TITLE:
The Marangudzi Alkaline Ring Complex, Southeastern Zimbabwe: Part I. Mineral Compositions and Formation Conditions of Genetically-Related Gabbro, Nepheline Syenite and Quartz Syenite
AUTHORS:
C. Michael B. Henderson
KEYWORDS:
Marangudzi Alkaline Complex, Microprobe Analyses, K-Rich Gabbro, Nepheline Monzonites/Syenites, Quartz Syenites, Ba-Ti-Rich Oxybiotite, Ti-Rich Oxohastingsites, Ba-Rich Alkali Feldspars, Geothermometry
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.10,
October
14,
2026
ABSTRACT: The Marangudzi gabbro is intruded by quartz syenite, nepheline monzonite and nepheline syenite “ring” units. Ortho-cumulate gabbros are richer in intercumulus primary biotite (Ti, Ba-rich) and alkali feldspar than mesocumulates. Clino- and ortho-pyroxenes are present, indicating transitional tholeiitic/alkaline basalt affinities. Least-evolved orthogabbros contain olivine (Mg/(Mg + Fe + Mn) mg # 62), clinopyroxene (mg# 78), orthopyroxene (mg# 67), plagioclase (An 92), with intercumulus Ti-Ba-rich biotite (mg# 69, TiO2 5.6, BaO 0.8 wt.%) and alkali feldspar (Or 88, BaO 1.1%). The rock series nepheline monzonite—nepheline-bearing syenite—foyaite probably formed by fractionation of parental K-Ti-Ba-rich alkaline basalt, as reflected in decreasing mg#, An and Ba with evolution. Late-intruded leuco-malignites (± pseudoleucites) have minerals less evolved than those in nepheline-bearing-syenites/foyaites. The main quartz syenite mafic mineral is hastingsite/edenite with ~2% TiO2 and higher Si/Al than amphiboles in undersaturated rocks. Mineral compositions suggest that a parental magma intruded at ~1150˚C, with olivine, plagioclase progressively equilibrating over the temperature range from 1000˚C to 950˚C with biotite at ~900˚C ± 50˚C in the gabbros and early nepheline monzonites. Feldspar/amphibole data give ~880˚C for amphibole equilibration in nepheline monzonite, while biotite shows similar textural relations but calculated formation temperatures of 750˚C suggest subsolidus re-equilibration. For nepheline-bearing syenites and foyaites, feldspar and amphibole over the range from ~900˚C to 850˚C to subsolidus (720˚C nepheline-bearing-syenites) and 620˚C (microfoyaites). Leuco-malignite amphiboles suggest magmatic formation formed from ~900˚C to 850˚C. Feldspar and amphibole in the quartz syenites suggest late magmatic/solidus (800˚C - 750˚C), with biotite re-equilibrating subsolidus at ~675˚C, and with magnetite-ilmenite subsolidus at ~600˚C (log fO2-18 bar) in all rocks.