Crust-mantle interactions and generation of silicic melts: insights from the Sarmiento Complex, southern Patagonian Andes

Abstract

A Late Jurassic seafloor remnant of the Rocas Verdes basin in southern Chile, the Sarmiento Complex {ca. 52°S), bears lithological layers with bimodal meta-igneous rocks appropriate for a comprehensive investigation of magma genesis in part of a lateral lithological transition from continental rifting to initial seafloor spreading. Metamorphosed mafic rocks collected from different layers in the ophiolite pseudostratigraphy and a plagiogranite have positive sNd₁₅₀ values (+ 1 and +2). Granophyres, which are crosscut by ophiolitic mafic dikes, have negative sNd₁₅₀ values (-5). Dacitic dikes within thick successions of pillow basalt have the least negative sNd₁₅₀ values (-3 and -4). Although mafic and felsic igneous rocks show contrasting isotopic signatures, thermochemical modeling (EC-AFC) suggests they can share a common origin. Models consider an arbitrary composition of the crustal assimilant (mostly metapelite with an average eNd₁₅₀ value of-7) and evaluate the feasibility for generation of silicic melts through the interaction of mafic magmas and metasedimentary rocks. A quantitative evaluation of basaltic magma contaminated by crustal wall rocks requires a Ma7 Mc (mass of anatectic melt/ mass of cumulates) ratio of 0.04. Analyses using dikes of dacite (with MaVMc ratios ranging between 0.28-0.35) and granophyres (with MaVMc ratios of 0.63-0.89) require the silicic magmas to contain higher proportions of anatectic melts derived from metamorphic rocks. Isotopic differences among granophyres and dacites could be controlled by eruption dynamics, regional stress field and/or differences in thermal regimes in magma chambers. Bimodal magmatism in the earliest tectonic evolution of the Rocas Verdes basin could reflect regimes of slow extension of the continental crust along the Jurassic southwestern margin of Gondwana.