Metamorphic reactions and textural changes in coronitic metagabbros from the Tepla Crystalline and Marianske Lazne complexes, Bohemian Massif
Abstrakt
Coronitic metagabbros occur along the contact between the Mariánské Lázně and the Teplá Crystalline complexes in the NW part of the Teplá-Barrandian Unit in the Bohemian Massif. They show variable metamorphic and textural characteristics with respect to their magmatic mineral assemblage and degree of metamorphism.
Magmatic mineral assemblages represented by plagioclase, orthopyroxene, clinopyroxene, amphibole, biotite, ilmenite, olivine, spinel or quartz are well preserved. Metamorphism is reflected by the formation of coronas at the contact of plagioclase with the other primary minerals.
The coronas consist of amphibole, garnet and orthopyroxene. The breakdown of magmatic plagioclase is reflected by the formation of a mixture of An(40) and An(90) plagioclase associated with spinel, corundum and kyanite.
The calculated P-T conditions show an increase in metamorphic grade towards the structural footwall, i. e. from the ESE (-600 +/- 50 degrees C; 10 +/- 1.5 kbar) to the WNW (-700 +/- 50 degrees C; 13.5 +/- 1.5 kbar), which corresponds to the 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 trace element 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.