Ultramafic Granulites Research Articles

Petrology and geochemistry of metamorphosed rocks associated with iron formations of the Toko‐Nlokeng iron deposit, (Southern Cameroon): Implications for geodynamic evolution and mineralization

Detailed petrographic, lithostratigraphic, and geochemical data of metamorphosed orthogneisses and mafic‐ultramafic metavolcanic rocks intruded into the greenstone belt of the Toko‐Nlokeng iron deposit have enabled the reconstruction of the tectonic and geodynamic setting and crustal evolution of the Nyong Complex. Samples were collected from the drill holes TNF11_01 and TNF11_02 from 17.16 m to 335.85 m depth. The lithostratigraphy supported by field and petrographic observations outlines two main lithologies: Iron formations (IFs) and metamorphosed host rocks. The IFs (granular iron formations (GIF) and banded iron formations (BIF)) are intercalated with host rocks consisting of orthogneisses and mafic‐ultramafic rocks. New and published geochemical data of metamorphosed associated IFs from Anyouzok (TNF08 prospect) of the Toko‐Nlokeng iron deposit, Mewengo iron deposit, Bipindi, and Kribi metavolcanic rocks in the Nyong Complex, suggest that these rocks were formed by material from intrusive and extrusive magmatic episodes with mid‐ocean ridge basalts (MORB) contaminated by either subduction or crustal components. CaO/Al2O3 and (Gd/Yb)N ratios >1 and positive Nb anomalies in ultramafic rocks indicate a mantle plume source contaminated by metasomatized subduction mantle lithosphere. These data also show that the mafic‐ultramafic metavolcanic rocks derive from magma of basaltic and basaltic andesite compositions, with a tholeiitic to calc‐alkaline tendency characteristic of the upper mantle. These data reveal that gneisses derive from granite and diorite subalkaline, peraluminous, and ferroan to magnesian compatible with Cordilleran magmas and island arcs with polygenetic crustal signatures. All samples of mafic granulites, garnet‐amphibolite, metabasites, and ultramafic granulites in the Nyong Complex reveal no residual garnet and predominantly show ca. 4% partial melting of an amphibole‐spinel‐peridotite source in the Dy/Yb versus La/Yb. Otherwise, hornblendites of Toko‐Nlokeng display ca. 5% partial melting of an amphibole‐garnet‐peridotite source, average (Gd/Yb)CN ratio of the hornblendites is (Gd/Yb)CN = 7.56 > 2. This result suggests these hornblendites do not have the same magmatic source as other mafic‐ultramafic rocks of the Nyong Complex. Mafic‐ultramafic host rock protoliths are classified as E‐MORB, P‐MORB, and G‐MORB (hornblendites) compositional types and arc, back‐arc ‘B’ in subduction unrelated, rifted margin setting with minor crustal contamination. The tholeiitic to calc‐alkaline and peraluminous affinity of these rocks indicate a mature arc and thickened crust during the Eburnean Trans‐Amazonian orogenic belts of the Congo Craton. This study shows that the host rocks were emplaced in a convergent tectonic setting and affected by a syn‐collisional episode where melts were derived from the partial melting of thick basaltic crust into amphibolite–eclogite facies in the subduction zone. The geochemical signatures of the mafic‐ultramafic rocks support the tectonic accretionary of the Palaeoproterozoic plume arc in Nyong Complex. Furthermore, the resulting hydrothermal alteration process was accompanied by an increase in Fe; particularly under conditions dominated by seawater, the origin and source of iron and silica in the Toko‐Nlokeng deposit.

Abiotic passive nitrogen and methane enrichment during exhumation of subducted rocks: Primary multiphase fluid inclusions in high‐pressure rocks from the Cabo Ortegal Complex, NW Spain

AbstractPrimary multiphase fluid inclusions (MFI) were studied in one eclogite and two granulites from the Cabo Ortegal Complex (COC, NW Spain) by means of Raman imaging, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM‐EDS) and Focused Ion Beam ‐ Scanning Electron Microscopy (FIB)‐SEM. Complementary, secondary MFI in pyroxenites from COC were also investigated. MFI hosted in eclogite and granulites occur along growth zones or in 3D clusters in garnet porphyroblasts suggesting a primary origin at high‐pressure (HP) metamorphic conditions. The mineral assemblage of MFI is mainly composed of Fe‐Mg‐Ca‐carbonates and phyllosilicates ± graphite ± quartz ± corundum ± pyrite ± apatite ± rutile and a fluid phase composed of nitrogen ± methane ± carbon‐dioxide. The mineral proportions vary among the lithologies. Dominant carbonates and hydrous silicates are interpreted as step‐daughter minerals (crystals formed in the MFI after entrapment as a result of fluid–host interaction), whereas apatite, quartz and rutile are considered in part as accidentally trapped minerals since they also occur as crystal inclusions together with MFI in each rock type. Quartz and corundum occur together in MFI in ultramafic granulite and are regarded as step‐daughter minerals in this lithology. These observations suggest that the MFI are products of post‐entrapment reactions of a homogeneous COHN fluid system with the host mineral. Thermodynamic calculations in the CaFMAS‐COHN system confirmed that bulk composition of the MFI in eclogite is similar to the host garnet+COHN composition except for a potential lost of H2O. Carbonation and hydration reaction between the host (i.e. garnet or pyroxene) and the fluid inclusion results in the consumption of all CO2 and part of the H2O from the fluid phase producing Ca‐Fe‐Mg‐carbonates and hydrous step‐daughter minerals, mostly pyrophyllite and chlorite. Nitrogen content of the originally trapped COHN fluid in eclogite was estimated to have a maximum value of 10 mol% at peak HP conditions and 30–40 mol% at retrograde conditions that is within the range of the observed MFI in the residual fluid (13–68 mol%). Pseudosection modelling confirmed the stability of the phase assemblage in the MFI in a specific low‐pressure, low‐temperature stability field (between 300°C and 400°C at pressures < 1 GPa), caused by H2O‐ and CO2‐consuming reactions possibly in a single step. Our findings indicate that such processes in the exhuming HP units may play a role in global nitrogen and carbon cycling as well as potentially contributing to nitrogen and methane supply to subsurface–surface environments during devolatilization in the forearc regions of convergent plate margins.

Eoarchean rock association in the Dniester-Bouh Domain of the Ukrainian Shield: A suite of LILE-depleted enderbites and mafic granulites

We present the results of a study of an Eoarchean rock assemblage in the Dniester-Bouh Domain of the Ukrainian Shield. This comprises granulite-facies granitoids intercalated with mafic and ultramafic granulites. Zircon U-Pb geochronology indicates enderbite crystallisation at 3786 ± 32 Ma, followed by a subsequent event at ca. 3500 Ma. Several events can be tentatively identified that affected these rocks between ca. 3000 and 2700 Ma. The last zircon growth event took place in response to granulite facies metamorphism and included two separate episodes at ca. 2000 and ca. 1900 Ma. The oldest two zircon populations in enderbites have εHf values around 0, indicating their crystallisation from a protolith with a short crustal residence time. Zircons that crystallised during the 3000–2700 Ma event(s) vary in Hf isotope systematics from εHf ~ 1 at ca. 3000 Ma to εHf ~ −14 at c. 2700 Ma. Paleoproterozoic zircons reveal even more significant variations in εHf value from +6 to –22. Such variations are indicative of juvenile input and mixing with old non-radiogenic Hf.All Eoarchean rocks are depleted in incompatible trace elements and have negative Ta-Nb, P, and Ti anomalies. Compared to the typical TTG associations, enderbites record depletion in felsic components (SiO2, Na2O, K2O, Rb, Th), and enrichment in mafic ones (TiO2, MgO, CaO, V), allowing them to be defined as “mafic” or “depleted” TTG.Geochemical data indicate that mafic and ultramafic rocks of the Dniester-Bouh Domain formed by shallow high-degree melting of the mantle, with the absence of garnet in their source, and the presence of residual Ti-bearing minerals and/or amphibole. In contrast, enderbites were formed from a mixed garnet-bearing amphibolite – eclogite source, i.e. melting over a range of pressures/depths. Our preferred model for the formation of the Eoarchean rock association involves the shallow melting of mantle and formation of basalts and accompanying ultramafic cumulates at a spreading centre, with subsequent underthrusting of one segment of oceanic crust beneath the other, and partial melting of hydrated metamorphosed (eclogitized) mafic rocks in the underthrust plate, leading to the formation of the TTG melts.

The U-Pb Zircon Geochronology (LA-ICP-MS) of Geological Processes in Granulites of Middle Bouh Area. Article 2. Rock association in the Chausove open pits

The LA-ICP-MS method was applied to investigate U-Pb and Lu-Hf isotope systematics of zircon crystals from charnockitoids that crop out in the open pits near Chausove village, Pervomaisk district. The association of enderbite gneisses, mafic and ultramafic (pyroxenite) granulites and vein bodies of pink biotite granites occur in the open pits. Enderbites are rather heterogeneous, and their composition varies from leucocratic quartzite-like rocks to enderbite-diorite. The Pervomaisk open pit is located within charnockite massif. Our data indicates that the association of enderbites and charnockites crystallised at 2850-2760 Ma from juvenile material (Hf isotope composition in zircons from enderbite-diorite is 176Hf/177Hf = 0.280915 ± 12, εHf2850 = +2; from leucocratic charnockite 176Hf/177Hf = 0.280875 ± 12, εHf2760 = –1; and from charnockite 176Hf/177Hf = 0.280875 ± 12, εHf2760 = –1). These rocks represented the newly formed Meso-Neoarchean crust, in contrast to the reworked older continental crust. This association has experienced a significant transformation in Paleoproterozoic. The last stage of transformation was accompanied by crystallisation of a new generation of zircon and formation of monazite at 2.04-2.06 Ga.

Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica: Constraints from melt inclusions and thermodynamic modeling

Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica: Constraints from melt inclusions and thermodynamic modeling

Ultramafic to mafic granulites from the Larsemann Hills, East Antarctica: Geochemistry and tectonic implications

The Larsemann Hills area is part of a reworked early Neoproterozoic metamorphic terrain in southwestern Prydz Bay, East Antarctica. Ultramafic and mafic granulites, whose origins remain controversial, occur as lenses, boudins or layered bodies within the para- and ortho-gneiss in the region. The ultramafic and mafic granulites show spinel-olivine-bearing and two-pyroxene-bearing mineral assemblages recrystallized at 860–900°C. Their bulk rock analyses indicate an origin as igneous cumulates, with high Mg# (molar MgO/(MgO+FeO)) from 0.73 to 0.84 for ultramafic granulite and from 0.46 to 0.78 for mostly mafic granulite as well as high Cr and Ni contents ([Cr] and [Ni] up to 1826ppm and 1400ppm respectively for ultramafic granulite and [Cr] of 1460ppm for mafic granulite). Trace element patterns show pronounced negative Nb anomalies, suggesting a subduction-related tectonic setting for their precursors, consistent with derivation from arc basalts, also suggested by low TiO2 and K-enrichment in mafic granulites. The ultramafic to mafic granulites may have been formed in a subduction-related continental back-arc basin environment simultaneously with peak metamorphism associated with arc-continent collision during the early Neoproterozoic (990–900Ma) Rayner orogeny.

Metamorphic evolution of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa

A large number of published accounts are available on the metamorphic evolution of the Central Zone of the Limpopo Belt. However, most investigations on the P-T evolution of this zone are restricted to pelitic and calc-silicate rocks. In this paper we present for the first time a detailed mineralogical and petrological investigation on an ultramafic granulite of komatiitic composition, which occurs in the vicinity of Musina in the Central Zone. These rocks are composed mainly of magnesium-rich Ca-amphibole (pargasite: MgO up to 18 wt%), together with Mg-rich olivine (X Mg = 0.82 to 0.83) and orthopyroxene (X Mg = 0.82 to 0.84), Cr-rich spinel, magnetite and some sulfide minerals. This set of minerals represents the peak metamorphic assemblage formed during the early stage of the metamorphic evolution of the Central Zone and is different from the mineral assemblage of less altered komatiites. This assemblage was subjected to minor retrogression and alteration to chlorite and serpentine during the waning stage of the metamorphic evolution of the Central Zone. Thermodynamic calculations combining classical conventional methods as well as equilibrium assemblage modelling using Theriak-Domino, in conjunction with textural relationships and mineral chemistry, indicate that the studied rocks went through granulite facies metamorphic conditions at a temperature of up to 820°C and an intermediate pressure of up to 8.5 kbar. These P-T conditions are in good agreement with previous investigations of metamorphic conditions on other rock types (mainly metapelites) in the Central Zone of the Limpopo Belt.

Tectonic juxtaposition of crust and continental growth during orogenesis: Example from the Rengali Province, eastern India

The southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the E–W trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S1M) foliation that formed during D1M deformation. This rock, along with the host charnockite, were intensely deformed by the D2M thrusting event and resulting S2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclase-orthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10−12 kbar, 860 °C) and was overprinted by a fine-grained assemblage of clinopyroxene-plagioclase ± hornblende that developed during decompression (down to 5.5–7.5 kbar). Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at ∼6.0 kbar, 730 °C which resulted from heating of the mid crust magma during the D2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower- and mid-crustal section during the D2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.

Partial melting of the Archaean Thrym Complex of southeastern Greenland

High-grade Archaean rocks of the remote Thrym Complex of southeastern Greenland expose a lower crustal section that formed at a depth of >30km at temperatures reaching 1000°C and pressures approaching 1.5GPa. The complex is part of the North Atlantic Craton, which is bound to the north by the Trans Hudson–Nagssugtoqidian–Ammassalik Orogen and to the south by the Makkovik–Ketilidian Orogen. The rocks in the complex include orthogneiss, migmatitic orthogneiss with banded or nebulitic fabrics, migmatite with a breccia-like structure (agmatite), and narrow bands of lensoidal mafic and ultramafic rocks (granulites) with associated widespread garnet-bearing mafic granulite, and a minor proportion of garnet–cordierite–sillimanite–biotite gneiss. The latter high-grade rocks form part of the substrate for the emplacement of the protoliths for orthogneiss that was synchronous with the ca. 2790–2700Ma Skjoldungen Orogeny.At least two suites have been recognised in the Thrym Complex based on geochemistry. The oldest is a tholeiitic suite of mafic to ultramafic granulites, and the other is a calc-alkaline suite that includes felsic to intermediate orthogneiss. Most of the orthogneiss has a granodioritic (oldest) to quartz monzodioritic (younger) composition with rare monzogranitic, tonalitic, monzonitic and quartz dioritic protoliths. Historically, the orthogneiss units have been called TTGs, although monzogranite and tonalite are rare, and no trondhjemite is observed. In addition, they could also be called adakitic on geochemical grounds. Instead of using this terminology, which has genetic connotations, the orthogneiss is referred to as predominantly granodioritic to quartz-monzodioritic.Assuming that modern tectonics operated during the Neoarchaean, at least some of the mafic and ultramafic granulites and orthogneiss exposed in the Thrym Complex represent the root zone of a magmatic-arc that might have been relaminated around 2760Ma during the collisional Skjoldungen Orogeny.

Geochemistry of mafic to ultramafic granulites from the Larsemann Hills, east Antarctica

Geochemistry of mafic to ultramafic granulites from the Larsemann Hills, east Antarctica

Crustal development at the pre-Grenvillian Laurentian margin: a record from contrasting geochemistry of mafic and ultramafic orthogneisses in the Chochocouane River area, Quebec

Mafic and ultramafic orthogneisses from the western Grenville Province of Quebec adjacent to the Archean Superior Province (Canada) display geochemical characteristics that highlight processes that shaped the pre-Grenvillian Laurentian margin through late Archean to Mesoproterozoic time. Group I mafic and ultramafic granulites of the Parautochthonous Belt compositions overlap Archean Abitibi metavolcanic rocks (komatiites to Fe-rich andesites) supporting their autochthonous character. Limited variation of incompatible trace-element ratios with increasing differentiation suggests minimal sialic crust assimilation during petrogenesis and probable formation in an oceanic setting. Group II mafic tholeiites occur within both the Parautochthonous Belt and the Allochthonous Polycyclic Belt. Their compositions differ significantly from those of Archean rocks but overlap those of Proterozoic and modern continental tholeiites. Group III mafic calc-alkaline amphibolites, predominantly in the Allochthonous Polycyclic Belt, display chemical characteristics expected of magma derivation from subducted oceanic crust. In contrast to group I Archean meta-igneous rocks metamorphosed during the Kenoran orogeny, group III rocks do not record an early phase of large-ion lithophile element (LILE) metasomatism and granulite-facies metamorphism. As such, their emplacement is interpreted to postdate the Archean Kenoran metamorphism and to be Proterozoic in age. Group I rocks reflect an Archaean ensimatic greenstone belt environment; group II tholeiites reflect a Proterozoic continental rift environment; and group III rocks reflect a subduction-related source, possibly related to Andean-style, calc-alkaline magmatism.

High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica

Pargasites (F/(F+Cl+OH) ratio ( X F) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100 °C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free pargasite is below 1050 °C, thermodynamic calculations for the present pargasite+quartz assemblage indicate that the thermal stability limit of pargasite with X F=0.5 is about 150 °C higher than that of the hydroxyl end member. Fluorine substitution in the pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island. A positive correlation between the F content of pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F–OH distribution. Although the fluorine composition of pargasites ( X F=0.12–0.48) and bulk rock (300–2500 ppm) varies widely, the log( f H 2O / f HF) values calculated for these rocks are relatively constant (3.2–3.7), which is consistent with infiltration of an F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high- X Mg pargasite in MgO-rich rocks.

Timing of magmatic and metamorphic events in the Jijal complex of the Kohistan arc deduced from Sm-Nd dating of mafic granulites

Abstract Mafic to ultramafic granulites in the northeastern part of the Jijal complex include two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite. Field and textural relations indicate that two-pyroxene granulite is a relict left after formation of the garnet-clinopyroxene granulite and garnet hornblendite was an originally intrusive rock which dissected the protoliths of mafic granulites. Sm-Nd mineral isochron ages of 118 ± 12 Ma, 94.0 ± 4.7 Ma and 83 ± 10 Ma were determined for two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite respectively. These ages, together with previously reported chronological data, led to the following tectonic implications: (1) crystallization of the granulite protoliths predates, or is coeval with, the tectonic accretion of the Kohistan arc to the Asian continent; (2) crustal thickening related to the accretion was probably responsible for the high-pressure granulite-facies metamorphism in the Jijal complex; (3) formation of the garnet hornblendite assemblage was probably after crystallization of garnet-clinopyroxene granulite.

The high-pressure granulites of the Bacariza Formation: an earlier stage in the exhumation of other eclogites in the Cabo Ortegal Complex (Hecynian belt, NW Spain)

This paper deals with the metamorphic evolution of the Bacariza Fm that outcrops in the two uppermost structural units of the Cabo Ortegal Complex (NW Iberian Massif). This formation includes ultramafic and mafic granulites, garnet amphibolites and garnet trondhjemitic gneisses. Although mineral associations characteristic of high pressure granulites predominate in the least retrogressed of these rocks, the presence of relic kyanite along with the fact that plagioclase only appears in symplectitic textures resulting from de-jadeitization of pyroxenes point to an earlier eclogite facies metamorphism. Thermobarometric estimations indicate higher P-T conditions for the rocks in the uppermost structural unit.

Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H 2O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H 2O=hornblende+quartz and plagioclase+hornblende+H 2O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.

Pan-African high pressure isobaric cooling: evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru Mountains, eastern Tanzania

Granulite- and amphibolite-facies rocks in the Uluguru Mountains of the polyorogenic Mozambique Belt are composed of basic granulites, meta-anorthosites, paragneisses and marbles as major lithotypes. Both granulite-facies metamorphism and exhumation took place during the Pan-African Orogeny, as indicated by the published UPb and RbSr data. In this work minerals of the garnet + clinopyroxene + ilmenite + plagioclase + hornblende ± orthopyroxene ± quartz bearing ultramafic granulites, meta-anorthosites, meta-anorthositic granulites, garnet pyroxene granulites and amphibolites from the belt were analysed for thermobarometry. Pressures and temperatures obtained by several calibrations, using suitable mineral parageneses, give temperatures ranging from ∼800 to ∼900°C and pressures from ∼9 kbar to ∼12 kbar. Rim compositions of analysed minerals consistently give temperatures and pressures of ∼800°C and ∼9–10 kbar, which are comparable to those obtained from the Furua Granulite Complex 200 km south of the study area. Mineral cores give higher PT values. Thus, core-rim relationships record a nearly isobaric cooling path. Meta-anorthosites give rim temperatures and pressures comparable to garnetiferous granulites, but their cores give temperatures of up to ∼950°C and pressures from ∼13 to ∼15 kbar. The highest temperatures of ∼1100°C and pressures of ∼15 to ∼17 kbar were obtained from mineral cores of the ultramafic granulites. The obtained PT data imply a minimum depth of about 35 km for the formation of these granulite-facies rocks. The preservation of the zoning and exsolution lamellae in some minerals considered to result from a relatively rapid exhumation of the granulite-facies rocks in the Uluguru Mountains soon after isobaric cooling in the lower crust.

Petrogenesis of a high‐temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic‐ultramafic blocks, and abundant Jurassic‐Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high‐pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene‐bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near‐isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase‐amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite‐amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km‐1 at intermediate to low pressures, and may represent the Sino‐Korean hangingwall during Triassic subduction for formation of the ultrahigh‐ and high‐P units to the south. Post‐collisional intrusion of a mafic‐ultramafic cumulate complex occurred due to breakoff of the subducting slab.

Characterization of fluids associated with gold mineralization and with regional high-temperature retrogression of granulites in the Limpopo belt, South Africa

The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal craton and the granulite terrane of the Limpopo belt. It dips steeply northward and flattens to the south and was responsible for allowing hot granulites to spread up and over the cooler greenstone terrane. This shear zone acted as a conduit for deep-seated infiltrating fluids into still hot granulites in the hangingwall producing a regional retrograde orthoamphibole isograd and an associated zone of rehydrated granulites. Thermodynamic constraints on the composition of fluids involved in retrograde anthophyllite formation in pelitic granulites on the isograd, supported by the results of a fluid inclusion study and by direct textural evidence in which olivine in closely associated ultramafic granulites has been partially replaced by hypersthene and magnesite, suggest that the hydrating fluid contained at least 70 mol% CO 2 and that this fluid infiltrated at P-Tconditions of about 620°C and 6 kbar pressure. The δS 13 C values of CO 2 extracted from magnesite vary between −5.5 and 6.0%. and suggests a deep-seated (possibly mantle) source for the CO 2-rich retrograde fluid. Fluid inclusion studies from two lode-gold deposits (Osprey and Louis Moore) in the same high-grade area, and three deposits from the immediately adjacent Sutherland greenstone belt (Fumani, Klein Letaba, and Birthday) at the northern edge of the Kaapvaal Craton, show that CO 2-rich fluids with similar characteristics were also associated with epigenetic gold mineralization. All of the ore bodies are located within EW-trending northward dipping, ductile shear zones with an oblique to reverse sense of movement. These relationships suggest that there is a direct link between the source of fluids involved in gold mineralization, the fluids that formed the zone of rehydration in the Southern Marginal Zone of the Limpopo belt, and southward verging faulting during the exhumation of the granulite terrane at about 2670 Ma.

High-temperature hydration of ultramafic granulites from the Southern Marginal Zone of the Limpopo Belt by infiltration of CO 2-rich fluid

The peak metamorphic assemblage of olivine + orthopyroxene + calcic amphibole + spinel is replaced by a variety of high-temperature retrograde assemblages in ultramafic granulites from the Southern Marginal Zone of the Limpopo Belt. The observed replacement of olivine according to the reaction forsterite + CO 2 = enstatite + magnesite is restricted to the zone of rehydration which bounds the granulite zone in the south. This observation is consistent with the post-peak metamorphic infiltration of an externally derived CO 2-rich fluid during the high-temperature post-peak metamorphic history of the Limpopo Belt. This event established a retrograde orthoamphibole isograd. Conditions of the retrogression are estimated to be near 670°C, 6 kbar, and X CO 2 (fluid) ⩾ 0.67. C-isotopic compositions of CO 2 extracted from magnesite produced by the high-temperature breakdown of olivine supports a deep-seated (mantle) source for the CO 2-rich retrograde fluid.

Geothermometry in the Kiglapait Aureole: Part II. Evaluation of Exchange Thermometry in a Well-Constrained Thermal Setting

Previous determination of a well-constrained thermal profile representing peak conditions of metamorphism for the contact aureole of the Kiglapait Intrusion, Labrador, provides the basis for critical comparative geothermometry of cation exchange thermometers applied to mafic and ultramafic granulites. Knowledge of the shape of the profile and constraints on the temperature of the intruding magma allow calculation of cooling rates which range from 150°C/m.y. at the contact to 30°C/m.y. at 2500 m from the contact. Substitution of these rates in diffusion equations permits discrimination between geothermometers which are either adequately or poorly calibrated, as well as thermometers which are probably inappropriate for use in metamorphic terranes. Our results suggest that the two-pyroxene thermometer (Lindsley & Andersen, 1983) is accurate from 950 to 750°C. Below 750°C, both limbs of the miscibility gap require at least minor recalibration, or the projection scheme of Lindsley & Andersen (1983) requires adjustment. Results from the cpx-ilm thermometer are erratic and imply that the solution models for pyroxene and ilmenite, as presently formulated in this thermometer, are inadequate. Results from the opx-ilm, ol-ilm, and opx-ol thermometers are inconsistent. Theoretical calculations at both high and low temperatures show that these three thermometers cannot give internally consistent results. Results from Fe-Ti oxide thermometry suggest that these minerals are easily re-equilibrated due to cation interdiffusion between grains. However some grain pairs appear to retain compositions appropriate to peak thermal conditions.