Primary positive Eu anomaly in clinopyroxenes of low-crust gabbroic rocks

Positive Eu and Sr anomalies have been found in whole rocks and clinopyroxenes, as well as feldspars, in gabbros of the deep crustal mafic intrusion of the Ivrea Zone (northwestern Italy). These anomalies are particularly pronounced at one stratigraphic level of the intrusion, the upper part of the Upper Zone of the Layered Series (HUZ). The Eu/Eu* ratios range from 19 to 40 in plagioclases and from 3 to 14 in the coexisting clinopyroxenes. Mass balance calculations show that metamorphic redistribution of Eu from plagioclase cannot account for the positive Eu anomalies of the clinopyroxenes and that a corresponding anomaly in the parent melt is required. Speculatively, this feature is attributed to mixing of a mantle component with crustal, highly polymerized, melts deriving from advanced anatexis of granulite-facies metasediments. The existence in the deep crust of hybrid melts having a positive Eu anomaly may be important for the Eu balance between the lower and upper crust.