Metamorphic evolution of a very low- to low-grade metamorphic core complex (Danubian window) in the South Carpathians

Abstract

Abstract The Danubian window, characterized by diagenetic to low greenschist facies conditions at a high thermal gradient, is evidently of great interest for methodological studies, because high metamorphic thermal gradient conditions during low grade metamorphism have received little attention so far. The general increase in metamorphic grade from SW to NE in the Danubian window is indicated by mineral Parageneses studies, as well as by illite Kübler index (KI) measurements and organic matter reflectance (OMR). For the first time, this study distinguishes between metamorphic conditions related to Jurassic ocean floor, Cretaceous nappe stacking, post-collisional accommodation and syn-kinematic Getic detachment metamorphism and cooling after Oligocene exhumation. The occurrence of the prehnite–pumpellyite facies in the Severin–Cosustea units in the southeastern area is the result of Cretaceous metamorphism. Remnants of ocean floor metamorphism prevailed. The highest pressure is constrained by the upper stability limit of prehnite to be at around 4.0 kbar. The Danubian units situated within the diagenetic zone were not below 200 °C, due to epidote formation. The KI, OMR and mineral data, indicate diagenetic conditions. Assuming temperatures between >200 and <250 °C, pressures between 1.8 and 2.6 kbar were calculated using kinetic and numerical maturity models. Orogenic collisional Cretaceous peak pressure conditions of 4.0±1.0 kbar are found in the Danubian nappes not altered by a subsequent syn-detachment metamorphic overprint. Highest temperatures in chloritoid schists and epidote–hornblende-bearing mylonites have been inferred for samples from the northern border of the Danubian window (between >300 and <400 °C). Along a syn- to post-detachment retrograde pressure path, post-dating the chloritoid formation, the occurrence of clinozoisite+chlorite+quartz suggests temperatures >300 °C in the northwest, while the association andalusite+quartz and biotite+muscovite indicates temperatures between 370 and 400 °C at <3.5 kbar in the northeast. It is demonstrated that the slope of the regression lines between KI and OMR data gives valuable qualitative information about the relative magnitudes of P and T: the slope of the regression line for the Danubian window samples indicates normal heat flow conditions during nappe stacking and hyperthermal conditions during the formation of the Getic detachment. High thermal gradient conditions can easily be explained by partly isothermal decompression during the Getic detachment event, the elevation of the geotherm being caused by crustal thinning and rapid exhumation of the Danubian units. Probably, also a higher heat-flux prevailed at the end of the Getic detachment, at a time when the retrograde chloritoid decomposition reactions took place, documenting late-stage HT greenschist facies metamorphism.