ABSTRACT Blocks of variably serpentinized oceanic mantle peridotite (harzburgite, olivine orthopyroxenite, and dunite) are entrained within a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust. Correspondingly, the Cemetery Ridge peridotite contains four serpentinization-related sulfide or intermetallic minerals quite unusual for an inland, continental tectonic setting: pentlandite, cobalt pentlandite, heazlewoodite, and awaruite. The peridotite also contains two Ni-arsenide minerals, orcélite and maucherite; and, less commonly, the sulfides pyrrhotite, bismuthinite, bornite, and parkerite(?). These minerals form tiny to small (~3–100 μm) grains sparsely scattered amongst the profusion of serpentine and magnetite produced by serpentinization of olivine; many grains are enclosed in magnetite. The three principal sulfide minerals at Cemetery Ridge are pentlandite [(Ni,Fe) 8.93 S 8 ] in all samples studied, and cobalt pentlandite [(Co,Ni,Fe) 9.01 S 8 ] and heazlewoodite (Ni 2.98 S 2 ) in many or most. Pentlandite and cobalt pentlandite form a discontinuous series to 74 molar percent of the Co end member. High-Co pentlandite has systematically elevated Ni/Fe. Pyrrhotite (Fe 7.88 S 8 ) occurs only in one high-S sample. Although orcélite (Ni 4.71 As 2 ) and maucherite (Ni 11.06 As 8 ) are volumetrically rare, one or both are found in most samples. Awaruite (Ni 5 Fe) is extremely rare, a few minute blebs in two samples. Sulfide assemblages at Cemetery Ridge indicate the highly reduced pore fluid typical of serpentinization. Sulfur was introduced into Cemetery Ridge peridotite during the early stages of serpentinization, as one aspect of a general enrichment in fluid-mobile elements in a suprasubduction (mantle-wedge) environment. Further serpentinization was accompanied by progressive desulfurization, with concomitant transformation from minerals of higher to lower sulfidation, and partial transfer of first Fe then Ni from sulfide to oxide phases. Five Ni or Ni-Co sulfide or arsenide minerals at Cemetery Ridge are new and unexpected for Arizona, where serpentinite is quite rare and Ni or Co deposits unknown. Sulfide minerals and assemblages at Cemetery Ridge are comparable to those of serpentinites in such places as the California and Oregon Coast Ranges and Mid-Atlantic Ridge, emphasizing the uniqueness of the tectonic setting of the subducted oceanic peridotite at Cemetery Ridge, and enhancing the status of the Orocopia and related schists as the world’s type (first-recognized, best-known) low-angle paleosubduction complex.

Among supracrustal sequences of the Jurassic magmatic arc of the southwestern Cordillera, the Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona, is remarkable for its lithologic diversity and substantial stratigraphic thickness, ≈8 km. The Topawa Group comprises four units (in order of decreasing age): (1) Ali Molina Formation—largely pyroclastic rhyolite with interlayered eolian and fluvial arenite, and overlying conglomerate and sandstone; (2) Pitoikam Formation—conglomerate, sedimentary breccia, and sandstone overlain by interbedded silt-stone and sandstone; (3) Mulberry Wash Formation—rhyolite lava flows, flow breccias, and mass-flow breccias, with intercalated intraformational conglomerate, sedimentary breccia, and sandstone, plus sparse within-plate alkali basalt and comendite in the upper part; and (4) Tinaja Spring Porphyry—intrusive rhyolite. The Mulberry Wash alkali basalt and comendite are genetically unrelated to the dominant calcalkaline rhyolite. U-Pb isotopic analyses of zircon from volcanic and intrusive rocks indicate the Topawa Group, despite its considerable thickness, represents only several million years of Middle Jurassic time, between approximately 170 and 165 Ma. Sedimentary rocks of the Topawa Group record mixing of detritus from a minimum of three sources: a dominant local source of porphyritic silicic volcanic and subvolcanic rocks, identical or similar to those of the Topawa Group itself; Meso-proterozoic or Cambrian conglomerates in central or southeast Arizona, which contributed well-rounded, highly durable, polycyclic quartzite pebbles; and eolian sand fields, related to Middle Jurassic ergs that lay to the north of the magmatic arc and are now preserved on the Colorado Plateau. As the Topawa Group evidently represents only a relatively short interval of time, it does not record long-term evolution of the Jurassic magmatic arc, but rather represents a Middle Jurassic “stratigraphic snapshot” of the arc. This particular view of the arc has been preserved primarily because the Topawa Group accumulated in deep intra-arc basins. These nonmarine basins were fundamentally tectonic and extensional, rather than volcano-tectonic, in origin. Evidence from the Topawa Group supports two previous paleogeographic inferences: the Middle Jurassic magmatic arc in southern Arizona was relatively low standing, and externally derived sediment was introduced into the arc from the continent (northeast) side, without appreciable travel along the arc. We speculate that because the Topawa Group intra-arc basins were deep and rapidly subsiding, they became the locus of a major (though probably intermittent) fluvial system, which flowed into the low-standing magmatic arc from its northeast flank.

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Full article available in PDF version.