General geology and petrography of the precambrian basement around Zalingei town, Darfur Province, Sudan

Abstract

Abstract The exposed Precambrian rocks of the greater Zalingei area consisted originally of a thick succession of psammites and semipelites. The upper exposed part of this succession included some probable bituminous intercalations that are now represented by graphitic quartz schists. The succession also contained few carbonate beds and minor intrusions of dolerite. The entire succession had later suffered a major protracted cycle of tectonic deformation, regional metamorphism and migmatism. The metasediments were open-folded into synforms and antiforms that now have ENE-WSW axial trends. The migmatites around Zalingei town display repeated events of deformation and recrystallization, as well as partial melting. Earlier metamorphic assemblages point to granulite facies conditions (sillimanite, orthorhombic pyroxene) but these assemblages were largely replaced by later amphibolite facies minerals (kyanite, staurolite, almandite, hornblende). Partial melting accompanied by extensive flow-folding on all scales affected the more susceptible semipelites and produced a wide variety of migmatitic gneisses. At first these migmatites were oligoclasic and produced melt segregates and small syntectonic injections of trondhjemite, tonalite or granodiorite. Later, more potassic migmatites evolved, mainly through microclinization of the earlier migmatites. A monzogranitic intrusion with very large porphyritic potash feldspars was also mobilized from a lower crustal level. A further episode of some strong deformation followed and left a variable foliate and rather erratic imprint on the different rocks. This deformation might have accompanied the uprise of intrusive granitoid diapirs mantled from outside by diverse concentrically foliated gneisses with boudinaged minor pegmatite intrusions. The diapirs have inner massive central cores of leucocratic monzogranites and their intrusion ended the major plutonic activities. Quartz—tourmaline veinlets and massive dyke-like bodies accompanied and survived the diapir intrusions and represented the terminal penumatolytic and hydrothermal stages. The large quartz dykes, the master-joints and other late lineaments are crudely oriented in a NW-SE trend. From apparently similar lithologies and geologic events in the adjacent basement rocks of Eastern Chad and the Central African Republic, the original age of the Zalingei sedimentary succession is provisionally assigned to Precambrian D (older than 2500 Ma). The migmatitic to late diapiric activities, however, represented reactivation (rejuvenation) of the basement rocks as the Zalingei basement falls within the broad NE-SW mobilized belt running between the West African and Congo cratons in Pan-African times (500–600 Ma).