Aluminous Metasediments Research Articles

Cordierite-Bearing Orbicules Record Bulk Assimilation and the Pulsating Nature of Magma Ascent: The Lafaye Orbicular Granite (France)

AbstractThe Lafaye orbicular body was emplaced in the Villatange tonalite-granodiorite unit of the Guéret magmatic complex (Massif Central, France). It consists of plagioclasic orbicules (4–35 cm diameter) embedded in homogeneous cordierite granodiorite. Orbicule cores consist mostly of residual metasedimentary xenoliths or autolithic plagioclasic cumulates. Rims (0.7–8 cm thickness) are single- or multi-layered; layers, mostly comb-textured, comprise alternating sheets dominated by cordierite (XFe = 0.32–0.37) or plagioclase (mostly An25–30). Additional mineral phases are minor biotite (XFe = 0.52; AlVI = 0.58–0.92 atoms per formula unit) and interstitial quartz. Plagioclase and cordierite morphologies (needle-like, skeletal, branching or fan-shaped) indicate growth under high initial supersaturation. However, the final polyhedral shapes and primary zoning of many individual plagioclase crystals, as well as evidence of partial recrystallization, imply significant textural maturation. Whole-rock major and trace element data (A/CNK = 1.12–1.46) and Sr and Nd isotopic compositions (εNd(355 Ma) = −8.6 to −7.4; 87Sr/86Sr(355 Ma) = 0.7110–0.7147) suggest that the parental magma of the orbicules resulted from bulk assimilation of aluminous metasediments by a Villatange-type granodioritic magma. Heterogeneous nucleation and growth of plagioclase and cordierite around xenoliths/autoliths are interpreted in terms of (1) adiabatic decompression of magma pulses ascending in dykes leading to superheating and resorption of early solids, and (2) volatile exsolution, inducing undercooling, supersaturation, and rim crystallization. The variability of layers (number, thickness, mineral distribution, and texture) is considered to result from oscillatory crystallization combined with variable plagioclase growth rates linked to changes in the degree of supersaturation as a function of the extent of melt degassing, itself linked to magma transfer dynamics.

Clockwise, low-[formula omitted] metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny

The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5kbar and 825°C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼550–600°C at below 4kbar, whereas the replacement of sillimanite by garnet–cordierite and subsequent partial replacement of garnet by cordierite at near-peak conditions indicates a clockwise trajectory for this segment of the P–T path. Subsequent near-isobaric cooling below the solidus resulted in secondary, texturally distinct sillimanite partially replacing cordierite. U–Pb zircon dating of pre-, syn- and post-tectonic granitoid intrusions indicates that granitoid plutonism occurred at c. 1120–1085Ma, whereas metamorphic overgrowths on these zircons constrains metamorphism to have occurred from c. 1065 to 1045Ma. A post-tectonic granite dyke intruded after the terrain cooled to subsolidus temperature has an age of 1004±6Ma. The timing and style of metamorphism is similar to that recorded in other parts of the NMC, suggesting that the various crustal fragments that constitute the NMC are not as allochtonous as previously suggested. The emplacement of voluminous granitoid magmas into this terrain triggered initial andalusite grade metamorphism through advective heating, with further slow burial of this juvenile crust leading to low-P–high-T peak metamorphic conditions. Magmatism and metamorphism appear to form a continuum, implying the presence of a long-lived (>50–70Ma), crustal-scale thermal anomaly as the cause of both. Such a primary anomaly most likely involves lithospheric delamination, although direct evidence for mantle involvement, such as contemporaneous mafic magmatism, does not occur in the Aus terrain.

Experimental simulations of anatexis and assimilation involving metapelite and granitic melt

Assimilation of foreign material into a granitic magma body entails mixing between two end-member components: the initially solid assimilant, and the original magma, whose composition is mostly represented by a granitic liquid. In this study, we assess the interactions between a haplogranite liquid made to the composition of the minimum at 200MPa H2O (Ab38.23Or28.72Qtz33.04Crn00.01) with a schist composed of muscovite and quartz with minor biotite and garnet and traces of ilmenite, plagioclase, apatite, and graphite, from 700° to 800°C at 200MPa±aqueous-carbonic fluid. The primary focus of this work is to assess the changes in compositions of granitic liquids that form in each reservoir as a result of the assimilation of schist by granitic magma. Before doing so, we evaluate the experimental database for the compositions of granitic liquids in equilibrium with peraluminous mineral assemblages. Most experiments that entail partial melting of peraluminous mineral assemblages begin with finely milled powders. This method yields melt pools (glass) too small for accurate analysis by electron beam methods. As a result, most published analyses of glasses from such experiments are more aluminous and less sodic than their actual compositions. The analytical errors are large, up to 50% relative to the mean value for some elements. In addition, the experimental melts are commonly more hydrous than reported, and vapor-saturated, even in experiments that are cited as “vapor” or “fluid-absent.” Experiments that combine sand-sized mineral grains and powdered granitic glass yield the best approach to chemical equilibrium and facilitate accurate analysis of the glass. Glasses from such experiments define the compositions of liquids that may be derived either from the anatexis of aluminous metasediments, or by the assimilation of aluminous rocks into a granitic magma whose composition is approximately represented by the minimum composition in the haplogranite system Ab–Or–Qtz–H2O.New experiments designed to simulate the assimilation of metapelites by granitic melt juxtaposed a solid core of anhydrous granitic glass with a solid core of mica schist, both as described above. At comparable conditions, melting within the mica schist alone is far less extensive than when coupled with the granitic liquid. This is because field diffusion, the long-range and highly organized migration of H, Na, and K throughout the entire volume of interconnected melt, quickly brings the composition of silicate liquid to chemical equilibrium across the metapelite–granite interface with respect to these components and in regard to the ASI, Aluminum Saturation Index and the H2O content of the melt. The most significant change entails the diffusion of Na from granitic liquid into the metapelite, which shifts the bulk composition of the metapelite toward the minimum in the granite system, and, in turn, promotes a greater proportion of melting within the metapelite. A weaker counter-flow of K back into the haplogranite raises its K* (molar K/[Na+K]) and lessens the increase in ASI (molar Al/[Na+K]) due to Na loss. The solubility of H2O in granitic liquid is positively correlated with the ASI value. Hence, changes in the ASI of melt that follow from field diffusion of H, Na, and K ions also strongly affect the aH2O within the coupled metapelite–granite system.

Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg–carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation

This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P≥1.1GPa and T=370°–420°C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite±chlorite±paragonite±Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks.

2.62 Ga high‐temperature metamorphism in the central part of the Eastern Dharwar Craton: implications for late Archaean tectonothermal history

Aluminous metasediments occurring in the central part of the Eastern Dharwar Craton (EDC), southern India, develop contrasting mineral assemblages in closely spaced domains. Detailed petrographic, mineral chemical and geothermobarometry showed ‘peak’ metamorphic conditions of 780–820 °C and ~5 kbar that resulted in partial melting of biotite‐bearing protoliths. Initial bulk compositional heterogeneity and different degrees of melt extraction produced diverse restitic mineral assemblages. P–T pseudosections, constructed from effective bulk composition, and consideration of the sequence of mineral reactions, deduced from textural criteria, define a nearly isobaric heating–cooling P–T trajectory. Monazite in the metasediments has a detrital core age of 3161 ± 78 Ma and a metamorphic age of 2625 ± 26 Ma (EPMA chemical age), which is the first record of a pre‐2500 Ma high temperature–low pressure metamorphism event in the EDC. Based on age contemporaneity with the 2.7–2.62 Ga hot juvenile magmas of tonalite–trondhjemite–granodiorite (TTG)‐greenstone accretion and the deduced P–T history, we conclude that heat advecting from rising TTG magmas and mafic magmas (greenstones) as a consequence of mantle melting, resulted in a hot transient mid‐crust during a major late Archaean tectonothermal event in the Eastern Dharwar Craton. All the phenomena are consistent with a recently proposed model involving an arc setting during ridge subduction and consequent opening of a slab window. This study therefore provides new constraints on the thermal evolution of the Eastern Dharwar Craton. Copyright © 2011 John Wiley & Sons, Ltd.

The Heerenveen Batholith, Barberton Mountain Land, South Africa: Mesoarchaean, Potassic, Felsic Magmas Formed by Melting of an Ancient Subduction Complex

The Heerenveen batholith exemplifies an aerially extensive group of � 3·1 Ga felsic potassic batholiths that mark the final magmatic event associated with the cratonization of the central^eastern Kaapvaal Craton, and the transition from the earlier sodic tonalite^ trondhjemite^granodiorite (TTG) to the more recent style of monzogranitic and granitic magmatism. As is typical of these bodies, it is geochemically and mineralogically heterogeneous, sheeted in structure, and was assembled incrementally, with eight main units in four assembly stages. Apart from a metaluminous quartz monzonite, all the Heerenveen rocks are peraluminous.There are few identifiable geochemical lineages among the rocks of the batholith. The geochemical heterogeneity is largely due to protolith internal heterogeneity, tapping of multiple sources, pulsed magma extraction related to sequential occurrence of discrete melting reactions, and varying degrees of entrainment of peritectic minerals formed during partial melting of the crustal protoliths, together with varying structural controls on magma extraction and ascent, which allowed distinct magma sources to be tapped, at different periods during batholith assembly. Fractional crystallization, magma mixing and wall-rock assimilation were not significant processes. TTG rocks are not suitable source materials for any of the Heerenveen magmas; nor are highly aluminous metasediments. For magmatic rocks with SiO2570 wt %, the likely source rocks were intermediate to mafic igneous rocks, with variable degrees of K-enrichment. For those rocks with SiO2472 wt %, the sources were K-enriched intermediate to felsic rocks. It seems likely that the partial melting reactions that produced the Heerenveen magmas occurred at moderate temperatures and that hornblende was present in the residues. This suggests that initial melting may have occurred in the presence of an H2O-rich fluid. There is no evidence that the genesis of the potassic granitic magmas involved either recycling of TTG crust or the presence of aluminous metasediments in the deep crust. The K-enrichment in the protoliths was most probably due to their ultimate derivation from mantle rocks metasomatized by fluids evolved in subduction-like processes. The monzonite in the batholith may have been derived directly by partial melting of this metasomatized mantle.

Strain localisation and high-grade metamorphism at Broken Hill, Australia: a view from the Southern Cross area

Progressive localisation of strain within Proterozoic high-grade rocks in the Southern Cross area of the Willyama Supergroup, Broken Hill, has resulted in the formation of shear zones of highly strained D 2 fabrics separated by less strained D 2 zones that preserve pre-tectonic relationships, complex fold interference patterns and abundant pegmatites related to a D 1/M 1 deformation/metamorphic event. The metamorphic peak (M 1) is characterised by biotite–sillimanite–quartz–alkali feldspar-plagioclase with garnet or cordierite in metapelites, formed at about 750°C and 400–500 MPa, presumably relatively late in M 1, as garnet has overgrown the S 1 biotite–sillimanite fabric. The D 2 strain is concentrated in high-strain zones that form sub-planar and anastomosing networks of apparently simple S–L tectonite fabrics associated with subhorizontal shearing and are locally overprinted by D 2 leucosomes in the more aluminous metasediments. M 2 involved identical metamorphic conditions as M 1. Late D 2 muscovite-bearing pegmatites intruded the area. A further Proterozoic D 3/M 3 event localised in the sites of D 2 high strain is characterised by the growth of staurolite and garnet across a muscovite fabric at about 600°C at 400–600 MPa, resulting from the addition of H 2O to the original upper amphibolite facies assemblages. Younger phases of retrogression, generally involving thrusting, are usually localised in sites of pre-existing high strain. They are characterised by chlorite-muscovite bearing assemblages. It is proposed that the events D 1/M 1,D 2/M 2 and D 3/M 3 occur within separate heating events separated by periods of repose at the ambient geotherm.

Fluid-Absent Melting Behavior of a Two-Mica Metapelite: Experimental Constraints on the Origin of Black Hills Granite

We have studied experimentally the vapor-absent melting behavior INTRODUCTION of a Proterozoic metapelite (HP-60-1) from the Black Hills, South Peraluminous leucogranites such as the Miocene granites Dakota, to assess whether the high-dO, tourmaline-bearing granites of the Himalaya and tourmaline granites of the Harney of the Harney Peak suite resulted from muscovite-dominated dePeak suite in South Dakota are widely considered to be hydration-melting at 10 kbar. Samples were sealed with no added the product of muscovite dehydration-melting; yet few H2O in gold capsules, then run in a 0·5 inch (1·27 cm) pistonexperimental studies of muscovite dehydration-melting cylinder apparatus over the temperature interval 812–975°C. reactions have been performed, because the low temExperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies. Mass balance calculations imply librium. In addition, melt fractions generated are typically the melting reaction muscovite + biotite + plagioclase + quartz low, resulting in small melt pockets which are very difficult → melt + alkali feldspar + aluminosilicate + garnet until to analyze with the electron microprobe. In this study, muscovite is exhausted (<812°C), then the reaction biotite + we assess the possible genetic relationship between the plagioclase + quartz + alkali feldspar ± aluminosilicate → tourmaline-bearing granites of the Harney Peak suite melt + garnet. The inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude. 812°C and produces 1–2 wt % melt. Melt production of the Although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature, hydration-melting reaction was hindered by the surto 32 wt % in our highest temperature run. The low melt prisingly low melt productivity, we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction, illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid-absent muscovite breakdown under the fractions by muscovite-dominated reactions in our starting material. conditions investigated. In addition, melts of HP-60-1 are considerably more mafic than Isotopic and trace element studies indicate a genetic the tourmaline-bearing granites, with average combined TiO2 + relationship between S-type granites and aluminous metaMgO+ FeO contents of 1·85 wt % and 0·84 wt %, respectively. sediments in the Black Hills of South Dakota, where the In particular, melts of HP-60-1 are enriched in Ti, and would Harney Peak Granite (HPG) intruded Proterozoic and probably stabilize biotite rather than tourmaline upon cooling, Archean schists and metagreywackes undergoing regional arguing against HP-60-1 as a potential source rock for the metamorphism. The timing and mode of emplacement of tourmaline granites. the HPG, and the deformation and regional metamorphic

Carpholite, sudoite, and chloritoid in low-grade high-pressure metapelites from Crete and the Peloponnese, Greece

In the high-pressure metamorphic Phyllite-Quartzite Unit of Crete and the Peloponnese, aluminous metasediments contain assemblages with Fe-Mg-carpholite, sudoite, chloritoid, pyrophyllite, chlorite, garnet, white mica and quartz. Variations of parageneses and mineral reactions in response to varying pressure, temperature and activity of H 2 O have been studied in detail. In Eastern Crete (≃300 ° C, 8 kbar), metapelites with high Mg/Fe 2+ are characterized by sudoite-bearing assemblages with chlorite or pyrophyllite. In rocks with low Mg/Fe 2+ , chlorite+pyrophyllite occur instead, suggesting the (continuous) reaction sudoite+quartz=chlorite+pyrophyllite+H 2 O. Additional carpholite rarely occurs in chlorite-pyrophyllite schists, probably through the hydration reaction chlorite+pyrophyllite+H 2 O=carpholite+quartz. Progress of this reaction requires relatively high a H2O , otherwise chlorite+pyrophyllite persist. In Central Crete (≃350 ° C, 9 kbar), the paragenesis chlorite+pyrophylite is no longer stable and chloritoid appears as a widespread rock-forming mineral, occasionally associated with carpholite, through the reactions carpholite=chloritoid+quartz+H 2 O, chlorite+pyrophyllite+I2O=chloritoid+carpholite+quartz. As in Eastern Crete, reactions involving carpholite proceed only in the case of relatively high a H2O ; the estimated values range between 1.0 and 0.7. This variability of H2O is reflected in AFM diagrams by the compositional shift of the three-phase assemblages carpholite-chloritoid-pyrophyllite and carpholite-chloritoid-chlorite. At lower a H2O , probably the normal case inCrete, no carpholite appears and chloritoid directly forms after chlorite+pyrophyllite=chloritoid+quartz+H 2 O. In this case, no minerals or parageneses diagnostic for high-P/low-T metamorphism appear. The same parageneses as in Central Crete have been observed in Western Crete (400 ° C, 10 kbar) and in the Peloponnese (≃450 ° C, 17 kbar), but with carpholites richer in Mg. For these rocks, AFM three-phase parageneses show fixed mineral compositions, suggesting equilibrium with a hydrous fluid phase of constant composition. The highest grade of metamorphic evolution of metapelites from the Phyllite-Quartzite Unit is reflected by the appearance of almandine-rich garnet in the Peloponnese

Zincian högbomite as an exploration guide to metamorphosed massive sulphide deposits

AbstractZincian högbomite (ZnO 3.5–10.5wt.%) occurs as an accessory phase in garnet quartzite that is intimately associated with the Broken Hill and Black Mountain Cu-Pb-Zn-Ag deposits, Aggeneys, South Africa. Högbomite coexists with a number of minerals including quartz, gahnite, sillimanite, sphalerite, pyrrhotine, pyrite, magnetite, and ilmenite, suggesting that högbomite may have formed by sulphidation and oxidation reactions. Such reactions may account for the high Zn content of högbomite. Where associated with metamorphosed massive sulphide deposits högbomite is enriched in Zn relative to that found in ultramafics, Fe-Ti deposits, Fe ores, aluminous metasediments, and skarns. This enrichment in högbomite constitutes a potential exploration guide for metamorphosed massive sulphide deposits.

Epigenetic tourmalinite and arsenopyrite in the aureole of the Leinster batholith, SE Ireland

Metasomatic tourmaline-quartz-hydromuscovite/phengite-fluorite rocks which locally contain in excess of 40% disseminated arsenopyrite and lesser amounts of pyrrhotite, chalcopyrite, pyrite, molybdenite, rutile, ilmenite and Fe-rich chlorite, occur near the village of Inistioge in SE Ireland. They are developed within Lower Palaeozoic phyllites close to the northwest margin of the southern termination of the Leinster batholith. The mineralization occurs within a belt of anomalously high (Caledonian) strain which was active during batholith-intrusion and which is essentially a product of heterogeneous shortening. Tourmaline constitutes up to 60% of the volume of the metasomatic material and although the lithology exhibits bed-parallel lamination, both detailed field and textural relationships demonstrate its origin to be epigenetic. The mineralization is spatially related to pegmatitic (K-feldspar, fluorite and muscovite-bearing) quartz veins with greisen alteration envelopes composed of coarse-grained muscovite, fluorite, apatite and tourmaline. The mineralized rocks were produced during alteration of aluminous metasediments by magmatic and/or meteoric fluids which were channelled through the high strain zone during cooling of the batholith.

The chemistry and origin of zincian spinel associated with the Aggeneys Cu-Pb-Zn-Ag deposits, Namaqualand, South Africa

Zincian spinel or gahnite [(Zn,Fe,Mg)Al2O4] occurs in metamorphosed sulphide-rich rocks, garnet quartzites, quartz-magnetite rocks, aluminous metasediments, barite-magnetite rocks, quartz veins, and pegmatites associated with the Aggeneys base metal deposits, Namaqualand, South Africa. Zincian spinel in, sulphide-bearing rocks, is considered to have formed predominantly by desulphurization reactions involving a member of the system Fe-S-O and sphalerite with sillimanite or garnet. Gahnite in sulphide-free garnet quartzites, quartz-magnetite rocks and barite-magnetite rocks probably formed from Zn and Al that were hydrothermally derived whereas gahnite in aluminous metasediments was derived from the metamorphism of metalliferous shales, in which Zn may originally have been linked to organic material. Gahnite is Zn-rich in sulphide-bearing rock, but is Fe-rich in sulphide-free garnet quartzites and quartz-magnetite rocks. Although Zn-rich spinels represent guides to ore in the Aggeneys area and elsewhere in the Namaqualand Metamorphic Complex, Fe-rich spinels should not be discounted because Zn-rich and Fe-rich spinels occur within metres of sulphides at Aggeneys.

Tectonometamorphic evolution of the gneissic Kidal assemblage related to the Pan-African thrust tectonics (Adrar des Iforas, Mali)

In the central part of the Adrar des Iforas (Mali), the 2 Ba Eburnean granulatic unit has been thrust above a high-grade gneissic unit, the so-called ‘Kidal assemblage’, during an early event of the Pan-African orogeny. The Kidal assemblage can be defined as a tectonic mixing of an Eburnean granulitic basement, its sedimentary cover of Middle to Upper Proterozoic age (quartzites, marbles, basalts and metavolcanics) and various pretectonic rocks: ultrabasic to basic rocks, diorites, tonalites. All these rocks have been deformed during at least four main events and metamorphosed together. Thrusting of the Iforas Granulitic Unit above the Kidal assemblage happened during the first event D1. The movement direction was roughly N–S, as shown by the stretching lineation. Some field criteria indicate a sense of displacement towards the north. The lattice preferred orientation of quartz c- and 〈a〉 axes indicate that the slip was dominantly on prismatic and probably pyramidal planes along an 〈a〉 direction; consequently D1 deformation was achieved at high temperature or low-strain rate. The quartz c- and 〈a〉 axes do not show any constant asymmetry, so they do not indicate a sense of shear. Two metamorphic stages have been found in the Kidal assemblage: the first one is characterized by kyanite in aluminous metasediments and by the occurrence of garnet-clinopyroxene-bearing boundis of basic rocks. The P–T range of this event is located at 700 ± 50°C and around 10 Kb. The second event is a syntectonic high temperature (600–650°C) low pressure (3.5 Kb) stage accompanied by migmatization. Such a tangential deformation in barrowian-type metamorphic conditions and with N–S transport direction is known along the entire Trans-Saharan belt and cannot be related in a simple way to the collision between West African Craton and the mobile belt.

The stability of zincian spinels in sulfide systems and their potential as exploration guides for metamorphosed massive sulfide deposits

Zincian spinel [(Zn,Fe,Mg)Al 2 O 4 ] is found in a variety of geologic settings including metamorphosed massive sulfide deposits, aluminous metasediments, pegmatites, and marbles. Many spinels in and near metamorphosed massive sulfides have formed by desulfidation and oxidation reactions that involve members of the system Zn-Fe-Al-Si-S-O. Experimental reversals of the simplified reaction: ZnS + FeS + 2Al 2 O 3 + O 2 = ZnAl 2 O (sub 4/) + FeAl 2 O 4 + S 2 , using hydrothermal techniques, have been obtained at 2 and 5 kb and various temperatures using Fe (sub 1-x) S + FeS 2 + Fe 3 O 4 as a buffer. Calculated positions of the spinel-sphalerite equilibria in f (sub O 2 ) -f (sub S 2 ) -X space, for ideal behavior along the ZnAl 2 O 4 -FeAl 2 O 4 join as is suggested by octahedral site preference energies, agree with experimental results and theoretical calculations depending on the choice of the free energy of formation of hercynite. The compositions of spinels in equilibrium with sphalerite and either pyrite or pyrrhotite become more Zn-enriched with increasing f (sub S 2 ) and f (sub O 2 ) values and can be used as a measure of f (sub O 2 ) during metamorphism.Spinel associated with metamorphosed massive sulfide deposits, marbles, and aluminous metasediments is enriched in the gahnite (ZnAl 2 O 4 ), spinel (MgAl 2 O 4 ), and hercynite (FeAl 2 O 4 ) molecules, respectively, whereas spinel associated with pegmatites is depleted in the spinel molecule. Such variability in composition constitutes a potential exploration guide for metamorphosed massive sulfide deposits.

Charnockites and the genesis of gem minerals

Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terrains correspond to regions of crustal thickening and are often associated with continental collision processes. The charnockites of the Highland Group of Sri Lanka represent this type and show genetic relationships to the central High-land Group. Geochemical investigations indicate that charnockites play a key role in the formation of the gem minerals. It seems probable that the contact metamorphic effects of charnockitic plutonic activity in the highly aluminous metasediments such as those found in Sri Lanka result in the formation of an assemblage of gem minerals such as sapphirine, cordierite, spinel, ruby and corundum. The pegmatites with which gem minerals like beryl, chrysoberyl and tourmaline are associated can also be considered as being derived from a charnockitic parent.

The Arunta Inlier: a complex ensialic mobile belt in central Australia. Part 1: Stratigraphy, correlations and origin

The Arunta Inlier is a 200 000 km2 region of mainly Precambrian metamorphosed sedimentary and igneous rock in central Australia. To the N it merges with similar rocks of lower metamorphic grade in the Tennant Creek Inlier, and to the NW it merges with schist and gneiss of The Granites‐Tanami Province. It is characterized by mafic and felsic meta‐igneous rocks, abundant silicic and aluminous metasediments and carbonate, and low‐ to medium‐pressure metamorphism. Hence, the Arunta Inlier is interpreted as a Proterozoic ensialic mobile belt floored by continental crust. The belt evolved over about 1500 Ma, and began with mafic and felsic volcanism and mafic intrusion in a latitudinal rift, followed by shale and limestone deposition, deformation, metamorphism and emergence. Flysch sedimentation and volcanism then continued in geosynclinal troughs flanking the ridge of meta‐igneous rocks, and were followed by platform deposition of thin shallow‐marine sediments, further deformation, and episodes of metamorphism and granite intrusion.

Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Abstract Preliminary isotopic data for Late Proterozoic (∼ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (∼ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (∼ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons. As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K 2 O 2 , Rb, Pb, and Th, and decreases in CaO, Na 2 O 2 and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.

ORIGIN OF THE CORUNDUM DEPOSITS OF RENFREW COUNTY, ONTARIO, CANADA

Research Article| December 01, 1957 ORIGIN OF THE CORUNDUM DEPOSITS OF RENFREW COUNTY, ONTARIO, CANADA H. D CARLSON H. D CARLSON DEPARTMENT OF GEOLOGY, UNIVERSITY OF SOUTH DAKOTA, VERMILLION, S. D. Search for other works by this author on: GSW Google Scholar Author and Article Information H. D CARLSON DEPARTMENT OF GEOLOGY, UNIVERSITY OF SOUTH DAKOTA, VERMILLION, S. D. Publisher: Geological Society of America Received: 18 Feb 1955 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1957, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1957) 68 (12): 1605–1636. https://doi.org/10.1130/0016-7606(1957)68[1605:OOTCDO]2.0.CO;2 Article history Received: 18 Feb 1955 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. D CARLSON; ORIGIN OF THE CORUNDUM DEPOSITS OF RENFREW COUNTY, ONTARIO, CANADA. GSA Bulletin 1957;; 68 (12): 1605–1636. doi: https://doi.org/10.1130/0016-7606(1957)68[1605:OOTCDO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In Renfrew County corundum is erratically distributed through a banded complex of rocks rich in scapolite, andeclase, and nepheline but is more abundantly developed and uniformly distributed in contact zones between this banded complex and a younger hybrid alkaline syenite.The scapolite-andeclase-nepheline gneiss complex may have formed by (1) the metasomatic alteration of intercalated calcareous and aluminous metasediments, or (2) the intrusion of sill-like masses of an anorthositic magma into such metasediments. No positive evidence supporting either of these hypotheses was obtained in this study. In both hypotheses it is assumed that some corundum formed by the transformation of alumina-rich parts of the metasediments into corundum-bearing rock either by metasomatism or by magmatic assimilation.The hybrid alkaline syenite appears to have developed by metasomatism of the rocks of the earlier complex. In this process potassium and silica were added, and lime, soda, and alumina were released from the scapolite, andeclase, and nepheline rocks. Much of the corundum in the contact zones is attributed to the crystallization of some alumina released from the older rocks during the transformation into syenite. Some of this alumina may have migrated, with the hydrothermal solutions responsible for the metasomatism, to loci favorable for the development of corundum-rich pegmatitic facies of the hybrid alkaline syenite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Charnockitic and Related Cordierite-Bearing Rocks from Dangin, Western Australia

VI. SummaryA series of charnockitic rocks and associated cordierite-bearing rocks from Western Australia has been described and compared with charnockitic rocks from other localities. This charnockitic suite is considered to be a series of basic intrusions into an older series of paraschists. It has, in places, been considerably contaminated by assimilation of the aluminous metasediments and in such cases is now represented by cordierite-bearing rocks. The basic charnockitic rocks and their contaminated equivalents have been subsequently invaded, under deep-seated conditions by granitic magma or emanations, and now appear as recrystallized remnants of the original masses. They have suffered extensive granitization which has yielded the acid charnockites (hypersthene-alkali felspar gneisses) in which the hypersthene is considered to have been derived from the older hypersthene-bearing basic charnockites. In view of the similarity to the Indian and Antarctic charnockites it appears possible that the charnockitic rocks of these areas are of similar origin, and that charnockite (hypersthene granite) rather than being the result of crystallization from a magma, is due to the granitization of older hypersthene-bearing rocks. If charnockite is of magmatic origin then it is most probable that this magma was of palingenetic origin and that the hypersthene is a relict from the earlier rocks and not the product of crystallization of the charnockite magma. Dunn (1942, p. 23) considers that the evidence afforded by Indian rocks is not opposed to the view that the hypersthene gneisses (charnockites) are the product of partial or complete palingenesis of very deep-seated rocks which had lost their water content.