Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride,N. Italy) |
| |
Authors: | E Rampone A W Hofmann G B Piccardo R Vannucci P Bottazzi L Ottolini |
| |
Institution: | (1) Dipartimento di Scienze della Terra, Universita’ di Genova, Corso Europa 26, I-16132 Genova, Italy, IT;(2) Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany, DE;(3) Dipartimento di Scienze della Terra, Universita’ di Pavia, via Abbiategrasso 209, I-27100 Pavia, Italy, IT;(4) CNR—Centro di Studio per la Cristallochimica e la Cristallografia, via Abbiategrasso 209, I-27100 Pavia, Italy, IT |
| |
Abstract: | Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere
of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh
samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new
data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type
mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies
stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole
rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements light rare
earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk
rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree
(<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism.
In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined
on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (87Sr/86Sr=0.702203–0.702285; 143Nd/144Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They
could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response
to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced
a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites
also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence
of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies
reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions.
Received: 15 February 1995/Accepted: 4 August 1995 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|