GEOCHEMICAL CONSTRAINTS ON THE MULTISTAGE MELT MIGRATION THROUGH OPHIOLITIC PERIDOTITE OF LANZO SOUTH BODY (WESTERN ALPS)

Abstract

The ophiolitic peridotites of Lanzo Massif (Sesia-Lanzo Zone, Western Alps) record an amazing structural, petrologic and geochemical complexity mainly established during the Jurassic rifting stages of the Ligurian branch of the Tethys ocean. In the southernmost Lanzo body, the following three main peridotite units have been recognised: i) the “reactive” spinel peridotites; ii) the “impregnated” plagioclase peridotites; iii) the “replacive” dunite-harzburgite bands and bodies (Piccardo et al., 2005). Ongoing geochemical investigation has revealed the presence of marked heterogeneity in terms of trace element composition of bulk rock and minerals, which enables us to place valuable constraints on the change of the physical and chemical parameters governing the melt migration through an upwelling mantle. Most of clinopyroxenes (Cpx) from “reactive” spinel peridotites show C1-normalised REE patterns progressively varying from strongly LREE-depleted (CeN ~0.35, CeN/SmN ~0.06, CeN/YbN ~0.14, with YbN ~9) to sinusoidal L/MREE-enriched (CeN = 4.3-5; LaN/NdN ~0.50; NdN/HoN ~3.0; YbN ~2). A further peculiar clinopyroxene composition is characterised by strongly humped REE patterns with maximum at EuN (>12) and very large MREEN/LREEN (CeN = 0.3; CeN/SmN = 0.04) and MREEN/HREEN (GdN/LuN = 3.85; YbN = 2.7) ratios. Petrologic and geochemical modelling suggests that the composition of the “reactive” spinel peridotites records the interaction between melt increments of fractional melting and a strongly refractory mantle column. The latter had a geochemical composition very depleted, which approached, at least for the moderately incompatible elements (e.g. HREE), that expected for residual peridotite after 15-20% of fractional melting under spinel facies condition. The heterogeneity of the trace element composition of clinopyroxenes mainly records the development of transient geochemical gradients affecting the percolating melt(s) as a consequence of the reaction with the host peridotite. Melt fractionation was mainly governed by chemical exchange between melt and peridotite minerals, although the effects of the assimilation of peridotite minerals or of the melt mass reduction due to fractional crystallisation are subordinately apparent. Complete geochemical recovering was only locally attained. Where it occurs, trace element composition of the clinopyroxene unravels the injection of melt increments produced by 5% of fractional melting of DMM placed at both garnet-and spinel-facies condition. The occurrence of melt increments produced by lower degrees of fractional melting (<1% to 4%) is suggested by the findings of less LREE-depleted clinopyroxene composition. As a whole, geochemical modelling also suggests that this event developed under low melt/rock (porosity = <1-3%) conditions and was assisted by the mineral/melt partition coefficients considered typical for spinel-facies mafic-ultramafic system at high-T condition (e.g. those listed by Ionov et al., 2002). Field evidence demonstrates that the “reactive” spinel peridotites were the ambient lithology during the melt migration leading to the plagioclase crystallisation. Clinopyroxenes from plagioclase peridotites are relatively enriched in REE, HFSE and Sc with respect to those of “reactive” spinel peridotites. They have humped REE patterns, which are characterised by strong to moderate LREE depletion (LaN/SmN = 0.005-0.25; LaN/YbN = 0.006-0.38) and M-HREEN up to 30. Plagioclase composition is rather variable, with An in the range of 67-90. Na decrease is matched by a decrease in Sr (from 154 to 5.7 ppm), LREE and LREE/M-HREE ratios (LaN/SmN = from 3.6 to 0.10). Geochemical features indicate that the migrating melts were yet the product of £ 5% fractional melting of spinel-facies DMM. During the segregation of the plagioclase-bearing mineral assemblage, the importance of the interstitial fractional crystallisation became definitely the dominant, according to a significant change of both intensive and extensive parameters of the system with respect to the stage recorded by the “reactive” spinel peridotites. Geochemical modelling also suggests that the decrease of melt mass produced by the fractional crystallisation was accompanied by a significant increase of both porosity and mineral-melt partition coefficients to account for the large trace element content shown by bulk rock and mineral chemistry. Most of the concordant and all the discordant “replacive” harzburgite-dunite bodies possess Cpx showing a trace elements composition consistent with a their segregation from aggregate MORB s.l. (LaN/SmN = 0.1-0.26; LaN/YbN ~0.07- 0.25; YbN ~7-10. Hypothetical liquid in equilibrium with cpx range from N-MORB to compositions slightly enriched in highly incompatible elements (LREE). The N-MORB Cpx are especially found in harzburgites, whereas the LREE-enriched compositions occur in dunite channels. This heterogeneity can be explained by two alternative processes: a) an early N-MORB injection was followed by upward migration of melts with T-MORB affinity; b) harzburgites record mainly the formation of the replacive bodies, which were determined by N-MORB, whereas dunite also record late differentiation processes affecting the residual melts during the cpx crystallisation process. Clinopyroxenes from concordant “replacive” bodies record a large chemical heterogeneity pointing to more complex history. In fact, harzburgitic-dunite bodies with cpx showing N-MORB geochemical character are associated with harzburgite-dunite bodies with cpx showing, alternatively, “depleted” or “strongly enriched” geochemical character. “Depleted” clinopyroxenes from “replacive” harzburgites have a geochemical composition characterised by low to very low content of moderately incompatible elements, such as HREE, Y, Ti and V (e.g. YbN = 7.1-1.9). The content in highly incompatible elements, such as LREE, is low (e.g. LaN = 1.2-0.4). However, it is less depleted and fractionated (LaN/NdN = 0.5-1) than that expected on the basis of the HREE concentration level. As a whole, the composition of the moderately incompatible elements is by far lower than those documented in Cpx crystallised in equilibrium with MORB s.l., being much more similar to those modelled for Cpx in refractory residual after large degrees of fractional melting (in the range 8-18%). This data indicates that focalised melt percolation induced a dissolution process of the mantle minerals that had geochemical features similar to those related to fractional melting. This process must have been accompanied by a significant increase of the whole melt mass. “Strongly enriched” clinopyroxenes from replacive channels are characterised by large content in all the highly incompatible elements, such as LREE, Sr, Nb, Ta, Th, U, Zr and Hf. Conversely, HREE and Y contents are consistent with those of Cpx from MORB. Relatively large concentration are also shown by moderately incompatible elements, such as Ti, Sc and V. These fractionations cannot be related to any kind of primitive melt and are interpreted as the product of the percolation small volume of extremely differentiated melts, in which highly compatible elements are concentrated by the effects of fractional crystallisation, whereas the moderately ones are more efficiently buffered by chemical exchange with the ambient peridotites. This crystallisation stage likely represented the last breath of the MORB activity in the dunite channels.