Zircon U--Pb geochronology and Sr-Nd-O isotopic constraints on the petrogenesis of the Jálama pluton (Central Iberian Zone, Spain)

Abstract

In this paper we report a combined zircon U–Pb–O isotopes and bulk-rock Sr–Nd–O isotopes study on granitic rocks from the Jálama pluton, Central Iberian Zone. Zircon U--Pb ages of representative granitic rock samples are bracketed in the range of 306–304 ± 3 Ma, but within the level 2σ errors of U--Pb ages. Therefore it is not possible to establish differences in intrusion ages between the main granitic units that make up the Jálama pluton. Microgranular enclaves appear to be slightly older (309 ± 2 Ma), but they do overlap with the age of the granitic rocks. Sr--Nd isotopic data for enclaves of tonalitic composition preclude a simple mantle or crustal origin, but high mean zircon δ18O values of 8.65 ± 0.9‰ and δ18O(WR) values of 9.96–10.56‰ reflect a largely crustally contaminated magma. The combination of field relations, petrography, bulk-rock geochemistry, Sr-Nd-O isotopic data, zircon δ18O values and U--Pb ages provide compelling evidence that the granites from the Jálama pluton: (i) were coevally formed involving assimilation and mingling/mixing concomitant to fractional crystallization; and (ii) represent typical S-type granites that evolved from a same crustal source, potentially metasedimentary rocks from the Schist Greywacke Complex by considering the Neoproterozoic ages of the inherited zircons.