Andalusite Porphyroblasts Research Articles

Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette di Ries, eastern Alps, Italy

Metapelitic hornfelses in the contact aureole of the Vedrette di Ries pluton exhibit the terminal decomposition of Zn-poor Fe-staurolite in a muscovite-quartz-free domain. The reaction takes place only within coarsegrained sillimanite that has replaced andalusite porphyroblasts during prograde metamorphism. The product is a gahnite-poor hercynitic spinel, which occurs as very small grains closely associated in space with resorbed staurolite. Microstructural observations indicate that bereynite growth postdates the pseudomorphs of sillimanite after andalusite. The textural evidence for a genetic relationship between hercynite and staurolite is confirmed by the identical Fe/Mg/Zn ratios of the two minerals, which causes the collinearity of hercynite, staurolite and Al2SiO5 in FeO−MgO−ZnO−Al2O3−SiO2−H2O composition space (FMZASH), and indicates hercynite formed by the reaction: Fe-staurolite = 3.85hercynite + 5.1sillimanite + 2.55quartz + 2H2O Staurolite inclusions within andalusite did not break down to form hercynite, indicating a kinetic control, as well as little overstepping of the equilibrium conditions, of the reaction forming hercynite. Assuming overstepping did not occur, modelling of the reaction with existing thermodynamic data in the simplified FASH system suggests that the terminal breakdown of staurolite to form hercynite occurred at 2.5–3.75kbar and 585–655°C.

A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland

Regional mapping demonstrates that S2 is the main fabric in the Central Highland Dalradian and that it is this fabric, not S3 as previously thought, which overprints the 590 Ma Ben Vuirich granite. The granite appears to have been emplaced following a tectonic event (D1) in the Dalradian rocks, for a finely-spaced penetrative fabric seen in newly-discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts (now both altered) which predate the D2 metamorphism. The Ben Vuirich granite cuts the Neoproterozoic Dalradian sequence of the Central Highlands of Scotland (Fig.1, inset), and was first recognized as an older foliated granite by Barrow et al. 1905. Later workers ( Bradbury et al. 1976 ; Bradbury 1978 ) concluded that the granite was intruded between D2 and D3 in the Dalradian rocks ( Harris et al . 1976 ) and for many years an U-Pb zircon age of 514 ±7 Ma from the intrusion (Pankhurst & Pidgeon 1976) provided a valuable datum point in the evolution of the Scottish Caledonides. However, Dalradian chronology has recently undergone a radical revision following the determination of a precise U-Pb age of 590 ± 2 Ma for the emplacement of the Ben Vuirich granite ( Rogers et al. 1989 ). In accepting the interpretation of Bradbury et al. 1976 that the regional penetrative fabric which affects the country rocks (and is continuous with that in the granite) is of D3 age, Rogers et al. were also guided by the conclusions of Harte et al. 1984 and Dempster & Harte

Sigmoidal inclusion trails, punctuated fabric development, and interactions between metamorphism and deformation

ABSTRACTPorphyroblast inclusion fabrics are consistent in style and geometry across three Proterozoic metamorphic field gradients, comprising two pluton‐related gradients in central Arizona and one regional gradient in northern New Mexico. Garnet crystals contain curved ‘sigmoidal’ inclusion trails. In low‐grade chlorite schists, these trails can be correlated directly with matrix crenulations of an older schistosity (S1). The garnet crystals preferentially grew in crenulation hinges, but some late crenulations nucleated on existing garnet porphyroblasts. At higher grade, biotite, staurolite and andalusite porphyroblasts occur in a homogeneous S2 foliation primarily defined by matrix biotite and ilmenite. Biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded S1 schistosity. Staurolite and andalusite contain distinctive inclusion‐rich and inclusion‐poor domains that represent a relict S2 differentiated crenulation cleavage. Together, the inclusion relationships document the progressive development of the S2 fabric through six stages. Garnet and biotite porphyroblasts contain stage 2 or 3 crenulations; staurolite and andalusite generally contain stage 4 crenulations, and the matrix typically contains a homogeneous stage 6 cleavage.The similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation. Three end‐member relationships may be involved: (1) tectonic linkages, where similar P‐T‐time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures; (2) deformation‐controlled linkages, where certain microstructures, particularly crenulation hinges, are favourable environments for the nucleation and/or growth of porphyroblasts; and (3) reaction‐controlled linkages, where metamorphic reactions, particularly dehydration reactions, are associated with an increase in the rate of fabric development. A general model is proposed in which (1) garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges, and (2) chlorite‐consuming metamorphic reactions lead to pulses in the rate of fabric evolution. The data suggest that fabric development and porphyroblast growth may have been quite rapid, of the order of several hundreds of thousands of years, in these rocks. These microstructures and processes may be characteristic of low‐pressure, first‐cycle metamorphic belts.

A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States: Discussion

Ririe states that the purpose of his comparison of the mineralogy and chemistry of gold deposits from Western Australia (Sons of Gwalia and Mount Celia) and the Carolina slate belt (Haile and Brewer) is illustrate that alteration in these two terranes is the result of similar processes that were also responsible for the introduction of gold into the altered (p. 1561). The alteration assemblages contain highalumina minerals such as andalusite, kyanite, pyrophyllite, kaolinite, and sericite, plus local alunite. Ririe interprets these rocks as having ... been altered by acid hydrothermal solutions prior to regional (p. 1561) based upon relations [that] indicate that the rocks had their alteration composition by the time the suite of aluminosilicate minerals was forming (p. 1574) and the absence of documented low-pH regional metamorphic fluids capable of producing alunite plus a suite of high-alumina minerals such as andalusite-pyrophyllite (p. 1574). He concludes that the best analogs for these gold deposits are acid-sulphate epigenetic gold deposits such as Goldfield, Nevada, and modern geothermal systems. The important question to be asked is, Does the presented evidence convincingly support the interpretation? In this writer's opinion, the presented evidence does not positively answer this question. ~easons why the two principal arguments proposed above by Ririe inadequately support his interpretation follow below. The textural evidence consists of photomicrographs of randomly oriented andalusite porphyroblasts, fractured and rotated andalusite crystals, and folded quartz-pyrophyllite + pyrite banding in the Western Australia deposits (Fig. 3, p. 1564), randomly oriented andalusite porphyroblasts at Brewer, and deformed and altered argillite at Haile (Fig. 7, p. 1568). It is reasonable to suggest that the high-alumina assemblages reflect metamorphism of a previously altered and 1 aluminous rock. But that is all that it proves; the products of hydrothermal alteration have been metamorphosed! However, I the critical questions is the timing of hydrothermal alteration relative to metamorphism and regional deformation? and What is the metamorphic and deformation history of the surrounding country rocks? are not addressed. ree ens chi st facies regional metamorphism is implied for Western Australia or stated (Carolina slate belt, p. 1563) with only one period of deformation and metamomhism imolied. ow ever. more than one period of deformationkd metahorphism occurred in both the Norsemen-Wiluna greenstone belt (Archibald et al. 1978; Bickle and Archibald 1984) and the Carolina slate belt (Williams and Hatcher 1983).

High chromium and vanadium in andalusite, phengite and retrogressive margarite in contact metamorphosed Ba-rich black slate from the Abercrombie Beds, New South Wales, Australia

AbstractGraphitic, quartz-rich black slate within the Late Ordovician Abercrombie Beds, Lachlan Fold Belt, southeast Australia, has undergone contact metamorphism adjacent to the Siluro-Devonian Wyangala Batholith. This produced porphyroblasts of andalusite and cordierite, and smaller grains of pale green phengitic mica. Later regional metamorphism caused complete retrogression of cordierite and partial retrogression of andalusite, with margarite replacing some andalusite.The aluminous minerals andalusite, margarite and phengite all contain V and Cr substituting for Al. Andalusite has up to 1.39% V2O3 and 1.09% Cr2O3, margarite has up to 1.07% V2O3 and 0.37% Cr2O3, and phengitic mica has up to 6.93% V2O3 and 1.52% Cr2O3. This mica also has BaO contents of up to 1.96%.Chemical analyses reveal very high SiO2 contents for these rocks (about 89%), carbon contents of about 2%, and extremely low CaO, FeO, MgO and Na2O. Although V and Cr are prominent in aluminous minerals, their concentrations in the rock are only about average for black shales. However, Ba values range from 2000 to 6000 p.p.m., well above average for black shales. It is suggested that V and Cr probably precipitated from sea water, but Ba may have been concentrated by planktonic organisms such as radiolaria.

Synvolcanic and syntectonic gold mineralization at the Bousquet Mine, Abitibi greenstone belt, Quebec

The Bousquet pyrite gold deposit, consisting of disseminated- and vein-type mineralization in sheared and altered greenschist facies Archean volcanic rocks, is hosted by two vein sets: foliation-parallel veins in ore zone 3 and foliation-oblique veins in zones 1, 2, 4, and 5. Foliation-parallel veins are composed of pyrite and lesser amounts of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, galena, bornite, rutile, tellurides, and native gold. The foliation-oblique veins of zones 1 and 2 are composed of pyrite, chalcopyrite, and sphalerite, with subordinate amounts of pyrrhotite, arsenopyrite, galena, stannite, gudmundite, rutile, ilmenite, and gold. Quartz and muscovite are the principal gangue minerals in both vein types in ore zones 1, 2, and 3. In zone 4, foliation-oblique veins are composed of pyrite, magnetite, pyrrhotite, and minor amounts of chalcopyrite and gold. The gangue minerals are calcite, ankerite, white mica, chlorite, and tourmaline. The veins of zone 5 contain pyrite, chalcopyrite, sphalerite, bornite, magnetite, rutile, and gold in a gangue of quartz, carbonate, white mica, and chlorite. Disseminated auriferous pyrite occurs in zones 3 and 5.Mineralogical and textural features indicate that gold emplacement and associated alteration occurred during two events, one predating peak regional metamorphism and another postdating peak metamorphism. Early, premetamorphic mineralization and related synvolcanic (?) hy-drothermal alteration produced initial auriferous sulfides and an advanced argillic alteration. Subsequent deformation, recrystallization, and metamorphic remobilization of early sulfides resulted in the currently observed, structurally controlled vein-type gold deposits. The vol-canogenic event is suggested by highly aluminous rocks closely associated spatially with the mineralized zones, by textures preserved through the overprint of metamorphic textures, and by the occurrence of stannite in ore zones 1 and 2.The late, postmetamorphic gold mineralization is considered to be contemporaneous with retrograde alteration of metamorphic minerals formed during late deformation. This alteration produced a pervasive kaolinite-pyrophyllite-diaspore gangue mineral assemblage involving the replacement of syntectonic andalusite porphyroblasts in felsic rocks. In mafic host rocks, retrograde alteration is characterized by carbonate, chlorite, muscovite, pyrite, and gold. The formation of a white mica-diaspore-carbonate-chlorite assemblage and the emplacement of associated gold are interpreted as epigenetic metamorphic events because these late retrograde minerals overprint the greenschist facies mineral assemblage.

Lack of porphyroblast rotation in noncoaxially deformed schists from Petrel Cove, South Australia, and its implications

Inclusion trails representing an S 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non-coaxial deformations. The pelitic schists of the Kanmantoo Group at Petrel Cove, South Australia contain two generations of porphyroblasts. The first one consists of cordierite porphyroblasts that formed early in D 2 and contain straight to slightly sigmoidal inclusion trails of S 1 . The second generation consists of andalusite porphyroblasts that overgrew crenulated S 2 late during D 3. Several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection. Hence, S 1 was planar and horizontal prior to D 2. S 1 measurements on limbs and hinges of a mesoscale D 2 foldpair show that folding had little effect on porphyroblast orientation as the S 1 orientation remained constant and subhorizontal around the fold. Hence, porphyroblasts have not rotated during any of the non-coaxial deformations accompanying and following their growth. This is interpreted as a result of the partitioning of the deformation around them. S 1 in the matrix has been totally destroyed by the formation of S 2 as a fully differentiated crenulation cleavage. What has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning. The presence of a subhorizontal S 1 foliation suggests horizontal movements (e.g. thrusting or detachment faulting) during the earliest phase of the Adelaidian orogeny.

Microstructural controls and the role of graphite in matrix/ porphyroblast exchange during synkinematic andalusite growth in a granitoid aureole

Abstract In the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event (D2) synchronous with the emplacement of muscovite‐bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse‐grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth.Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non‐graphitic layers, were preferentially replaced by quartz. This resulted in muscovite‐depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation of D2 crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite.Lack of deflection of the pre‐existing mylonitic foliation and anastomosing of the axial planes of D2 crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existing foliations.

Sequential growth of cordierite and andalusite porphyroblasts, Cooma Complex, Australia: microstructural evidence of a prograde reaction

Abstract Low‐pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south‐east Australia, has produced cordierite‐andalusite schists at intermediate grades. The first foliation (S1) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation‐foliation (S2) and that andalusite porphyroblasts grew during the development of a later crenulation‐foliation (S3). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in ‘beard’structures between cordierite microboudins formed by extension in S3 confirms that the andalusite grew during the development of S3. The investigation shows that porphyroblast‐matrix relationships can preserve the orientation of an early S‐surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.

Pre-tectonic metamorphic crystallization in the Mt. Lofty Ranges, South Australia

SUMMARYEvidence for the growth of andalusite and staurolite porphyroblasts prior to the first penetrative movements in the rocks of the Mt. Lofty Ranges is presented. The implications of this unusual observation are discussed.

Structures in the Lower Paleozoic Meguma Group, Nova Scotia

At least three generations of folds and faults affect slates, schists, and quartzites of the Meguma Group and other Lower Paleozoic rocks over a large part of mainland Nova Scotia. The main folds ( F 1 ) are upright and low-plunging and trend northeast to east closely parallel to the strike of a steep axial-plane foliation ( S 1 ). Mid-Devonian granite batholiths intersect the F 1 folds with little disruption of the trends. Cross-folds ( F 2 and F 3 ), although widespread, are less pervasive than the F 1 folds; most, but not all, are small-scale structures restricted to the foliated argillites. The cross-folds deform the S 1 foliation and are therefore younger than the F 1 folds. The F 2 folds are typically interfolial or interstratal, angular, and Z -shaped in profile; nearly vertical F 2 axial planes strike northward, and the axes of the F 2 folds in S 1 are steep. In metamorphic aureoles of granites, quartz-feldspar veins and andalusite porphyroblasts cross F 2 folds and crinkles, indicating that the granites are younger than the F 2 folds. Nearly all F 3 folds are open S -shaped kinks with steep axial planes striking northwest. The kinks deform porphyroblasts in the granite aureoles and pass into small sinistral faults which cut the granites. However, large sinistral faults parallel to the kinks offset the folded Meguma rocks, but not the granites. Thus the sinistral faults, and possibly the kinks, were formed before and after the emplacement of the granites. The Z -shape of the pregranite F 2 folds and the occurrence of postgranite dextral offsets along and across the S 1 foliation suggest that northeast- to easterly directed dextral movements may also have been repeated. Possibly the maximum compression was originally oriented northwest to north perpendicular to the F 1 axes and changed to an east-west direction to form the northerly trending F 2 folds; a continuation of this compression, both before and after granites were intruded, led to the F 3 S -kinks and sinistral faults striking northwestward, and to dextral faults striking northeast to eastward.

The structure and metamorphism of the mantling Karagwe–Ankolean sediments of the Ntungamo gneiss dome and their time-relation to the development of the dome

The Ntungamo gneiss dome has a quartzo-feldspathic core that has an abrupt and generally concordant margin with the overlying Pre-Cambrian Karagwe–Ankolean metasediments. The sediments, which dip off the dome on all sides, lie in synclinal belts around it and between the Ntungamo dome and adjacent domes. There is no good evidence of the intrusive character of the granitic rocks of the dome, and it is suggested that they are deformed Karagwe-Ankolean basement. The deformation history of the metasediments is divisible into two phases separated by a time-interval during which porphyroblasts of andalusite, kyanite, and staurolite developed. The schistosity of the sediments was formed in the first phase and was deformed in the second; the dome and synclines were produced in the second phase. This interpretation differs from earlier ones in which all the Karagwe-Ankolean structures were described as forming in one deformational episode, the schistosity being produced during dome-forming movements that were themselves the result of granite intrusion.