Basic Granulites Research Articles

EVOLUÇÃO PETROLÓGICA E ESTRUTURAL DA PORÇÃO ORIENTAL DO ESTADO DE MINAS GERAIS E SUAS IMPLICAÇÕES GEOTECTÔNICAS

A metamorphic terrain with high-grade rocks of the Atlantic Metamorphic Belt underlies the eastern part of Minas Gerais State, from south of the town of Manhuaçu to Caratinga. This terrain comprises peraluminous gneisses, igneous and meta-igneous rocks. Granulites occur as small nucleus and vary in composition between peraluminous and basic to intermediate, the latter represented by enderbitic mobilizate. Their formation, as well as that of migmatites of granitic composition, is considered to be related to mafic and ultramafic intrusions. In basic granulites, garnet-bearing mineral assemblages, with the development of corona textures, attest the effects of granulite facies metamorphism, although igneous assemblages and textures are still well preserved. Retrograde alteration assemblages are locally preserved. Despite of the diversity of metamorphic phenomena in this area, T and P calculations reveal consistent results. Temperature and pressure calculations were undertaken in basic granulites slightly affected by the retrograde process. Using Fe +²/Mg exchange between garnet and ortopyroxene as geothermometers and the exchange reaction: An +En = 2/3Pyr + 1/3Grs + Qz as geobarometers peak metamorphic temperatures in the range of 660 to 760°C, at 4,8 to 6,6 Kbar are obtained. Mineral, textural and geochemical evidences indicate that the metamorphic conditions have changed with time and suggest that the formation of the granulites is caused by the underplating of magmas, probably mantle-derived, at the base of the crust. Several rations between major, trace and rare earth elements have been employed. The basic rocks are similar in composition to tholeiites generated in within-plate tectonic settings. Positive correlations netween K2O and SiO2 and negative between MgO and SiO2 in fresh gabbro-noritic rocks and enderbites indicate magmatic differentiation. The geochemical character of altered basic rocks displays an unsystematic dispersion in correlations diagrams. This lack of correlation coupled with field and petrographic suggest the effects of a late metasomatic event on these rocks. This metasomatism comprises the dispersed development of charnockitic rocks with large K-feldspars and quartz crystals. Later dynamic processes gave place to subvertical shear zones with a well defined foliation.

Structural relations of charnockites of the Archaean Dharwar craton, southern India

Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch‐and‐swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near‐coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non‐coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non‐coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.

Granulite facies middle-lower crustal xenoliths from Nushan alkali basalt in northeastern Anhui Province, China

Metamorphic xenoliths within the Nushan alkali basalt of northeastern Anhui (NEA), China, are from the middle-lower crust. They could be divided into two end-members: basic and acid. Intermediate xenoliths are scarcely found. Basic two-pyroxene granulites (pyriclasites) were formed at 720–810° C and 7–8 kb. Petrological and geochemical studies indicate that the primary magma of the protoliths of basic granulites was derived from the metasomatized upper mantle, while the parental magma of the acid end-member was probably produced by partial melting of the basic rocks. The protoliths of charnockites and grey gneisses represent respectively the early and late crystallization products of the granitic magma. The Nushan granulites are much different in many aspects from the granulites exposed in the northern part of North China, which implies the inhomogeneity regarding to the early evolution of the North China terrane.

The Redan Geophysical Zone: Part of the Willyama Supergroup? Broken Hill, Australia

The Redan Geophysical Zone forms a regional magnetic high in contrast to the regional magnetic low defined by the main part of the Broken Hill Block. The magnetic rocks are interpreted to dip below the remainder of the Broken Hill Block and there has been speculation that they are significantly older than the Early Proterozoic Willyama Supergroup. Evaluation of lithological mapping and aeromagnetic data permitted interpretation of a stratigraphic sequence within the Redan Geophysical Zone, consisting of three new formations: the Redan Gneiss, Ednas Gneiss and Mulculca Formation, plus the Lady Brassey Formation, part of the Thackaringa Group. The rocks are considered to belong to the lower part of the Willyama Supergroup and are not an older basement. Although the Redan Geophysical Zone contains some rock types not found elsewhere in the Broken Hill Block, there are some lithological similarities with the lower part of the Willyama Supergroup: an abundance of albite‐rich rocks, the presence of quartz‐magnetite rocks with Cu and trace Co, and abundant amphibolite/ basic granulite in the Lady Brassey Formation. The boundary between the Redan Geophysical Zone and the remainder of the Broken Hill Block appears to be conformable, with no evidence of major faulting. Similarly no evidence of unconformities or major displacement of stratigraphic boundaries has been found within the Redan Geophysical Zone. Structural history, fold style and orientation, and metamorphic grade within the Redan Geophysical Zone are similar to adjacent areas of the Broken Hill Block. It is concluded that the Broken Hill Block contains no outcropping equivalent of the first cycle of sedimentary/ igneous rocks recognized in the Early Proterozoic of northern Australia. Albite‐quartz‐hornblende‐magnetite rocks unique to the Redan Geophysical Zone most likely comprised detritus derived directly from an intermediate volcanic suite. Some were altered considerably, while other rocks retained the dacite/andesite composition, except for the addition of Na, an increase in the oxidation state, and partial leaching of some of the more mobile elements. These modifications could have taken place in shallow alkaline evaporitic lakes. The Redan Geophysical Zone contains some of the elements of a foreland basin adjacent to a continental volcanic arc: a thick stratigraphic sequence, oxidizing evaporitic conditions, and intermediate volcanic detritus. The change from intermediate‐acid volcanism in the earliest formations, to bimodal acid/basic volcanism in the Thackaringa and Broken Hill Groups could correspond with a change from initial continental arc volcanism into bimodal rift volcanism. The case for the arc volcanism is weakened, however, by the relative scarcity of rocks with andesitic compositions and the lack of basaltic andesite compositions. The alternative is that the intermediate to acid volcanism represents only a variation on the later bimodal rift volcanism.

GEOLOGICAL THERMOMETRY AND BAROMETRY OF METAMORPHIC COMPLEXES: CENTRAL KOLA ARCHEAN GRANULITE-GNEISS REGION

The paper distinguishes two complexes of high-grade meta-morphic rocks in the central portion of the Kola Peninsula (Baltic Shield): 1)the Vyezhe Tundra complex, mainly enderbites and basic granulites with subordinate plagiogneisses, iron formation, and metapyroxenites, and 2)the Kola series (mainly plagiogneisses with subordinate iron formation and basic granulites). Textural and paragenetic relationships point toward multi-cyclic metamorphism in the region. Metamorphic temperature conditions were estimated using a consistent system of geothermometers and geobarometers. Temperature maxima (670, 565 and 500° C) were ob-tained, which correspond to different stages of metamorphism in the re-gion. The results show that metamorphism in the two sequences occurred at similar temperatures; furthermore, no differences were found between unsheared and sheared rocks. The findings support the view that equilibra-tion between minerals during metamorphism is controlled by breakdown-crystallization reactions. The obtai...

Granulites in the Tongbai Area, Qinling Belt, China: Geochemistry, petrology, single zircon geochronology, and implications for the tectonic evolution of eastern Asia

The Tongbai area of the eastern Qinling belt in China includes granulite‐grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton). Our petrological and geochemical data characterize these rocks as two‐pyroxene granulites and garnet granulites which formed at temperatures of 757°–840°C and pressures of about 9.5 kbar and are now found as xenoliths in granodioritic gneisses. The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro. The 207Pb/206Pb ratios derived from evaporation of single zircons yield ages of 470±20 and 470±14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement. These are intruded by a 435±14 Ma granodioritic gneiss post‐dating granulite formation. A metaquartzite sample contains detrital zircons as old as 2555±8 Ma. Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single‐zircon 207Pb/206Pb evaporation ages of 776±8 and 746±10 Ma, respectively, and document late Proterozoic igneous activity. We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic igneous activity.

Geobarometers involving clinopyroxene, garnet, plagioclase, ilmenite, rutile, sphene and quartz: estimation of pressure in quartz-absent assemblages

Enthalpy of formation and third law entropy for andradite (Ca3Fe2Si3O12) and hedenbergite (CaFeSi2O6) at standard state have been retrieved from experimental P-T-f(O2) data on the stability relations of these phases. The data for hedenbergite were combined with the thermodynamic data for related phases (Berman 1988) to formulate the geobarometers based on the reactions CaFeSi2O6+2CaAl2Si2O8+2FeTiO3= Ca3Al2Si3O12+Fe3Al2Si3O12+2TiO2 (CPG-IR), 3CaFeSi2O6+3CaAl2Si2O8+3FeTiO3=Ca3Al2Si3O12 +2Fe3Al2Si3O12+3CaTiSiO5 (CPG-IS) and 3CaFeSi2O6+3CaAl2Si2O8=2Ca3Al2Si3O12+ Fe3Al2Si3O12+3SiO2 (CPG-O). The geobarometers involving rutile and sphene are proposed for the first time. In several granulite terranes, pressures have been estimated from the three geobarometers for basic granulites and charnockitic suite of rocks containing the above assemblages. In clinopyroxene-garnet-plagioclase-quartz-ilmenite-rutile assemblages, pressures obtained from CPG-IR are within ±500 bars of pressures calculated from the CPG-Q barometer. However, if rutile is absent, the pressures computed from the CPG-IR barometer are 300 to 1,200 bars higher than those calculated from the CPG-Q barometer. For maximum differences in pressures obtained from the two equilibria TiO2 activity [a(TiO2)] in the rocks is calculated to be 0.8. The sphene-bearing geobarometer (CPG-IS) also registers pressures comparable (<±500 bars) to those obtained from the CPG-Q barometer. The close agreement in pressure values obtained from the CPG-IR and the CPG-IS equilibria with those registered by the CPG-Q geobarometer in assemblages containing quartz implies that the rutile- and sphene-bearing geobarometers which do not involve quartz, can also be applied to estimate pressures from quartz-absent assemblages-assemblages which are not amenable to quantitative geobarometry using existing formulations.

CARACTERIZAÇÃO PETROGRÁFICA E METAMÓRFICA DOS GRANULITOS DO ARQUEANO/PROTEROZÓICO INFERIOR DA REGIÃO DE ITABUNA - BAHIA

The Archean/Lower Proterozoic high-grade rocks ofthe Itabuna region in southern Bahia were separated into five assemblages. The first one was named Ilheus Complex, being formed by intercalations of basic granulites with hornblende and felsic granulites with plagioclase. The second, Sao Jose Complex, is composed of intermediate to basic granulites with intercalations of garnet bearing basic granulites. The third, Ibicarai-Buerarema Complex is formed essentially of intermediate granulites and a fourth assemblage that comprises the Agua Sumida (metamonzonites), Rio Paraiso (meta-syenites) and Sao Geraldo granitoids (this latter one of granitic composition). All the rocks ofthe former four assemblages are of the granulite facies of metamorphisrn. The fifth and last assemblage was named Rio Japu, being formed mostly of biotite and hornblende gneisses re-adjusted to the amphibolite facies conditions. Besides the amphibolite and granulite progressive facies the paragenetic and textural studies ofthe metamorphic reactions and mineral chemistry allowed to recognize the granulite, amphibolite and greenschist regressive facies in the area. The temperatures and pressure values for the maximum metamorphism were estimated in 6 kbar and 850°C. The geotectonic model for the area is in agreement with the one interpreted for a northern area. In that place is taken into account the existence of a subduction zone closer to the Atlantic coastal zone and, an arc or a series of island arcs as one moves inboard the continent.

P-wave velocity in granulites from South India: implications for the continental crust

P-wave velocities ( V p ) were measured in 160 high-grade metamorphic rocks from the South Indian granulite terrain (SGT). The wide variations observed in the V p of charnockites and gneisses could be due to the complex prograde and retrograde metamorphic histories of the two major rock types of the SGT. The velocity-density relation showed distinct trends for charnockites and gneisses. Initial stages of retrograde metamorphism in charnockites significantly affected their magnetic properties, however, its effect on velocity and density is not diagnostic. Contrasting physical properties on either side of the Palghat-Cauvery (P-C) shear zone lends support for the contention that the P-C shear zone is a major paleosuture. The laboratory mean V p of the rocks from the northern SGT are comparable with the mid-crustal DSS velocity in the adjacent granite greenstone terrain (GGT), suggesting that the GGT is possibly underlain by a felsic granulite basement. The physical properties of the high-grade metamorphic rocks from SGT are significantly lower than that of the lower crust. The physical properties and tectonic considerations show that the granulites of South India may not be of lower crustal origin and hence not representative of the lower crust, as generally thought. A simplified two-layer crustal model with a predominantly felsic granulite upper crust and a mafic granulite lower crust, is suggested for the SGT.

Tectonics of the Verkhoyansk-Kolyma Mesozoides in the context of plate tectonics

The Verkhoyansk-Kolyma Mesozoides, located to the northeast of the ancient Siberian platform, form a part of a Mesozoic orogenic belt which lies between the Siberian and North American platforms, the Pacific Ocean and Arctic Ocean. The outer part of the Verkhoyansk-Kolyma Mesozoides is adjacent to the Siberian platform and comprises the Verkhoyansk foldbelt, composed primarily of Carboniferous-Middle Jurassic clastic sequences and lower-middle Paleozoic and Riphean terrigenous-carbonate series deposited on the Siberian passive continental margin. Northeastward, the passivemargin shelf deposits grade into Upper Permian to Lower Jurassic black-shale sequences of the Kular Nera slate belt which formed at a continental rise and in a marginal sea. Thick (up to 8–10 km), largely carbonate Ordovician-Devonian deposits of the Cherskiy, Selennyakh and Ulakhan-Tas ranges lie allochthonously over the Mesozoic and represent formations of the Omulevka carbonate platform. Ophiolites of uncertain age and a serpentinite melange have been tectonically juxtaposed against the latter. The time of emplacement for the Ophiolites is constrained by their occurrence in a Bathonian-Callovian olistostrome. The northeast-trending Alazeya volcanic arc, of Carboniferous to early Jurassic age, is located in the central part of what was once considered as the Kolyma massif. An accretionary wedge associated with the arc is buried beneath Late-Cenozoic deposits and can be traced by a large magnetic anomaly which extends to the south and east. The Omolon and Okhotsk massifs have an Archean granulite basement and are independent microcontinents. The available paleomagnetic data do not allow of the quantitative assessment of the amount ofseparation of the various allochthonous terranes, either from each other or from the Siberian continent. An affinity between Paleozoic and Mesozoic faunas of the Mesozoides and those ofthe Siberian platform indicates that their relative separation did not exceed a few thousand kilometers. Remnants of basins with oceanic crust found in the inner areas of the Verkhoyansk-Kolyma Mesozoides indicate that they are more comparable to present-day marginal seas of eastern Asia than to the open Pacific Ocean. Early folding occurred as a result of the pre-mid-Jurassic attachment of the Prikolyma and Omolon terranes to the Alazeya arc, which led to the formation of the large, heterogeneous, Kolyma-Omolon microcontinent. Later folding was due to a collision of the Kolyma-Omolon microcontinent with the Omulevka platform and the Verkhoyansk passive margin. This deformation propagated from the inner (Cherskiy) areas, in Bathonian time, towards the Siberian platform, where it culminated in frontal thrusts in the latest Cretaceous. Transverse and longitudinal belts of Upper-Mesozoic granitoids are closely related to collision processes. Their long history (about 90 m.y.) reflects the duration of the collision.

A Pan African age for the HP-HT granulite gneisses of Zabargad island: implications for the early stages of the Red Sea rifting

Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, Sm Nd and Rb Sr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined Rb Sr and Sm Nd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the Sm Nd and Rb Sr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the Rb Sr isotopic system of the mafic granulite. The initial 143Nd/ 144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.

Coexisting pyroxenes from the basic granulites of Visakhapatnam area in the Eastern Ghats terrain

Fifteen pairs of coexisting pyroxenes from basic granulites associated with leptynites in the khondalite suite of rocks are analysed and the distribution of Mg and Fe2+ ratios is presented. Temperature estimates for the coexisting pyroxenes from the basic granulites of Visakhapatnam may be expressed as 750±100†C corresponding to intermediate pressure granulites.

Isotopic (RbSr, UPb and SmNd) and trace element geochemistry of eclogites from the pan-African Belt: A case study of REE fractionation during high-grade metamorphism

Major and trace element analyses [including rare earth elements (REE)], as well as RbSr (whole rocks and micas), UPb (zircons) and SmNd (whole rocks) isotopic data for eclogites and basic granulites from the pan-African Belt suggest the existence of two distinct tectonic settings. An occurrence of high grade rocks, located along the pan-African suture zone in Togo, involves basic protoliths derived from depleted mantle source(s) with ϵ Nd( T) generally close to +7, or +2 in cumulate rock-types. The ages of their protoliths are not precisely determined but are probably Proterozoic (younger than 1.15 Ga). They are thought to have been involved into subduction-obduction processes (640-580 Ma ago) which affected pan-African (s.l.) oceanic and continental crust. A second occurrence of high-grade rocks located in the Aleksod area of the central Hoggar (Algeria) has also been investigated. It corresponds to older (≅2.5 Ga) basic protoliths metamorphosed during the pan-African orogeny. These rocks are considered to belong to an intracontinental context as proposed for this part of the belt. A model is proposed to account for the “step-shaped” REE patterns sometimes observed in certain eclogite whole-rock samples. In the case of eclogites from the Mount Lato area, these REE patterns are not representative of magmatic protoliths. Some LREE-loss occurred during high-grade events particularly evidenced in syn-eclogite blastomylonites.

High-grade metamorphic rocks and peridotites along the Leiza Fault (Western Pyrenees, Spain)

Acid and basic granulites, migmatites, and lherzolites outcrop along the Leiza Fault (Navarra, Spain) in the western extremity of the Pyrenean Belt. The protoliths of the acid granulites have granodioritic composition. Textural and mineralogical data suggest that the acid granulites evolved from a first, syn kinematic medium-pressure granulite-facies stage [garnet-biotite I-Kfsp] to a post-kinematic granulite-facies stage of lower pressure and higher temperature [garnet-biotite II-cordierite-Kfsp-(spinel?), kinzigites]. Basic granulites were formed from protoliths with composition of tholeiitic to alkaline basalts. Basic granulites exhibit millimeter size subidiomorphic garnets dispersed through the matrix and smaller coronitic garnets between opaques or orthopyroxenes and plagioclase. Thermo-barometric estimates for the peak of the granulite-facies metamorphism are c. 800°C and 8 kbar. The migmatites presumably represent a shallower level of metamorphism (andalusite/sillimanite-Kfsp). Scapolite in the basic granulites was formed during a post-granulitic metamorphic episode. The lherzolites have been intensely brecciated and serpentinized, which makes difficult the comparison of their evolution with that observed in the other rocks associated with the Leiza Fault. Acid and basic granulites, migmatites, and lherzolites along the Leiza Fault may be correlated with similar rocks outcropping elsewhere in the North-Pyrenean Zone and along the North-Pyrenean Fault as tectonic slices and massifs. By analogy with those rocks, the granulitefacies metamorphism observed in the rocks studied must be Hercynian in age. The Leiza Fault constitutes, therefore, the western continuation of the North-Pyrenean Fault, and the rocks studied (except perhaps the lherzolites) may be considered as remnants of an Hercynian metamorphic massif, dismembered as a consequence of the activity of the fault at the end of the Hercynian cycle and during the Alpine tectonometamorphic events.

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre‐Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.(a) A medium‐ to low‐Pgranulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K‐feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.(b) A post‐granulite re‐equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.(c) An eo‐Alpine eclogite stage giving rise to the crystallization of high‐Pand low‐Tassemblages.In an effort to quantify this evolution, independent well‐calibrated thermobarometers were applied to basic and acid rocks. For the granulite event,P‐Testimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution,P‐Twas 4–5 kbar and 600° C. For the eo‐Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.The inferredP‐Tpath is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre‐Alpine granulite and post‐granulite retrograde evolution to the Permo‐Jurassic extensional regime. The complex granulite‐eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.

Sr‐Nd isotope compositions of cenozoic granitoids along a traverse of the central Peruvian Andes

AbstractSr‐Nd isotopic compositions of late Eocene to Pliocene intrusive rocks from a traverse (⋍11° lat. S) of the central Peruvian Andes are presented. The data set suggests that the calc‐alkaline magmatism is mainly the end‐product of the evolution at deep crustal levels of magmas derived from a slightly depleted, but not MORB‐type, subcontinental mantle, which have assimilated crustal material and have mixed with new batches of primary magma. Assimilation of crustal material increased eastwards and with time. The evolution of Sr‐Nd isotope compositions in time and space along the traverse reflect both the lateral heterogeneities in the subcontinental mantle, the lower crust composition, and the progressive tectonic thickening of the crust during the Cenozoic. No granulitic basement akin to the early Proterozoic south Peruvian Arequipa massif has participated in the genesis of the central Peruvian calc‐alkaline and alkaline magmas.

Hydration of basic granulite to garnet-epidote amphibolite in the Sanbagawa metamorphic belt, central Shikoku, Japan

Granulite in the Iratsu metagabbro of central Shikoku, with color-banding defined by the presence and absence of plagioclase, is surrounded by a narrow epidote amphibolite, which itself is encased in garnet-epidote amphibolite. The epidote amphibolite formed metastably under low P H 2O conditions based on thermodynamic considerations. The granulite-epidote amphibolite boundary is formed by the hydration reaction of plagioclase to zoisite, kyanite and quartz, and that of clinopyroxene and orthopyroxene to amphibole. If it is assumed that the temperature range of the Sanbagawa epidote amphibolite facies is 600–650°C, we obtain a lithostatic pressure of 12–15 kbar and a H 2O pressure of 1–3 kbar for the hydration conditions. The boundary between epidote amphibolite and garnet-epidote amphibolite is defined by the formation of garnet, which is more stable than the epidote amphibolite assemblages. Pargasitic amphibole occurs at the garnet rim. Up to 0.32 wt.% Cl is present in the amphibole from both of the hydrated facies, but is < 0.03 wt.% Cl in the granulite amphiboles. During hydration, there was an overall increase in bulk-rock Na 2O and Cl contents, and localized migration of Mg and Fe across the layering of the hydrated derivatives. It is concluded that hydration did not take place in the presence of an excess fluid phase, based on the difference of the Cl contents of amphibole between observed and expected values from a qualitative infiltration metasomatic model and also on the estimated low fluid pressure. The formation of a sharp boundary between the granulite and epidote amphibolite suggests intracrystalline diffusion.

Premetamorphic lateritisation in Proterozoic metabasites of Rogaland, SW Norway

Heterogeneous layers of granulite facies metamorphic basites of the Proterozoic supracrustal Faurefjell Formation in Rogaland, S.W. Norway, display an extreme chemical variation. Within a single layer the bulk chemical composition gradually changes from approximately basaltic in basic granulites to alumina-iron-rich in granofelses. Component-ratios and composition-volume relations indicate open-system chemical reactions mainly involving the extraction of silica. Apparent enrichment in Fe, Ti, P, Al, Zr, Ni, Co, Zn, Y, Nb, Hf and REE and variations in resulting metamorphic mineral assemblages are related to premetamorphic progressive lateritisation of a basaltic protolith. The weathering generated a continuous chemical suite from SiO2=48 wt%, Fe2O3=10 wt% and Al2O3=19 wt% in the basic granulites to 14 wt%, 40 wt% and 25 wt% in the Fe-Al granofelses. Metasomatism during diagenesis and during (very) high-grade metamorphism (1200–900 Ma) further perturbed the concentrations of relatively mobile elements Ca, Mg, K, Rb, Sr, Ba, Na and Li in the laterites without affecting the transition metal ratios. In particular, the REE did not fractionate differentially during the supracrustal and metasomatic alteration.

The stability of sapphirine + clinopyroxene: implications for phase relations in the CaO-MgO-Al2O3-SiO2 system under deep-crustal and upper mantle conditions

Reaction textures and chemographic relations in sapphirine-bearing basic granulites at Finero, Italy, suggest that sapphirine and aluminous diopside were formed in mutual equilibrium from an inferred early spinel-bearing assemblage. Finero appears to be the only known locality where this association has been found in situ, although it is also known from kimberlite and breccia pipe nodules elsewhere. The reactions deduced to have occurred in these rocks suggest the existence of stable invariant points involving the phases sapphirine-spinel-orthopyroxene-clinopyroxene-garnet-anorthite and sapphirine-two pyroxenes-garnetanorthite-kyanite (or sillimanite) in the CMAS end-member system. P-T estimates for the relevant rocks, and the available experimental data, suggest that these points lie at around 800°–900° C, 9–11 kbar. A semi-quantitative petrogenetic grid, incorporating these invariant points with previously determined univariant reactions, is proposed. It is inferred that sapphirine+diopside are stable relative to spinel-bearing assemblages below 900° C. The relatively low temperature explains why sapphirine has not to date been reported from experimental work on the CMAS system. It also suggests that sapphirine may be an important aluminous phase in Mg-rich metagabbros under conditions corresponding to the base of the continental crust, despite the observed rarity of such rocks at the surface.

Basic rocks of the inner Fiskefjord area, southern West Greenland

During mapping for the Fiskefjord map sheet (64 V.1 N - 1:100000) observations were made on the basic rocks. Earlier reports on the mapping of the Fiskefjord area have been given by Garde (1984, 1986) and Garde et al. (1987), and the reader is referred to these for more detailed descriptions af the general geology. Most of the inner Fiskefjord area is made up of a variety af gneisses, correlated with the late Archaean Nûk gneisses. The most common type is a weakly banded, tonalitic gneiss, usually grey, sometimes with a Iilac tint. Metagranites and more mafic gneisses of dioritic to quartz dioritic composition form mappable units as well as bands of inclusions in the tonalitic gneisses. The gneisses show intrusive relationships to a group of basic and ultrabasic rocks, mainly two-pyroxene basic granulites (named 'pyribolites' by Berthelsen, 1960) with minor dunite and norite. Small amounts of metasedimentary gneiss are associated with the norite.