UNCOVERING THE DEEP ULTRAMAFIC ROOTS OF THE ACADIAN OROGEN: IMPLICATIONS FOR THE GENESIS OF THE COEVAL EVOLVED MAGMATISM

The traditional notion for the production of the intermediate to silicic magmatism in the New England portion of the Acadian orogen postulates that it dominantly originated after partial melting of a meta-sedimentary crust during crustal thickening, with limited to none mantle-derived magmatic contribution. The scarcity of ultramafic or mafic magmatic components in the Acadian orogen has bolstered the case for such model. The intense tectonism and weathering associated with terrains of complex settings such as accretionary orogens often obfuscates field relations, making difficult to ascertain crystal-fractionation links between evolved magmatism and its complementary—and often meagre—ultramafic or mafic roots. In this communication, we uncover and examine a suite of ultramafic/mafic rocks within a mélange complex brought to the surface by deep-reaching thrust faults in the Acadian Orogen (Connecticut, southern New England, U.S.A.). A combination of field and micro-textural relations, whole-rock and mineral major element compositions, and in-situ zircon isotope geochemistry indicate that the studied rocks are ultramafic/mafic magmatic cumulates formed after assimilation fractional crystallization processes from an originally mantle-derived calc-alkaline hydrous parental melt in a lower crustal MASH zone (40–45 km depth) during the Early Devonian. We interpret that they represent the magmatic cumulate roots of the Acadian orogen. As such, these rocks posit the need for a re-evaluation of the deep links between a primitive mantle-derived component and the generation of the Acadian evolved magmatism in New England. Moreover, in-situ zircon U-Pb and trace element geochemical signatures indicate a remarkable temporal affinity and a potential crystal fractionation genetic link between the studied rocks and the coeval intermediate to silicic plutonism in New England. On that basis, we discuss a scenario in which the genesis of the intermediate to silicic Acadian magmatism in New England involved the differentiation of mantle-derived parental melts and the segregation of amphibole-dominated hydrous cumulates in a deep crustal hot zone, in contrast to classic models involving crustal melting processes only.