| Description |
Light green, silvery green and greenish-brown mafic, feldspathic, and calcareous schist. Typically occurs as frost-riven slabs and flakes that underlie rounded hills and dark greenish-black tors and rubble piles several meters across. Tors of metabasite, abundant plagioclase porphyroblasts in dark-green, chlorite-rich schist, and the quartz-poor nature of the rocks are characteristic of this unit. The most common lithologies are dominated by components of mafic, feldspathic, and calcareous composition that are intermixed and interlayered on a scale of tens of centimeters; the layering may occur in repetitive couplets. Medium- to pale-grayish green weathering pelitic schists are common. Plagioclase, chlorite, white mica, and quartz in subequal amounts dominate these rocks; epidote, carbonate, and glaucophane (or pseudomorphs of chlorite and plagioclase after glaucophane) are typical of many of these schists. Titanite (sphene), rutile, and sulfides are present in minor amounts. Based on major element chemistry, the protoliths of these schists were shales and graywackes (Werdon and others, 2005a). Carbonate-rich schists or layers are typically buff or pale brown weathering and tend to be more recessive in outcrop than other lithologies. Pure carbonate layers are rare and thin but include both pure and impure varieties; they weather pale brown, black, or gray. Dark green weathering schists are rich in chlorite, epidote, actinolite, and plagioclase, and represent metamorphosed mafic material. Dark green weathering chlorite-rich schists spotted with white equant plagioclase grains typically contain few to no calcium-bearing phases. These are probably mafic rocks that were altered or weathered previous to metamorphism. Boudins, lenses and layers of fine- to coarse-grained, massive metabasite comprise the greenish-black tors of the unit. In thin section these rocks are found to be composed of glaucophane, actinolite, chlorite, epidote, garnet, albite, white mica, titanite, and locally quartz, Fe-carbonate, pyroxene, and barroisite. Coarser-grained varieties have textures suggestive of a coarse-grained gabbroic protolith. Mafic schist layers in the surrounding rocks have mineral assemblages similar to the metabasite pods. The metabasites comprise two compositional groups (Werdon and others, 2005a,c). One group has weakly developed arc-like signatures (e.g., slight Nb depletion in spidergrams) reflecting crustal contamination, and another group exhibits features associated with enriched mantle (E-MORB) and alkaline intercontinental rifts (no Nb depletion, small positive Ti anomalies in spidergrams) (Ayuso and Till, 2007). Metabasites from unit DOx fall into the same two compositional groups. The chemical characteristics are thought to indicate a tectonic setting related to the early stages of continental, rift-related magmatism (Ayuso and Till, 2007); crustal contamination during rifting is thought to have produced the weak arc-like signatures. In the western Solomon quadrangle, the contact between Ocs and the overlying impure marble unit (Oim) is exposed. Near the contact, on all sides of a synform cored by Oim, a thin (few meters - tens of meters) layer of black weathering, platey, fine grained and finely laminated quartz-graphite schist occurs. The amount of graphite in the rock is variable, though it is always black-weathering; thin laminae of lenses (mm- to cm-scale) that are more quartz-rich are common. Graphite occurs as fine disseminated material in the quartz-rich matrix, as well as in lozenges several mm across. White mica is disseminated and minor. Semi-quantitative spectrographic analyses of a few samples from this layer show elevated values of Mo, V, Ag, and Zn (B. Gamble, written commun., 1985). Thin layers of mafic schist separate the graphitic layer from the overlying impure marble. No direct evidence for the depositional age of Ocs exists. Seven detrital zircon samples collected from widely distributed parts of the unit contain very similar grain populations. Most grains fall into the range of 600-700 Ma; several samples contain small populations of Ordovician or Cambrian grains (Amato and others, 2003a; Till and others, 2006; 2008a). The depositional age of the unit must be younger than 600 Ma (latter part of the Neoproterozoic), and is likely Ordovician or younger. Werdon and others (2005a) considered the unit to be Cambrian in age, based on an Rb-Sr isochron. The samples included in the isochron are a mix of mafic and pelitic rocks, so their assumption that the samples shared the same initial strontium isotopic composition is likely not correct; the isochron represents a mixing line between mafic and sedimentary protoliths. The Ordovician age assigned here is based on the detrital zircon geochronology and on the occurrence in both this and the impure marble unit (Oim) of both metabasite schist layers and unfoliated metabasite pods. The protoliths of both units apparently contained pyroclastic or redeposited mafic material as well as intrusive mafic rocks. The metabasite layers indicate that production of mafic material was at latest syn-depositional - not simply post-depositional. We postulate that Oim and Ocs were formed in the same basin. Because the impure marble unit yielded Early through Middle Ordovician conodonts, we believe that basin was formed during the Ordovician. The unit is 0.6 to 1.6 km thick and is best exposed in southeastern Solomon D-5 quadrangle, north of the Nome-Council road on the ridge north and northwest of Horton Creek; in the central part of the Solomon D-5 quadrangle at the headwaters of Alma and Venture Creeks; and in east-central Solomon D-6 quadrangle on the ridgeline between Eldorado and Nelson Creeks, including hills 2144 and 2067. The Casadepaga schist was named and first described by Smith (1910). Partially equivalent to the "slate of the York region" of Sainsbury (1974), and "pCqms" of Miller and others (1972); equivalent to "Ocs" of Till and others (1986) |
