Tectonic setting, metamorphism an deformation of the Sukuma land archaean greenstone rocks around Geita

The main supracrustallithotypes at Geita from bottom to top of the stratigraphy are volcanic flows, tuff, BIF chert bed and classic sedimentary rocks. There are supracrustal rocks are intruded by gramites and quartz porphyries and are crosscut by dolerite dykes. The metavolcanics of suumaland greenstone bet belong to tholelitic ( for mafic rocks) and calc-alkalie ( for felsic rocs) series. The patterns of REE and trace clements of the mafic rocks suggest back are or ocean are tectoni settings whereas the REE patterns of granites (all calk-alkline)suggest a volcanic tectonic setting. The main two types of metamorphism at Geita are regional greenchist facies and local contact epidote-amphibolite facies thermal metamorphism. The greenschist facies minerals assembages is associated with shear zones and its mineral assemblage stabilized between 300C and 500C temperatures at low pressures. Locally highly sheared zones have some epidote-amphibolite facies mineral assembliages that stabilized between of 250C to 350C in metagranites and 450C to 460C in metabolities Temperatures in contact metamorphism range between 441 to 544C in met ironstone and between 500C to 725C in metabasite. The local contact metamorphism in the supracrustal rocks associated with granite emplacement whereas the greennschist facies metamorphism came later during the shearing deformation that affected both granites structures in BIF were syn- depositional polycline flexural folds without axial plane cleavage implying no significant deformation to the supracrustal rocks prior to granite emplacement.