Geochronology of older Precambrian rocks in Gila County, Arizona

Abstract

A sequence of Older Precambrian volcanic and sedimentary rocks more than 15,000 feet thick occurs in the Blackjack Mountains and White Ledges, 20 miles north of Globe, Arizona. This sequence consists of (from older to younger) the Redmond formation (acidic volcanic rocks) and the Hess Canyon group (clastic sedimentary rocks). The Hess Canyon group is subdivided into: the White Ledges formation (interbedded quartzites and argillaceous rocks); the Yankee Joe formation (argillaceous strata with interbedded graywackes and arkoses); and the Blackjack formation (argillaceous quartzites). These rocks have been intruded by the Ruin Granite (a porphyritic quartz monzonite) and subsequently eroded to approximately the present level of exposure prior to the deposition of the Younger Precambrian Apache Group. The unconformity between the Older and Younger Precambrian strata is well exposed at Butte Creek north of Haystack Butte. Diabase has intruded the Blackjack formation, the Ruin Granite and the Apache Group. No Paleozoic or Mesozoic rocks are known to occur within the surveyed area. Sediments and volcanic and sedimentary rocks of Tertiary and Quaternary age partly conceal the older rocks. The Hess Canyon group can be correlated with the Deadman Quartzite, Maverick Shale, and Mazatzal Quartzite of the Mazatzal Mountains (Wilson 1939a) and also the Houden Formation of the Diamond Butte Quadrangle (Gastil 1958). Whole rock Rb-Sr dating indicates an age of 1,510 ± m.y. for the Redmond formation. Isotopic dating of the Ruin Granite near the Blackjack Mountains and of the granitic rocks intruded the Mazatzal Quartzite of Four Peaks in the southern Mazatzal Mountains indicates that the Mazatzal Orogeny (the Mazatzal Revolution of Wilson, 1939a) occurred 1,425 to 1,380 m.y. ago in central Arizona. This orogeny followed the deposition of the Mazatzal Quartzite and the Hess Canyon group, terminating older Precambrian time in Arizona and was followed by the deposition of the Younger Precambrian Apache Group. Isotopic dating of volcanic metamorphic and plutonic rocks in the Pinal and Tortilla Mountains and near Roosevelt Dam on the Salt River indicate that portions of the Pinal Schist in the type locality are greater than 1,730 m.y. old and that these rocks have experienced a complex series of events in Older Precambrian times. The Madera Diorite of Ransome (1903) consists of rocks 1,730 ± 30 m.y. old as well as rocks about 1,500 m.y. old. The Older Precambrian igneous rocks in this part of Arizona appear to have developed from material similar in Rb to Sr ratio to average shallow continental crust. These rocks formed during the interval 1,730 to 1,370 m.y. ago. The continental crust in this region probably originated no earlier than about 1,800 m.y. ago. Igneous rocks younger than 1,370 m.y. have not been derived soley from average shallow crustal material.