Mineral chemistry of Plio-Quaternary subvolcanic rocks, southwest Yazd Province, Iran

Abstract

The NW–SE-trendingLate Cretaceous–Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA) in southwest Iran hosts numerous Plio-Quaternary subvolcanic porphyritic andesitic to rhyodacitic domes intruded into a variety of rock sequences. Bulk-rock geochemical data show that the calc-alkaline dacitic to rhyodacitic subvolcanic rocks share compositional affinities with high-silica adakites, including high ratios of Na2O/K2O >1, Sr/Y (most >70), and La/Yb (>35), high Al2O3 (>15 wt.%), low Yb (<1.8 ppm) and Y (<18 ppm) contents, no significant Eu anomalies, and flat to gently upward-sloping chondrite-normalized heavy rare-earth element (HREE) patterns. All analysed rocks are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). They also display typical features of subduction-related calc-alkaline magmas. In chondrite-normalized rare-earth element patterns, the light rare-earth elements (LREEs) are enriched ((La/Sm) N = 3.49–7.89) in comparison to those of the HREE ((Gd/Yb) N = 1.52–2.38). Except for the G-Aliabad Dome, plagioclase crystals in the Shamsabad, Ostaj, Abdollah, and Bouragh Domes are mostly oligoclase to andesine (An19–49). Amphibole and biotite are abundant ferromagnesian minerals in the subvolcanic rocks. Calcic amphiboles are dominantly magnesiohornblende, magnesiohastingsite, and tschermakite with Mg/(Mg + Fetot) ratios ranging from 0.58 to 0.78. In all the studied domes, amphiboles are typically ferric iron-rich, but that those the Shamsabad Dome have the highest Fe3+/(Fe3+ + Fe2+) ratios, between 0.69 and 0.98. Amphiboles from the Ostaj and Shamsabad Domes are relatively rich in F (0.39–1.01 wt.%) in comparison to the other studied domes. This phase commonly shows pargasitic and hastingsitic substitutions with a combination of tschermakitic and edenitic types. Temperature-corrected Al-in-hornblende data show that amphibole phenocrysts from the Ostaj, Abdollah, and G-Aliabad Domes crystallized at pressures ranging from 2.14 to 3.42 kbar, 3.49 to 3.96 kbar, and 2.02 to 3.47 kbar, respectively. Temperatures of crystallization calculated with the amphibole–plagioclase thermometer for the Ostaj, Abdollah, and G-Aliabad subvolcanic domes range from 735°C to 826°C (mean = 786 ± 29), 778°C to 808°C (mean = 791 ± 13), and 866°C to 908°C (mean = 885 ± 12), respectively. In the annite–siderophyllite–phlogopite–eastonite quadrilateral, biotite from the G-Aliabad, Bouragh, and Ostaj Domes are characterized by relatively low total Al contents with variable Fetot/(Fetot + Mg) values from 0.26 to 0.43. All biotite analyses define a nearly straight line in the X Mg versus Fetot plot, with r = –0.96 correlation coefficient. In comparison to other domes, the F content of biotite from the G-Aliabad Dome shows high concentrations in the range of 1.80–2.57 wt.% (mean = 2.20). Inferred pre-eruptive conditions based on the calcic amphibole thermobarometric calculations for the Shamsabad, Abdollah, and Ostaj Domes show that the calc-alkaline subvolcanic magma chamber, on average, was characterized by a water content of 6.10 wt.%, a relatively high oxygen fugacity of 10–10.66 (ΔNNO + 1.28), a temperature of 896°C, and a pressure of 2.75 kbar.