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Crustal Growth in Southern Arizona: U-Pb Geochronologic and Sm-Nd Isotopic Evidence for Addition of the Paleoproterozoic Cochise Block to the Mazatzal Province

Department of Geosciences, University of Arizona, Tucson, Arizona, 85721

Terranes composed of arc-related igneous and sedimentary rocks were accreted onto the continental margin of southwestern North America from 1.9 to 1.6 Ga. They formed the Yavapai-Mazatzal province in Arizona, of which the so called ‘Pinal Schist’ and associated rocks in southeastern Arizona form an integral part. 148 U-Pb analyses on magmatic and detrital zircons were performed on 21 samples to constrain timing of geologic events in Paleoproterozoic time in the area of southeastern Arizona. 25 Sm-Nd whole rock analyses were made to infer sources of sedimentary and igneous rocks. Our results allow us to delineate a suture zone between the Pinal tectonostratigraphic unit (Pinal block), which consists primarily of basinal metaturbiditic rocks, and a southeastern allochthonous juvenile volcano-sedimentary domain. The location of the suture is constrained by lithologic contrast across the boundary, an increase in age of detrital zircons, a marked decrease of _Nd(t), and an intervening subduction complex (Swift and Force, this issue). We propose the name Cochise block for the southeastern part, because it represents a faultbound, allochthonous tectonic unit.

In the Cochise block, we document volcanic activity from about 1630 to 1647 Ma. Quartzitic sedimentary rocks were largely derived from almost contemporaneous volcanic rocks and show detrital zircon ages from 1630 to 1729 Ma, with the majority (85 percent) between 1630 to 1674 Ma. Quartzites and intermediate to felsic metavolcanic rocks show a tight cluster of _Nd(t)-values (2.9–3.8) close to the depleted mantle model and therefore represent juvenile additions to the crust. Post-tectonic metabasalts with _Nd(1500) = 4.5 are interpreted as being derived from a depleted upper mantle source around 1.5 Ga.

In the Pinal block, detrital zircons yield values from 1678 to 1731 Ma. _Nd(t)-values of metapelites and wackes range from –0.2 to 2.3. Values decrease from southeast to northwest, indicating progressive input of more evolved material toward the craton. Our data suggest that sedimentation within the Pinal block was proximal to the continental margin and occurred later than 1.68 Ga and ended before the intrusion of granitoids at 1.65 Ga.

We show that the North American crust grew rapidly and progressively by addition of juvenile but evolved metavolcanic arc rocks and derived quartz-rich metasedimentary rocks as documented in the Cochise block. Timespans between depleted mantle model age and zircon crystallization on the order of 50 my attest to rapid crustal processing during this significant time of crustal growth.

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