MINERALOGICAL AND GEOCHEMICAL STUDY OF GRANULAR XENOLITHS FROM THE ALBAN HILLS VOLCANO, CENTRAL ITALY - BEARING ON EVOLUTIONARY PROCESSES IN POTASSIC MAGMA CHAMBERS

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ul tramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alk alies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is high ly magnesian (up to Fo% = 96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indica te a genesis of ultramafic ejecta by thermal metamorphism of a siliceo us dolomitic limestone, probably with input of chemical components fro m potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive eq uivalents of lavas or cumulates crystallized from variably evolved ult rapotassic magmas. One sample of the former group has major element co mposition resembling ultrapotassic rocks with kamafugitic affinity. So me cumulitic rocks have exceedingly high abundances of Th (81-84 ppm) and light rare-earth elements (LREE) (La+Ce = 421-498 ppm) and extreme REE fractionation (La/Yb = 288-1393), not justified by their modal mi neralogy which is dominated by sanidine, leucite and nepheline. Fine-g rained phases are dispersed through the fractures and within the inter stices of the main minerals. Semiquantitative EDS analyses show that T h and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the h igh concentration of incompatible trace elements in the whole rocks. T he interstitial position of these phases and their association with fl uorite support a secondary origin by deposition from fluorine-rich flu ids separated from a highly evolved potassic liquid. The Nd isotopic r atios of the ejecta range from 0.51182 to 0.51217. Sr-87/Sr-86 ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster a round 0.7105 +/- 3. This indicates either the occurrence of several is otopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires select ive assimilation of host rocks in order to explain isotopic and geoche mical characteristics of lavas and xenoliths. The new data indicate th at the evolutionary processes in the potassic magmas of the Alban Hill s were much more complex than envisaged by previous studies. Interacti on of magmas with wall rocks may be an important process during magmat ic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during th e latest stages of magmatic evolution. Such a process was able to prod uce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurren ce of xenoliths with kamafugitic composition points to the existence o f this type of ultrapotassic magma at the Alban Hills.