Abstract

In west-central Arizona near the northeast margin of the Basin and Range Province, the Rawhide detachment fault separates Tertiary and older rocks lacking significant effects of Tertiary metamorphism from Precambrian, Paleozoic, and Mesozoic rocks in the Harcuvar metamorphic core complex below. Much of the northern part of the Harcuvar complex in the Buckskin and eastern Harcuvar Mountains is layered granitic gneiss, biotite gneiss, amphibolite, and minor pelitic schist that was probably deformed and metamorphosed in Early Proterozoic time. In the eastern Buckskin Mountains, Early and Middle Proterozoic plutons having U-Pb zircon ages of 1,683±6.4 mega-annum (Ma) and 1,388±2.3 Ma, respectively, intruded the layered gneiss. Small plutons of alkaline gabbro and diorite intruded in Late Jurassic time. A sample of mylonitized diorite from this unit has a U-Pb zircon age of 149±2.8 Ma. In the Early Cretaceous, amphibolite facies regional metamorphism was accompanied by partial melting and formation of migmatite. Zircon from a granitic layer in migmatitic gneiss in the eastern Harcuvar Mountains has a U-Pb age of 110±3.7 Ma. In the Late Cretaceous, sills and plutons of the granite of Tank Pass were emplaced in both the Buckskin and eastern Harcuvar Mountains. In the Buckskin Mountains those intrusions are locally numerous enough to form an injection migmatite. A pluton of this granite crops out over almost half the area of the eastern Harcuvar Mountains.

Paleozoic and Mesozoic sedimentary rocks were caught as slices along south-vergent Cretaceous thrusts related to the Maria fold and thrust belt and were metamorphosed beneath a thick sheet of Proterozoic crustal rocks.

Inception of volcanism and basin formation in upper-plate rocks indicates that regional extension started at about 26 Ma, in late Oligocene. The Swansea Plutonic Suite, composed of rocks ranging from gabbro to granite, intruded the lower-plate rocks in the Miocene and Oligocene(?). Granite and a gabbro from the suite have a U-Pb zircon age of 21.86±0.60 Ma. Previously published ⁴⁰Ar/³⁹Ar ages of hornblende suggest that some of the Swansea Suite is Oligocene. The felsic rocks contain numerous inclusions ranging from porphyritic granite to porphyritic granodiorite. A sample from one inclusion has a U-Pb zircon age of 1,409±6.3 Ma. A discordia line for the U-Pb zircon data from the Swansea Plutonic Suite has an upper intercept at 1,408±3.4 Ma. The Swansea Plutonic Suite probably formed by interaction between mantle material and plutonic rocks at least as old as Middle Proterozoic. An irregular layer in the middle crust, which is thickest under and adjacent to the Buckskin Mountains, may be the level where that interaction took place.

During extensional deformation these rocks and all the older rocks were displaced southwest from beneath the rocks of the Colorado Plateau transition zone below an area extending 50–80 kilometers northeast of the Buckskin Mountains as far as Bagdad, Arizona, or beyond. At that time the rocks were variably mylonitized, and a northeast-trending lineation formed. Much of the evidence for the complex sequence of structural events preserved in these rocks in the western Harcuvar Mountains has been obliterated in the northern Harcuvar complex by Miocene deformation.

Version 1.0

Posted January 2008

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