PINAL SCHIST OF SOUTHERN ARIZONA: EVIDENCE FOR SPREADING RIDGE-TRENCH INTERACTIONS IN THE PALEOPROTEROZOIC?

The Pinal Schist is the basement rock of southeastern Arizona. It shows many characteristics consistent with the hypothesis that it was part of a subduction complex associated with a Paleoproterozoic (1.64-1.7 G.y.) Mazatzal volcanic arc. In a (SE) direction perpendicular to the Mazatzal arc, the Pinal Schist is exposed over a distance of at least 300 km in southeastern Arizona (from the Phoenix area to Bisbee). This “exposure width” is in the range of modern arc-trench gaps. Thus, the Pinal Schist could represent a largely intact Paleoproterozoic fore-arc terrane. The Pinal Schist is composed largely of interbedded meta-wackes and schists interpreted as turbidites. The Pinal meta-wacke/schist sequences locally contain various types of meta-volcanic rocks some of which appear to have oceanic affinities (for example, MORB) while others appear to have arc affinities. The metamorphic grade of this terrane ranges normally from subgreenschist facies to upper greenschist facies although amphibolite facies assemblages are found locally. Neither high P/T facies rocks (e.g., blueschists) nor ultramafic rocks have been identified in the Pinal terrane in over 100 years of mapping. Various granitic intrusions also of Paleoproterozoic age are present within the Pinal terrane. These granitic bodies were intruded into the Pinal meta-wacke/schist terrane tens to hundreds of kilometers south of the Mazatzal volcanic arc. Structural/tectonic features of Paleoproterozoic age associated with the Pinal terrane include mélanges, thrust faults, shear zones, and other structures typical of subduction complexes. Kinematic indicators generally suggest northwest-directed transport. In many ways, the various lithologic and tectonic features of the Pinal terrane are similar to those observed in Phanerozoic fore-arc terranes that have experienced spreading ridge-trench interactions. For example, the Prince William Terrane of south-central Alaska shows many similarities to the Pinal terrane including fore-arc granitic intrusions and an absence of blueschists that appear to have resulted from ridge-trench interactions in Early Tertiary time. The Neogene subduction complex of western California also experienced ridge-trench interactions and offers potential analogs for various features of the Pinal complex.