Metamorphic reactions, deformation localization and rheological evolution of gabbro to eclogite transformation: A case study from Lofoten Anorthosite Complex, Norway

Eclogitization of lower crustal rocks during subduction processes and subsequent retrogression during exhumation will produce a range of mineral assemblages over widely spaced P-T conditions affecting the rheological behavior of the rock significantly. In this contribution, we describe the formation of syn-kinematic mineral assemblages which promotes development of centimeter- to meter-scale shear zones in undeformed leucogabbro from Lofoten, Norway. Plagioclase (Pl₀), olivine (Ol₀) and orthopyroxene (Opx₀) form the igneous texture in the leucogabbro. Opx₁ and Grt₁ corona indicate the onset of metamorphism (M₁) during subduction processes. Cm- wide alternating bands of a mixture of Pl-Amphibole(Amph) ± Spl and Grt_I(M₁) - Opx -Omph (M_2A) phase mixtures characterize prograde metamorphic shear zones in the leucogabbro. Elongated Omph overgrow deformed Opx grains (D₁). Relicts of Opx grains are surrounded by recrystallized opx indicating that the Opx grains survived in the Omph stability field. Grt- Cpx - Pl symplectite (M_2B) rim the Omph± Opx porphyroblasts. Grt_II(M₃) occurs along the contact of deformed Opx± Omph and Pl. Amph ± Spl (M₄) overgrow the finer grained pyroxene. Conventional thermobarometry indicate that metamorphism (M₁) starts at 700⁰C at 1 GPa pressure. The peak PT-condition (650-750⁰C, > 1.8 GPa) was attained during the formation of Omph. The retrograde PT-path is represented by Cpx-Pl symplectite (600-650⁰C, 1 GPa) and Grt corona (600⁰C, 0.8 GPa). Replacement of fine-grained Cpx by Amph represents the last stage of metamorphism. Crystal orientation of Opx relicts is different from the CPO of the newly formed Opx (10 - 15 μm in size) indicating that Opx did not recrystallize by progressive subgrain rotation. The dramatic grain size reduction without significant change in chemical composition is probably caused by cracking of Opx at the onset of deformation. Small new grains may nucleate from fragments. In the fine grained Pl+ Amph± Spl aggregates, the random CPO of Pl suggests dominant diffusion creep deformation. The CPO of Amph suggests that Amph re-orients by rigid body rotation during deformation forming aligned (100) planes with the shear plane. Mineral chemistry and EBSD analysis shows that shear zone formation in the gabbro is associated with initial stage of cracking and grain size reduction under water deficient conditions. The cracking of Opx at lower crustal conditions (700 ^OC, 1 GPa) requires high differential stresses which indicate a higher strength of leuco-gabbro at the onset of subduction. Absence of newly formed Opx in the Omph corona suggests that cracking and recrystallization of Opx begins before peak PT-conditions. During exhumation, Omph is replaced by fine grained Cpx+Pl. The drastic change in grain size due to breakdown of Omph and replacement of Cpx by Amph facilitates diffusion creep deformation that may accommodate large strains during deformation. This study demonstrates the change in deformation mechanism from initial cracking of orthopyroxene to grain size sensitive diffusion creep due to high-pressure metamorphism and fluid infiltration.