New, high-precision P-T estimates for Oman blueschists: implications for obduction, nappe stacking and exhumation processes

摘要

Oman blueschists and eclogites lie below the obduction nappe of the Semail ophiolite in one of the key areas on Earth for the study of plate convergence. Here new metamorphic and tectonic constraints are provided for the central, yet poorly constrained Hulw unit, sandwiched between the low-grade units (similar to 10 kbar, < 300 degrees C) and the As Sifah eclogites (P-max similar to 23 kbar; T-max similar to 600 degrees C).TWEEQU multi-equilibrium thermobarometry, using both compositional mapping and spot analyses, and Raman spectroscopy of carbonaceous material yield a high-precision P-T path for the Hulw and As Sheikh units and reveal that they shared a common P-T history in four stages: (i) a pressure decrease from 1012 kbar, 250-300 degrees C to 7-9 kbar, 300-350 degrees C; (ii) almost isobaric heating at similar to 8-10 kbar from 300-350 degrees C to 450-500 degrees C; (iii) a pressure decrease at moderate temperatures (similar to 450-500 degrees C); and (iv) isobaric cooling at similar to 5-6 kbar from 450-500 to 300 degrees C. No significant pressure or temperature gap is observed across the upper plate-lower plate discontinuity to the north and west of the Hulw unit. The combination of tectonic and P-T data constrains the stacking chronology of the three main metamorphic units comprising the Saih Hatat window (i.e. the Ruwi-Quryat, the Hulw-As Sheikh and the Diqdah-As Sifah units). These results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple, N-vergent continental subduction of the passive Arabian margin below the obduction nappe along a cold P-T gradient.