Origin and geodynamic significance of the Siuna Serpentinite Mélange, Northeast Nicaragua: Insights from the large-scale structure, petrology and geochemistry of the ultramafic blocks

Abstract

The Siuna Serpentinite Mélange (SSM) is located in the boundary zone between continental Chortis block and oceanic Mesquito Composite Terrane in NE Nicaragua. In this paper, we present new large-scale structure, meso-scale kinematic analysis, 40Ar39Ar geochronology, mineral chemistry, in situ trace element composition of clinopyroxene, and bulk rock geochemical data for representative ultramafic rocks preserved as blocks in the SSM. Structurally, the mélange is a > 2.5 km-thick tectonic unit characterized by a disrupted block-in-matrix fabric with occurrence of exotic blocks, which have igneous and sedimentary protoliths and were variably metamorphosed to high-pressure (P) conditions. There are also blocks of ultramafic rocks, which consist mainly of harzburgite and clinopyroxene (Cpx)-rich harzburgite, with minor lherzolite and pyroxenite, and rare dunite and chromitite. In these rocks, relics of bastite and mesh textures of lizardite are replaced by antigorite blades and overprinted by antigorite schistosity.The presented data give rise to argue the following points. (1) The serpentinite matrix of the mélange resulted from hydration, ductile deformation and metamorphism at high-P conditions of abyssal and supra-subduction zone (SSZ) mantle peridotites in a subduction channel. (2) The geochemical characteristics of the ultramafic protoliths can be explained by at least two-stages of melting and refertilization processes: the first-stage occurred in a mid-ocean ridge (MOR) environment and produced Cpx-rich harzburgite and the second-stage took place in an SSZ setting and resulted in harzburgite, dunite and chromitite. (3) The second-stage magmatic processes produced melts that range in composition between island-arc tholeiites (IAT) and boninites, similar to arc-like protoliths of some mafic blocks in the mélange, which establishes a genetic link between blocks and matrix. (4) The presence of ultramafic and mafic blocks in the SSM, derived from both an island-arc upper plate and a MOR-like lower plate sources, suggests its position in an intra-oceanic subduction zone during the late Jurassic-early Cretaceous. (5) By the Hauterivian, the SSM became exhumed and tectonically emplaced onto the Chortis block as a consequence of the collision between the intra-oceanic arc and the southern continental margin of North America.