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Silicate-Carbonate-Salt Liquid Immiscibility and Origin of the Sodalite-Haü

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Silicate-Carbonate-Salt Liquid Immiscibility and Origin of the Sodalite-Haü Cent. Eur. J. Geosci. • 1(4) • 2009 • 377-392 DOI: 10.2478/v10085-009-0036-1 Central European Journal of Geosciences Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy Research Article Liya I. Panina1∗, Francesco Stoppa2 1 Institute of Geology and Mineralogy: Siberian Branch, Russian Academy of Sciences, Koptyug pr. 3, Novosibirsk, 630090, Russia, 2 Dipartimento Scienze della Terra, Università G.d’Annunzio, via dei Vestini, 30, 66013 Chieti Scalo (CH), Italy Received 27 June 2009; accepted 22 October2009 Abstract: Melt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 – 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite. Keywords: salt-silicate-carbonate inclusions • immiscibility • olivine foidite • plume metasomatism • Vulture-Italy © Versita Warsaw ∗E-mail: [email protected] 377 Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy 1. Introduction sulphate mineralization is represented by the haüyne- sodalite group rocks. Most of them are volcanic rocks Nowadays, most researchers favor the igneous origin of composed of haüynites and haüyne phonolites. The best carbonatite. Discussions focus mainly on the sources and known occurrences are in Western Germany (E.Eifel), mechanisms of formation of carbonatite melts, their initial Central and Southern Italy (Vulture volcano and Grotta composition, and character of transformation during crys- del Cervo), and Southern Armenia. Genesis of these rocks tallization. There are three most popular concepts on the is related by their very high crystallization temperature sources and mechanisms of carbonatite melt formation: a) and chemistry to primitive mantle magmas initially en- carbonatite melts are formed during melting of carbon- riched in alkali chlorides, sulphates and carbonates. A ate mantle; b) carbonatite melts appear owing to liquid classical example of spatial coexistence of carbonatites immiscibility in parental nephelinites or melilitite magma with the sodalite haüyne group of rocks is the Vulture during its high-temperature (> 800°C) evolution; c) car- volcano (Southern Italy). bonatite melts are residual low-temperature (< 800°C) Melt inclusions in the minerals of olivine foidite bomb from products of the crustal differentiation and fractionation of the Serra di Costantinopoli pyroclastic flows of the Vul- alkaline magmas. ture volcano can add a further piece of precious informa- Most of the previous authors are focused on melilitite- tion about concomitant genetic causes for haüyne-sodalite carbonatite association [1]. As to the origin of carbon- group mineral appearance in carbonatite and alkaline- atites, most researchers think that two-phase silicate- mafic volcanoes. Inclusions have the unique property carbonate immiscibility plays a leading role in their for- to retain information about PT-parameters, compositions mation (see discussion in Stoppa et al. [2]). This assump- (initial and derivative), conditions of evolution (behavior tion is supported by numerous experimental data, results of some elements during the ascent and crystallization of studies of melt inclusions in minerals, and geological of magma, evolution trend, crystallization differentiation observations. and fractionation processes, liquid immiscibility and mix- Experimental studies of synthetic and natural alkaline ing of melts), and degree of fluid saturation in mineral- systems [3–7] evidenced that liquid silicate-carbonate im- forming systems [25, 26]. Studies of melt inclusions in miscibility occurs in a wide range of temperatures and minerals provide us information about the reasons of ori- pressures, and depends on the composition of parental sil- gin of haüyne-sodalite group of minerals in carbonatites icate melt, degree of fluid saturation, and oxygen fugacity. and alkaline-mafic volcanic rocks. An increase in the amount of alkalies and CO2 pressure expands the immiscibility field [8–11]. Volatiles cannot separate from the melt in the form of gas phase because 2. Geological background of the high activity of alkalies. This produces a structural change of the alkaline aluminosilicate melts and initiates Vulture is located on the Eastern side of the southern silicate-carbonate immiscibility. Moreover, experiments in Italian peninsula, in the Lucania region and is well sep- a carbonate-silicate system at critical concentrations of arated from the Recent leucititic volcanoes of the Roman alkalies, S, P, Cl, and F revealed that separated carbon- Region. The volcano lies on relatively thick lithosphere. ate melt contains immiscible portions of alkali-phosphate, This area is far away from the intermediate and deep alkali-chloride, alkali-fluoride composition [12]. seismicity of the southern Tyrrhenian Sea and is char- Abundant occurrences of silicate-carbonate and multi- acterized by a zone of intense, crustal, dip-slip earth- phase silicate-carbonate-salt immiscibility were detected quakes. Tectonic and volcano-tectonic processes have in thermometric experiments with melt inclusions in the produced a small horst and graben system within the minerals of different alkaline rocks [2, 13–23]. Of espe- pre-volcanic flysch substratum, and partially within the cial interest was multiphase carbonate-salt immiscibility volcano itself which presently reaches 1 326 m.s.l. Vul- in silicate-salt inclusions in perovskite of melilite rocks ture belongs to Province of kalsilite-bearing melilitites from the Krestovskaya intrusion (Polar Siberia). Here for and foidites (i.d. kamafugites), melilities and carbonatites the first time in thermometric experiments both silicate- named Intramountain Ultra-alkaline Province whose gen- carbonate and carbonate-salt immiscibility were observed. eral geological features are fully addressed in Lavecchia The latter was found in immiscible carbonate-salt glob- and Creati [27]. ules, which, on heating, separated into immiscible por- Vulture is a long-lived volcanic complex dating back to c. tions of alkali-sulphate, alkali-chloride, alkali-phosphate, 700 ka, formed during periods of relatively short activity and Mg, Ca, Fe-carbonate compositions [24]. separated by long resting periods. During the early pe- A very interesting association of carbonatites and alkali- riod, a phonolitic caldera, about 6×4.5 km in size filled 378 Liya I. Panina, Francesco Stoppa with co-caldera deposits, domes and epiclastites were formed (710 and 750 ka). At the same time the sedimen- tary substratum was intruded by nepheline syenite dikes. At about 610 ka the Vulture-San Michele strato-volcano was formed by emission of foidite and phono-foidite pyro- clastics and lava flows with minor melilititic rocks forming a volcanic pile being approximately 600 m thick. The size of the volcano was then largely reduced by tectonic offset. A locally important strike-slip fault system was particu- larly active between 625 and 550 ka, and was likely re- sponsible for triggering a large sector collapse of the SSW flank of the volcano, associated with large pyroclastic flow emission. At approximately the same time, several satel- lite vents formed around the main volcano. A consider- able period of inactivity was terminated by a maar swarm which formed at 141±11 ka. The younger products are co- eruptive carbonatite and melilitite, which cover an area of approximately 15% of the volcano, although these rocks represent a small volume in comparison to the foidite- phonolite
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