Paper No. 3
Presentation Time: 8:40 AM
THE GROWTH OF THE NORTHERN YAVAPAI PROVINCE: THE CASE FOR A RETREATING ACCRETIONARY OROGEN
JONES, Daniel S., Geosciences, Western Carolina University, Cullowhee, NC 28723, SNOKE, Arthur W., Department of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 E. University Avenue, Laramie, WY 82071 and BARNES, Calvin G., Department of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409-1053, dsjones@wcu.edu
Rocks of the northernmost Yavapai province, traditionally assigned to the Green Mountain arc, include two temporally and compositionally distinct, bimodal, igneous suites. Gabbroic rocks of the older (~1780-Ma) suite have the geochemical characteristics of arc magmas, consistent with previous interpretations of the coeval Green Mountain Formation volcanic rocks. Felsic intrusive rocks of this suite are mostly granodioritic and have geochemical characteristics of island-arc granitic rocks. Gabbroic rocks of the younger (~1763-Ma) bimodal suite are rich in Fe and incompatible trace-elements compared to the older gabbroic rocks and exhibit both arc-like and MORB/WPB-like geochemical characteristics. The felsic rocks of this suite (including the Sierra Madre Granite) have the geochemical signatures of within-plate granites. Both intrusive suites were deformed by a major shortening event at ~1750 Ma, which juxtaposed them against the Archean Wyoming province along the Cheyenne belt. The 1780-Ma bimodal suite is interpreted as reflecting island-arc magmatism, whereas the younger suite is interpreted as having an extensional origin, possibly reflecting the rifting of the 1780-Ma arc to form a back-arc basin. The 1750-Ma shortening event has no obvious proximal cause and is interpreted to reflect far-field effects of tectonic activity to the south (present coordinates).
Correlating geologic events from the Cheyenne belt region and central Colorado shows that the ~1763-Ma extensional event is coeval with both Dubois magmatism and extensive fine-grained sedimentation in north-central Colorado, possibly reflecting slab rollback and the formation of a back-arc basin related to a Dubois arc. The ~1750-Ma contractional event coincides with the end of Dubois magmatism suggesting a genetic link such as slab shallowing. The end of contraction corresponds to the onset of Cochetopa magmatism. It is suggested that the northern Yavapai province formed as a result of cycles of convergence and extension within an environment of continuous subduction. Key implications of this hypothesis are that: (1) exposed “arcs” may be rifted fragments, not individual arcs; (2) extensional magmatism played a volumetrically major role in crustal growth; and (3) arc-arc collisional sutures may not exist within the province.
