Hybrid granitic magma originated at the advancing front of basaltic underplating: inferences from the Sesia Magmatic System (Wesyern Alps)

Most rhyolitic and granitic rocks of the large Permo-Carboniferous province of Europe show a restricted range in isotopic compositions, intermediate between mantle and crustal values. We propose an explanationto the relative homogeneity of these hybrid granitic magmas based on geochemistry and field observationsof the Sesia Magmatic System, which includes a deep crustal gabbroic complex, upper crustal granite plutons and a volcanic field dominated by rhyolitic caldera fill tuff (Quick et al., 2009). Isotopic compositionsof the deep crustal gabbro overlap those of coeval andesitic basalts, whereas coeval granites define adistinct, more radiogenic cluster (Sri ≈ 0.708 and 0.710, respectively). AFC computations starting from thebest mafic candidate for a starting melt show that isotopic compositions and trace elements of andesitic basalts may be modelled by assimilation of about 30% of partially depleted crust and about 15-29% fractionation. Trace elements of the deep crustal gabbro cumulates require a further ≈ 60% fractionation of the andesitic basalt and loss of about 40% of silica-rich residual melt (Sinigoi et al., 2016). The compositionof the granite pluton is consistent with a mixture of almost equal parts of residual melt delivered from the gabbro and the anatectic melt. Chemical and field evidence lead to infer a conceptual model which links the production of the two granitic components to the evolution of the gabbroic complex. During the growth of the gabbroic complex, progressive incorporation of packages of crustal rocks resulted in a roughly steady-state rate of assimilation. Upwards segregation of anatectic melts delivered from the hot zone above the advancing mafic intrusion facilitates reactive bulk assimilation of the restite by density-driven stoping. At each cycle of mafic intrusion and incorporation of roof layers, residual and anatectic melts are produced in more or less constant proportions, because the amount of anatectic melt produced at the provisional roof is a function of volume and latent heat of crystallization of the intruded mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt. Such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the Permo-Carboniferous province of Europe and elsewhere.