Chemistry of minerals and mass changes of elements during sodiccalcic alteration of the Panj-Kuh intrusive body (Damghan, Iran)

Exact petrography and mineral chemistry investigations of pristine and altered rocks of the post-Eocene Panj-Kuh pluton show thatsubsequent to emplacement and crystallization, rocks were subjected to two secondary composition equivalents, sodic-calcic andpotassic alterations, respectively. Fresh rocks have syenitic composition and contain plagioclase (labradorite), K-feldspar, magmaticdiopside, biotite and minor magnetite, sphene and apatite. The rocks, subjected to Na-Ca alteration, are characterized by a bleachedappearance and in point of modal classification referred to as monzonite. Albitization of the corner of plagioclase (An10Ab85Or5),pervasive scapolitization of the plagioclases (marialite, Ma97Me2) and albite with chessboard twinning are some common properties ofrocks subjected by this type alteration. Magmatic magnetite is enclosed in pyroxene and biotite and is also dissolved and partiallyremoved. This type of metasomatism is structurally controlled by fracturing, with filling minerals of albite, scapolite, analcime,clinopyroxene, sphene and apatite (scapolite-albite veins). The pyroxene has the same composition in syenites, monzonite and inscapolite-albite veins (on average, Wo48En38Fo12). This means that the secondary hydrothermal effects do not tend to alter thecomposition of primary clinopyroxene. The average Or content of potassium feldspar crystals in altered and unaltered rocks range fromOr90 to Or84. According to the isocon method and the obtained plots, Ti and REEs were relatively immobile during Na-Ca alterationand the mass change calculations varied from 1 to 20%, depending on the intensity of alteration. By contrast, the washed-out contentsof K and Fe invaded the other rocks and created potassic zone alteration. These rocks were characterized by the conversion ofmagmatic pyroxene and biotite to shreddy biotite (with dark green pleochroism and secondary fine sphene), as well as plagioclasesurrounded by rinds of potassium feldspar.