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| Title: | The 40Ar/39Ar geochronology and thermochronology of the Latir volcanic field and associated intrusions: Implications for caldera-related magmatism |
| Authors: | Zimmerer, Matt |
| Issue Date: | 26-Aug-2009 |
| Abstract: | Volcanic and plutonic rocks exposed in the Latir volcanic field, Sangre de Cristo
Mountains of northern New Mexico, provide a unique opportunity to study caldera-related
magmatism and understand the spatial and temporal relationship between the
volcanic and plutonic record. Fifty-one samples were dated using 40Ar/39Ar method. The
results indicate a 10 Ma period of Latir volcanic field related magmatism. The volcanic
geochronology provides point-in-time information about magmatism whereas the
thermochronology of plutonic rocks establishes their emplacement and cooling history.
Volcanic rocks provide information about the earliest magmatism associated with
Latir volcanic field. Precaldera volcanism began at 28.3 Ma and ended at 25.3 Ma, based
on 40Ar/39Ar analysis of hornblende, biotite, and sanidine from exposed volcanic rocks.
Combining the published geochemistry with ages of precaldera volcanism from this study
indicates that the earliest magmatism was characterized by multiple, small magma
chambers, rather than a single, large magma chamber. Peak magmatism occurred during
the eruption of the 500 km3 peralkaline Amalia Tuff from the Questa caldera. Sanidine
analyses from eleven samples yielded a mean age of 25.23 Ma for the Amalia Tuff.
Following the eruption of the Amalia Tuff, four resurgent plutons were emplaced
in the shallow crust near the center of the caldera. K-feldspar multiple diffusion domain
(MDD) thermal models indicate that the plutons cooled rapidly after emplacement. By
24.7 Ma, within 500 ka of caldera eruption, all the plutons cooled to 150°C. A biotite
from the previously undated Canada Pinabete pluton, a resurgent pluton chemically
similar to the Amalia Tuff, yields an age 25.28 Ma. Because the Canada Pinabete pluton
and Amalia Tuff are geochemically similar and their ages are analytically
indistinguishable, the Canada Pinabete pluton is interpreted as non-erupted Amalia Tuff.
This supports the idea that ignimbrite magma chambers may not completely drain during
eruption and plutons can be directly correlated to large-scale ignimbrite sheets. The
remainder of the resurgent plutons are slightly younger than the Amalia Tuff and record a
compositional transition to lesser-evolved magmas. Three postcaldera rhyolites yield
sanidine ages between 24.9 and 25.0 Ma indicating coeval volcanism with emplacement
of the resurgent plutons.
After resurgent plutonism, three plutons, probably cupolas of a larger, single
intrusion, were emplaced and are now exposed along the southern caldera margin. Biotite
ages from the Red River, Sulfur Gulch, and Bear Canyon plutons are 24.8, 24.5, and 24.3
Ma, respectively, suggesting incremental emplacement of the larger intrusion along the
southern caldera margin. K-feldspar monotonic MDD thermal histories from the
individual plutons display differences of rates and timing of cooling. MDD models
suggest the Red River pluton experienced a period of isothermal cooling at 300°C
between 24 and 22 Ma, followed by rapid cooling at 21 Ma. One K-feldspar MDD
thermal model from the Bear Canyon indicates rapid cooling at 21 Ma, but another Bear
Canyon K-feldspar thermal model indicates rapid cooling at 23 Ma, followed by
isothermal conditions at 200°C between 22 and 18 Ma. The unconstrained MDD thermal
models suggest reheating by younger thermal events possibly related to magma
emplacement.
The two youngest plutons, Rio Hondo and Lucero Peak, were emplaced 5-15 km
south of the caldera. An associated study of U-Pb zircon ages suggest that the Rio Hondo
pluton was possibly incrementally emplaced between 23 and 22.5 Ma. Biotite collected
from multiple locations in the Rio Hondo pluton yield ages of ~21 Ma, indicating that
following incremental emplacement, the different increments comprising the pluton
cooled to 350°C at the nearly the same time. K-feldspar MDD monotonic cooling models
indicate a period of slow to isothermal cooling between 21 and 16 Ma. Alternatively, the
unconstrained modeling results show a thermal perturbation at 16.5 Ma, which
corresponds to the age of a Rio Hondo hosted rhyolite dike. A single age of 22.5 Ma
from a postcaldera andesite on Brushy Mountain suggests coeval volcanism with the
emplacement of the Rio Hondo pluton. Biotite ages are ~19 Ma from both the interior
and margin of the Lucero Peak pluton. Similarly, K-feldspar cooling histories from the
interior and margin of the pluton both suggest slow cooling between 19 and 16 Ma. The
similarity of cooling histories between marginal and interior unites, combined with the
lack of robust reheating models, is interpreted to be the result of a complex emplacement
history, rather than simple batch emplacement of a pluton. In summary, 40Ar/39Ar results
from this study describe magmatism at different times associated with caldera-volcanism,
and provide insight into the relationship between the volcanic and plutonic record. |
| URI: | http://hdl.handle.net/10136/505 |
| Appears in Collections: | Independent Studies
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| Zimmerer, Matt.pdf | | 23953Kb | Adobe PDF | View/Open |
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