Lupongola (Chitado)

Emplaced into a complex of gabbros and anorthosites, which is cut by numerous large dolerite dykes and sills, Lupongola appears to be a ring structure defined by a circular peripheral fault. However, the partial ring-like form of outcrops of carbonatitic and syenitic rocks may be fortuitous depending, in part at least, on the blanket of recent deposits that obscures most of the central area of the structure. Lapido-Loureiro (1973) distinguishes two types of carbonatitic rocks. The first comprises several arcuate areas lying along the ring-fault that consist of a breccia in which xenoliths, up to 30 cm in diameter, of syenite, and possible granite, are set in a matrix of feldspar-bearing carbonatite. Homogeneous carbonatite is only found as lenses and pockets, particularly in outcrops towards the centre of the structure. The carbonatite is calcitic, may be apatite-rich and generally contains alkali feldspar. The syenites are brecciated alkali feldspar rocks, generally with abundant carbonate, and in some varieties amphibole and biotite. Lapido-Loureiro (1973) suggests that the syenites represent an earlier intrusion that was disrupted by later carbonatite injection. However, it seems possible that the syenites are some sort of fenite. The second group of carbonatitic rocks consists of a complex of dykes and veins which extends for several kilometres into the country rocks, notably to the north and south. Again Lapido-Loureiro (1973) considers that initially syenite was intruded which was followed by carbonatitic and other fluids. The carbonatite forms veins and small discontinuous lenses and occupies fractures and crush zones. The syenitic rocks are variable consisting of various combinations of alkali feldspar, amphibole and biotite with accessory titanite and apatite. A syenite from the southeastern part of the structure is reported to contain sodalite. An average of six analyses of carbonatites is given by Issa et al. (1991) and a feldspar-bearing carbonatite with 31.85 wt% SiO2 has 1.71 wt% Na2O but 5.49 wt% K2O. Analyses of six carbonatites and a carbonatitic trachyphonolite, including trace elements and O, C, Sr and Nd isotope data, are given by Alberti et al. (2000). This is a rather puzzling occurrence.