Abstract

The 1,550-m.y.-old Paraguaza granite of the northwestern Guayana Shield in Venezuela represents one of the larger, apparently anorogenic rapakivi intrusive rocks of the world. The massive rapakivi granite intrudes foliated granitic rocks and associated volcanic rocks of trans-Amazonian age in a structural setting transcurrent to the general northeast-southwest trend of the older basement rocks of the Guayana Shield. Age relations and the geochemistry of the rapakivi suggest an anatectic origin from tensional effects developed by internal distortions within a continental mass.

The extensive 1,550-m.y.-old Parguaza intrusion in Venezuela is correlated with 1,550-m.y.-old basement rocks underlying the Amazon Basin in Brazil and suggests a widespread “Parguazan” event 1,500 to 1,600 m.y. ago which affected a large part of the northwestern and southern Guayana Shield, extending as far south as the Guapore craton of Brazil. The Parguazan event therefore marks an important Proterozoic episode in the tectonic evolution of the widespread Precambrian Shield area of northern South America.

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