Mark E. Holland

An imbricate midcrustal suture zone: The Mojave-Yavapai Province boundary in Grand Canyon, Arizona

The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed ɛHf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile ɛHf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile ɛHf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains (ɛHf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains (ɛHf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ∼200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.

Polyphase Proterozoic deformation in the Four Peaks area, central Arizona, and relevance for the Mazatzal orogeny

For more than 25 yr, the Mazatzal orogeny has been a central component of virtually all tectonic models involving the Proterozoic rocks of the southwestern United States. Recent recognition that some sedimentary sequences and some major structures are Mesoproterozoic rather than Paleoproterozoic has led to new questions about the nature, even the existence, of the Mazatzal orogeny. This study aims to clarify the relationship between Mazatzal (ca. 1.65 Ga) and Picuris (ca. 1.45 Ga) orogenic activity. New U-Pb geochronology of variably deformed igneous and metasedimentary rocks constrains several periods of deformation at ca. 1.68 Ga, 1.66 Ga, and 1.49–1.45 Ga in the Four Peaks area of central Arizona. Detrital zircon analyses and field relationships indicate the deposition of a rhyolite-sandstone-shale assemblage at ca. 1.660 Ga with renewed deposition at 1.502–1.490 Ga and a significant disconformity, but no recognized angular unconformity, between these episodes. Three populations of monazite growth at 1.484 ± 0.003 Ga, 1.467 ± 0.004 Ga, and 1.457 ± 0.005 Ga indicate prolonged Mesoproterozoic metamorphism. The ca. 1.485 Ga population is associated with the formation of the Four Peaks syncline during Mesoproterozoic orogenesis and subsequent amphibolite-facies contact metamorphism. Rocks in the Four Peaks area record polyphase deformation, sedimentation, and plutonism from the Paleoproterozoic to Mesoproterozoic. Hf-isotopic data suggest the involvement of older, nonjuvenile crust. In this area, effects of the Mazatzal (ca. 1.65 Ga) and Picuris orogenies (ca. 1.49–1.45 Ga) are entwined and involved sedimentation, deformation, pluton emplacement, and pluton-enhanced metamorphism.