Chad

A symmetrical intrusion Chad is concentrically zoned with a nucleus 2 km in diameter of dunites which is surrounded by a ring of peridotite and clinopyroxenite. The peripheral part is formed by gabbroic rocks, syenites and diorites. Veins of peralkaline pegmatite, aplite and granite porphyry are present, particularly towards the centre of the intrusion. The dunites are medium- grained, compact, dark green rocks of olivine with some serpentine, chrome spinel and platinum group minerals also present. The peridotites are formed predominantly of diopside and olivine, sometimes with a little titanomagnetite. The composition of the peridotites varies from the internal to the external contacts. In the zone adjacent to dunites they are coarse-grained bright green rocks. Closer to the periphery the peridotites become darker, finer grained and titanomagnetite appears. Bogomolov and Kitsul (1964) regarded the peridotites as being generated by reaction around the dunite core during the introduction of the gabbroic intrusions. Moderately alkaline rocks of monzonitic type occur in the gabbroic intrusion, but olivine gabbro is also present. The moderately alkaline gabbros have poikilitic textures with large, up to 1 cm diameter, grains of K-feldspar with inclusions of pyroxene and olivine. In addition, plagioclase (An50-55), titanomagnetite, biotite, hornblende and apatite are also present. The olivine gabbros are similar to the alkaline ones but K-feldspar is absent and the plagioclase has a more basic composition (An60-65). Bodies of alkaline syenite and shonkinite form an arcuate body around the southern margin of the intrusion against the country rock limestones. The syenites consist of clinopyroxene, andesine and K-feldspar while the shonkinites comprise clinopyroxene and K- feldspar.