Francesco P. Sassi

The paragonite-muscovite solvus: II. Numerical geothermometers for natural, quasibinary paragonite-muscovite pairs☆

Three parametric (nonthermodynamic) equations have been developed to calculate final equilibration temperatures for natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction margarite in Pg < 0.05, Si/formula unit ≤ 6.2 and/or Σ(Mg + Fe2+ + Fe3+) ≤ 0.35 for coexistent Ms). The first two equations are paragonite-based and muscovite-based, because they yield equilibration temperatures that depend on the Na-K compositions of K-saturated Pg and Na-saturated Ms, respectively. The third equation is closure-based, because it yields equilibration temperatures that vary with the width of the gap between the Na-K compositions of coexisting Pg and Ms (i.e., the degree of solvus closure). Equilibration temperatures can be estimated, using just one of the geothermometers, or by averaging the temperatures obtained from any combination of the three equations. Comparison of the three calculated temperatures yields information on the mutual consistencies of the Na-K compositions of coexisting Pg and Ms with respect to their utility in Pg-Ms solvus thermometry. Geothermometric inconsistencies in the Na-K compositions of quasibinary Pg-Ms pairs can be caused by (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H3O+ → K+ and vacancy → K+ substitution in muscovite and/or paragonite (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Due to the effects of pressure on the stability relations of Pg-Ms pairs and the K/Na ratios of Nasaturated muscovites, practical applications of Pg-Ms solvus thermometry are restricted to quasibinary Pg-Ms pairs that equilibrated at pressures between approximately 2 and 8 kbar. Within this pressure range, the utility of Pg-Ms solvus thermometry is limited thermally to approximately 300 ≤ T ≤ 700°C by (1) the decomposition of K-saturated paragonite at temperatures between approximately 580 and 700°C, and (2) steepening of the solvus limbs at temperatures below 300°C. Graphical and calculated Pg-Ms solvi based on solvus data for synthetic, binary Pg-Ms micas are inconsistent with solvus data for natural, quasibinary Pg-Ms micas. Consequently, these solvi should not be used to estimate equilibration temperatures for natural, quasibinary Pg-Ms pairs.

Acritarch evidence for the Cambrian age of phyllites in the agordo area (south-alpine basement of eastern Alps, Italy)

Abstract Abundant but poorly preserved acritarchs have been found in black metapelites. The assemblage consists mainly of small (5–15 μm), smooth, thin-walled specimens, without processes. Eight subgroups are represented by 19 genera. Stratigraphically diagnostic acritarchs are: Ovulum saccatum, O. lanceolatum, Volkovia flagellata, Aranidium cf. confusum, Veryhachium minutum, Acanthodiacrodium sp. (div. sp.), Cymatiogalea sp., Impluviculus sp.. The assemblage suggests an age in the range of late Early Cambrian to Early Ordovician (Tremadoc), the Late Cambrian being the more probable. The acritarchs described in this paper are the first Cambrian fossils discovered in the Eastern Alps, where until now no fossils older than the Late Ordovician have been found, and only one other record of Cambrian fossils is known from the whole of the Alps.

The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

The statistical b 0 method is based on the fact that the cumulative b 0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b 0 values (and related celadonite content) in WKM increase with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b 0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made a large amount of data available, revision of this b 0 scale is now opportune, and the primary need is a better definition of the reference b 0 values and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustres of W Alps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilufer area of NW Turkey. In addition, WKM chemical and b 0 data were used from various sources. The new chemical and b 0 data (EMPA and XRPD) allowed us: ( i ) to establish the reference b 0 and chemistry of WKM from HP/LT metapelites: b 0 turns out to be around 9.040 A; ( ii ) to confirm and extend to HP/LT metapelites the petrological basis of the b 0 method, particularly the control of mineral assemblages on b 0 values; ( iii ) to glimpse more complex relationships between b 0 and Si than those currently known, and particularly the importance of the Fe 2+ /(Mg + Fe 2+ ) ratio; ( iv ) to ascertain the validity of the b 0 method for detecting generic HP/LT conditions; (ν) to ascertain the inability of the b 0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b 0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation.

Miscellaneous isomorphous substitutions in Na-K white micas: a review, with special emphasis to metamorphic micas

Numerous new studies have been done over the last ten years on the minor isomorphous substitutions in Na-K white micas. These results, from over a hundred papers, are reviewed and critically discussed here. Special emphasis is given to the new advances which the authors perceive as having a direct petrogenetic significance for understanding metamorphic rocks.RiassuntoNegli ultimi dieci sono stati compiuti molti studi sulle sostituzioni isomorfe minori nelle miche sodico-potassiche. I relativi risultati sono contenuti in un centinaio di pubblicazioni. Il presente lavoro è una rassegna critica di questi risultati, e pone particolare attenzione a quelli che vengono ritenuti rilevanti per la comprensione petrogenetica delle rocce metamorfiche.

Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine

Abstract A crystal-chemical study of thirteen biotite (twelve of 1M polytype and one of 2M1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VIMg2+/ VI(Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VIAl ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VIAl ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters (T and fO₂) on mica crystal structure, and not just chemical composition.

The Austridic eclogites, metabasites and metaultrabasites from the Pohorje area (Eastern Alps, Yugoslavia): 1. The eclogries and related rocks

Petrography, petrology, mineral chemistry, metamorphic evolution and geothermo-barometric estimations concerning the Pohorje eclogites are discussed. These rocks record a three-stage metamorphic evolution: (0 an older, dry, high-pressure/high-temperature recrystallization (eclogite stage);(ii) a later, lower-pressure/lower temperature symplectite stage, partly developed under granulite faciès conditions;(iii) a younger, epidote-amphibolite overprint. The latter water-controlled recrystallization is believed to be Alpine. However, the main part of the metamorphic history is to be considered pre-Alpine: such chronological classification is consistent with the prevailing interpretation concerning similar Austridic rocks in the Eastern Alps.RiassuntoLe eclogiti dell’area di Pohorje vengono studiate dal punto di vista petrografico, petrologico, della composizione delle fasi mineralogiche e dell’evoluzione metamorfica. Tale evoluzione risulta articolata in tre stadi: (i) una antica ricristallizzazione in ambiemte anidro, sotto alte T eP (stadio eclogitico);(ii) un successivo stadio simplettitico, a più basseT e P, svoltosi nella sua ultima parte in condizioni granulitiche;(Hi) una sovrimpronta recente in facies delle anfiboliti ad epidoto. Quest’ultima ricristallizzazione in presenza di H2O viene ritenuta Alpina, mentre la parte principale della storia metamorfica di queste eclogiti viene ritenuta pre-Alpina. Tale classificazione cronologica è in accordo con le prevalenti interpretazioni riguardanti simili rocce del basamento Austridico delle Alpi Orientali.

Sulla variability, compositiva dell’ amiche nellöambito di uno stesso affioramento di metapeliti di basso grado: un esempio. 1: Le biotiti

RiassuntoSono state analizzate 140 lamelle di miche (76 muscoviti e 64 biotiti) da 4 campioni di metapeliti di basso grado provenienti da un grande affioramento di basamento sudalpino in Pusteria. Le lamelle sono state scelte:(i) in ski microstrutturali diversi (fianchi e cerniere di micropieghe);(ii) in microsistemi chimici diversi (domini granoblastici e bande lepidoblastiche; lamelle a contatto con fasi diverse); e(iii) in modo da poter essere riferite per caratteri microstrutturali o allo stadio di cristallizzazione sincinematica Cr2 o a quello postcinematico Cr3. Vengono qui riportati i risultati analitici relativi alle biotiti, mentre si rimanda ad uno specifico lavoro per quanto riguarda le muscoviti e le considerazioni conclusive. Le biotiti dei letti granoblastici risultano sostanzialmente uguali a quelle dei letti lepidoblastici, così come le biotiti situate in zone di cerniera rispetto a quelle lungo i fianchi. Invece le biotiti cristallizzate durante Cr3 sono compositivamente diverse da quelle cristallizzate durante Cr2, e denotano temperature lievemente piu alte (pur essendo rimaste invariate le compatibilità mineralogiche). Sono state inoltre rilevate alcune peculiarità compositive delle biotiti in funzione delle fasi mineralogiche a contatto con le lameile analizzate.Abstract140 mica flakes (76 muscovites and 64 biotites) have been analyzed in 4 samples of lowgrade metapelites from a large outcrop of the Southalpine basement in Pusteria (Pustertal, Eastern Alps). The analyzed flakes have been selected:(i) in different microtextural sites (hinges vs. limbs of microfolds);(ii) in different chemical microsystems (granoblastic vs. lepidoblastic domains; flakes in contact with different phases); and(iii) in such a way that each flake could be either referred to the synkinematic crystallization stage Cr2 or to the postkinematic stage Cr3. The present paper reports the results obtained from the biotites, while the muscovites are considered in a separate paper, which also proposes some concluding remarks concerning both micas. Biotites from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as biotites flakes located in the hinges of microfolds are identical to the flakes in the limbs. On the contrary, mica flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3; in the latter case, biotite chemistry suggests slightly higher temperatures although the mineral compatibilities are unchanged. Some other chemical peculiarities of biotite flakes have been also detected: they depend on the different mineral phases in contact with the analyzed flake.

What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks

Abstract Petrologic, chemical, and polytype data are presented for dioctahedral potassic micas from Kfeldspar- bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure.

Sulla vanabilità compositiva delle miche nell’amhito di uno stesso affioramento di metapeliti di basso grado: un esempio. 2: Le muscoviti e considerazioni conclusive

The present paper reports and discusses the results on compositional variability of muscovites, obtained in the frame of a research project on the chemistry of biotites and muscovites in low-grade metapelites. The results on biotites have been reported in a separate paper, to which readers are deferred also because it gives the statement of the problem, the strategy of research and the analytical procedures. Muscovite flakes from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as muscovite flakes located in the hinges of microfolds in comparison with those in the limbs. On the contrary, muscovite flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3, in the latter case, muscovite and biotite chemistries suggest slightly higher temperatures although the mineral compatibilities are unchanged. The analytical results indicate a similar behaviour for muscovites and biotites. In particular, within rock samples in which Cr3 effects are widespread, the chemistries of Cr2 muscovites and biotites turn out to be changed. This fact, as well as some chemical peculiarities of the mica flakes depending on the different mineral phases in contact, indicates that use of geothermobarometry in low-grade metapelites requires caution: microtexturally identical mica flakes may give differentT,P values depending on their size, the adjacent mineral phase, and the extent of chemical re-equilibration during the thermal climax.RiassuntoVengono qui riportati e discussi i risultati sulla variabilità compositiva delle muscoviti, ottenuti nelTambito di una ricerca sul chimismo di biotiti e muscoviti in metapeliti di basso grado. I risultati sulle biotiti sono stati l’oggetto di uno specifico lavoro, al quale si rimanda anche per 1a deiinizione del problema, 1a strategia di ricerca e 1e procedure. Le muscoviti dei letti granoblastici risultano sostanzialmente uguali a quelle dei letti lepidoblastici, così come 1e muscoviti situate in zone di cerniera rispetto a quelle lungo i fianchi. Invece 1e muscoviti cristallizzate durante Cr3 sono compositivamente diverse da quelle cristallizzate durante Or2, e denotano temperature lievemente più alte (pur essendo rimaste invariate le compatibilità mineralogiche). Quindi i risultati analitici indicano comportamenti analoghi per 1e muscoviti e 1e biotiti. In particolare, nei campioni nei quali gli etfetti di Cr3 sono vistosi, il chimismo delle muscoviti Cr2 così come quello delle biotiti risulta modificato. Questo fatto, insieme ad alcune peculiarità compositive delle miche in funzione delle fasi a contatto, indica che occorre grande cautela nell’uso dei geotermobarometri in metapeliti di basso grado: lamelle di miche microstrutturalmente identiche possono fornire valori diT oP diversi a seconda delle fasi mineralogiche a contatto, delle dimensioni, e della maggiore o minore efficacia della riequilibrazione chimica durante il culmine termico.

Growth history of two garnet porphy rob lasts of the Cima Dura- Durreck Complex and its implication on the polymetamorphic evolution of this complex (Austrides, Eastern Alps)

Detailed microstructural and microprobe analyses were carried out on some small garnet idioblasts from phyllonites and two garnet porphyroblasts, respectively from a representative amphibolite and phyllonite rock sample. A consistent sequence of growth stages has been ascertained in this way in the two porphyroblasts, suggesting, combined with the structural reasoning, that their multistage growth is entirely pre-Alpine in both cases. The results discussed in the present paper indicate that, as the amphibolite lithons are related to deformation partitioning on the mesoscale, the persistence of Variscan garnet porphyroblast in the phyllonites is due to deformation partitioning on the microscale.RiassuntoAlcuni piccoli idioblasti di granato provenienti da filloniti e due porfiroblasti di granato selezionati rispettivamente da due campioni rappresentativi di anfibolite e di fillonite, sono stati sottoposti a dettagliate analisi microstruttura-li e alla microsonda elettronica. In questo modo è stata evidenziata una sequenza di stadi di crescita simile nei due porfiroblasti, il che suggerisce, unitamente a motivi strutturali, che 1a loro crescita multistadiale è interamente pre-Alpina in entrambi i casi. I risultati discussi nel présente lavoro indicano che, come i litoni di anfibolite sono collegati a ripartizione delia deformazione su scala mesoscopica, 1a persistenza di porfiro-blasti di granato Varisici nelle fiiloniti è dovuta a ripartizione della deformazione su scala microscopica.