Amphibolite-granulite Facies Transition Research Articles

Petrology of the meta-mafic rocks from the Lolodorf area, Nyong complex (Southwest Cameroon): implication for the origin and emplacement conditions

This study presents the new petrology and geochemistry data for the comprehension of the origin and emplacement conditions of the Lolodorf mafic rocks whitin the Nyong Complex. The meta-mafic rocks consist of fine-grained garnet meta-mafic (FGM) and coarse-grained garnet meta-mafic (CGM) rocks presenting nearly similar mineral composition. They are made up of quartz, plagioclase, garnet orthopyroxene, clinopyroxene, amphibole, opaques, apatite and rutile. They also present recrystallization features such as quartz ribbons, corroded grain mineral, cuspate plagioclase boundary and necklace garnets in the matrix. These mineral assemblages suggest that the recrystallization took place under granulite facies condition. The meta-mafic rocks have basaltic and tholeiitic composition like the magmas (SiO2 FGM = 48.20–50.70 wt% and SiO2CGM = 46.68–50.27 wt% and MgFGM = 43.24–62.27 and MgCGM = 40.85–69.70) emplaced in the active continental margin context. The low La/Th ratio observed in the FGM and the high La/Th ratio in the CGM together with low Hf content imply that the FGM are reworked plutonic rocks that recorded both high grade partial melting and metamorphism whereas CGM recorded only metamorphism imprints. Thermobarometry results suggest that the partial melting and metamorphic peak occurred around the amphibolite-granulite facies transition. The fine-grained garnet meta-mafic rocks emplaced under pressure estimated at around 2.7–14.89 kbar with temperature interval between 672 and 952 °C while the coarse-grained garnet meta-mafics settled at around 1.5–16.09 kbar and 631–909 °C. The selected amphiboles display low water content (1.92–2.10 wt%) and plot between NNO and NNO + 2 buffers indicating that they crystallized under high oxidizing conditions. These results coupled with previous geochronological study suggest that the FGM may have occurred at the Mesoarchean and have been reworked during Eburnean whereas the CGM may have been formed at the Eburnean.

Age, provenance and tectonic setting of the high-grade Jequitinhonha Complex, Araçuaí Orogen, eastern Brazil

ABSTRACT: The Jequitinhonha Complex of the northeastern Araçuaí orogen is an extensive sedimentary unit metamorphosed in the amphibolite-granulite facies transition around 580-545 Ma. The unit consists of Al-rich (kinzigitic) paragneisses with decametric intercalations of graphite gneisses and quartzites, and centimetric to metric lenses of calcsilicate rocks. A new detrital zircon U-Pb age spectrum is reported for a sample of quartzite, and whole-rock geochemical (major and trace elements, 9 samples) and Sm-Nd isotope data (10 samples) for Jequitinhonha Complex paragneiss. Together with published data these show that: (1) the geochemistry of paragneiss samples of the Jequitinhonha Complex are similar to those of passive margin sedimentary protoliths; (2) detrital zircon data yield U-Pb age populations between ca. 0.9 and 2.5 Ga; and (3) Sm-Nd TDM model ages range from 1.6 to 1.8 Ga and εNd(575 Ma) around -7.5. The data reveal a mixture of Cryogenian to Mesoproterozoic rift-related igneous rocks with the Palaeoproterozoic-Archaean basement rocks of the São Francisco-Congo palaeocontinent as the main source areas, and also support the correlation between the Jequitinhonha Complex and the passive margin units of the upper Macaúbas Group, constituting the precursor basin of the orogen. Our results, with the absence of ophiolites in the Jequitinhonha Complex, reinforce the interpretation that the São Francisco-Congo palaeocontinent was not divided to the north of the focused region, suggesting an ensialic termination of a gulf during the Neoproterozoic.

Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: Interplay of coeval contractional deformation and extensional gravitational collapse

We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25–2.00Ga) West African Craton in northern Ghana, based on the interpretation of field observations and airborne geophysical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and low-grade volcano–sedimentary belts, separated by shear zones and assembled during the Paleoproterozoic Eburnean orogeny (2.15–2.07Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite–granulite facies transition, during a (D1) deformation event, consistent with N–S horizontal shortening, which generated reverse shear zones. High and low metamorphic grade rocks were brought in contact along extensional shear zones formed during N–S extension (D2). These structures were overprinted by contractional deformation associated with E–W shortening and N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolution (D4–D7) is characterised by strain localisation in successive generations of narrow shear zones, and the re-activation of inherited structures in a dominantly transcurrent regime. U–Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the collision of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics.

Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton

AbstractNew petrological and geochronological data are presented on high‐grade ortho‐ and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25–2.00 Ga) West African Craton. The study area is located in the interference zone between N–S and NE–SW‐trending craton‐scale shear zones, formed during the Eburnean orogeny (2.15–2.00 Ga). High‐grade metamorphic domains are separated from low‐grade greenstone belts by high‐strain zones, including early thrusts, extensional detachments and late‐stage strike‐slip shear zones. Paragneisses sporadically preserve high‐pressure, low‐temperature (HP–LT) relicts, formed at the transition between the blueschist facies and the epidote–amphibolite sub‐facies (10.0–14.0 kbar, 520–600 °C), and represent a low (~15 °C km−1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite–granulite facies transition. They reveal a clockwise pressure–temperature–time (P–T–t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0–8.0 kbar, which fit a 30 °C km−1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterized by peak metamorphic conditions of 7.0–10.0 kbar at 550–680 °C, which match a 20–25 °C km−1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in‐situ U–Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono‐metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

Geochemical evolution of rocks during ultrametamorphism in the amphibolite-granulite facies transition zone, central part of the Aldan shield

Geochemical evolution of rocks during ultrametamorphism in the amphibolite-granulite facies transition zone, central part of the Aldan shield

Multistage metamorphic evolution and nature of the amphibolite–granulite facies transition in Lofoten–Vesterålen, Norway, revealed by U–Pb in accessory minerals

The nature of the amphibolite- to granulite-facies transition zone exposed in the Lofoten–Vesterålen islands of northern Norway, and the relationships between high-grade metamorphism and the genesis of an anorthosite, mangerite, charnockite and granite suite (AMCG) are studied with the help of ID-TIMS U–Pb dating of zircon, monazite and titanite in polyorogenic gneisses. The western part of the region is dominated by granulite facies gneisses intruded by a 1800–1790 Ma AMCG suite and characterized by a strong magnetic and gravity anomaly. To the east this zone borders a crustal section with amphibolite facies gneisses, migmatites and granites. The transition has been considered to reflect the amphibolite to granulite facies metamorphic gradient of a Palaeoproterozoic orogenic event. The results of this study show that the region underwent multiple high-grade events, one in the late Archean at ca. 2640 Ma, one in the period slightly before (< 10 m.y.) the main pulse of AMCG magmatism at 1800–1790 Ma, and another during the main AMCG event, with less distinct evidence pointing to an earlier episode sometime between 1880–1810 Ma. The major Archean and Proterozoic events are recorded on both sides of the metamorphic transition, thus supporting an affinity of these crustal segments. In detail adjacent parts across the metamorphic transition give distinct results. For example at Sigerfjord, a 1800 Ma granulite is matched with Archean migmatites, the latter showing little evidence of having undergone amphibolite facies metamorphism at 1800 Ma. Paired with differences in the timing of Caledonian events these relationships suggest that the transition may be a tectonic boundary. The successive metamorphic events left a very unequal record in various rocks that can be explained by the role played by compositional factors, presence of fluids, specific mineral reactions and as an integration of these factors the ability of each rock to make, modify and preserve zircon, monazite or titanite.

Magnetic anomalies and metamorphic boundaries in the southern Nagssugtoqidian orogen, West Greenland

Within the southern Nagssugtoqidian orogen in West Greenland metamorphic terrains of both Archaean and Palaeoproterozoic ages occur with metamorphic grade varying from low amphibolite facies to granulite facies. The determination of the relative ages of the different metamorphic terrains is greatly aided by the intrusion of the 2 Ga Kangâmiut dyke swarm along a NNE trend. In Archaean areas dykes cross-cut gneiss structures, and the host gneisses are in amphibolite to granulite facies. Along Itilleq strong shearing in an E–W-oriented zone caused retrogression of surrounding gneisses to low amphibolite facies. Within this Itivdleq shear zone Kangâmiut dykes follow the E–W shear fabrics giving the impression that dykes were reoriented by the shearing. However, the dykes remain largely undeformed and unmetamorphosed, indicating that the shear zone was established prior to dyke emplacement and that the orientation of the dykes here was governed by the shear fabric. Metamorphism and deformation north of Itilleq involve both dykes and host gneisses, and the metamorphic grade is amphibolite facies increasing to granulite facies at the northern boundary of the southern Nagssugtoqidian orogen. Here a zone of strong deformation, the Ikertôq thrust zone, coincides roughly with the amphibolite–granulite facies transition. Total magnetic field intensity anomalies from aeromagnetic data coincide spectacularly with metamorphic boundaries and reflect changes in content of the magnetic minerals at facies transitions. Even the nature of facies transitions is apparent. Static metamorphic boundaries are gradual whereas dynamic boundaries along deformation zones are abrupt.

Metabasites along the amphibolite-granulite facies transition in the Okiep Copper District, South Africa

At least two types of metabasites occur along the granulite amphibolite facies transition in the Okiep Copper District in western Namaqualand. Low-pressure mafic granulites are mainly found in metagranitoids (Nababeep Gneiss) and supracrustal sequences (Khurisberg Subgroup) in the southern granulite facies Okiep domain; they include low-MgO as well as Mg-rich ol-normative types. Prograde amphibolites and rare relics of mafic granulite occur in orthogneisses of the −1.8 Ga Gladkop Suite and associated volcano-sedimentary rocks of the Lammerhoek Subgroup in the northern Steinkopf domain. All metabasites were metamorphosed during the late Narnaqua (1.02 to 1.03 Ga) high-grade event and are of orthomagmatic origin: precursors of mafic granulites were of subalkaline tholeiitic, while those of amphibolites of transitional alkali olivine basaltic to tholeiitic composition. MORB-normalised spidergrams and REE patterns are vely similar for the two groups and indicate prominent large ion lithophile (LIL) element and LREE enrichment compared to MORB ((La/Lu)N mafic granulites, 1.2–16.0; amphibolites, 0.6–5.1). Geochemical differences were, however, confirmed for LIL elements; the lower and more erratic concentrations of K2O, Na2O, Rb, and Pb in the mafic granulites may be the result of mobile behaviour of LIL elements during high-grade metamorphism. However, observed increased LREE concentrations cannot be explained with post-magmatic mobilisation alone. REE modelling demonstrates that some REE patterns of mafic granulites may be explained with about 20 % of partial (batch) melting of a fertile mantle source with subsequent variable degrees of fractional crystallisation of clinopyroxene, olivine, plagioclase ± hornblende; higher melting rates up to 40 % need to be assumed for successful modelling of the Mg-rich granulite. 10–15 % partial melting of fertile mantle can reproduce the most primitive amphibolite REE patterns. However, partial melting of a primitive mantle source and subsequent fractionation could not reproduce LREE-rich patterns documented for most amphibolites; an enriched mantle source is indicated. Geochemical data exclude an intra-plate or a depleted MORB-type mantle source for the various types of mafic granulites as well as for amphibolites but do not allow discrimination between extensional (e.g. continental flood basalt) and subduction-related (e.g. arc type) geotectonic settings. Our data establish strong geochemical similarities to older 1.65 Ga amphibolites from central Bushmanland.

Seismic properties of rock samples from the Pikwitonei granulite belt – God's Lake domain crustal cross section, Manitoba

Laboratory measurements of compressional and shear wave velocity to confining pressures of 600 MPa for a suite of representative samples collected from the Pikwitonei granulite belt and God's Lake domain, an Archean crustal cross section in the northwestern Superior Province, provide the basis of comparison of these terranes with the seismic characteristics of Archean lower crust. We found that felsic rocks in the Pikwitonei granulite belt and God's Lake domain, which make up the bulk of these terranes, have a similar average compressional wave velocity of 6.5 km/s at 600 MPa, indicating that felsic rocks show little velocity change across the amphibolite–granulite facies transition. Compressional wave velocities for mafic rocks from each terrane are between 7.1 and 7.3 km/s. Apparent Poisson's ratio ranges from 0.24 to 0.26 and 0.26 to 0.28 for felsic and mafic rocks, respectively. These velocity data compare favorably with data for similar lithologies from the Kapuskasing uplift. Using the relative abundances of the constituent lithologies, the weighted average compressional wave velocities of the God's Lake domain and Pikwitonei granulite belt at 600 MPa are 6.56 and 6.63 km/s, respectively. These values, coupled with velocity distribution functions based on the population statistics and relative abundance for each lithology, show that there is no correspondence between the seismic characteristics of the Pikwitonei granulite belt and typical Archean and Proterozoic lower crust. The average properties of the Pikwitonei granulite belt and God's Lake domain, however, correspond well with typical Archean and Proterozoic middle crust. This suggests that either the Pikwitonei granulite belt represents an extreme felsic end member of Archean lower crust or that the deepest levels of the Superior Province crust are not exposed in the Pikwitonei granulite belt. Similar distribution function diagrams for acoustic impedance show that the Pikwitonei granulite belt is characterized by high acoustic impedance contrasts, but the high-impedance component is low in abundance. If the strong reflections observed under the Pikwitonei granulite belt in recent Lithoprobe surveys are not due to other causes, such as favorably oriented bodies of metamorphosed banded iron formation, diabase, or rock units not exposed in this region but present at depth, then they are caused by surprisingly small volumes of mafic metavolcanic rocks.

Redistribution of rare earth elements, thorium, and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism: The role of apatite and monazite in orthogneisses from southwestern Norway

The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE) 2 O 3 in hornblende and litanite + 3 ( LREE) 2 in allanite O 3 + 2 Ca 5 (PO 4 ) 3 apatite ( F,OH ) + 6 SiO 2 quartz ⇔ 6 ( LREE)P monazite O 4 + 2 Ca 2 (M-HREE) 3 (SiO 4 ) 3 ( F,OH) lessingite in apatite + 6 CaO in plagioclase . At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3 ( LREE)PO 4 monazite + 3 SiO 2 quartz + 4 CaO in plagioclase + (F 2 ,H 2 O) fluid ⇔ Ca 5 (PO 4 ) 3 (F,OH) + apatite Ca 2 (LREE) 3 (SiO 4 ) 3 (F,OH) less ingite in apatite . or the breakdown of the remaining allanite: 3 ( LREE) 2 O 3 in allanite + 6 SiO 2 quartz + 4 CaO in plagioclase + (F 2 ,H 2 O) fluid ⇔ 2 Ca 2 (LREE) 3 (SiO 4 ) 3 (F,OH) lessingite in apatite . The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.

Tectonic evolution of lower crustal rocks in an exposed magmatic arc section in the Hidaka metamorphic belt, Hokkaido, northern Japan

Abstract : The Hidaka metamorphic belt consists of an island‐arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D0rs, D1, D2rs, and D3, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D0) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D1 is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S‐type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D1 stage, mafic and intermediate magmas supplied and transported heat to form the arc‐type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D2 stage is characterized by subhorizontal top‐to‐the‐south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S‐type tonalite. Deformation structures and textures of high‐temperature mylonites formed along the décollement, as well as the duplex structures, show that the D2 stage movement occurred under a N‐S trending compressional tectonic regime. The depth of intra‐crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust‐mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral‐reverse‐slip (dextral transpression) movement accompanied by low‐temperature mylonitization with retrograde metamorphism, the stage defined as D3. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D3 stage. During D2 and D3 stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.

An infrared spectroscopic (IR) and light element (Li, Be, Na) study of cordierites from the Bamble Sector, south Norway

Infrared absorption spectra and light element contents of several natural cordierites across the Arendal amphibolite-granulite facies transition, Bamble Sector, south Norway are presented. The infrared spectra record H 2O (both type-I and type-II) and CO 2 as fluid constituents in the channels, CO and hydrocarbons were not detected. Type-II water and (Ca+Na+K) are correlated and show a 2:1 molar ratio. The infrared and light element data indicate a decrease of Na, type-II H 2O, CO 2 and Li with metamorphic grade, while type-I H 2O and Be are variable. The decrease of these volatiles and alkalies and the low total volatile contents of the granulite facies cordierites are best explained by progressive dehydration and decarbonation processes, possibly related to partial melting. A progressive change of the X H 2O = H 2O (H 2O+CO 2) ratio in the cordierites across the transition, as suggested by previous studies, is not observed. Greenschist to low-amphibolite facies re-equilibrated cordierites show a significant increase in either CO 2, or H 2O+CO 2. Na is introduced at some localities. The retrograde fluid phase is calculated to be CO 2-rich.

High- P—high- T anatexis of Archaean tonalitic grey gneisses: the Eseka migmatites, Cameroon

The Eseka migmatites provide a good example of high- P—high- T anatexis of Archaean quartz-plagioclase-biotite grey gneisses. Partial melting resulted in the development of neosomes comprising a quartzo-feldspathic fraction of granodioritic to trondhjemitic composition and ferromagnesian minerals represented by hornblende mainly (15–45 vol.%), garnet in lesser amounts (4–8 vol.%) and scarce clinopyroxene. Thermobarometry suggests that anatexis occurred close to the amphibolite-granulite facies transition, under conditions estimated at around 9 kbar and 750°C. A late retrogression is documented by the growth of epidote and chlorite and by plagioclase albitization. Textural relations, mineralogical data and recalculation of the primary mineral assemblages suggest that the biotite dehydration melting conditions were reached leading to the generation of granodioritic melts in equilibrium with K-feldspar, hornblende and garnet (±clinopyroxene). In some cases, this was followed, during melt crystallization, by a retrograde reaction involving the granodioritic melt and hornblende to produce biotite-epidote assemblages and trondhjemitic neosomes. Compared to similar migmatites, the Eseka neosomes are characterized by higher Al and Ti contents of hornblende, the presence of garnet and the lack of sphene which can be related to higher P, T conditions. Geochemical data point to the segregation and removal of some melt in the Eseka migmatites resulting in limited concentration of hornblende and garnet at the site of melting. This together with previous field observations and experimental data, could suggest that some hornblende-rich gneisses or amphibolites found in the lower middle crust may represent a residue left after extraction of anatectic melts. Geochronological and petrological data further suggest that large portions of the Archaean TTG rocks from the Congo craton have been reworked under high-grade conditions during the Paleoproterozoic.

Halogen geochemistry of fluid during amphibolite-granulite metamorphism as indicated by apatite and hydrous silicates in basic rocks from the Bamble Sector, South Norway

The halogen chemistry of apatites, biotites, amphiboles and titanites from metabasites in the Proterozoic Bamble Sector amphibolite-granulite facies transition zone (Norway) has been studied to gather data on the behaviour of halogens during original magmatism and subsequent metamorphism. Three groups of apatites could be recognized: (1) FOH-apatites with more than 50 mole % F and virtually no Cl. (2) FOH-apatites with up to 15 mole % of Cl. (3) Cl-rich apatites with up to 6.73 wt.% Cl. The latter are the most Cl-rich metamorphic apatites ever reported. The F-contents of coexisting biotite do not follow this division, but their Cl-contents do. Log( z.sfnc;HF z.sfnc;H 2O ) values for the coexisting fluid phase calculated from biotite are slightly lower than those calculated from apatite. Log( z.sfnc;HCl z.sfnc;H 2O ) values in the coexisting fluid phase calculated from biotite are substantially higher than those calculated from apatite. The log( z.sfnc;HF z.sfnc;H 2O ) reflects the composition of the peak metamorphic fluid phase. The apatites show three types of patterns of F-contents: (1) W-shaped, (2) Rimward increasing and (3) Rimward decreasing. Pattern (2) reflects progressive metamorphism. Pattern (3) is thought to reflect growth starting near peak metamorphism and going on during retrograde dilution of the peak metamorphic fluid by a newly introduced aqueous one. Some apatites show local resorption effects along cracks. No regional trend in calculated log( z.sfnc;HF z.sfnc;H 2O ) values is present. As different from other granulite terrains, the Bamble biotites and amphiboles from granulite facies samples do not have higher halogen contents as those from the amphibolite facies area. The samples show that halogen bearing fluids were controlled at a very restricted scale during peak as well as retrograde metamorphism.

Petrophysical properties of a prograde amphibolite-granulite facies transition zone at Sigerfjord, Vesterålen, northern Norway

The Vesterålen-Lofoten area in northern Norway is a province of deep-seated origin with associated regional magnetic and gravimetric anomalies. A regional transition zone between amphibolite and granulite-facies gneisses has been examined in detail with regard to interpretation of aeromagnetic and gravimetric data in northern Fennoscandia. The location of this petrographical boundary is defined by the first appearance of orthopyroxene in the intermediate gneisses. This orthopyroxene isograd is situated within an approximately 3 km wide magnetic transition zone where the average magnetite content increases fairly gradually from approximately 0.05% in the amphibolite facies ta 1.0% in the granulite facies. These estimates are based on in-situ susceptibility measurements which show an increase from 150 · 10 −5 SI to 3000 · 10 −5 SI. Magnetite is by far the most common Fe-oxide. The orthopyroxene isograd is situated approximately 500 m from the point where the susceptibility starts to increase. A few of the samples in the granulite fades have garnet occurring as coronas around Fe-Ti oxides and pyroxenes, indicating high-pressure retrograde metamorphism. Prograde textures, however, predominate. Density shows a moderate increase across the profile, varying from 2.75 · 10 3 kg/m 3 in the amphibolite-facies rocks, through 2.77 · 10 3 kg/m 3 in the transition zone, to 2.81 · 10 3 kg/m 3 in the granulite-facies rocks. The Q-values are 2.0 for the amphibolite facies, 0.45 for the transition zone and 0.27 for the granulite facies. The remanence is predominantly viscous, and probably influenced by coarse-grained multidomain magnetite. Based on susceptibility measurements and thin section studies it is concluded that the prograde metamorphism occurred at the same time as or before the development of numerous fractures in which fluids such as CO 2 had easy access. Sparse retrograde metamorphism in the granulite zone took place along more localized microshears that appear to have been carriers of H 2O.

Petrophysical properties (density and magnetization) of rocks from the northern part of the Baltic Shield

Density, magnetic susceptibility and remanent magnetization of more than 30,000 samples of crystalline rocks from the northern part of the Baltic Shield have been measured by the national geological surveys of the Nordic countries. These data were compiled into a database as part of a project involving Finland, Norway and Sweden — the Nordkalott Project. In addition, more than 20,000 in-situ susceptibility measurements are available for a large part of the Baltic Shield. From all these measurements, a number of general conclusions can be drawn regarding the distribution of magnetic minerals in the different lithologies: 1. (1) The Precambrian metasupracrustal rocks show a rather typical bimodal distribution in susceptibility. The spatial distribution of these susceptibility modes gives rise to the banded patterns observed in high-resolution (low-altitude) aeromagnetic measurements. 2. (2) The amount of remanence in the various rock types is generally low, as indicated by Q-values (Königsberger ratios) of around 0.2 on average. Thus, magnetization is parallel to the Earth's magnetic field, simplifying the interpretation ol the total field magnetic survey results. 3. (3) Plutonic rocks as a whole have the same bimodal susceptibility distribution, but individual plutons have usually unimodal susceptibility distributions and thereby create characteristic local anomaly patterns. 4. (4) Metamorphic processes significantly affect the distribution of magnetite. Well-documented examples include the amphibolite-granulite facies transition (Tectonophysics, Vol. 192). the serpentinization of ultramafic rocks (Pure Appl. Geophys., Vol. 114) and oxidation in fracture zones (Geoexploration, 15). The metamorphic development of magnetite appears to reflect in many cases the variation in primary composition. 5. (5) The range in density variation and magnetic susceptibility in crystalline rocks is now well defined. In local structures it is, however, always necessary to determine the actual petrophysical contrasts in order to minimize the ambiguity in quantitative interpretation of potential field anomalies.

Micro-analysis of sulfur-isotope ratios and zonation by laser microprobe

The University of Wisconsin laser microprobe/mass spectrometer system combines high spatial resolution with precise, accurate, and rapid analysis of sulfur-isotope ratios. Spot sizes of 100–200 μm are routine and much higher spatial resolution may be possible. Analytical precision ranges from 1 σ = ±0.15%. (pyrite, pyrrhotite, sphalerite) to 1 σ = ±0.43%.. (galena, chalcopyrite). Comparison of laser microprobe and conventional combustion analyses indicates that laser δ 34S values are approximately equal to or lower than combustion values for these minerals, and an empirical correction of −0.1‰ (sphalerite) to +1.4%. (pyrrhotite) is required. Variations in δ 18O of SO 2, laser power density, and burn duration affect the magnitude of this correction; thus, consistent technique is critical. Natural samples were analyzed by laser microprobe to evaluate the extent of sulfur-isotope ratio zonation. Coexisting sulfide mineral pairs from the Rua Cove Mine, Alaska, show that isotopic equilibrium exists in the stockwork feeder zone, while disequilibrium characterizes coexisting pairs from the overlying, more rapidly cooled massive sulfide blanket. Conventional sulfur isotope extraction data on these samples are not valid due to the fine-grained, intergrown nature of the sulfide phases, which precludes satisfactory mineral separation. Isotopically homogeneous sphalerite and pyrite crystals were found in samples from the amphibolite facies Balmat Mine, while isotopically heterogeneous pyrite crystals were found in a sample from the upper greenschist facies Sullivan Mine. Sulfur-isotope zonation up to 2.9%. in 200 μm was found in banded sphalerite blacksmoker chimney material from the Bayda Mine (Oman). Retrograde sulfides from the Marcy Anorthosite Massif vary by 4.2%. over <1 cm, and finely disseminated sulfides from an amphibolite-granulite facies transition zone sample, southern India, are in disequilibrium across the 25 cm facies transition. These results show that sulfur isotope homogeneity may be attained in the cores of annealed high-grade metamorphic minerals while lower temperature, retrograde, or hydrothermally deposited minerals are often isotopically zoned due to temporal variations in fluid chemistry and temperature.

Evidence for fluid pathways through Archaean crust and the generation of the Closepet granite, Karnataka, South India

In the Archaean amphibolite—granulite facies transition zone of southern Karnataka anatexis of amphibolite facies Peninsular gneiss occurred. Based on cross-cutting relationships this melting event, which produced the Closepet granite, was contemporaneous with the formation of charnockitic rocks. This formation of charnockite took place initially due to channelized CO2-rich volatiles moving along an anastamosing network of shears and micro-fissures. These fluid pathways are now represented by brown veins of charnockite. Later, the fluid became more pervasive and resulted in areas of total inversion to charnockite. A lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening, can be traced through the country rocks. These different effects along the same, laterally continuous, vein are interpreted to represent the lateral change of volatile composition which, in front of the CO2-rich fluids, were increasingly H2O-dominated.

Water contents of silicic to intermediate magmas

Silicic to intermediate magmas have water contents that range from < 1 wt. % to around 7 wt. % depending on the character and depth of the magma source, type of melting reaction and the degree of differentiation. The condition of water saturation is rare, implying that crustal magma genesis generally takes place under fluid-absent conditions or that any fluid present must contain volatile species other than H 2O. The source and quantity of water present during partial melting profoundly affect the types of melting reactions, residues and magmas as well as subsequent magma evolution. For S-type granitic magmas, melting reactions involving muscovite breakdown in pelites yield low-temperature, “wet” melts that form veins, pockets and small plutons in regional metamorphic terranes. Biotite breakdown in pelites near the amphibolite-granulite facies transition yields hotter magmas with variable water contents. These are emplaced at medium to high levels. Under granulite facies conditions, biotite breakdown in quartzofeld-spathic rocks yields high-temperature, “dry” magmas emplaced as subvolcanic batholiths and volcanics.

Water activity changes across an amphibolite-granulite facies transition, Broken Hill, Australia

Phase compositions in pelitic and mafic gneisses place tight constraints on pressure (ranging from 3 up to 6 kb), and, to a lesser extent, on temperature (500° up to 800° C) during prograde regional metamorphism of the Willyama Complex, Broken Hill, SE Australia. These limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses. The stability of cummingtonite and biotite over much of the terrain places upper limits on temperature, and the presence of syn-metamorphic partial melts in the metasediments places lower limits on a(H2O). Garnet-biotitesillimanite-K feldspar-quartz relations combined with the partial melting data suggest a decrease in water activity from near 1.0 in the lower grade zones to 0.5±0.2 in the Broken Hill — Little Broken Hill part of the two pyroxene zone. This result is compatible with less precise hornblende-orthopyroxene-clinopyroxene-quartz relations.