Geology, geochemistry, and genesis of the Alvand sapphire-bearing syenitic pegmatite, Sanandaj-Sirjan Zone, Hamedan Province, northwest Iran
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Abstract Alvand blue sapphire is related to the igneous rocks of Alvand pluton, located in the Sanandaj–Sirjan zone, Hamadan area. The main geological unit is the Alvand batholith including gabbro, granite, and granitoid. Sapphire occurs within the desilicated pegmatite dykes intruding magmatic-metamorphic lithologies, closely to metasomatic zones. Sapphire-bearing rocks are situated in the central zone of aplitic-pegmatitic composite dykes. In addition to sapphire, the rocks contain albite, microcline, and orthoclase as major minerals, the absence of quartz is noticeable. The whole-rock geochemical characteristics show a relatively silica-poor and alumina-rich composition of syenitic sapphire-bearing rocks. According to peraluminous and S-type signatures, aplite and pegmatites indicate crustal magma origin. The slight increase of Al 2 O 3 /TiO 2 ratio in the sapphire-bearing pegmatites may suggest mobilization of Al, which either by melt mobilization through alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers. Also based on strong enrichment in Th, sapphire may be formed by late-stage fluids. The high Ti, Fe, and Ga parallel to the low Cr and V contents indicate a magmatic-metasomatic origin of the Alvand sapphire.Keywords:
Pegmatite
Batholith
Metasomatism
Quartz monzonite
Alkali feldspar
Diorite
Microcline
Emerald Lake Pluton is a newly remapped member of the mid-Cretaceous Tombstone plutonic suite in Yukon. It is crudely concentrically-zoned comprising: augite syenite, hornblende quartz syenite, hornblende quartz monzonite, and biotite granite, from west to east and in order of relative age. Density and magnetic susceptibility values of samples from each phase decrease, and Shand Indices increase, from west to east and correlate with mapped intrusive phases. The intrusion is also characterized by the presence of large miarolitic cavities distributed along the southern and eastern margins of the intrusion. They are spatially and temporally associated with a sheeted system of aplite dykes, quartz K-feldspar pegmatites, and molybdenite bismuthinite gold quartz veins. Miarolitic cavity and pegmatite formation are coeval with the main intrusive events, and were followed by continued pegmatite development, fracturing, and injection of aplite dykes and veins.
Pegmatite
Quartz monzonite
Hornblende
Alkali feldspar
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This is the first detailed description of amazonitic alkali feldspar in Japan. The amazonitic feldspar is from pegmatite in the Tanakami Granite, southwest Japan. Macroscopically, the feldspar is characterized by the coexistence of pale blue and white parts. The two parts of microperthite are distinctly different in terms of their microscopic texture. The pale blue part has clear crosshatched twin patterns, albite laths, and few albite patches with relatively few micropores. In contrast, the white part has unclear twin patterns, few albite laths, and many albite patches with abundant micropores. The pale blue color is due to the host microcline containing 103 ppm of PbO at the maximum, and the white color is due to both the two albite phases and turbid microcline causing strong diffuse reflection. It is deduced that plagioclase laths were first crystallized and then enclosed by pale blue microcline that crystallized later. Crosshatched twins were formed in the microcline during cooling, and albite replaced the microcline forming perthitic patches with many micropores. The composition of plagioclase laths is very close to that of pure albite, which is same as that of albite patches. The microcline part having undergone extensive albitization appear white to the naked eye; the part that did not undergo albitization has retained its pale blue color. The amazonitic microcline crystallization and albitization are estimated to have occurred at very low temperatures around 200 °C.
Microcline
Pegmatite
Alkali feldspar
Orthoclase
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Abstract Alvand blue sapphire is related to the igneous rocks of Alvand pluton, located in the Sanandaj–Sirjan zone, Hamadan area. The main geological unit is the Alvand batholith including gabbro, granite, and granitoid. Sapphire occurs within the desilicated pegmatite dykes intruding magmatic-metamorphic lithologies, closely to metasomatic zones. Sapphire-bearing rocks are situated in the central zone of aplitic-pegmatitic composite dykes. In addition to sapphire, the rocks contain albite, microcline, and orthoclase as major minerals, the absence of quartz is noticeable. The whole-rock geochemical characteristics show a relatively silica-poor and alumina-rich composition of syenitic sapphire-bearing rocks. According to peraluminous and S-type signatures, aplite and pegmatites indicate crustal magma origin. The slight increase of Al 2 O 3 /TiO 2 ratio in the sapphire-bearing pegmatites may suggest mobilization of Al, which either by melt mobilization through alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers. Also based on strong enrichment in Th, sapphire may be formed by late-stage fluids. The high Ti, Fe, and Ga parallel to the low Cr and V contents indicate a magmatic-metasomatic origin of the Alvand sapphire.
Pegmatite
Batholith
Metasomatism
Quartz monzonite
Alkali feldspar
Diorite
Microcline
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Abstract Petrochemical studies were conducted on granitoids in Sibolga and surrounding areas in North Sumatra, Indonesia. The granitoids presented the characteristics of A‐ and I‐type ilmenite series. The syenogranites from Sarudik, the syenogranite, quartz syenite, and quartz alkali feldspar syenite from Sibuluhan Sihaporas, and the monzogranites from Sibolga Julu are A‐type granitoids that have high SiO 2 , CaO, Al 2 O 3 , Na 2 O + K 2 O, and Rb content, a high FeO/MgO ratio, and low Ba, Sr, and Zr content. The alkali feldspar syenite from Sarudik, the quartz alkali feldspar syenite from Tukka, and the quartz syenite from Adian Koting show I‐type characteristics. I‐type granitoids have lower SiO 2 , Na 2 O + K 2 O, Rb, Yb, and total REE (ΣREE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I‐type characteristics, has high ΣREE content but a low FeO/MgO ratio. The granitoids consisted mainly of K‐feldspar megacrysts, quartz, plagioclase, and biotite with hornblende in some cases. Accessory minerals were zircon, apatite, allanite, and titanite. Cordierite and corundum occurred as xenocrysts and inclusions in K‐feldspar in syenogranite (Sarudik) and quartz syenite (Adian Koting). SiO 2 and ΣREE content and the Rb/Sr ratio are positively correlated to each other. The REE are enriched in quartz alkali feldspar syenites from Sibolga Julu and Sarudik, quartz syenite from Tarutung, and quartz alkali feldspar syenite, syenogranite, and quartz syenite from Sibuluhan Sihaporas. These highly‐differentiated granitoids were formed within the plate settings. In contrast, the ΣREE content of hornblende‐bearing granitoids that were formed in volcanic arc settings is low. The coexistence of cordierite and corundum with biotite and hornblende indicates that the metaluminous magma was contaminated by metasedimentary basement rocks.
Alkali feldspar
Quartz monzonite
Hornblende
Pegmatite
Allanite
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Alkali feldspar
Quartz monzonite
Orthoclase
Microcline
Recrystallization (geology)
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Abstract Metamorphosed Precambrian sedimentary rocks and associated late-kinematic granites of the Västervik area in southeastern Sweden contain abundant potassium feldspar. The alkali feldspar from the metasediments shows zero to low obliquity and contains from 8 to 10 % albite in solid solution. In the granitic rocks, and in the accompanying pegmatites and aplites, the potassium feldspar is microcline, containing 1 to 3 % albite The potassium feldspar of contact rocks and inclusions in granite is also microcline The formation of monoclinic potassium feldspar in the metasediments is the result of regional metamorphism in the upper part of the amphibolite facies. The widespread association with sillimanite suggests a temperature of formation above that of the triple point of andalusite—sillimanite—kyanite. In the granites, a monoclinic ancestry of the potassium feldspar is indicated, but transformation to the triclinic form has taken place, and unmixing has resulted in a nearly pure potassium feldspar. Solid-state transformations in the granitic potassium feldspars may have been facilitated by abundance of volatile constituents.
Microcline
Sillimanite
Alkali feldspar
Pegmatite
Orthoclase
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Felsic
Alkali feldspar
Microcline
Quartz monzonite
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Microcline
Pegmatite
Orthoclase
Metasomatism
Alkali feldspar
Muscovite
Leucite
Fluorite
Analcime
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The bulk compositions of the groundmass alkali feldspar from the Hell Canyon Pluton is 0.146mole% albite. The composition of the outermost zone of the oscillatory zoned plagioclase is 0.686 mole% albite, whereas the most calcic cores have a composition of 0.43 mole% albite. The structural state of the alkali feldspar is near orthoclase. Both composition of coexisting feldspars and structural state of the alkali feldspar are nearly constant throughout the pluton.
Orthoclase
Alkali feldspar
Microcline
Batholith
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