Graduate Thesis Or Dissertation


A petrogenetic model for the Caribbean large igneous province Public Deposited

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  • The Caribbean Plateau is an oceanic large igneous province (CLIP). A widely accepted model for LIP petrogenesis proposes that these large bodies of igneous rock are formed by decompression melting associated with upwelling mantle plume heads during the initiation of hotspot activity. According to this classical model, petrogenesis occurs over a relatively short timeframe (i.e. a few million years). Recently published ⁴⁰Ar/³⁹Ar ages for the CLIP, however, suggest a much larger age range (~30 m.y.) despite relatively limited geochemical variation. This raises questions about the applicability of the "mantle plume impact" model to the Caribbean Plateau. Although previous research has examined the processes and mantle sources involved in the construction of the CLIP, there is currently not a petrogenetic model that attempts to reconcile the geochemistry of the CLIP with this extended age range. The research presented in this thesis utilizes major element, trace element, and isotopic chemistry of intrusive and extrusive basaltic samples from three localities--the Curaçao Lava Formation (CLF), Dumisseau Formation (DF) and Beata Ridge (BR)--in order to develop a petrogenetic model for the CLIP capable of accounting for the relatively limited variation in geochemistry over ~30 m.y. of volcanic activity. This model integrates the results of MELTS and rare earth element (REE) modeling to provide constraints on the nature of the parental magmas, melting processes, and mantle source(s) of the CLIP. The results of the MELTS modeling demonstrate that nearly the full range of major element compositions observed in the CLIP samples can be generated by fractional crystallization of magmas with similar major element compositions with a range of water contents (0-1 wt%), and crystallizing over a range of pressures (1-2.5 kbar). This suggests a magma storage system with multiple shallow crustal magma chambers in which efficient mixing resulted in a relatively restricted range of compositions. Despite the large age range observed for the CLIP lavas, the geochemistry of the samples is not inconsistent with a plume-related origin, and isotopic compositions show significant overlap with those of the Galápagos plume. Age-corrected ɛNd for the CLIP samples ranges from ~5.1 to 9.4, suggesting a mantle source that has components of depleted (MORB) and plume mantle. Indeed, batch melting and fractional crystallization models using REEs suggest that the geochemistry of the CLIP samples is consistent with variable degrees of melting of a hybrid of enriched and depleted mantle sources. The difference in degree of melting indicated by the model for the CLF and BR (~15-30%) relative to the DF (~5-10%) is consistent with a model in which the CLF and BR lie near the plume axis, while the DF is situated near the plume margin. Paradoxically, although the Nd isotopic compositions of the CLIP samples suggest a variably depleted source, the results of the batch melting model suggest contributions from source material with greater trace element enrichment than primitive mantle. The combination of a variably depleted isotopic signature with depleted to enriched trace element compositions suggests that the enriched material was generated by the relatively recent metasomatism of a long-term depleted mantle source. The results of these models are consistent with a formation mechanism for the CLIP in which melting is mediated by the interaction between plume head material and asthenospheric mantle flow associated with nearby subduction zones. Changes in the mantle flow regime due to variations in the orientation (e.g. polarity reversals, slab rollback) of nearby subduction zones would allow for localized upwelling beneath the initially formed CLIP (~94 Ma), allowing for ~30 million years of intermittent magmatism through the repeated tapping of this mantle source. The petrogenetic model for the CLIP presented here demonstrates that the classical paradigm for LIP petrogenesis is not universally applicable, and suggests that the formation of LIPs may be significantly influenced by lithosphere-asthenosphere dynamics (such as subduction-driven mantle flow) in addition to plume-driven melting.
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