Péter Árkai

Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 jC and 1.8– 3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at V550 jC, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu* 0.4). Small differences have been observed in Yand HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+ XTh 4+ +Ca 2+ . In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Yand Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Yor HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE. D 2002 Elsevier Science B.V. All rights reserved.

Timing of low-temperature metamorphism and cooling of the Paleozoic and Mesozoic formations of the Bükkium, innermost Western Carpathians, Hungary

K-Ar ages of illite-muscovite and fission track ages of zircon and apatite were determined from various lithotypes of the Bükkium, which forms the innermost segment of the Western Carpathians. The stratigraphic ages of these Dinaric type formations cover a wide range from the Late Ordovician up to the Late Jurassic. The grade of the orogenic dynamo-thermal metamorphism varies from the late diagenetic zone through the ‘anchizone’ up to the ‘epizone’ (chlorite, maximally biotite isograd of the greenschist facies). The K-Ar system of the illite-muscovite in the < 2 μm grain-size fraction approached equilibrium only in ‘epizonal’ and high-temperature ‘anchizonal’ conditions. The orogenic metamorphism culminated between the eo-Hellenic (160-120 Ma) phase connected to the beginning of the subduction in the Dinarides, and the Austrian (100-95 Ma) phase characterized by compressional crustal thickening. No isotope geochronological evidence was found for proving any Hercynian recrystallization. The stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic units. A temperature less than 250°C and effective heating time of 20–30 Ma had only negligible effects on the tracks, whereas total annealing was reached between 250 and 300°C. Apatite fission track ages from the Paleozoic and Mesozoic formations show that the uplift of the Bükk Mountains occurred only in the Tertiary (not earlier than ca. 40 Ma ago). Thermal modeling based on apatite fission track length spectra and preserved Paleogene sediment thickness data proved that the Late Neogene burial of the recently exhumed plateau of the Bükk Mountains exceeded 1 km.

Two-, three- and four-feldspar assemblages with hyalophane and celsian: Implications for phase equilibria in BaAl2Si2O8-CaAl2Si 2O8-NaAlSi3O8-KAlSi 3O8

The occurrences of natural coexisting feldspars including hyalophane and also celsian delineate two-, three- and possibly four-phase fields in the system BaAl2Si2O8—CaAl2Si2O8—NaAlSi3O8—KAlSi3O8. Hyalophane occurs with albite and microcline in a very low grade (anchizonal to epizonal) metasedimentary association from the Uppony Mountains, Hungary, and in Grenville marbles from Ontario. Analyses show very little Ba in albite and only limited Na in hyalophane. One marble from the garnet zone has albite (Ab95–98), oligoclase (An17Sl3Ab80), hyalophane (Cn65Sl3An2Ab9Or24) and celsian (Cn92Sl3An1Ab2Or2). The albite and oligoclase are complexly intergrown and may indicate unmixing during cooling. A marble in the sillimanite zone contains albite (Ab95–98Or2–3), oligoclase (An17An2Sl3Ab33Or24), hyalophane (Cn65Sl3An2Ab9Or24), and an inclusion of celsian (Cn67An1Sl2Ab2Or91) in an albite. Sanidine from the Peshtigo monzonite in Wisconsin unmixed to a symplectic perthite with barian microcline (Cn8–11An2Ab8–13Or75–80) and oligoclase (CnlAn18Ab79Or2). The former compositions of the ternary igneous feldspars (Cn3An9Ab46Or42, Cn1An18Ab69Or12) were obtained by reintegration. The Na content of hyalophane equilibrated with albite is correlated with metamorphic grade. Hyalophane has 5 ± 2 mol % Ab in very low-grade associations, 10 ± 3 mol % Ab in the greenschist facies, 16 ± 2 mol % Ab in the low to middle amphibolite facies, and 30 mol % Ab in the upper amphibolite to granulite facies even when not buffered with albite. The limited Na content of celsian equilibrated with albite in the greenschist facies is in striking disagreement with the narrow solvi obtained from unreversed experiments on the join BaAl2Si2O86-NaAlSi3O8. Up to 8 four-feldspar and 24 three-feldspar assemblages may be stable in the system BaAl2Si2O8-CaAl2Si2O8-NaAlSi3O8-KAlSi3O8. In contrast, the repeatedly observed and variably located discontinuities within zoned hyalophane grains may represent changes in the environment during mineral growth rather than internal miscibility gaps. Given its miscibility gaps with microcline and celsian, the name hyalophane is justified for intermediate feldspars near the Cn-Or join.

White mica domain formation: A model for paragonite, margarite, and muscovite formation during prograde metamorphism

Abstract Scanning transmission electron microscopy images of the 00l white mica planes in crystals from central Switzerland and Crete, Greece, reveal that domains of paragonite, margarite, and muscovite are ordered within the basal plane. Energy dispersive X-ray analyses show that both cations in the interlayer and in the 2:1 layer have ordered on the scale of tens to hundreds of nanometers. Domain boundaries can be both sharp and crystallographically controlled or diffuse and irregular. A model outlining the domain formation process is presented that is consistent with X-ray powder diffraction and transmission electron microscopy data. The domain model incorporates aspects of a mixedlayered and a disordered compositionally intermediate phase models. The main feature of the model is the formation of mica species that segregate within the basal plane and contradict the notion of homogeneous layers within mixed-layer phases. Implications for the formation of all diagenetic and very low-grade metamorphic 2:1 sheet silicates are discussed

COMPARISON OF EVOLUTION OF TRIOCTAHEDRAL CHLORITE/ BERTHIERINE/SMECTITE IN COEVAL METABASITES AND METAPELITES FROM DIAGENETIC TO EPIZONAL GRADES

The evolution of texture, structure and chemical composition of chloritic clays in coeval pairs of metabasites and metapelites of a prograde sequence from the Bükk Mountains has been investigated using electron microscopy techniques. Samples are from the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine metamorphism, ranging from late diagenesis to epizone for pelites and from prehnite-pumpellyite to greenschist facies for the metabasites.Although bulk-rock compositions, textures and primary minerals are different, chlorite evolved at similar rates in coeval metabasites and metasediments, but along different paths. The principal similarities in the prograde sequence are a decrease in the percentage of interstratified material in both dioctahedral and trioctahedral phyllosilicates and increase in thicknesses of chlorite and illite crystallites. The principal difference is in the type of interstratification in chlorite, with berthierine in metapelites, and smectite (saponite) in metabasites, although smectitic mixed layers also occur in the former. The evolution of trioctahedral phyllosilicates is marked by a decrease in the number of mineral species with increasing grade, chlorite, sensu stricto, being the only trioctahedral mineral at higher grades. This is consistent with the trend in reaction progress where both metastable systems (metabasites and metapelites) tend toward the same end-member, thermodynamically stable chlorite, as well as texture (crystal size), and where all intermediate states are metastable, and determined by the Ostwald step rule.

Effects of lithology and bulk chemistry on phyllosilicate reaction progress in the low-T metamorphic Graz Paleozoic, Eastern Alps, Austria

Structural and chemical evolution paths of white K-mica and chlorite from low-temperature metamorphic rocks are studied by X-ray powder diffractometry and electron probe micro-analysis in order to determine the effects of lithology expressed by bulk rock major element chemistry on the phyllosilicate reaction progress. The sample set representing three main lithotypes, namely pelitic and marly slates, metatuffites and massive meta-igneous rocks of basic to intermediate compositions, derived from the Graz and Sausal Paleozoic of the Upper Austroalpine unit, Eastern Alps, Austria. Reaction progress, modelled in terms of proportion of swelling mixed-layers, crystallinity indices, mean crystallite size, lattice strain and mineral chemistry, shows grain-size dependent variations in all lithologies. Referring to disequilibrium conditions, the larger authigenic grains reflect more evolved stages than the

White micas with mixed interlayer occupancy: a possible cause of pitfalls in applying illite Kübler index ( crystallinity ) for the determination of metamorphic grade

Integrated microstructural observations, X-ray powder diffractometric (XRPD) modal composition, illite Kubler index and chlorite “crystallinity” determinations and vitrinite reflectance measurements were carried out on marly slates from selected profiles of the Helvetic zone of the Central Alps, Switzerland. The studied profiles were: Upper Jurassic from the Wildhorn nappe, Brienz, Upper Jurassic from the Parautochthonous of the Aar massif, Glarus Alps, Eocene from the Griesstock nappe of the Glarus Alps and Upper Jurassic from the Axen nappe, Rhine Valley. In some of the localities studied, illite Kubler index (“crystallinity”) values were anomalously high, yielding only diagenetic conditions, while chlorite “crystallinity” and vitrinite reflectance showed anchi- and epizonal metamorphic conditions. Detailed XRPD observations carried out on Ca-saturated and glycolated mounts indicated subordinate amounts of swelling (smectitic) interstratifications in white mica. In addition to the dominant K-white mica, traces of discrete paragonite and paragonitic phases and tobelitic impurities in the form of either regular interstratifications or micas with mixed (K≫Na>NH4) interlayer compositions could be detected from the XRPD (00,10) basal reflections. On the basis of the organic maturity assumed from vitrinite reflectance, probable partitioning of N and H between organic and inorganic phases, and the results of elemental (C, H, N, S) analyses carried out on the <2 μm fraction decarbonated and oxidized samples, the amount of NH4+ fixed in inorganic phases could be estimated. Small, but systematically appearing, absorption bands between 1400 and 1440 cm−1 in the FTIR spectra unequivocally proved the presence of ammonium in the rocks studied. Small amounts of N within the mica flakes were detected by electron energy loss spectrometry (EELS), confirming that NH4+ is indeed fixed in the interlayer site position. Energy dispersive spectroscopy (EDS) using scanning transmission electron microscopy (STEM) revealed heterogeneities in the interlayer cation occupancies. Although K is always dominant, irregular, local, domain-like enrichments in Na could be seen. The most accurate model to describe the disequilibrium state of the incipient metamorphic white micas studied is that of a dioctahedral mica structure with irregularly varying interlayer occupancies combined with subordinate amounts of swelling mixed-layers. The present work shows that, if these white micas are to be used for metamorphic petrogenetic purposes, special attention should be paid to their detailed characterization, especially in organic matter rich lithologies often characterized by high Al/Si bulk chemical ratios. Seemingly, white micas with mixed interlayer occupancies may be more widespread than has generally been anticipated so far.

Two-, three- and four-feldspar assemblages with hyalophane and celsian implications for phase equilibria in BaAl2Si2O8—CaAl2Si2O8—NaAlSi3O8—KAlSi3O8*Contribution No. 518 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan 48109–1063.

The occurrences of natural coexisting feldspars including hyalophane and also celsian delineate two-, three- and possibly four-phase fields in the system BaAl2Si2O8—CaAl2Si2O8—NaAlSi3O8—KAlSi3O8. Hyalophane occurs with albite and microcline in a very low grade (anchizonal to epizonal) metasedimentary association from the Uppony Mountains, Hungary, and in Grenville marbles from Ontario. Analyses show very little Ba in albite and only limited Na in hyalophane. One marble from the garnet zone has albite (Ab95–98), oligoclase (An17Sl3Ab80), hyalophane (Cn65Sl3An2Ab9Or24) and celsian (Cn92Sl3An1Ab2Or2). The albite and oligoclase are complexly intergrown and may indicate unmixing during cooling. A marble in the sillimanite zone contains albite (Ab95–98Or2–3), oligoclase (An17An2Sl3Ab33Or24), hyalophane (Cn65Sl3An2Ab9Or24), and an inclusion of celsian (Cn67An1Sl2Ab2Or91) in an albite. Sanidine from the Peshtigo monzonite in Wisconsin unmixed to a symplectic perthite with barian microcline (Cn8–11An2Ab8–13Or75–80) and oligoclase (CnlAn18Ab79Or2). The former compositions of the ternary igneous feldspars (Cn3An9Ab46Or42, Cn1An18Ab69Or12) were obtained by reintegration. The Na content of hyalophane equilibrated with albite is correlated with metamorphic grade. Hyalophane has 5 ± 2 mol % Ab in very low-grade associations, 10 ± 3 mol % Ab in the greenschist facies, 16 ± 2 mol % Ab in the low to middle amphibolite facies, and 30 mol % Ab in the upper amphibolite to granulite facies even when not buffered with albite. The limited Na content of celsian equilibrated with albite in the greenschist facies is in striking disagreement with the narrow solvi obtained from unreversed experiments on the join BaAl2Si2O86-NaAlSi3O8. Up to 8 four-feldspar and 24 three-feldspar assemblages may be stable in the system BaAl2Si2O8-CaAl2Si2O8-NaAlSi3O8-KAlSi3O8. In contrast, the repeatedly observed and variably located discontinuities within zoned hyalophane grains may represent changes in the environment during mineral growth rather than internal miscibility gaps. Given its miscibility gaps with microcline and celsian, the name hyalophane is justified for intermediate feldspars near the Cn-Or join.

Two-, three- and four-feldspar assemblages with hyalophane and celsian

The occurrences of natural coexisting feldspars including hyalophane and also celsian delineate two-, three- and possibly four-phase fields in the system BaAl2Si2O8—CaAl2Si2O8—NaAlSi3O8—KAlSi3O8. Hyalophane occurs with albite and microcline in a very low grade (anchizonal to epizonal) metasedimentary association from the Uppony Mountains, Hungary, and in Grenville marbles from Ontario. Analyses show very little Ba in albite and only limited Na in hyalophane. One marble from the garnet zone has albite (Ab95–98), oligoclase (An17Sl3Ab80), hyalophane (Cn65Sl3An2Ab9Or24) and celsian (Cn92Sl3An1Ab2Or2). The albite and oligoclase are complexly intergrown and may indicate unmixing during cooling. A marble in the sillimanite zone contains albite (Ab95–98Or2–3), oligoclase (An17An2Sl3Ab33Or24), hyalophane (Cn65Sl3An2Ab9Or24), and an inclusion of celsian (Cn67An1Sl2Ab2Or91) in an albite. Sanidine from the Peshtigo monzonite in Wisconsin unmixed to a symplectic perthite with barian microcline (Cn8–11An2Ab8–13Or75–80) and oligoclase (CnlAn18Ab79Or2). The former compositions of the ternary igneous feldspars (Cn3An9Ab46Or42, Cn1An18Ab69Or12) were obtained by reintegration. The Na content of hyalophane equilibrated with albite is correlated with metamorphic grade. Hyalophane has 5 ± 2 mol % Ab in very low-grade associations, 10 ± 3 mol % Ab in the greenschist facies, 16 ± 2 mol % Ab in the low to middle amphibolite facies, and 30 mol % Ab in the upper amphibolite to granulite facies even when not buffered with albite. The limited Na content of celsian equilibrated with albite in the greenschist facies is in striking disagreement with the narrow solvi obtained from unreversed experiments on the join BaAl2Si2O86-NaAlSi3O8. Up to 8 four-feldspar and 24 three-feldspar assemblages may be stable in the system BaAl2Si2O8-CaAl2Si2O8-NaAlSi3O8-KAlSi3O8. In contrast, the repeatedly observed and variably located discontinuities within zoned hyalophane grains may represent changes in the environment during mineral growth rather than internal miscibility gaps. Given its miscibility gaps with microcline and celsian, the name hyalophane is justified for intermediate feldspars near the Cn-Or join.