Regionally and compositionally coherent magma sources for Mesozoic and Tertiary granitoids in Nevada and eastern California north of 37 degrees latitude

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5022366

Robinson, A C ^[1]

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1. 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--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALKALINE EARTH ISOTOPES
CALIFORNIA
CENOZOIC ERA
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC STRUCTURES
GEOLOGY
GRANITES
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
MAGMA
MAGMATISM
MESOZOIC ERA
NEVADA
NORTH AMERICA
PETROGENESIS
PETROLOGY
PLUTONIC ROCKS
RIFT ZONES
ROCKS
STRONTIUM ISOTOPES
TERTIARY PERIOD
USA