Phyllite member
Phyllite member
Jurassic and Late Triassic(?
Primarily phyllite, phyllitic sandstone, and metaconglomerate chiefly derived from sandstone, shale, andesite tuff, and sparse marble. Forms an elongate belt east of the Melones fault and west of the Sonora fault from the Mokelumne River southeast to the Oakhurst pendant at Coarsegold. Unit includes part of phyllite of Briceburg (Triassic) of Bateman and Krauskopf (1987). Part of the Don Pedro terrane of Schweickert (2015). Unit locally structurally interleaved with volcanic flows and lesser pyroclastic rocks unit (unit MRDGS). This unit also includes a limestone block (olistolith) that has yielded Late Triassic conodonts (Paterson and Wainger, 1991). This unit is intruded by the ~167-177 Ma plutons of the Don Pedro intrusive suite (unit Gdp), thus much of the unit must be that age or older. In addition, detrital zircon analysis from this unit has yielded MDAs in two groups, ~168-171 Ma and ~255-350 Ma (Attia and others, 2021). The younger MDAs match the Jurassic age of the other age indicators, but the older MDAs are much older, and so perhaps reflect recycling of older rock, as suggested by the limestone olistoliths. However, part of the greenstone (see unit MRDGS below), so also probably the interleaved phyllite of this unit, is Late Jurassic, postdating the plutonic intrusion. Metamorphosed at lower greenschist facies. Metamorphic Ar-Ar age for greenstone of 163-161 Ma is probably the same as phyllite metamorphism of this unit. Primary structures in most areas obliterated by intense deformation. This unit, along with units of volcanic flows and lesser pyroclastic rocks (unit MRDGS) and granitic rocks (unit Gdp) interpreted as parts of the Early to Middle Jurassic Don Pedro island arc overlap assemblage deposited mainly on the western margin of Calaveras Complex (unit MRCS).