Gabbro-derived granulites from the Northern Apennines (Italy): Evidence for lower-crustal emplacement of tholeiitic liquids in post-Variscan times

Abstract

The mafic granulites from the Northern Apennines commonly contain significant amounts of either olivine or Fe-Ti-oxide phases. On the basis of mineralogy and whole-rock major and trace element compositions, their protoliths are recognized as cumulus rocks derived from variably evolved tholeiitic liquids. Clinopyroxenes from the olivine-bearing rocks show peculiar trace element compositions (e.g. low amounts of Cr, Zr and lanthanides, coupled with relatively high Sr concentrations) that record a process of granulite-facies trace element redistribution controlled by a reaction between olivine and plagioclase. The trace element compositions of clinopyroxenes from the Fe-Ti-oxide-bearing rocks point to igneous geochemical trends that argue for a common igneous parentage. The εnd of the mafic granulites calculated at the age of emplacement of the gabbroic protoliths (290 Ma) ranges between +6.8 and -4.5. 87Sr/ 86Sr (290Ma) varies between 0.7031 and 0.7077. The least evolved rocks show initial Nd-Sr isotopic compositions close to typical depleted mantle values. Two samples of felsic granulites associated with the mafic rocks yield age-corrected εNd of -8.0 and -5.7, and Sr isotope ratios of 0.7107 and 0·7109. As a whole, the Nd and Sr isotope data at the time of emplacement form a hyperbolic array. Whole-rock rare earth element and Nd-Sr isotopic compositions indicate that the parental liquids of the mafic granulite protoliths were tholeiites derived from a depleted mantle source. Isotopic compositions of the least evolved rocks are consistent with those of residual mantle lherzolites from the ophiolites of the Northern Apennines and the Western Alps. The protoliths of the Fe-Ti-oxide-bearing mafic granulites can be related to the least evolved rocks through fractional crystallization and assimilation of crustal material. The felsic granulites probably represent anatectic and migmatitic rocks resulting from multi-stage melting of lower-crustal basement rocks.