Contrasting response of accessory ilmenite and magnetite to magma mixing and postmagmatic alteration in Petrohan pluton, Western Balkan, Bulgaria

Series of representative samples of granodiorite, diorite, hybrid diorite, hybrid gabbro and mafic magmatic enclaves (MME) from Petrohan pluton, Western Balkan, Bulgaria, were chosen to clarify the effects of magma mingling and mixing and postmagmatic alteration onto the chemical composition, microstructure and stability of accessory magnetite and ilmenite. Although the two oxide phases were found to crystallize simultaneously – after and, probably, during the mixing of magmas as late magmatic minerals, they demonstrated much contrasted behaviour during magmatic and postmagmatic processes. In all the studied rocks, magnetite occurs in the form of well-preserved homogeneous crystals with sporadic, very fine exsolutions of ulvöspinel and coarser oxy-exsolutions of ilmenite. The microstructure of accessory ilmenite in the rocks is more complicated, being a result of several post-magma-mixing processes: (i) prevailing part of the ilmenite grains has relic outlook due to intensive replacement by late-magmatic and postmagmatic titanite; (ii) each ilmenite grain contains numerous micro- and nanoscale hematite exsolutions distributed parallel to (0001) crystallographic plane of ilmenite; (iii) significant part of the ilmenite grains is locally decomposed into hematite and rutile as a result of postmagmatic oxidation alteration. Magnetite from different rocks does not show any essential differences in its chemical composition: the most important minor elements in the mineral as Al, Ti and V vary in similar ranges (Al2O3 -0.3–0.9 wt.%, TiO2 – 0.5–1.4 wt. %, V2O3 – 0.1–0.45 wt. %). The composition of ilmenite grains (point analyses of ilmenite and area analyses of ilmenite with hematite exsolutions) is more variable and reflects effects of magma mixing – similar range of Fe3+ content (5–18 mol.% of Fe2O3) and anomalously high content of Mn (∼40 mol.% of MnTiO3) in part of ilmenite grains of hybrid diorite as well as the specific influence of the chemical composition of each of the rocks onto the content of Mn in the mineral. It is suggested that the magnetite composition records more general characteristics of the mixing and mingling magmas in the Petrohan pluton, while the ilmenite composition rather emphasizes their differences