The nature of differentiation trends in some volcanic rocks from the Galapagos Spreading Center

TLDR: The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron-and titanium-enriched basalt and low-alumina andesite, and (3) rhyodacites.

Abstract: The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron- and titanium-enriched tholeiites, (3) andesites, and (4) rhyodacites. Interpretation of major element compositions determined for these rocks, associated glassy selvedges, and mineral phases suggests that the entire suite can be produced by fractional crystallization of the observed low-pressure phenocrysts. Early fractionation to extreme iron and titanium enrichment requires 50–75% crystallization of five parts plagioclase, three parts augite, and one part olivine. In the highly fractionated basalts, pigeonite replaces olivine as a phenocryst phase and is a minor phase in the fractionation. At extreme iron and titanium enrichment, titanomagnetite becomes a phenocryst phase, and fractionation produces a silica enrichment trend to andesite (79% total crystallization) and rhyodacite (87% total crystallization). Apatite occurs as a microphenocryst phase in the silicic rocks. This suite of iron- and titanium-enriched basalt and low-alumina andesite differs from the calc-alkaline suite of island arcs and continental margins. It is similar to the suite of fractionated lavas of the Galapagos Islands and Iceland, though these rocks are generally more alkaline than the Galapagos spreading center suite, which is similar to other ocean floor basalt suites though more fractionated. Magma mixing is suggested by the presence of basaltic xenoliths in the silicic rocks; the presence of high-silica glass inclusions in the glassy selvedges of the basalts, andesite, and rhyodacite; the common occurrence of reversely zoned phenocrysts in the silicic rocks; and the occurrence of some highly fractionated yet aphyric rocks. The unusual extent of fractionation for this region may be related to the size, morphology, and eruption and resupply rates of the magma chambers associated with the Galapagos spreading center. This and the regional distribution of these highly fractionated rocks suggest an important, though indirect, influence of the Galapagos hot spot.