Garnet Clinopyroxenite Research Articles

A treasure chest full of nanogranitoids: an archive to investigate crustal melting in the Bohemian Massif

Abstract The central European Bohemian Massif has undergone over two centuries of scientific investigation which has made it a pivotal area for the development and testing of modern geological theories. The discovery of melt inclusions in high-grade rocks, either crystallized as nanogranitoids or as glassy inclusions, prompted the re-evaluation of the area with an ‘inclusionist’ eye. Melt inclusions have been identified in a wide range of rocks, including felsic/perpotassic granulites, migmatites, eclogites and garnet clinopyroxenites, all the result of melting events albeit over a wide range of pressure/temperature conditions (800–1000°C/0.5–5 GPa). This contribution provides an overview of such inclusions and discusses the qualitative and quantitative constraints they provide for melting processes, and the nature of melts and fluids involved in these processes. In particular, data on trace-element signatures of melt inclusions trapped at mantle depths are presented and discussed. Moreover, experimental re-homogenization of nanogranitoids provided microstructural criteria allowing assessment of the conditions at which melt and host are mutually stable during melting. Overall this work aims to provide guidelines and suggestions for petrologists wishing to explore the fascinating field of melt inclusions in metamorphic terranes worldwide, based on the newest discoveries from the still-enigmatic Bohemian Massif.

Recent crustal foundering in the Northern Volcanic Zone of the Andean arc: Petrological insights from the roots of a modern subduction zone

Periodic loss of the lower lithosphere into the convecting mantle due to gravitational instability is postulated to be a major mechanism for lithosphere recycling in orogenic zones, but unequivocal petrologic evidence of this process is elusive. The Granatifera Tuff, located in the Mercaderes–Rio Mayo area of the southern Colombian Andes, contains a wide variety of crustal and mantle xenoliths. Here we focus on the thermobarometry and Lu–Hf isotope systematics of crustal garnet clinopyroxenite xenoliths, the results of which offer the first evidence of recent, and likely active, crustal foundering in the Northern Volcanic Zone of the Andean arc. We find that most of these xenoliths equilibrated between 60–80 km depths, ∼7–27 km below the seismically determined Moho in this region, and that at least one crustal garnet clinopyroxenite re-equilibrated at depths exceeding 95 km. A second garnet clinopyroxenite equilibrated at ∼150 km depths, and is either foundered lithospheric material or the product of reaction between peridotite and a mobile component (either silicic melt or fluids) at >4 GPa. All of the investigated garnet clinopyroxenites are negatively buoyant relative to the upper mantle asthenosphere. The presence of minor amounts of secondary amphibole and orthopyroxene, coupled with the lack of major-element retrograde zonation in primary phases within these xenoliths, indicates that these rocks were rapidly transported to, and briefly resided at, shallow depths before eruption. Lu–Hf ages from two garnet clinopyroxenites and one garnet–clinopyroxene hornblendite are <5 Ma, and approximate the time at which these xenoliths were transported to shallow depths prior to eruption. A large-magnitude positive geoid anomaly and relatively low mean surface elevations indicate that the gravitationally unstable crustal root is still largely attached to the overriding crust in this part of the Northern Volcanic Zone. Thermobarometric calculations indicate that the lowermost crust in this region is a partial melt zone, and we argue that rheological weakening in the presence of melt has led to the foundering of relatively small parcels of gravitationally unstable crustal material, which the Mercaderes xenoliths document, without catastrophic removal of the crustal root.

Eclogites and garnet clinopyroxenites in the Anrakhai complex, Central Asian Orogenic Belt, Southern Kazakhstan: P-T evolution, protoliths and some geodynamic implications

The Anrakhai Metamorphic Complex (AMC), located in the SE part of the Chu-Ili Mountains of Southern Kazakhstan in the western part of Central Asian Orogenic Belt, exhibits occurrences of HP metamorphic rocks in the form of eclogites and garnet clinopyroxenites with peak metamorphic conditions of 750–850° and 15–19kbar estimated with both conventional geothermobarometric methods and phase diagram modeling. P-T estimates as well as intimate field relations evidently imply a common metamorphic history for eclogites and garnet clinopyroxenites of the AMC. These high-pressure, medium temperature eclogite facies P-T conditions are indicative of a collision or subduction tectonic setting. Major and trace element geochemistry suggests that they probably had a common magmatic origin as part of a suite of differentiated tholeiitic intrusions. Furthermore, distinctive mineral and chemical compositions of these eclogites and garnet clinopyroxenites correspond to the Fe-Ti type of ultramafic rocks suggesting that they may have been derivatives of intraplate tholeiitic melts, introduced into continental crust before HP metamorphism.

Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen, Svalbard Archipelago

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of −1.5 to +0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe2+ between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH−, crystal hydrate water H2Ocryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H2Ocryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (>80% H2O) to aqueous–carbonic (~60% H2O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.

Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)

Research Article| October 01, 2015 Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy) Alessandra Montanini; Alessandra Montanini 1DIFEST, Università di Parma, Parco Area delle Scienze 157a, 43100 Parma, Italy Search for other works by this author on: GSW Google Scholar Riccardo Tribuzio Riccardo Tribuzio 2Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 9, 27100 Pavia, Italy3CNR–Istituto di Geoscienze e Georisorse–U.O.S. (Unità Operativa di Supporto) Pavia, via Ferrata 1, 27100 Pavia, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Alessandra Montanini 1DIFEST, Università di Parma, Parco Area delle Scienze 157a, 43100 Parma, Italy Riccardo Tribuzio 2Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 9, 27100 Pavia, Italy3CNR–Istituto di Geoscienze e Georisorse–U.O.S. (Unità Operativa di Supporto) Pavia, via Ferrata 1, 27100 Pavia, Italy Publisher: Geological Society of America Received: 12 Apr 2015 Revision Received: 07 Aug 2015 Accepted: 09 Aug 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (10): 911–914. https://doi.org/10.1130/G36877.1 Article history Received: 12 Apr 2015 Revision Received: 07 Aug 2015 Accepted: 09 Aug 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alessandra Montanini, Riccardo Tribuzio; Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology 2015;; 43 (10): 911–914. doi: https://doi.org/10.1130/G36877.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The pyroxenite-peridotite sequence from the External Ligurian (northern Apennines, Italy) ophiolites is evidence of the evolution of recycled crust within the mantle. We present new major and trace element and Nd-Hf isotopic compositions of garnet clinopyroxenites and websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk-rock rare earth element patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that underwent negligible interaction with the host peridotites. The garnet websterites are interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ca. 220 Ma caused melting of eclogites originating from a mid-oceanic ridge basalt–type gabbroic sequence. These mafic protoliths underwent a long-lived evolution of recycling in the mantle (1.5–1.0 Ga). We show that heterogeneity of crustal protoliths, age of recycling, and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ18O values in the range 4.3 to 5.4‰ (N=10) with a mean of 4.9±0.4‰, and rubies from 5.5 to 5.6‰ (N=3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ18O values from 4.6 to 5.2 ‰, with a mean of 4.9±0.2‰ (N=9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ18O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.

Pyroxenite Dykes in Orogenic Peridotite from North Qaidam (NE Tibet, China) Track Metasomatism and Segregation in the Mantle Wedge

The early stages of magmatic processes operating at mantle depths beneath continental arcs are poorly known. The chemical compositions of minerals and rocks, mineral Sr^Nd^Hf^O isotopes and zircon U^Pb ages of garnet clinopyroxenite dykes from the Shenglikou peridotite massif (North Qaidam Orogen, NE Tibet, China) were studied to constrain their sources and genesis, and the dynamic processes that controlled pyroxenite formation beneath an early Paleozoic active continental margin. Major-element compositions of bulkrocks suggest that the pyroxenitic protoliths were cumulates segregated from a melt, which was extracted from a peridotite-dominated mantle source. Bulk-rock and mineral trace-element patterns show strong enrichment in fluid-mobile elements (e.g. Cs, Rb, Ba,Th, U, K, Pb and Li) and marked negative anomalies in the high field strength elements relative to rare earth elements, similar to the characteristics of melts derived from a volatile-rich subarc mantle. Enriched Sr and Nd initial isotopic compositions at 500Ma (Sr/Sr of 0 70919^0 71774 and eNd of 16 3 to 3 4) are in contrast to the highly radiogenic Hf isotope compositions (similar to those of the depleted-mantle reservoir) and to the uncontaminated upper-mantle dOV-SMOW (garnet: 5 6 0 3o, 2SD, n1⁄4 61; zircon: 5 9 0 3o, 2SD, n1⁄4 28). These decoupled isotopic signatures suggest that the melt source was located in a convective mantle wedge (controlling the Hf and O isotopes) that had been pervasively metasomatized by fluids from a subducted ProtoTethys oceanic slab (controlling the Sr^Nd isotopes and highly incompatible elements). Zircons with two groups of U^Pb ages (430 5Ma and 401 7Ma) were generated by recrystallization events, corresponding to UHP metamorphism and a major uplift stage during the North Qaidam orogeny, respectively.The combined evidence reveals a picture of continental arc magmatism at mantle depths and subsequent continental collision. The subduction of the Proto-Tethys oceanic slab beneath the southern Qilian margin triggered flux melting of the metasomatized convective mantle wedge and generated hydrous arc magmas. These primitive magmas intruded into the overlying lithospheric mantle and segregated the cumulates parental to the Shenglikou pyroxenites. Subsequent continental subduction incorporated fragments of the mantle-wedge peridotite (containing pyroxenite dykes) at 430Ma and carried them to shallow depths during exhumation at 400Ma.

Amphibole genesis in pyroxenites from the Beni Bousera peridotite massif (Rif, Morocco): Evidence for two different metasomatic episodes

Abstract The presence of variable amounts of amphibole ± phlogopite in a garnet websterite and a garnet clinopyroxenite from the Beni Bousera peridotite massif provides evidence for post-formation metasomatism. Textural observations associated with major- and trace-element mineral compositions allowed us to distinguish two metasomatic episodes, which occurred at different stages of the Beni Bousera massif evolution. The garnet websterite has recorded interaction with LREE-rich silicate melts before the uplift of the massif. Amphibole/clinopyroxene and amphibole/garnet trace-element ratios closely approach partition coefficient values, indicating that chemical equilibrium was attained between amphibole and pyroxenite matrix minerals. The geochemical signatures of the putative alkaline interacting melts are similar to those of recent basaltic magmas erupted in Morocco, suggesting a common peridotite mantle source. In contrast, amphibole from the garnet clinopyroxenite is in chemical disequilibrium with the pyroxenite matrix minerals. In this clinopyroxenite the crystallization of amphibole and plagioclase occurred at lower T (and P) conditions, most probably during the ascent of the Beni Bousera massif and its emplacement into the crust. The melt responsible for this later metasomatic episode was LREE-depleted and HREE-enriched, suggesting that it resulted from decompression melting of a garnet-bearing source (with garnet as a melting phase), similar to the garnet-bearing pyroxenites outcropping in the Beni Bousera massif.

Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif (northern Morocco): Petrography, mineral chemistry and thermobarometry

A detailed mineralogical and textural study of two pyroxenites from the Beni Bousera massif, a garnet clinopyroxenite (GP) and a garnet clinopyroxenite containing graphite pseudomorphs after diamond (GGP), indicates a strong metamorphic overprint associated with the massif exhumation. In both pyroxenites, the primary assemblage [Cpx(I)+Gt(I)±Opx] originally stable with diamond, records temperatures in excess of 1200°C. Along the exhumation path, Cpx(I) has decomposed according to the reaction:CpxI=0.55–0.60CpxII+0.30–0.35OpxII+0.03–0.05GtII+0.03–0.5Pl.This breakdown reaction occurred under sub-solidus conditions in at least two stages, which first led to Cpx and Opx exsolution lamellae and second to garnet crystallization at the expense of the newly formed Cpx lamellae. Successive garnet generations, Gt(II), can be distinguished by their REE content; primary garnets Gt(I) have the highest HREE content. Secondary garnets, Gt(II), have grown in the 850–950°C temperature range. We show that these conditions are below the blocking temperature of Mg–Fe exchange between garnet and pyroxene (ca. 1050°C) and above the blocking temperature of Mg–Fe interdiffusion in garnet. Consequently, the original composition of the secondary garnet has not been modified upon further cooling; equilibrium with the appropriate pyroxene can be used to retrieve meaningful P–T conditions. This is however not the case for pairs of pyroxene inclusion and their porphyroclastic garnet host, formed earlier in the Beni Bousera history (i.e. at higher grade) and for which decoupling between Al diffusion and (Mg, Fe) interdiffusion (in pyroxene) has occurred. In that particular case of decoupling, conventional thermometry which yield apparent conditions around 2GPa and 1000–1100°C based on Al-content in pyroxene on one hand and Mg–Fe partitioning between the pyroxenite silicates on the other, will intrinsically fail at providing reliable P and T estimates. Detailed chemical inspection of Gt–Cpx–Opx inclusions in the graphite pseudomorphs indicates that (1) these silicates are genetically related to the same minerals in the bulk rocks and (2) they were chemically isolated from the bulk rock (included in diamond and/or graphite). Finally, the pyroxenite samples from Beni Bousera have recorded two major events pertaining to the late evolution of this massif: (1) the increase of the water activity in GP as shown by the crystallization of pargasitic amphibole which suggests the proximity to crustal units, and (2) a temperature increase up to ca. 1050°C, which led to water-present partial melting of both GP and GGP.

Garnet pyroxenite in the Biskupice peridotite, Bohemian Massif: anatomy of a Variscan high-pressure cumulate

*Corr esponding author A strongly layered garnet pyroxenite, consisting of cm-scale layers of garnetite, garnet orthopyroxenite, garnet clinopyroxenite, and websterite, occurs in the Biskupice garnet peridotite, which is located in migmatitic gneiss of the Gfohl Assemblage in the Vysocina District, western Moravia. Major- and trace-element compositions and REE patterns of minerals and layers in the pyroxenite are consistent with origin of the layers by HT–HP crystallization and accumulation of variable proportions of garnet, orthopyroxene, and clinopyroxene. The present compositions of the minerals (pyroperich garnet, low-Al, low-Ca enstatite and low-Al, high-Ca diopside) are the result of extensive subsolidus re-equilibration at ~900 oC and ~37 kbar, during the Variscan (332 Ma) Orogeny. Neodymium and strontium isotopic values (e 3 N 3 d 5 , +1.8

Aspectos petrográficos e petrológicos de um gnaisse calciossilicático (meta-rodingito) em alto grau na base da Klippe Carvalhos, Faixa Brasília, borda Sul do Cráton São Francisco, em Minas Gerais

Petrographic and petrologic features of a high-grade calcium-silicate gneiss (metarodingite?) at the base of Carvalhos Klippe, Brasília Belt, Southern border of the São Francisco Craton, Minas Gerais State. In the southern region of Minas Gerais State, in the southern border of the Neoproterozoic Tocantins Orogen, at São Francisco Craton south, several ultramafic bodies occur among oceanic metassediments, turbiditic and pelagic to hemipelagic, in medium to high-grade metamorphism and positioned along deep thrust faults associated with granulites, retroeclogites, and basement slivers. Among these ultramafic rocks of controversial origin and with a probable ophiolitic nature, the most important are the Morro do Corisco and Fazenda da Roseta, in the Liberdade region. The Fazenda da Roseta ultramafic body is made up by an association of meta-spinel harzburgite, meta-websterite, serpentinite carbonate, pegmatitic metabronzitite, and a garnet clinopyroxenite with a gneissic fabric, a possible meta-rodingite. This lithotype shows high density, ultramelanocratic character, banded granoblastic gneissic texture, which is constituted by mineral associations with varied proportions of garnet, clinopyroxene, epidote, and hornblende as the association of metamorphic peak, and plagioclase, epidote, ilmenite, magnetite, carbonate, which forms incomplete simplectitic reactions around garnet, among hornblende, with apatite and carbonate, the accessory minerals. Chemically, these rocks show a characteristic enrichment in CaO and major Na2O and SiO2 depletion when is compared with meta-mafic and meta-ultramafic rocks of the region. These rocks are positioned on the fields of meta-mafic rocks related to N-MORB and IAT. It has curved upward patterns for large-ion lithophile elements (LILE) and flat patterns for HFES and heavy rare earth elements (HREE), when compared with the abundance of these elements in N-MORB basaltic compositions. When this rock is analyzed together with the meta-ultramafic rocks of the region, it is possible to observe possible parental relations with meta-websterite, meta-harzburgite and meta-bronzitite, which show similar patterns of LILE, with negative Rb anomaly, and similar REE patterns, with enriched patterns related to chondrite and a Ce strong negative anomaly. From litho-textural and geochemical considerations together with the aloctone character of the associated ultramafics, retroeclogites and deep thrust faults, it is suggested that this (meta)ultramafic rock is a register of a calcium metassomatism (formation of rodingite), a probable meta-rodingite, in a probable oceanic environment, in a former episode to the process of subduction and collision, attributed to the closing of the Brasilíades Ocean, in a scenario of the Brazilian Orogen formation, synchronous to the formation of structural nappes.

Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean–continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66–71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (CeN/SmN=0.1–0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet+clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis, Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm–Nd and Lu–Hf isotope correlations. The Nd–Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (εNd=+4.7 to +7.6, εHf=+4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (δ18O=+4.9‰ to +5.2‰) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts.

Deformation processes and rheology of pyroxenites under lithospheric mantle conditions

We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0GPa and 950–1000°C and spinel pyroxenites at 1.8GPa and 1100–1150°C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle.

Comments on “Garnet-bearing ultramafic rocks from the Dominican Republic: Fossil mantle plume fragments in an ultra high pressure oceanic complex?” by Gazel et al. [Lithos 125 (2011) 393-404

The Rio San Juan Complex in the Dominican Republic hosts a suite of enigmatic corundum-bearing garnet peridotite rocks. Two hypotheses for their origin exist, an ultrahighpressure (UHP) hypothesis, in which corundum, spinel and garnet crystallised simultaneously at UHP magmatic conditions (>3.2 GPa and >1500°C), and a low pressure (LP) hypothesis, in which the rocks are metamorphosed equivalents of low-pressure plagioclase-bearing cumulates. Distinction between the two is hampered by extensive metamorphic recrystallisation of the rocks under UHP conditions. In a recent paper, Gazel et al. [Gazel, E., Abbott, R.N., Draper, G., 2011, ultramafic rocks from the Dominican Republic: Fossil mantle plume fragments in an ultra high pressure oceanic Lithos 125, 393-404] reported trace element data for minerals from a garnet clinopyroxenite rock and used garnet-clinopyroxene partitioning coefficients to demonstrate an ultrahigh formation temperature of the rock, thus supporting the UHP hypothesis. However, the interpretations contained many unwarranted assumptions and were not consistent even with their own new data. This Comment addresses the shortcomings of the paper and demonstrates that (1) the authors’ temperature estimates are invalid, (2) their garnet compositional data are best explained by its formation by metamorphic recrystallisation after plagioclase. Therefore, an UHP igneous origin of the rocks is highly unlikely. Author Final Draft or ‘Post-Print’ Versionthe final version was subsequently published in Lithos copyright of Elsevier (2012) available online. Cite As: De Hoog, C-J 2012, 'Comments on Garnet-bearing ultramafic rocks from the Dominican Republic: Fossil mantle plume fragments in an ultra high pressure oceanic complex? by Gazel et al. [Lithos 125 (2011) 393-404]' Lithos, vol 134-135, pp. 330-334.

Aspectos petrográficos e petrológicos de um gnaisse calciossilicático (meta-rodingito) em alto grau na base da Klippe Carvalhos, Faixa Brasília, borda Sul do Cráton São Francisco, em Minas Gerais

Petrographic and petrologic features of a high-grade calcium-silicate gneiss (metarodingite?) at the base of Carvalhos Klippe, Brasília Belt, Southern border of the São Francisco Craton, Minas Gerais State. In the southern region of Minas Gerais State, in the southern border of the Neoproterozoic Tocantins Orogen, at São Francisco Craton south, several ultramafic bodies occur among oceanic metassediments, turbiditic and pelagic to hemipelagic, in medium to high-grade metamorphism and positioned along deep thrust faults associated with granulites, retroeclogites, and basement slivers. Among these ultramafic rocks of controversial origin and with a probable ophiolitic nature, the most important are the Morro do Corisco and Fazenda da Roseta, in the Liberdade region. The Fazenda da Roseta ultramafic body is made up by an association of meta-spinel harzburgite, meta-websterite, serpentinite carbonate, pegmatitic metabronzitite, and a garnet clinopyroxenite with a gneissic fabric, a possible meta-rodingite. This lithotype shows high density, ultramelanocratic character, banded granoblastic gneissic texture, which is constituted by mineral associations with varied proportions of garnet, clinopyroxene, epidote, and hornblende as the association of metamorphic peak, and plagioclase, epidote, ilmenite, magnetite, carbonate, which forms incomplete simplectitic reactions around garnet, among hornblende, with apatite and carbonate, the accessory minerals. Chemically, these rocks show a characteristic enrichment in CaO and major Na2O and SiO2 depletion when is compared with meta-mafic and meta-ultramafic rocks of the region. These rocks are positioned on the fields of meta-mafic rocks related to N-MORB and IAT. It has curved upward patterns for large-ion lithophile elements (LILE) and flat patterns for HFES and heavy rare earth elements (HREE), when compared with the abundance of these elements in N-MORB basaltic compositions. When this rock is analyzed together with the meta-ultramafic rocks of the region, it is possible to observe possible parental relations with meta-websterite, meta-harzburgite and meta-bronzitite, which show similar patterns of LILE, with negative Rb anomaly, and similar REE patterns, with enriched patterns related to chondrite and a Ce strong negative anomaly. From litho-textural and geochemical considerations together with the aloctone character of the associated ultramafics, retroeclogites and deep thrust faults, it is suggested that this (meta)ultramafic rock is a register of a calcium metassomatism (formation of rodingite), a probable meta-rodingite, in a probable oceanic environment, in a former episode to the process of subduction and collision, attributed to the closing of the Brasilíades Ocean, in a scenario of the Brazilian Orogen formation, synchronous to the formation of structural nappes.

Pyroxenites and megacrysts from Vitim picrite-basalts (Russia): Polybaric fractionation of rising melts in the mantle?

Picrite basalt tuffs and lavas from the Miocene basalt plateau of Vitim (Trans Baikal, Russia) contain abundant megacrysts and varied pyroxenite and mantle lherzolite xenoliths (spinel facies and upper part of the garnet-facies) and crustal cumulates. Black pyroxenites and megacrysts show decreasing temperatures from 1350 to 900 °C, and range from high-T dark green websterites and clinopyroxenites, to low-T black megacrystalline garnet clinopyroxenites and phlogopite–ilmenite-bearing varieties. Garnet-bearing Cr-diopside veins and zoned veins with mica and rare amphiboles cross-cut peridotite xenoliths. Veins consisting of almost pure amphibole are more common in spinel lherzolite xenoliths. P– T calculations for pyroxenites yield pressure intervals at 3.3–2.3, 2.2–2.0, 1.9–1.5 and 1.3–1.0 GPa, probably corresponding to the locations of dense magmatic vein networks in mantle. Major and trace elements for clinopyroxenes from the black megacrystalline series can be modeled by fractional crystallization of a picrite-basalt melt. In contrast, green high-temperature pyroxenites and black giant-grained garnet pyroxenites with lower LREE-enrichment and variable LILE and HFSE concentrations probably result from AFC processes and mixing with partial melts derived from older pyroxenites and peridotites. Gray low-Cr garnet clinopyroxenites with highly fractionated and inflected trace element patterns may have been formed by remelting of metasomatic veins within peridotites. Multistage melting of metasomatic assemblages with selective removal of clinopyroxene in vein contacts produce the REE patterns with low MREE concentrations and usually with elevated HFSE contents. Cr-diopside veins were most probably formed by partial melting of phlogopite- and/or amphibole-bearing lherzolites. The trace element and Sr–Nd isotopic features of the megacrystalline pyroxenites suggest that they crystallized from magma volumes that evolved in separate systems during formation of pre-eruption vein networks and magma chambers, which together formed the feeding system for the host basalts.

Petrology and geochemistry of UHP-metamorphosed ultramafic–mafic rocks from the main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH), China: Fluid/melt-rock interaction: Mafic–ultramafic complex from CCSD-MH

Mafic–ultramafic rocks recovered between 600 and 690 m in the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) Project in the Sulu terrane consist of garnet peridotite, garnet pyroxenite, garnet clinopyroxenite and eclogite. All of the rocks have undergone ultrahigh pressure (UHP) metamorphism with peak conditions of at least ∼750–850 °C/3.5–4.0 (GPa). The protoliths were mainly lherzolite, websterite, minor clinopyroxenite and harzburgite, with major and trace element compositions characteristic of crustal layered intrusions. The low Mg# of the whole rocks and mafic minerals, layered textures, and adjacent lithology pairs of garnet peridotite - garnet pyroxenite and garnet pyroxenite–eclogite pair probably reflect fractional crystallization of the protolith magma. Whole-rock MgO and FeO abundances decrease and Al 2O 3, CaO and SiO 2 increase toward the mafic endmembers. Extensive metasomatism and metamorphism influenced REE and trace element patterns. At least three episodes of fluid/melt metasomatism are recognized: a partial melting event that depleted K and Rb during the UHP metamorphism; a retrograde metasomatism that occurred during exhumation, leading to the formation of serpentine + phlogopite veins; and a late subgreenschistfacies hydrothermal event that formed carbonates and calc–silicate minerals. The protoliths of the garnet-bearing mafic–ultramafic rocks of the CCSD-MH are interpreted as crustal cumulates, which were subducted during collision between the Yangtze and Sino-Korean Cratons in the Triassic. During subduction, the original ultramafic rocks of the CCSD-MH were transformed to garnet peridotites and pyroxenites.

Intra-oceanic island arc origin for Iratsu eclogites of the Sanbagawa belt, central Shikoku, southwest Japan

New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O 2 and H 2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative ε Nd(t) value (~−5)), consistent with their origin as continent-derived trench-fill turbidites.

Rheological contrast between garnet and clinopyroxene in the mantle wedge: An example from Higashi-akaishi peridotite mass, SW Japan

Garnet clinopyroxenites occur within foliated dunite in the Higashi-akaishi peridotite mass, located within the subduction-type high-pressure/low-temperature Sanbagawa metamorphic belt. The garnet clinopyroxenites contain 3–80% garnet, and garnet and clinopyroxene are homogeneously distributed. Garnet crystals contain extensive, regular dislocation arrays and dislocation networks, suggesting that dislocation creep was the dominant deformation mechanism. Analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene, regardless of modal composition, indicating that these minerals deformed with similar degree of plasticity. Moreover, indexes of crystallographic fabric intensity (i.e., J-index and M-index) for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet. Fourier-transform infrared spectroscopy analysis revealed that water content in garnet is ∼60 ppm, whereas that in clinopyroxene is ∼70 ppm. Olivine crystal-preferred orientations in the Higashi-akaishi peridotite mass, characterized by [0 0 1] (0 1 0), are thought to have developed during deformation under wet conditions. Consequently, we argue that the presence of water could act to enhance garnet plasticity during deformation. The results reveal contrasting influences of water on the deformation of garnet and diopside: under wet conditions compared with dry, the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside. Given the influence of water on the creep strength of garnet, garnet within the Higashi-akaishi mass may have become significantly as weak as clinopyroxene during deformation.

An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and garnet inclusions in clinopyroxene

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [\( 1\bar{1}00 \)]Ilm//[\( 0\bar{1}0 \)]Cpx (0001)Ilm//(100)Cpx; and [110]Spl//[\( 0\bar{1}0 \)]Cpx (\( \bar{1}11 \))Spl//(100)Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.