Padrões isotópicos Sm-Nd no limite entre os terrenos Alto Pajeú e Alto Moxotó (PB)

Abstract

Sm-Nd isotopic studies were carried out to recognize and define two different groups of geologictectonic terranes of the eastern most part of the "Transversal Zone" (central sub-province) of the Borborema Province, northeast of the South American Platform. The first recognized group (I), formed by granitic orthogneisses and supracrustal rocks is the Alto Pajeú Terrane - APT - that exhibits an isotopic signature with T DM values between 2.28 and 1.37 Ga, high negative values for εNd(0) ( ; 0.12 that indicates a possible isotopic fractioning. The second group (II) is composed mostly by orthogneisses (a minor supracrustal belt -Sertânia complex), the Alto Moxotó Terrane -AMT - exhibits an isotopic signature with higher T DM values, between 3.0 and 2.0 Ga, very negative values for εNd , between -20 and -35, and 147Sm/144Nd lower than those of the group I, i.e. between 0.12 and 0.08. The εNd(t) values calculated for the assumed ages of both groups are respectively: group I εNd (950)= -14 up to +1.28; group II - εNd (2000) = -11.28 up to +1.73. Additionally, these distinct isotopic patterns worked out as a complementary tool for the geological mapping, as well as, they have gradually been reiterated by means of the study of the Brasiliano granitic plutonism (Ediacaran in age) that occurs piercing both, APT and AMT. Data obtained by other geochronological methods (U-PB and Rb-Sr) for this granites and for the pre-Brasiliano terranes themselves were in good agreement with these above discriminated patterns.