Giuseppe Liborio

The eclogite-bearing series of Isla Margarita, Venezuela: Geochemistry of metabasic lithologies in the La Rinconada and Juan Griego Groups

Abstract XRF analysis for major and minor elements in 94 samples of the metabasic La Rinconada Group and in 28 samples of metabasic lithologies in the predominantly metasedimentary, overlying Juan Griego Group of Isla Margarita, Venezuela, has been carried out to compare the bulk compositions of the two eclogite-bearing metabasic suites. The two units differ in the age and geological setting of the protolith. The La Rinconada Group has a monotonous basaltic character, but shows irregular variations in bulk composition which are interpreted to reflect variations in the type of protolith, ranging from basaltic lava flow to gabbroic, cumulitic dyke. As a whole, the La Rinconada Group is related to a source of tholeiitic composition in a MORB-type volcanic-hypabyssal environment. The Juan Griego metabasic rocks are compositionally more homogeneous and uniformly basaltic in their major elements, again reflecting a tholeiitic source and a MORB-type environment. Despite their contrasting geological setting, the two suites of metabasic rocks can be inferred to have had a similar source and have probably developed in a passive, “Atlantic”-type continental margin setting. We infer that the La Rinconada Group represents early oceanic crust later covered by the siliceous, continent-derived clastics of the Juan Griego Group, in which the minor basaltic intrusions represent the last stages of waning magmatic activity.

The ecologite-bearing metabasaltic sequence of Isla Margarita, Venezuela: A geochemical study

Abstract XRF analyses for twenty elements in 41 samples and RNAA for eight REE in ten samples have been carried out as part of a continuing project to elucidate the geological history of the La Rinconada Group metabasites of Margarita Island, Venezuela, through the study of continuous and discontinuous metamorphic reactions, the study of variations in amphibole composition and by means of a large-scale survey of bulk-rock geochemistry. All together, twelve sampling points taken along a continuous metamorphic profile were considered, with grade increasing from chlorite-bearing amphibolites to paragonite-bearing amphibolites to end up into amphibole-bearing eclogites. The entire section lies within the P — T field of conditions of the epidote-amphibolite facies. The La Rinconada Group as a whole has a basaltic character, although SiO 2 may reach as much as 54%. Despite being macroscopically monotonous the metabasites are chemically variable, with fractionation indices (i.e. FeO ∗ MgO ) varying by a factor of 5 from 0.48 to 2.66. There is no correlation between the zonation in the metamorphic mineral assemblages and the bulk-rock chemistry, so that this zonation must reflect only variations in the intensive parameters of metamorphism. A definitive distinction between MORB and IAT, or the PIA classification of Donnelly and Rogers (1978), is not yet possible on the basis of the present partial data set. The LIL elements, which are notoriously mobile, suggest an IAT affinity. However, the relatively immobile elements Ti, Cr, Zr, and Y favour MORB. REE patterns and enrichment factors, which are also considered fairly reliable criteria, display characteristics between those typical of MORB and PIA.

Equilibration conditions of eclogite lenses from Isla Margarita, Venezuela: Implications for the tectonic evolution of the metasedimentary Juan Griego Group

Abstract The constituent primary and secondary minerals in a suite of fifteen samples of eclogite, amphibole-eclogite and garnet-amphibolite, scattered as boudins and pods in metapelitic schists and gneisses on Isla Margarita, Venezuela, have been analysed by electron microprobe to augment existing bulk-rock chemical data. The large sample population available allows spurious effects of stoichiometric Fe3+ calculation procedures and bulk-rock influence to be recognised and eliminated. All samples belong to one population with a relatively homogeneous character. The equilibration temperatures are derived from Fe 2+ Mg fractionation between garnet and clinopyroxene and range from 525–650 °C. Jadeite contents of clinopyroxene and the persistence of minor stable albite constrain pressures to between 13 and 19 kbar. Combined with new evidence for high-pressure metamorphism in the enclosing metapelites and with existing data on an eclogitic metabasic unit exposed in north-eastern Isla Margarita (La Rinconada Group), these data show convincingly that the Margarita crustal block is and has been a relatively coherent unit ever since the entire complex suffered high-pressure metamorphism in Late Mesozoic times.

Petrology of the amphibolitized eclogites of Gorduno, Lepontine Alps, Switzerland

Abstract The amphibolite-facies gneisses outcropping west of Bellinzona, from Gorduno up to Alpe Arami, include boudins and layers of amphibolite with relics of eclogite. Well-preserved eclogites crop out at the margin of the garnet—peridotite of Alpe Arami and are included within the chlorite—peridotite bordering it. The chemical compositions of eclogites and relic-bearing amphibolites are so similar as to suggest isochemical retrograde metamorphism. However, a minor change can be traced, because the chemical affinity slowly trends from clearly tholeiitic to alkali-basaltic, while the trace elements still suggest derivation from “ocean-floor basalts”. Garnets in the eclogites are homogeneous almandine—pyrope (Alm 41 Pyr 36 Gro 23 ) but with increasing amphibolitization they become rimmed by more ferrous compositions (Alm 53 Pyr 27 Gro 20 ). Clinopyroxenes are omphacites with cores richer in Jd (Jd 35–40 ) than the rims (Jd 23–26 ). Symplectites contain diopside (Jd 6–8 ) in equilibrium with plagioclase (An 15–30 ). Jd-rich omphacites display the ordered P2/n symmetry, whereas Jd-poor omphacites are disordered C2/c. Primary brown amphiboles show extreme alumino-ferroanpargasitic compositions; green and colourless amphiboles are either hastingsites or Mg-hornblendes. The petrogenetic evolution of these rocks is bracketed between a maximum P — T condition of 720°C, 16 kbar and a minimum of 500°C, 4 kbar. The latter condition reflects declining stages of the amphibolite facies, later than the climax of the Lepontine phase. The former suggests derivation from oceanic mafic material subducted during the Eo-alpine phase, rather than from cognate mafic layers of upper-mantle origin. The geochemical evidence supports the possibility that they are neither mantle-derived nor in situ eclogites but tectonically emplaced foreign slices of oceanic material.

Petrogenesis of the eclogites from Soazza, Switzerland

Abstract The metabasite bodies interbedded in the kyanite—staurolite-subfacies gneisses of the Adula Nappe in the vicinity of Soazza (Mesolcina Valley) preserve the eclogite assemblage in their cores and grade into common amphibolite towards their margins. Garnets vary irregularly in composition from sample to sample. Omphacites included in garnets have Jd 32 , those occurring as large crystals Jd 45 , but intergranular laths again Jd 35 . They all display the ordered P2/n space group. Amphibole formed late in the eclogite assemblage is a pargasitic hornblende while those occurring in amphibolites are edenites or Mg-hornblendes. Garnet—clinopyroxene pairs equilibrated first at T = 675° C and P ≥ 15 kbar then at a constant T = 760° C under P conditions decreasing from 15 to 10 kbar; the amphibolite-facies assemblages equilibrated at ≈ 570° C , 6 kbar. Chemically, the eclogite relics plot within the field of “common” ( B -type) eclogites in high-grade metamorphic rocks. Magmatic trends are still indicated by their minor elements, and discrimination diagrams suggest that these eclogites were originally tholeiitic basalts of oceanic affinity. In spite of now being within a crustal sequence, the Soazza eclogites are believed to have been a part of an oceanic crust that underwent subduction and “Eo-alpine” metamorphism to be emplaced later, as disrupted solid wedges, during the Alpine tectonism and then recrystallized under Barrovian amphibolite-facies regional conditions during the “Lepontine” phase.

Cummingtonite in the amphibolites of the South-Alpine basement complex (upper Lake Como region, Italy): Its relationships with hornblende

SummaryCummingtonite and hornblende occur together in metamorphosed basic rocks interlayered with lower-amphibolite facies sillimanite-muscovite-bearing schists and gneisses in the upper Lake Como region, South-Alpine basement complex of the Alps. Textural, chemical and spectroscopic studies indicate that they do not represent equilibrium pairs, but that hornblende was overgrown by cummingtonite during a late stage of either temperature increase or, more likely, of isothermal pressure release, by a continuous reaction which also produced the reverse zoning of plagioclase.ZusammenfassungIm Gebiet des oberen Como-Sees, im südalpinen Basement-Komplex, kommen Cummingtonit und Hornblende zusammen in metamorphen, basischen Gesteinen mit Zwischenlagen von Sillimanit-Muskovit-führenden Schiefern und Gneisen der unteren Amphibolit-Fazies vor. Untersuchung der Texturen, der chemischen Zusammensetzung und Infrarot-Spektroskopie zeigen, daß diese nicht Gleichgewichts-Paare darstellen, sondern daß Hornblende während eines späten Stadiums, entweder bei zunehmender Temperatur, oder eher während isothermalen Druck-Nachlasses durch eine kontinuierliche Reaktion von Cummingtonit überwachsen wurde, die auch für die reverse Zonierung der Plagioklase verantwortlich ist.

Caratteri chimici e mineralogici del «vitrofiro di Gràntola» (Varese)

Rare outcrops of a glassy rock very similar to an obsidian occur among the Permian volcanics of the Lugano region, about 15 km North of Varese (Italy). Such a rock is known as «vitrofiro di Gràntola» (the «vitrophyric dacite» of Stille and Buletti, 1987, 262 Ma in age). XRF analyses for major and some trace elements performed on four selected samples indicate a rhyodacitic chemistry with a calcalkaline affinity for these rocks. Microprobe analyses of Mg-Fe and Fe-Mg-orthopyroxene, Fe-augite, Fe-pargasite, Fe-edenite, Fe-olivine, plagioclase, sanidine and allanite are given as a support for the pre-eruption differentiation history.RiassuntoCirca 15 km a Nord di Varese (Italia), si osservano limitati affioramenti di una roccia dall’aspetto vetroso, molto simile a quello di un’ossidiana, facente parte delle vulcaniti permiane del Luganese. Tale roccia, conosciuta come «vitrofiro di Gràntola», corrisponde allavitrophyric dacite datata 262 Ma da Stille e Buletti (1987). La composizione (determinata con XRF) in element! maggiori, minori ed in traccia di 4 campioni opportunamente selezionati, dimostra il loro carattere riodacitico con affinità calcalcalina. La composizione, determinata alla microsonda elettronica, dei minerali femici (orto-e clinopirosseni, anfiboli, olivine), sialici (plagioclasi e sanidino) e di un’allanite ha fornito un contribute) alla comprensione del processo evolutivo precedente alla messa in posto.

Cummingtonite nelle plutoni ti della Val Blandino (Como)

Cummingtonite occurs in the marginal rind of the Val Biandino intrusion (Orobic Alps, Como, Italy), both as the only amphibole phase in association with pyroxene, and as linked to calcian amphiboles of different types. Crystalchemical data point out the relationship of cummingtonite with the early stage of intrusion of the Val Biandino masses.RiassuntoViene segnalata la presenza di cummingtonite sia come unico anfibolo, associato talora a pirosseni, sia concresciuta con Ca-anfiboli in zone marginali, prevalentemente gabbrodioritiche, delle plutoniti della Val Biandino. Ľinterpretazione dei dati cristallochimici ha permesso di chiarire il significato di questo minerale in relazione alle ipotesi di messa in posto ed evoluzione delle plutoniti stesse.