Crustal Evolution Of The Neoproterozoic Mozambique Belt Rocks In Matuu-masinga Area, Central Kenya: An Implication To Their Tectonic Setting And Potential Mineralization

The Matuu-Masinga study area, located about 70 km north-east of Nairobi, lies at the intersection of latitude 1010'S and longitude 37° 30E. The area is predominantly underlain by rocks of the Mozambique Belt (MB) in central Kenya. The MB itself exhibits a general N-S trend extending from Mozambique in the South, through East Africa and beyond to Ethiopia in the North. The MB has a complex history of superimposed deformation and metamorphism which consists of a high grade reworked or reactivated basement. Presently this trans-continental belt is regarded as a product of continent-continent collision. This mobile belt is believed to be polyorogenic, with the latest Neoproterozoic (the Pan-African) orogeny being superimposed onto the Mesoproterozoic (Kibaran) and Palaeoproterozoic (the Ubendian) orogenies. The study area offers a striking geological region composed of granitoids, ultramafic/mafic rocks and the field association of granulite-(meta)-anorthosite-amphibolite rocks. These rocks are of greater importance insofar as the understanding of the tectonic evolution of the belt and petrogenesis of the deep continental crust is concerned. The present study has drawn evidence from trace element and isotope geochemistry of the rocks and from analysis of the characteristics and sequence of the structures. The Matuu-Masinga area firstly experienced- regional prograde granulite facies metamorphism which was later retrograded to the amphibolite and greenschist-facies. The structural trend varies from NNW -SSE to NW -SE with westerly dips ranging from 50° to the vertical. Several shear zones, v.arious foliation surfaces, linear structures and complex folds suggest a complex tectonic history of the region with at least three phases of deformation. Distribution of aluminum among the tetrahedral and octahedral sites in the pyroxene analyses from dioritic rocks shows the c1inopyroxenes to have crystallized under higher pressure than their corresponding iron rich orthopyroxenes. The almost equal amounts of the jadeite (AI VI) and Ca-Tschermak (Allv) in the c1inopyroxenes of gabbroic rocks are interpreted to be as a result of the combined effects of high pressure and high temperature. The characteristic coronas around clinopyroxene crystals are interpreted to have developed under metamorphic conditions. Thermobarometric PT calibrations obtained from various mineral pairs range from 750 -c and pressures between 6.55 to 6.98 kbar for the amphibolite rocks, to temperatures between 879-904 "C and pressures between 5.89 to 6.31 kbars for the gabbroic rocks, to temperatures between 843-854 -c and pressure of 15 kbar for the dioritic rocks. The use of the AI-in-amphibole thermo-barometry data for the .granitic rocks gave an average equilibrium temperature of 750 "C and a pressure of 5.97 kbar. The rocks of the study area are mainly metaluminous to slightly peraluminous. Geochemical data indicate the granites are of magmatic origin with calc-alkaline affinity. The data indicate a dominantly island arc-tectonic setting with subordinate within-plate environment. A Rb-Sr whole rock age of 558 +/- 16 Ma is-given on the Mavoloni hills granite. From the low initial 87 Sr/86Sr ratio of 0.70398 recorded from the granite, the age is interpreted to indicate the time of original emplacement for granite magma from a juvenile mantle-derived material. The relatively high concentration of Cu (av. 1960 ppm) and zinc (av. 155 ppm) in the mafic rocks of the study area compared with the average for ultramafic rocks of 30 and 50 ppm respectively invokes further exploration of their ore minerals. The granites and diorites in the study area are of potential economic use as dimension stone in the construction and building industry. Keywords: Mozambique Belt, Tectonic evolution, Petrography, Mineral chemistry, Thermobarometry, Geochemistry, Geochronology, Structures, Metamorphism, Mineralization, Matuu-Masinga area.