Phlogopite compositions as an indicator of both the geodynamic context of granitoids and the metallogeny aspect in Memve'ele Archean area, northwestern Congo craton

Abstract

A barium bearing phlogopite (celsian) has been found for the first time within the charnockitic and tonalitic suites that compose Archean mineral belt in Cameroon. Electron microprobe analyses of these phlogopites are reported and contain moderate contents of BaO (0.42–1.26 wt. %) and up to 5.95 wt. % TiO2. Micas are Mg-rich and their compositions indicate phlogopites rich-meroxenes. Phlogopites from Memve'ele are characterized by a nearly horizontal trend of increasing total aluminium (2.494–2.931 a.p.f.u.) and relatively constant Fe/(Fe + Mg) suggesting contributions of aluminous supracrustal material to the magmas by anatexis or assimilation. Compositions of the barium titanium bearing phlogopite vary systematically according to rock types. It seems that the substitution scheme include Ba + Al + VI (Mg, Fe)2+ + 2 IVSi = K + Si + VITi + 2IVAl was dominant in the Memve'ele area thus, this scheme has made easy incorporation of Ba into phlogopite structure. The binary diagram of aluminium vs. titanium shows that phlogopites from the Memve'ele area have been formed by the same metasomatic mechanism as phlogopites from Canary Island xenoliths and Mezitler andesites but Ba enrichment of phlogopites from the Memve'ele area implies an early Ba-metasomatism contrary to those from Mezitler. The estimated temperature of the studied phlogopites indicated mainly two groups: (1) temperature range from 662 to 688 °C (average 676 °C) for phlogopite grains with High Mg# in the trondhjemite sample and (2) temperatures with interval limits from 757 to 800 °C (average 777.07 °C) for remnant phlogopites; reflecting primary and late crystallization respectively from slightly to highly oxidized magma (−17.30 to −13.87 Kbars). The geothermal gradient with average temperatures are 35.57–53.360 °C/Km and 30.95–46.42 °C/Km corresponding to 14.56–21.84 Km and 14.56–30.58 Km depth of below crust respectively. The crystallizing melt is enriched in Ba emanated from sea water at medium and high temperatures, low and high fugacity and high water fugacity generated Ba-bearing phlogopite grains both in trondhjemite samples (S13W6) and remaining granitoids during partial melting of the mantle. The phlogopite grains with low Mg# in S13W6 sample have more Ba than all remain phlogopites. The presence of high Ba contents in these phlogopite grains can be explained by (1) the admixture of residual and new Ba rich melts or (2) the presence of both low Mg and Mn contents in the octahedral site that generate a large interlayer site which accommodate more Ba or (3) both mechanisms are displayed to crystallize these phlogopite grains. Thus Ba rich phlogopites occur in the fresh granitoids depending only on early magmatic processes or hydrothermal alteration at high temperature. Contents of titanium are only controlled by temperature during phlogopite crystallizations. Moreover, igneous phlogopites are used as metallogenic indicator. The study is useful to the exploration efforts for barium ore and shows that the Ba-rich parent granitoids can produce Ba-rich soils which can be potential economic interest for Cameroon.