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Close up of Syenite rock, Presa del Parralillo, Gran Canaria, Ca

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Photo ID: 474
Gallery ID: 58
Photo Title: Close up of Syenite rock, Presa del Parralillo, Gran Canaria, Ca
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Keywords:
syenite, alkaline syenite, nepheline syenite, presa del parralillo, gran canaria, canary islands, artenara, mesa de acusa, barranco de la aldea, san nicolas de tolentino, roque bentaiga, volcanic explosion, miocene, caldera, caldera de tejeda, fataga formation, volcano, volcanic, stratovolcano, ignimbrites, lava flows, cone sheet, dikes, igneous intrusion, intrusive complex, plutonic, plutonic rock, phaneritic, potassium feldspar, anorthoclase phenocrysts, alkali feldspar, plagioclase, clinopyroxene, aeririne, augite, amphibole, biotite, nepheline, rocks, chemically, trachyte, silica, contents, aluminium, alkalis,
Description:
Sample of plutonic alkaline syenite rock from an outcrop just below the Presa de Parralillo dam on the road from Artenara to San Nicolas de Tolentino. These syenites form part of the Tejeda formation, a large intrusive complex of some 500 trachytic cone sheet dikes, hypabyssal syenite stocks and trachyphonolitic subordinate radial dikes that formed a 12 km diameter intrusive complex within the Miocene Caldera de Tejeda between 12.3 and 7.3 million years ago, These syenites are part of a dome shaped intrusion (or intrusions ?) into the lavas and ignimbrites that had previously filled in this caldera, intruded into a 3-5 km central core area from the shallow Fataga system magma chamber, prior to the several hundred cone sheet dikes which were to be emplaced later and which were to create a structural uplift of the core zone by as much as 1,400 metres, this core zone being where the Roque Bentaiga stands today and where the fragmented blocks of syenites have been located at up to 1200 metres above sea level. The rock is a nepheline syenite, a course grained rock phaneritic of a very light grey colour which is primarily composed of large potassium feldspar and anorthoclase phenocrysts, in between which have crystallized the other minerals, these being alkali feldspar and Na rich plagioclase, together with irregular micro-aggregates of opaques and oxides which are the products of transformed primary ferromagnesium clinopyroxene and amphibole minerals. In some cases these crystals of dark greenish aeririne augite and brown-green amphibole appear unoxidised. Biotite and nepheline also appear to a lesser degree. These rocks are chemically a trachyte and have been dated as 11.8 to 11.9 million years old. The various samples of syenites which have been analysed from various parts of the formation, have silica contents of between 59% and 64%, aluminium content of 13% to 20% and alkalis of 11.5% to 13%. The syenites have a wide range of textures from some very fine grained to those which are phaneritic. These rocks are the plutonic equivalent of the Fataga formation trachyte and phonolite extrusives and have been located further away from the core zone, such as at the base of the Mesa de Acusa at 750 metres above sea level, where it is suggested that they may have ascended within fractures and slowly crystallized in a subvolcanic environment. The many outcrops and inter dike screens are not only the fragments of the dome shaped intrusion of the central core area but are in fact the fragmented portions of the upper part of the crystallized Fataga formation magma chamber, which were structurally uplifted by the cone sheet dikes which were emplaced well after the cessation of extrusive volcanic activity.
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