Abstract
Petrographic analysis identified two distinct rock types. The first type is quartz syenite, a silica-saturated rock dominated by alkali feldspar, plagioclase, quartz, and garnet, with inequigranular, porphyritic, poikilitic, myrmekitic, and perthitic intergrowth textures. The second type is nepheline syenite, a silica-undersaturated rock containing significant amounts of feldspar and feldspathoid minerals (nepheline, sodalite and cancrinite), with mafic minerals such as amphibole, biotite, and aegirine-augite, which displays holocrystalline, inequigranular, poikilitic, and perthitic intergrowth textures. Alterations include cancrinite pseudomorphs, albitization, and sericitization.
Electron microprobe analysis (EPMA) reveals the key mineral composition: K-feldspar is potassium-rich, predominantly orthoclase. The plagioclase composition in both rock samples is sodium-rich, predominantly consisting of albite and oligoclase. Garnets are Fe-rich almandine in quartz syenite, and the clinopyroxene is mainly of Fe-augite, and amphiboles are Fe-rich ferro-tschermakite/pargasite composition and very high temperature minerals in nepheline syenite. The presence of feldspathoid minerals (nepheline, low-crystallization temperatures) and the presence of cancrinite and sodalite in nepheline syenites indicate post-magmatic alteration and fluid-rock interaction.
Whole-rock geochemistry classifies both rock suites as alkaline and exhibits a calc-alkaline affinity, characterized by higher SiO2 in quartz syenite (64.2-68 wt.%) than nepheline syenite (57-59.3 wt.%) and elevated K2O + Na2O. REE patterns reveal enrichment in LREE relative to HREE, with higher ∑REE in quartz syenite (106.6–383 ppm) than in nepheline syenite (7.7–145.3 ppm). Negative Eu anomalies indicate plagioclase fractionation and limit crustal contamination, while positive Eu anomalies in some nepheline syenite samples reflect plagioclase accumulation. High Rb/Sr ratios suggest that the crust was assimilated from pelitic and psammitic sources.
Tectonic discrimination diagrams indicate that the Bulfat syenites formed in a continental arc to post-collisional setting, associated with the subduction of the Neo-Tethys Ocean and the subsequent collision between the Arabian and Iranian plates. Derived from mantle-sourced alkaline magmas, these syenites underwent extensive fractional crystallization and were substantially modified by crustal contamination during the late stages of Neo-Tethys subduction.
