Total-rock U [sbnd]Pb and Rb [sbnd]Sr systematics in the Imataca Series, Guayana Shield, Venezuela

Abstract

A combination of data from two isotopic systems may be useful in “seeing through” an intense metamorphic event to the primary age of an ancient terrane. Rb Sr secondary isochrons on thin-slice samples make it possible to select domains within the rock in which Sr isotopes were equilibrated during the high-temperature stage of the metamorphism. A single domain may then be tested for Pb isotopic homogeneity at the same time by use of secondary U Pb isochrons. These monitor the possible effect of recent U loss, and if successful yield values for the common Pb ratios at the time of the thermal event. If the thermal event is both ancient and well-dated by the secondary isochrons, a model age for the protolith may then be established. The method is illustrated here with reference to the Imataca Series, about which we conclude the following: (1)|The primary or protolith age of the Imataca Series is in the range of 3.4–3.7 b.y. (2)|At approximately 2 b.y. ago, a regional metamorphic event up to granulite grade in intensity affected the Imataca. (3)|This metamorphism was accompanied by complete local homogenization of Sr isotopes over distances of centimeters; equilibration of Pb isotopes was apparently less perfect at the same scale. (4)|The granulite event was further characterized by extensive depletion of U relative to Pb. Comparable Rb loss has not taken place, perhaps because of the abundance of K-bearing phases. (5)|Deformation of the rock unit investigated may have taken the form of separate shear surfaces, between which isotopically equilibrated domains on either side of a shear plane are identified by differences in initial Sr isotope ratio.