Granitoids from Fornos de Algodres and Castro Daire areas (Northern Central Portugal): a comparison and some problems

Abstract

A comparison is made between the most representative granitoid units of the Castro Daire zoned pluton and some of the granotoid units that occur in the Fornos de Algodres area, the areas being close to each other. In each area one amphibole-bearing granodiorite and four granites have been selected.The granites of both areas show the same internal order of cristalization. Contrary to the granodiorites, that are metaluminous rocks, they plot in the aluminous domain of Debon & Le Fort's "characteristic minerals" diagram.The granodiorite also plot apart from the granites in Rb-Sr diagram. The zoned pluton granite units show compositional continuity in this diagram which is not the case of the rocks of the other area.Rb-Sr whole rock isochron dta show two of the Fornos de Algodres units to belong to a 370 +- 10 Ma age group and two other units to be ca, 255 Ma in age, whereas four units of the zoned pluton yielded ages ranging from 322 Ma up to 291 Ma. The age of 285 +- 5 Ma is the only one shared by rocks that outcrop in both areas and it relates to coarse grained porphyritic biotite granites. Granodiorites are the oldest dated rocks in both areas, but they differ in age from each by about 50 million years.Initial 87Sr/86Sr ratios are either low (< 0.704) or high (> 0.712) in the Fornos de Algodres rocks and intermediate (0.707 +- 0.002) in the case of zoned pluton rocks.Most of the granites of both areas show S-type granite features wheares the granodiorites are probably I-type granitoids with some crustal contamination.Some granites evidence moscovitization and albitication effects, as well as megacrysts, for which a metasomatic origin related to the intrusion of later granites cannot be clearly deduced from the data.The granites are not considered to be related to the granodiorites by fractional crystallization, but the granites of the zoned pluton could be mutually related by the process, contrary to the granites of the Fornos de Algodres area.