Resetting of whole rock and mineral Rb-Sr ages by subsequent proterozoic orogenies

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6577406

Bickford, M E; Van Schmus, W R

The U-Pb age range for Wopmay, Trans-Hudson, and Penokean Orogens is 1.83-1.90 Ga, but in each orogen Rb-Sr isochron ages are commonly 1.74-1.78 Ga. Orogenic rocks in Colorado, northern New Mexico, and northern Arizona yield U-Pb ages of 1.73-1.80 Ga, but Rb-Sr isochron ages of these rocks are commonly 1.64-1.68 Ga. This, in turn, is the age range of zircons from orogenic rocks in southern New Mexico and Arizona. Granite and rhyolite in central Wisconsin yield zircon U-Pb ages of 1.76 Ga but give excellent Rb-Sr isochron ages of 1.63 Ga, the zircon age of granitic plutons and volcanic rocks in the subsurface of northeastern Kansas and northern Missouri. Rocks in Colorado whose zircon ages are 1.63-1.78 Ga commonly have yielded Rb-Sr mineral ages of 1.34-1.45 Ga. Recently, a terrane of younger granite and rhyolite whose zircon age is 1.34-1.40 Ga has been discovered in the subsurface of the western midcontinent; at least one small pluton of this age intrudes the older volcanic rocks in the St. Francois Mountains. These relationships indicate that Rb-Sr ages are reset by later igneous events related to renewed orogenic activity and do not necessarily record the age of metamorphism associated with primary arc accretion. In contrast, U-Pb ages normally record the time of primary crystallization. The heat that resets Rb-Sr ages is presumably transferred via renewed magmatism, possibly by eruption of volcanic cover, later removed by erosion, upon older terranes. These relationships suggest subduction dipping beneath newly accreted rocks during most orogenies on the southern margin of North America.

• Research Organization: Univ. of Kansas, Lawrence (USA)
• OSTI ID: 6577406
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580100* -- Geology & Hydrology-- (-1989)
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COMPARATIVE EVALUATIONS
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