Jökull - 01.01.2001, Blaðsíða 59

Caldera formation at Cerro las Cumbres, Mexico
Figure 9. Photomicrograph of the dome rocks. The section is 4.5 mm across. The three phenocrysts present
are pyroxene absorbed on its edges, rounded quartz and sanidine feldspar. Groundmass is made up of pla-
gioclase, clinopyroxene and titano-magnetite. – Ljósmynd af þunnsneið úr bergi hraungúlsins. Ljósmyndin er
45 mm í þvermál. Þrír kristallar sjást á myndinni, pýroxen með ávalar hlíðar, ávalt kvars og sanidine feldspat.
Grunnmassi er úr plagíóklas, clínópýroxen og títanómagnetíti.
9). This is the same phenocryst assemblage observed
in the pumices, but the dome phenocrysts differ in
that they are subhedral to anhedral. In addition, the
dome lava contains subhedral olivine and clinopyrox-
ene. The volume of the lava dome is calculated as
1.5 km


DRE (Table I).
GEOCHEMISTRY
Whole rock composition of samples from the dome
and ash–flow deposits, and three pumices from the
pumice-fall deposits were analyzed by ICP-MS at
Clermont-Ferrand (Table II).
According to the anhydrous total alkalis–silica
chemical classification system, (Figure 10) (Le
Maitre, 1989), the ash–flow deposits and the dome
are dacitic, whereas the pumice–fall deposits are rhy-
olitic (Table II). The bulk composition of the the ash–
flow deposits may have been modified by the loss of
fine light components such as glass or feldspar etc.,
the concentration of dense minerals, or the incorpora-
tion of lithic clasts (Luhr, personal communication).
Figure 10 shows analyses of lavas from the neighbor-
ing volcano Pico de Orizaba and surrounding satelitic
cones (Höskuldsson, 1992), and depicts the general
geochemical trend of magmas from the region. The
plots shows the evolved nature of the rhyolites. If
the mineral assemblage of the dome lava, is related to
the mixing of two magmas, then the basaltic magma
would probably be within this compositional range.
RECONSTRUCTION OF EVENTS
In reconstructing the 16 ka Cerros las Cumbres erup-
tion it is necessary to consider the environmental
conditions prevailing at the time. Cerro las Cum-
bres has an altitude of 3,950 m at the highest peak
on the caldera rim. Just prior to the eruption and
collapse of the central part the cone, the volcano is
thought to have been considerably higher, perhaps
4,500-5,000 m (Figure 11–1). At present, mountains
in Mexico reaching this altitude, such as Popocatépetl,
Iztaccihuatl and Pico de Orizaba, are all capped by
small glaciers. Paleoclimatic studies from the Gulf
of Mexico show that during the late Wisconsinan, the
climate was much colder than it is today (Kennett
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