Ultrahigh‐pressure metamorphic rocks and the thermal evolution of continent collision belts

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Abstract
Coesite‐bearing eclogites exposed in the Alpine, Qinling‐Dabie (China), Caledonian, and Ural orogenic belts provide insight into the time‐dependent thermal structure of continent collision belts. Coesite‐bearing eclogites record peak metamorphic temperatures of 550‐900°C at pressures ≥ 2.5 GPa reflecting anomalously cool conditions at depths of 90 km or more. The low temperatures recorded by coesite‐bearing eclogites strongly suggest formation in a convergent plate margin where the downward advection of cool lithosphere depresses isotherms on a regional scale. Subduction zone pressure‐temperature (P‐T) paths calculated using a two‐dimensional finite‐difference model predict steady‐state temperatures of 450‐650°C at 100 km depth at the slab‐mantle interface for convergence rates of 10 to 100 mm/yr. Coesite‐bearing eclogites record peak temperatures ~100‐250°C higher, possibly reflecting (i) formation during the early stages of convergence prior to the achievement of thermal steady state; (ii) attainment of peak metamorphic temperatures during decompression (exhumation); (iii) formation during slow, <10 mm/yr, convergence; or (iv) uncertainties in the modeling parameters. Retrograde P‐T paths determined for coesite‐bearing eclogites from the western Alps and China indicate cooling during decompression from depths of ~100 km. Cooling of eclogite terrains during exhumation requires loss of heat downward into lithosphere that continues to subduct beneath the eclogites, loss of heat upward into the cooler hanging wall of a large‐scale normal fault/shear zone, or a combination of the two scenarios.