Here I try to chat about and list some definitions (as I understand them at least) associated with some loose geological terminology revealed by various groupings of minerals and rocks used by geologists.
Firstly, there are processes whereby groups of minerals are formed, and these are given rock names. Secondly, there names given to groups of rocks, which require an interpretation about the way they formed. Thirdly, there are groups of groups of rocks, which stretch the bounds of what we can say reliably.
(1) Groups of minerals
As an intro 'exsolution' is where phases (usually 2-3) crystallize at the expense of a host crystal and form as lamellae. The exsolved products as a whole is the same composition as host. The geometry of exsolved phases is typically crystallographically controlled. e.g. Perthite: 'Rock' name for exsolved feldpars.
Symplectite: phases (usually 3 or more) crystallize as a corona around host crystal. Composition may vary slightly, involving originally adjacent phases.
Kelyphite: Type of symplectite formed around garnet, typically from kimberlite where it reacts with volcanic liquids to form fine-grained intergrown serpentine+phlogopite+oxides
Myrmekite: Type of symplectite formed within K-feldspar, cosnisting of intergrown quartz and plagioclase.
There are also other kind-of-symplectites which are alteration products of particular minerals involving partial oxidation and/or hydration:
Pinite: Hydrous alteration of cordierite yielding intergrown muscovite+illite+smectite+other clay minerals±oxides
Bastite: is a textural term for platy replacement along cleavage planes in orthopyroxene and rarely clinopyroxene.
Leucoxene: Oxidation of titanium-bearing minerals yielding intergrown rutile+clay minerals+Fe-oxides
Iddingsite: Oxidation ± hydration of olivine yielding intergrown smectite+hematite±Fe-hydroxides
Saussurite: Oxidation + hydration of plagioclase yielding intergrown epidote+sericite±scapolite
Cancrinite: Single mineral formed as as result of hydration and cabronation of nepheline....countless other examples of hydration which forms single replacement minerals ...
(2) Groups of Rocks
On the other end of the spectrum there are many (often debatably used) rock names referring to groups of rocks. Typically the useage is process-oriented and hence interpretation rather than observation is implied:
Ophiolite: sequence of mafic-ultramafic volcanic and intrusive rocks produced as a result of sea-floor spreading, forming typical oceanic crust. This is then thrust onto continental crust and hence preserved resulting in 'ophiolite'.
Migmatite: literally 'mixed-rock' formed of host, melt and restite components. Strictly, melt is derived from in-situ melting of the host but term is also used generally for melts which are injected into hosts.
Amphibolite: a metamorphic facies describing all rocks that have been taken to medium pressure and temperature. Also, specifically referring to a metabasalt composed primarily of amphibole and plagioclase. Granulite, eclogite, blueschist also refer to facies.
Meteorite: wide compositional suite of extra-terrestrial rocks which have landed on earth.
Boninite/Komatiite/Kimberlite: (others): Suites of volcanic rocks defined by their geochemical composition and may easily be confused with similar rocks formed from very different tectonic processes.
Cataclasite/breccia/mylonite: fault/shear zone rocks of particular grain size and texture distributions.
Turbidite: sequence of greywackes and mudstones interlayered (may be formed of repeated 'Bouma sequences') as a result of turbidity currents in the middle depths of oceans.
Hyaloclastite: Volcaniclastic rock consisting of transported broken glass.
Biomicrite: Limestone made of shells set in carbonate mud.
Olistostrome/Olistolith/Melange: useage implying landslide/tectonic activity yielding a mixing of rocks.
(3) Groups of groups of rocks
Groups of groups of rocks are even more strange and interpretive leaving geoloists questioning such things as plate tectonics, plumes and long geological timescales. Some of these are:
Orogens
Continental/oceanic arcs
Continental/oceanic plateaux
Rifts
Coninental margins
Impactites/impact structures
Terranes (continent/microcontinent fragments)
Greenstone belt
Basin
No comments:
Post a Comment