The size-isotopic evolution connection among layered mafic intrusions; clues from a Sr-Nd isotopic study of a small complex

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doi: 10.1029/93JB03126
Authors:Poitrasson, Franck; Pin, Christian; Duthou, Jean-Louis; Platevoet, Bernard
Author Affiliations:Primary:
Universite Blaise Pascal, Departement des Sciences de la Terre, Clermont-Ferrand, France
Universite de Paris-Sud, France
Volume Title:Journal of Geophysical Research
Source:Journal of Geophysical Research, 99(B5), p.9441-9451. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Publication Date:1994
Note:In English. 53 refs.; illus., incl. 2 tables, geol. sketch maps
Summary:Several theoretical and experimental works have focused on the processes occurring in continental mafic magma chambers. In contrast, systematic isotopic studies of natural remnants of these latter remain scarce, although they can give fundamental constraints for theoretical studies. This is especially true if different layered complex with contrasting characteristics (e.g., different size) are compared. For this reason, we present the results of a Sr-Nd isotopic profile across a small layered mafic intrusion of Permian age exposed near Fozzano (SW Corsica). In the main zone of the layered section, decreasing 87Sr/86Sri and increasing 143Nd/144Ndi are observed from less evolved (bottom) to more evolved (top) rocks. This peculiar pattern precludes assimilation and fractional crystallization (AFC) as a dominant mechanism in the petrogenesis of this body. Instead, we interpret this trend as reflecting the dilution of an early stage contaminated magma by several reinjections of fresh basalt in the chamber. In agreement with mineralogical and structural data, every cyclic unit is interpreted as a new magmatic input. On the basis of rough refill and fractional crystallization (RFC) calculations, the average volume for each reinjection is estimated to have been about 0.04 km3. The cumulative volume of these injections would amount to about 75% of the total volume of the layered complex. This implies that reinjections were accompanied by an important increase of the volume of the chamber or by magma withdrawal by surface eruptions. The RFC mechanism documented within this small layered body contrasts with the isotopic pattern observed between several intrusions at the regional scale in SW Corsica, and within large continental mafic magma chambers elsewhere. In these cases the isotopic evolution is dominated by AFC processes, and there is no clear isotopic evidence for reinjections, unless major influx of fresh magma occurred. It is suggested that there is a close relationship between the assimilation rate and the magma chamber volume. Small magma chambers are quickly isolated from their country rocks and better preserve the subtle isotopic signature of reinjection processes. Copyright 1994 by the American Geophysical Union.
Subjects:Alkaline earth metals; Chemical composition; Continental crust; Crust; Fractional crystallization; Intrusions; Isotope ratios; Isotopes; Layered materials; Mafic composition; Magma chambers; Magmas; Metals; Nd-144/Nd-143; Neodymium; Paleozoic; Permian; Rare earths; Sr-87/Sr-86; Stable isotopes; Strontium; Corsica; Europe; France; Western Europe; Evolution
Coordinates:N413000 N415000 E0085000 E0083000
Abstract Numbers:95M/1823
Record ID:1995051544
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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