Nd- and Pb-isotope variations in the multicyclic central caldera cluster of the San Juan volcanic field, Colorado, and implications for crustal hybridization

Journal Article · Mon Oct 01 04:00:00 EDT 1990 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1990)018<0975:NAPIVI>2.3.CO;2· OSTI ID:6313787

Riciputi, L R; Johnson, C M ^[1]

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1. Univ. of Wisconsin, Madison (USA)

The {epsilon}{sub Nd} values for six large-volume (100-3000 km{sup 3}) ash-flow tuffs and associated lavas from the multicyclic central caldera cluster of the San Juan volcanic field in south-central Colorado are between those of Proterozoic crust in the region and mantle-derived basaltic magmas, and the values generally become progressively higher in progressively younger tuffs and lavas. The increase in the {epsilon}{sub Nd} values of the tuffs, from -8.0 to -6.0 with decreasing age, can be modeled by assimilation and crystal fractionation of a mantle-derived magma, accompanied by an increase of {approx} 4 units in {epsilon}{sub Nd} values of the assimilated crust. The postulated increase in {epsilon}{sub Nd} values of the crust is envisioned to have occurred by hybridization of the crust through continued injection of mantle-derived magmas during the life of the magmatic system. Decreasing {sup 206}Pb/{sup 204}Pb ratios observed in progressively younger tuffs following the initiation of caldera-related volcanism cannot, however, be solely explained by addition of mantle-derived magmas to the crust, but are more likely to reflect the transfer of lower-crustal Pb into the upper crust as the magmatic system evolved. Input of large volumes (> 300,000 km{sup 3} in the San Juan volcanic field) of mantle-derived magma resulted in extensive hybridization of preexisting crust, suggesting that large-scale silicic volcanism involves generation of large quantities of new crust.

• OSTI ID: 6313787
• Journal Information: Geology; (USA), Journal Name: Geology; (USA) Vol. 18:10; ISSN GLGYB; ISSN 0091-7613
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALPHA DECAY RADIOISOTOPES
BASALT
CALDERAS
COLORADO
CONTINENTAL CRUST
EARTH CRUST
EVEN-EVEN NUCLEI
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC MODELS
GEOLOGY
HEAVY NUCLEI
HOURS LIVING RADIOISOTOPES
IGNEOUS ROCKS
ISOMERIC TRANSITION ISOTOPES
ISOTOPE RATIO
ISOTOPES
LAVA
LEAD 204
LEAD 206
LEAD ISOTOPES
MAGMA
NEODYMIUM ISOTOPES
NORTH AMERICA
NUCLEI
PRECAMBRIAN ERA
RADIOISOTOPES
ROCKS
STABLE ISOTOPES
TUFF
USA
VARIATIONS
VOLCANIC REGIONS
VOLCANIC ROCKS
VOLCANISM
YEARS LIVING RADIOISOTOPES