Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Abstract

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value ⩾ 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO 2, Al 2O 3, Na 2O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al 2O 3, CaO and variable enrichment in K 2O and incompatible elements. LMP rocks are saturated in silica and have high SiO 2, K 2O, K 2O/Na 2, MgO, Ni and Cr and low Al 2O 3, CaO, Na 2O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS. All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K 2O, whereas different trends of Sr and HFSE vs. K 2O are defined by LMP-TRANS and KS-HKS-KAM. 87Sr/ 86Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar 87Sr/ 86Sr values clustering around 0.710. LMP and TRANS rocks have the highest 87Sr/ 86Sr values. Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes. Different trends of incompatible elements vs. K 2O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na 2O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.