Journal of the Virtual Explorer

The plagioclase phenocrysts up to 25% modal are the most abundant mineral in the dacite and rhyodacitic rocks. Microlithic plagioclases are found in glassy or felsitic matrix. The plagioclase phenocrysts are typically zoned and show euhedral to subhedral shapes. Compositional determination shows that the plagioclase crystals in these rocks are oligoclase to andesine in core (An₂₀ to An_44.7) and andesine in rim (An_23.1 to An_40.4).

Two types of plagioclases are observed in studied rocks. The first type is clear having normal zoning.

The second type indicates resorption textures such as sieved texture (with a clear core and a sieved rim, or vice versa), dusty core or rim and reverse zoning. In these minerals, the observed maximum compositional range from core to rim is 13.2 mole %An. Moreover, the zoning is abrupt (Table 1). Sieved texture in plagioclase phenocrysts is described as the interconnected inclusions of glass or other matrix materials (Shelly, 1993). It also formed by decompression when magma rises towards the surface (Nelson and Montana, 1992). According to Stamatelopoulou-Seymour, et al. (1990), if sieved plagioclases were formed due to mixing, a calcic sieve-texture core should be followed by a resorption surface associated with a sudden change in An content of at least 10%. The heterogeneity of plagioclase rims within a single sample indicate a complex origin and may be due to magma mixing event (Wallace and Carmichael, 1994).

In dusty plagioclases, groundmass glass is sometimes detected in the dusty rim of plagioclase. The maximum An% in the dusty rim is about 46% whereas it is approximately 26% in the clear part. Several lines of evidence such as the corrosion of plagioclases, the variations in compositional zoning, the sieved texture and the dusty rims suggest that precipitation of plagioclase crystals are inconsistent with physico-chemical conditions of magma (Tsuchiyama, 1985; Stamatelpoulou-Seymour, et al. 1990)

Based on the classifications of Leake (1978), amphiboles from dacite and rhyodacites fall in the calcic amphibole field (except for opacitized rims which classified as Mg-Fe-Mn-Li amphiboles). The amphiboles from different regions display different composition. Those from Surk, Shamsabad, Ostaj and Abdollah regions vary from tschermakite to magnesio-hornblende, whereas the ones from Ernan and S-Ernan are mainly magnesio-hastingsite (Table 2).

The amphiboles in particular, those from Surk, Ernan and S-Ernan show normal or reverse zoning. The Mg-number in reverse zoning amphibols is increased from core to rim. These minerals occasionally have reaction rims partially to completely oxidized forming “opacite” pseudocmorphs caused by replacing of amphiboles with Fe-Ti oxides.

If the water content of the magma is too low for amphibole stability, reaction rims are developed (Rutherford and Hill, 1993; Feeley and Sharp, 1996). Thus, amphiboles in the studied rocks display disequilibrium characteristics such as reverse zoning, reaction rims and pseudomorphs after hornblende.

Several features particularly the existence of oscillatory and reverse zoning (Mg-number of the biotite increase from core to rim, Table 3), the reaction rims and Fe-Ti oxide pseudomorphs after biotite, document that the biotite in the studied volcanic rocks is in a disequilibrium phase.