Petrology, Vol. 11, No. 4, 2003, pp. 365–382. Translated from Petrologiya, Vol. 11, No. 4, 2003, pp. 405–424. Original Russian Text Copyright © 2003 by Panina, Stoppa, Usol’tseva. English Translation Copyright © 2003 by åÄIä “Nauka /Interperiodica” (Russia).
Genesis of Melilitite Rocks of Pian di Celle Volcano, Umbrian Kamafugite Province, Italy: Evidence from Melt Inclusions in Minerals L. I. Panina*, F. Stoppa**, and L. M. Usol’tseva* * Institute of Mineralogy and Petrography, Siberian Division, Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia e-mail:
[email protected] ** Dipartimento di Scienze della Terra, Perugia, Italy e-mail:
[email protected] Received September 6, 2002
Abstract—Syngenetic inclusions of silicate and carbonate melts were found and examined in olivine and melilite in fine-grained melilitite lavas, bombs, and lapilli tuffs of Pian di Celle volcano in central Italy. The silicate melts conserved in the inclusions are chemically different. The highest temperature (1360°C) melt type, corresponding to alkaline pyroxenite in composition, was found in inclusions in a fragment of the most magnesian phenocryst of Fo97, which could be a mantle xenocryst. There are good reasons to believe that the composition of these inclusions reflects the composition of either the primitive melts or the conserved relics of the depleted mantle. Fo92–91 and melilite most often contain inclusions of melilitite melts and their derivatives, while Fo91–88 also bears inclusions of trachybasalt–trachytic melts. Their evolutionary trends have different directions: the melilitite trend is directed toward hyperagpaitic melts and is characterized by an enrichment in alkalis and depletion aluminum, while the trachybasaltic melts become enriched in silicon and aluminum in the course of their evolution, i.e., their trend is directed toward miaskitic aluminous residues. The melilitite melt inclusions are higher temperature (≥1360°C), whereas the trachybasalt inclusions homogenize at 1100–1200°C. The presence of the latter suggests that the magmatic chamber could contain trachybasaltic melts, along with volumetrically predominant kamafugite magma. The inflow of the former was insignificant and did not notably affect the evolutionary trend of the melilititic melts. The trachybasaltic melts could be genetically related to the alkaline basaltoid magma of the Roman comagmatic province. The carbonate inclusions comprise silicate–carbonate and predominantly carbonate varieties, whose presence is obviously related to silicate–carbonate immiscibility of the magma, a phenomenon that seems to have first occurred at significant depths and temperatures above 1300°C. The salt melts evolved from silicate-bearing alkaline to predominantly calcic carbonate liquids with a simultaneous enrichment in Ba, Sr, F, and Cl. Our results and analysis of literature materials led us to conclude that the liquidus crystallization of melilite in natural larnite-normative systems can proceed only at carbonate–silicate immiscibility.
INTRODUCTION The genesis of kamafugite rock series remains a complicated petrological problem, awaiting its settling. Even the origin of the widely known and exhaustively examined kamafugites in the East African Rift remains a matter of heated discussion (Bailey, 1989; Edgar and Arima, 1981; and others). A consensus was reached only concerning the origin of kamafugites in the upper mantle and that the leading role in the origin of the rock series was played by crystal fractionation. These conclusions were underlain by the results of geological, mineralogical, and petrological research and data on the experimental melting of komafugitic rocks at elevated P–T parameters (Arima and Edgar, 1983; Lloyd, 1985) and were also corroborated by evidence from inclusions in minerals from various kamafugitic series, indicating that the parental melts started to crystallize at depths of approximately 20 km and temperatures some-
what higher than 1400°C, under pressures of 5–6 kbar. Crystallization proceeded in intermediate chambers and was controlled by crystallization differentiation (Naumov and Polyakov, 1971; Naumov et al., 1972). However, later data obtained on inclusions testified to lower crystallization temperatures of minerals in ugandites from the Toro–Ankole province (Gurchenko et al., 1989): the temperatures were 1065–1000°C for small olivine grains and 1140–1100°C for leucite. These authors reported the chemical composition of the parental magma during the crystallization of the small olivine grains and noted that it was strongly undersaturated in SiO2 (~42 wt %), enriched in alkalis, incompatible elements (K, Ba, Sr, Ti, and P), and was close to ugandite composition. It was also noted that the magma was fairly high in volatiles, among which a significant part was played by CO2, with the H2O concentration close to 1 mol %. The minimum fluid pressure attained 4–5 kbar. This led the authors to conclude that the
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magma was generated under unusual conditions, related to the metasomatic recycling of the mantle material, and it was also admitted that ultrapotassic magmas can be produced by the partial melting of orthopyroxene-bearing ultramafic mantle rocks. When studying kamafugitic series in the East African Rift, their researchers noted that these series include relatively silicic rocks like leucite basanite and trachybasalt, whose occurrence cannot be explained within the framework of the crystallization differentiation concept alone. It was also remarked that there was no single tendency in the distribution of elements: the melanocratic rocks contained, along with Fe-group elements, significant amounts of lithophile elements, such as REE, Nb, Zr, and Ba. The conclusion was made that these inconsistencies were related to either certain specific features of the melting processes and mantlesource composition, in combination with the differentiation of the original parental melts under low pressures or the mixing of the mantle melts (ugandite, olivine– nepheline basaltoid, or carbonatite) with both acid sialic Precambrian rocks (from which K and trace lithophile elements came) and basanitic and melanocratic trachybasaltic melts (which were derived directly from the mantle and differentiated independently) (Belousov et al., 1974). The genesis of the melilite and carbonatite rocks of kamafugite series and, speaking more broadly, conditions of melilitite and carbonatite formation remain uncertain. Egorov (1969) argued that the coexistence of these rocks exclusively in alkaline–ultramafic intrusions and volcanic complexes cannot be coincidental and believed that this fact “provides a clue to the genesis of alkaline–ultramafic magmatic series” (Egorov, 1969, p. 3). Our research was centered on melilitite rocks from Pian di Celle volcano, Umbria, which are kamafugitic rocks of central Italy. These are fine-grained melilitite lavas, bombs, and lapilli tuffs produced during early stages of the volcanic activity. The genesis of kamafugitic rocks in this province is thought to be related to a mantle source. Some researchers (Cundari and Ferguson, 1991) believe that the P–T crystallization regime was the same as that of lamproites. Others (Stoppa and Cundari, 1995) maintain that the rocks were derived in a tectonic environment analogous to that of African rocks and were related to sources of the same nature. Kamafugitic melts are thereby believed to have been produced either by the partial melting of a mantle source, which could be potentially phlogopitized or characterized by a high CO2/(CO2 + H2O) ratio (Gallo et al., 1984), or by the melting of residual phlogopite-bearing upper-mantle material, which was enriched in LILE and radiogenic Sr (Piccerillo et al., 1988). It is thought that the kamafugite province could be related to the high-K leucitebearing rock association of the Roman Comagmatic Region (Stoppa and Cundari, 1995).
All conclusions concerning the mantle nature of kamafugitic melts are based on mineralogical, geochemical, petrological, and isotopic data, i.e., in a sense, on indirect evidence. The aim of our research was to examine melt inclusions in minerals in order to obtain direct information on the chemical composition and temperature of the parental melts and to trace their evolution within the Earth’s crust, where the melts differentiated and their minerals were fractionated under relatively low pressures. One of our objectives was to obtain information on the possibility of melt mixing and liquid immiscibility in them. It should be mentioned that the youngest rocks of Pian di Celle volcano are pegmatoid melilitolites, which were earlier characterized thermometrically by Sharygin (2001), Stoppa et al. (1997), and other researchers. This opens a real possibility of tracing the evolution of the parental melts from early to the latest magmatic stages by comparing earlier and newly obtained data. Moreover, there is a unique opportunity to compare these results with the physicochemical conditions of melilite crystallization in intrusive rocks in a variety of magmatic series and in different regions, which are fairly extensively described in the literature. In contrast to volcanic rocks, melilite in intrusive series is distributed unevenly and occurs as irregular segregations and poikiloblasts with pyroxene and olivine relics, i.e., seems to bear evidence of reactions or replacement. Hopefully, the comparison of these data will provide insight into melilite crystallization in nature. GEOLOGY Pian di Celle volcano and San Venanzo volcano, which is located 800 m north of it, are situated near the village of San Venanzo (Terni) approximately 30 km to the south-southwest of Perugia. A few small volcanic centers are scattered around the village over an area of approximately 1.5 km2 and are often collectively referred to as San Venanzo. These volcanic centers are parts of the Umbria–Latium Late Pleistocene kamafugite–carbonatite province, which lies in the peripheral part of the Tyrrhenian extensional system and is spatially restricted to the northwestern margin of the Roman Comagmatic Region in central Italy. This area started to actively change in the Pleistocene, when grabens and small depressions were formed, which are now filled with continental sediments. The volcanic centers were mostly restricted to graben margins and were characterized by short volcanic events and the development of diatremes and tuff ring bodies at an insignificant amount of lava flows and dikes. Volcanic activity in the area began with the eruptions of fluid-rich carbonatite–phonolite magma and the development of the San Venanzo Maar. Later, the eruption center was shifted southward, where an explosive eruption of degassed melilite–olivine magma occurred and a tuff ring body of Pian di Celle volcano was formed. Two flows of melilitite lava were subsePETROLOGY
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quently erupted in the northwestern and southern flanks of this ring, and a thick network of pegmatoid melilitolite veins developed in their northeastern part. The regional magmatic activity ended with the intrusion of two dikes in the central portion of the tuff body. The pyroclastics of Pian di Celle cover an area of approximately 1.5 km2 at an average thickness of about 20 m and a total volume of 3 × 106 m3. The lavas belong to the pahoehoe type, with the northwestern flow extending for approximately 600 m at an average thickness of 20 m and a width of 200 m, and the southern flow is 250 m long and 6 m thick. The pegmatoid melilitolite veins (which were previously referred to as pegmatoid venanzite) are relatively short and 0.05–1 m thick. The K–Ar isotopic age of Pian di Celle rocks is 265 ± 3 ka (Stoppa and Lavecchia, 1992; Stoppa, 1996). The pyroclastic material of Pian di Celle volcano is diverse and varies from agglomerates and massive breccias to variably sized lapilli tuffs, coarse-grained sand, and ash. The peripheral portion of the tuff body is dominated by crumpled lapilli tuff, in which variably sized and shaped authigenic and xenogenic fragments appear closer to the center, and the rock acquires a breccia-like crystalloclastic appearance typical of deep rocks in diatremes. Lapilli in the tuffs consist mostly of carbonatized melilitite, have rounded or irregular elongated shapes, and are surrounded by a carbonatized matrix with crystal fragments and glass sand. Lapilli often accentuate a trachytic-like texture of the tuff and sometimes “flow” around large fragments. The latter can be minerals and their clusters, as well as xenoliths of the host or deep-seated rocks and bombs. The host rock fragments are MZ–PZ quartzites, granites, granite-dacites, pelites, and granulites. The deep xenoliths comprise dunite nodules and crystals and crystal aggregates of forsterite and diopside. According to Stoppa et al. (1996) and other researchers, the structure, deformed morphology, and composition of the latter provide evidence for their mantle origin. Numerous bombs contained in the pyroclastic material belong to the breadcrust type and consist of carbonatized melilitite, which is porous closer to the bomb cores and becomes notably enriched (up to 40%) in calcite. The active release of CO2, which passed through the diatreme, sometimes caused a peculiar intrusive appearance of the pyroclasts. The lavas are variably crystalline, have porphyritic textures, and contain olivine phenocrysts. These lavas were originally referred to as venanzite, but later they were classed with kalsilite–olivine–leucite melilitites. According to the mineralogical and chemical composition, they affiliate with the kamafugite family (Cundari and Ferguson, 1991; Gallo et al., 1984). PETROLOGY
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PETROCHEMISTRY, PETROGRAPHY, AND MINERALOGY OF THE OLIVINE MELILITITES All melilite rocks composing Pian di Celle and San Venanzo volcanoes show unusual geochemical and mineralogical features. They belong to ultramafic larnite-normative rocks strongly undersaturated in Si and Na, with high Mg# and high K/Na ratios, enriched in incompatible components, and containing moderate concentrations of Ti, V, Co, and U (Gallo et al., 1984; Peccerillo et al., 1988). The fine-grained melilitite lavas and lapilli tuffs from Pian di Celle volcano, described in this paper, contain (in wt %) up to 12.5 MgO, 15 CaO, 8 K2O, 42–44 SiO2, 10–11 Al2O3, and 5 in the late residual pegmatoid melilitolites; Sharygin, 2001). The 100 Fe/(FeO + MgO) ratio of the melilito-
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Fig. 2. Correlations between the concentrations of SiO2 and major oxides in melt inclusions in olivine and melilite from fine-crystalline melilitites and lapilli tuffs and in minerals from pegmatoid melilitolites of Pian di Celle volcano. (1) Heated inclusions in olivine from melilitite; (2) unheated inclusions in olivine from melilitite; (3) unheated inclusions in melilite from melilitite; (4) heated inclusions in minerals from pegmatoid melilitolite; (5) unheated inclusions in minerals from pegmatoid melilitolite (Sharygin, 2001). Fields of inclusions: (I) melilitite melts and inclusions of alkaline pyroxenite composition; (II) carbonatite-rich melts; (III) trachybasalt melts. Arrows indicate the evolutionary trends of conserved melts: in field I, the concentrations of Al, Mg, Ca, and K decrease from early to late melt portions and the Ti, Fe, and Na concentrations simultaneously increase, i.e., the melts evolve toward hyperagpaitic SiO2undersaturated compositions. In field III, the late derivatives of trachybasaltic magma become richer in Si, Al, and Ti and poorer in Mg, Fe, and Ca, i.e., the trend is directed toward miaskitic melts and corresponds to the normal trend of crystallization differentiation of high-Al alkaline basaltic magmas. In field II, the melts evolve from silicate-bearing alkaline to calcite-rich liquids.
lite melts did not come higher than 40–60 during any evolutionary stage, while the trachytic melts have 100 Fe/(FeO + MgO) = 62–70. The CaO concentration of the melilitite melts is always higher than the sum of alkalis by factors of 1.5–2 and only the residual derivatives have this oxide ratio smaller than one. The CaO/(Na2O + K2O) ratio of the trachybasalt-trachytic melts is equal to 0.4–0.7 at a remarkable K dominance
over Na. Melts of such composition are usually believed to be residual products of the crystallization differentiation of high-K basaltoid magmas (Panina, 1983). In a variation plot in terms of major components versus silica (Fig. 2), different types of the inclusions are clearly grouped within three fields: field I comprises PETROLOGY
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Fig. 3. Correlations between the concentrations of SiO2 and major oxides in kamafugitic rocks from Italy and the East African Rift. (1–6) Rocks from Italy. Pian di Celle volcano: (1) melilitite lapilli from tuff; (2) tuff matrix; (3) fine-grained melilitite lava (our data); (4) pegmatoid melilitolite (Stoppa et al., 1997); (5) venanzite from San Venanzo volcano (Cundari and Ferguson, 1991); Polino volcano: (6) carbonatite tuffisite (Stoppa and Lupini, 1993). (7–11) Rocks from the western branch of the East African Rift (Belousov et al., 1974): (7) melilitite; (8) trachybasalt; (9) trachyte; (10) carbonatite from Fort Portal; (11) calcite carbonatite from Kerimasi. Fields: (I) melilitites, venanzites, and pegmatoid melilitolites; (II) tuffisites and calcite carbonatites; (III) trachybasalts and trachytes.
melilitite melts and inclusions of alkaline pyroxenite composition, field II includes predominantly carbonatite melts, and field III is defined by trachybasaltic melts. Field I comprises type-Ia inclusions in olivine, whose composition is close to melilitite, and alkaline pyroxenites (type Ib of inclusions). The same field contains the data points of Ic inclusions in melilite, which characterize more differentiated portions of the melt and inclusions in the pegmatoid melilitolites (Sharygin, 2001) corresponding to the latest residual melts. Field II encompasses carbonate-bearing and carbonate inclusions in olivine (types IIa and IIb), with the former and PETROLOGY
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the later plotting within the regions of high-Si and predominantly calcic carbonate melts, respectively. Field III comprises the high-Si inclusion of type III found in ferrous olivine. The composition of this inclusion corresponds to trachybasalt and trachyte. The composition of rocks of the kamafugite series in Italy and the East African Rift cluster within three fields (Fig. 3): I—melilitites, venanzites, and pegmatoid melilitolites; II—tuffisites and calcite carbonatites; and III—trachybasalts and trachytes. The contours of the fields practically coincide with the configuration of the
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fields for the melt inclusions, a fact illustrating similarities between their compositions. The fact that the data points of alkaline pyroxenite inclusions plot in the field of melilitites and their derivatives suggests their potential genetic relations. In this context, the main problem is the nature of the alkaline pyroxenite melts themselves. There are experimental data (Lloyd et al., 1985) that the partial melting (20– 30%) of phlogopite pyroxenite nodules from high-K rocks in southwestern Uganda at P ≈ 30 kbar gave rise to melts of composition close to that of the San Venanzo melilitite lavas. Furthermore, some researchers of Italian kamafugitic rocks (Cundari and Ferguson, 1991) believe that the San Venanzo and Pian di Celle melilitite melts resulted from the partial melting of wehrlites–clinopyroxenites, i.e., mantle rocks of intermediate composition between lherzolite and harzburgite. It was also proposed (Peccerillo et al., 1988; Gallo et al., 1984) that the melting process was notably affected by metasomatic agents, such as H2O, CO2, F, and LILE, which came from the deep mantle and/or the subducted oceanic ± continental crust and associated sediments (the Tertiary time in the Apennines was marked by very active subduction). Hence, both the hypotheses presented above and the results on experimental melting suggest that the alkaline pyroxenite inclusions can be regarded either as primitive melts or the partly depleted mantle, which were conserved in deep (mantle) Fo97 xenocrysts. At the same time, it is known that their close intrusive analogues in alkaline ultramafic carbonatite complexes (Kukharenko et al., 1965; Egorov, 1969) are usually members of normal (mafic-to-acid) evolutionary magmatic series, in which these rocks are early crystallization products of the parental magma. Not making any final conclusions concerning this problem because of the scarcity of information on the inclusions, we are quite positive that melts (or rocks) of alkaline pyroxenite composition should have played a notable part early during the origin of the Pian di Celle and San Venanzo kamafugitic rocks. Conceivably, that is why the venanzite (olivine melilitite) compositions plot on the trend of the Oldoinyo Lengai pyroxenites (Cundari and Ferguson, 1991) in an [(Al + 6Ca + 2Mg)]– [(Fe + Ti) + 7(Na + K)]–[4(Si + Na + K)] diagram. The occurrence of trachybasalt–trachyte inclusions (along with melilitite inclusions) in olivine suggests that the magmatic chamber contained both the volumetrically dominant alkaline larnite-normative magma and silicic miaskitic melts, which, perhaps, were not in equilibrium with the magma and mixed with it. The absence of trachybasalts and trachytes in the Pian di Celle edifice seems to be at variance with this concept, but it can be hypothesized that these rocks associations were not found because of their small volume and relatively poor exposures. It cannot also be ruled out that these melts did not attain the surface, because their mixing with the kamafugitic magma could occur at significant depths. The genesis of the trachybasalt–trachyte melts can be related to the derivation of trachyba-
saltic composition directly from a mantle source and their further independent evolution. It is also possible that the trachybasalt–trachyte melts could be derivatives of the high-K alkaline basaltic magma that gave rise to the Roman Comagmatic Region. Arguments for possible genetic relations between alkaline basaltic and kamafugitic magmas were presented by Stoppa and Cundari (1995) and other researchers. The concept of melt mixing can readily account for some chemical features of minerals in these rocks, for example, the complicated zoning of the melilite or unsystematic variations in the forsterite concentration in olivine phenocrysts and fine olivine grains in the groundmass. Based on the Nd and Sr isotopic composition, Castorina et al. (2000) also arrived at the conclusion that carbonatite– kamafugite rocks in Umbria Province were produced by the simple mixing of a mantle material like oceanicisland basalt and a component with low εNd and high εSr. At the same time, it can be definitely concluded that the supposed mixing of the melts did not notably modify either the evolutionary trend of the predominant melilitite magma or the chemical composition of its derivatives. The appearance of carbonatite melts in the magma chamber was definitely related to carbonate–silicate liquid immiscibility, with the carbonate liquid genetically related to the predominant melilitite magma but not trachybasaltic melts. This follows from the isotopic equilibrium between the silicate and carbonate fractions of the melilitites (Castorina et al., 2000) and from the onset of liquid immiscibility at 1240–1180°C in the inclusions contained in minerals of the late pegmatoid melilitolites (Sharygin, 2001). The initial segregation and spatial separation of carbonate melt from the silicate magma occurred, perhaps, at deep levels of the crust or even in the mantle at temperatures higher than 1360°C. At least during the growth of the outer rims of the olivine megacrysts and the crystallization of small olivine grains, carbonatite melt had already segregated from the melilitite melt and occurred in equilibrium with it. This follows from the fact of the occurrence of carbonatite inclusions and their coexistence with silicate inclusions in olivine from the melilitolites, as well as from the carbonatized composition of the matrix of the lapilli tuff and the calcite-dominated composition of bomb cores. At the same time, we did not observe the onset of liquid immiscibility when homogenizing inclusions of melilitite composition in olivine of the rocks. This led us to conclude that liquid immiscibility occurred at higher temperatures and during earlier evolutionary stages of the parental magma, i.e., in its weakly differentiated portions. The possibility of silicate–carbonate liquid immiscibility in alkaline ultramafic and mafic systems under elevated P–T parameters is suggested by extensive data, including experimental results (Wallace and Green, 1988; Hamilton and Kjarsgaard, 1993; Lee Who-Jer and Wyllie, 1997; Chalot-Plat and Arnold, 1999; and others). It was noted that the main factors inducing silicate–carbonate liquid PETROLOGY
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immiscibility in undersaturated silicate melts are critical concentrations of Ca and alkalis under high pressures of volatile components, predominantly CO2, as well as F, Cl, and S, which catalyzed the onset of immiscibility. A temperature decrease is also usually favorable for the widening of the immiscibility region. There are good reasons to believe that equilibrium in the magmatic system was often disturbed during Pian di Celle eruptions, and the silicate and carbonate melts were fully or partly mixed. It was only during the origin of the pegmatoid melilitolites at reestablished equilibrium that carbonate–silicate liquid immiscibility was clearly pronounced again and persisted over a wide temperature interval (from 1240 to 670°C) under a low pressure (of approximately 1 kbar) (Sharygin, 2001). The spatially separated carbonate liquid evolved from low-alkaline silicate-bearing melts to calcite-dominated liquids rich in Ba, Sr, F, Cl, and REE, which contained, according to Sharygin (2001), trace amounts (0.07–0.08 wt %) of H2O and crystallized at 720– 670°C. According to our thermometric data, the silicate-bearing carbonate melts crystallized at temperatures slightly above 1180–1100°C. Summarizing the results, the following conclusions can be made. The parental melilitite melt (type I of inclusions) was heated to high temperatures. It was low in silica, had a larnite-normative composition, and was enriched in salts (first of all, carbonates, and salts of S, Cl, and, perhaps, F), with which there was silicate–carbonate liquid immiscibility (inclusions of type II). In this melt, the outer rims of forsterite megacrysts and small groundmass olivine grains grew at temperatures slightly higher than 1360°C. The cores of high-Mg forsterite megacrysts seem to have crystallized from a more primitive melt, which, judging from the composition of Ic inclusions, could be compositionally close to phlogopite pyroxenite. Melilite started to crystallize late during the crystallization of small olivine grains, after which the silicate melt evolved toward hyperagpaitic, Ca- and alkali-enriched melts. Our results are pretty well compatible with published data on inclusions in melilite rocks of kamafugite series and genetically close deep rocks of alkaline ultramafic carbonatite complexes. According to data on the homogenization of inclusions, melilite crystallized in rocks of alkaline ultramafic complexes at temperatures of 1230–1000°C. For example, these temperatures are 1230–1180°C for the turjaites and uncompahgrites of Cape Turii (Panina and Podgornykh, 1974); 1235–1160°C for the melilite–monticellite rocks of the Krestovskaya intrusion (Panina et al., 2001); 1080– 970°C for the Kovdor turjaites (Veksler et al., 1998); and 1100–1060°C for the melilitolites of the Gardiner Complex (Nielsen et al., 1997). In all of these rocks, melilite crystallized after olivine, usually immediately after pyroxene but before feldspathoids. It is worth noting that melilite in these rocks either carries silicate– salt inclusions that exhibit silicate–carbonate liquid PETROLOGY
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immiscibility when heated or contains two syngenetic types of inclusions: silicate and salt. It is reasonable to believe that the salt constituent and silicate–carbonate immiscibility played a significant, or even determining, part in the origin of melilite. Microprobe analyses of such inclusions (Panina, 1985; Panina et al., 2001; Nielsen et al., 1997; Veksler et al., 1998; and others) indicate that the composition of the silicate melts from which melilite crystallized is always low in silica (38–42 wt % SiO2) and alumina (5– 10 wt % Al2O3) but is significantly enriched in CaO (up to 17–20 wt % and even higher) and alkalis (8–10 wt % of the sum of alkalis with Na dominating over K, according to the chemistry of the alkaline series). Most researchers regard melilitite melts as derivatives of a parental mantle ultramafic magma enriched in alkalis, but there is still no consensus about its composition. At the same time, the causes of melilite crystallization are more or less clear: all researchers admit that a significant role in this process is played by carbonatite melts. The crystallization of liquidus melilite in a larnite-normative alkaline magma usually proceeds in the presence of a carbonatite melt. It is also possible that melilite can crystallize as a reaction mineral as a consequence of interaction between a carbonatite melt and earlier minerals: pyroxene, olivine, and wollastonite. Carbonatite melts can supposedly be produced during various evolutionary stages of the parental magma by carbonate–silicate liquid immiscibility or be the residual products of the differentiation of this magma and the fractionation of its minerals. Also, it cannot be ruled out that there could be an independent deep-seated source of carbonatite magma (Seifert and Thomas, 1995). By way of illustration, let us consider the conclusions of some researchers. Nielsen et al. (1997) argued that the melt responsible for the origin of melilitolites in the Gardiner Complex was a low-temperature fraction of a more magnesian larnite-normative ultramafic magma of the lamprophyre type with mantle origin. The melilite crystallized in the presence of immiscible carbonate and silicate liquids. The melt further evolved toward larnite-normative nephelinites. Veksler et al. (1998) examined melilite rocks in the Kovdor and Gardiner complexes and arrived at the conclusion that their parental magma was CO2-bearing melanonephelinite. It is thought that amphibole and phlogopite crystallization and fractionation were favorable for CaO enrichment in the derivative melts. Later, depending on the stability of settling melilite, the evolutionary trend of the melts was directed either toward a two-phase carbonate–silicate liquid immiscibility and crystallization of liquidus melilite (Gardiner Complex) or toward carbonate-bearing ijolite and the appearance of reaction melilite and residual carbonate melts (Kovdor). The genesis of melilite rocks in the Krestovskaya intrusion is thought to have been related to the crystallization of melilitite melts (Panina et al., 2001), which were deriv-
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atives of a parental alkaline ultramafic magma enriched in Ca and salts. The liquidus crystallization of melilite proceeded simultaneously with carbonate–silicate liquid immiscibility, a phenomenon that resulted in the spatial separation of the carbonatite constituent and the further evolution of the silicate melt toward agpaitic larnite-normative alkaline residues. In the Malomurunskii ultrapotassic synnyrite-bearing massif, melilite in olivine–melilite–monticellite rocks is ascribed to reaction magmatic minerals (Panina and Usol’tseva, 1999). This mineral was supposedly produced by the interaction of pyroxene and olivine (early liquidus minerals of alkaline basaltic magma) with high-temperature carbonatite melts, which were immiscible with and separated from a Ca-enriched magma of presumably lamproite composition. Finally, the origin of combeite– melilite rims around pyroxene and wollastonite phenocrysts in lapilli of Oldoinyo Lengai volcano is also thought to have been related to the action of carbonatite melts on earlier phenocrysts contained in ijolite melt (Dawson et al., 1992).
Egorov (1969), who argued that melilite can be formed in melts only at high potentials of alkalis and a relatively low CO2 activity in the parental melilitite melt. The latter is thought to be compositionally approximated by turjaite, appeared early in the course of the deep-seated differentiation of the parental alkaline ultramafic magma, and enriched in Ca because of olivine crystallization and fractionation. An increase in the CO2 potential in the melt precludes magmatic melilite synthesis. Excess Ca is incorporated in carbonates, which are immiscible with the silicate constituent and separate from it. It has been demonstrated (Eggler, 1973, 1974) that the presence of CO2 in simple systems results in the origin of low-Si melts, and the high-pressure partial melting of peridotite in the presence of CO2 gives rise to larnite-normative melts. In other words, carbon dioxide is considered important for the highpressure generation of some magmas, whose crystallization under relatively low pressures gives rise to melilite-bearing rocks.
In studying experimentally the factors that constrain melilite crystallization in alkaline low-silica melts, it was determined that akermanite is stable only under relatively low (1300°C) evolutionary stage at a deep level in the crust (or even the mantle), the primitive melts seem to have experienced the onset of silicate–carbonate liquid immiscibility with the subsequent spatial separation of small volumes of carbonate liquids. 3. The melilitite melts seem to have been a silicate derivative of these primitive melts. Their CaO concentration remained at a high level (up to 15–17 wt %), the alkali concentrations were as high as 8–9 wt %, with K dominating over Na, the agpaitic coefficient was close to unity, and the melts were enriched in Sr, Ba, Zr, F, and Cl. These melts produced small forsterite grains and outer rims around forsterite phenocrysts. The CO2 partial pressure in the melts was, perhaps, relatively low because of the separation of the carbonate constituent, so that favorable conditions existed for the liquidus crystallization of melilite. This process was likely initiated at temperatures slightly above 1240°C, i.e., at temperatures comparable with the crystallization temperatures of this mineral in the pegmatoid melilitolites (Stoppa et al., 1997). Melilite crystallization and fractionation caused an increase in the Ti, Fe, alkali, Ba and Sr concentrations in the melts, while the Al concentration dramatically decreased, and the agpaitic coefficient became greater than one.
Experiments were also conducted in order to elucidate the effect of CO2 on melilite genesis. In the presence of CO2, akermanite is stable within a fairly narrow temperature field under pressures below 6 kbar. At lower temperatures, CO2 contained in the melt reacts with melilite to give rise to diopside and calcite. A decrease in the CO2 partial pressure widens the melilite stability field (Yoder, 1983). Carbon dioxide is generally thought to play a significant part in alkaline ultramafic systems. Tomkiev (1962) believes that CO2 can affect the equilibrium crystallization of a magma and preclude the appearance of Ca silicates and aluminosilicates. Moreover, this component controls the binding of alkali earth elements in carbonates with the development of immiscible silicate–carbonate melts. A similar ideas were put forth by
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4. The composition of the crystallizing melilitite melts was insignificantly affected by their mixing with derivatives of a trachybasaltic magmas that were brought to the magma chamber. The latter magma exhibited a miaskitic evolutionary trend, which is typical of Al-enriched alkaline basaltic magmas. The crystallization temperatures of the trachybasalt–trachyte melts were within the range of 1100–1120°C. These melts could be genetically related to alkaline magmatism in the Roman Comagmatic Region. 5. The carbonatite melts, which were immiscible with the melilite magma, evolved from silicate-bearing alkaline toward calcite-rich liquids enriched in Ba, Sr, F, and Cl. Equilibrium between the immiscible liquids seems to have been repeatedly disturbed during the eruptions of the volcano, and the liquids partly or fully mixed. The immiscibility phenomenon clearly manifested itself only during the origin of the pegmatoid melilitolites at 1240°C and gave rise to carbonate globules in the silicate melt. Liquid immiscibility persisted to a temperature of 670°C (Sharygin, 2001). 6. Data obtained on melt inclusions in minerals of melilitites from Pian di Celle volcano clearly demonstrate that liquidus melilite can be produced by natural alkaline larnite-normative melts only when there are immiscible carbonate and silicate liquids, with the former, perhaps, widening the stability field of this mineral through a decrease in the CO2 partial pressure in the system. From this viewpoint, it is easy to explain the systematic coexistence of melilite rocks and carbonatites. REFERENCES Arima, M. and Edgar, A.D., High Pressure Experimental Studies on a Kamafugitic and Their Bearing on the Genesis of Some Potassium-Rich Magmas of the West Branch of the African Rift, Petrol., 1983, no. 24, pp. 166–187. Bailey, D.K., Carbonate Melts from the Mantle in the Volcanoes of Southeastern Zambia, Nature (London), 1989, vol. 338, pp. 415–418. Barker, D.S. and Nixon, P.H., High-Ca, Low-Alkali Carbonatite Volcanism at Fort Portal, Uganda, Contrib. Mineral. Petrol., 1989, vol. 103, pp. 166–177. Belousov, V.V., Gerasimovskii, V.I., Goryachev, A.V., et al., Vostochno-Afrikanskaya riftovaya sistema (The East-African Rift System), Moscow: Nauka, 1974, vol. 3. Castorina, F., Stoppa, F., Cundari, A., and Barbieru, M., An Enriched Mantle Source for Italy’s Melilitite–Carbonatite Association as Inferred by Its Nd–Sr Isotope Signature, Mineral. Mag., 2000, vol. 64, no. 4, pp. 625–639. Chalot-Prat, F. and Arnold, M., Immiscibility between Calcio-Carbonatitic and Silicate Melts and Related Wall Rock Reactions the Upper Mantle: A Natural Case Study from Romanian Mantle Xenoliths, Lithos, 1999, vol. 46, no. 4, pp. 627–659. Cundari, A. and Ferguson, A.K., Petrogenetic Relationships between Melilitite and Lamproite in the Roman Comagmatic Region: The Lavas of St. Venanzo and Cuppaello, Contrib. Mineral. Petrol., 1991, no. 107, pp. 343–357. PETROLOGY
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