Plume-lithosphere interaction at Santiago Island (Cape Verde) : implications for ocean island magma genesis models

• Author(s): Martins, Sofia Maria Marques
• Date: 2014
• Persistent ID: http://hdl.handle.net/10451/16025
• Origin: Repositório da Universidade de Lisboa
• Subject(s): Teses de doutoramento - 2014
• Document Type: Doctoral thesis
• Language: English
• Advisor(s): Mata, João Manuel Lima da Silva, 1957-; Munhá, José Manuel Urbano, 1951-
• Contributor(s): Repositório da Universidade de Lisboa

Description

Tese de doutoramento, Geologia (Geoquímica), Universidade de Lisboa, Faculdade de Ciências, 2014

Santiago Island is the largest one in the Cape Verde archipelago with a total area of 991 km2. It belongs to the southern group with Maio, Fogo and Brava Islands. Apart from the substrate (Complexo Antigo) Santiago's volcanostratigraphy is largely composed of subareal and submarine alkaline lavas outcrops, which cover a time span of circa 5 Ma. Santiago alkaline lavas are testimony of a complex magmatic history, where chemical and lithological heterogeneous mantle sources and the interaction of advecting magmas with thick metasomatized oceanic lithosphere is preponderant in the observed isotopic and trace element signatures. The distinctive major and trace element characteristics of Santiago lavas stand as a strong indicator of a carbonated mantle source with contributions from peridotitic and silica deficient garnet pyroxenite domains. The correlation of these elemental evidences with radiogenic isotopic data attests the presence of these domains within the plume, as well as their ancient signature. Young (< 3.3 Ma) primitive Santiago lavas show evidence for the equilibration with modal and cryptically metasomatized oceanic lithospheric mantle. The metasomatism is responsible for the inferred hydrous mineral assemblage (amphibole and phlogopite) as well as specific geochemical tracers. Interestingly this signature is not ubiquitous in Santiago lavas mantle source, since the older formations do not show strong evidences for interaction with metasomatized domains of the lithosphere. The carbonatitic nature of the metasomatic agent is attested from indirect geochemical signatures in alkaline lavas and direct evidence portrayed by lherzolitic xenoliths. It is important to acknowledge that the metasomatic agent composition is elemental and isotopically distinct from outcropping carbonatites. In accordance with the overall isotopic characteristics of the Southern Cape Verde Islands, elemental and isotopic signatures of Santiago alkaline lavas suggest the involvement of HIMU and EM1-type mantle end-members, with the contribution a relatively undegassed and unradiogenic reservoir (Lower Mantle). The geochemical characteristics of Santiago allow us to consider the EM-1 like end-member as resulting from the involvement of subcontinental lithospheric mantle in Santiago’s magma genesis. Tholeiitic outcropping lavas in the Santiago’s northern area are considered as formed in a mid oceanic ridge environment. The lavas are highly depleted in LREE and incompatible elements, even when compared with D-MORB values. This depletion seems to be ubiquitous in Central Atlantic MORB erupted between 160 to 118 Ma. The eruption of highly depleted lavas at the initial stages of the Atlantic opening is here interpreted as the result of high degrees of partial melting induced by sub-lithospheric elevated temperatures brought on by continental thermal insulation. Santiago gabbroic xenoliths show geochemical similarities with Santiago MORB affinity lavas attesting their cumulate formation from abyssal-type magmas. The enrichment observed in some is most certainly derived from the interaction of ascending alkaline magmas with the pre-existing N-MORB crust.

Fundação para a Ciência e a Tecnologia (FCT)