Hornblende gabbro (Monte Mattoni, Adamello)

The Adamello batholith, generated, during the Oligocene, in an arc-related continental margin setting, is the largest Tertiary intrusive complex along the Pedriadriatic lineament, and is located in the Eastern Southern Alps. The Adamello batholith is exposed over an area of about 700km² at the intersection of the Periadriatic Lineament with the Giudicarie Line and consists of multiple intrusive units (up-to 98 % tonalites and granodiorites and minor ultramafic and gabbroic rocks) ranging in age from about 42 Ma to about 29 Ma. Mafic and ultramafic bodies are found locally, especially in the southernmost Re di Castello unit. The mafic bodies are mainly concentrated at the border of this unit and best exposed in the Val Fredda and Cornone di Blumone areas.

Fig.1: Geologic map of the Adamello batholith.

Monte Mattoni

The Val Fredda Complex consists of granodiorites, tonalites and quartz-diorites intruded by several mafic sheets of metre to decametre thickness. Two types of mafic sheet have been recognized in the Val Fredda Complex, specifically in the Mt. Cadino and Mt. Mattoni sequences. The Mt. Cadino mafic rocks range from hornblende gabbros to quartz-diorites, mostly with equigranular textures. The Mt. Mattoni mafic rocks consist of hornblende-phyric, pyroxene-bearing gabbros (hereafter amph-gabbros) with subordinate hornblendite blocks or bands. Subhorizontal layering is locally recognizable and is defined by an irregular succession of different types of amphibole gabbro, in places associated with hornblendite. The chilling of the mafic intrusions against the host tonalite suggests that the mafic sheets of both Mt. Cadino and Mt. Mattoni were emplaced into the pre-existing Val Fredda tonalite.

The Mt. Mattoni mafic to ultramafic rocks are interpreted as deep-seated cumulates crystallized from a hydrous melt with a basalt or picro-basalt composition that were disrupted by new injections of mafic magma and then transported to the emplacement level. The trace element signature of the parental melt of the Mt. Mattoni mafic intrusive rocks has been attributed to partial melting of spinel-facies subcontinental mantle metasomatized by slab-derived fluids.

Hornblende gabbro
The amphibole gabbro crystallized at c. 40 Ma and has porphyritic structure. In these rocks euhedral zoned amphibole (up to 1 cm), similar to the amphibole from the hornblendite, is dispersed into a matrix consisting of plagioclase, clinopyroxene, amphibole (hornblende to edenite) and accessory minerals, including quartz, titanite and zircon. The amphibole gabbro is thus interpreted to represent the product of a melt–rock reaction between the hornblendite and the infiltrating basic melt.

Fig.2: Simplified sketch map showing the location of the major mafic and ultramafic intrusive rocks (black areas) in the southern part of the Re di Castello unit (modified after Ulmer et al.,1983).

Porphyritic structure in the Monte Mattoni Gabbro. (black = Hornblende, white = plagioclase).

Cumulitic structure in the Monte Mattoni Gabbro.

bibliography

• Tiepolo, M., Tribuzio, R., Ji, W. Q., Wu, F. Y., & Lustrino, M. (2014). Alpine Tethys closure as revealed by amphibole-rich mafic and ultramafic rocks from the Adamello and the Bergell intrusions (Central Alps). Journal of the Geological Society, 171(6), 793-799.
• Tiepolo, M., Tribuzio, R., & Langone, A. (2011). High-Mg andesite petrogenesis by amphibole crystallization and ultramafic crust assimilation: evidence from Adamello hornblendites (Central Alps, Italy). Journal of Petrology, 52(5), 1011-1045.

Foto
Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>
Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>
Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>
Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>
Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>
Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (green and rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 1x (Field of view = 9mm)/td>	Big hornblende crystals (rich in inclusions) in a fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 1x (Field of view = 9mm)/td>
Pyroxene crystals in the fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 2x (Field of view = 7mm)/td>	Pyroxene crystals in the fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 2x (Field of view = 7mm)/td>	Pyroxene crystals in the fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. PPL image, 2x (Field of view = 7mm)/td>
Pyroxene crystals in the fine-grained matrix. Hornblende gabbro from Monte Mattoni, Adamello, Italy. XPL image, 2x (Field of view = 7mm)/td>