GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 290-8
Presentation Time: 3:45 PM


KEITH, Stanley B. and RASMUSSEN, Jan, MagmaChem Exploration, PO Box 950, Sonoita, AZ 85637

Evidence for tectonic burial by SW directed thrusting/subduction is present in the lower plate of the Santa Catalina crystalline complex. Here the Oracle Ridge/Marble Peak porphyry copper skarn system is present adjacent to the Leatherwood quartz diorite-granodiorite intrusive complex dated at 69 Ma. The Marble Peak Cu skarn is typical of Laramide copper skarns (eg. Twin Buttes, Mission-Pima, and Christmas formed between 450 and 300oC at 2-5 km depth (Force, 1997). However, the Leatherwood intrusions unlike other Laramide intrusions of similar age and petrochemistry in the upper plate contain an epidote-alkali feldspar assemblage and secondary amphibole produced from plagioclase breakdown (Hanson, 1966) interpreted as metamorphic and Anderson and others 1988 interpret as magmatic and equilibrated with a 21 km paleodepth. We accept the metamorphic model of Hanson and the 21 km paleodepth indicated by the mineral chemistry and argue that this deep metamorphism is induced by thrust burial between 65 and 55 Ma on a former shallowly emplaced porphyry copper system. The thrust burial was achieved by SW directed thrusting along the regionally distributed Maricopa Thrust system proposed by Keith (1983) and Keith and Wilt (1986). Formation of the Maricopa Thrust coincided with emplacement of very hydrous peraluminous magmatism and contact hydrous alkali metasomatism widely present in lower plate windows exposed between the leading edge of the Maricopa Thrust and the Colorado Plateau. In the Santa Catalina lower plate the local representative of the peraluminous magmatism is the Wilderness batholith complex of Keith and others (1980) where geobarometry indicates an emplacement of about 15 km (Anderson and others, 1988). In effect a significant amount of Arizona's porphyry copper metallogeny was subducted during plate scale flat subduction of the Farallon Plate that reached very shallow levels between about 60 and 34 Ma. Upward strain transfer from the SW vergent interface between the NE subducting the Farallon slab and the overriding North American plate resulted in at least 2 major trench/SW directed thrust systems within the North American plate: the Peninsular Range and Maricopa thrust systems. The effect of post-porphyry intraplate subduction has yet to be fully evaluated by the copper exploration industry.