Rapid uplift and crustal growth in extensional environments: An isotopic study from the Death Valley region, California
- Harvard Univ., Cambridge, MA (USA)
- Univ. of Maine, Orono (USA)
The Willow Spring Diorite, in the Black Mountains of the central Death Valley extended terrain, yields a U-Pb zircon age of 11.6 {plus minus} 0.2 Ma. {sup 40}Ar-{sup 39}Ar analyses of hornblende and U-Pb analyses of sphene from this sample give ages of about 10 Ma, indicating that the batholith remained above about 500 C for about 1.5 m.y. after crystallization. Geologic relations indicate that the diorite was exposed to erosion by about 5 Ma, bracketing the evolution of the diorite within the time between onset of extension and uplift of the Black Mountain crustal block. Initial {sup 87}Sr/{sup 86}Sr ratios range from 0.7060 (mafic diorite) to 0.7083 (felsic diorite) in samples collected from an area 200 x 100 m. These data, combined with structural and petrologic evidence, suggest that the batholith represents a rare view of a mid-crustal zone of mixing between mantle-derived magma and crustal material, often suggested to exist on the basis of observations of intermediate volcanic rocks. The Black Mountains may therefore expose a cross section through a continental rift magmatic system, from partially contaminated mafic to intermediate intrusive rocks in the deep crust up to their volcanic equivalents. The relatively low initial {sup 87}Sr/{sup 86}Sr and high {epsilon}{sub Nd} ({minus}1.4) of the diorite, which is within Proterozoic basement with {epsilon}{sub Nd} {approximately}{minus}18, is consistent with significant amounts of mantle input in continental rifts inferred from geophysical data and measurement of He isotopic ratios. Such additions to the crust in continental rights may represent a significant process of crustal growth. Furthermore, the emplacement of igneous bodies with a large mantle component may help reconcile the large crustal pull apart in the Basin and Range (in excess of 140 km) with the fact that the crust still has as normal thickness of about 30-35 km.
- OSTI ID:
- 6357614
- Journal Information:
- Geology; (USA), Journal Name: Geology; (USA) Vol. 18:3; ISSN GLGYB; ISSN 0091-7613
- Country of Publication:
- United States
- Language:
- English
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580000* -- Geosciences
ACTINIDE ISOTOPES
ALKALINE EARTH ISOTOPES
AMPHIBOLE
ARGON 39
ARGON 40
ARGON ISOTOPES
BASEMENT ROCK
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CALIFORNIA
CONTINENTAL CRUST
DIORITES
EARTH CRUST
EARTH MANTLE
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION IX
GEOLOGIC AGES
GROUND UPLIFT
HORNBLENDE
HOURS LIVING RADIOISOTOPES
IGNEOUS ROCKS
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPE RATIO
ISOTOPES
LEAD ISOTOPES
LIGHT NUCLEI
MAGMA
MINERALS
MOUNTAINS
NEODYMIUM ISOTOPES
NORTH AMERICA
NUCLEI
PLUTONIC ROCKS
PRECAMBRIAN ERA
RADIOISOTOPES
ROCKS
SILICATE MINERALS
STABLE ISOTOPES
STRONTIUM 86
STRONTIUM 87
STRONTIUM ISOTOPES
TECTONICS
TITANITE
URANIUM ISOTOPES
USA
VALLEYS
VOLCANIC ROCKS
YEARS LIVING RADIOISOTOPES