Regionally and compositionally coherent magma sources for Mesozoic and Tertiary granitoids in Nevada and eastern California north of 37 degrees latitude
- Geological Survey, Menlo Park, CA (United States)
Since 1977, more than 2,000 Mesozoic and Tertiary granitoid samples in Nevada and eastern California north of 37[degree] latitude have been analyzed for [sup 87]Sr/[sup 86]Sr. Though widely different in age, the samples plotted simultaneously on the measured [sup 87]Sr/[sup 86]Sr versus [sup 87]Rb/[sup 86]Sr diagram relate functionally to inferred source isochrons. Limiting lines, defined by minimum [sup 87]Rb/[sup 86]Sr per [sup 87]Sr/[sup 86]Sr per [sup 87]Sr/[sup 86]Sr coordinates of coherent data fields, approximate loci of parent magmas derived from time-integrated Rb-Sr sources. Source isochrons can be modeled from the limiting lines, assuming appropriate Rb/Sr melt-solid partitioning and degree of partial melting. Three inferred regional sources are: (1) an Early Proterozoic lower continental crust of approximately tholeiitic composition, yielding initial [sup 87]/[sup 86]Sr (Sr[sub i]) > 0.705; (2) slightly more alkalic (higher Rb/Sr) Late Proterozoic rift-generated lower crust, underplated during incipient rifting beneath uplifted and attenuated continental crust along the locus of the rift, yielding Sr[sub i] between 0.705 and 0.707; and (3) a Late Proterozoic rift-generated lower crust, of composition similar to (1) and emplaced as oceanic crust as the continental segments moved apart, yielding Sr[sub i] between 0.704 and 0.705. In this interpretation, the boundary between S[sub n] greater than 0.705 and less than 0.705 conceptually locates on rifted continental margin. Between Winnemucca and Austin, this boundary lies approximately 45 kilometers west of the Sr[sub i]=0.706 isopleth, and in the vicinity of Lovelock, takes a ridge-transform like 90[degree] bend from a NE-SW to NW-SE trend. Source (2), yielding of Sr[sub i] between 0.705 and 0.706, mostly occupies the intervening space.
- OSTI ID:
- 5022366
- Report Number(s):
- CONF-9305259-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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MAGMA
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580000* - Geosciences