A mantle-derived dolomite silicocarbonatite from the southwest coast of Ireland

2012 ◽  
Vol 76 (2) ◽  
pp. 357-376 ◽  
Author(s):  
A. E. Brady ◽  
K. R. Moore
Keyword(s):  
Mineral Assemblage ◽  
Magma Source ◽  
Chrome Spinel ◽  
Volatile Components ◽  
Mantle Material ◽  
Rock Composition ◽  
Physical Conditions ◽  
Mineral Assemblages ◽  
Ferroan Dolomite ◽  
Metasomatized Mantle

AbstractThe magma source and evolution of a zoned breccia pipe on the southern Beara Peninsula in southwest Ireland are investigated using the geochemistry of the host mineral assemblages. The clast-poor inner zone of the pipe has a magnesium-rich silicocarbonatite whole-rock composition (14.30 wt.% MgO; 31.80 wt.% SiO2). The silicocarbonatite has retained an ultimate mantle source 13C isotopic composition after metamorphism, consistent with the presence of mantle debris. The silicocarbonatite is Cr-, Ni- and Co-rich (847 ppm, 611 ppm and 60 ppm, respectively) but REE depleted compared with volcanic dolomite carbonatites worldwide. The mineral assemblage consists of Sr-rich (0.55 wt.% SrO) ferroan dolomite, magnesite and pseudomorphs of chlorite after phlogopite, consistent with derivation from a carbonated and hydrated mantle. However, chrome spinel crystals (≤4 40.14 wt.% Cr2O3) are compositionally indistinguishable from unmetasomatized spinel macrocrysts in kimberlites. The silicocarbonatite is inferred to represent a magma produced by partial melting of metasomatized mantle at physical conditions between those in which primary dolomite carbonatite and ultramafic magmas of high-pressure origin form. The primary silicocarbonatite magma ascended and sampled mantle material in a manner similar to kimberlite, and subsequently lost volatile components due to release of metasomatic fluids and later metamorphism.

scholarly journals ULTRABAZICKÉ HORNINY TĚŠÍNITOVÉ ASOCIACE V ZÁPADNÍ ČÁSTI SLEZSKÉ JEDNOTKY

2013 ◽  
Vol 20 (1-2) ◽  
Author(s):  
David Buriánek ◽  
Kamil Kropáč ◽  
Zdeněk Dolníček
Keyword(s):  
Mineral Assemblage ◽  
Igneous Rocks ◽  
Chrome Spinel ◽  
Crystallization Sequence ◽  
Carpathian Flysch ◽  
Mineral Assemblages ◽  
Southwest Part

The teschenite association occurring in southwest part of the Silesian Unit (Carpathian Flysch Belt) consists of several types of predominantly alkaline basic to ultrabasic intrusive and extrusive igneous rocks. Ultrabasic rocks such as peridotites and picrites represent geochemically relatively primitive melt. Primary magmatic mineral assemblages of ultrabasic rocks are characterized by a large amount of olivine (Fo 81–86 mol. %), clinopyroxene (predominantly diopside: XMg = 0.65–0.85, Na = 0.02–0.04 apfu; aegirine-augite sometimes forms small rims around diopside: XMg = 0.00–0.14, Na = 0.40–0.98 apfu) and rare chrome spinel to chromite with Cr/(Cr+Al) values 0.55–0.62 in peridotite or 0.26–0.69 in picrites. Olivine, linopyroxenes or spinelides in these rocks appear as suitable for calculation of PT conditions. The temperatures obtained using the olivine-clinopyroxene thermobarometry (1 294–1 322 °C; 8–11 kbar) are consistent with the appearance of these two minerals in the crystallization sequence of peridotite. However, the temperatures calculated for spinelide inclusions in olivine are ~ 500 °C lower than the experimentally derived liquidus for ultrabasic melt. This difference can be explained by subsolidus re-equilibration during cooling. Younger mineral assemblage in peridotite consists of phlogopite, pargasite, and magnetite.

The occurrence of gold in Voia deposit, South Apuseni Mountains, Romania

2021 ◽  
Author(s):  
Elena-Luisa Iatan
Keyword(s):  
Iron Oxides ◽  
Base Metal ◽  
Mineral Assemblage ◽  
Native Gold ◽  
Metal Sulfides ◽  
Volcanic Complex ◽  
Apuseni Mountains ◽  
Base Metal Sulfides ◽  
Mineral Assemblages ◽  
Phyllic Alteration

<p>Voia deposit belongs to the Săcărâmb-Cetraș-Cordurea Miocene volcano-tectonic alignment of the South Apuseni Mountains, Romania. This large volcanic complex represents a Sarmatian-Pannonian magmatic-hydrothemal mega-system of around 5 km<sup>2</sup> with an estimated 3–4 Ma time-space evolution, consisting of seven andesitic volcanic structures grouped in a circle, three subvolcanic andesite-quartz porphyry microdiorite and associated porphyry Cu-Au(Mo), pyrite Ca-Mg skarns and epithermal Au-Ag-Pb-Zn-Cu mineralizations.</p><p>The mineral assemblages of alteration and mineralization processes belong to several mineralized zones on a vertical scale, according to sampling evidence and laboratory studies. HS products are found in the upper part of the structure (300-500 m), with dominant advanced and intermediate argillic alterations and sulfide-sulfate gold-poor veins (pyrite, marcasite, base metal sulfides, Fe-Ti oxides, vuggy quartz, alunite, gypsum, anhydrite). Within the 500-1200 m depth, the HS mineral assemblages gradually decrease in favor of IS and LS products. It is characterized by the coexistence of gold-rich LS assemblage (native gold, base metal sulfide, adularia, sericite-illite, chlorite, carbonates ± anhydrite veins), with the IS assemblage (iron oxides, chalcopyrite, pyrite, quartz, anhydrite). These assemblages overprint the HS mineral associations, resulting in a transition zone characterized by gold - pyrite - chalcopyrite - iron oxides - quartz - anhydrite mineral assemblage characteristic for HS and native gold - pyrite - base metal sulfides - carbonates - quartz mineral assemblage corresponding to IS+LS type.</p><p>Gold is present in all of the identified mineralization forms: porphyry-epithermal Cu-Au, epi-mesothermal carbonate veins with gold - base metal sulfides, quartz veins with pyrite - chalcopyrite - magnetite ± hematite ± anhydrite, anhydrite veins with base metal sulfides and sulfosalts, anhydrite veins with pyrite - anhydrite ± quartz, vuggy quartz (silica residue) with gold-poor pyrite veins and impregnations in porphyry systems.</p><p>Drilling core samples revealed that in Voia deposit, gold is concentrated in chalcopyrite (drills no. 7, 19, 37) along with pyrite - magnetite - hematite - quartz assemblage from the late potassic stage. The major amount of gold associated with chalcopyrite tends to be mainly submicroscopic. Pyrite from anhydrite veins of the early potassic stage ± phyllic alteration is relatively poor in gold (drills no. 1-6, 8-14). However, the highest gold contents are present in pentagonal dodecahedron pyrites (drills no. 33, 38, 39) of pyrite-chalcopyrite-magnetite ± hematite-quartz assemblage from late potassic stage ± phyllic alteration. Pyrite associated with magnetite from anhydrite veins tends to be poor in gold (drills no. 8, 11, 15, 28, 29). A carbonate vein containing gold-bearing base metal sulfides that was intercepted at 960,00-960,30m depth by drill no. 17 is one of the richest in gold.</p><p>Native gold occurs as fine inclusions in ore minerals (5-20 μm). Large irregular grains of native gold (>50 μm) appear at mineral boundaries and along the fissures. The gold color is bright yellow and has a measured Au:Ag ratio of 5:1, suggesting that native gold has been formed at a relatively high temperature.</p><p>Acknowledgments: This work was supported by two Romanian Ministry of Research and Innovation grants: PN-III-P4-ID-PCCF-2016-4-0014 and PN-III-P1-1.2-PCCDI-2017-0346/29.</p>

scholarly journals Petrogenesis of the synplutonic high-magnesian porphyritic dykes from the mafic-granitoid plutons of the East-European Craton, Voronezh crystalline massif

2019 ◽  
Vol 64 (4) ◽  
pp. 356-371
Author(s):  
R. A. Terentiev
Keyword(s):  
Trace Elements ◽  
Magma Chamber ◽  
Earth Element ◽  
Parent Magma ◽  
Chrome Spinel ◽  
Calc Alkaline ◽  
East European ◽  
Fine Grained ◽  
Mineral Inclusions ◽  
Metasomatized Mantle

This paper documents the data on high-Mg porphyrite dykes (PDs) from the mafic to felsic (~2.09 Ma) plutons of Elan complex (EC). The low-thickness (first centimeters) synplutonic dykes are characterized by sharp straight contacts without visible chilling zones, in contrast to the larger (up to 119 m) dykes that have gradual transitions. The dykes are fresh, porphyritic (bronzite, Al-enstatite, labradorite) and has fine-grained mainly quartzo-feldspathic (+biotite, sulfides, accessories, ±hypersthene) matrix. Based on geochemistry data the PDs are intermediate rocks (SiO2 = 58.9–60.3 wt. %) and plot into calc-alkaline series with high magnesian of whole rock (Mg# ~0.7) and felsic (68.9–70.2 wt. %) matrix (Mg# ~0.5). The PDs show differentiated rare-earth element patterns with negligible Eu anomalies. The bronzite phenocrysts varying sizes are characterized by block zoning and contain irregular inclusions of olivine (Mg# ~0.85), clinopyroxene (Mg# ~0.88), phlogopite (Mg# up to 0.94), labradorite, chrome spinel, graphite and sulfides. The Al-enstatite phenocrysts are practically sterile with respect to trace elements and mineral inclusions. The geochemical features as well as diffusion zones, reaction rims, and resorbed faces of the phenocrysts such as orthopyroxene and plagioclase indicate processes of recrystallization and/or partial dissolution of nonequilibrium crystals in the melt and indicate intratelluric nature of the dyke phenocrysts that cores are inherited from the EC derivatives/cumulate. The mineral thermometry estimates are: (1) the parent magma starting temperatures of 1200–1400 °С and (2) the EC crystallization temperatures 1080–1155 °С, (3) the PD emplacement temperatures 910–1070 °С. The petrogenetic model supposes the generation of EC high-temperature magmas similar to boninites from an upper metasomatized mantle. The melt is contaminated with continental crust lithologies. It implies the half-way evolved magma chamber in the crust. The PD melt, as a result of ending of the half-way magma chamber evolution, was emplaced into the still unheated EC plutons.

On the provenance of flysch deposits in the External Hellenides of mainland Greece: results from heavy mineral studies

1998 ◽  
Vol 135 (3) ◽  
pp. 421-442 ◽  
Author(s):  
P. FAUPL ◽  
A. PAVLOPOULOS ◽  
G. MIGIROS
Keyword(s):  
Heavy Mineral ◽  
Frequent Occurrence ◽  
Chrome Spinel ◽  
Mineral Distribution ◽  
Mineral Assemblages ◽  
Subordinate Role ◽  
East West

The terrigenous materials of the flysch deposits of the External Hellenides of mainland Greece have been characterized by their heavy mineral assemblages, based on 194 samples. Three major source types were distinguished. (1) A metamorphic source is shown by abundant garnet accompanied by traces of staurolite and chloritoid. In the source of the Pindos and Ionian zones, blueschist complexes were incorporated within the metamorphic terrains, demonstrated by the frequent occurrence of blue amphiboles. (2) The existence of ophiolitic sources is indicated by the occurrence chrome spinel. Pyroxenes, green amphiboles and partly epidote are related to volcanic/metavolcanic complexes. High ophiolitic detritus was especially found in Mid-Cretaceous turbiditic layers supplied from internal terrains. (3) Granitoid and gneiss source terrains are indicated, predominantly represented by zircon, tourmaline and apatite. This type of source is characteristic for Mid-Cretaceous turbidites sampled in western parts of the Pindos zone. In the terminal flysch deposits, granitoid detritus played only a subordinate role. An extensive recycling of Pindos Flysch material into the younger Western Hellenic Flysch can be excluded. Stratigraphic trends in the heavy mineral distribution of the terminal Pindos Flysch give insights into the changing tectonic situation of the source terrains. A regional east–west trend with changing ophiolitic detritus, observed in the Parnassos-Ghiona Flysch, points to a complex feeder system.

The system Pd–Ag–S: phase relations and mineral assemblages

2020 ◽  
Vol 84 (1) ◽  
pp. 125-130
Author(s):  
Anna Vymazalová ◽  
František Laufek ◽  
Alexandr V. Kristavchuk ◽  
Dmitriy A. Chareev
Keyword(s):  
Mineral Assemblage ◽  
Glass Tube ◽  
S Phase ◽  
Mineral Deposits ◽  
Mineral Assemblages ◽  
Pd Alloy ◽  
Stable Association ◽  
Silver Palladium ◽  
New Phase ◽  
Silica Glass Tube

AbstractPhase equilibria in the system Pd–Ag–S were studied using the silica-glass tube method at 400°C and 550°C. In the system we synthesised three ternary phases: coldwellite (Pd3Ag2S), kravtsovite (PdAg2S) and a new phase Pd13Ag3S4. At 400°С, coldwellite forms a stable association with vysotskite (PdS) and vasilite (Pd16S7); vysotskite and kravtsovite; phase Pd4S and a Ag–Pd alloy; it also coexists with a new phase Pd13Ag3S4. Kravtsovite is stable up to 507°C; the presence of kravtsovite in the mineral assemblage reflects its formation below this temperature. The occurrence of coldwellite, vysotskite and Ag2S together in equilibrium reflects the formation of this mineral assemblage above this temperature (507°C). Coldwellite is stable up at 940°С. Mineral assemblages defined in this study can be expected in Cu–Ni–PGE mineral deposits, associated with mafic and ultramafic igneous rocks, in particular in mineralisations with known silver–palladium sulfides.

Isograds and mineral assemblages in the eastern axial zone, Montagne Noire (France): implications for temperature gradients and P–T history

1982 ◽  
Vol 19 (1) ◽  
pp. 129-143 ◽  
Author(s):  
Peter H. Thompson ◽  
Jean-Pierre Bard
Keyword(s):  
Metamorphic Grade ◽  
Temperature Gradients ◽  
Low Grade ◽  
Reaction Sequence ◽  
Axial Zone ◽  
Rock Composition ◽  
Mineral Assemblages ◽  
Montagne Noire ◽  
Sequence Study

Detailed petrography across a metapelitic sequence in the eastern axial zone of the Montagne Noire, France, is the basis for a sequence of isograds marking the first appearance of biotite–cordierite, staurolite, andalusite, and sillimanite. The juxtaposition of low-grade biotite-free rocks against medium-grade rocks at the gently dipping biotite–cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence. Study of mineral assemblages with respect to an AFM reaction sequence indicates the staurolite isograd is related to changes in rock composition, and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower. These assemblages are interpreted to contain a record of part of the P–T history during which pressure decreased as temperature increased. P–T profiles show that temperature gradients of 200–300 °C/km suggested by previous workers are not required to explain the isograd pattern; gradients of 37 °C/km or less are sufficient.

Mineralization in the Bonser vein, Coniston, English Lake District: mineral assemblages, paragenesis, and formation conditions

1982 ◽  
Vol 46 (340) ◽  
pp. 343-350 ◽  
Author(s):  
C. J. Stanley ◽  
D. J. Vaughan
Keyword(s):  
Electron Microprobe ◽  
Mineral Assemblage ◽  
Oxygen Activity ◽  
British Isles ◽  
Lake District ◽  
Coexisting Phases ◽  
Mineral Assemblages ◽  
English Lake District ◽  
Formation Conditions ◽  
Native Bismuth

AbstractThe Bonser vein, one of the most productive sources of copper in the English Lake District, contains a mineral assemblage comprising quartz, chlorite, calcite, dolomite, stilpnomelane, magnetite, pyrrhotine, pyrite, marcasite, native bismuth, bismuthinite, laitakarite, joseite, arsenopyrite, chalcopyrite, sphalerite, galena, and cosalite. The phases pyrrhotine, sphalerite, arsenopyrite, laitakarite, joseite, and cosalite were analysed by electron microprobe. The occurrence of laitakarite is the first reported in the British Isles. Textures of the ore and gangue minerals show that the vein minerals were deposited successively in open voids. The sequence of deposition was of quartz forming throughout, early chlorite and stilpnomelane with some dolomite and calcite, magnetite as the earliest opaque phase, followed by arsenopyrite, pyrrhotine, sphalerite, chalcopyrite, pyrite, and marcasite; the bismuth- and lead-bearing minerals were the last to form.The data on the coexisting phases and their compositions have been used to estimate the temperatures of mineralization and the activities of sulphur and oxygen. Magnetite and arsenopyrite appear to have formed at 350–400% withaS2reaching 10−12to 10−14atm. andaO2around 10−24to 10−29atm. The assemblage of quartz, chlorite, stilpnomelane, calcite, dolomite, pyrrhoine, chalcopyrite, sphalerite, and (late) arsenopyrite was probably deposited at ∼ 240°C, withaS2decreasing to 10−15to 10−16atm andaO2to 10−38) to 10{su−44atm. The later minerals probably formed at temperatures as low as 200°C and under similar conditions of sulphur and oxygen activity.

scholarly journals Metamorphism of metachert from the Southern Alps, New Zealand. A thermodynamic forward modelling study

2021 ◽  
Author(s):  
◽  
Jill Fernandes
Keyword(s):  
Mineral Assemblage ◽  
Bulk Composition ◽  
Carbonaceous Material ◽  
Southern Alps ◽  
Forward Modelling ◽  
Bulk Rock ◽  
Rock Composition ◽  
Bulk Rock Composition ◽  
Garnet Composition ◽  
Garnet Core

<p>Scattered, scarce occurrences of garnet- and quartz-rich metamorphic rock, probably derived from Mn- and Fe-rich chert, occur within metamorphosed greywacke sequences worldwide. The metamorphism of such garnetiferous metacherts has not previously been investigated using modern thermodynamic forward modelling techniques due to the lack of appropriate, internally-consistent activity-composition (a–x) models for Mn-bearing minerals. The present study applies thermodynamic forward modelling using the recently-proposed a–x models of White et al. (2014) to investigate the metamorphism of garnetiferous metachert samples from the Southern Alps, New Zealand.  Pressure-temperature (P–T) pseudosections are used in combination with results from petrography, element composition mapping using micro X-ray fluorescence (µXRF) and scanning electron microscope (SEM) methods, and garnet composition data from analytical transects by electron probe microanalysis (EPMA), to study metachert metamorphism. All the samples are compositionally layered, so the possibility exists that an input bulk rock composition might not match the effective bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection matched the mineral assemblage in the rock, this was taken as an initial indication of a permissible input bulk rock composition. In that case, refined constraints on the P–T conditions were sought by comparing calculated and measured garnet compositions. The studied rocks include samples that are carbonate-bearing, which require consideration of the effects of fluid composition in mixed H₂O–CO₂ fluids, as well as a sample in which the garnet is strongly zoned, texturally-complex, and inferred to be of polymetamorphic origin. The effects of element fractionation by that garnet were investigated by recalculating the P–T pseudosection using a new bulk rock composition with the garnet core content removed. In none of the samples did the calculated and observed composition isopleths for the garnet cores match, suggesting that initial garnet nucleation in these Mn-rich rocks was locally controlled. For most samples in which the calculated and observed mineral assemblages matched, successful estimates of the peak metamorphic conditions were obtained. A garnet chert (A12E) from the mylonite zone of the Alpine Fault at Vine Creek, near Hokitika, gave a tight intersection of composition isopleths, indicating peak metamorphic conditions of 510 °C/5.5 kbar, after recalculation to correct for element fractionation by the strongly-zoned garnet. This tight, modern constraint is within error of previously-reported results from traditional geothermobarometry (420–600 °C/5.9–13 kbar) and Raman spectroscopy of carbonaceous material (RSCM T = 556 °C) from nearby sites. A peak metamorphic estimate of 520–550 °C/7–10 kbar was obtained from a dolomite-bearing sample from the garnet zone near Fox Glacier (J34), in good comparison with published temperatures from Raman spectroscopy of carbonaceous material in nearby metagreywacke samples (526–546 °C). The prograde metamorphic P–T path was probably steep, based on growth of the garnet core at ~475535 °C/5–9 kbar. The successful results for these garnet chert samples show that the new a-x models for Mn-bearing minerals extend the range of rock types that are amenable to pseudosection modelling.  Results obtained in this study also serve to highlight several possible concerns: a) garnet nucleation and initial growth in very Mn-rich rocks may be subject to local compositional or kinetic controls; b) bulk rock compositions may not always mimic the effective bulk composition; c) the existing a–x models for Mn-bearing minerals and white micas may need refining; and d) some rocks may simply be ill-suited to thermodynamic forward modelling. Items a) and b) may be indicated by the common observation of a mismatch between predicted and measured garnet composition isopleths for garnet cores, and by a mismatch between garnet composition isopleths and the appropriate mineral assemblage field for sample AMS01, from the mylonite zone, Hari Hari, Southern Alps. For item c) every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals showed garnet stable to very low temperatures below 300 °C. In addition, the P–T pseudosection for an oligoclase-zone metachporphyroblasts of Fe-Ti oxides (magnetitert (Sample J36) from Hari Mare stream, Franz Josef - Fox Glacier, indicated that the white mica margarite should be present instead of plagioclase (oligoclase), for a rock in which oligoclase is present and margarite is absent, a problem previously noted elsewhere. Item d) is exemplified by a very garnet-rich ferruginous metachert sample (J35, garnet zone, headwater region, Moeraki River, South Westland) which proved impossible to model successfully due to its complex mineral growth and deformation history. This sample contained multiple generations of carbonate with differing compositions, amphibole (not incorporated for modelling with the new a–x models for Mn-bearing minerals), large e associated with smaller, possibly later-formed ilmenite), and the garnet bands were offset by late deformation.  The garnetiferous metachert samples studied here preserve in their textures and compositions clues to their growth mechanism and metamorphic history. The textures in at least two of the samples are consistent with the diffusion controlled nucleation and growth model for garnet. This research has successfully used state of the art thermodynamic modelling techniques in combination with the latest internally consistent a-x models on Mn-rich metachert, for the first time, extracting P–T conditions of the metamorphism of garnetiferous metachert from the Southern Alps.</p>

Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

2000 ◽  
Vol 64 (4) ◽  
pp. 593-613 ◽  
Author(s):  
A. P. Jones ◽  
T. Kostoula ◽  
F. Stoppa ◽  
A. R. Woolley
Keyword(s):  
High Pressure ◽  
Heat Flow ◽  
Southern Italy ◽  
Mineral Chemistry ◽  
Continental Rift ◽  
Mantle Xenoliths ◽  
Chrome Spinel ◽  
Partial Melt ◽  
Host Magma ◽  
Mineral Assemblages

AbstractWe present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90–92), orthopyroxene (∼En89, Wo2.0), clinopyroxene (Mg90–92, 3–6% Al2O3, 1–1.5% Cr2O3), and chrome-spinel (14–20% MgO, ∼30–40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite) Some contain primary mica (phlogopite with ∼4% FeO, 1.8% Cr2O3, 1.4–2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (∼0.5–1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14–22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm−2) but it compares well with the high-pressure end of a typical alkaline continental rift.

Chapter 2 The Early Stages - Scourian and Inverian

2002 ◽  
Vol 26 (1) ◽  
pp. 11-20
Keyword(s):  
Mineral Assemblage ◽  
Dyke Swarm ◽  
Small Bodies ◽  
Mafic Mineral ◽  
Partial Recrystallization ◽  
Early Stages ◽  
Mineral Assemblages

Archaean gneisses occupy large areas on each side of the central NW-SE-trending belt formed by the outcrop of the Loch Maree Group (Fig. 2.1). They are cut by numerous amphibolite dykes of the 'Scourie dyke' swarm (see Chapter 3). The gneisses are predominantly granodioritic to tonalitic, quartzo-feldspathic biotite gneisses but large areas of more mafic hornblende gneiss occur in the NE, and small bodies of amphibolite are enclosed within the gneisses in all parts of the area.The gneisses have undergone a long and complex history, having experienced Scourian, Inverian, and Laxfordian thermotectonic events. Although Scourian structures have been preserved locally, little of the original Scourian mineral assemblage remains, and the mineral assemblages mainly reflect Inverian and Laxfordian recrystallizations (see Section 2.4).The quartzo-feldspathic biotite gneisses are pale grey to pinkish-weathering, banded or massive, granodioritic (or less commonly tonalitic) gneisses containing biotite as their main mafic mineral. The banded varieties show partial or complete segregation of micas into seperate layers or lenses (Fig. 2.2). Typical examoles contain oligoclase, quartz, microcline and a dark brown biotite, in varying proportions. Muscovite, chlorite or epidote may be present in addition, together with traces of opaque ore and apatite. The gneisses within several kilometres of the outcrop of the Loch Maree Group exhibit evidence of partial recrystallization from a coarsergrained assemblage, the larger feldspar grains being surrounded by granular aggregates of smaller grains feldspar and quartz. Chlorite and epidote are clearly replacive.Retrogressive recrystallization to an epidote-bearing assemblage is particularly marked on the SW

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