Sr and Nd Isotopic and Geochemical Analysis of the Vanda Dike Swarm, Antarctica
Abstract
The Vanda dike swarm, in the Dry Valleys of Antarctica, crosscuts synorogenic Cambro-Ordovician granitoid plutons produced during the Ross Orogeny, circa 500 Ma. The emplacement of the dikes broadly coincides with the transition between the cessation of subduction and the onset of extensional magmatism. Recent fieldwork included field documentation of over 600 dikes, with subsequent whole rock major and trace element analysis and Sr and Nd isotopic analyses of a representative subset. Although originally characterized as being compositionally bimodal, the compositions of the dikes range continuously from 48% to 78% SiO2. New radiogenic isotopic data from 24 samples exhibit a surprisingly wide range of ɛNd and 87Sr/86Sri values; the bulk of the samples have an age-corrected ɛNd from -12.7 to -3.0, however a subset of four of the dikes (both mafic and felsic) have ɛNd values ranging from +2.1 to 8.0. The 87Sr/86Sri ratios vary from 0.7085 to 0.7118. Notably, there is significant overlap between the isotopic signatures of the mafic and felsic rocks and no strong correlation between ɛNd and 87Sr/86Sri. Likewise, there is no correlation between either isotopic ratio and SiO2. These variations indicate that the Vanda dikes may be derived from compositionally heterogeneous sources. Additionally, Nd model ages (τDM) of the felsic dikes yield an average of ~900 Ma, suggesting derivation from a Neoproterozoic crustal source. The Vanda dike samples’ 87Sr/86Sri overlap with the high end of the range defined by the youngest plutons in the region emplaced during the Ross Orogeny (adakitic DV1b, monzonitic DV2; Cox et al., 2000). Similarly, ɛNd values of the Vanda samples most closely resemble those of the DV1b and DV2 plutons, but also extend to significantly more negative and more positive values. None of the isotopic signatures of the dikes are similar to those of the older, orogenic granites in the region (DV1a), which have lower 87Sr/86Sri averages than both the dikes and the DV1b and DV2 plutons. Strontium isotope systematics define an errorchron that yields an age comparable to but slightly older than the emplacement ages of the DV1b and DV2 plutons (491 Ma). Conversely, the samples do not form a single line on a Nd isochron diagram. The absence of a strong correlation between either 87Sr/86Srm or 143Nd/144Ndm and SiO2 precludes simple assimilation of continental crust as an explanation for the isotopic variation. Several additional factors may be responsible for the wide range of isotopic signatures in the Vanda dikes: a) derivation of the dikes from the subduction-altered sub-continental mantle lithosphere may produce mafic magmas with variable ɛNd; or b) there may be differences in basement composition throughout the region, accompanied by assimilation during dike emplacement. Both of these hypotheses are evaluated in light of the interpretation that the dike swarms likely reflect different phases of the tectonic transition from a convergent to an extensional zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51C1685B
- Keywords:
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- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8413 VOLCANOLOGY / Subduction zone processes;
- 9310 GEOGRAPHIC LOCATION / Antarctica