Crustal origin of Hercynian peraluminous granitic batholiths of Central Spain: petrological, geochemical and isotopic (Sr, Nd) constraints

Abstract

In Central Spain, it is possible to distinguish two main types of peraluminous late-Hercynian granites: the PI types and the PS types. The distinction between both types is made on the basis of differences in peraluminosity (PS types are more peraluminous than PI types) and also on the appearance of some characteristic mafic minerals; PS types have biotite, cordierite and monazite as the most typical accessory phase, whereas PI types have biotite, amphibole (in the less evolved facies) and allanite as the accessory mineral. Both granite types have similar trace element ratios and initial Sr, Nd and Pb isotopic signatures. Initial 87Sr/86Sr ratios of both types exhibit a large range from 0.7073 to 0.7193, whereas initial εNd varies in a restricted range from -5.4 to -6.6. The scarce associated basic rocks do not play a significant role in the chemical variability of these peraluminous granites which follow low pressure crystal fractionation trends from granodiorite/monzogranite parental magmas. Mixing and AFC modelling of Sr and Nd isotopic data reveal an unrealistically high mantle contribution. Based on major and trace element and isotopic data, an orthogneissic protolith for both granitic series is proposed. Nevertheless, none of the metamorphic country rocks of Central Spain has the appropriate Sr isotopic composition to satisfy the origin of these granitic rocks, and so, it is argued that progressive isotopic re-equilibration of crustal material during the granulization of the lower Hercynian crust, together with the possibility of isotopic disequilibrium during melting as has been demonstrated in migmatitic terranes in nearby areas. may explain the isotopic differences between the granites and the high level metamorphic country rocks