Download
1 / 21
210 likes | 361 Views
Endogeeniset prosessit. Magmakivet 1. Volcanic rock units in Lapland: Salla Group (blue), Onkamo Group (dark green), Kittilä Group (light green). Drill hole penetrating a 250-m-thick volcanic sequence between the basement complex and quartzite metasediments, Möykkelmä, Sodankylä area.
E N D
Endogeeniset prosessit Magmakivet 1
Volcanic rock units in Lapland: Salla Group (blue), Onkamo Group (dark green), Kittilä Group (light green)
Drill hole penetrating a 250-m-thick volcanic sequence between the basement complex and quartzite metasediments, Möykkelmä, Sodankylä area
Glassy textures result from rapid cooling. There are few if any mineral grains; the rock is chiefly or entirely comprised of natural volcanic glass, or obsidian. Most obsidian is felsic (high in silica), but it also forms from other magma types as well. (For example, in Hawai’i, basaltic lavas flow into the ocean and chill into basaltic glass.) Frequently, there are tiny phenocrysts, called microlites, in glasses. Over time, these glasses are unstable and will recrystallize into aphanitic volcanic rocks. Volcanic glass that is frothy as well, from trapped gas bubbles, is called pumice. Aphanitic textures characterize volcanic rocks that have cooled rapidly but nonetheless have allowed for mineral crystals to form. However, the individual crystals are too small to be seen or identified with the naked eye. These can be differentiated from glassy textures by the fact that they are far less “glassy” or shiny in appearance; the surface is dull. Fine-Grained textures are very common in volcanic rocks, particularly basalts. In these, all the rock is crystallized, but the individual grains are very small (typically in the range of 0.25-1.0 mm across, the size of fine to medium sand, and commonly even smaller). Porphyritic textures may be the most common volcanic texture, especially for volcanic rocks on the continents. In these, large grains, called phenocrysts are embedded in a finer groundmass. Most commonly, the phenocrysts are grains that began crystallizing while the magma was still underground, then became entrapped in the fine-grained groundmass that formed following eruption and rapid cooling. Phenocrysts are commonly minerals higher in Bowen's Reaction Series than groundmass minerals, and may include minerals such as hornblende or biotite, which will not occur in the groundmass. Plagioclase feldspar phenocrysts will commonly be more calcium-rich than the plagioclase feldspars in the groundmass. Pyroclastic textures result from the fusion of solid volcanic ejecta that have been welded into a solid, coherent mass. These often include shards of glass and many irregular rock fragments, ash, etc.
Q Classification of Igneous Rocks Quartzolite 90 90 Quartz-rich Granitoid 60 60 Grano- Tonalite Granite Alkali Feldspar Granite diorite Alkali Fs. Qtz. Diorite/ 20 20 Quartz Syenite Qtz. Gabbro Quartz Quartz Quartz Monzonite Syenite Monzodiorite Alkali Fs. 5 Diorite/Gabbro/ 5 Syenite A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. Syenite Monzodiorite Monzonite Anorthosite 90 35 10 65 A P (Foid)-bearing (Foid)-bearing (Foid)-bearing Syenite Monzonite Monzodiorite (Foid)-bearing 10 10 Diorite/Gabbro (Foid)-bearing Alkali Fs. Syenite (Foid) Syenite (Foid) (Foid) Monzosyenite Monzodiorite (Foid) Gabbro Q = quartz (kvartsi) A = alkali feldspar (alkalimaasälpä) P = calcic feldspar (kalkkirikas maasälpä) F = feldspathoids (maasälvän sijainen) 60 60 (Foid)olites F
Classification of Igneous Rocks A classification and nomenclature of volcanic rocks. After IUGS. Q = quartz (kvartsi) A = alkali feldspar (alkalimaasälpä) P = calcic feldspar (kalkkirikas maasälpä) F = feldspathoids (maasälvän sijainen)
Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Classification of Igneous Rocks A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. (c)
Classification of Igneous Rocks A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) Journal of Petrology, 27, 745-750.
Phase diagrams A. Systems with Complete Solid Solution Example: Plagioclase(Ab-An, NaAlSi3O8 - CaAl2Si2O8) Fig. 6-8. Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1913) Amer. J. Sci., 35, 577-599.
B. Eutectic Systems Example: Diopside – Anorthite (pyroxene – plagioclase) No solid solution Fig. 6-11. Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40, 161-185.
