Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism |
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| Authors: | R Tribuzio M F Thirlwall B Messiga |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy Fax: 382 505890; E-mail: tribuzio@crystal.unipv.it, IT;(2) C.N.R – Centro di Studio per la Cristallochimica e la Cristallografia (CSCC), Via Ferrata 1, I-27100 Pavia, Italy, IT;(3) Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, UK, GB |
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| Abstract: | The origin of the Sondalo gabbroic complex has been unravelled by means of a petrological study of the least evolved rocks,
troctolites to norites containing up to 20% of anhedral clinopyroxene and titanian pargasite. Pyroxenes and titanian pargasite
from the troctolites have higher Mg, Al and Cr, and lower Mn than those from the norites, whereas plagioclase does not show
systematic compositional variations (An ca. 65 mol%). The variation trend of anorthite content of plagioclase versus the forsterite
content of olivine differs from that of arc-related gabbroic rocks. Plagioclase, clinopyroxene, orthopyroxene and titanian
pargasite were analyzed for REE and selected trace elements by ion microprobe. Application of crystal/liquid partition coefficients
to trace element mineral compositions suggests that the parental liquids of both troctolites and norites had tholeiitic affinity
and were slightly LREE and LILE enriched relative to N-MORB. A troctolite and a norite give Sm-Nd mineral isochron ages of
300 ± 12 Ma and 280 ± 10 Ma. Plagioclase-amphibole Rb-Sr isochron ages are 266 ± 10 Ma and 269 ± 16 Ma for the same rock samples,
and they are interpreted to represent cooling ages. The Nd-Sr-O isotopic compositions indicate that a substantial crustal
contribution was involved in the petrogenesis of the norite, which has low ɛNd(290 Ma), high 87Sr/86Sr(290 Ma) and high δ18OPx (−2.6, 0.7057 and +7.9‰, respectively) compared with the troctolites. We thus conclude that the troctolite/norite association
formed by concomitant fractional crystallization and crustal assimilation. The somewhat elevated δ18OCpx (+6.4‰) and the relatively low forsterite contents in olivine suggest that the parental liquids of the troctolites had already
been evolved through an AFC type process. The ɛNd(290 Ma) and 87Sr/86Sr(290 Ma) of these rocks (ranging from +2.8 to +4.4 and from 0.7037 to 0.7040, respectively) probably do not reflect the
ɛNd and Sr isotopic compositions of their mantle source, and it is thus unclear whether the primary melts were derived from a
slightly enriched or from a depleted mantle source. The Sondalo gabbroic complex was most likely associated with the post-Variscan
gabbroic complexes of the Alpine belt. These gabbroic complexes can be ascribed to the intrusion at different crustal levels
of tholeiitic mantle-derived melts and were emplaced in the time span of 300–270 Ma.
Received: 14 September 1998 / Accepted: 4 January 1999 |
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