Petrology and geochemistry of late-hercynian granites from the Western Central System of the Iberian Massif
Rottura, Alessandro; Bargossi, Giuseppe Maria; Caironi, Valeria; D'amico, Claudio; Maccarrone, Eleonora
European Journal of Mineralogy Volume 1 Number 5 (1989), p. 667 - 684
published: Nov 16, 1989
manuscript accepted: Apr 17, 1989
manuscript received: Sep 16, 1988
ArtNo. ESP147050105007, Price: 29.00 €
Abstract A geological, petrographic and geochemical study, involving mineral chemistry, zircon typology as well as whole-rock major, trace and rare earth element data, has been carried out on the granites occurring N and W of Bejar in the western zone of the Spanish Central System (CSB). These consist mainly of porphyritic biotite monzogranites-granodiorites (BG), with minor muscovite ± cordierite-bearing types (MCG), clearly intrusive at a high-level into a thick sequence of greywackes and pelites metamorphosed at greenschist facies, and into cordierite + K-feldspar-rich migmatites. Magmatic mafic microgranular enclaves commonly occur mostly in the BG. Both granite types are ilmenite-bearing. The textural and compositional data suggest at least a partly magmatic origin for muscovite ; cordierite could be primary and partly exogenic in origin. The chemical data show the following features: 1) moderately peraluminous affinity for both granite types, more pronounced in the MCG (A/CNK = 1.19 vs. 1.13 ; normative C = 2.40 % vs. 1.86 % ) , which is also more acidic (SiO2 = 70% vs. 68%) ; 2) element variation trends rather scattered for the MCG, but linear and suggestive of cumulus-intercumulus mixtures in the dominant BG ; 3) LREE-enriched patterns with a moderate to pronounced negative Eu-anomaly (Eu/Eu* = 0.57 to 0.23), lower total REE contents (135 vs. 170 ppm) and greater HREE fractionation (mean normalized Gd/Yb = 2.29 vs. 1.89) in the MCG. In the zircon typology grid (Pupin, 1980), the BG plot in the domain of hydrid calc-alkaline granitoids and the MCG in that of aluminous anatectic granites. The field, petrographic and chemical data rule out genetic processes involving a single homogeneous source and suggest an origin via mantle-crust mixing processes. Further data, in particular isotopic, are necessary to constrain the granite genesis in terms of source materials and to choose between: (i) a pure intracrustal melting process triggered by basic intrusions and, mainly for the BG, (ii) a model involving mantle derivatives.