Abstract
Aillikite magmas (a carbonate-rich ultramafic
lamprophyre; UML) erupted through the Archean crust of the
Torngat Mountains in northern Labrador between 610 and 550
Ma. These dykes are characterized by long-term isotopic
depletion (87Sr/86Sr = 0.70323-0.70377; HNd = +1.2 to +1.8;
HHf = +1.4 to +3.5; 206Pb/204Pb = 18.2-18.5), but grade into
carbonate-poorer mela-aillikites which show isotopic
enrichment (87Sr/86Sr = 0.70388-0.70523; eNd = -0.5 to -3.9;
eHf = -0.6 to -6.0; 206Pb/204Pb = 17.8-18.2). This long-term
enriched signature is coupled to distinctively higher Rb/Cs and
Zr-Hf-Ti. Increased partial melting involving old phlogopiteand
rutile-bearing source material reminiscent of MARID
probably imparted this signature to carbonate-rich aillikite
magmas, thereby shifting compositions towards melaaillikites.
Channelling of CO2-rich melts into pre-existing
MARID vein networks in the lower reaches of the cratonic
lithosphere and their varying interactions best explains the
aillikite/mela-aillikite continuum. As there is no evidence for
independent melting of the low-T fusible MARID veins,
temperatures during aillikite magma generation may never
exceed 1200°C. Thus, decompression and volatile-fluxing are
the principal causes of UML/carbonatite magma generation
within rifting cratonic mantle. Strong interactions between
CO2-rich melts and lithosphere are unavoidable, explaining
the rarity of direct eruptions of primary carbonatite magmas.
The degree of interaction controls whether CO2 (i) remains
dissolved in UML magmas, enabling separation of carbonatite
intrusions at crustal levels, or (ii) largely reacts out at mantle
depths resulting in carbonate-poor UML magmas, as in the
Torngat examples.
lamprophyre; UML) erupted through the Archean crust of the
Torngat Mountains in northern Labrador between 610 and 550
Ma. These dykes are characterized by long-term isotopic
depletion (87Sr/86Sr = 0.70323-0.70377; HNd = +1.2 to +1.8;
HHf = +1.4 to +3.5; 206Pb/204Pb = 18.2-18.5), but grade into
carbonate-poorer mela-aillikites which show isotopic
enrichment (87Sr/86Sr = 0.70388-0.70523; eNd = -0.5 to -3.9;
eHf = -0.6 to -6.0; 206Pb/204Pb = 17.8-18.2). This long-term
enriched signature is coupled to distinctively higher Rb/Cs and
Zr-Hf-Ti. Increased partial melting involving old phlogopiteand
rutile-bearing source material reminiscent of MARID
probably imparted this signature to carbonate-rich aillikite
magmas, thereby shifting compositions towards melaaillikites.
Channelling of CO2-rich melts into pre-existing
MARID vein networks in the lower reaches of the cratonic
lithosphere and their varying interactions best explains the
aillikite/mela-aillikite continuum. As there is no evidence for
independent melting of the low-T fusible MARID veins,
temperatures during aillikite magma generation may never
exceed 1200°C. Thus, decompression and volatile-fluxing are
the principal causes of UML/carbonatite magma generation
within rifting cratonic mantle. Strong interactions between
CO2-rich melts and lithosphere are unavoidable, explaining
the rarity of direct eruptions of primary carbonatite magmas.
The degree of interaction controls whether CO2 (i) remains
dissolved in UML magmas, enabling separation of carbonatite
intrusions at crustal levels, or (ii) largely reacts out at mantle
depths resulting in carbonate-poor UML magmas, as in the
Torngat examples.
| Original language | English |
|---|---|
| Pages (from-to) | A1003-A1003 |
| Number of pages | 1 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 71 |
| Issue number | 15 supplement |
| Publication status | Published - Aug 2007 |
| Externally published | Yes |
| Event | Goldschmidt Conference (17th : 2007) - Cologne, Germany Duration: 19 Aug 2007 → 24 Aug 2007 Conference number: 17 |