Mineralogy of a rare Ediacaran epidote-bearing trondhjemitic complex from the Nubian Shield: Insights into P-T conditions of magma emplacement

Abstract

Igneous epidote occurring in tonalite–trondhjemite–granodiorite (TTG) rocks provides valuable insight into the physiochemical parameters of their magmatic system. Epidote has been identified within a newly discovered, rare, Ediacaran, high-Al TTG-suite at the Fannani Igneous Complex (FIC), Nubian Shield. The objective is to assess P–T conditions of magma emplacement using petrographic-, mineral chemical data, and thermobarometry methods. The TTG-suite is made up of ubiquitous oligoclase, variable contents of quartz, K-feldspar, amphibole, biotite, epidote, accessory titanite, zircon, apatite and magnetite. Feldspars vary from An14 to An26 in plagioclase, with K-feldspar having near-end-member compositions (Or91–Or95). Amphiboles are Al-rich (AlT = 1.65–2.04 atoms per formula unit [apfu]), calcic (Ca = 1.83–1.93, apfu), with average Mg/(Fe + Mg) of 0.44, and are ferroedenite–edenite–ferropargasite. Biotites are moderately Fe-rich (Fe/(Fe + Mg) = 0.50–0.58) characteristic of type ‘C’–biotite of calc–alkaline orogenic rocks. Epidote exhibits pistacite components [Ps = Fe3+/(Fe3++Al)] of Ps24–Ps33, typical of magmatic epidote. The average crystallization pressure of 5.41 kbar (calculated using the Al-in-Hb barometer) is consistent with the presence of magmatic epidote and reflects the mesozonal crustal levels of emplacement. The Hb–Plag–geothermometer produced an average crystallization temperature of 721°C. These values fall within experimentally-determined P–T ranges of stability of magmatic epidote with fO2 buffered from nickel-nickel-oxide (NNO) to hematite (HM). The FIC rocks were developed from a wet, subsolvus, orogenic, Al-rich magmatic system. Findings reported here could have implications for conditions of magma emplacement and mineralogical characterization of TTGs occurring within various continental masses. © 2022 John Wiley & Sons Ltd.