GEOCHEMICAL STUDY OF LIKELY CAMBRIAN OR ORDOVICIAN AGE ROCKS OF THE AVERY BROOK FORMATION AND AN UNNAMED TONALITE NEAR CAUCOMGOMOC LAKE, NORTHERN MAINE

The Avery Brook Formation is a thick unit of deformed massive greenstones and greenschists, locally pillowed, and likely of Cambrian or Ordovician age. It contains metamorphic chlorite, plagioclase, actinolite, epidote, and opaques, with some relict igneous plagioclase and clinopyroxene in some samples. In fault contact with the Avery Brook Formation is the Hurd Mountain Formation, also likely of Cambrian or Ordovician age. It is mélange with knockers of greenstone and limestone, locally intruded by metagabbro and meta-diabase, metamorphosed to greenschist grade. The Hurd Mountain Formation is also intruded by a large stock of tonalite that is undeformed and only slightly metamorphosed. The tonalite contains plagioclase, quartz and hornblende partially recrystallized to chlorite, actinolite and epidote.

Avery Brook and tonalite samples were analyzed by XRF and ICP-MS. Geochemical discrimination diagrams show that the Avery Brook plots in mid-ocean ridge (MOR) and volcanic arc (VAB) fields. On Zr/4-Y-Nb*2, Th-Ta-Ha/3, Th/Yb-Ta/Yb, and Zr-Y*3-Ti diagrams, the Avery Brook plots in both VAB and MOR fields. On Cr-Y and Ti/1000-V discrimination diagrams the Avery Brook plots in the MOR field. On Ce-Nb and Y/15-La/10-Nb/8 diagrams, the Avery Brook plots in backarc basin and VAB fields. The MORB-normalized diagram of the Avery Brook shows a Ta-Nb negative anomaly indicating a supra-subduction zone setting. The chondrite-normalized diagram shows no enrichment of incompatible elements indicating a depleted mantle source. Overall, samples from the Avery Brook Formation display characteristics of both MOR and VAB tectonic settings, and is similar to previous analyses of metabasalt knockers and intrusive metagabbros in the Hurd Mountain Formation.

The tonalite samples have higher silica contents ranging from 69% to 72% and plot in volcanic arc fields of the Rb-Y-Nb and Y-Nb granite discrimination diagrams.

The geochemical data of the Avery Brook and tonalite, and their relationship with the Hurd Mountain mélange is consistent with a ridge subduction event, although a collapsed backarc basin setting is also permissible. The tonalite likely intruded shortly after deformation, but while temperatures were still at greenschist facies conditions.