Magmatic evolution of the southern Coast Belt: Constraints from Nd-Sr isotopic systematics and geochronology of the southern Coast Plutonic Complex

Igneous rocks of the southern Coast Belt (SCB) and adjacent Insular Belt developed within a Jurassic-Quaternary magmatic are built across accreted juvenile-are and oceanic terranes. SCB plutons are mostly of intermediate composition, with I-type characteristics and major element, trace element, and rare earth element geochemistry consistent with genesis in a subduction-related magmatic are. Ubiquitous xenoliths and migmatitic zones at pluton - county rock contacts indicate that assimilation of crustal rock was an important magmatic process. U-Pb zircon crystallization ages for SCB and Insular Belt igneous rocks indicate an overall eastward migration of the magmatic axis from Middle Jurassic through Late Cretaceous time. Although absent in most rocks, traces of old inherited zircon are present in several Middle Jurassic - Upper Cretaceous plutons in the southeastern Coast Belt. The primitive character and restricted range of Nd-Sr isotopic data for Middle Jurassic to Quaternary igneous rocks of the SCB (epsilon(Nd) = +2.4 to +8.0; Sr-i = 0.7030-0.7042) indicate they were generated in an isotopically juvenile magmatic are. The distribution of isotopic values along the mantle array and the wide range of f(Sm/Nd) values suggest magma was derived from depleted mantle within a mantle wedge, with little or no contribution from old, isotopically evolved continental material. Although field evidence suggests that assimilation of juvenile crust was an important process during magma ascent, isotopic and geochemical data do not permit discrimination between direct mantle derivation of magmas followed by fractionation and crustal assimilation, and wholesale melting of mafic are-derived lower crust.