Metamorphic reactions and textural changes in coronitic metagabbros from the Teplá Crystalline and Mariánské Lázně complexes, Bohemian Massif

Coronitic metagabbros occur as small isolated bodies along the contact between the Mariánské Lázně and the Teplá Crystalline complexes in the northwestern part of the Teplá-Barrandian Unit in the Bohemian Massif.Metagabbros show variable metamorphic and textural characteristics with respect to their magmatic mineral assemblage and degree of metamorphism.The aim of this study has been to characterize the mineralogical, chemical, and microstructural changes related to metamorphism in selected samples.Magmatic mineral assemblages in the metagabbros represented by plagioclase, orthopyroxene, clinopyroxene, amphibole, biotite, ilmenite and occasionally involving olivine, spinel or quartz are generally well preserved.Metamorphism is mainly reflected by the formation of single or multiple coronas at the contact of plagioclase with the other primary minerals.The coronas typically consist of amphibole, garnet and orthopyroxene.The progressive breakdown of magmatic plagioclase is reflected by the formation of a mixture of An 40 and An 90 plagioclase associated with spinel, corundum and locally also kyanite.The calculated P-T conditions show an increase in metamorphic grade towards the structural footwall, i.e. from the east-southeast (~600 ± 50 °C; 10 ± 1.5 kbar) to the west-northwest (~700 ± 50 °C; 13.5 ± 1.5 kbar), which corresponds to the previously reported Variscan metamorphic field gradient in this area.Since there is no gap in P-T conditions between metagabbros included in MLC and TCC and showing similar age and geochemical signature, it is concluded that both complexes were brought together before the gabbro intrusion at ~500 Ma.In addition, the maximum pressure of ~14 kbar estimated for the metamorphism of the gabbro occurring in the eclogite-bearing Mariánské Lázně Complex suggests that the eclogite-facies metamorphism might have been pre-Variscan.The chemistry of the studied metagabbros corresponds to subalkaline basalts with traceelement signatures characteristic of E-MORB, which is consistent with an interpretation that the intrusion of these rocks was related to an intracontinental rifting of the Teplá-Barrandian Unit during Late Cambrian and was not connected to any subduction processes.