What is andesite porphyry?
Andesite-porphyry is a porphyry with andesitic chemism. The term “porphyry” is used for igneous rock composed of large, conspicuous crystals (phenocrysts) and a fine grained to glassy groundmass (matrix) in which the phenocrysts are embedded.
How are andesites formed?
Andesite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magmas usually when it erupts onto the Earth’s surface and forms lava flows. Andesite forms from magma that contains less quartz (silica) than rhyolite but more than basalt.
What type of rock is dacite?
dacite, volcanic rock that may be considered a quartz-bearing variety of andesite. Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes, and sometimes massive intrusions in the centres of old volcanoes.
What is basalt made of?
Basalts are common aphanitic igneous extrusive (volcanic) rocks. Basalts are composed of minute grains of plagioclase feldspar (generally labradorite), pyroxene, olivine, biotite, hornblende and <20% quartz.
How is porphyry rock formed?
The porphyry deposits are formed by differentiation and cooling of a column of rising magma in stages. The different stages of cooling create porphyritic textures in intrusive as well as in subvolcanic rocks.
Is andesite porphyry plutonic or volcanic?
Andesite (/ˈæn. dɪˌzaɪt, -də-/) is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
How are ultramafic rocks formed?
Most ultramafic metamorphic rocks are formed by alteration of peridotite or pyroxenite to serpentinite or talc–amphibole–chlorite rocks that commonly are schistose or sheared.
How fast does dacite magma flow?
Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes over an erupting vent. Lava domes often grow by the extrusion of many individual flows >30 m (100 ft) thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.
Why is basalt dark in color?
Basalt is usually dark grey to black in colour, due to its high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. The physical properties of basalt reflect its relatively low silica content and typically high iron and magnesium content.
Can gold be found in basalt?
The abundance of gold in crustal rocks is an important constraint on the formation of gold deposits. Mid ocean ridge basalt has lower gold concentration than ocean-island and volcanic-arc basalt, due mainly to lower oxygen fugacity at MOR settings that causes sulphur saturation.
What kind of mineral is a trachyte rock?
Trachyte is an extrusive rock, belonging to the alkali series of intermediate volcanic rocks. The major mineral component of trachyte is alkali feldspar (e.g. orthoclase ), and it generally contains no quartz. Trachyte often displays trachytic texture, where acicular to tabular feldspar phenocrysts align in one direction,…
How are trachyte and syenite similar to each other?
See TAS diagram below to see how these rock types differ in their chemistry. Trachyte is a volcanic equivalent of syenite. Syenite is a feldspar-rich plutonic rock which is similar to granite but lacks or contains very little quartz.
Where is trachyte most likely to be found?
It is the volcanic equivalent of syenite. Trachyte is common wherever alkali magma is erupted, including in late stages of volcanism of ocean islands and in continental rift valleys and above mantle plumes. Trachyte has also been found in Gale crater on Mars.
Which is rare tridymite or quartz in trachyte?
Quartz is rare in trachyte, but tridymite (which likewise consists of silica) is not uncommon. It is rarely in crystals large enough to be visible without the aid of the microscope, but in thin sections it may appear as small hexagonal plates, which overlap and form dense aggregates, like a mosaic or like the tiles on a roof.