Showing papers on "Granulite published in 1988"
TL;DR: In this article, the melting of a natural metapelite under fluid-absent conditions was studied experimentally and a series of P-T, T-XH2O, and liquidus diagrams were proposed.
Abstract: In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.
1,017 citations
TL;DR: In this paper, the authors considered that Coronites formed near T = 900°C and P = 10 Kb by two stages of subsolidus reaction from an anorthositic gabbro parent.
Abstract: Coronite-bearing anorthositic granulites consisting of olivine, orthopyroxene, clinopyroxene, garnet and plagioclase assemblages are particularly well preserved at Gaupas and Holsnoy in the Bergen Arc of West Norway. The coronites are considered to have formed near T= 900° C and P=10 Kb by two stages of subsolidus reaction from an anorthositic gabbro parent. The first reaction involved ol+plag→cpxI+opxI+sp and the second cpxI+opxI+sp+pl→cpxII+opxII+gn. The incomplete reaction products are preserved to varying extents in different corona structures. Sm-Nd isotopic data for each of four coronas yield precise isochrons, and demonstrate isochronism both between the constituent phases of the corona assemblages and dispersed ground mass phases. Three individual coronas not associated with shear zones yield ages of 907±9 my, 912±18 my and 905±37 my. Eclogite facies mineralogy is developed locally in shear zones, which are shown by Sm-Nd and Rb-Sr analyses to be Caledonian in age. Where relict corona structures survive unsheared within these zones, Nd exchange between the constituent phases cannot be resolved. This observation together with sympathetic Ca/Mg and Sm/Nd zoning preserved at T∼900° C in the garnet mantles of coronites places a limit on the diffusivity of Nd in pyropic garnets which is no higher than published experimental values for Mg in pyrope-almandine garnets. Consequently even in slowly-cooled granulite terrains, garnet grains are expected to yield Sm-Nd chronologies very close to the time of mineral growth.
298 citations
TL;DR: The Yaounde series is composed of low to high-grade garnet-bearing schists and gneisses belonging to the Pan-African North-equatorial fold belt as discussed by the authors.
285 citations
TL;DR: Granulite facies metapelite, biotite gneiss and metabasic granulite have been found to contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals.
Abstract: Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body.
Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization.
This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.
222 citations
TL;DR: Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known as discussed by the authors.
Abstract: Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known. The xenoliths are predominantly pelitic gneisses and mafic orthogneisses that were brought to the surface on the eastern edge of the Sierra Madre Occidental by recent alkalic basalts. The pelitic gneisses are uniform in mineral assemblage and contain garnet+quartz+plagioclase+sanidine+rutile +sillimanite/kyanite+graphite. The orthogneisses are plagioclase, garnet and/or spinel bearing two pyroxene granulites. Available geothermometric and geobarometric data show that the xenoliths equilibrated at temperatures and pressures consistent with those of the mantle/crust boundary in those areas. The xenoliths range from 46.2 to 67.2 SiO2. Paragneisses are in general more silicic than the orthogneisses. The xenoliths have Rb concentrations between 0.4 and 97 ppm but most samples are very low, with less than 3 ppm Rb. The Sr isotopic ratios of orthogneisses from the lowermost crust throughout most of northern Mexico are very similar and range from ca. 0.705 to 0.706. Previous studies indicate that these rocks have measured ɛ(inNd) values between+2 and −5. Paragneiss xenoliths are generally more radiogenic in Sr isotopic ratio, up to 0.730, and have lower ɛ
Nd values of−11. The Nd and Sr isotopic characteristics of the orthogneisses are similar to those of the voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental. The xenoliths cannot represent cumulate material produced during the mid-Tertiary volcanism because they are Paleozoic or older. Consequently, based on Sr and Nd isotopic data, the silicic ignimbrites could comprise up to 100% lower crustal material.
139 citations
TL;DR: Fiordland, southern New Zealand, contains two primary geologic components: an elongate core comprising high-pressure (>12 kbar) granulite facies orthogneisses (western Fiordland orthognes) and a structurally overlying mid-Paleozoic plutonic-metasedimentary complex with subordinate metavolcanics (Tuhua Sequence) as discussed by the authors.
Abstract: Fiordland, southern New Zealand, contains two primary geologic components: an elongate core comprising high-pressure (>12 kbar) granulite facies orthogneisses (western Fiordland orthogneiss) and a structurally overlying mid-Paleozoic plutonic-metasedimentary complex with subordinate metavolcanics (Tuhua Sequence). The contact between these two components is a major structural discontinuity or decollement hitherto defined as the Doubtful Sound thrust, but reinterpreted here as a ductile shear zone separating lower and upper crustal plates generated during continental extension. Strongly lineated Lower Cretaceous mylonites occur throughout this zone; they formed prior to separation of New Zealand from Australia (at ∼80 Ma). Continental rifting was accompanied by increased heat flow, silicic to mafic magmatism, and the widespread resetting of mineral isotopic systems. Thus, in terms of its overall tectonic setting and tectonothermal history, Fiordland bears a striking resemblance to the metamorphic core complexes of the North American Cordillera.
137 citations
Journal Article•
TL;DR: In this paper, the interchange energy of Fe-Mg mixing in cordierite and the free-energy change of the exchange equilibrium in the temperature range of 700-850 'C have been extracted using standard methods of linear regression analysis.
Abstract: From chemicaldata on 108 pairs of naturally occurring garnet and cordierite, the interchange energy of Fe-Mg mixing in cordierite (Wo*-\") and the free-energy change of the exchange equilibrium %FerAlrSirO,, -t t/zMgrAIoSirO,, : Y:MgrAlrSirO,, * YzFerAloSirO,, in the temperature range of 700-850 'C have been extracted using standard methods of linear regression analysis. The input thermochemical parameters for multicomponent CaMg-Fe-Mn garnet solid solutions are based on the updated and revised values after Ganguly and Saxena (1984), e.g., Wr\"_.\": 2500 cal, W*: W*\": 3000 cal calculated on the basis of one-third formula unit. On the basis of the derived values at 710, 760, 800, and 850 \"C, Fe-Mg mixing in cordierite has been approximated to a symmetric regular solution. The Fe-Mg substitution shows positive deviation from ideality in cordierite, with tzi:1., : 9.34 + 4.54kJ/mol of divalent cation. The values derived for the free energy of mixing, at Xi:d : Xi;ds : 0.5, are -3.26 kJ at 700 'C and -4.39 kl at 900 \"C. The AG,.. value (in joules) for the above equilibrium can be expressed as 15079 8.552. Incorporating these parameters, the geothermometric expression for the exchange equilibrium is
124 citations
TL;DR: The Great Slave Lake Shear Zone (GSLSZ) as discussed by the authors is a type example of a crustal-scale fault zone profile, where the metamorphic grade decreased from granulite to greenschist facies and the locus of high strain narrowed and jumped laterally, abandoning relatively older mylonites.
121 citations
01 Sep 1988-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: Following the India-Asia collision, intracrustal movements along the Main Central Thrust (MCT) and Main Boundary Thrust (MBT) in a piggy-back-style, thrust duplexes developed that uplifted the Vaikrita (Central) crystallines of the basement to more than 8000 m elevation as discussed by the authors.
Abstract: Following the India--Asia collision, intracrustal movements along the Main Central Thrust (MCT) and Main Boundary Thrust (MBT) in a piggy-back-style, thrust duplexes developed that uplifted the Vaikrita (Central) crystallines of the basement to more than 8000 m elevation. Blocking of subduction on the suture and slowing down of movement on the MCT led to the formation of the Trans-Himadri (Malari) Thrust between the Vaikrita basement and the Tethyan cover sediments, and to gravity-induced backfolds and backthrusts in the latter. The Vaikrita crystallines underwent upper amphibolite to lower granulite facies metamorphism at 600-650 degrees C and more than 5 kbar (1 kbar = 10$^{8}$ Pa) and migmatistation associated with 28--20 Ma old S-type granites that formed at 15--30 km depth during the culmination of metamorphism and thrust deformation. Delimited by the MCT and MBT, the Lesser Himalaya is made of Proterozoic sediments beneath the Almora nappe constituted of low- to medium-grade metamorphics and 1900 $\pm $ 100 Ma old granitic gneisses and 560 $\pm $ 20 Ma old granites. The Lesser Himilaya underwent considerable neotectonic rejuvenation during differential movements along the MBT. The frontal Siwalik molasse below the MBT was severely thrusted and folded in the late Holocene, and continued underthrusting of the Indian Shield beneath the Himalaya is manifest in the development and activation of the deep Himalayan Front Fault (HFF), which separates the Siwalik from the subRecent--Recent alluvial plain of the Ganga Basin.
111 citations
109 citations
TL;DR: The Archean was the principal period for gold mineralization as discussed by the authors, and these rocks compose ∼12% of the exposed crust but have produced more than half of the world9s gold.
Abstract: The Archean was the principal period for gold mineralization.These rocks compose ∼12% of the exposed crust but have produced more than half of the world9s gold. Most major lode deposits formed at 2.7 ±0.2 Ga. This corresponds to the principal time of crustal thickening and stabilization, an essential part of which was formation of granulite, the anhydrous, refractory base for the crust. Upward streaming of mantle CO 2 along shear zones in the lower crust is one means for dehydrating amphibolite to form granulite and is accompanied by removal of large ion lithophile (LIL) elements to higher crustal levels. Archean lode-gold deposits were formed along major shear zones at intermediate crustal depths. They are enclosed by zones of CO 2 metasomatism, many of them kilometres in extent; δ 13 C data indicate that the CO 2 is juvenile. Some major Archean deposits have been shown to have formed from relatively oxidized hydrothermal fluids. CO 2 streaming with LIL-element depletion could only have occurred under relatively oxidized conditions that permitted a free CO 2 vapor to exist in the lower crust. Such conditions would also have favored dissolution of gold and associated sulfide and their transfer to the mid-crust in a CO 2 -H 2 O fluid, the H 2 O coming from dehydration of amphibolite.
TL;DR: In the D'Entrecasteaux islands of Papua New Guinea, the metamorphic and ultramafic structures are similar to the core complexes of western North America as discussed by the authors.
Abstract: Compositionally layered metamorphic rocks of the D'Entrecasteaux Islands, Papua New Guinea, are folded into domes and antiforms bounded by faults parallel to metamorphic layering and foliation. The structures are broadly similar to the metamorphic “core complexes” of western North America. Lenses of ultramafic rock lie on the bounding faults, and the same faults have served as loci for Quaternary andesitic volcanic activity. Metamorphic grade in the northern islands (Goodenough and Fergusson) is amphibolite facies, with pockets of eclogite (Fergusson Island only) and granulite, and is greenschist facies in the southern island (Normanby). In all three islands there is a characteristic tectonostratigraphic sequence (FMU sequence) from felsic metamorphic rocks at base, or internally, through mafic metamorphic rocks to ultramafic rocks at top, or externally. The association of metamorphic and ultramafic rocks apparently developed in a north dipping Paleogene subduction system and was exhumed to upper crustal level in the Oligocene--Early Miocene, possibly by reversal of movement on faults in the former subduction system. Vigorous uplift and development of domes and antiforms in the Pliocene was triggered by westward propagation of the Woodlark Basin spreading ridge and was accompanied by rifting, rift-related magmatism, rapid erosion, and deposition of coarse sediment in the adjacent Trobriand Basin.
TL;DR: The Narryer Gneiss Complex as discussed by the authors consists of two major groups of quartzo-feldspathic gneiss, namely Meeberrie and Dugel, which were deformed and metamorphosed in granulite facies about 3350-3300 Ma ago.
TL;DR: In this paper, the authors present abundances and carbon isotope compositions of CO2 in fluid inclusions in quartz and garnet from gneiss-granulite pairs from the incipient char-nockite quarries of southern India, determined by a stepped combustion/thermal decrepitation carbon extraction technique.
Abstract: Models for the formation of crustal granulites, which include the metamorphism of anhydrous lithologies1,2, the extraction of hygroscopic granite melts3,4 and fluxing by CO25–7 constrain the thermal and geochemical evolution of the lower crust and its interaction with mantle-derived volatiles. Here we present abundances and carbon isotope compositions of CO2 in fluid inclusions in quartz and garnet from gneiss–granulite pairs from the incipient char-nockite quarries of southern India, determined by a stepped combustion/thermal decrepitation carbon–extraction technique, which quantifies problematic sample contamination and allows precise lithological control. The results show that the CO2 in incipient charnockite is more abundant and isotopically heavier than the CO2 in associated gneiss, providing evidence for the influx of CO2-rich fluids during their formation. The inferred carbon isotope compositions of these fluids are consistent with a mantle source, and at Ponmudi, the isotopic composition of coexisting graphite and CO2 implies fluid/rock volume ratios in excess of 0.15.
TL;DR: In this article, velocity measurements of representative rock samples from across a nearly complete crustal section from the Serre Mountains of southern Calabria have been carried out as an aid to interpreting in-situ seismic velocity data.
TL;DR: In this paper, the authors show that the Willyama supergroup is volcanic exhalative in origin, probably formed from seawater convected through the sequence, and driven by heat from high-level magma chambers.
Journal Article•
TL;DR: In this article, Orthopyroxene (Opx) and augite (Aug) in metasedimentary and meta-igneous gneisses in a supracrustal succession at Fyfe Hills exhibit intimate exsolution intergrowths.
Abstract: Orthopyroxene (Opx) and augite (Aug) in metasedimentary and meta-igneous gneisses in a supracrustal succession at Fyfe Hills exhibit intimate exsolution intergrowths. The textural relations and compositional data suggest the following phase relations at peak conditions: Opx-Aug (for 031XFeOpx 0.5), Aug-Pgt (for 031XFeAug >0.48), where XFe = Fe2+/(Fe2+ + Mg). A peak metamorphic T of = or >1000oC at 9 kbar is indicated. -J.A.Z.
TL;DR: In this article, the lower crust was sampled by bringing mafic granulite, amphibolite, and pyroxenite (eclogite) xenoliths to the surface.
Journal Article•
TL;DR: Schylek et al. as mentioned in this paper showed that the D2-5 tectonothermal activity, rapid cooling and uplift in the Sowie Gory block of the Middle Sudetes pre-date the early Carboniferous granulite facies metamorphism and granitic plutonism further S in the Moldanubian zone of the Hercynides in Czechoslovakia.
Abstract: Dewonska aktywnośc tektonometamorficzna w gnejsach bloku Gor Sowich w świetle badan izotopowych Rb-Sr i U-Pb
In widely developed biotite-oligoclase-quartz paragneiss a U-Pb upper intercept age of 1750 ±270 is interpreted as a (possibly mixed) sedimentary provenance age. U-Pb monazite and Rb-Sr biotite ages of 381 ± 2 Ma and ca. 370-360 Ma, respectively, indicate rapid Devonian cooling from upper amphibolite to greenschist facies temperatures. The isotopic data are consistent with stratigraphic evidence for rapid Devonian uplift. U-Pb zircon isotopic data support, but do not prove conclusively, that the metamorphism associated with D2 in the Sowie Gory block was Devonian in age. The Z)4 event has been dated at 370 ± 4 Ma using Rb-Sr in large muscovite books from a pegmatite in an F4 hinge zone. Low-mid-amphibolite facies Ds activity cannot be much younger in view of the biotite cooling ages. The D2-5 tectonothermal activity, rapid cooling and uplift in the Sowie Gory block of the Middle Sudetes pre-date the early Carboniferous granulite facies metamorphism and granitic plutonism further S in the Moldanubian zone of the Hercynides in Czechoslovakia.
Schylek progresywnego metamorfizmu, aktywnośc magmową (pegmatyty) i podniesienie kompleksu gnejsow sowiogorskich datowano odpowiednio na 381+2 Ma, 370 + 4 Ma i 370 - 360 Ma. Szybkie stygniecie kompleksu, wiązące sie z inwersją tektoniczną przy koncu dewonu, trwalo okolo 10 - 20 mln lat.
TL;DR: Granulite facies lithologies from the Adirondack Mountains of New York contain alteration assemblages composed dominantly of calcite +/- chlorite +/- sericite as discussed by the authors.
Abstract: Granulite facies lithologies from the Adirondack Mountains of New York contain alteration assemblages composed dominantly of calcite +/- chlorite +/- sericite These assemblages document fluid infiltration at middle to upper crustal levels Cathodoluminescence of samples from the Marcy anorthosite massif indicates that the late fluid infiltration is more widespread than initially indicated by transmitted-light petrography alone Samples that appear unaltered in transmitted light show extensive anastomosing veins of calcite (< 005 mm wide) along grain boundaries, in crosscutting fractures, and along mineral cleavages The presence of the retrograde calcite documents paleopermeability in crystalline rocks and is related to the formation of high-density CO/sub 2/-rich fluid inclusions Recognition of this process has important implications for studies of granulite genesis and the geophysical properties of the crust
TL;DR: In this paper, the fluid-buffering mineral assemblages: amphibole, micas, and pyroxenes were found to have low activity of water during the peak of granulite-facies metamorphism.
Abstract: Samples located near the Oregon Dome anorthosite massif in the south-central Adirondack Mountains, New York contain the fluid-buffering mineral assemblages: amphibole + clinopyroxene + orthopyroxene + quartz or biotite + quartz + orthopyroxene + K-feldspar. These rocks were metamorphosed under granulite-facies conditions (T=725°–750°C, P=7.5 kbar) during the Grenville orogeny. The Mg-rich nature of amphiboles, micas, and pyroxenes allow accurate calculation of water activities because corrections for the effects of solid solution are relatively small. The activity of water was low during the peak of granulite-facies metamorphism, with αH2O≈0.15±0.14. Wollastonite occurrences indicate that the αCO2 was low (<0.3) in nearby rocks, demonstrating that large quantities of CO2 did not infiltrate in a pervasive manner. The combination of low αH2O with low αCO2 is consistent with the hypothesis that magmatic processes were dominant, generating dry, fluid-absent conditions.
TL;DR: In this paper, the Lewisian whole-rock isochron of 2,920 ± 50 Myr was interpreted as the time of protolith formation, which is ∼260 Myr older than estimates for the timing of high-grade metamorphism in the complex at ∼ 2,660 Myr2'3.
Abstract: A published Sm–Nd whole-rock isochron of 2,920 ± 50 Myr, obtained from a wide range of lithologies in the Lewisian complex of north-west Scotland, was interpreted1 as the time of protolith formation. This date is ∼260 Myr older than estimates for the timing of high-grade metamorphism in the complex at ∼ 2,660 Myr2'3, and this period is considered to represent the duration of the Lewisian crustal accretion-differentiation superevent (CADS)4. Here we give new Sm-Nd data, obtained specifically from granulite facies tonalitic gneisses, that yield a date of 2,600 ±155 Myr. Although depleted-mantle model ages (tDM suggest >200 Myr of premetamorphic crustal residence, the regression date and its associated initial Nd-isotopic parameters demonstrate Nd-isotopic homogenization during the high-grade event, as well as the probability of general rare-earth-element (REE) mobility. Models for selective element depletion in the complex have previously assumed REE immobility since 2,920 Myr, but the data presented here suggest that a reappraisal of the depletion mechanism is required.
TL;DR: Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700-750°C and pressures in the vicinity of 6-8 kbar as mentioned in this paper.
Abstract: Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid‐levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO2‐rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re‐equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi‐isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan‐African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton. Copyright © 1988, Wiley Blackwell. All rights reserved
TL;DR: Van Reenen et al. as mentioned in this paper made micro-thermometric analyses of over 1000 fluid inclusions in four samples, representing a traverse across the retrograde orthoamphibole isograd, with the goal of characterizing the hydrating fluid.
TL;DR: A Miocene volcanic pipe that cuts Cretaceous granodiorite is exposed in the central Sierra Nevada near the town of Big Creek, California as discussed by the authors, where a diverse assemblage of igneous and metamorphic xenoliths including peridotites, eclogites and granulites, schists and hornfels, and gabbroids and granitoids are restricted to the central area of the pipe.
Abstract: A Miocene volcanic pipe that cuts Cretaceous granodiorite is exposed in the central Sierra Nevada near the town of Big Creek, California. The volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent, resulting in a "pseudo-andesitic" composition. The conduit cut through the entire thickness of the Sierra Nevada batholith, as well as the underlying lower crust and upper mantle, and the magma entrained a diverse, polygenetic assemblage of igneous and metamorphic xenoliths, including peridotites, eclogites and granulites, schists and hornfels, and gabbroids and granitoids. Some xenoliths of the high-grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the Sierra Nevada. Garnet granulites, the most abundant rock type of the mafic high-grade metamorphic assemblage, and eclogites are restricted to the central area of the pipe. These xenoliths are essentially bimineralic, consisting of colorless to pale green clinopyroxene and garnet; eclogites contain brilliant emerald green clinopyroxene. The clinopyroxenes, which have a broad range of compostions [Mg/ (Mg + Fe) = 0.45-0.90], have a >20% jadeite component in eclogites. Garnets also have a range of compositions [Mg/(Mg + Fe) = 0.30-0.70], with those from eclogites being the most Fe rich. One of the xenoliths contains Ca-rich garnet (grs + adr > 90%). Feldspathic granulites contain plagioclase of An 45-70 , and some lack garnet. Hornblende occurs in some granulites, and a few amphibolites are present. Likely protoliths are ocean-floor basalts for the eclogites and Precambrian limestone for the grossular-clinopyroxene rock. The Big Creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western Sierra Nevada.
Journal Article•
TL;DR: In this article, the stable isotopes of carbon in cordierite (crd) or graphite (gr) can provide a means to discriminate between two controversial models of granulite formation: (1) large-scale CO2 infiltration7, or (2) magmatic processes8-11, which might result in fluid-absent metamorphism.
Abstract: Carbon dioxide is a common constituent in the channels of high-grade metamorphic cordierites1–6 Here we show that analysis of the stable isotopes of carbon in cordierite (crd) or graphite (gr) can provide a means to discriminate between two controversial models of granulite formation: (1) large-scale CO2 infiltration7, or (2) magmatic processes8–11, which might result in fluid-absent metamorphism. The fractionation of carbon isotopes between cor-dierites and graphites in high-grade rocks approximates the predic-ted equilibrium value. This suggests that cordierites can preserve peak metamorphic carbon isotope and channel gas compositions. Isotopically light carbon values in many granulite-facies cordierites (δ13Ccrd< −15) and graphites severely restrict theories of massive CO2 infiltration (δ13Cfluid≳−7). CO2rock ratios from a number of granulite terranes are estimated to be ≲0.01 by weight (∼0.016 on a molar oxygen basis), unless the rocks contain sub-stantial additional carbon. These data emphasize the importance of local fluid buffering in the granulite facies, consistent with dehydration by partial melts.
TL;DR: The Magondian Mobile Belt as mentioned in this paper is a typical thin-skinned thrust belt with Magondi Supergroup rocks thrust SE onto the Zimbabwe Archaean craton, and the structural style changing to a more thick-skinned type, metamorphic grade increasing from greenschist to granulite facies and with increasing amounts of early Proterozoic basement gneisses imbricated within the sedimentary sequence.
TL;DR: The southern part of the Cretaceous Kohistan island arc is occupied by an extensive belt dominantly comprised of amphibolites as mentioned in this paper, which consist essentially of hornblende, plagioclase and/or epidote.
Abstract: The southern part of the Cretaceous Kohistan island arc is occupied by an extensive belt dominantly comprised of amphibolites. These include banded amphibolites of partly meta-volcanic parentage, and non-banded amphibolites derived from intrusive rock. In addition to being relict, banding has also been produced by shear deformation, metamorphic/metasomatic segregation and, possibly, by lit-par-lit injection of plagiogranitic material. Non-banded amphibolites also occur as retrograde products of noritic granulites forming the lopolithic Chilas complex. The chemistry of 37 rocks has been compared with those of known tectonic environments. The amphibolites have chemical characteristics similar to volcanic rocks found in island arcs and most of the analyses apparently support affinity with the calc-alkaline series. The amphibolites consist essentially of hornblende, plagioclase and/or epidote. Garnet and clinopyroxene have developed locally in rocks of appropriate bulk composition. Metamorphism may have taken place during the mid-Cretaceous under conditions of 550 to 680 ~ and 4.5 to 6.5 kbar PH2o. The metamorphic grade appears to increase from the centre of the southern belt toward the Chilas complex to the north and Indus-Zangbo suture (IZS) to the south. In the vicinity of the IZS, garnet-clinopyroxene + amphibole assemblage developed locally in response to high P-T.