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| Titel |
Evaluating the relative roles of crustal growth and recycling through continental arc magmatism in the Ross orogen, Antarctica |
| VerfasserIn |
Graham Hagen-Peter, John Cottle |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250147488
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| Publikation (Nr.) |
 EGU/EGU2017-11659.pdf |
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| Zusammenfassung |
| There remains much debate about the mechanisms of the growth and differentiation of
continental crust over geologic time, although the geochemical resemblance between
continental crust and arc magmatism around the world make subduction-related magmatism a
conspicuous candidate. It’s clear that both juvenile magmatism and crustal recycling occur at
convergent margins, but is it difficult to quantify the roles of these two end-member
processes. This is particularly challenging in continental arc settings, where magmas ascend
through and interact with thick continental lithosphere of variable—and usually
unknown—age and composition. We assess the relative magnitudes of crustal growth
and recycling in a 500-km-long segment of the Ross orogen of Antarctica—an
archetypal example of a long-lived “Cordilleran-style” continental arc—utilizing an
extensive set of zircon Hf (∼70 samples) and whole rock Nd and Sr (15 samples)
isotopic data for igneous rocks ranging from gabbro to granite. Initial εNd and
87Sr/86Sr values range from +0.1 to –10.3 and ∼0.7044 to 0.7137, respectively.
Initial εHf values (weighted means of individual analyses from each sample) are
predominately negative—ranging from +3.5 to –12.3—potentially interpreted as
reflecting a dominant crustal component in the source of the granitoids. However,
inherited zircon domains provide evidence for significantly less radiogenic ancient
crust in the unexposed basement of the orogen. Additionally, primitive samples
(SiO2 < 52 wt%; Mg# > 69) range in initial εHf from approximately +0.8 to –4.0,
representing juvenile magmatism with enriched isotopic compositions. While a
broader range and lower εHf values (+3.5 to –12.3) in more evolved samples from
large granitic plutons likely reflect variable degrees of crustal assimilation during
differentiation, overlap with the isotopic compositions of primitive samples permits
differentiation with relatively minor degrees of crustal assimilation. This qualitative
interpretation is corroborated by binary mixing and assimilation fractional-crystallization
(AFC) models, suggesting that the large granitic (sensu lato) batholiths of the Ross
orogen reflect significant additions of new, evolved continental crust with subordinate
crustal reworking. Precise quantitative estimates of the end-member processes of
crustal reworking versus closed-system fractionation from juvenile magmas are
hampered by the numerous assumptions in the mixing and AFC models. Instead,
the aim of this study is to explore a permissible range of crustal growth/recycling
scenarios. Our extensive radiogenic isotope dataset also expands our knowledge of
the Ross orogen in the greater context of tectono-magmatic processes along the
margin of East Gondwana in the late Neoproterozoic through the early Paleozoic. |
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