Journal of Petrology, Volume 39, Issue 5: May 1 1998.

Origin of Differentiated Volcanic and Plutonic Rocks from Ascension Island,South Atlantic Ocean

A. KAR¹, B. WEAVER¹, J. DAVIDSON² AND M. COLUCCI³

¹SCHOOL OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF OKLAHOMA, NORMAN, OK 73019, USA ²DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF CALIFORNIA LOS ANGELES, LOS ANGELES,CA 90095, USA ³DEPARTMENT OF GEOLOGY, SOUTHERN METHODIST UNIVERSITY, DALLAS, TX 75275, USA

The first phase of felsic magmatism on Ascension Island, in the form of trachyte and rhyolite domes, coulées, lava flows, and pyroclastic deposits, created the central and eastern parts of the island between about 1·0 and 0·56 my ago. The geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallization of high Zr/Nb mafic magmas as evidenced by identical ¹⁴³Nd/¹⁴⁴Nd and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitute one of the four distinct basalt and hawaiite suites identified from Ascension based on trace element characteristics. Syenite, monzonite, and granite xenoliths associated with the felsic magmatism are interpreted as cumulate rocks from, and intrusive equivalents of, fractionating felsic magmas. Many of the felsic rocks are characterized by high ⁸⁷Sr/⁸⁶Sr (>0·704) compared with mafic rocks (⁸⁷Sr/⁸⁶Sr <0·703), even when corrected for in situ decay of ⁸⁷Rb since eruption. Such high ⁸⁷Sr/⁸⁶Sr coupled with high ¹⁴³Nd/¹⁴⁴Nd signatures do not correspond to known suboceanic mantle reservoirs and in the most part appear to reflect sub-solidus addition of a high ⁸⁷Sr/⁸⁶Sr component. This component is probably a seawater-derived fluid that might be added at the surface from wind-blown spray, or, more likely, at depth through hydrothermal circulation (fluids with high Sr contents have been recovered from fractures in a 3126-m-deep geothermal well). In either case, the extremely low Sr contents of the felsic rocks make them particularly susceptible to Sr-isotope modification. Internal (mineral) isochrons for two granite xenoliths give ages of ~0·9 and ~1·2 Ma, with initial ⁸⁷Sr/⁸⁶Sr >0·705. Even though the high ⁸⁷Sr/⁸⁶Sr signature of most of the volcanic rocks is demonstrably introduced after solidification, the high initial ⁸⁷Sr/⁸⁶Sr values of the granite xenoliths suggest that hydrothermally altered pre-existing volcanic basement may have been melted or assimilated during differentiation of some of the felsic magmas.

Keywords: felsic magmatism; fractional crystallization; radiogenic isotopes; rock-magma interaction

Pages 1009-1024

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