Multiple sources for the Coastal Batholith of central Chile (31-34 degreesS): geochemical and Sr-Nd isotopic evidence and tectonic implications

The Coastal Batholith of central Chile between 31 and 34 degrees S represen ts a locus of long-lived (c. 200 Ma) plutonic activity. It is composed of f our plutonic complexes: Santo Domingo, Limari, Papudo-Quintero and Illapel. These complexes, which occur as north-trending discontinuous belts that de crease in age eastward, were emplaced during four major episodes of are mag matism. The Santo Domingo Complex (Carboniferous) comprises hornblende-biot ite bearing tonalites and granodiorites with abundant mafic enclaves, and e nclave-free granites, The association of enclave-bearing granitoids and enc lave-free granites resulted from different degrees of mixing of crustal- an d mantle-derived magmas, The Limari Complex (Lower Jurassic) is a bimodal a ssociation of crustal leucogranites and mantle-derived gabbros. The litholo gy of the Papudo-Quintero Complex (Middle Jurassic) varies from homblende-p yroxene diorite to biotite granite. The main rock types, namely tonalites a nd granodiorites, commonly contain mafic enclaves. The granitoids and the e nclaves have remarkably similar REE patterns and are similar in Sr-Nd isoto pe composition, which suggest that they represent a cogenetic suite of mant le origin. The Illapel Complex (Cretaceous) consists of hornblende bearing tonalites, trondhjemites and granodiorites that resemble the rocks of TTG s uites. The epsilon(Nd) values and initial Sr-87/Sr-86 ratios (Sr-i) for the mafic rocks of the Carboniferous Complex (epsilon(Nd): -2.0 and -3.5; Sr-i : c, 0.7057) differ markedly from those of the Mesozoic mafic rocks (epsilo n(Nd): +1.9 to +5.4; Sr-i: 0.7033 to 0.7039), the Cretaceous rocks being th e most depleted with regard to Sr-Nd. A change from Carboniferous lithosphe re-dominated sources to Mesozoic asthenosphere-dominated sources of the mag mas that started in the Early Jurassic is recognized. Slab components can b e recognized in the source of the Cretaceous rocks, The crustal source of t he Lower Jurassic leucogranites was more isotopically depleted and refracto ry than that of the Carboniferous granites. On the other hand, no crustal p articipation in the origin of the Middle Jurassic and Cretaceous felsic gra nitoids is recognized; they are differentiated from mantle-derived magmas. Progressive removal of old lithospheric mantle during the Mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source, as well as the melting and modification towards a mo re refractory nature of the continental crust. (C) 1999 Elsevier Science B. V. All rights reserved.