Le Colme, Brosso, Metropolitan City of Turin, Piedmont, Italyi

Regional Level Types
Le Colme	Mountain
Brosso	Commune
Metropolitan City of Turin	Metropolitan City
Piedmont	Region
Italy	Country

Le Colme (1655 m) is a mountain on the ridge connecting Monte Gregorio (1953 m) with Monte Cavallaria (1478 m).

The upper part of this ridge consists of "eclogitic micaschists", with interbedded omphacitites and eclogites; on the west and south the micaschists have been intruded by the Tertiary monzodiorite stock of Traversella and converted into hornfelses. Andesite dykes, mostly a few dm thick, are found in the micaschist, especially near Grange Piani.

Essential components of the "eclogitic micaschists" are quartz (20-30 %), phengite, and garnet; glaucophane may locally be important; rutile, apatite, clinozoisite-epidote (often with zoisite cores) and rare tourmaline and zircon are found as minor accessories. A few mica schists (Grange Marlette area) contain stilpnomelane.

Jadeite megablasts crop out in "eclogitic micaschists" southern slope of Le Colme as intercalations ranging in thickness from a few dm to about 1 m. The contact between the intercalations and the paraschists is sharp, and is marked by a thin layer of phengite. Jadeite megablasts make as much as 70 % of the intercalations, the remainder being a quartz-phengite aggregate with minor garnet and glaucophane; the nodules are typically rounded to elongate and slightly flattened along the foliation; they range in length from 2 to 15 cm and average about 8 cm; their colour is light grey on weathered, and bluish on freshly broken surfaces. The jadeite megacrysts are typically poikiloblastic, containing abundant inclusions of quartz, phengite, pink garnet (Alm₅₅Grs₁Prp₆Sps₃₈) and minor accessories. Among the accessory minerals rutile and apatite are very common as inclusions, while pyrite, ilmenite, and zircon are quite rare. The jadeite megablasts of Le Colme are commonly altered, being partly to completely replaced by complex pseudomorphs consisting mainly of albite.

The garnet omphacitites occur as conformable intercalations, up to a few metre thick, in the "eclogitic micaschists"; they are dark-green, massive rocks, consisting of omphacite, with subordinate white mica, quartz, and glaucophane as major constituents; rutile, clinozoisite (with cores of zoisite), apatite, and zircon occur as minor accessories. Where garnet (Alm₆₀Grs₃₁Prp₉) is plentiful, omphacitites grade into eclogites and glaucophane eclogites.

Two other rock types, both probably derived from igneous rocks, are observed in the Colme area: a) metagranitic rocks with cognate inclusions and b) leucocratic gneisses.

a) The metagranitic rocks with inclusions crop out on the southwest slope of Le Colme, between the elevation of 1500 m and the saddle west of the summit; they are conformably interlayered in the eclogitic micaschists and, like these, dip steeply to the south. K feldspar is absent in these rocks. Quartz and phengite are the most abundant constituents, colourless omphacite forms a network of unoriented prisms eclosing the other minerals; oriented phengite marks a poorly-developed schistosity. Glaucophane, rutile, apatite, zircon, clinozoisite, and opaque minerals are minor accessories. Allanite relics occur as cores in clinozoisite. The mineral association of the inclusions (sodic pyroxene, phengite and subordinate garnet) is similar to that of the metagranitic rocks; the inclusions are, however, fine-grained and almost devoid of quartz.

The leucocratic gneisses occur as conformable bodies (up to a few tens of metres long) interbedded in the "eclogitic micaschists". The largest outcrops in the area are found on the road to Colle Pian del Muli, northeast of point 1170 m and northwest of Le Colme, along the ridge to Monte Gregorio. In a few outcrops the leucocratic gneisses show conspicuous augen of K-feldspar and, rarely, also megablasts of jadeite. Quartz, albite, jadeite, white mica ± K-feldspar are the essential constituents of the leucocratic gneisses; among the accessory minerals apatite and locally fluorite are abundant. Jadeite is found only rarely, since most outcrops of leucocratic gneisses lie in the contact aureole of the Traversella stock and most of the jadeite has been replaced by a brownish symplectite, consisting of very fine-grained albite and while mica, or by aggregates of granoblaslic albite enclosing skeletal white mica.

With in the contact aureole of the Traversella stock the high-pressure, low-temperature assemblages of the "eclogitic micaschists" have been overprinted by contact-metamorphic assemblages: In the innermost part of the aureole they are replaced by mineral assemblages of the hornblende-hornfels facies, whereas in the outermost part the thermal effect is marked by crystallisation of albite, biotite, chlorite, actinolite, epidote, and titanite.

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References