A SEVERED HEAD: CHEMICAL LINKS BETWEEN SILETZIA AND THE COLUMBIA RIVER FLOOD BASALTS

Mafic large igneous provinces (LIPs), oceanic plateaus and continental flood basalts, are produced from the convecting heads of deep-seated mantle plumes, and related hotspots tracking away from individual LIPs are created by the plume tails [1]. The late Cenozoic Yellowstone hotspot (YHS) trace along the Snake River plain is related to the Columbia River flood basalt (CRFB) LIP, however the latter is a small province (2.1 x 10⁵ km³) with a lower eruption rate than other mafic LIPS. An alternative candidate for the YHS plume head volcanic accumulation is the much larger Siletzia oceanic plateau (2 x 10⁶ km³), accreted to the western margin of North America while still active during the Eocene [2,3]. In this model, the plume head was cut off by Siletzia accretion and the establishment of an eastward-dipping subduction zone to its west. The CRFB lavas are then melts from a secondary head, fed by the plume tail, that accumulated beneath the subducting slab after Siletzia accretion, and broke through during the Miocene [4]. If so, it should be possible to trace some affinity between Siletzia and CRFB lavas, despite the well-documented chemical heterogeneity of mantle plumes globally [5]. Siletzia rocks fall into two distinct areas in Pb-Pb isotope space. The dominant population lies on the Northern Hemisphere Reference Line (NHRL), while a second group has high delta-7/4. The most primitive main-phase CRFB lavas belong exclusively to the high delta-7/4 group, with which they share affinities in key trace element ratios. These relations are consistent with production of both Siletzia and the CRFB from a single heterogeneous plume. Siletzia is dominated by melts of convectively-entrained NHRL upper mantle in the plume head, plus a small contribution from the high delta-7/4 plume tail. In contrast, the CRFB are dominated by melting products of the plume tail, which conforms to the EM II type ocean island basalt mantle source [6]. Chemical data therefore provide support for the single plume Siletzia–CRFB–YHS model.

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