HYDROTHERMAL AMPHIBOLE IN SUBGREENSCHIST FACIES METAVOLCANIC ROCKS, NORTHERN SIERRA NEVADA, CALIFORNIA

Unusual sub-calcic amphiboles are associated with Pmp+Prh+Chl+Ep+Ab+Qtz in volcanic rocks and intrusive diabases in northern California. Similar amphibole in other arcs and ophiolites has been attributed to very low grade, regional metamorphism. However, the sub-calcic amphiboles are distinct from the actinolite that is commonly reported in the Prh-Pmp facies and we interpret them to have formed during hydrothermal metamorphism associated with the intrusion of the abundant diabases. Amphibole in the hydrothermal assemblages is sub-calcic and rich in ferric iron (Ca=0.96-1.42, Na+K=0.20-0.73; Fe=1.97–5.0; Fe³⁺=0.70-1.91; 15eK calculation) Deficiencies in the "B" site are satisfied primarily with Fe. In rocks that are not affected by the hydrothermal alteration, actinolite is the common amphibole (Ca > 1.92; Na +K < 0.08; Fe < 2.22; Fe³⁺ < 0.5). In the hydrothermal association, the amphibole is less magnesian than either Chl or Pmp and Pmp is more magnesian than coexisting Chl. In rocks unaffected by the hydrothermal alteration, Act is more magnesian than both Chl and Pmp and Pmp is mostly less magnesian than coexisting Chl. Independent evidence suggests that the sub-calcic amphibole assemblage was formed in an hydrothermal environment. This assemblage is common to intrusive diabases and volcanic rocks within a five km²area, which is completely surrounded by Pmp-Prh facies rocks. The sub-calcic amphiboles are found as veins that cut Cpx phenecrysts, as euhedra in hydrothermal Qtz and Prh, and as needles around amydules in the Pmp-bearing groundmass of volcanic flows that otherwise lack amphibole. The location of the amphibole is apparently controlled by the original porosity of the rock. Monomineralic patches of amphibole are cut by late Pmp veins. Thus, we infer that the sub-calcic amphibole was formed by hydrothermal interactions during the intrusion of the mafic diabases into the volcanic sequence.