EarthRef.org Reference Database (ERR) -- Gee et al. 1991

Fifteen dredges on the summit and upper flanks of Jasper Seamount (122¿44'W; 30¿27'N) recovered a wide variety of lithologies, including pillow lavas, vesicular lapillistones from shallow submarine explosive volcanism, and a range of xenoliths. On the basis of dredge locations, geochemical characteristics, and 40Ar/39Ar age data, three distinct phases of volcanism can be distinguished, a shield-building tholeiitic/transitional phase (Flank Transitional Series, FTS), followed by a flank alkalic series (FAS), and a late-stage Summit Alkalic Series (SAS). All three series consist exclusively of differentiated 2+/(Mg2++Fe2+)> compositions. The FTS represents a low-pressure differentiation trend from tholeiitic/transitional basalts to quartz-normative residual liquids and probably accounts for more than 90% of the volume of Jasper. 40Ar/39Ar age data, the dominant reversed polarity of Jasper, and a plausible duration (<1 m.y.) for shield construction suggest FTS volcanism began about 11 Ma and ended about 10 Ma. FTS lavas probably erupted from a NW trending, hotspot track-parallel rift system. The intermediate alkalinity FAS lavas, which probably comprise 3--8% of the volume of Jasper, erupted from 8.7 to 7.5 Ma, possibly after a brief volcanic hiatus or period of reduced eruptive activity. Normative projections suggest the FAS Lavas are the product of fractionation or equilibration at elevated pressures. The hawaiites and mugearites of the SAS erupted between 4.8 and 4.1 Ma, after a probable 2.7 m.y. period of volcanic quiescence, and probably constitute <1% of the seamount volume. A suite of xenoliths incorporated in SAS lavas includes (1) tholeiitic basalt fragments from either the ocean crust or seamount interior, (2) a range of differentiated gabbros largely derived from the ocean crust, (3) residual mantle spinel lherzolites, and (4) pyroxenite and peridotite cumulates. The abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the SAS suggests that these lavas probably fractionated in a magma chamber at the crust-mantle boundary. The occurrence of orthopyroxine-bearing alkalic cumulate xenoniths in these lavas, however, is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates. The SAS vents of Jasper define a NE-SW volcanic trend which is orthogonal to the FTS rift. The pattern of volcanic activity, including periods of volcanic quiescence, and the general increase in alkalinity, as well as the structural reorganization of magmatic feeder systems of Jasper Seamount, is strikingly similar to the patterns observ Fifteen dredges on the summit and upper flanks of Jasper Seamount (122¿44'W; 30¿27'N) recovered a wide variety of lithologies, including pillow lavas, vesicular lapillistones from shallow submarine explosive volcanism, and a range of xenoliths. On the basis of dredge locations, geochemical characteristics, and 40Ar/39Ar age data, three distinct phases of volcanism can be distinguished, a shield-building tholeiitic/transitional phase (Flank Transitional Series, FTS), followed by a flank alkalic series (FAS), and a late-stage Summit Alkalic Series (SAS). All three series consist exclusively of differentiated 2+/(Mg2++Fe2+)> compositions. The FTS represents a low-pressure differentiation trend from tholeiitic/transitional basalts to quartz-normative residual liquids and probably accounts for more than 90% of the volume of Jasper. 40Ar/39Ar age data, the dominant reversed polarity of Jasper, and a plausible duration (<1 m.y.) for shield construction suggest FTS volcanism began about 11 Ma and ended about 10 Ma. FTS lavas probably erupted from a NW trending, hotspot track-parallel rift system. The intermediate alkalinity FAS lavas, which probably comprise 3--8% of the volume of Jasper, erupted from 8.7 to 7.5 Ma, possibly after a brief volcanic hiatus or period of reduced eruptive activity. Normative projections suggest the FAS Lavas are the product of fractionation or equilibration at elevated pressures. The hawaiites and mugearites of the SAS erupted between 4.8 and 4.1 Ma, after a probable 2.7 m.y. period of volcanic quiescence, and probably constitute <1% of the seamount volume. A suite of xenoliths incorporated in SAS lavas includes (1) tholeiitic basalt fragments from either the ocean crust or seamount interior, (2) a range of differentiated gabbros largely derived from the ocean crust, (3) residual mantle spinel lherzolites, and (4) pyroxenite and peridotite cumulates. The abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the SAS suggests that these lavas probably fractionated in a magma chamber at the crust-mantle boundary. The occurrence of orthopyroxine-bearing alkalic cumulate xenoniths in these lavas, however, is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates. The SAS vents of Jasper define a NE-SW volcanic trend which is orthogonal to the FTS rift. The pattern of volcanic activity, including periods of volcanic quiescence, and the general increase in alkalinity, as well as the structural reorganization of magmatic feeder systems of Jasper Seamount, is strikingly similar to the patterns observ