The nature and petrogenesis of intra-oceanic plate alkaline eruptive and plutonic rocks: King's Trough, Northeast Atlantic

Basalts, diorites, and gabbros dredged from the side of King's Trough — a mid-plate fracture on the northeast Atlantic sea floor — are alkaline in character based on major- and trace-element chemistry and mineralogy. The variation in bulk chemical composition and in mineral paragenesis and composition suggest differentiation in a large magma chamber beneath a seamount volcano. Fractional crystallization took place in a hydrous magma and is characterized by early formation of a Ti-rich biotite as rims on olivine. Thus, pyroxene compositions are unusual in having relatively low molar Ti/Al ratios, and increasing differentiation is marked by progressively decreasing ratios of K to Na.