SYSTEMATIC GEOCHEMICAL ZONING IN MINERALS FROM HIGH-PRESSURE LOW-TEMPERATURE METAMORPHIC ROCKS: DO UBIQUITOUS PATTERNS REFLECT A UBIQUITOUS PROCESS?

Petrological and geochemical characteristics of rocks metamorphosed at high-pressure (HP) and low temperature (LT) provide vital insight into processes operating in subduction zones, both across geological time and at the timescale of individual subduction cycles. Oscillatory core-to-rim zoning—short wavelength peaks and troughs of elemental concentration—in minerals is a common feature in the global record of exhumed HP–LT metamorphic rock record, and might therefore reflect some common process operating in the vicinity of the slab interface. We explore this using data from blueschist–eclogite blocks from California’s Franciscan Complex, As Sifah, Oman, Syros, Greece and Punta Ballandra, Dominican Republic, and eclogite–amphibolite blocks from Puerto Cabello, Venezuela. All settings host HP–LT garnet containing oscillatory major element (e.g., Mn, Mg) and rare-earth element (REE) zoning that is equivalent between grains of a given sample. Diffusion modelling places an upper bound of < c. 300 Kyr metamorphic cycles for the preservation of these steep compositional gradients in major elements, but it remains unclear why, in some samples,major element oscillations are absent. All garnets in eclogite–amphibolite from Russia’s relatively warm (peak metamorphic conditions of c. 600 ˚C) Maksyutov Complex preserve oscillatory zoning only in REE; those in major elements are interpreted to have been homogenized during post-peak modification and fluid infiltration.

Contrary to long-held opinion, recent work has indicated that this oscillatory zoning in HP–LT rocks may be unrelated to infiltration of externally derived and compositionally distinct fluids during prograde–peak metamorphism. If these ubiquitous geochemical features of subduction zone rocks do not record fluids then how do they form? Do they systematically reflect fluctuating P–T conditions in/around the slab–wedge interface? Equilibrium responses to changing rates of garnet growth? Matrix-wide disequilibrium processes? We correlate directional changes across suites of elements in these Proterozoic subduction zones, and explore these possibilities using phase equilibria and comparative approaches.