Petrology and geochemistry of shield-building and post-erosional lava series of Samoa: implications for mantle heterogeneity and magma genesis

Thesis/Dissertation · Wed Jan 01 00:00:00 EST 1986

OSTI ID:6409119

Wright, E

Samoan shield-building lavas are more alkalic than Hawaiian shield basalts, but a transition from less- to more-alkalic flows may be identified in the Pago Shield of Tutuila. This transition is not gradational, but rather consists of alternating less- and more-alkalic interbedded flows, indicating little magma mixing during this period, a conclusion supported by extreme isotopic heterogeneity in these flows. Further evolution of the Pago shield lava compositions is controlled by fractionation of olivine to produce increasingly Si-enriched differentiates. Samoan post-erosional lavas are distinct from Samoan shield-building lavas. They are strongly Si-undersaturated and unfractionated, and show higher YXSr/YWSr and SXPb/SUPb and lower UTNd/ UUNd isotopic ratios than shield-building lavas. Whereas shield-building lavas have isotopic compositions similar to the Society Islands, post-erosional lavas may represent mixing of this shield source with an enriched end member, tentatively identified as recycled crustal material. Compositional variation within the post-erosional lavas is controlled by source differences evident in rare earth element and isotopic compositions, and minor fractionation of olivine, Ti-magnetite, and clinopyroxene. The close spacing of vents along a 282-km-long rift zone parallel to the trend of the island chain, and the anomalously large volume of post-erosional lavas are inconsistent with a plume origin. Two types of ultramafic xenoliths are included in Samoan post-erosional lavas: harzburgite-lherzoilite nodules showing porphyroclastic and equigranular-mosaic textures, and wehrlite-dunites showing tectonized cumulate textures.

• Research Organization: California Univ., San Diego, La Jolla (USA)
• OSTI ID: 6409119
• Resource Relation: Other Information: Thesis (Ph. D.)
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GEOLOGIC DEPOSITS
GEOCHEMISTRY
GEOLOGIC HISTORY
PETROLOGY
OCEANIA
HETEROGENEOUS EFFECTS
ISLANDS
ISOTOPE RATIO
LAVA
LEAD
MAGMA
MINERALS
NEODYMIUM
ORIGIN
PLATE TECTONICS
RIFT ZONES
STRONTIUM
ALKALINE EARTH METALS
CHEMISTRY
ELEMENTS
GEOLOGIC STRUCTURES
GEOLOGY
METALS
RARE EARTHS
TECTONICS
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)
580400 - Geochemistry- (-1989)