جستجوی مقالات مرتبط با کلیدواژه « leucogranite » در نشریات گروه « زمین شناسی »

Garnet and tourmaline have been recognized as minor minerals in Marziyan leucogranites of the North of the Azna region as a part of the Sanandaj-Sirjan structural zone. These felsic rocks outcrop in metamorphic rocks such as micaschist and andalusite schist. Garnets are mostly anhedral to euhedral, without reaction rim as well as inclusion and they have uniform composition and rich in spessartine. Based on the petrographic and chemical studies, garnets have a magmatic origin and they are derived from a felsic melt because their CaO amount is <4wt%. Tourmalines are anhedral to subhedral, coarse to medium grains and they are classified in shorlite-dravite type as well as alkali group in composition. Tourmalines are categorized into two groups of magmatic with a ratio higher than Al, Fe/Fe+Mg and hydrothermal with a low Fe#. Mineral chemistry of garnet and tourmaline minerals signifies the S-type Marziyan granite magma.

Gasht metamorphic complex crops out in the Talesh Mountains (Western Alborz). This complex mainly consists of metapelitic and granitic rocks. Tourmaline is prevalent as an accessory phase within these rocks. In schist samples, dravitic tourmaline is abundant as nucleation component and fine size, and displays compositional zoning. Dravitic tourmaline of gneissic samples are coarser and homogenous. Tourmaline of the migmatites are restricted to both leucosome and melanosome parts. Brown tourmalines of garnet-bearing melanosome are magnesiofoitite in composition and similar to the other mafic phases of melanosome, indicate a residuum nature. However, dravitic tourmalines of the garnet – free melanosome are coarse crystalline and have grown over biotite with green pleochroism. The composition of the leucogranite tourmalines are schorlitic and they become dravitic toward rims. Different types of replacement are observed in the tourmaline composition of Gasht complex. According to the compositional signatures, metapelites tourmaline were formed mainly under metamorphic conditions, while the one of the leucogranites have been formed in the magmatic conditions. Probably B-bearing fluids of Gasht complex in the high grade metamorphism caused lowering of the temperature of water-bearing solidus of granite. This resulted in facilitating of migmatitization process. Furthermore, boron element caused decreasing of leucosome viscosity, mobilization and easier fractionation of melt from melanosome and creation of adjacent leucogranites.

The studied area is located in the Binalud Zone and comprises Mashhad leucogranites (g2) that include pegmatites and tourmaline veins. According to studies of the Mashhad leucogranites¡ these granites are peraluminous¡ highly differentiated and resulted from melting metapelites. In these granites¡ tourmaline can be seen in various forms of dendritic¡ nodules¡ vein and pegmatitic forms. For geochemical studies of vein and pegmatitic tourmalines¡ it was used microprobe analysis (EPMA)¡ XRF and ICP- MS. Based on these analyses¡ pegmatitic tourmalines include schorl and vein tourmalines which are magnesium-rich- schorl and both are classified in the range of alkaline tourmalines. They have oscillatory chemical zoning and bluish green to brown pleochroism that is probably due to changes in concentration of Ti¡ Ca¡ and Fe. In the studied tourmalines¡ major substitutions took place including Na(Fe)= X-Vacancy and Na(Fe)OH=Na(O). Both types of tourmalines are resulted from magma differentiation with this difference that pegmatitic tourmalines show magmatic system characteristics but vein tourmalines have magmatic-hydrothermal system characteristics. These properties are related to the pegmatitic development stages and fluids obtained during their evolution.

Tourmaline nodules in the Mashhad granites (g2) formed of two parts: dark core and white halo which are scattered in the leucocratic granites. Petrographic studies of these parts (core, white halo and host granite) show tourmaline accumulation in core, quartz, muscovite, microcline and orthoclase accumulation in white halo and quartz, plagioclase, microcline, muscovite and biotite accumulation in host granite. Based on the microprobe and geochemical analysis, the combination of tourmaline nodules is schorl and close to dravite. Features such as oscillatory zoning, Ca, Mg increase in rim, medium Fe/Fe+Mg (56-63) and the REE distribution pattern compatible to metapelite melting that show these tourmalines have double origin. In first stage, tourmaline have schorl composition, then with upward movement of magma and mixing to enriched liquid and boron originated of reaction to host metapelites, open system formed and tourmaline composition changed to dravite.

The Mishu garnitoid complex, located NW of Mishu Mountains, southwest of Marand and northwest of Iran (East Azarbaidjan), and intruded into the metamorphosed Precambrian rocks (Kahar Formation). Based on petrographic studies, the pluton composed of acid rocks including granodiorite, monzogranite, syenogranite and two-mica granite, all cut by acid and diabasic dykes. The presence of surmicaeous xenoliths are evidences of refractory minerals and restite. Petrographic and geochemical studies indicate that the Mishu plutonic rocks can be classified as S-type granite having calc-alkaline and peraluminous nature. On chondrite and primitive mantle-normalized diagrams, almost all the rocks show enrichment in LILE and LREE, HREE, HFSE depletion as significant negative depletion in Nb, Ta, Eu, Ti, Sr and Ba, which is typical of calc-alkaline magmatism from subduction-zone environment. The geochemical data and position of the studied samples on discriminative tectonic diagrams show that the rocks are compatible with orogenic granitoids.