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.