Lauro Morten

P–T evolution of ‘crustal’ garnet peridotites and included pyroxenites from Nonsberg area (upper Austroalpine), NE Italy: from the wedge to the slab

Abstract Up-to-date thermobarometers have been applied to garnet-bearing peridotites and pyroxenites from Nonsberg area (upper Austroalpine, NE Italy), a remnant of the lower crust of the Central European Variscan belt. The Nonsberg ultramafic rocks record an unusual metamorphic history, from high-temperature, moderate-pressure, spinel-facies conditions to low-temperature, high-pressure, garnet-facies conditions. This peculiar P – T evolution is interpreted as a result of subduction processes, which involved mantle-wedge peridotites overlying a subducting continental slab. The following scenario is proposed based on our P – T estimates and available physical models for subduction zones. Relatively high-temperature (∼1200°C, 1.31–1.58 GPa) mantle-wedge peridotites (spinel lherzolites) were intruded by hot (>1400°C) hydrous melts rising from deeper, inner portions of the wedge. Coarse-grained, garnet-free pyroxenites were segregated from these melts and formed veins in the host peridotites. Convection within the wedge induced by the downgoing slab caused the peridotites to flow towards the slab while cooling at essentially constant pressure. The peridotites were then dragged to greater depths at slightly decreasing, or nearly constant, temperature by the downward mantle flow near the wedge corner and near the wedge-slab interface and eventually emplaced into the underlying slab. Entrainment in the continental crust, either as tectonic slices or as sinking mantle blobs, was probably favored by density contrast between overlying mantle and underlying crust. Mantle flow and entrainment in the cold continental crust caused the peridotites to cool down to ∼850°C before, or while, being subducted together with the slab to depths of about 90 km. During this T -decrease, P -increase path, garnet formed at the expense of spinel and pyroxenes, giving rise to garnet ± spinel-bearing lherzolites and pyroxenites.

Geochemistry of metabasites from the Nevado-Filabride complex, betic cordilleras, Spain: Relics of a dismembered ophiolitic sequence

Abstract Major- and trace-element distribution, including REE data, in the metabasites of the Nevado-Filabride Complex (Betic Cordilleras, Spain) suggests that the whole mafic-ultramafic association represents a dismembered ophiolitic sequence. All the analyzed samples have been affected by sea-floor alteration. Yet only the fine-grained rocks have suffered some significant mobilization of the LREE. Life the other Western Mediterranean ophiolites, this sequence is related to the opening of the Liguro-Piemontese basin during Mesozoic times. However, the metabasites from Betic Cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios. The entire variation of these ratios in the Liguro-Piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites. possibly up to the spinel-plagioclase facies transition. In this process, the ophiolites of the Betic Cordilleras would record the deepest stages and the lowest degrees of melting. This feature is related to the peculiar location of these ophiolites at the western end of the Liguro-Piemontese basin.

The Nevado-Filabride metaophiolitic association in the Cobdar region (Betic Cordillera, SE Spain): preservation of pillow structures and development of coronitic eclogites

AbstractThe Nevado-Filabride ophiolilic association is made up of the dismembered and metamorphosed remains of a Jurassic ocean floor. This associatiion is exceptionally well preserved, and locally unmetamorphosed, around the village of Cobdar (prov. of Almeria). It crops out in this area as lenticular metabasite and metasedimentary bodies, about 7 km in length and several hundred metres thick, tectonically intercalated between the Caldera and the Sabinas Units of the Mulhacen Group of Nappes. The ophiolitic association in this region is made up of three distinct sequences: a plutonic sequence consisting of cumulitic troctolites, olivine-pyroxene gabbros and dolerites, cut through by numerous dykes of porphyric basalts; a volcanic sequence composed of massive or vesicular olivine-pyroxene basalts, sporadically intersected by basaltic dykes and locally containing pillow-structures; and a frequently laminated sedimentary sequence made up mostly of ankerite-garnet calc-schists with interbedded lenses of mica...

Rare earth abundances in superferrian eclogites from the Voltri Group (Pennidic Belt, Italy)

Abstract Ten samples of blueschist facies eclogites occurring as dykes in the ultramafic rocks of the Voltri Group were analysed for eigth rare earth elements (REE). Their patterns are clearly different from those of cognate eclogites enclosed in ultramafic rocks and similar to those of the volcanic rocks dredged from the Mid-Atlantic Ridge (MAR). This similarity confirms both the tholeiitic nature of the magma injected into the Pennidic oceanic basement and metamorphosed to eclogite, and the immobile character of REE during metamorphism up to 10 Kbar and 450°C. Depletion of LREE vs HREE and a positive Eu anomaly in the coarse-grained central portions of the former dykes suggest their derivation from a mush consisting of pyroxene and plagioclase plus trapped superferrian liquid. The high total REE content and the flat pattern of the fine-grained eclogites occurring at the borders of the dykes are consistent with their derivation from a rapidly crystallized superferrian liquid.

The evolution of the magmatism in the external zones of the Betic Cordilleras during the mesozoic

AbstractThe Mesozoic igneous rocks from the External Zones of the Betic Cordilleras extend for some 300 km along the Subbetic Zone. They are poorly differentiated basic rocks which, altogether, correspond to a transitional series containing tholeiitic and sodium-alkaline terms. They crop out as small ophite stocks and dykes intruded into middle- and upper-Triassic rocks, or as submarine flows and sills interlayered with Jurassic materials.Geological and radiometric evidence points to an upper-Triassic-Liassic age for the ophite-generating magmatism, while the fissure volcanism began locally in the early-Liassic and extended throughout the Dogger. It reached its climax in the Tithonian and ceased abruptly in the lowermost Cretaceous.The magmas that generated the two groups of rocks originated within the mantle. During the ascent through a continental crust they were contaminated by deep-crust granitoid rocks and by the assimilation of pelites from the basement. The chemical composition and fractional-cryst...

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.

Origin of a mica megacryst in an alkaline dyke from the Veneto Volcanic Province, Italy

We report the first finding of a mica megacryst included in an alkaline basic dyke belonging to the Tertiary Veneto Volcanic Province (northern Italy). The megacryst is surrounded by a corona with a mineral assemblage consisting of fine-grained mica flakes + Ti-magnetite. The megacryst is a biotite with a mg# of 0.75, containing 1.35 wt. % TiO 2 , 1041 ppm Ba and 3.8 wt. % H 2 O. The mica flakes have a mg# of 0.79 and contain 4.33 wt. % TiO 2 ,21 968 ppm Ba and 3.0 wt. % H 2 O. Both mica types show high concentrations of LILE and Nb (461–135 ppm). REE contents are typically below the analytical detection limit. Fractional crystallization modelling shows that the mica megacryst crystallized at depths in the mantle consistent with the spinel-facies stability field. The unusually high Nb N /Sr N and Sr N /Zr N The calculated structural formulae based on 24(O+F+Cl) indicate that full occupancy of the tetrahedral site can only be attained by allowing the entry of a small fraction of Fe 3+ or Ti 4+ . In addition, Ti-oxy (likely coupled with Ba-oxy) substitution occurs to provide charge balance with the excess negative charge at the O4 position. The occurrence of such exchange mechanisms can enhance the thermal stability of the mica flakes that most likely crystallized at shallower depths under high temperature and high f (O 2 ) conditions.

Spinels in Harzburgite and Lherzolite inclusions from the San Giovanni Ilarione Quarry, Lessini Mountains, Veneto Region, Italy

SummaryChrome-bearing spinels in 21 ultramafic nodules contained in a basanite from San Giovanni Ilarione (SGI), Veneto Region, Italy, have been grouped from a textural and chemical standpoint into five types: (1) Interstitial, surrounded by silicates, homogeneous, with a Cr/(Cr + Al) ratio averaging 0.11; (2) Within alteration pods and veinlets, in contact with either silicates or alteration products, showing zonation with Mg/(Mg + Fe2+), Cr, and Ti increasing and Al decreasing from core to rim; (3) In contact with the basalt or surrounded by basalt but distinct from ground-mass grains. The former are strongly zoned while the latter, referred to as xenocrysts, are relatively homogeneous. Both belong to a trend distinct from that of (2); (4) Symplectic, intergrown with pyroxenes and homogeneous, or in contact with plagioclase and/or reaction products and showing more variable composition than the former. Both are generally richer in Cr than all other spinet types and define a trend parallel to and with higher Cr/(Cr + Al) and Cr values than that of (3); and (5) groundmass grains within the host basalt, showing lower Mg/(Mg + Fe2+) and Cr/(Cr + Al), and higher Fe values than for all other, types.Group 1 spinels and the cores of group 2 and group 3 spinets display very similar compositions, suggesting a common origin, modified by later events such as partial melting, solid-liquid reactions, and subsolidus reactions which occurred prior to, during, and subsequent to interaction with the host basalt. Group 2 and 3 spinels define trends diverging from a common composition—i.e., the same origin. Group 4 spinels are probably due to exsolution which occurred after formation of the original spinels, while group 5 spinels are not directly related to types 1–4.ZusammenfassungChromhaltige Spinelle 21 ultramafischer Knollen aus einem Basanit von San Giovanni Ilarione (SGI), Region Veneto, Italien, wurden nach texturellen und chemischen Gesichtspunkten in fünf Gruppen eingeteilt: 1. Interstitiell, von Silikaten umgeben, mit einem durchschnittlichen Cr/(Cr + Al)-Verhältnis von 0,11.2. In Umwandlungstaschen und -gängchen, in Kontakt entweder mit Silikaten oder mit Umwandlungsprodukten, zoniert mit Mg/(Mg + Fe2+), Cr und Ti vom Kern zum Rand zunehmend, Al aber abnehmend. 3. Im Kontakt zum Basalt oder vom Basalt umgeben, aber von den Körnern der Grundmasse verschieden. Die ersteren sind stark zonar, während die Letzteren, die als Xenokrysten angesehen werden, relativ homogen sind. Beide gehören zu einem Trend, welcher von jenem der Gruppe 2 verschieden ist. 4. Simplektitisch, mit Pyroxenen verwachen und homogen, oder im Kontakt zu Plagioklas und/oder Reaktionsprodukten und von variablerer Zusammensetzung ab die ersteren. Beide sind allgemein reicher an Cr als alle anderen Spinelltypen und sie definieren einen Trend parallel zu und mit höheren Cr/(Cr + Al)- und Cr- Werten als jene von Gruppe 3. 5. Körner der Grundmasse des Wirtsbasaltes, die niedrigere Mg/(Mg + Fe2+)- und Cr/(Cr + Al)- Werte und höhere Fe-Werte zeigen als alle anderen Typen.Die Spinelle der Gruppe 1 und die Kerne der Spinelle der Gruppen 2 und 3 zeigen sehr ähnliche Zusammensetzungen, die auf einen gemeinsamen Ursprung hinweisen, der später modifiziert wurde durch Ereignisse wie partielles Schmelzen, fest-flüssig-Reaktionen und subsolidus-Reaktionen, die vor, wärend und nach der Wechselwirkung mit dem Wirtsbasalt stattfanden. Die Spinelle der Gruppen 2 und 3 definieren Trends, die von einer gemeinsamen Zusammensetzung und damit von einem gemeinsamen Ursprung her auseinanderlaufen Spinelle der Gruppe 4 entstanden wahrscheinlich durch Entmischung, die nach Bildung der Ursprungsspinele eintrat, während die Spinelle der Gruppe 5 zu denen der Gruppen 1 bis 4 in keiner direkten Beziehung stehen.

FeMn crusts from the southern Tyrrhenian Sea

Abstract Dark crusts and concretions on volcanic and calcareous rocks dredged from seamounts west of the Aeolian Islands, southern Tyrrhenian Sea, consists mainly of amorphous FeMn hydroxides, with some goethite, todorokite, birnessite and clastic minerals. Chemically they consist of predominant Fe (up to 45%) and Mn (up to 12%), with significant amounts of Zn, Ni, Co and Cu. That the REE distribution patterns are similar to those of shallow Pacific derivation — from hydrothermal waters cognate or cycled through volcanic rocks of intermediate composition — is tentatively suggested.

Phlogopite from the Serre ultramafic rocks, Central Calabria, Southern Italy

A 6–7 meter thick phlogopite-bearing peridotite “body” interlayered within a 200 m thick sequence of layered metagabbro and pyroxene-bearing felsic granulite outcrops in the Serre Massif, southern sector of the Calabrian Peloritan Arc (southern Italy). Phlogopite occurs throughout the whole peridotitic body, but in different modal amounts and in various textural sites. It occurs as Type-I, isolated and oriented grains in textural equilibrium with the other phases, either in the peridotite or in included orthopyroxenite veins; Type-II, in a kind of melange, as millimetric smearings of flakes which separate anastomosing phlogopite-rich and orthopyroxene-rich peridotitic portions; Type-III, in centimetric phlogopitite layers alternating with phlogopite-rich layers. The phlogopite has mg values [mg = Mg/(Mg+Fe)] in the range 0.90 to 0.94. It also shows appreciable variations of TiO 2 , Cr 2 O 3 and BaO contents, i.e., 1.26–2.44, 0.21–1.35, 0.15–2.02 wt%, respectively. The primitive mantle-normalized REE pattern of Type-I phlogopite shows a steep negative slope from La to Eu, followed by a slight Gd positive anomaly and a moderately positive slope from Tb to Lu. The REE patterns of Types-II and -III show negative slopes from La to Lu, but the Type-II pattern displays a marked Eu negative anomaly, and Type-III phlogopite has lower M- and HREE contents. The trace element spider diagrams show a generally decreasing trend in elemental amounts, in accordance with decreasing incompatibility, with evident K, Pb, Mo, P, Sn and Ti positive anomalies. The occurrence of phlogopite in peridotite from the Serre Massif probably indicates that the rocks underwent a severe metasomatic event in upper mantle and/or lower crust-mantle boundary conditions.