35 Petrology, Metamorphic

Sunday, 16 October 2005
3:30 PM - 5:30 PM

PETROLOGIC CHARACTERIZATION OF GUATEMALAN LAWSONITE ECLOGITE: ECLOGITIZATION OF SUBDUCTED OCEANIC CRUST IN A COLD SUBDUCTION ZONE

Paper number: 35-5
Program: Technical Sessions
Time: 4:30 PM – 4:45 PM

Authors

Abstract

Petrologic and microtextural analyses of Guatemalan lawsonite eclogite reveal three metamorphic stages formed during four deformational phases. The prograde eclogite stage represents an incipient eclogitization below 300°C and is preserved mainly in garnet, along with older S1-S2 foliation. The prograde eclogite-facies assemblage is Grt (XMg= ~0.22) + Omp (jd~52) or Jd (jd~83) + Lws + Rt + Qtz ± Phe (3.65 Si) ± Chl. The presence of syn-metamorphic brittle deformation, precursor pumpellyite inclusions within lawsonite in garnet, Fe2+-Mg distribution coefficients between omphacite inclusions and adjacent garnet with Ln(KD)= 2.7–4.5, and Grt–Cpx–Phe thermobarometry suggest that eclogitization initiated at T= ~300°C and P> 1.1 GPa, and continued to T= ~480°C and P= ~2.6 GPa. In contrast, the retrograde eclogite-facies assemblage is characterized by reversely zoned garnet rims and Omp ± Gln + Lws + Rt + Qtz ± Phe (< 3.5 Si) along S3 foliation. Grt–Cpx–Phe thermobarometry yields P= ~1.8 GPa and T= ~400°C. The youngest, garnet-bearing lawsonite blueschist-facies assemblage (Gln + Lws + Chl + Ttn + Qtz ± Jd ± Phe) locally replaces earlier mineral assemblages along S4 crenulations.

The inferred prograde P–T trajectory lies near a geotherm of ~5°C km-1, comparable to the calculated thermal and petrologic structures of the present-day NE Japan subduction zone. These petrologic characteristics indicate: i) the basalt–eclogite transformation may have occurred at T= ~300°C in a cold subduction zone, ii) glaucophane-bearing prograde assemblages are rare during incipient eclogitization in cold subduction zones (abundant retrograde glaucophane formed instead by hydration during exhumation), and iii) the chlorite-consuming reactions to form Fe–Mg–Mn garnet are more effective than the lawsonite-consuming reaction to form the Ca component. At depths of ~100 km in cold subduction zones, dehydration embrittlement may be caused by such chlorite-consuming reactions.