Migmatitic Rocks Research Articles

Estruturação geológica da região da Serra do Juqueriquerê, São Sebastião, SP

The crystalline basement in the region of the Juqueriquerê plateau, São Sebastião, SP, is made up of para- and orthoderived high- to médium grade metamorphic rocks, and as by variably deformed and gneissic granitoid rocks. In the northern part of the region the Pico do Papagaio gneiss granitoid occurs, and to the south, separated by the Camburu transcurrent shear zone, is a set of mainly paraderived migmatitic rocks, with an important contribution of metabasic rocks. Structuraily, the region exhibits a fan-like distribution of the foliation (schistosity, gneissic layering, mylonitic foliation) around the vertical Camburu shear zone, a dextral transcurrent ductile shear zone. Both to the north and south of this shear zone, the foliation flattens out, with localized thrust or oblique shear zones with displacement to the north in the northern block, and to the south in the southern block. This fan-like configuration of the structures around the Camburu shear zone, with divergent thrusts in the northern and southern blocks, suggests a transpressive character for the ductile deformation, which probably occurred between the end of the Neoproterozoic and the early Paleozoic.

Rate and time controls on metamorphic processes

AbstractAmong the most important controls on metamorphism is the rate at which tectonic processes operate. The purpose of this paper is to review briefly a couple of recent advances in the field of geochronology and metamorphic petrology which have helped to elucidate these processes. Re‐equilibration at high temperatures often eradicates information that might have been recorded by rocks during their prograde and high‐temperature evolution. One of the major advances of recent years has been the recognition of the utility of garnet in preserving information on prograde thermal histories. A case study involving garnet‐bearing rocks from New England is described: In this area, garnet grew during the latter stages of prograde metamorphism and records heating through 100°C during decompression of around 2.5 kbar. Combined with chronological data on garnet cores and rims, which yield a time interval for garnet growth of 10 Ma, these data yield a heating rate of ∼ 10°C Ma‐1 and an exhumation rate of about 1 mm a‐1. Comparison with simple thermal models suggests that the heating rate is too high to be accounted for by simple conductive relaxation. At temperatures appropriate to crustal melting, even garnet re‐equilibrates such that any chronological information is lost. At these grades one approach is to use the extent of diffusional modification of growth zonation profiles to provide time constraints on the process operating. Mn distributions in garnets from the Zanskar Himalaya are used to limit the time available for high‐grade metamorphism. The times obtained of 1–2 Ma are extremely short and require cooling by emplacement of cold upper crustal rocks adiacent to migmatitic rocks.

Strontium and oxygen isotope profiles across marble-silicate contacts, Lizzies Basin, East Humboldt Range, Nevada: constraints on metamorphic permeability contrasts and fluid flow

Sr isotope profiles across marble-silicate rock contacts are used in conjunction with previously published oxygen isotope profiles to constrain fluid movement, porosity and permeability contrasts in migmatitic rocks from Lizzies Basin in the East Humboldt Range, Nevada. The 18O/16O systematics in the high-grade sequence have been interpreted to reflect infiltration of ∼2×102 m3/m2 of a relatively low 18O hydrous fluid through the sequence, but with preservation of δ18O anomalies in thin marble bands due to a 30-fold lower porosity in the marble compared with silicate rocks (Wickham and Peters 1992). The Sr isotope profiles confirm that tracer exchange between marble and silicate rock was primarily by diffusion, and in one case, indicate that porosities differed by less than a factor of four in the ∼10 cm boundary layer which exhibits diffusive modification of 87Sr/86Sr ratios. This contrasts with modelling of the oxygen isotope profiles which imply porosity contrasts >10 for one marble band and >50 for a second marble band. Either strontium and oxygen isotope diffusion reflect different events (possible if fluid Sr contents varied with time) or porosity varied substantially with the silicate rocks. Oxygen isotope profiles in the deeper part of the metamorphic section in which δ18O values of silicate rocks have been homogenised and lowered, indicate similar diffusion distances (and thus porosity-time evolution) to oxygen isotopic profiles higher in the section. Comparison of strontium and oxygen isotope diffusion distances constrains fluid Sr contents to between ∼50 and ∼500 ppm deep in the section, but less than ∼10 ppm higher in the section. The difference is related to release of relatively saline, Sr-rich fluids, by the abundant leucogranites and associated skarns deep in the section (cf. Peters and Wickham 1995).

Granite magmatism

Abstract Read’s two presidential addresses to the Geological Society (1948, 1949) heralded the end of the coherent rearguard action by the ‘granitizers’ against the ‘magmatists’. They were the distillation of his thoughts on the genesis of granite and culminated in his concept of the ‘Granite Series’. In this, he identified a continuity from metamorphic through migmatitic rocks to granite. Although he was wrong on granitization, the general idea remains intact and granites are produced by high-temperature metamorphism leading to partial melting. However the role of migmatites is still contentious. Not all granites belong to the granite series as he presented it; this particularly applies to Cordilleran (Andean) type granites. The genesis of this type will be discussed in the context of the earlier, classic work of Nockolds (1940) on the Garabal Hill complex where he demonstrated fractional crystallization was a major process in producing the diversity present and was similar to that seen in volcanic rocks, which had clearly been liquids. He also proposed that the source was basaltic with the implication, strongly supported by modern isotopic studies, that granites of this type are essentially mantle derived. Nockolds contribution was a geochemical confirmation of Bowen’s belief in the importance of closed-system fractional crystallization in the differentiation of plutonic rocks. Most authorities today would accept this but would not necessarily follow Bowen in the belief that major granitic batholiths formed by differentiation of basalt or that the system was closed. Recent models of the generation of the two main types of granite are presented, incorporating many of the ideas of Nockolds and Read. The discussion focuses on high-level differentiation, partial melting and intrusion in an extensional regime, high-T/low-P metamorphism associated with the magmatism, and the relation of granite to the plate tectonic setting.

A stable isotope study of the migmatitic rocks in the Ballachulish contact aureole, Scotland

ABSTRACTThe highest grade pelitic and semipelitic rocks of the Ballachulish aureole are dominantly potash feldspar + cordierite + biotite hornfelses with widely variable amounts of quartz, plagioclase, andalusite, sillimanite and corundum (together with accessory phases). On a microscopic scale these hornfelses show textural evidence of the presence of melt, whilst on a mesoscopic scale they contain a variety of leucosomes. Oxygen isotope studies have been carried out on both whole rocks and mineral separates in order to: (1) assess the sources of molten and volatile constituents and (2) determine the extents of isotopic homogenization and equilibration. Data from localities with both restricted and extensive evidence of leucosomes and melt development are compared, as well as one locality with petrographic evidence of melt incursion from the igneous complex.The whole‐rock δ18O values of the leucosomes (10.5–14.9%.) are in general similar to the immediately adjacent mesosomes (9.9–14.5%.) which are typically cordierite‐ and feldspar‐rich hornfelses. Isotopic evidence is thus consistent with an in‐situ partial melt origin for the leucosomes, without the substantial addition of externally derived components. In the area of extensive melt development, the ‘chaotic zone’, it is possible there was addition of an H2O‐rich fluid phase (6‐13 wt%) from the igneous complex which resulted in a slight lowering of δ18O values by 0.5–1.0%.Quartz mineral separates were used to assess the degree of local isotopic homogenization. In the extensively molten area (chaotic zone) there is extensive homogenization between rock layers (quartz δ18O usually within 1.0%), whilst in less molten areas δ18O quartz has a range of c. 3.0%. The greater homogenization in the chaotic zone is attributed to the increased degree of melting and infiltration of H2O‐rich fluid from the igneous complex.

Role of melt during deformation in the deep crust

ABSTRACTDeformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr).Partial melting of deep crustal rocks can lead to melt‐enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture.Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt‐enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt‐enhanced embrittlement occurs. The occurrence of melt‐enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt‐enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.

Granite magmatism

Read’s two presidential addresses to the Geological Society (1948 Geological Society (1949) heralded the end of the coherent rearguard action by the ‘granitizers’ against the ‘magmatists’. They were the distillation of his thoughts on the genesis of granite and culminated in his concept of the ‘Granite Series’. In this, he identified a continuity from metamorphic through migmatitic rocks to granite. Although he was wrong on granitization, the general idea remains intact and granites are produced by high-temperature metamorphism leading to partial melting. However the role of migmatites is still contentious. Not all granites belong to the granite series as he presented it; this particularly applies to Cordilleran (Andean) type granites. The genesis of this type will be discussed in the context of the earlier, classic work of Nockolds (1940) on the Garabal Hill complex where he demonstrated fractional crystallization was a major process in producing the diversity present and was similar to that seen in volcanic rocks, which had clearly been liquids. He also proposed that the source was basaltic with the implication, strongly supported by modern isotopic studies, that granites of this type are essentially mantle derived. Nockolds contribution was a geochemical confirmation of Bowen’s belief in the importance of closed-system fractional crystallization in the differentiation of plutonic rocks. Most authorities today would accept this but would not necessarily follow Bowen in the belief that major granitic batholiths formed by differentiation of basalt or that the system was closed. Recent models of the generation of the two main types of granite are presented, incorporating many of the ideas of Nockolds and Read. The discussion focuses on high-level differentiation, partial melting and intrusion in an extensional regime, high- T /low- P metamorphism associated with the magmatism, and the relation of granite to the plate tectonic setting.

Structural evidence supporting a remnant origin of patchy charnockites in the Chilka Lake area, India

AbstractDark patches of charnockitic rocks characterized by orthopyroxene occur within garnetiferous granite gneisses (leptynites) in a granulite-migmatite suite around the Chilka Lake, Orissa, within the Eastern Ghats belt in the Indian Precambrian. Analysis of structures of different scales observed in this terrain establishes the presence of three phases of deformation. S1 is pervasive in the metapelitic granulites (mainlykhondalite), while in the migmatite complex composed of leptynites, charnockites and quartzofeldspathic veins, S1 is present exclusively within the charnockite lenses and bands, and shows different stages of obliteration in the associated leptynites. Thus, the charnockite patches must be earlier than the surrounding migmatitic rocks. The charnockite patches and the surrounding leptynitic gneisses are chemically quite different and the two rock types are not related by any prograde or retrograde transformation. The shapes and disposition of charnockite patches in the mixed exposures are found to be largely controlled by the third phase of folding and locally associated shearing. The kinematics of this late deformation are not favourable for fluid ingress from deeper levels.

Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia

The recognition of large areas of gneiss within granitic complexes of the northeastern part of the Eastern Goldfields Province of Western Australia, based on analysis of regional aeromagnetic data, has resulted in a re-evaluation of the structural data for that region. Five magnetic domains have been established in the granitic complexes, which are marginal to the main greenstone belts. At least two of these domains have a distinctive domal form, and also have internal domeshaped regions. The Ballard and Laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes. Boundary relationships at the Laverton and Ballard domains show that the domes are concordant sheets, and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level. Strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower-grade greenstones was initially along relatively gently-dipping shear zones which were active during uplift. Structural relationships around both the domal granitic complexes and the later granite batholiths, however, show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks. The tectonic hypothesis presented for the Eastern Goldfields is similar to that proposed for the formation of Cordilleran metamorphic core complexes. The uplift event responsible for the crustal extension required to allow the emplacement of the gneiss, may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts.

Segregation of leucogranite microplutons during syn‐anatectic deformation: an example from the Taylor Valley, Antarctica

ABSTRACTA suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn‐anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre‐scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre‐thick stromatic leucosomes. Vein‐ and dyke‐like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome‐to‐rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite ‘microplutons’ up to 50 m across beneath refractory ‘cap‐rock’ layers.Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite‐bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50‐m‐wide ‘microplutons’ of xenolith‐rich leucogranite.Lack of vapour‐absent melting and consequent low melt‐to‐rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome ‘microplutons’ did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic‐scale granitoids.

RbSr age of Pan-African migmatites in the Oban massif of southeastern Nigeria

Migmatites consisting mainly of lit-par-lit gneisses form one of the most dominant metamorphosed rocks in the Oban massif of southeastern Nigeria. Banded schists and agmatites are the other mixed rocks recognized in the area. The lit-par-lit gneisses are composed of quartz, feldspar (leucocratic portion) and biotite, chlorite, garnet (melanocratic portion). They are extensively sheared, fractured and folded. The dominant fold axes trend N-S parallel to the regional foliation of the rocks. Chemical data show that the lit-par-lit gneiss is granodioritic in composition. The agmatite is restricted in occurrence. Banding is rare in the agmatite but dissection of the paleosome by quartzo-feldspathic veins is common. The agmatite is quartz-dioritic in composition. The banded schist is coarse-grained and contains index minerals: garnet, kyanite and sillimanite, indicating that the area underwent high-grade metamorphism of the Barrovian type. It is pelitic in composition. These migmatitic rocks were formed by partial melting, metamorphic segregation and injection due to high-grade metamorphism. The migmatitic gneiss yielded a RbSr isochron age of 150±11 Ma whilst the migmatitic schists gave an age of 527±16 Ma.

Cordierite + spinel parageneses in pelitic gneiss from the contact aureoles of the Mistastin batholith (Quebec) and the Taylor Brook gabbro complex (Newfoundland)

The contact aureoles of the Mistastin batholith and the Taylor Brook gabbro complex contain cordierite + spinel-bearing assemblages derived from quartz-, K-feldspar-, sillimanite-, garnet-, and biotite-bearing pelitic gneiss. Andalusite occurs locally. As a result of continuous, cordierite-forming reactions, garnet and biotite have recrystallized to relatively Fe-rich compositions compared with their counterparts outside the aureoles.Mosaics of fine-grained cordierite and spinel replace sillimanite and biotite throughout the Mistastin aureole. Similar assemblages and textures are preserved in metapelite in the outer part of the Taylor Brook aureole, where garnet adjacent to sillimanite is replaced by cordierite + spinel. The formation of spinel-free cordierite porphyroblasts by garnet-, sillimanite-, biotite-, and quartz-consuming reactions depleted metapelites in both aureoles in quartz, permitting the formation of the undersaturated assemblages. The undersaturated cordierite + spinel assemblages formed on a domainal scale (individually, a few hundred cubic millimetres in volume) and coexist metastably with saturated assemblages elsewhere in the same rock. Peak temperatures (700–750 °C) determined for both aureoles were insufficient to stabilize quartz + spinel parageneses owing to the low gahnite content of the oxide (ZnO < 2 wt.%).Metapelites in the innermost part of the Taylor Brook aureole appear to have been desilicified by the formation and fractionation of anatectic melt, yielding migmatitic rocks virtually devoid of quartz and K-feldspar. In contrast, migmatite in the inner part of the Mistastin aureole retains a quartz + two-feldspar mineralogy, so, as in the nonmigmatitic paragneiss, undersaturated assemblages are present only on a domainal scale.These aureole rocks demonstrate that undersaturated assemblages are not only characteristic of thermally- recrystallized low-grade pelites but also can occur in contact aureoles developed in granulite-facies paragneiss. Aluminous rocks can become undersaturated by subsolidus, quartz-consuming reactions yielding cordierite, or by the formation of anatectic liquids.

Per migma ad magma downunder

AbstractAccording to some early definitions, ‘magma’ had to be completely molten so that granitic rocks containing residual material from their source (restite) were not considered to be magmatic. Granites and volcanics with abundant recognizable restite were called migmatites or migmatitic rocks, and this terminology appears to be still in use by some. However, many felsic granites and volcanics commonly contain at least some restite, and for this reason we suggest that magma be defined either as melt or as rock material with enough melt for it to become mobile and intrusive or extrusive. The rocks crystallizing from these magmas are igneous. They are not migmatites. On the other hand, we suggest that migmatites be defined as rocks that did not have sufficient melt to become mobile; they retain a structural coherency even though the structures may be complex. Differences between the two rock types result from the amount of melt phase. When a critical melt fraction is reached during partial melting, there is a rapid change from migma to magma (migmatite to granite).The geochemistry and mineralogy of migmatites tell us little about reactions that produce more melt and hence about granite formation because of back reactions and other processes that occur during the cooling of migmatite. Back reactions mostly involve H2O that is not lost from the migmatitic system. Because H2O is lost by processes of second boiling and pressure quenching during the crystallization of granite magmas at or near the surface, it is these rocks and their inclusions, not migmatites, that provide the most information about sources and processes of granite formation.

Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)

The Montagne Noire is interpreted as a Variscan metamorphic core complex composed of: 1. (1) an uplifted core of assumed Proterozoic gneisses and migmatitic rocks associated with anatectic granites related to a late Hercynian LP/HT metamorphism and 2. (2) an upper plate composed of low-grade or non-metamorphic folded Palaeozoic sediments. These two units are separated by normal and wrench/normal ductile shear zones and detachment faults. These fault zones are characterized by synmetamorphic S- C type mylonites and stretching lineations superimposed on the earlier compressional deformation features. Early orogenic shortening is marked by polyphase, large-scale folding in the core rocks (Axial Zone) and in the thrust nappes of the upper plate. The late Variscan evolution of this massif is in general characterized by extensional tectonics. Kinematic analysis of the extensional deformation shows that displacement on the fault zones is parallel to retrogressive stretching lineations. Shear sense criteria indicate an upper plate movement towards the northeast in the northeastern part of the massif, and towards the south-southwest in the southwestern part. At the same time, transcurrent right-lateral movements occurred along the subvertical ENE-trending boundaries of the core zone. The late orogenic extensional tectonism in the Montagne Noire is attributed to uplift of the previously thickened core zone controlled by an extensional stage between two right-lateral wrench fault zones. Associated with the late Hercynian metamorphic core complex, a Stephano-Permian continental sedimentary basin was formed along the active detachment to the northeast of the uplifted core. The tectonic fabric of the basal coarse clastic Stephanian sediments shows an extension parallel to the ductile stretching in the underlying extensional shear zones. The geometry of unconformably layered clastic sequences, synsedimentary brittle faults, and the various deformation features are consistent with emplacement in a detachment-controlled basin with a NE-oriented extension direction. Together with regional radiometric data the occurrence of very low-grade metamorphism and intense carbonatization within the basal sediments in the vicinity of the core suggests that the basin was formed during the extensional tectonism related to the uplift of the metamorphic core. The Montagne Noire is a well-defined example of late orogenic extensional tectonics on the southern edge of the Variscan Massif Central. Crustal extension and Stephano-Permian basin formation is related to collapse of the thick, wide Hercynian chain during late Palaeozoic times.

The performance of some soils under stabilization in Ondo State, Nigeria

This study was aimed at classifying and improving soils from some parts of Ondo State, Nigeria, using cement and lime as stabilizing agents. The brownish-red clay (Ak-1) and brownish gravelly clay (Ak-2) were derived from migmatitic rocks (consisting of granite gneisses and metasediments) and charnockites in the north and central part, while the reddish clayey sand (Ok-1) was derived from the Quaternary coastal plain sands and alluvial sands in the southern flank of Ondo State. They have high “fines” ranging between 50 % and 80 % thereby requiring improvement before use as pavement material. Their maximum dry densities were achieved at an average of between 4 % and 6 % cement, although peak CBRs were obtained at between 8 % and 10 % lime. It was observed that cement is a more economical additive in pavement construction.

Geochronology of metamorphic and eruptive rocks of southeastern Neuquén and northwestern Río Negro Provinces, Argentine Republic

A synthesis of geologic and geochronologic information on metamorphic and eruptive rocks cropping out in the region located between 70°W and 71°W and 40°S and 41°S in Neuquén and Río Negro Provinces, Argentine Republic, is presented. In particular the metamorphites, migmatites, and granitic rocks of the so-called “Crystalline Basement” have been investigated and some data are presented on the Cenozoic basalts. The studies carried out so far permit the following preliminary conclusions. The metamorphic rocks belong to two different events. The oldest, with an age of 1190 ± 16 Ma and an initial 87Sr/ 86Sr ratio of 0.7043 ± 0.0002, corresponds to a regional metamorphic event. The youngest, with an age of 615 ± 26 Ma and an initial ratio of 0.7061 ± 0.0005, belongs to a second metamorphic event that has been recognized only locally. Some of these rocks have undergone a calco-sodic migmatization process. The Rb/Sr whole-rock isochrons for these rocks give ages between 490 and 420 Ma and initial 87Sr/ 86Sr ratios from 0.7045 to 0.7100. The best isochron has an age of 480 ± 10 Ma and an initial ratio of 0.7049 ± 0.0002. The KAr dates obtained for samples from the three metamorphic events are, in general, non-concordant with the results given by the Rb/Sr method, indicating that the KAr systems were modified by thermal and deformational processes that occurred after crystallization of the metamorphites. However, a KAr date on biotite-schist gives an age of 643 ± 20 Ma in accord with the second metamorphic event. Furthermore, samples collected from some of the localities studies have given KAr ages between 450 and 420 Ma, in agreement with the Rb/Sr whole-rock ages obtained for the migmatization process. A KAr age of 440 ± 20 Ma was obtained for a tonalite body, the emplacement of which would seem to be related to the migmatitic event. The initial 87Sr/ 86Sr ratios of the metamorphic and migmatitic rocks are low in comparison with those of similar rocks from other areas of Argentina. The Rb/Sr ages obtained for the granites show the existence of two cycles (or subcycles). The oldest took place in the Late Devonian, with an age of 370 ± 6 Ma. The other cycle occurred in the Early-Middle Permian and has an age of 280 ± 5 Ma. The KAr ages obtained on the same granite samples are concordant with these Rb/Sr ages. Preliminary study of the Cenozoic basalts reveals three different cycles: in the Eocene (39–44 Ma), Oligocene (28–32 Ma), and Miocene (16–18 Ma). Previous studies of these rocks had assigned them to the Oligocene.

Finding of olivine- and garnet-bearing biotite quartz norite from the Hidaka metamorphic belt, Hokkaido, Japan.

We found olivine- and garnet-bearing biotite quartz norite from the upper stream of the Niobetsu River of the southern Hidaka Mountains. Until now no peraluminous igneous rocks have been reported from the Main Zone of the Hidaka metamorphic belt except migmatitic rocks. Garnets of the norite are similar in composition to those from dacites and rhyolites. Orthopyroxenes contain 1.0-2.3wt% Al2O3 and 0.2-0.5wt% CaO, resembling those in peraluminous igneous rocks. The texture, mineralogy and other evidence suggest that the norite crystallized from a peraluminous magma at 940-1070°C and 4-5kbar.

Metamorphic petrology of the northeast Front Range, Colorado: The Pingree Park area

The Pingree Park area in the northeast Front Range, Colorado, is underlain by Precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1.75 b.y. ago, forming a distinct zonal pattern of minerals indicating an increase in grade of metamorphism to the west and north. Three isograds have been identified. The andalusite-sillimanite isograd is defined by the first occurrence of sillimanite in the prograde direction and is interpreted to mark the reaction of andalusite to sillimanite. A band as much as ∼1 km across contains both andalusite and sillimanite and represents a divariant band or reaction overstepping of 20 °C or less. The K-feldspar–sillimanite isograd separates microcline + sillimanite from prograde muscovite + quartz assemblages and is interpreted to represent the reaction (K, Na) muscovite + quartz = (K, Na) microcline + sillimanite + H 2 O. Despite the presence of significant amounts of sodium in both muscovite and microcline, the reaction shows no perceptible divariant band. There has been extensive reaction of microcline + sillimanite to retrograde muscovite + quartz. The migmatite-in isograd is defined by the first occurrence of migmatitic rocks in the prograde direction and is interpreted to represent the inception of partial melting. The isograds form an eastward-plunging set of synformal surface the south limb of which has a gentle dip and the north limb of which apparently has a much steeper dip. Based on the chemical composition of the minerals and comparison with experimentally calibrated equilibria, the conditions of metamorphism at the K-feldspar–sillimanite isograd are estimated to have been P H 2 O ≅ P total = 3 to 4 kbar, T = 650 ± 30 °C, f O 2 ≅ 10 −12 to 10 −15 bars, and f HF ≅ 1/4 to 1 bar. The geothermal gradient at the peak of metamorphism was ∼60 °C/km.

As rochas granulíticas da Faixa Paraíba do Sul

Granulitic and migmatitic rocks of the Paraiba do Sul granulitic terrains are petrographically and structurally characterized. Charnockites, enderbites and two pyroxene granulites are described. Blestomilonitic textures are widespread all over the area. All studied rocks are strongly folded and deformed, showing transposition foliation, boudinage and conspicuous metamorphic banding. The granulites belong to the Juiz de Fora Complex, and are considered of Archean ages, with a polimetamorphic evolution.

Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Reconnaissance field studies have identified a regional metamorphic complex, at least 200 km long and 25 to 50 km wide, in the eastern Chugach Mountains, southern Alaska. The metamorphic complex was developed within the Cretaceous Valdez Group, a flysch sequence that was accreted to the continental margin during the Late Cretaceous or early Tertiary. The metamorphic complex shows the progressive development from subgreenschist and green-schist facies slate, phyllite, and semischist that is regionally characteristic of the Valdez Group, to biotite-plagioclase-quartz schist (± muscovite, cordierite, andalusite, staurolite, garnet, and sillimanite) in an intermediate zone, and amphibolite facies muscovite-biotite-plagioclase-quartz schist and gneiss (± sillimanite and garnet) in a higher-grade central zone. K-feldspar is characteristically absent in the amphibolite facies schists, but it is present in migmatitic rocks that are common to the central zone. The mineral assemblages and petrographic relations indicate that metamorphism took place under low-pressure intermediate-facies series conditions; partial melting took place at the highest grade of metamorphism. Melting produced granodioritic magmas in migma-titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex. The timing of metamorphism as indicated by field relations and K-Ar data is about 50 to 65 m.y. ago and overlaps or is coeval with the emplacement, throughout the Gulf of Alaska region, of early Tertiary plutons that are thought to be anatectic in origin. The distribution of these plutons suggests that a high-temperature low-pressure metamorphic belt, about 1,800 km long, developed along the leading edge of the continental margin in late Paleocene to early Eocene time.