Magma chamber evolution of the Ardestan pluton, Central Iran: evidence from mineral chemistry, zircon composition and crystal size distribution

Abstract

The Oligo–Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh–Dokhtar Magmatic Assemblage of Iran. The Ardestan pluton is composed mainly of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2–2.8 apfu) over very restricted Fe# ratios (0.42–0.56) which are characteristic of continental arc magmatic suites. High Ti2O contents of biotite (<6.1 wt.%) suggest a magmatic origin. Ti-in-biotite geothermometery gives a mean crystallisation temperature of 730 ± 56°C, slightly higher than calculated TZr.Ti°C (716 ± 50°C) and similar to the average TZr.sat°C (735 ± 26°C). These results are consistent with the low bulk-rock SiO2 contents, which provide minimum estimates of temperature and indicate zircon crystallised from a fractionated magma. Zircons from the Ardestan pluton have high (Sm/La)N (>10) ratios suggesting a magmatic origin. T–$f_{{\rm O}_{\rm 2}}$ calculations of oxygen fugacity between –13.6 to –16.9 indicate oxidising crystallisation conditions between the Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers. Tight linear trends of log (XF/XOH), log (XCl/XOH) and log (XCl/XOH) vs. XMg represent a narrow range of $f_{{\rm H}_2O}$, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of basic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.

Zircon U–Pb–Hf and whole‐rock Sr–Nd isotopic insights on the Silurian syenite–monzonite shoshonitic magmatism of Central Iran

“…Transferring melts from a steady‐state (e.g., closed system in deeper levels) into an unsteady‐state (e.g., an open system in shallower levels) magmatic chambers is usually accompanied by assimilation and/or mixing processes (AFC) that give rise to the modification of primary magma compositions (Babazadeh, Furman, et al, 2019; Babazadeh, Ghalamghash, Furman, et al, 2021; Kuritani et al, 2005). Therefore, it is vital to appraise the effect of open system processes before using the elements to decipher the potential mantle sources.…”

“…Incompatible trace element ratios shed light on the origin of metasomatic components of subduction‐related magmas (Babazadeh, Furman, et al, 2019; Babazadeh, Ghalamghash, Furman, et al, 2021; Hawkesworth et al, 1997). Accordingly, subducted sediment components give rise to the enrichment of Th concentration, (Th–Ce–La)/Yb ratios and deviation of data towards higher 87 Sr/ 86 Sr ratios (Hauff et al, 2003; Staudigel et al, 1996).…”

Petrogenesis of Miocene igneous rocks in the Tafresh area (central Urumieh‐Dokhtar magmatic arc, Iran): Insights into mantle sources and geodynamic processes

“…An important episode of plate margin extension and magmatic flare‐up affected Iran from ~55 Ma (latest Palaeocene‐Early Eocene) until ~37 Ma (late Eocene) (Agard et al, 2011; Chiu et al, 2013; Moghadam et al, 2016; Verdel et al, 2011). Late Eocene (~35 Ma) to Early Miocene (~22 Ma) magmatism recorded in the central UDMA is inferred as a result of melt production before the collision between the Arabian and Eurasian tectonic plates (Babazadeh, Furman, Cottle, Raeisi, & Lima, 2019; Babazadeh, Ghorbani, Cottle, & Bröcker, 2019). Reported ~10 Ma ages from Ardestan plutons represent the youngest magmatic phase so far recognized in the central part of the UDMA (Babazadeh, Ghorbani, et al, 2019).…”

Hydrothermal alteration in Eshtehard volcanoes, Iran: Constraints from trace elements redistribution and stable isotope geochemistry

“…20-18 Ma magmatism is attributed to the final stages of a low-angle subduction system before the Arabian and Eurasian lithospheric plates collided. The <12 Ma episode, which is hitherto known as the youngest magmatic phase in the area (Babazadeh et al, 2019a(Babazadeh et al, , 2019b, is the product of post-collisional magmatic activity within Zagros orogen.…”