Minette inclusions in the rhyodacitic lavas of Mt. Amiata (Central Italy): Mineralogical and chemical evidence of mixing between Tuscan and Roman type magmas

Abstract

Abstract Major-element, trace-element and mineralogical data are reported for a series of mafic inclusions occurring in the rhyodacitic lavas of Mt. Amiata (southern Tuscany, Italy), a volcanic complex belonging to the Tuscan Magmatic Province. The inclusions form, together with some subordinate mafic latitic lavas and the host rock rhyodacites, a continuous series, which varies from olivine and minor nepheline normative to quartz, hypersthene and corundum normative rock-types over a range of 48–67% SiO 2 . All of the rocks are characterized by high contents of potassium (5–6% K 2 O) and many trace elements, particularly the incompatible elements. Following straight-line correlation trends, Al, Fe, Mg, Ca, Ti, P, Sr and Ba decrease with increasing SiO 2 , whereas Na and Li increase, and K, Rb, Zr, La, Ce, Nb and Y remain virtually constant. The primary mineral assemblage of the inclusions consists of diopsidic pyroxene, forsteritic olivine and mica, and is representative for minettes. Xenocrysts of plagioclase, sanidine, orthopyroxene clinopyroxene, biotite, ilmenite and olivine occur in increasing amounts in the more siliceous samples. The composition of these xenocrysts is identical to that of the phenocryst assemblage in the rhyodacites. The latitic lavas are rich in xenocrysts and differ from the minettes mainly by the absence of primary mica. The data strongly suggest a magma mixing origin for the minettes, latites and possibly part of the rhyodacites. Injection of mafic magma into a siliceous magma chamber has resulted in the formation of hybrid inclusions and a latitic magma that could only reach the surface after eruption of the voluminous rhyodacitic lavas. The rhyodacites which erupted early and are slightly richer in SiO 2 than most of the other rhyodacites are considered to represent the acid mixing component, whereas the mafic end-member has close affinities to the potassic alkaline lavas of the adjacent Roman Province. The inclusions have the modal composition of a minette and this implies that volatiles were important constituents of the hybrid magmas. This may be attributed to the primary character of the Roman-type end-member involved, or explained by a process of selective enrichment operating during mixing. Although upper mantle derived Roman magmas have interacted with Tuscan magmas of crustal anatectic origin at Mt. Amiata and probably other volcanic centres in central Italy, the specific chemistry of lavas in both provinces cannot be explained by simple mixing of these magmas in shallow chambers.