Petrochemical Study of Lherzolitic Rocks from the Western Alps

An integrated geologic and petrochemical comparative study offive peridotite complexes in the western Alps has been undertaken.Investigated bodies are exposed at Alpe Arami in southern Switzerland,and at Finero, Balmuccia, Baldissero and Lanzo in northwesternItaly. The Alpe Arami mass has been tectonically emplaced withinthe Lepontine gneiss terrane of the in part subducted, morenortherly European lithospheric plate, whereas the other fourultramafic masses represent original portions of the non-subductedSouth Alpine plate. Eighty samples were examined petrographically. Most are lherzolites,but a few are clinopyroxene-bearing harzburgites, olivine websteritesor clinopyroxenite. Alpe Arami peridotites locally contain primarygarnet (± minor spinel); in contrast, the South Alpinelherzolites are spinel bearing, and in addition, the Lanzo massifcontains widespread plagioclase. All masses display strain effectssuch as bent lamellae in pyroxenes, gradational or sectoraloptical extinction, and minor recrystallization. Twenty-fourbulk XRF analyses demonstrate that the investigated rocks fairlyclosely match pyrolite composition, but are slightly impoverishedin alumina. The normative olivines of all analyzed specimenshave Fa contents ranging between 8 and 10 mole per cent. Electronmicroprobe analyses for 26 olivines, 27 orthopyroxenes, 23 calcicpyroxenes, three garnets, 18 spinels, three plagioclases, 13calcic amphiboles, two chlorites and two phlogopites are presented.Phases in a particular sample are remarkably homogeneous, anobservation consistent with an inferred close approach to chemicalequilibrium. Use of various two-pyroxene geothermometers allows the followingassignments of grand average apparent temperatures for the fiveperidotite complexes: Alpe Arami, 966±78°C; Finero,893 ± 94°C; Balmuccia, 973 ± 50°C; Baldissero,1002 ± 37°C; and Lanzo, 1069 ± 85°C. Pressureestimates, determined using the lherzolitic petrogenetic grid,Al₂O₃^cpx and Al₂O₃^opx isopleths, involve large uncertainties.The Alpe Arami ultramafic body evidently crystallized at a pressureof 40 ± 10 kilobars, the nominal value depending on themethod of computation. In contrast, the South Alpine spinellherzolites seem to have last equilibrated at pressures of approximately5–20 kb. The deep upper-mantle source region of the AlpeArami body apparently was subjected to a subcontinental-typegeothermal gradient, consistent with solid-state rise of thismass and Late Alpine tectonic insertion in the old Lepontinesialic crust of the European lithospheric plate. The mechanismand driving force of this process remain obscure. In contrast,the investigated spinel peridotite complexes of the South Alpinelithospheric plate seem to have been derived from shallow, uppermostmantle sections characterized by oceanic geothermal gradients.For this reason it is conjectured that these massifs were derivedfrom the northern margin of the southern plate where continentalcrust evidently was moderately thin or absent. Their upliftand overthrusting involved P-T paths which in most cases alloweddecompression partial recrystallization and incipient fusion,the latter indicated by the presence of transecting mafic dikesand segregations associated with the peridotites. The Finerobody appears to have reached its present position at the baseof the South Alpine continental crust in pre-Mesozoic time,whereas the Lanzo complex was tectonically involved in the EarlyAlpine orogeny.