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

Abstract

Exact petrography and mineral chemistry investigations of pristine and altered rocks of the post-Eocene Panj-Kuh pluton show that<br />subsequent to emplacement and crystallization, rocks were subjected to two secondary composition equivalents, sodic-calcic and<br />potassic alterations, respectively. Fresh rocks have syenitic composition and contain plagioclase (labradorite), K-feldspar, magmatic<br />diopside, biotite and minor magnetite, sphene and apatite. The rocks, subjected to Na-Ca alteration, are characterized by a bleached<br />appearance and in point of modal classification referred to as monzonite. Albitization of the corner of plagioclase (An10Ab85Or5),<br />pervasive scapolitization of the plagioclases (marialite, Ma97Me2) and albite with chessboard twinning are some common properties of<br />rocks subjected by this type alteration. Magmatic magnetite is enclosed in pyroxene and biotite and is also dissolved and partially<br />removed. This type of metasomatism is structurally controlled by fracturing, with filling minerals of albite, scapolite, analcime,<br />clinopyroxene, sphene and apatite (scapolite-albite veins). The pyroxene has the same composition in syenites, monzonite and in<br />scapolite-albite veins (on average, Wo48En38Fo12). This means that the secondary hydrothermal effects do not tend to alter the<br />composition of primary clinopyroxene. The average Or content of potassium feldspar crystals in altered and unaltered rocks range from<br />Or90 to Or84. According to the isocon method and the obtained plots, Ti and REEs were relatively immobile during Na-Ca alteration<br />and the mass change calculations varied from 1 to 20%, depending on the intensity of alteration. By contrast, the washed-out contents<br />of K and Fe invaded the other rocks and created potassic zone alteration. These rocks were characterized by the conversion of<br />magmatic pyroxene and biotite to shreddy biotite (with dark green pleochroism and secondary fine sphene), as well as plagioclase<br />surrounded by rinds of potassium feldspar.<br /><br />