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| Titel |
Timing and evolution of Late Oligocene to Miocene magmatism in the southern Sierra Madre Occidental silicic large igneous province: insights from zircon chronochemistry and Ar/Ar geochronology |
| VerfasserIn |
Aldo Ramos Rosique, Scott Bryan, Luca Ferrari, Charlotte Allen, Margarita López Martínez, Andrew Rankin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040256
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| Zusammenfassung |
| The Sierra Madre Occidental silicic large igneous province (SLIP) of western México (SMO)
is one of the largest SLIPs known. Here we present new field, U-Pb zircon and 40Ar/39Ar
geochronologic and chemical data from the central Bolaños graben (southern SMO); together
with previous work (Bryan et al, 2008; J Petrol) on the SMO that sheds light on the temporal
evolution of silicic magmatism. The exposed succession in the Bolaños graben spans at least
10 Myr, with a ~1 km thick Oligocene volcanic pile dominated by ignimbrites interbedded
with resedimented pyroclastic units (Zuloaga Series). The Zuloaga Series includes
both crystal-poor (30%) welded ignimbrites, with the
crystal-poor types often being densely welded, high-grade ignimbrites. A distinctive
ignimbrite (the Alacrán ignimbrite) and many rhyolitic domes were then emplaced at
~24-23 Ma, which are also interbedded with basaltic lavas. These rhyolites are
very crystal-poor (-¤5%) with the ignimbrites being distinctly poorly welded and
they show distinctive zircon ages and chemistries in contrast with the preceding
Oligocene Zuloaga Series and ignimbrites from the northern SMO. Capping the erupted
succession is the non-welded Chimal Tuff (18.4 ± 0.4 Ma, 40Ar/39Ar) and basaltic
lavas.
U-Pb age data from the northern SMO rhyolites show well-defined unimodal peaks in the
age distribution, although there is some zircon inheritance in some units. In contrast,
ignimbrites from the Bolaños area, particularly the ~24-23 Ma suite of rhyolite ignimbrites
and domes have more complex and polymodal zircon age distributions, and the U-Pb zircon
population age can be up to 6 Myr older than the associated 40Ar/39Ar age. The combined
zircon age and whole-rock chemical data reveal a trend toward polymodal and inherited
zircon age distributions and zircon undersaturation of magmas through time. This is
well-expressed in the ~23 Ma (40Ar/39Ar) Alacrán ignimbrite with its low zircon abundance
but high antecrystic zircon content (clustering at ~29 Ma), and where ~100% of the zircons
are inherited. The inherited zircon ages suggest the Alacrán ignimbrite magma was sourced
from igneous crustal rocks formed during previous stages of SMO magmatism.
The ~2 Myr age gap between the youngest zircon ages in this ignimbrite and the
40Ar/39Ar age argues against mush remobilisation and instead suggests re-melting
has occurred. In addition, the Alacrán ignimbrite also has anomalous whole-rock
chemistry such as enrichments in Rb, Cs, K and Al characteristic of clay compositions,
suggesting that the crustal source for this ignimbrite may also have been hydrothermally
altered.
The combination of zircon chemistry, U-Pb geochronology and whole-rock chemistry
indicates the Early Miocene Bolaños graben rhyolites are the result of re-melting of highly
differentiated granites generated during preceding phases of SMO activity, and which in part,
may also have been hydrothermally altered. A long-term change in the locus and source of
silicic magma generation from the Oligocene to Miocene is thus suggested by these data,
whereby the source of melt generation appears to have moved upwards through the crustal
profile. Crustal extension and shallow emplacement of significant volumes of basalt
associated with Early Miocene graben formation in the southern SMO may have promoted
crustal melting at shallower depths. The petrogenetic evolution appears to also be
reflected in the deposit characteristics of the ignimbrites, which change from moderate
to high-grade and weakly lava-like welded ignimbrites erupted in the Oligocene
(reflecting deeper, higher-temperature melt generation) to an Early Miocene suite
of poorly to non-welded but sintered, crystal-poor, Zr undersaturated ignimbrites
(reflecting mid- to upper-crustal and lower temperature melting) that notably will have a
low preservation potential in the geologic record. This temporal trend in changing
magma source depths may thus be poorly preserved in older silicic igneous provinces. |
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