FORMATION OF INTERMEDIATE AND ULTRABASIC METASOMATIC ROCKS WITHIN A REACTION ZONE SURROUNDING SUBDUCTED PERIDOTITE, CEMETERY RIDGE, SOUTHWEST ARIZONA

Blocks of subducted peridotite (partially serpentinized harzburgite, serpentinite from dunite, tremolite serpentinite) at Cemetery Ridge are enclosed within quartzofeldspathic schist of the Late Cretaceous (to early Paleogene?) Orocopia Schist. These peridotite blocks are enveloped by metasomatic rocks formed by mechanical and metasomatic interaction of chemically contrasting schist and ultramafic rock, in the presence of a Ca-bearing subduction-zone fluid. Field relations, whole-rock compositions, and mineral compositions determined by EMP indicate a two-stage process.

Stage 1 produced the major metasomatic rock unit, the spotted gneiss (SG), which is gradationally derived from quartzofeldspathic schist (QFS), and forms a chemical and mechanical boundary layer between schist and peridotite. Spotted gneiss (SiO₂ 60 %) comprises variable proportions of porphyroblastic albite (An₀₃) and actinolite (mg 70), including abundant veins and pods of actinolite-rock and local albitite. SG formed by mechanical mixing of peridotite with QFS, metasomatic addition of Ca and Na from subduction-zone fluid, and comprehensive loss of LILE.

Stage 2 generated several other varieties of metasomatic rocks, considerably less voluminous than SG but genetically informative. Most significant is the blackwall-like chlorite-tremolite schist (CTS), composed of dominant clinochlore, with subordinate tremolite (mg 85), olivine (Fo₇₅), spinel (pleonaste, mg 60), and (locally) anthophyllite. Although ultrabasic (SiO₂ 35 %) and ultramafic (MgO 27 %), CTS contains detrital zircon and has REE patterns and abundances of HFSE (Ti, Zr, Hf, Nb, Ta, P, Sn) indicative of derivation from SG and, ultimately, QFS. Metasomatic fluids that converted SG to CTS were generated by alteration of serpentine to tremolite within the serpentinized peridotite, a reaction that consumed Si and Ca while releasing Mg. The resulting Mg-rich, Si-poor fluid interacted with SG, stripping Si to convert albite to pseudomorphous chlorite and adding Mg to convert actinolite to tremolite. Upon diminution of fluid flow, partial dehydration of chlorite led to the formation of olivine and pleonaste.

The metasomatic rocks at Cemetery Ridge provide critical insights into fluid flow and metasomatism at high metamorphic grade within an exhumed subduction zone.