Petrography and geothermobarometry of quartz diorite from Pohorje Mountains, Slovenia

Abstract

The mineral composition and pressure–temperature conditions of Pohorje quartz diorite were investigated to reconstruct crystallization sequence and integrate the results with previous data on the Pohorje igneous complex, providing insights into its petrogenesis. Pohorje quartz diorite has phaneritic texture and is medium- to coarse-grained. The major minerals include light green clinopyroxene, dark green amphiboles, and white feldspars; the first two give the rock its characteristic colour. The proportion of dark- to light-colored minerals is approximately 4:1. Clinopyroxene predominates and correspond to diopside (XCa = 0.47–0.51, XMg = 0.41–0.49, XFe = 0.05–0.09). Amphiboles are Ca-amphiboles and are divided into two types: Type I amphiboles occur as single grains with distinctive core and rim zones, whereas Type II amphiboles replace clinopyroxene grains. Type I amphibole cores are classified as magnesiohornblende, tschermakite, edenite, pargasite, or magnesiohastingsite; Type I amphibole rims are classified as magnesiohornblende and actinolite; and Type II amphiboles are classified as magnesiohornblende. The dominant feldspars are oligoclase to andesine (XAb = 0.61–0.73), often replaced by potassium feldspar orthoclase. Minor minerals include quartz, biotite group minerals, apatite group minerals, titanite, epidote group minerals (allanite), and magnetite, while secondary minerals comprise chlorite group minerals and calcite. Various thermometers and barometers were applied to reconstruct the crystallization history of the quartz diorite and link it to the evolution of the host granodiorite intrusion. Thermobarometric data indicate that clinopyroxene in the quartz diorite, which is considered the earliest cumulate product from basaltic melts, crystallized under the highest P–T conditions (840–905 °C; 6.70–7.70 kbar), consistent with petrographic evidence. Subsequent crystallization of Type I amphibole cores occurred at 675–730 °C and 6.45–6.50 kbar, conditions comparable to those of the less evolved granodiorite, suggesting coeval formation. Later stages involved the formation of Type I amphibole rims at 585–640 °C and ~2.00 kbar, Type II amphiboles at 615–680 °C and 2.59–2.79 kbar, and biotites at 670–690 °C, associated with the emplacement of more evolved granodiorite at shallower crustal levels.