Importance of crustal relamination in origin of the orogenic mantle peridotite-high-pressure granulite association: example from the Namest Granulite Massif (Bohemian Massif, Czech Republic)

摘要

A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle-crust interface of HP-UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Namest Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 +/- 4.4 and 337.2 +/- 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr-Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT-HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies.