Sr and Nd isotopic study and Rb-Sr geochronology of the Bejar granites, Iberian Massif, Spain
Pinarelli, Laura; Rottura, Alessandro
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 577 - 589
published: May 19, 1995
manuscript accepted: Dec 22, 1994
manuscript received: Nov 23, 1993
ArtNo. ESP147050703012, Price: 29.00 €
Abstract The Spanish Central Massif (SCM) is a major granitoid complex in the southern Central-Iberian Zone of the Hercynian Iberian belt. Its western part (the Bejar region, Gredos sector) consists of mafic enclave bearing porphyritic biotite monzogranite-granodiorites (BG) with minor cordierite ± muscovite-bearing granites (CMG). Some granites also occur whose geochemical and mineralogical characteristics are intermediate between those of the BG and CMG. No intrusive relationships have been observed between the BG and the CMG. The BG and the CMG are geochemically similar, though the latter have higher Sr contents. Both are moderately peraluminous (on average, the Aluminum Saturation Index is 1.13 in the BG and 1.19 in the CMG), defining trends that overlap or are in continuity with each other in most major and trace element variation diagrams. They exhibit LREE enriched patterns with moderate to pronounced negative Eu anomalies and show affinities with post-collision late-orogenic granitoids. Overall, the granites and their enclaves exhibit many major and trace element variation trends that reflect a liquid line of descent. The Rb/Sr biotite ages from both granite types are consistent with each other, and range between 295 ± 4 and 307 ± 5 Ma. The granites all have variable initial 87Sr/86Sr ratios (taking 300 Ma as the intrusion age) which range between similar values (0.70838-0.70910 for the BG, 0.70799-0.70897 for the CMG). The dioritic enclave has a significantly lower initial Sr isotope ratio (0.70718), while the granodioritic enclave has an initial 87Sr/8 Sr ratio (0.70797) very close to that of the host granites. The initial 8Nd values of the granites are also variable, but again show similar ranges (-3.9 to -8.0 for the BG, -4.7 to -7.8 for the CMG). The εNd of both the dioritic (-4.3) and the granodioritic (-5.5) enclaves falls within the range of the host granites. The isotopic data presented here, and the close association of the granites with gabbroic rocks of mantle origin, rule out a purely crustal source, rather suggesting a mantle-crust mixed origin. A two-stage petrogenetic process may be envisaged: a mantle derived magma accumulated at the base of the crust and evolved through crustal interaction to an intermediate magma with the Sr and Nd isotopic characteristics of the dioritic enclave and coeval gabbros. The BG and CMG were probably generated from the coalescence of distinct magma batches derived from the hybrid intermediate melt. This process involved fractional crystallization combined with contamination by a melt produced by a low degree of partial melting of the surrounding metapelitic rocks.