The contribution of the young Cretaceous Caribbean Oceanic Plateau to the genesis of late Cretaceous arc magmatism in the Cordillera Occidental of Ecuador


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The contribution of the young Cretaceous Caribbean Oceanic Plateau to the genesis of late Cretaceous arc magmatism in the Cordillera Occidental of Ecuador
Journal of South American Earth Sciences
Allibon J., Monjoie P., Lapierre H., Jaillard E., Bussy F., Bosch D., Senebier F.
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The eastern part of the Cordillera Occidental of Ecuador comprises thick
buoyant oceanic plateaus associated with island-arc tholeiites and
subduction-related calc-alkaline series, accreted to the Ecuadorian
Continental Margin from Late Cretaceous to Eocene times. One of these
plateau sequences, the Guaranda Oceanic Plateau is considered as remnant
of the Caribbean-Colombian Oceanic Province (CCOP) accreted to the
Ecuadorian Margin in the Maastrichtien.
Samples studied in this paper were taken from four cross-sections
through two arc-sequences in the northern part of the Cordillera
Occidental of Ecuador, dated as (Rio Cala) or ascribed to (Macuchi) the
Late Cretaceous and one arc-like sequence in the Chogon-Colonche
Cordillera (Las Orquideas). These three island-arcs can clearly be
identified and rest conformably on the CCOP.
In all four localities, basalts with abundant large clinopyroxene
phenocrysts can be found, mimicking a picritic or ankaramitic facies.
This mineralogical particularity, although not uncommon in island arc
lavas, hints at a contribution of the CCOP in the genesis of these
island arc rocks.
The complete petrological and geochemical study of these rocks reveals
that some have a primitive island-arc nature (MgO values range from 6 to
11 wt.%). Studied samples display marked Nb, Ta and Ti negative
anomalies relative to the adjacent elements in the spidergrams
characteristic of subductionrelated magmatism. These rocks are
LREE-enriched and their clinopyroxenes show a tholeiitic affinity
(FeO(1)-TiO(2) enrichment and CaO depletion from core to rim within a
single crystal).
The four sampled cross-sections through the island-arc sequences display
homogeneous initial Nd, and Pb isotope ratios that suggest a unique
mantellic source for these rocks resulting from the mixing of three
components: an East-Pacific MORB end-member, an enriched pelagic
sediment component, and a HIMU component carried by the CCOP. Indeed,
the ankaramite and Mg-basalt sequences that form part of the
Caribbean-Colombian Oceanic Plateau are radiogenically enriched in
(206)Pb/(204)Pb and (207)Pb/(204)Pb and contain a HIMU component similar
to that observed in the Gorgona basalts and Galapagos lavas. The
subduction zone that generated the Late Cretaceous arcs occurred far
from the continental margin, in an oceanic environment. This implies
that no terrigenous detrital sediments interacted with the source at
this period. Thus, the enriched component can only result from the
melting of subducted pelagic sediments.
We have thus defined the East-Pacific MORB, enriched (cherts, pelagic
sediments) and HIMU components in an attempt to constrain and model the
genesis of the studied island-arc magmatism, using a compilation of
carefully selected isotopic data from literature according to rock age
and paleogeographic location at the time of arc edification.
Tripolar mixing models reveal that proportions of 12-15 wt.% of the
HIMU component, 7-15 wt.% of the pelagic sediment end-member and 70-75
wt.% of an East-pacific MORB end-member are needed to explain the
measured isotope ratios. These surprisingly high proportions of the
HIMU/CCOP component could be explained by the young age of the oceanic
plateau (5-15 Ma) during the Late Cretaceous arc emplacement. The CCOP,
basement of these arc sequences, was probably still hot and easily
assimilated at the island-arc lava source. (C) 2008 Elsevier Ltd. All
rights reserved,
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