Wollastonite melilitolite
The Colle Fabbri leucite-wollastonite-melilitolite ("Euremite")
The Colle Fabbri igneous center is part of the Intra-mountain Ultra-alkaline Province (IUP) of Central Italy characterized by kamafugite and carbonatite monogenetic volcanoes: diatremes, maars, and tuff-rings. The IUP igneous centers are settled in a system of Pleistocene/Quaternary continental tectonic depressions which cros- cut the Pliocene Appennine thrust-fold system. Colle Fabbri body is located close to the town of Spoleto (
Fig.1) and covers a total surface area of 10,000 m
2. Radiometric dating suggests a maximum age of around 0.7 m.y. Upper Miocene marls and sandstones (Marnoso-Arenacea Formation) and Villafranchian (Late Pliocene-Early Pleistocene) siltstones and claystones are the main country rocks, along with widespread limestones and marls of Late Triassic-Miocene age.
Fig.1: a) Geological map of Umbria region (central Italy) showing main geological and geographical feature. b) Colle Fabbri outcrop. 1 - volcanics of the Vulsini caldera, 2 - carbonatitic-kamafugitic igneous centers, 3 - Pleistocene extensional faults, 4 - sedimentary deposits filling up Plio-Pleistocene grabens; b) the Colle Fabbri volcano, 5 - clays, 6 - A-breccia, 7 - B-breccia, 8 - sill and dykes, 9 - melilitolite plug, 10 - detritus; 11 - hypothesized conduit-crater rims, 12 - fractures, 13 – houses. From Panina, L. I., Nikolaeva, A. T., & Stoppa, F. (2013).
The Colle Fabbri outcrop is comprised of:
1- An association of layered extrusive breccias, composed of thermally metamorphosed clay and mineralized travertine.
2- A melilitolite stock plus smaller sills and dykes and their contact facies.
3- Pelitic country rock and conglomerates.
The extrusive breccias rest on conglomerate, which is unconformably lying on pelites, intruded by the melilitolite stock. The extrusive breccias consist of two layers which were described by Stoppa and Rosatelli (2009) as the lower A-breccia and upper B-breccia. The lower A-breccia is a hardened, dense, reddish rock composed of thermometamorphic, argillite fragments and lumps of mud. The matrix of breccia is dense, fine-grained, and, in places, vesicular. The upper B-breccia consists mainly of fragments of travertine of different sizes, small red-clay clasts surrounded by aphyric glassy and vesicular black material, and scarce aphyric lava clasts.
The stock of melilitolite outcrops on an area of 500 m
2, intrudes the breccias with sharp vertical contacts. The rock is a medium grained, equigranular melilitolite (intrusive rock with about 40% vol. of melilite), composed of melilite euhedral crystals up to 5 mm across, poikilitically enclosing wollastonite prisms. Leucite and kalsilite are the main intergranular constituents together with accessory Ti-garnet, Ti-clinopyroxene, magnetite, perovskite, rankinite, apatite, Fe-Ni sulphides, and carbonate/zeolite. Scattered anorthite crystals are also present. According to Le Maitre et al. (1989), the rock, formerly named
euremite by Stoppa 1988, should classify as a leucite-wollastonite-melilitolite. The melilitolite plug shows irregular cooling joints and becomes progressively more fine-grained and ocellar towards the contacts. Sill and dykelet swarm are unlikely to be directly related to the main melilitolite stock. Their contacts with the wall-rocks are marked by a distinct quenched porphyritic rock, including abundant, elongated, dark green clinopyroxene and tiny plagioclase crystals. The groundmass consists of fine-crystalline laths of plagioclase, acicular aggregates of clinopyroxene, ore phases (magnetite, sulfides), and glass (Stoppa and Rosatelli 2009).
The origin of the Colle Fabbri melilitolites
The origin of kamafugites and associated carbonatites of the Intra-mountain Ultra-alkaline Province (IUP) of Central Italy was the subject of a long-lasting debate due to their unusual composition and isotopic characteristics. Genetic relationship of kamafugites with the hosting tectonic structure and geodynamic setting was considered ambiguous by some researchers. Some researchers (Peccerillo, 1998; Conticelli et al. 2002) relate the peculiar nature of Italian kamafugites to the contribution of crustal rocks. Kamafugites and carbonatites of Italy resulted from the entrapment of sedi-mentary carbonates by silicate melts during their ascent through the thick layers of limestones and marls that occur along the peninsula of Italy. Other authors refer the unique features of Italian rocks to a magmatism related to subduction (Lustrino, 2000; Serri, 1997) or, vice versa, consider it the result of intraplate magmatism and mantle metasomatism (Cundari, 1994). Among kamafugites of Italy the Colle Fabbri melilitolite is one of the most unusual. It is the only IUP center showing notable signs of hydrovolcanism and hydrothermal activity. Melilitolite contains essential wollastonite and does not contain phlogopite which is ubiquitous in kamafugites. However, it contains leucite and the kamafugite key mineral: kalsilite. Wollastonite is commonly frequent mineral in thermally metamorphosed impure limestones but is rare in igneous rocks and mostly found in ijolites, melilitolites and phonolites associated with carbonatites. Some researchers reject the magmatic origin of Colle Fabbri rocks, and believe that Colle Fabbri rocks are paralavas formed as a result of melting and recrystallization of marls during "lignite burning".
Although Colle Fabbri mineralogy may partially resemble that of paralavas, the geology of the outcrop, the presence of an explosion breccia, the composition similar to that of other Italian leucite-wollastonite-bearing melilitolite (found in volcanic context, where the igneous origin of the rocks is not challenged), strongly contradict the idea of combustion metamorphism origin. At last, the data obtained by analyzing the melt inclusions in wollastonite, clinopyroxene and anorthite, showed high homogenization temperatures (1230-1190°C) of inclusions, resulted from the interaction of pelitic wall-rock with magma, probably, of melilititic composition, from which melilitolites of the core crystallized.
Sample doned by F.Stoppa (University of Chieti, Italy)
Bibliography
• Melluso, L., Conticelli, S., D’Antonio, M., Mirco, N. P., & Saccani, E. (2004). Petrology and mineralogy of wollastonite-and melilite-bearing paralavas from the Central Apennines, Italy. American Mineralogist, 88(8-9), 1287-1299.
• Panina, L. I., Nikolaeva, A. T., & Stoppa, F. (2013). Genesis of melilitolite from Colle Fabbri: inferences from melt inclusions. Mineralogy and Petrology, 107(6), 897-914.
• Stoppa, F., & Rosatelli, G. (2009). Ultramafic intrusion triggers hydrothermal explosions at Colle Fabbri (Spoleto, Umbria), Italy. Journal of Volcanology and Geothermal Research, 187(1), 85-92.
Foto
Melilite (colorless) and wollastonite (elongated prismatic crystals). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (elongated prismatic crystals). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (elongated prismatic crystals). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (elongated prismatic crystals). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (pale brown basal section). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (pale brown basal section). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (pale brown basal section). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (colorless) and wollastonite (pale brown basal section). Wollastonite-melilitolite, Colle Fabbri, Italy. PPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |
Melilite (with anomalous, berlin-blue colors) and wollastonite (orange-yellow birefringence). Wollastonite-melilitolite, Colle Fabbri, Italy. XPL image , 2x (Field of view = 7mm) |