Ignimbrite

Luganese Porphyry-district

The Southern Alps host volcano-sedimentary basins (Fig.1) that formed during post-Variscan extension and strike-slip in the Early Permian. Basin formation and magmatism are closely related to each other and occurred during a short time span, between 285 and 275 Ma.

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Fig.1: Distribution of pre-Early Permian basement and cover rocks of the Southern Alps. Modified from Schaltegger (2007).



The Luganese Porphyry-district (red box in Fig.1) in the Varese province, Northern Italy, is situated in the southern Alps and it lies some 25 km south of the east-west Insubric fault-line that separates the central Alps from these southern Alps. The Lugano porphyries rest unconformably on a complex of crystalline basement rocks that have been folded and metamorphosed during the Hercynian orogeny. On top of the volcanic series a thick Triassic sedimentary series has been deposited, starting with the Lower Triassic. This stratigraphic position makes a Permian age for the volcanics plausible.

The Lugano-Valganna complex (Luganese Porphyry-district) is composed of approximately 1.800 m of mainly extrusive volcanic rocks, intruded by the subvolcanic, pink-coloured Ganna granite stock (281 ± 9 Ma), which most likely represents the latest magmatic event in the region. The Lugano-Valganna succession consists of a series of tuffs, ignimbrites and lavas including andesitic to dacitic flows and thin intercalations of clastic rocks. The age of the Lugano-Valganna complex remains controversial.

The GrÓntola Vitrophyre

The GrÓntola Vitrophyre crop out 15 Km North of Varese (Fig.1 and Fig.2). The macroscopic appearance on a fresh fracture of this rock resembles that of obsidian: it shows a lustrous black color and to the naked eye it is difficult to detect the phenocrysts which it contains in large quantity. Only on closer investigation, with the pocket lens, many prismatic crystals of feldspar and olivines are observed. Feldspar are so clear that the black groundmass is seen through them. Their subparallel arrangement of the phenocrysts suggests a banded texture typical of explosive volcanic rocks.
The GrÓntola Vitrophyre was already known since the end of 1700, and it was defined as "Pechstein von Grantola" (Pitchstone in English). It was described from Harda (1883) and Harloff (1926) but the only most recent work is that of Bulletti (1987). The most extensive outcrop, which is however limited to a few m2, is located near Cunardo, where the Vitrophyre lies between ignimbrites and porphyritic tuffs.

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Fig.2: Geological map of the GrÓntola area. Lugano-Valganna volcanic rocks in red-orange colors. Modified from Harloff (1926)



Under the microscope the structure of the rocks is porphyric (30-40%) with phenocrysts embedded in a highly glassy groundmass. Among the sialic phenocrysts, the most abundant is plagioclase (An38-55); less abundant is sanidine and quartz (with corroded rims). Femic phenocrysts are mainly represented by Fe-olivines (often altered by iddingsite and serpentine) and orthopyroxene. Common accessories amphiboles, opaque minerals (magnetite and ilmenite) and allanitic epidotes.


Bibliography



• Harloff, C. (1926). The geology of the porphyry district of Lugano between Ponte Tresa and Luino. Leidse Geologische Mededelingen, 2(1), 115-222.
• Stille P., Bulletti M., 1987. Nd-Sr isotopic characteristics of the Lugano volcanic rocks and constraints on the continental crust formation in the South Alpine domain (N-Italy-Switzerland). Contrib. Mineral. Petrol., 96: 140-150.
• Bocchio, R., Crespi, R., De Capitani, L., Liborio, G., & Schiavinato, S. G. (1991). Caratteri chimici e mineralogici del vitrofiro di GrÓntola (Varese). Rendiconti Lincei, 2(1), 39-49.
• Schaltegger, U., & Brack, P. (2007). Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps. International Journal of Earth Sciences, 96(6), 1131-1151.

Photo
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Feldspar (colorless), olivine (altered by red iddingsite) crystals and pumice fragment (beige) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and pyroxene crystals and pumice fragment (beige) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite) and pyroxene crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite) and pyroxene crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystals and "fiamme" (lenses of glassy material) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and pyroxene crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
grantola(71).jpg
Feldspar (colorless) and pyroxene crystals and pumice fragment (beige) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and pyroxene crystals and pumice fragment (beige) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and pyroxene crystals and pumice fragment (beige) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystals and "fiamme" (lenses of glassy material) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystals and "fiamme" (lenses of glassy material) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystals and "fiamme" (lenses of glassy material) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite), pyroxene crystals and xenolith embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystals and "fiamme" (lenses of glassy material) embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite) and pyroxene crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless), olivine (altered by red iddingsite) and pyroxene crystals embedded in a eutaxitic groundmass. PPL image, 2x (Field of view = 7mm)
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Feldspar (colorless) crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Pyroxene crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless), pyroxene and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Feldspar (colorless) and olivine (altered by red iddingsite) crystals embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Pyroxene crystal embedded in a eutaxitic groundmass. PPL image, 10x (Field of view = 2mm)
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Eutaxitic groundmass. PPL image, 20x (Field of view = 1mm)
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Eutaxitic groundmass. PPL image, 20x (Field of view = 1mm)
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Eutaxitic groundmass. PPL image, 20x (Field of view = 1mm)
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Feldspar (colorless) embedded in a eutaxitic groundmass. PPL image, 20x (Field of view = 1mm)
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Feldspar (colorless) embedded in a eutaxitic groundmass. PPL image, 20x (Field of view = 1mm)