FLUID INCLUSION EVIDENCE FOR HYDROTHERMAL ACTIVITY AND DEEP METEORIC WATER CIRCULATION IN THE CORDILLERA BLANCA DETACHMENT, PERU

The Cordillera Blanca detachment (CBD) fault bounds the western flank of the Cordillera Blanca (CB) massif, Peru, and is a well exposed example of active extension in a synconvergent orogen. The CB is situated over the modern flat-slab segment of the Nazca plate, and there is ongoing debate surrounding mechanisms facilitating extension and mid-crustal exhumation in this compressional setting. Contextualizing fluids channeled along fault interfaces during deformation can aid in deciphering processes effecting fault lubrication, fluid sourcing, near-surface connectivity, and reactivation history. Here, we report new petrographic analysis and Raman spectroscopy of 99 fluid inclusions from three samples of mylonitic granodiorite from Quebrada Rajururi. These samples are representative of shallow, intermediate, and deep structural positions into the footwall of the northern segment of the CBD fault. Results provide evidence for infiltration of low salinity, hydrocarbon-bearing meteoric waters during syn-convergent extensional shearing and brittle fracture near the brittle-ductile transition in quartz. Three generations of fluid inclusions were identified, which correlate to depths of 44, 104, and 212 m into the footwall of the 360-m-thick shear zone. The first generation includes secondary to pseudosecondary inclusions that are oriented parallel to the σ₁-σ₂ plane of each section. The second generation of secondary inclusions is approximately normal to the σ₁-σ₂ plane, arguing for preservation of fluid flux through tensional fractures in addition to flow along the foliation plane of the ductile fabric. The third generation is a series of unoriented annular inclusions with significant water loss and relative CO₂ enrichment that are interpreted as relict primary inclusions. Raman spectroscopy of secondary inclusions provides strong evidence for a gradient in fluid composition with respect to depth into the shear zone. Deeper structural positions show increasing prominence of NaCl, KCl, and MgCl₂ on O-H stretching bands. This contrasts with shallower positions with high SO₄ and hydrocarbon peaks. This supports channelized meteoric fluid flux penetrating the ductile segment of the northern portion of the CBD that remained active during early-stage brittle overprinting of quartz.