A Gondwana pre-break up related magmatism in Bafoussam area, west Cameroon: source characteristics

The Gondwana pre-break up related formations in Bafoussam area are transitional calc-alkaline doleritic dykes exhibiting high Alumina and low Ti-Mg contents. Their REE compositions are similar to those of E-MORB (Nd/Nb≈1, Zr/Nb≤20). A high partial melting of about 20% of Garnet peridotites source having primitive mantle composition, is inferred to the studied rocks. They are slightly evolved 50<Mg#<54, and its clinopyroxenes are augite of high temperature (600°C-1100°C). The studied dykes were emplaced in a within-plate tectonic setting and yield a Ar-Ar plateau ages of 229±7Ma. These ages are slightly different from those of dolerites from the ObanObudu massif (Nigeria), from the basaltic dyke of the Cameroon Volcanic Line and from the Karoo-Ferrar mafic magmatism, all interpreted as magmatism that preceded the break-up of Gondwana. The emplacement age of 229±7Ma of dolerites from Bafoussam area relates them to the very early stage of Gondwana pre-break up magmatism in central Africa.


Introduction
In Cameroon, the basement formations are made up of Archean to Paleo-Proterozoic rocks related to the Congo Craton and of Neoprotorozoic rocks related to Pan-African orogeny; this basement is overlain in some places by tertiary to recent volcanic rocks or by sedimentary formations (Fig. 1A). Mafic doleritic dykes have been reported at the northern edge of the Congo craton in the Southeasthern part of Cameroon (Vicat et al., 1997), these intrusions of about 1Ga display continental tholeitic composition generated from spinel lherzolite source and are related to a pre-Panafrican Orogeny extensional phase in the area. Doleretic dykes have been reported by Vicat et al., (2001) Fig. 1 A&B). Kouankap et al., (2013) reported two distinct petrochemical types of doleritic dykes in Bafoussam area. In this paper, after recalling the main petrochemical characteristics, we present the pyroxene chemistry and the Argon-Argon geochronology of dolerites dykes in Bafoussam region. These new data will constraint their sources and will contribute to understand the geodynamic evolution of the Central Africa within the Gondwana geological setting.

Previous studies and rationale to this work
Previous work carried out on the dolerites studied in the Bafoussam area reveals two distinct petrochemical types of dolerites: olivinebearing dolerite and calcite-bearing dolerite (Kouankap et al., 2013). The olivine-bearing dolerites are massive, compact and present a distinct doleritic texture; they are composed of plagioclase, olivine, pyroxene and opaque minerals. The matrix constitutes more than 40% of the total volume of the rock. The calcite-bearing dolerites are massive and appear greenish. The rock is composed of plagioclase, calcite, pyroxenes, amphiboles and opaque minerals. The calcite crystals (0.5 to 2 mm) are sub-rounded and are associated with plagioclase and amphiboles. Amphiboles are very rare and are only observed around the rim of calcite crystals. Their crystals are fine grained; most of them constitute the matrix elements which make up about 60% of the total volume of the rock. The studied dolerites which have a basaltic composition, were less differentiated, and were different from proterozoic dolerites studied in the south of Cameroon (Vicat et al., 1998). According to the preliminary petrochemical data of dolerites from Bafoussam area, Kouankap et al., (2013) suggested that the contrast between the two types is probably due to a diverse source. This work aims at verifying this assertion, by providing their ages and more characterization of the rocks and their sources.

Analytical methods
Standard thin sections were made by Geotech lab (Canada) and the whole-rock geochemistry was performed at the commercial analytical laboratory ACME (Vancouver Canada), methods and results are presented in Kouankap et al., (2013). The pyroxene chemistry of the elements (Si, Ti, Al, Cr, Fe, Mn, Mg, Ni, Na, K and P) was carried out using a SX100 and SX five Camebax microprobe at the "Pierre et Marie Curie" University, Paris 6, France. The analytical conditions were as follows for clinopyroxene: 15k V and 40 nA, 20s, except Ti (30 s). The "PAP" correction program used is defined by Pouchou and Pichoir (1991). The mineral phases were recalculated on a stoichiometric basis of oxygen and cations following the recommendations of the International Association Subcommittee on the nomenclature of minerals. The clinopyroxene phenocrysts from dolerite samples were analyzed ( Table 1). The compositions have been recalculated on the basis of 4 cations and 6 oxygen atoms. Fe 3+ is estimated using stoichiometric criteria according to Droop (1987 Fig 2C) show no negative anomalies in Eu and are similar to those of E-MORB (Nd/Nb≈1, Zr/Nb≤20). Plots of the rocks on the Nb/Y versus Zr/Y log-log diagram of Fitton et al. (1997) clearly indicate that they are related to mantle non-plume source (Fig 2D). The Zr/Nb ratios for all rocks are less than 20 which correspond to primodial mantle characteristics (Middlemost 1975). The Nd/Nb ratios are ≈1, indicating an enriched Mantle source (Fig 3). The Dy/Yb versus La/Yb diagram (Fig 4) indicates that the parental magma of the studied transitional dolerites from Bafoussam area was probably formed by high degree of partial melting of about 20% of garnet peridotite at a depth relatively shallow. This high degree of partial melting (≈20%) is so far the highest recorded till date from the basaltic igneous rocks in Cameroon (Fig 4) which are about 16% for transitional lavas and less than 15% for alkaline lavas occurring in a tertiary volcano-plutonic complex of Cameroon (Kuepouo et al., 2006;Fosso et al., 2005).

Mineral chemistry of pyroxenes
The data of clinopyroxene phenocrysts chemistry is presented in table 1.  Kushiro (1960) showed that during magmatic crystallization, the proportion of Si increases in the pyroxene structure whereas that of Al in the tetrahedral site decreases. Al IV >Al VI is somewhat typical of pyroxene formed at high temperature. The estimation of temperature for Bafoussam dolerites is made using pyroxene thermometry (Lindsley, 1983).

Geochronology, 39 Ar-40 Ar dating
The summary of the Ar-Ar geochronological data of the representative samples of dolerites from Bafoussam area is presented in table 2.
It should be noted that the gas is released by progressive crushing and not by progressive heating as is more commonly the case in 40 Ar/ 39 Ar dating experiments. The results of these stepwise-crushing experiments are presented in apparent age spectrum plots and in 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar diagrams (Fig. 10). The age spectrum is a convenient way to present the data in terms of progressive gas release (Qiu and Wijbrans, 2006). The data from dolerites crushing experiments define a homogenous line on the normal isochron diagram but showing a disparity in the inverse isochron diagram. Dolerites from Bafoussam yielded an Ar-Ar age of 229.03 ± 7.07 Ma (Fig. 7), the patterns of cumulative 39 Ar released %, show a slight variation in the age plateau

Conclusion
The main conclusions of this paper are: 1) Dolerites from Bafoussam area are transitional calc-alkaline basalts, with high Alumina and low Mg contents. Their parental magma evolved mostly by fractional crystallization and their REE compositions are similar to those of the E-MORB.
2) The clinopyroxenes of dolerites from Bafoussam area are similar to those of transitional basaltic lavas and other basaltic dykes described in Cameroon. They yielded a wide range of temperatures from 600°C to 1100°C. 3) Dolerites from Bafoussam area are relatively highly fractionated; the parental magmas were probably formed by high degree of partial melting of about 20% of garnet peridotite, which is so far the highest degree of partial melting recorded till date from the basaltic igneous rocks in Cameroon. 4) Dolerites from the Bafoussam area were emplaced in a within plate tectonic setting and yield an 40 Ar-39 Ar plateau age of 229±7Ma. These ages are slightly different from the ages of dolerites of the Oban-Obudu massif of southeastern Nigeria, from the basaltic dykes of the southern continental part of the Cameroon Volcanic Line and from the Karoo and ferrar mafic magmatism, but all interpreted as intrusions preceding the break-up of Gondwana.