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 that subsequent to emplacement and crystallization, rocks were subjected to two secondary composition equivalents, sodic-calcic and potassic alterations, respectively. Fresh rocks have syenitic composition and contain plagioclase (labradorite), K-feldspar, magmatic diopside, biotite and minor magnetite, sphene and apatite. The rocks, subjected to Na Ca alteration, are characterized by a bleached appearance 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 of rocks subjected by this type alteration. Magmatic magnetite is enclosed in pyroxene and biotite and is also dissolved and partially removed. 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 in scapolite-albite veins (on average, Wo48En38Fo12). This means that the secondary hydrothermal effects do not tend to alter the composition of primary clinopyroxene. The average Or content of potassium feldspar crystals in altered and unaltered rocks range from Or90 to Or84. According to the isocon method and the obtained plots, Ti and REEs were relatively immobile during Na-Ca alteration and the mass change calculations varied from 1 to 20%, depending on the intensity of alteration. By contrast, the washed-out contents of K and Fe invaded the other rocks and created potassic zone alteration. These rocks were characterized by the conversion of magmatic pyroxene and biotite to shreddy biotite (with dark green pleochroism and secondary fine sphene), as well as plagioclase surrounded by rinds of potassium feldspar.