Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a 207Pb/206Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite).
Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago.
Initial Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. 87Sr/86Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between 87Sr/86Sr and K2O and weaker correlations between initial Nd and K2O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative Nd and high 87Sr/86Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks. 相似文献
Implementing ecosystem based fisheries management (EBFM) will require a suite of ecological indices to track the state of an ecosystem, in addition to monitoring species-abundance for both economically important species, as well as species that are not targeted for harvest. However, EBFM implementation requires an examination of current methods and applications of ecological indices that are being used. This paper identifies four key uses for ecosystem indices in the context of EBFM and discusses the implications of each: (1) motivation for socio-political action, (2) information for individual users to modify their behavior, (3) implementation of decision rules for management evaluation, and (4) discovery of ecosystem functions to advance scientific knowledge. In a fisheries management context, ecological indicators will be linked to decision rules based on the definitions of both “ecosystem overfishing” and the current single-species definitions of overfishing. Two components of common ecological indicators are species weightings (i.e. catch or abundance data by species) and the species-specific ecological attributes (e.g. mortality rates, body size, trophic level). We discuss statistical issues that arise from estimating the parameters in ecological index calculations from both fishery-dependent and independent data and the potential biases introduced by using catch-per-unit-effort (CPUE) data, catch data, and abundance estimates. Given the range of biases that arise, often the best estimates of species weightings (abundances) are those derived from fish stock assessments. This implies that progress in EBFM may be best served by increasing the number of species for which single-species assessments are done and to expand the list of species to include species which may not be economically important. The policy framework exists to carry out EBFM; however, future works needs to focus on empirical management strategy simulations, as well as theoretical works to identify management criteria based on those indicators. 相似文献