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 a new regional large-scale structure, a kinematic analysis of the meso-structures, 40Ar-
39Ar 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-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 an
antigorite schistosity.
The presented data allow us to argue the following points. (1) The serpentinite matrix of the
mélange results 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 harzburgites and the second-stage took place in an SSZ
setting and resulted in harzburgites, dunites and chromitites. (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.