A (meta)sedimentary window into ocean–continent subduction: the Raspas Ophiolitic Complex (SW Ecuador)

Metasedimentary rocks in high-pressure complexes offer a complementary record of subduction processes by capturing mineralogical and chemical features that are highly sensitive to P-T evolution and strongly interact with fluids. Their variability helps constrain the nature and extent of fluid–rock interaction, provenance, and the mechanical and thermal structure of the subduction environment. The Raspas Complex (SW Ecuador), a well-preserved oceanic unit exhumed without continental collision, provides a unique opportunity to evaluate how sediments register burial and peak metamorphic conditions within a cold-subduction setting and how these records compare with those preserved in associated mafic lithologies.

Our integrated approach — combining petrography, bulk-rock and mineral chemistry, thermodynamic modelling, and Zr-in-rutile thermometry — defines a coherent prograde-to-peak metamorphic evolution. Garnet zoning shows strong decoupling between major and trace elements. Fe–Mn–Ca–rich cores and Mg-rich rims define two main stages (prograde, M1; and peak, M2), while HREE–Y distributions preserve a depleted inner core and limited diffusion during growth. Trace-element patterns (e.g., Sc following Mn; V showing the inverse trend; Cr decreasing outward; fracture-hosted enrichments in Zn) reflect episodic release from reacting phases and locally fracture-controlled modifications. Thermodynamic models that account for garnet fractionation constrain a clockwise P–T path from ~525 °C, 17–18 kbar (M1) to ~570 °C, ~21.5 kbar (M2). Zr-in-rutile temperatures of 483–630 °C (at 12–25 kbar) are consistent with the independently modelled P–T conditions. Retrograde chlorite at garnet rims and fractures marks subsequent cooling and decompression.

The results demonstrate that sedimentary slices can retain discrete growth stages and subtle overprints that complement the metamorphic information recorded in mafic blocks. Together, these data refine the thermal structure, fluid regime, and burial–exhumation dynamics of the Raspas subduction system. Forthcoming U–Pb in-situ dating of key phases, garnet oxygen-isotope analyses, diffusion-based modelling, and integration with parallel results from the metamafic rocks will further constrain the rates and conditions of subduction and exhumation, advancing reconstructions of deep-crustal recycling in cold-subduction settings.