Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Peru (Tarma province)

摘要

High-pressure conditions of 11-13 kbar/500-540 ℃ during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm-Nd mineral-whole rock isochron at 465 ± 24 Ma dates fluid influx at peak temperatures of ～600 ℃ and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12-13 ℃/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Maranon Complex. Here, low PT conditions at 2.4-2.6 kbar, 300-330 ℃ were estimated for a phyllite-greenschist assemblage representing a contrasting metamorphic geotherm of 32-40 ℃/km characteristic for a magmatic arc environment.