UNUSUAL METASOMATIC ROCKS ASSOCIATED WITH SUBDUCTED MANTLE PERIDOTITE IN THE OROCOPIA SCHIST AT CEMETERY RIDGE, SOUTHWEST ARIZONA

Recently discovered blocks of oceanic mantle peridotite (serpentinized dunite, partially serpentinized harzburgite) in the early Paleogene Orocopia Schist subduction complex at Cemetery Ridge in southwest Arizona are accompanied by diverse, mineralogically and chemically unusual metasomatic rocks produced by intensive interaction of the peridotite and host quartzofeldspathic schist (QFS). These metasomatic rocks are of four types (most to least voluminous): (1) Abundant spotted gneiss (SG): porphyroblastic albite + actinolite. (2) Chlorite-tremolite schist (CTS): chlorite + tremolite + secondary olivine + pleonaste + relict serpentine. (3) Veins and pods of massive actinolite or tremolite, some with selvages of tremolite + secondary olivine + pleonaste. (4) Minor (and enigmatic) highly aluminous gneiss (HAG): hornblende + anorthite + epidote ± diopside ± pleonaste. SG, derived from QFS, forms a ductilely deformed boundary zone between schist and peridotite. CTS adjoins and is derived from peridotite. We infer that conversion of QFS to SG and peridotite to CTS were effected largely or entirely by fluid-mediated elemental transfer, as field evidence for mechanical mixing is minimal.

Conversion of QFS to SG involved substantial (≥ 2 ×) gain of Mg, Ca, Mn, Co, Cr, and Ni; and substantial loss of K, Rb, Cs, Ba, Li, Cu, Mo, W, Pb, and Bi. CTS is unusual because it is ultrabasic and ultramafic (SiO₂ 35 %, MgO 27 %) but also aluminous (Al₂O₃ 15 %). Transformation of peridotite to CTS took place by deserpentinization accompanied by advanced Al metasomatism, with pronounced (≥ 5 ×) gain of Al, Ca, Ti, Sr, Th, U, Pb, Zr, Nb, Sn, and REE; and substantial loss of B, Cs, Cr, Ni, As, and Bi. Formation of SG from QFS and CTS from peridotite probably were chemically complementary, at least for some major elements. Specifically, alteration of QFS to SG provided Al and Ca to convert peridotite to CTS, while this transformation released Mg added to QFS to produce SG. The proportion of Al lost from QFS in making SG was small, but compensated by the much greater volume of QFS and SG than CTS. Roles of Ca and Si are still uncertain. Behavior of many trace elements was controlled primarily by availability of mineral hosts.

The metasomatic rocks at Cemetery Ridge provide insights, possibly unique, into fluid processes deep within a low-angle subduction zone.