Showing papers on "Granulite published in 1987"
TL;DR: In this article, the authors present a model for calculating the amounts of melt that may be formed by fluid-absent breakdown of micas and amphiboles in common crustal rock types (pelitic, quartzofeldspathic, intermediate and mafic).
720 citations
TL;DR: In this paper, a detailed mapping of three localities of the Caledonian mountain chain was carried out and it was shown that eclogites account for up to 30-45% of the volume of the rock volume.
529 citations
TL;DR: Petrologic studies and application of well-calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains: (1) "Peak" metamorphic pressure and temperature conditions cluster around values of $$7.5 \pm 1 kbar$$ and $$800 \pm 50°C$$, implying average geothermal gradients of 30-35°C/km as discussed by the authors.
Abstract: Petrologic studies and application of well-calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains: (1) "Peak" metamorphic pressure and temperature conditions cluster around values of $$7.5 \pm 1 kbar$$ and $$800 \pm 50°C$$, implying average geothermal gradients of 30-35°C/km. (2) In comparison with upper amphibolite facies rocks, especially in paired, amphibolite-granulite terrains, it is evident that granulites are distinguished from amphibolites by the former's higher metamorphic temperatures but not higher pressure. (3) Initial cooling of granulite terrains from "peak" conditions is nearly isobaric. For garnet closure temperatures of $$600 \pm 50°C$$, the dP/dT retrograde paths inferred from compositionally zoned garnet rims are 2-8 b/°C in most granulites. (4) Primary, coarse sillimanite is found in most granulites, and retrograde kyanite is known in a few. These data suggest that granulites form as a result of anomalous thermal gradients...
436 citations
TL;DR: In this article, the Seve Nappes can be followed for more than 700 km along the Scandinavian Caledonides, and zircon data from a migmatitic paragneiss leucosome give almost concordant Caledonian U-Pb ages, and define together with strongly discordant zircons from the paragneis a discordia line with the intercept ages 423±26 Ma and 1512±36 Ma.
Abstract: The Seve Nappes can be followed for more than 700 km along the Scandinavian Caledonides. Seve paragneisses are in part migmatitic, metamorphosed to granulite facies and eclogite-bearing. This probably reflects an ancient continent-continent collision. Zircon data presented here from a migmatitic paragneiss leucosome give almost concordant Caledonian U-Pb ages, and define together with strongly discordant zircons from the paragneiss a discordia line with the intercept ages 423±26 Ma and 1512±36 Ma. Strongly discordant zircon from another paragneiss, composed of cores overgrown with thick transparent mantles, also fits this chord. A stepwise dissolution experiment on these zircons demonstrates the existence of a component with Caledonian age in the crystals, and 3 of 5 dissolution steps plot along the discordia line defined by the conventional analyses of the same sample. The zircon data reflect high grade Caledonian metamorphism of sediments with Proterozoic protoliths. Sm-Nd model ages (TDM and TCHUR) for whole rock samples cluster around 1.9 and 1.55 Ga, respectively, demonstrating a Proterozoic average crustal provenance age for the detritus in these sediments. The zircon data support a correlation between the Seve Nappes and the Western Gneiss Region in Norway.
404 citations
TL;DR: The amphibolites of Caozhuang and Huangbaiyu in eastern Hebei have been dated by the Sm-Nd isochron method as mentioned in this paper, and the results yielded: T = 3470 ± 107 Ma, INd = 0.50827 ± 11, and ϵNd (T) = +2.7 ± 0.6.
305 citations
TL;DR: In this paper, the authors show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO2 and Na2O Rb and Y are depleted in the charnockite.
Abstract: Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka Two major kinds of orthopyroxene-producing reactions are recognized Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic “gray gneiss” produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of “mixed-facies” exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO2 and Na2O Rb and Y are depleted in the charnockite Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar Lower P(H2O) in the Ponmudi-type metamorphism was probably the definitive factor CO2-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO2 from some deep-seated source The open-system behavior and high oxidation states of the metamorphism are in accord with the CO2-streaming hypothesis CO2-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations These inclusions may have suffered post-metamorphic H2 leakage or some systematic contamination Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO2 drove charnockite formation of the Ponmudi type The possibility remains that orthopyroxene and CO2-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin Further evidence, such as oxygen isotopes, is necessary to test the CO2-streaming hypothesis for the Ponmudi-type localities
218 citations
TL;DR: In this article, the authors used major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, to constrain the petrogenesis of these rocks.
Abstract: Granulite facies lower crustal xenoliths from a single basaltic vent (Hill 32) in the McBride volcanic province, north Queensland, Australia, illustrate the extreme lithologic diversity of the deep crust. These xenoliths are dominantly mafic, but intermediate and felsic granulites represent a significant proportion (∼20%) of the xenolith population. All xenoliths have high-grade mineralogies and generally well-equilibrated textures, as well as decompression features which are indicative of derivation from deep crustal levels (0.7–1.0 GPa). Major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, are used to constrain the petrogenesis of these rocks. In the smaller, layered samples, metamorphic differentiation may lead to nonrepresentative sampling. However, in most instances such secondary processes can be identified and the original chemical characteristics delineated. The mafic xenoliths formed through a variety of processes, including crystallization of mafic magma, crystal accumulation from mafic and felsic magmas, and partial melting of intermediate compositions to yield a mafic residuum. The two analyzed intermediate xenoliths are metasediments, based upon high alumina to alkali ratios and rare earth element patterns, whereas the two felsic xenoliths have compositions similar to igneous rocks. Comparisons of noncumulate and/or restite xenoliths with unmetamorphosed rock types show that K and Rb are variably depleted whereas Th and U are strongly depleted, in all rock types. The large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains, suggesting similar processes were operative. Suggestions of an anomalously Ba- and Sr-rich lower crust are not supported by the data. By using the observed lithologic proportions of xenoliths at Hill 32, and the chemical analyses presented here, a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions. The weighted mean composition is mafic, and formed through a combination of basaltic underplating, crystal accumulation, tectonic underplating, and partial melting. No simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in Phanerozoic continental margin settings.
192 citations
TL;DR: In this paper, the authors proposed that many intermediate and high-pressure granulites form in the lower parts of thickened crust and therefore require a two-stage cycle of tectonism to be uplifted and exposed at the earth9s surface.
Abstract: Rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies. They can be exposed at the earth9s surface within this one erogenic cycle. For example, the granulites exposed in British Columbia have concordant U/Pb zircon ages of 65–85 Ma. In contrast, rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger, lower grade rocks. At the cessation of orogeny, such rocks will undergo near-isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness. These granulites require a second orogeny to uplift and expose them at the surface. As an example, the Archean granulites of Enderby Land, Antarctica, were metamorphosed at 3070 Ma at 1000 °C at 8–10-kbar pressure. They then followed at least a 400 °C isobaric cooling path. They also had a prolonged residence time of 2000 m.y. near the base of the crust before uplift. No doubt a continuum exists between these extremes, but it is proposed that many intermediate- and high-pressure granulites form in the lower parts of thickened crust and therefore require a two-stage cycle of tectonism to be uplifted and exposed at the earth9s surface. Although most granulites probably form in thickened crust at active plate margins, underplating of normal (30–40-km thickness) crust by mantle melts during extension can also produce granulites. These granulites would also undergo isobaric cooling and require a later orogeny to expose them. An example is the Paleozoic Lachlan Fold Belt of eastern Australia, which has undergone only minor uplift since the major ultrametamorphism at about 400 Ma. However, isotopic, petrologic, and geochemical differences are to be expected between these extremes of granulite formation.
192 citations
TL;DR: The Khondalite Belt of southernmost India is an immense section of granulite facies metasediments of probable early Proterozoic age as discussed by the authors, and the dominant rock types are migmatized khondalites, acid biotite-garnet graphitic gneisses and charnockites.
Abstract: The Khondalite Belt of southernmost India is an immense section of granulite facies metasediments of probable early Proterozoic age. Dominant rock types are migmatized khondalites (garnet-sillimanite $$\pm$$ cordierite metapelites), acid biotite-garnet graphitic gneisses and charnockites (quartzofeldspathic gneisses with orthopyroxene). Throughout much of southern Kerala, the biotite gneisses are in a state of arrested conversion to charnockite. Partially decrepitated $$CO_{2}$$ inclusions in quartz at the Ponmudi mixed biotite gneiss-charnockite quarry have significant $$CH_{4}$$ and/or $$N_{2}$$ and are texturally late. The probable sequence of events in metasediments was (1) migmatization, (2) charnockite formation, (3) symplectitic cordierite formation, and (4) late $$CO_{2}$$ entrapment. Phase equilibrium diagrams of anatexis in aluminous systems and Fe-Mg exchange between garnet and biotite indicate metamorphic temperatures of 650-850°C for the khondalites. Temperatures based on garnet-orthopyroxene...
170 citations
TL;DR: In this paper, the authors suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust, and that granulite-facies metamorphism is a necessary complement of crustal melt ing.
Abstract: The discovery that granulites contain CO2-rich fluid inclusions while amphibolites in the same terrane contain H2O-rich or mixed CO2–H2O inclusions1 has led to a debate over the processes that produce granulite metamorphism. One theory2–4 maintains that granulites form as the result of a massive influx of CO2 from lower crustal or mantle sources. Such an influx could be responsible for the marked depletion many granulite terranes show in large-ion lithophile elements2,5. Others, however, maintain that granulite-facies metamorphism is a necessary complement of crustal melt ing6–9. Recently, Lamb and Valley10,11 have argued that granulite metamorphism may be a product of any one of three processes: CO2-streaming, partial melting, or recrystallization of originally dry rocks. Here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust.
166 citations
TL;DR: In this paper, the effects of shear heating and displacement of cool hanging-wall rocks against hotter footwall rocks are calculated, and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed.
TL;DR: The high-grade gneisses of Sri Lanka display spectacular in-situ granulitization phenomena similar to those observed in southern India and of current interest for evolutionary models of the lower continental crust as mentioned in this paper.
Abstract: The high-grade gneisses of Sri Lanka display spectacular in-situ granulitization phenomena similar to those observed in southern India and of current interest for evolutionary models of the lower continental crust. The absolute ages of these rocks are poorly constrained and so, using the SHRIMP ion microprobe, we have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments. Detrital grains from a metaquartzite of the Highland Group preserve premetamorphic U-Pb ages of between 3.17 and 2.4 Ga and indicate derivation of the sediment from an unidentified Archean source terrain. The Pb-loss patterns of these zircons and the other samples suggest severe disturbance at ca. 1100 Ma ago, which we attribute to high-grade regional metamorphism. Two pelitic gneisses contain detrital zircons with ages up to 2.04 Ga and also record an \approx 1100 Ma event that is also apparent from metamorphic rims around old cores and new zircon growth. A granite intrusive into ...
TL;DR: In this paper, the cordierite-bearing gneisses occurring as elongate patches in an 8 to 10 km-wide zone along the Achankovil faultlineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India.
Abstract: The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO2 (0.80–0.95 g/cm3) as the dominant fluid phase, with traces of probable CH4 (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO2 and CO2-H2O (±CH4?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics.
TL;DR: In this paper, an extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies.
Abstract: The present geology of southern Scandinavia offers the unique opportunity to sample deep and intermediate levels from the same crustal section for both heat flow and heat production. In the central part of southern Norway, amphibolite facies terranes appear to lie on top of the same deeper crustal formations which crop out on their western and eastern margins. An extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies. Using constraints from heat flow data in the same area allows a reliable model of the distribution of crustal heat production. The average heat production of granulite facies terranes is 0.4 µW/m³, similar to values in other parts of the world. For amphibolite facies rocks, the value is 1.6 µW/m³. The present shield also includes heat producing element enriched granites formed in later events and the average heat production of presently exposed crust is 2.7 µ/m³. Using heat flow and radioactivity data from ten stations, the reduced heat flow in the area is 22 ± 2 mW/m². This corresponds to the heat flow at the top of 28 km of deep crustal facies, implying that the mantle heat flow is probably as low as 10 mW/m². Over the whole crustal thickness, the average amount of radiogenic heat is 31 mW/m².
TL;DR: The results of U-Pb zircon and Sm-Nd whole-rock analyses are reported on three meta-igneous granulitic samples from the Central African Republic basement, formerly ascribed to an Archaean complex as discussed by the authors.
TL;DR: Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton.
TL;DR: The Faeringehavn-Tre Brodre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event as discussed by the authors.
Abstract: The Faeringehavn–Tre Brodre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Faeringehavn terrane, largely composed of early-Archaean Amitsoq gneiss cut by younger Archaean granitic gneiss defined here as the Satut gneiss; (2) Tre Brodre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nuk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nuk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Faeringehavn and Tre Brodre terranes which at c. 2800 Ma experienced a lower grade of metamorphism. Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages. This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amitsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nuk gneiss precursors described from Godthabsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthabsfjord region.
TL;DR: Sapphirine-cordierite-quartz and spinel-cramerite-spinel-cementitequartz are found in the Labwor Hills, Uganda as discussed by the authors, where they are interpreted as pseudomorphs after osumilite.
Abstract: Sapphirine-cordierite-quartz and spinel-cordierite-quartz form relic assemblages of probable Archaean age in Fe-rich aluminous metapelites from Labwor Hills, Uganda, and reflect an unusually high temperature metamorphism (∼1,000° C) at pressures in the vicinity of 7–9 kbars and a(O2) near the magnetite-hematite buffer. Subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine-hypersthene-K-feldspar-quartz symplectites which are interpreted as pseudomorphs after osumilite. A petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a(O2), or decreasing a(O2) at constant temperature and pressure. The former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures, and by the comparatively low Al2O3-contents of secondary hypersthene. This pressure-temperature path implies that: (1) metamorphism occurred at deep levels within normal thickness crust, probably less than 40–45 km thick, due to an extreme thermal perturbation induced either by emplacement of mantle-derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime, (2) the excavation and surface exposure of the granulites is due to a subsequent, postgranulite facies metamorphism, crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the Mozambique foldbelt.
TL;DR: In this article, whole-rock samples are defined for granulite facies orthogneisses from Fiordland southwest New Zealand and a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4.7 indicating decoupling of Sr-Nd isotope systems.
Abstract: Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. ɛ
Nd values (at 120 Ma) show a relatively wide range of from −0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and ɛ
Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured ɛ
Nd(0) value of −10.2, 87Sr/86Sr ratio of 0.73679 and a T
Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below ∼300° C by ∼100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration. Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher ɛ
Nd values (3.9 to 4.6) and slightly lower 87Sr/ 86Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (LaN/ YbN=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of ɛ
Nd values, but uniform initial 87Sr/86Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by ∼20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.
TL;DR: In this article, the authors considered the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al2O3-SiO2 (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa].
Abstract: Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al2O3-SiO2 (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low PH2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.
TL;DR: In this paper, pressure-temperature estimates on garnet websterites, and temperature estimates on two-pyroxene granulites and spinel lherzolites, allow definition of a stratigraphic sequence and the ambient geotherm.
TL;DR: In this article, the ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the age of major orogenic episodes manifest at the earth's surface.
TL;DR: In this paper, the main crystalline rocks of the study area are (1) the intrusive suite of Bear Valley, a mid-Cretaceous tonalite batholith complex with coeval gabbroic intrusives, and (2) the gneiss complex of the Tehachapi Mountains, which consists of Early Cretaceous orthogneiss and subordinate paragneiss with local domains having granulite facies metamorphic assemblages.
Abstract: The southernmost Sierra Nevada offers a view into the deep levels of the Mesozoic batholithic belt which constitutes much of the range to the north, and represents one of the major tectonic features of western North America. The main crystalline rocks of the study area are (1) the intrusive suite of Bear Valley, a middle Cretaceous tonalite batholith complex with coeval gabbroic intrusives, and (2) the gneiss complex of the Tehachapi Mountains, which consists of Early Cretaceous orthogneiss and subordinate paragneiss, with local domains having granulite facies metamorphic assemblages. The orthogneisses are dominantly tonalitic in composition, with significant layers of granodioritic to granitic and lesser dioritic to gabbroic gneiss. Quartz-rich and psammitic metasedimentary rocks with subordinate marble constitute the main framework assemblage into which the plutonic rocks were emplaced. Field relations demonstrate assimilation of metasedimentary material into the orthogneiss and tonalite batholith magmas, and magma mixing between mafic, tonalitic, and granitic materials. Significant domains of both homogenization and inhomogenization are recognized isotopically within the mixed rocks. U/Pb zircon studies have resolved two major igneous suites and a third suite of postdeformational intrusives, all lying between 90 and 120 Ma. The first suite (gneiss complex of the Tehachapi Mountains) was emplaced at ∼115 Ma, and exhibits penetrative high-temperature deformation developed at or near solidus conditions. A number of discordance patterns, along with the physical properties of the zircon, suggest minor inheritance of Proterozoic zircon and limited open system behavior in response to a major 100 Ma plutonic event. The 100 ± 3 Ma intrusive suite of Bear Valley crosscuts the older suite, but also exhibits significant synplutonic deformation. Mainly concordant zircon ages indicate the igneous crystallization age, but some discordances occur due to inheritance or entrainment of Proterozoic zircon. The high-temperature deformation fabrics in these suites and within the metasedimentary framework rocks were crosscut by the granodiorite of Claraville (90 Ma) and pegmatite dikes (∼95 Ma). The granodiorite of Claraville shows strong inheritance of Proterozoic zircon and high initial ^(87)Sr/^(86)Sr and δ^(18)O. Zircon populations from paragneiss and quartzite samples are dominated by Proterozoic detrital grains. Strontium and oxygen isotopic data on the zircon geochronology sample suite suggest simple twocomponent mixing of mantle-derived gabbroic to tonalitic magmas with partial to complete melt products from the metasedimentary framework rocks. Sedimentary admixtures for some granitic rocks may be as high as 45%, but for the tonalitic batholithic complex are no higher than about 15%. Modeled values of 10–20% metasediment are typical for the orthogneisses. Initial ^(87)Sr/^(86)Sr correlates directly with δ^(18)O, and generally correlates inversely with Sr content. Some subtle complexities in the Sr and O isotopic data suggest the involvement of a third cryptic component. Such a component could be early Phanerozoic ensimatic accretionary terranes that were structurally beneath the observed metasedimentary sequence, or altered oceanic crust and sediments introduced into the mantle magma source area by subduction. One of the initial aims of this study was to seek out remnants of Proterozoic sialic crystalline rocks within the gneiss complex of the Tehachapi Mountains. No such remnants were found, and our studies strongly suggest that sialic components within this link of the Mesozoic batholithic belt were introduced into mantle-derived magraatic systems by anatexis of continent-derived sedimentary rocks.
TL;DR: Gneisses from the Rauer Group, near Davis Base, east Antarctica, preserve a complex structural metamorphic and igneous history as discussed by the authors, including two early deformation episodes (D1 and D2) under granulite facies conditions.
Abstract: Gneisses from the Rauer Group, near Davis Base, east Antarctica, preserve a complex structural‐metamorphic and igneous history. Mafic granulites and paragneisses, preserved as relics and rafts within Proterozoic felsic orthogneisses, were affected by two early deformation episodes (D1 and D2) under granulite facies conditions. Partial melting of the mafic rocktypes occurred subsequent to these events and partly synchronous with the intrusion of syntectonic (D3) felsic orthogneisses. D3, a major episode which resulted in close to isoclinal folding of most of the gneisses in the area, progressed under granulite facies conditions of greater than 800°C and 7–8 kbar. Preliminary isotopic data indicate that D3 can be no older than 1400 Ma. A period of near‐isothermal decompression to 4 kbar at 700∘C late in and following D3 resulted in the production of leucocratic melts from metasedimentary gneisses, prior to the intrusion of discordant basic dykes of probable Late Proterozoic age (1200–1000 Ma). Further defor...
TL;DR: A suite of granulites including a meta-ironstone, pyroxenites, and spinel-lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution-recry-stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphics temperatures of nearly 1000°C as discussed by the authors.
Abstract: A suite of granulites including a meta-ironstone, pyroxenites, and spinel-lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution-recry-stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000°C. Reintegrated pre-exsolution and pre-reaction grain compositions in the meta-ironstone indicate the former coexistence of metamorphic pigeonite (Wo12En38Fs50) and ferroaugite (Wo35En31Fs34) at temperatures in excess of 980°C for pressures of 7 kbar (0.7 GPa) using pyroxene quadrilateral thermometry (Lindsley, 1983). Intra-grain lamellae relationships indicate the exsolution of a second pigeonite (Wo12En35Fs53) from the ferroaugite at temperatures in the range 930–970°C, prior to the c. 720–600°C exsolution of orthopyroxene and clinopyroxene (100) lamellae and later partial recrystallization at similar temperatures. Although pyroxenitic and iherzolitic granulites preserve a much less complete history, reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone. Pyroxene thermometry based on neoblast compositions suggests that recrystallization post-dating a late, low intensity, deformation phase (D3) occurred at temperatures greater than 600°C. These results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the Napier Complex followed by near-isobaric cooling and later deformation under lower-grade granulite facies conditions. Comparison with similar pyroxene data from Fyfe Hills (Sandiford & Powell, 1986) demonstrates further the regional significance of these high temperatures, and implies broadly isothermal metamorphic conditions over a large area (∼ 5000 km2) and thickness (6–9 km) of lower crust at c. 3070 Ma.
TL;DR: In this article, the authors presented an isotopic analysis of early Proterozoic and Archaean clastic metasediments from Finland and used it to calculate crustal residence ages and to constrain the sedimentary provenance.
TL;DR: The vein graphite deposits of Sri Lanka are located in a Precambrian high grade metamorphic terrain dominated by granulite facies rocks and the origin of the graphite is proposed to be the direct consequence of granulitic facies metamorphism in the presence of a CO2 rich fluid under low fO2 conditions as mentioned in this paper.
Abstract: The vein graphite deposits of Sri Lanka are located in a Precambrian high grade metamorphic terrain dominated by granulite facies rocks. The vein graphite has been interpreted as being of solid phase lateral secretion origin, derived by hydrothermal solutions or of biogenic origin. Based on what is known on the composition of the fluids under granulite facies conditions and the role of these fluids in their transport through the crust, the origin of the graphite is proposed to be the direct consequence of granulite facies metamorphism in the presence of a CO2 rich fluid under low fO2 conditions. This CO2 rich fluid could promote hydraulic fracturing and precipitation of vein graphite. Textures and structures of the vein graphite indicate syntectonic deposition by a crack-seal process under granulite facies metamorphic conditions. This model is supported by temperature estimates on graphite based on XRD data and stable carbon isotopes of graphite that suggest a deep-seated crustal origin.
TL;DR: In this paper, major and trace elements including REE as well as SmNd isotope analyses of eight granulite samples from the Madras area are presented.
TL;DR: In this paper, the ages of the Agawa migmatite terrain in the southern Superior Province, Canada, together with published ages from neighbouring high- and low-grade terrains, reveal a pattern of progressive younging of plutonic and metamorphic processes with depth and provide insights into the mechanisms controlling late Archaean cratonisation.