The geochronology of Guyana

Abstract

The Precambrian rocks of Guyana occur in two geological provinces separated by a structural break followed in part by an infilled rift valley which traverses the country to the immediate south of latitude 4°N. North of this structure the folded and more or less metamorphosed sedimentary and volcanic rocks of the Barama-Mazaruni Assemblage and the gneissose granitic rocks of the Bartica Assemblage are intruded by members of the Younger Granite Group, and all are overlain with marked unconformity by the tabular, continental deposits of the Roraima Formation. Both the Roraima Formation and the pre-Roraima rocks are cut by dolerite dykes and sills which constitute the Younger Basic Intrusive Group, including the Roraima Intrusive Suite and a minor dyke suite. Age determinations indicate a younger limit to the post-Barama-Mazaruni-Bartica diastrophism of c.2000 m.y. This diastrophism is now termed the Akawaian episode and is believed to correspond closely in time to the emplacement of most of the Younger Granites. One member of the Mazaruni Group is possibly older than c.2500 m.y. A younger limit to the Roraima Formation of c.1700 m.y. is set by K-Ar and Rb-Sr age determinations on the dolerites of the Roraima Intrusive Suite and on micas of a contact hornfels in the Roraima Formation. In the southern geological province the major rock unit is the South Savanna Granite which intrudes high-grade gneisses (granulite facies) of the Kanuku Group and metasediments of the Marudi Group. A uranium-lead determination, though affected by uranium leaching, sets a maximum limit to the age of this granite of 2075 m.y., and Rb-Sr determinations indicate emplacement at c.1850 m.y. The granite shows evidence of dynamic metamorphism, and K-Ar determinations on micas suggest an age of c.1200 m.y. for this metamorphism. The Kanuku gneisses and South Savanna granites are cut by dolerite dykes, one of which has been dated at 450 m.y.