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| Titel |
Temporal constraints on crustal contamination: whole-rock and crystal-scale evidence from the Carlingford Igneous Centre, Ireland |
| VerfasserIn |
Fiona Meade, Valentin Troll, Robert Ellam, Laura Font, Carmela Freda, Jane Chadwick, Nicola Stephen |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049860
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| Zusammenfassung |
| Magmatism at the Palaeogene Carlingford Igneous Centre is represented by a major gabbro
laccolith and a microgranite ring-dyke, both of which are crosscut by a series of aphyric to
highly porphyritic basaltic cone-sheets with subordinate rhyolite, basaltic-andesite and
trachy-andesite components. These lithologies, plus local crust, were analysed for major
and trace elements and whole-rock Sr, Nd and Pb isotopes to assess petrogenetic
processes throughout these three magmatic episodes. All samples (87Sr/86Sr =
0.7057-0.7201(60Ma)) deviate markedly from mantle values towards local Silurian siltstones
(87Sr/86Sr = 0.7144-0.7276(60Ma)). The earlier microgranite ring-dyke (87Sr/86Sr =
0.7067-0.7127(60Ma)) seems to have incorporated partial melts of the Silurian crust, rather
than bulk material. In contrast, the majority of trends for mafic samples can be
explained by bulk contamination. The highly evolved cone-sheet rhyolites (87Sr/86Sr =
0.7098-0.7100(60Ma)) lie within the range of the basalt/basaltic-andesite, suggesting
fractionation after initial contamination of basaltic parental magmas. Evidence from
experimental petrology indicates that the meta-siltstones can be easily melted at basaltic
temperatures (~1000 degrees C) and magma chamber pressures (3 KBar). Melting of the
local country rock is therefore likely to have been commonplace where it came
into contact with hot magmas. Localised K-rich trachy-andesites from cone-sheet
intrusions (87Sr/86Sr = 0.7198-0.7201(60Ma)) appear to be sedimentary derived,
representing the (high-K) melt of hornfelsed xenoliths (S-type basalts!). The xenoliths
represent the restite remaining after partial melt loss from the local Longford-Down
meta-siltstone.
Plagioclase phenocrysts from porphyritic basalt cone-sheets show that An generally
decreases with crystal growth (An91-24), with resorption surfaces marking distinct
compositional steps. Convection and re-equilibration in a heterogeneous magma chamber
may therefore be an important process affecting these phenocrysts. This is consistent with
groundmass values, which are also highly variable and range in 87Sr/86Sr from 0.7064 to
0.7092, with matrix feldspar values of An62-46. Micro-drilled Sr isotope analyses of
individual zones of large plagioclase phenocrysts show strong variation, with high-An cores
yielding both high (0.7070-0.7077) and low (0.7063-0.7067) Sr ratios compared with low An
rims. These variations are attributed to the presence of inherited xenocrystic cores from
feldspars formed in the calc-silicate rocks of the contact aureole, as well as classical
replenishment and magma mixing processes.
Despite the involvement of a single major crustal contaminant, our data imply
petrogenetic evolution was not straightforward. Rather, we see a succession of variably
overprinting processes that form a time sequence of contamination; from initial formation of
microgranite by incorporation of crustal partial melts, to bulk contamination of gabbro and
basalt as the system heats up, and subsequent melting of the remaining restites during
late-stage cone-sheet emplacement to form a melt of trachy-andesite-type composition. |
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