Geochemical and isotopic evidence for the petrogenesis of the northeastern Idaho batholith, Bitterroot Range, Montana and Idaho
Isotopic analyses of igneous rocks, metasedimentary inclusions, and intruded metasedimentary country rocks of the northeastern portion of the Idaho batholith indicate a significant contribution from a lower crustal source region to the intrusive rocks. Rb-Sr and U-Pb isotopic data indicate that the source was depleted in Rb and U and was heterogeneous or the mixing process between crustal and mantle sources was complicated. Zircons from the interior of the batholith were found to have rounded cores and euhedral overgrowths. U-Pb isotopic analysis of these zircons yielded a lower intercept age of 69 Ma and an upper intercept age of approx.1760 Ma. This is interpreted to be a mixing line between the crystallization age of the granitic rocks (60 Ma old overgrowths) and the average age of the source (1760 Ma old cores). The zircon cores are most likely inherited from the source region of the igneous rocks and were not assimilated from the intruded country rocks. Rb-Sr and U-Pb isotopic data and REE data also show that the intruded country rocks had little effect on the chemistry of the intruding magmas and were not the source of this portion of the Idaho batholith. A model of partial melting of a lower crustal source region that is heterogeneous, Proterozoic in age, and Al-rich, and leaving garnet as a residual phase is presented. It appears that differences between the chemistry of the Sierra Nevada batholith and the Idaho batholith are the result of melting of different types of source regions, not different amounts of contamination of mantle-derived melts.
- Research Organization:
- Kansas Univ., Lawrence (USA)
- OSTI ID:
- 5216582
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GEOLOGIC DEPOSITS
GEOCHEMISTRY
GEOLOGIC HISTORY
GEOLOGIC MODELS
IDAHO
MONTANA
AGE ESTIMATION
IGNEOUS ROCKS
ISOTOPE RATIO
LEAD
RUBIDIUM
SEDIMENTARY ROCKS
STRONTIUM
URANIUM
ZIRCON
ACTINIDES
ALKALI METALS
ALKALINE EARTH METALS
CHEMISTRY
ELEMENTS
FEDERAL REGION VIII
FEDERAL REGION X
METALS
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
USA
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES
580400* - Geochemistry- (-1989)