Laurite and zircon from the Finero chromitites (Italy)

new insights into evolution of the subcontinental mantle

K. N. Malitch*, E. A. Belousova, W. L. Griffin, I. Yu Badanina, V. V. Knauf, S. Y. O'Reilly, N. J. Pearson

*Corresponding author for this work

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of 'unradiogenic' 187Os/188Os values for laurite at Alpe Polunia (0.1247-0.1251, mean 0.1249±0.0001). Re-Os model ages (TRD) of laurite reflect an Early Paleozoic partial melting event (ca 450Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ∼310Ma to 190Ma. Three age peaks at Rio Creves (220±4Ma, 234.2±4.5Ma and 277.5±3.2Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with εHf values ranging mainly from -3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8-1.0Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed.

Original languageEnglish
Pages (from-to)210-225
Number of pages16
JournalOre Geology Reviews
Volume90
DOIs
Publication statusPublished - Nov 2017

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Keywords

  • age
  • chromitite
  • Finero
  • Hf-Os-isotopes
  • laurite
  • metasomatism
  • subcontinental mantle
  • zircon

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