[1] We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3 He/ 4 He ratios (17-28 R A ) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3 He/ 4 He ratios are restricted to the preshield stage of Hawaiian magmatism. 3 He/ 4 He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3 He/ 4 He ratios (up to 8 R A in $10 2 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3 He/ 4 He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206 Pb/ 204 Pb, anomalously low 207 Pb/ 204 Pb relative to 206 Pb/ 204 Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3 He/ 4 He reservoir in the Hawaiian plume and suggest that the proportion of the different mantle components in the plume have changed significantly in the past 5 Myr. The long-term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas require more complex geochemical heterogeneities than suggested by radially zoned plume models. These complexities may arise from heterogeneities in the thermal boundary layer and through variable entrainment of ambient mantle by the upwelling plume.

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We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540øC) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the sitemean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56 ø ñ 10 ø) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (ñ 1300 kin) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes. Introduction The Canadian Cordillera, including southeastern Alaska, is divided into a series of physiographic/geologic provinces that are roughly parallel to the continental margin. Paper number 95TC01579. 0278-7407/95/95TC-01579510.00 Mountain Belt. Of these, both the Insular Belt (Terrane II) and the Intermontane Belt (Terrane I) are superterranes composed of smaller tectonostratigraphic terranes that are termed "suspect" because they are evidently allochthonous with respect to the North American craton [e.g., Monger et al., 1982]. The Coast Plutonic Complex and the Omineca Crystalline Belt are the broad loci of sutures joining these two allochthonous superterranes to each other and to the continent. The Intermontane superterrane accreted to North America in Middle Jurassic time, and the outboard Insular superterrane accreted to the Intermontane superterrane sometime between the Middle Jurassic and Late Cretaceous [Monger et al., 1982; van der Heyden, 1992; Rubin et al., 1990]. A regionall...

Thellier paleointensity results from an R‐N transition zone in Kauai, Hawaii, show that field intensity dropped from 0.431 Oe to 0.101 Oe while the field remained within 30° of the reversed axial dipole direction. A recovery in intensity and the main directional change followed this presumably short period of low field strength. As the reversal neared completion, the field had an intensity of 0.217 Oe while still 40° from the final direction. The relationship of paleointensity to field direction during the early part of the reversal thus differs from that toward the end, a feature that only some reversal models are consistent with. For example, a model in which a standing nondipole component persists through the dipole reversal predicts only symmetric intensity patterns. In contrast, zonal flooding models generate suitably complex field behavior if multiple flooding schemes operate during a single reversal or if the flooding process is itself asymmetric.

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