The first discovery of combeite and pectolite in kamafugitic rocks of Central Italy

Nikolaeva A.T.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

[e-mail]

The considered Сupaello volcano is part of the Intra-mountain Ultra-alkaline Province (IUP) of Central Italy characterized by kamafugitic and carbonatitic magmatism. This district consists of diatremes, maars and tuff rings [11]. IUP magmatic centers, such as the San-Venanzo (SV) volcano, the Colle Fabbri (CF) stock, and the Сupaello volcano are found in the Pleistocene/Quaternary continental tectonic depressions which cross cut the Pliocene Apennine thrust-fold system. The Cupaello volcano is located along the eastern border fault of the Rietti basin, Central Italy. This is represented by lava flow about 700 m long, 60-200 m wide, and up to 6 m thick [1,2,10].

The lava flow is composed of kalsilite melilitite (local name - cupaellite). The rock consists of phenocrysts of clinopyroxene, phlogopite and fine grains of melilite. Groundmass is represented by clinopyroxene, melilite, kalsilite, olivine, monticellite, perovskite, opaque minerals, and glass.

The chemical composition of kalsilite melilitite (cupaellite) is drastically SiO₂ undersaturated (~ 43,8 wt.%), has a low content of Al₂O₃ (~ 7,4 wt.%) and alkalis (4.4 wt.% K₂O and 0.27 wt. % Na₂O), and a high content of MgO (~ 11,3 wt.%), FeO (~ 6,7 wt.%), and CaO (~ 15.4 wt.%). This composition is very close to that of the SV olivine melilitites [1], but compared to the CF melilitolite contains less Al₂O₃ and CaO (about 11 wt.% and 38 wt. % in CF melilitolite, respectively).

Clinopyroxene phenocrysts in this rock have a short-columnar and prismatic habit. Their composition is diopside (Mg # 95) and similar to that of the SV clinopyroxene, and has more FeO and Al₂O₃ compared to the CF clinopyroxene [12]. Phlogopite phenocrysts are corroded in the rock. Their chemical composition is characterized by high TiO₂ (up to 2.3 wt.%) and Mg/Mg+Fe value (86-94). Melilite grains are euhedral in the rock. Their composition contains about 86% akermanite, up to 2% gehlenite, and about 12% Na-melilite components. This is similar to the SV melilite composition and very different from the CF melilite composition. Kalsilite from the groundmass is different from the SV kalsilite [1] due to a high content of FeO.

Combeite and pectolite was found in primary silicate-carbonate completely crystallized inclusions which are present in clinopyroxene phenocrysts. Previously, considered minerals were not detected among rock-forming minerals of carbonatite and kamafugite rocks of IUP. Inclusions in the clinopyroxene from kalsilite melilitite have a rounded, irregular, close to the prismatic form. Their size varies from 10-15 mμ to 50 mμ. The content of inclusions is represented by fine-grained aggregates of colorless, light green and brownish daughter minerals. Among the latter, with the exception of combeite (Na₄Ca₃[Si₆O₁₆](OH, F)₂) and pectolite (Ca₂NaH[SiO₃]₃), are present carbonates and sulphates of Ba, K, and Ca, phlogopite, and opaque minerals (Fig.).

Combeite is a rare mineral of alkaline rocks, but, nevertheless, it is typical magmatic mineral in combeite-, and wollastonite-bearing nephelinites from Oldoinyo Lengai volcano in Tanzania [7]. In these rocks combeite was found as phenocrysts, coronas around wollastonite and clinopyroxene [3], as well as globular patches in the groundmass of rock [5]. In addition, it was found in melilite-bearing rock – kugdite [9] as a daughter phase of melt inclusions present in the olivine and perovskite from Krestovskaya intrusion (Polar Siberia). The composition of combeite is characterized by 49-51 wt.% SiO₂, 26-27 wt.% CaO, and 18-20 wt.% Na₂O. Its stoichiometric formula, Na_{4, 39}Ca_{3, 49}[Si_{6, 06}O₁₆](OH, F)₂, is different from the standard one due to the fact that we have not detected F. The calculated Na/Na+Ca ratio is 0,55-0,57. Its composition is similar to that of Oldoinyo Lengai combeite (Tanzania), differing from it in low content of FeO (0,2-0,23 wt.% vs. 8 wt.%) and the presence of K₂O (0,61-0,67 wt.%). With a certain degree of conditionality it can be assumed that formation of combeite in the Italian kalsilite melilitite and in the Oldoinyo Lengai nephelenite has occurred under similar physico-chemical parameters.

Fig. Daughter phases of combeite (comb) and pectolite (pct) in silicate-carbonate inclusions from clinopyroxene (cpx).

The pectolite is a very rare mineral in primary magmatic rocks. It is occasionally found in tinguaites, mikrofoyaites, phonolites, and different nepheline syenites [4]. Pectolite is also observed as a reaction rim of mantle xenoliths in Gahcho Kue' kimberlites in Canada [6], in the melt inclusion from Cr-diopside of Inagli Deposit [8], as well as a crystallite and daughter phase of melt inclusions in pyroxenites and kugdites of Krestovskaya intrusion [9]. Chemical composition of pectolite is characterized by ~ 50 wt.% SiO₂, ~ 30 wt.% CaO, and ~ 9 wt.% Na₂O, its stoichiometric formula is Ca_2,04Na_1,14[Si_3,19O₈]OH. This composition is similar to that of pectolite from melilite-bearing rocks of Krestovskaya intrusion.

Thus, discovery of pectolite and combeite among crystalline phases in melt inclusions from clinopyroxene in melilitites suggests that initial melt was enriched in Ca and alkalis during clinopyroxene crystallization, and Na was predominant over K among alkalis. In addition, it can be assumed that the physico-chemical conditions of rock crystallization were quite comparable to those of rock crystallization of the Oldoinyo Lengai volcano.

References:

1. Cundari A., Ferguson A.K. (1991) Petrogenetic relationships between melilitite and lamproite in Roman Comagmatic Region: the lavas of S. Venanzo and Cupaello. Contrib Mineral Petrol 107: 343-357.

2. Gallo F., Giammetti F., Venturelli G., Vernia L. (1984) The kamafugitic rocks of S. Venanzo and Cupaello, Central Italy. Neues Jahrb Mineral Monatsh 5: 198-210.

3. Dawson J.B., Smith J.V., Steele I.M. (1989) Combeite (Na_2.33Ca_1.74others_0.12)Si₃O₉ from Oldoinyo Lengai, Tanzania. Journal of Geology 97:365–372.

4. Deer W.A., Howie R.A., Zussman J. (1963) Rock-forming minerals 2 – Chain Silicates. Longmans, London (in Rus).

5. Donaldson C.H., Dawson J.B., Kanaris-Sotiriou R., Batchelor R.A., Walsh J.N. (1987) The silicate lavas of Oldoinyo Lengai, Tanzania. Neues Jahrbuch für Mineralogie. Abhandlungen 156: 247–279.

6. Hetman C.M., Scott Smith B.H., Paul J.L., Winter F. (2004) Geology of the Gahcho Kue´ kimberlite pipes, NWT, Canada: root to diatreme magmatic transition zones. Lithos 76: 51– 74.

7.Klaudius J., Keller J. (2006) Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania. Lihos 91: 173–190.

8. Naunov V.B., Kamenetsky V.S., Thomas R., Kononkova N.N., Ryzhenko B.N. (2008) Inclusions in silicate and sulfate melts in chrome diopside from the Inagli Deposit, Yakutia, Russia. Geochemistry International 46(6): 554-564.

9. Panina L.I., Sazonov A.M., Usol’tseva L.M. (2001) Melilite- and monticellite-bearing rocks of Krestovskaya intrusion (Polar Siberia) and their genesis. Russian Geology and Geophysics 42(9): 1314 – 1332.

10. Stoppa F., Cundari A. (1995) A new Italian carbonanite occurrence at Cupaello (Rieti) and its genetic significance. Contrib Mineral Petrol 122: 275-288.

11. Stoppa F., Lavecchia G. (1992) Late Pleistocene ultra-alkaline magmatic activity in the Umbria – Latium region (Italy): An overview. Journal of Volcanology and Geothermal Research 52: 277 – 293.

12. Stoppa F., Sharygin V.V. (2009) Melilitolite intrusion and pelite digestion by high temperature kamafugitic magma at Colle Fabbri, Spoleto, Italy. Lithos 112: 306-320.

Silicate-carbonate inclusions in clinopyroxenes of shonkinites, Inagli massif (Aldan Shield, Russia)

Rokosova E. Y.^1,2, Vasil’ev Yu. R. ²

¹Novosibirsk State University, Novosibirsk, Russia, ²V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

[e-mail]

The Inagli massif belongs to the alkaline ultramafic complexes of potassic series. The massif is situated in the northwestern margin of the Aldan Shield (Yakutia, Russia). It is about 20 km² in area and is topographically manifested as a cupola structure with a central caldera. It is nearly isometric in shape and has a concentrically zoned structure. The central part of the massif is a stock, 16 km² in area, made up of dunite. The stock is surrounded by alkali gabbroids (shonkinites), melanocratic alkali syenites and pulaskites. A narrow (50 m) zone of peridotites is situated between the dunites and the alkali gabbroids. Sills of syenite porphyry occur at the periphery of the massif within the Cambrian carbonate sequence. The central dunite is threaded by numerous pegmatitic veins and veinlets of diverse mineral assemblages which include phlogopite, potassium feldspar, chrome diopside, richterite and Mg-arfvedsonite [1,2].

The genesis of rocks of alkaline ultramafic massifs is still a matter of wide debate. This concerns the identification of the parental magmas, the characteristics of mantle source and P-T parameters, as well as processes of evolution in the melts during crystallization.

Silicate-carbonate inclusions are found in clinopyroxenes of shonkinites of Inagli massif. Shonkinites consist of (vol.%) 50-55 clinopyroxene, 10-15 olivine, 5-10 serpentine, 10-20 potassium feldspar + pseudo-leucite, 3-7 biotite, 5 magnetite, 3 apatite. Clinopyroxene is prismatic, rarely irregular in the shape and has slightly greenish-yellow color. Clinopyroxene is often cracked and contains chadacrysts of olivine, titanomagnetite, apatite, potassium feldspar, phlogopite. Clinopyroxene is represented by diopside (wt.%: 49.7-55.2 SiO_2, 0.26-1.6 TiO_2, 1.5-3.1 Al₂O₃, 4.7-6.5 FeO, 0.14-0.26 MnO, 13.6-15.9 MgO, 21-21.98 CaO, 0.6-0.9 Na₂O, 0.05-0.1 P₂O₅; Mg# = 0.8-0.86).

Considered primary melt inclusions are arranged singly, in different parts of the grains of clinopyroxene. Inclusions are irregular or rounded in the shape and varies from 7 to 30 µm in size.

Daughter phases of inclusions are represented by pale brown laths of phlogopite (wt.%: 40 SiO₂, 13.72 Al₂O₃, 19.45 MgO, 9.3 FeO, 10.2 K₂O, 2.8 TiO₂, 2.1 F), colorless grains of potassium feldspar and/or albite (wt.%: 60.5 SiO₂, 18.6 Al₂O₃, 15.9 K₂O, 0.9 Na₂O and/or 63.8 SiO₂, 19.4 Al₂O₃, 2.67 СаO, 11.2 Na₂O), prismatic grains of apatite, grains of magnetite (wt.%: 86.1-88.9 FeO, 1.4-1.7 TiO₂, 1.7-5.1 Gr₂O₃, 1.3-1.95 Al₂O₃, 0.76-0.78 CaO), and fine-grained carbonate-salt phases. The latter are observed in the interstices between other daughter phases (Fig. a).

Inclusions were heated on a stage with a silite heater. At 800°C the carbonate-salt part of the inclusions started to melt and one or two oval gas bubbles appeared between the daughter phases. At 1000-1100°C gas bubbles became more rounded and disappeared in carbonate-salt melt. Melting of silicate glass occurred at the same temperatures. At 1230°C the carbonate-salt melt was transformed in a globule and then gradually decreased in size. The homogenization of carbonate-salt melt globule in a silicate melt occurred at 1280-1300°C. With a small decrease in temperature to 1270-1280°C the carbonate-salt globule appeared again and gradually increased in size during the cooling. After heating the inclusions consist of silicate glass, gas bubble and carbonate-salt globule at room temperature. The latter occupies up to 1/5 of the volume of the inclusion (Fig. b).

Fig. Inclusion in clinopyroxene of shonkinites of Inagli massif.

a) Inclusion before heating. The composition of daughter phases in inclusion was determined by the scanning microscope LEO1430VP.

b) Inclusion after heating. The composition of salt globule and silicate glass was determined by microprobe “Camebax-micro”.

The composition of homogenized silicate glasses of inclusions corresponds to trachyandesitobasalts (wt.%): 51-55.5 SiO_2, 0.6-0.8 TiO_2, 13.4-14.8 Al₂O₃, 4.3-6.7 FeO, 0.1-0.25 MnO, 12.5-6 MgO, 9.2-4.5 CaO, 2.2-2.8 Na₂O, 4.2-6.9 K₂O, 0.5-1 P₂O₅, 0-0.08 BaO, 0.45-0.5 Cl, 0.02-0.11 SO₃). The composition of carbonate-salt globules of inclusion is as follows (wt.%): 17.2-23.2 SiO_2, 0.4-1 TiO_2, 1.7-6.1 Al₂O₃, 2.6-4.9 FeO, 0.1-0.13 MnO, 3.1-5.1 MgO, 13.3-22.8 CaO, 1.8-4 Na₂O, 1.9-3 K₂O, 0.9-1.66 P₂O₅, 0-0.14 BaO, 0.06-0.5 SrO, 0.33-1.28 Cl, 0.19-1.3 SO₃. The composition of carbonate-salt globules is close to the carbonatite lavas of Fort Portal (Uganda) [3]. Normative composition of carbonate-salt globules includes diopside, phlogopite, potassium feldspar, albite, anorthite, apatite, calcite, barium and calcium chlorides, calcium and strontium sulfates.

Thus, the clinopyroxenes of shonkinites crystallized from homogeneous silicate carbonate-salt melt at 1280-1300°C. The melt separates into the silicate and carbonate-salt components under decreasing of temperature. It was shown in a review article about liquid immiscibility in deep-seated magmas [4] that the carbonate-salt melts spatially separated from silicate parental magma are enriched in Ca, alkalis, CO₂, S, F, Cl, P, H₂O and represent the original carbonatite melts. The letter separates into immiscible fractions of carbonate, alkaline-chloride, alkaline-sulfate, and alkaline-phosphate compositions under decreasing of temperature and pressure and nonequilibrium conditions. It should be noted that earlier Naumov et al [5] have concluded based on the study of polyphase inclusions in chrome diopsides of Inagli massif that the chrome diopsides crystallized from a silicate melt, which contained the emulsion salt globule of sulfate-dominated compositions.

References:

1. Korchagin A.M. (1996) Inagli pluton and its natural resources. Moscow: Nedra, 156 pp.

2. Kostyuk V.P., Panina L.I., Zhidkov A.Ya., Orlova M.P., Bazarova T.Yu. (1990) Potassium alkaline magmatism of Baikal-Stanovoy rifting system. Novosibirsk: Nauka. Siberian Branch, 239 pp.

3. Beloussov V V, Gerasimovsky V I, Goryatchev A V, Dobrovolsky V V, Kapitsa AP, Logatchev N A, Milanovsky E E, Polyakov A I, Rykunov L N, Sedov V V (1974) East – African rift system, 3 – Geochemistry, seismology: main results. Nauka, Moscow, 287 pp.

4. Panina L. I., Motorina I.V. (2008). Liquid immiscibility in deep-seated magmas and the origin of carbonatite melts. Geochemistry International 5, 487-504.

5. Naumov V.B., Kamenetsky V.S., Thomas R, Kononkova N.N., Ryzhenko B.N. (2008) Inclusions of silicate and sulfate melts in chrome diopside from the Inagli deposit, Yakutia, Russia. Geochemistry International, 46, 554-564.

Occurrence of I-type granitoid in the Paleo-Tethys ophiolite and associated metaflysch (Mashhad, NE Iran)

Samadi R. ¹, Shirdashtzadeh N. ²

¹Department of Geology, Science and Research Branch, Islamic Azad University, Tehran, Iran; ²Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

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The Dehnow igneous body is a part of the NW-SE trending granitoid-metamorphic complexes along with Binaloud structural zone which is cropped out at the west to south of Mashhad city (NE Iran) and it forms an individual intrusive body intruded into the metamorphic rocks and flysch in the west of Dehnow [1].

This part of Iran is regarded as the suture zone (SZ) of Paleo-Tethys ocean closures. In the Paleozoic, the Tethys Ocean was separating Eurasia in the north from Gondwana in the south. The Iranian plate, as a part of Gondwana, was located at the southern margin of Tethys Ocean and Turan plate in the north. The northward subduction and closure of the Paleo-Tethys and subsequent collision between the Iranian Cimmerian Microcontinent and the Turan plate resulted in the obduction of ophiolite complexes and generation of metamorphic rocks and granitic batholith in Mashhad area [2, 3] and the granitoid-metamorphic complexes intruded into the remnant of Paleo-Tethys meta-ophiolite and associated metaflysch [2, 3, 4]. [2] related the obduction of the accretionary assemblage over the Iranian microcontinent to the prior of Late Triassic time. [4] suggested that Paleo-Tethys Ocean opened during Silurian time and subduction under Turan plate was started in Late Devonian and then Turan plate obducted over Iran Plate by Late Triassic (225 Ma). Then, in Triassic to Cretaceous, the Paleo-Tethys remnants (meta-ophiolite and meta-flysch) were intruded by granitic rocks. The obducted remnants of the Paleo-Tethys Ocean in Binaloud range include several rock assemblages including ophiolite complexes, meta-flysch and some submarine pyroclastics. [5] compared the meta-flysch with similar less metamorphosed, fossil-bearing rock exposed 150 Km southeast of Mashhad and given a Devonian-Carboniferous age for them. The ophiolite complexes and metamorphic rocks with a NW-SE trending belt are in the northern flank of Binaloud, and in the northern part Mashhad granitoids are located.

The pale grey tonalite of Dehnow is a xenolith free igneous body. It is mineralogically include of quartz (~20-30 vol. %), plagioclase (~45-50 vol. %), ferro-hornblende (~10 vol. %), and secondary and accessory minerals of annite-siderophyllite (~10-15 vol. %), almandine (-pyrope), muscovite, chlorite, epidote, calcite and ilmenite. The microscopical texture is granular and the mineralogy show a tonalite (-granodiorite) composition. Lack of sedimentary or metamorphic enclaves or minerals (i. e. cordierite, sillimanite, etc.), in neither macro- nor micro scale, is the most significant characteristics in this studied tonalite.

A quantitative chemical analysis of minerals was carried out by wavelength-dispersive EPMA, model JEOL JXA-8500F and JXA-8800 (XDS), at Institute for Research on Earth Evolution (Japanese Agency for Marine-Earth Science and Technology, Yokosuka, Japan). The analysis was performed under an accelerating voltage of 15 kV and a beam current of 15 nA. The standard ZAF data corrections were performed. Natural and synthetic minerals of known composition are used as standards. Whole rock chemical analyses were conducted by X-ray fluorescence spectrometry at Geological Survey of Iran.

Petrography and mineral chemistry reveals that quartz occurred as xenomorphic interstitial grain, show wavy extinction, and rarely contains inclusions of plagioclase. Plagioclases show zonation and polysynthetic twining and suffered saussuritization to some extent. They are andesine to labradorite in composition. Amphiboles that are mostly replaced with biotite and chlorite in the rims, are mostly ferrohornblende to ferro-tschermakite. The biotites which are formed at the expense of amphiboles are annite to siderophyllite in composition. Phenocrystic garnet reported by [1] with almandine composition.

According to geochemical data set it is tonalite (to granodiorite) in composition with SiO₂ ~ 62.2-66.5 wt%. It is metaluminous (A/CNK = 0.93-1.00) and according to the mineralogy, geochemistry, and the isotopic data of [4], they are calcalkaline I-type granitoid formed in an oceanic - continental subduction setting [6, 7]. SiO2, Fe2O3t and Rb/Ba are higher in sample from centre of the tonalitic body while Al₂O₃, MgO, Na₂O/K₂O and Sr/Y content are progressively enriched toward the marginal samples.

Sr/Y ratio (~18-61-23.68) and Y (~17-31 ppm) indicate an island arc affinity rather than adakitic for the studied tonalitic magma. Low content of Sr/Y suggests low content or lack of garnet in the residual phases of tonalitic parental magma.

The more immobile trace elements of Zr and Zn against SiO₂ content (on plot by [8]) display I–type magma origin. [4] performed an isotopic research on the intrusive rocks of Dehnow and they obtained ⁸⁷Sr/⁸⁶Sr = 0.707949 to 0.708589, ¹⁴³Nd/¹⁴⁴Nd = 0.512059 to 0.512019, and εNd = -6.63 to -5.90 for them. These values on the diagram of εNd versus ⁸⁷Sr/⁸⁶Sr by [9] clearly are plotted within the I-types field.

The isotopic data of zircon [4] show the mean age of ~215±4Ma (Late Triassic) and [5] suggested the age of Devonian-Carboniferous for the area metaflysch. Therefore, absence of sedimentary aged zircons (from Devonian-Carboniferous meta-sediments of the area) in tonalite points out that the tonalitic magma source confidently could not be not connected to the melting of sourronding Paleozoic flysch and sediments in the area.

Dehnow igneous tonalite (- granodiorite) is a metaluminous I-type granitoids formed in an oceanic - continental subduction setting. Our conclusion is based on:

(1) Lack of sedimentary enclaves or nonmagmatic minerals (like cordierite, sillimanite and etc.) in the studied rocks simply describe that magma origin may not be related to the adjacent metasedimentary rocks.

(2) Based on geochemical data, we figure out that enigmatic nature of the Dehnow tonalite is definitely an I-type granitoid developed in a more deep magma origin or an extensional tectonic setting. It is metaluminous (A/CNK = 0.93-1.00) and I- type amphibole-bearing calcalkaline granitoids (ACG) formed in an oceanic - continental subduction setting. The tonalitic melt may experienced little contaminatation by the lower crust material and then undergone nearly shallow alterations.

(3) The isotopic data of zircon and whole rock as mentioned in previous paragraphs.

References:

1. Samadi, R. (2009) Petrology of tonalitic rocks of Dehnow (Northwest of Mashhad, Iran), M. Sc. Thesis, University of Tehran.

2. Alavi, M. (1991) Sedimentary and structural characteristics of the Paleo-Tethys remnants in northeastern Iran. Geological Society of America Bulletin 103(8), 983-992.

3. Alavi, M., Majidi, B. (1972) Petrology and geology of metamorphic and intrusive rocks of the Mashhad area. Geological Survey of Iran, 30 p.

4. Karimpour, M.H., Stern, C.R., Farmer, G.L. (2010) Zircon U-Pb geochronology, Sr-Nd isotope analyses, and petrogenetic study of the Dehnow diorite and Kuhsangi granodiorite (Paleo-Tethys), NE Iran, Journal of Asian Earth Sciences, 37, 384-393.

5. Majidi, B. (1981) The ultrabasic lava flows of Mashhad, North East Iran. Geological Magazine, 118, 49-58.

6. Samadi, R. Mirnejad, H. Shirdashtzadeh, N. and Kawabata, H. (2010) Petrology of tonalitic rocks of Dehnow (Northwest of Mashhad, Iran), The 1st International Applied Geological Congress. Islamic Azad University of Mashhad, Iran.

7. Samadi, R. and Shirdashtzadeh, N. (2011) A new debate on the origin of granitoid rocks from Dehnow area (NE Iran), based on isotopic data. Goldschmidt Conference Abstracts, Mineralogical Magazine, Vol. 75 (3), p 1785.

8. Chappell, B.W., Bryant, C.J., Wyborn, D., White, A.J.R., Williams, I.S. (1998) High- and low-temperature I-type granites. Resource Geology 48, 225-236.

9. Keay, S., Collins, W.J., McCulloch, M.T. (1997) A three component Sr–Nd isotopic mixing model for granitoid genesis, Lachlan fold belt, eastern Australia. Geology 25, 307–10.

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