ORIGIN OF DIFFERENTIATED VOLCANIC AND PLUTONIC ROCKS FROM ASCENSION ISLAND, SOUTH ATLANTIC-OCEAN

The first phase of felsic magmatism on Ascension Island, in the Son of trachyte and rhyolite domes, coulees, lava flows, and pyroclastic dep osits, created the central and eastern parts of the island between abo ut 1.0 and 0.56 my ago. The geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallizat ion of high Zr/Nb mafic magmas as evidenced by identical Nd-143/Nd-144 and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitut e one of the four distinct basalt and hawaiite suites identified from Ascension bared on trace element characteristics. Syenite, monzonite, and granite xenoliths associated with the felsic magmatism are interpr eted as cumulate rocks from, and intrusive equivalents of, fractionati ng felsic magmas. Many of the felsic rocks are characterized by high S r-87/Sr-86 (>0.704) compared with mafic rocks (Sr-87/Sr-86 <0.703), ev en when corrected for in situ decay of Rb-87 since eruption. Such hh S r-87/Sr-86 coupled tenth high Nd-143/Nd-144 signatures do not correspo nd to known suboceanic mantle reservoirs and in the most part appear t o reflect sub-solidus addition of a high Sr-87/Sr-86 component. This c omponent is probably a seawater-derived fluid that might be added at t he surface from wind-blown spray, or more likely, at depth through hyd rothermal circulation fluids with high Sr contents have been recovered from fractures in a 3126-m-deep geothermal well). In either care, the extremely low Sr contents of the felsic rocks make them particularly susceptible to Sr-isotope modification. Internal (mineral) isochrons f or two granite xenoliths give ages of similar to 0.9 and similar to 1. 2 Ma, with initial Sr-87/Sr-86 >0.705. Even though the high Sr-87/Sr-8 6 signature of mast of the volcanic rocks is demonstrably introduced a fter solidification, the high initial Sr-87/Sr-86 values of the granit e xenoliths suggest that hydrothermally altered pre-existing volcanic basement may have been melted or assimilated during differentiation of some of the felsic magmas.