribolite restite suggest this later event, which was restricted largely to the basin, attained conditions of hornblende-granulite facies. Open warps attributed to monzonite injection and the basin formation are superimposed on nappes west of the principal sheet. Normal faults with downthrow to east and west relate to the formation of troughs filled with Upper Palaeozoic and Mesozoic sediments in the Scoresby Sund region. The distribution of the faults suggests Renland was a horst area in Upper Palaeozoic times. Tertiary igneous activity in south Renland is represented by rare dykes of olivine dolerite and scattered plugs of pyroxenite which locally contain large blocks of host gneisses.

Geochemical investigations of deep-seated rocks in the Australian shield

Medium to high pressure granulite facies rocks are relatively uncommon at the earth's surface and are probably formed at considerable depth in the continental crust. Low pressure granulite facies rocks are rather common in shield regions and frequently grade into upper amphibolite facies rocks. Medium to high pressure granulite terranes have less ‘acidic’ average major element compositions, and have significant depletions of U, Th, and Rb compared with typical low pressure granulite and amphibolite facies terranes and the overall surface shield. It is considered that the medium to high pressure granulite terranes reflect chemical trends with increasing depth in the continental crust. These trends are the results of modifications of the original compositions by metamorphic (including melting) processes. The strong upward concentrations of Rb, U, and Th in the continental crust indicated by these studies will affect estimates of their crustal and terrestrial abundances. The U and Th distribution is in accord with surface heat flow measurements.