Geochemical considerations of the gehlenitic skarns from Valea Crişenilor, Oraviţa (Romania)
Résumé
A shallow-level pluton of Late Cretaceous age, belonging to the "banatitic" magmatic and metallogenetic belt, caused extensive contact methamorphism of Cretaceous limestones and marls sequences in the Oraviţa area. In Valea Crişenilor, a small elongate apophysis of this pluton mainly dioritic, with some variations toward quartz diorite and monzonite, intruded a series of calcareous deposits of Crivina Formation deposed in Reşiţa anticlinorum. The intruded formations experienced an extended contact metamorphism with local generation of high-temperature mineral species. The observed mineral assemblages reflect a wide range of temperature and fluid-composition space. In this respect, the metamorphic peak assemblages in the calcite-saturated rocks include wollastonite, melilite, ellestadite-(OH), titanian grossular and scarce spurrite. The estimated equilibra temperature for this mineral assemblage is about 820-870°C with a C-rich, internally buffered pore-fluid. The aperence and infiltration of the C-poor fluids, triggering the formation of retrograde mineral assemblages comprising monticellite, vesuvianite, grandite garnets, diopside, clintonite. These assemblages formed probably near 600°C. A low-temperature retrogression affected the previous mineral paragenesis producing scawtite, hydrogrossular, hibschite, xonotlite, thomsonite. As it is expected, the intensity of the contact metamorphism decreases from the innermost to the outermost parts of the aureole. In this respect, the mineral reactions involving carbonate and silicate minerals led to the formation of various of Si-poor silicates, but rich in Al and Ti oxides, while the next skarn zone can be circumscribed to a Si- , Mg-rich and Al-poor geochemical system, marked by the formation of gehlenite, vesuvianite ± wollastinite, garnet. The Ca-Si ratio value is close to that of Mg-Si, suggesting a weak circulation of the fluids and a thermodynamic closed system, followed by the retrograde stage, marked by a high mobility of the chemical elements into an open system.