An integrated geologic and petrochemical comparative study of five 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 northwestern Italy. The Alpe Arami mass has been tectonically emplaced within the Lepontine gneiss terrane of the in part subducted, more northerly European lithospheric plate, whereas the other four ultramafic masses represent original portions of the non-subducted South Alpine plate. Eighty samples were examined petrographically. Most are lherzolites, but a few are clinopyroxene-bearing harzburgites, olivine websterites or clinopyroxenite. Alpe Arami peridotites locally contain primary garnet (± minor spinel); in contrast, the South Alpine lherzolites are spinel bearing, and in addition, the Lanzo massif contains widespread plagioclase. All masses display strain effects such as bent lamellae in pyroxenes, gradational or sectoral optical extinction, and minor recrystallization. Twenty-four bulk XRF analyses demonstrate that the investigated rocks fairly closely match pyrolite composition, but are slightly impoverished in alumina. The normative olivines of all analyzed specimens have Fa contents ranging between 8 and 10 mole per cent. Electron microprobe analyses for 26 olivines, 27 orthopyroxenes, 23 calcic pyroxenes, three garnets, 18 spinels, three plagioclases, 13 calcic amphiboles, two chlorites and two phlogopites are presented. Phases in a particular sample are remarkably homogeneous, an observation consistent with an inferred close approach to chemical equilibrium. Use of various two-pyroxene geothermometers allows the following assignments of grand average apparent temperatures for the five peridotite complexes: Alpe Arami, 966±78°C; Finero, 893 ± 94°C; Balmuccia, 973 ± 50°C; Baldissero, 1002 ± 37°C; and Lanzo, 1069 ± 85°C. Pressure estimates, determined using the lherzolitic petrogenetic grid, Al2O3cpx and Al2O3opx isopleths, involve large uncertainties. The Alpe Arami ultramafic body evidently crystallized at a pressure of 40 ± 10 kilobars, the nominal value depending on the method of computation. In contrast, the South Alpine spinel lherzolites seem to have last equilibrated at pressures of approximately 5–20 kb. The deep upper-mantle source region of the Alpe Arami body apparently was subjected to a subcontinental-type geothermal gradient, consistent with solid-state rise of this mass and Late Alpine tectonic insertion in the old Lepontine sialic crust of the European lithospheric plate. The mechanism and driving force of this process remain obscure. In contrast, the investigated spinel peridotite complexes of the South Alpine lithospheric plate seem to have been derived from shallow, uppermost mantle sections characterized by oceanic geothermal gradients. For this reason it is conjectured that these massifs were derived from the northern margin of the southern plate where continental crust evidently was moderately thin or absent. Their uplift and overthrusting involved P-T paths which in most cases allowed decompression partial recrystallization and incipient fusion, the latter indicated by the presence of transecting mafic dikes and segregations associated with the peridotites. The Finero body appears to have reached its present position at the base of the South Alpine continental crust in pre-Mesozoic time, whereas the Lanzo complex was tectonically involved in the Early Alpine orogeny.
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E. Ernst (1978) studied this question.