The signature of amphibole in mafic alkaline lavas, a study in the northern Canadian Cordillera

Despite evidence for its involvement, the importance of amphibole in controlling the compositions of mafic alkaline magmas remains under-appreciated. Relatively small variations in large ion lithophile elements (LILE) with respect to other incompatible elements, such as light rare-earth elements (LREE) or Th, require that amphibole was an important residual phase during the production of Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmas beneath the northern Canadian Cordillera. The erupted mafic magma types define a continuous array from Ol-NEPH to hypersthene-normative olivine basalt (Hy-NORM AOB). The overall compositional array has a sense of curvature which is counter to binary mixing, but can be modeled by two distinct linear melting trends: one from Ol-NEPH to basanite (BASAN) compositions, during which amphibole controlled the composition of the melt, and the ratios of LILE/LREE change significantly, but the ratios of high field strength elements (HFSE) remain relatively constant; the other from BASAN to Hy-NORM AOB corresponding to the melting of a lherzolite assemblage, following the exhaustion of amphibole, across which the ratios of LILE/LREE remain relatively constant, but the ratios of HFSE change significantly.Other intraplate alkaline suites, such as those of the Hawaiian Islands, show similar evidence for the involvement of residual amphibole in the genesis of Ol-NEPH to BASAN magmas. The melting of any amphibole-bearing mantle assemblage is likely to be a two-step process, regardless of whether the amphibole is segregated as veins or distributed interstitially. In a water-undersaturated environment, the first stage of melting is controlled by the breakdown of amphibole, which produces silica-saturated liquids below 12 kbar and silica-undersaturated liquids at greater depths, with little contribution from other mineral phases. In the second stage, following the exhaustion of amphibole, the major element compositions of subsequent melts change rapidly to equilibrate with a lherzolite mineralogy, but the incompatible trace-element characteristics of the former amphibole persist.