AN ANOMALOUSLY FE-RICH BLUESCHIST FROM THE AMBLER DISTRICT, BROOKS RANGE, ALASKA

We are investigating a blueschist-greenschist sequence from a paleo-seafloor basin found in the Dead Creek prospect of the Brooks Range Schist Belt, Alaska (centered near 67.21, -156.51), within the Ambler Mineral District. This section comprises a stack of metasediments and metabasalts which includes an anomalously Fe-rich glaucophane-pargasite schist. The metabasalts from Dead Creek and nearby areas display relict pillow, fragmental, and porphyritic textures suggesting seafloor volcanism and possible shallow intrusions. They contain glaucophane cores variably replaced and rimmed by actinolite, garnet (Alm_55-69Grs_26-36), chlorite, phengite (with up to 6.9 MgO+FeO wt.% and up to 2.3 wt.% BaO), clinozoisite, albite, quartz, and calcite. The glaucophane-pargasite schist is dense, bluish-black, and crudely foliated; samples so far analyzed contain up to 35% garnet (Alm_60-75Grs_23-34), ferro-glaucophane as the main foliation-defining phase, ferro-pargasite locally replacing ferro-glaucophane and defining a cross-cutting foliation, clinozoisite, titanite, albite, quartz, and chlorite. The least Fe-rich amphibole and chlorite from the schist is more Fe-rich than any amphibole and chlorite from the metabasites above it. The glaucophane-pargasite schist plots as a subalkaline basalt along with its neighboring metabasalts in Nb/Y-Zr/Ti space (Pearce, 1996), but differs in other ways. The schist contains 19.4 wt.% FeO, on the high end of reported ranges for North American ferrobasalts (NAVDAT.org), and 1.6 wt.% MgO, yielding an Fe# (100*wt.% Fe/[Fe+Mg]) of 94, higher than ~300 regional non-mineralized metabasites in the Ambler Metals database (mean Fe# = 70.5). The schist’s high Fe/Mg, low Sr (68 ppm), low Ni (3 ppm), and high Y (120 ppm) might represent a residual magma after significant plagioclase and mafic mineral fractionation and possibly Fe-rich pyroxene or hornblende accumulation, or another, unusual magma, or a hydrothermally altered rock, or another species of protolith altogether. Work is ongoing to assess the schist’s protolith and the P-T conditions of the sequence’s metamorphism.