THE GENESIS OF FELSIC-MAFIC PLUTONIC ASSOCIATIONS - A SR AND ND ISOTOPIC STUDY OF THE HERCYNIAN BRAGA GRANITOID MASSIF (NORTHERN PORTUGAL)

The late tectonic Braga plutonic complex in the ''Centro lberian Zone' ', North Portugal, was emplaced during the Hercynian orogeny within a Silurian sedimentary sequence and displays an acid-basic association w hich consists of three well-defined intrusions: the biotite-dominant B raga granite, minor bodies of gabbro to granodiorite composition and t he mildly peraluminous Gonca leucogranite. These three plutonic suites present field relationships indicating a synchronous magmatic emplace ment for which a Rb-Sr age of 310 +/- 10 Ma is obtained. The distribut ion of the three plutonic units along regular curves in major and trac e element diagrams suggests that the different units can be geneticall y related. However, the Rb-Sr and Sm-Nd isotopic results do not indica te a unique homogeneous source and a simple fractional crystallization process. The gabbros have chemical characters (high K, Ba, Sr and lig ht rare-earth elements and low Nb, Ti and Zr contents) and isotopic co mpositions (Sr(i) = 0.70497 and epsilon(Nd) = -2.5), which suggest an alkaline magmatic affinity of shoshonitic type. They are probably deri ved from an enriched mantle source. In contrast, the peraluminous Gonc a leucogranite (Sr(i) = 0.70933 and epsilon(Nd) = -6.8) is more likely the result of crustal anatexis. The Braga granite and the evolved mem bers of the basic series have an intermediate isotopic composition (Sr (i) = 0.70532 to 0.70733 and epsilon(Nd) = -3.0 to -6.2) which can be interpreted in terms of an hybridization process between the two previ ous end-members. The chemical and Sr-Nd isotopic compositions of the B raga plutonic series can be explained by an assimilation-fractional cr ystallization (AFC) model between a mantle-derived magma (equivalent t o the gabbros) and a crust-derived magma (the Gonca granite).