Geochemical and structural evolution of micas in the Rožná and Dobrá Voda pegmatites,Czech Republic

Summary The chemistry, structural parameters, polytypism, optical properties and Rb-Sr isotopes were examined in 11 to 60 samples of biotite, muscovite and lepidolite from the pegmatites at Roná (the type locality of lepidolite; 323 ± 4Ma) and Dobrá Voda (306 ± 9Ma) in western Moravia. At both localities, early endocontact biotite is followed inwards by muscovite and lepidolite, which is concentrated in and around the core. At Roná, a 1M lepidolite follows after 2M₁ muscovite but all later generations of lepidolite are 2M₂, close to Tri₅₀ Ply₅₀ and in part associated with muscovite 2M₁. At Dobrá Voda, all lepidolite types are 1M and free of muscovite, and the late varieties approximate Tri₃₀ Ply₇₀. At both localities, a trend of increasing HF is indicated during the progress of mica crystallization, culminating in precipitation of topaz. Polytypism of lepidolite is not correlatable with any compositional or growth feature, or their combination. Throughout the mica crystallization, Rb/Cs decreases but K/Rb becomes reversed after an initial decrease. Boron is partitioned preferentially into muscovite (up to 1.10 wt.% B₂O₃) but Be, Zn, Mn and Sc are enhanced in lepidolite. A slight increase in Fe, Ba and Cl in the last generation of lepidolite might be possibly due to mixing of residual pegmatite fluids with metamorphic pore solutions.

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Geochemische und strukturelle Entwicklung der Glimmer in den Pegmatiten von Roná und Dobrá Voda, Tschechische Republik

Zusammenfassung In 11 bis 60 Proben von Biotit, Muskovit und Lepidolith aus den Pegmatiten von Roná (Typlokalität des Lepidoliths; 323 ± 9 Ma) in Westmähren wurden Chemie, Struktur-parameter, Polytypie, optische Eigenschaften und Rb-Sr-Isotopie untersucht. An beiden Lokalitäten wird früher Biotit an Endokontakten nach Innen von Muskovit und Lepidolith gefolgt, letzterer ist in und um den Kern konzentriert. In Roná folgt 1M-Lepidolith auf 2M₁-Muskovit, aber alle späteren Lepidolithgenerationen sind 2M₂, nahe Tri₅₀Ply₅₀ und zum Teil mit 2M₁-Muskovit vergesellschaftet. In Dobrá Voda sind alle Lepidolithe vom Typ 1M und frei von Muskovit, die späten Varietäten kommen Tri₅₀Ply₅₀ nahe. An beiden Lokalitäten ist während des Fortschreitens der Glimmerkristallisation eine Tendenz von steigendem HF angezeigt, die in der Ausfällung von Topas ihren Höhepunkt findet. Die Polytypie des Lepidoliths kann nicht mit irgendeiner Eigenheit der Zusammensetzung oder des Wachstums korreliert worden, auch nicht mit einer Kombination von diesen. Während der ganzen Glimmerkristallisation nimmt Rb/Cs ab, aber die Tendenz von K/Rb ändert sich nach anfänglichem Abfall. Das Bor verteilt sich bevorzugt auf den Muskovit (bis zu 1.10 Gew. -% B₂O₃), aber die Be-, Zn-, Mn- und Sc-Gehalte sind im Lepidolith erhöht. Ein leichter Ansteig von Fe, Ba und Cl in der letzten Lepidolithgeneration könnte vielleicht durch eine Mischung von pegmatitischen Restlösungen mit metamorphen Porenlösungen verursacht sein.

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With 9 Figures     

Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic

ABSTRACT The Bouvet Triple Junction (BTJ) region in the South Atlantic, where the African, South American and Antarctica plates meet, is affected by several topographic/melting anomalies. Causes of these anomalies were investigated through a study of mantle-derived serpentinized peridotites sampled from three sites in the BTJ region: (1) the Inner Corner High at the intersection of the America Antarctic Ridge (AAR) with the Conrad transform; (2) the south wall of the Bouvet transform (South West Indian Ridge, SWIR); and (3) the eastern Bouvet SWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.     

Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride,N. Italy)

 Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (⁸⁷Sr/⁸⁶Sr=0.702203–0.702285; ¹⁴³Nd/¹⁴⁴Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions. Received: 15 February 1995/Accepted: 4 August 1995     

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo_88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H₂O, S and Cl contents are relatively high, whereas magmatic CO₂ concentrations are very low, probably due to CO₂ loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K₂O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998     

Metasomatic events recorded in ultramafic xenoliths from the Hyblean area (Southeastern Sicily, Italy)

Three types of ultramafic xenoliths from the Hyblean area (Sicily) show prime evidence for mantle metasomatism, namely: 1) Spinel-facies depleted harzburgite veined by phlogopite-bearing clinopyroxenite; 2) Amphibole-bearing harzburgite; and 3) Al-spinel websterite. (2) and (3) exhibit glassy pockets having respectively mugearitic and basanitic compositions, but a little amount of glass with low Ca and very low alkalis in (2). Glasses generally show trace element distributions consistent with the partial melting of pargasite-dominated mineral assemblages. Abundant Ca-Mg-carbonate globules immersed in these glassy pockets testify to immiscibility between silicate and carbonatite melts. Silicate melts and hydrous-silicate supercritical fluids, which underwent phase separation during fluxing throughout the semi-brittle lithospheric mantle, may account for such metasomatizing processes. The nature and abundance of some fluid-mobile elements in glasses and hydrous minerals (especially the Ca-poor glass, with B?=?59 ppm, Li?=?27 ppm, Ba?=?700 ppm and phlogopite, with Ba?=?8,465 ppm, Sr?=?260 ppm, F?=?5,700 ppm) suggest that some hydrous fluids may derive from hydrothermally altered oceanic crust. Conversely, metasomatizing silicate melts probably have a deep-seated origin. These results confirm previous suggestions on the key role of mantle metasomatism in the origin of some alkaline Hyblean magmas.     

The Campanian Ignimbrite (southern Italy) geochemical zoning: insight on the generation of a super-eruption from catastrophic differentiation and fast withdrawal

More than ca 100 km³ of nearly homogeneous crystal-poor phonolite and ca 100 km³ of slightly zoned trachyte were erupted 39 ka during the Campanian Ignimbrite super eruption, the most powerful in the Neapolitan area. Partition coefficient calculations, equilibrium mineral assemblages, glass compositions and texture were used to reconstruct compositional, thermal and pressure gradients in the pre-eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption. Our petrologic data indicate that a wide sill-like trachytic magma chamber was active under the Campanian Plain at 2.5 kbar before CI eruption. Thermal exchange between high liquidus (1199°C) trachytic sill and cool country rocks caused intense undercooling, driving a catastrophic and fast (10² years) in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption. This process culminated in an abrupt reservoir opening and in a fast single-step high decompression. Sanidine phenocrysts crystal size distributions reveal high differentiation rate, thus suggesting that such a sill-like magmatic system is capable of evolving in a very short time and erupting suddenly with only short-term warning.     

Red micas from basal ignimbrites of Mt. Vulture (Italy): interlayer content appraisal by a multi-methodic approach

The crystal chemistry of red phlogopites from Mt. Vulture (Italy) ignimbrites has been studied by electron microprobe, secondary ion mass spectrometry (SIMS), single crystal structural investigation and Fourier transform infrared (FTIR) spectroscopy. The analysed phlogopite has Fe/(Fe + Mg) ∼ 0.35, TiO₂ (wt%): 2.8–5.0 and H₂O (wt%): 1.24–3.37. Infrared spectra revealed the presence of bands due to the NH₄⁺ and H₂O stretching and bending vibrations. The samples belong to the 1M polytype. The bimodal behaviour of several structural parameters allows red micas to be clustered into two distinct groups: K⁺ ↔ NH₄⁺, H₂O and M³⁺-vacancy substitutions dominate in the first group; M^3+,4+-oxy, in the second group. It has to be pointed out that quantitative analysis of hydrogen (via SIMS) together with the characterization of the local environment of the anionic site (via FTIR) are fundamental in assessing the correct structural formula and the substitution mechanisms in micas. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The relationship between websterite and peridotite in the Balmuccia peridotite massif (NW Italy) as revealed by trace element variations in clinopyroxene

 Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with “kamafugite” lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary or near-primary mantle silicate melts of “kamafugitic” composition are transitional with Ca-carbonatite liquid and provides direct evidence of immiscibility of carbonatite from “kamafugite” magma. It is inferred that a primary mantle origin of Ca-carbonatites is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface. The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that their genesis depends on similar source and associated tectonic conditions. Received: 17 January 1995 / Accepted: 14 June 1995     

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu^* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu^*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.