Fluid-rock interaction in metamorphosed andesitic dikes, Elat metamorphic complex, Israel

Basic dikes intruded Precambrian high T/P metamorphic rocks of the Elat Complex, and subsequently were metamorphosed and deformed. This work explores the fluid-rock interaction during metamorphism and deformation through petrographic, mineralogical, and oxygen isotope study of a profile across a 3.9-m-wide andesitic dike and its granite gneiss host rock. Samples from the center of the metamorphosed dike are undeformed, and relic igneous minerals are preserved. However, the dike margins show sheared, foliated textures, characterized by preferred orientation of elongated biotite grains and a matrix of polygonal quartz, feldspar, and amphibole. Correspondingly, the granite gneiss host rock transforms from a weakly deformed rock to a highly lineated, often schistose, rock as the contact with the dike is approached. The textural changes are accompanied by contemporaneous metamorphic mineralogical changes. The most important evidence for fluid infiltration from the schistose dike into the granite gneiss comes from the oxygen isotope studies of quartz and biotite separates. The dike has a uniform δ¹⁸O quartz value of about 13.5‰ (SMOW). These values are also found in the granite gneiss at the contact before dropping to lower δ¹⁸O values (12.9‰ at the southern contact; 12.2‰ at the northern contact). The quartz isotopic systematics are mirrored by those of the biotite, with the result that the quartz-biotite oxygen isotope fractionation shows clear metamorphic resetting at ca. 500 °C in the schistose dike and the immediate 0.5-1 m of granite gneiss. The δ¹⁸O quartz profile at the southern contact was modeled using a one-dimensional transport equation and indicates that the component of fluid infiltration normal to the contact advected 0.60 m into the granite gneiss. Corresponding integrated (total) fluid flux was about 1 m³/cm², connected porosity 2 x 10^-5 and integrated time of infiltration about 2000 a. The study suggests a synergetic relationship between water, metamorphism, and deformation. Ductile shearing is promoted by the presence of water (hydraulic weakening of rock to plastic deformation), while at the same time, the deformation increases permeability of the rock to fluid infiltration.