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Abstract

The paper brings new data on the petrography and geochemistry of the Muntele Mare Granitoid (MMG), from its northernmost outcropping area (Mănăstireni-Bedeci area, Gilău Massif, Apuseni Mountains), where MM\G is are actively quarried for quartz and feldspar that are used in the ceramics industry. The MMG mined in the Bedeci quarry has a pegmatitic hypidiomorphous – inequigranular fabric, and a low melanocratic index (ca. 7%). It consists of quartz, plagioclase (two generations: acidic andesine-basic oligoclase (34-35% An), and albite (9-11% An)), orthoclase ± perthite in various substitution stages by microcline (intermediary), as well as microcline maximum, muscovite and biotite. Accessories include: apatite, zircon, magnetite, rutile and titanite. Garnets occur in contact areas, and local concentrations of apatite and tourmaline are found in areas affected by metasomatic processes. Micas underwent micronization, iron leaching and hydration. Three textural varieties are identified: equigranular, microgranular, and gneissic granite. Subsequent alkaline metasomatism, cataclastic deformation, and silicification transformed these varieties into a pegmatitic granite. The contact of the MMG with the overlying Someş lithogroup is marked by a 0.5–1.0m thick biotite hornfels. Away from the contact, the hornfels gradually turns into granitic gneiss affected by metasomatic processes. The major element distribution in the Bedeci Valley granitoid is characteristic to a peraluminous, subalkaline, medium-potassic granite, affected by dominantly sodic alkaline metasomatism, and Al enrichment. Geochemical processing of the analytical data (major elements; 296 samples) suggests that the granite has been formed in a collision environment, which contrasts with the post-collision environment inferred by other authors. We state that this contradiction is to be explained by chemistry changes caused by metasomatism and silicification processes.

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