Continental to Oceanic Mantle Transition—REE and Sr-Nd Isotopic Geochemistry of the Lanzo Lherzolite Massif

J L Bodinier, Martin Menzies, M Thirlwall

Research output: Contribution to journalSpecial issuepeer-review


The Lanzo peridotite, Italian Alps, is a classic plagioclase lherzolite massif emplaced during the opening stages of the Liguro-Piemontese ocean basin, in the early Mesozoic. It consists of three bodies (i.e., northern, central and southern) separated by shear zones. Clinopyroxenes separated from
plagioclase lherzolites from the southern body show a limited range in present-day values of i43Nd/i44Nd (0-5131-0-5133) and 87Sr/86Sr (0-7022-0-7027), and plot within the field of Atlantic mid- ocean ridge basalt (MORB) asthenosphere. Clinopyroxenes from the northern body have higher 143
Nd/144Nd ratios (0-51335 and 0-51342) and a very low 87Sr/86Sr ratio (0-70180). Clinopyroxenes from the central body have isotopic ratios intermediate between these two extremes. The southern body is interpreted as asthenosphere that rose from the stability field of garnet as a high-temperature diapir. This evolution was accompanied by a significant degree of melt extraction
(6-12%, locally up to 20%). In contrast, the northern body is considered to be a fragment of the continental lithosphere (i.e., mechanical boundary layer) which became isolated from the convective mantle 400-700 m.y. ago. The central body is interpreted as a mantle hybrid which contains components from the sub-continental lithosphere modified by small-volume melts from the astheno- sphere (i.e., thermal boundary layer). The refractory peridotites show a wide range of 143Nd/144Nd and 87Sr/86Sr values (0-5127-0-5133
and 0-7024-0-7045, respectively). These facies are interpreted as percolation bands of magmas which evolved from 'plume type', possibly associated with the pre-rift evolution of the Liguro-Piemontese area, to MORB type associated with rifting of the Liguro-Piemontese basin and partial melting of the Lanzo massif. A similar evolution is suggested for the parental magmas of the vein-conduit segregates (pyroxenites and gabbros).
Original languageEnglish
Pages (from-to)191-210
Number of pages19
JournalJournal of Petrology
Publication statusPublished - 1991

Cite this