Eighteen unpublished whole-rock chemical analyses on Iblean deep-seated xenoliths, combined with twenty-nine analyses reported in previous papers, form a data set representative of the various xenolith types as fixed by field and laboratory investigations lasting more than ten years. All Iblean xenoliths can be split into two main groups: ultramafic (UL) and feldspar-bearing (FB). The former, mostly representing the lithospheric mantle, consists of peridotites (UL-a) and various pyroxenites (UL-b-c-d). The peridotites exhibit a moderately refractory character, with some enrichment in LREE and other incompatible elements due to metasomatic event(s). The pyroxenites (spinel-websterites in most cases) derive from crystallisation and later sub-solidus re-equilibration of basic melts intruding the peridotites at different mantle depths. We distinguish Cr-rich and Al-poor pyroxenites (group UL-b, green) from those Cr-poor and Al-rich (groups UL-c, UL-d, black). Only the latter group keep their original igneous texture, but minor sub-solidus transformations occur. Garnet is found in some Al-rich pyroxenites, especially in those of group UL-d. The garnet, where modally abundant, controls the whole-rock distribution of some trace elements, especially REE. Fully recrystallized two-pyroxene granulites are the most common feldspar-bearing lithotypes (FB-e) together with minor cumulitic gabbros (group FB-f, variously recrystallized anorthosites (FB-g), rare metadiorites (FB-h) and plagioclase-websterites (FB-i). The representative whole-rock chemical compositions confirm such a division on a petrographic basis for most of the FB xenoliths. The chemical data set considered here, coupled with estimates of relative abundances of the various xenolith groups, may suggest the lblean upper-mantle and inaccessible crust bulk compositions. The first exhibits a Primordial Mantle composition, due to the refertilizing effect of pyroxenites on the barren peridotites. Instead, the crust shows a roughly basaltic composition. Assuming that the Iblean xenoliths adequately represent all parts of the lithospheric column, which is probable for diatreme-related xenoliths, there is a marked absence of any felsitic upper crustal lithology. This fact may depend on (1) extreme lamination of a previous continental crust by rifting processes and concomitant magmatic intrusions and underplating, (2) severe erosion of the upper continental crust during an (unknown) period of exhumation, (3) the oceanic nature of the Iblean lithosphere.
|Number of pages||20|
|Journal||Periodico di Mineralogia|
|Publication status||Published - 2000|