Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition.
Syenites are products of alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith to a fairly low degree of partial melting. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite, adamellite or tonalite.
At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime.
Conversely in certain conditions, large volumes of anorthite crystals may precipitate from thoroughly molten magma in a cumulate process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.
A syenite is an excellent example of a cumulate rock type.
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.
Additionally, based on composition, cumulate rocks can be divided into four classes:
FELSIC- The light-coloured minerals predominate and are composed mostly of feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotite, hornblende).
INTERMEDIATE- It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar (mainly plagioclase) and dark minerals like amphibole (mainly hornblende, pyroxene, and/or biotite). There is little or no quartz.
MAFIC- are dominated by plagioclase and pyroxene and smaller amounts of olivine.
ULTRAMAFIC - Are rocks dominated by dark-coloured minerals mainly by olivine and/or pyroxene.
At the cache coordinates, you have syenite. This syenite shows clear evidence of being a cumulate rock. To validate your FIND, you need to answer the following questions and send them to me.
Questions:
- What two colours predominate in this rock?
- Is this rock, based on the above compositional differences, a felsic, intermdiate, mafic or ultramafic type of rock?
- What is the average crystal size of the two types of crystals in this rock?
- What shape are the crystals?
Sienito é uma rocha ígnea plutónica, de textura holocristalina grosseira, composta principalmente por feldspatos (microclina, ortoclase e alguma ou nenhuma plagioclase), anfibolas (geralmente hornblenda), clinopiroxena (augite, egirine), pouca ou nenhuma biotite.
Uma rocha cumulada é formada pela acumulação de cristais. Normalamente é criada por assentamento gravitacional ou por diferenciação de fluxo tal como na figura infra.
Além disso, com base na composição, as rochas cumuladas podem ser divididas em quatro classes:
FÉLSICA- Os minerais de cor clara predominam e este tipo de rochas são compostos principalmente por feldspato potássico, pelo menos 10% de quartzo e menos de 15% de minerais máficos (biotite, hornblende).
INTERMÉDIA- Geralmente contém quantidades aproximadamente iguais de minerais claros e escuros, incluindo grãos de feldspato (principalmente plagiocláse) e minerais escuros como anfibola (principalmente hornblende, piroxena e/ou biotite). Contém pouco ou nenhum quartzo.
MÁFICA- São riochas dominadas por plagiocláse e piroxena e quantidades menores de olivina.
ULTRAMÁFICA - São rochas dominadas por minerais de cor escura, principalmente por olivina e/ou piroxena.
No Local das coordenadas vais ver um sienito (com a provável origem da Serra de Monchique no Algarve) que exemplifica muito bem este conceito das rochas cumuladas. Para validares o teu registo, tens de me enviar as respostas às seguintes peguntas:
- Quais a s duas cores predominantes nesta rocha?
- Com base na classificação por composição, como classificarias esta rocha?
- Qual o tamanho médio dos dois tipos de cristais que consegues observar?
- Qual a forma desses cristais?
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