摘要：Abstract
Cenozoic basalts carrying ultramafic mantle
xenoliths occur in the Matilde, León and Chenque hills
in the Paso de Indios region, Argentina. The mantle xenoliths
from the Chenque and León hills mainly present
porphyroclastic textures, whereas the Matilde hill xenoliths
have coarse-grained to porphyroclastic textures. The
equilibrium temperatures are in the range of 780 to 940ºC,
indicating a provenance from shallow sectors of the lithospheric
mantle column that were subjected to a relatively
low heat ffiux at Cenozoic Era.
According to the modal compositions of xenoliths, the
mantle beneath Matilde and León hills was affected by
greater than 22% partial melting, while less depleted peridotites
occur in the Chenque suite (starting from 10%
partial melting). Such an observation is confirmed by the
partial melting estimates based on Cr#Sp, which vary from
8 to 14% for the selected Chenque samples and from 14 to
18% for the Matilde ones.
The common melting trend is overlapped by small-scale
cross cutting local trends that may have been generated by
open-system processes, such as open-system partial melting
and/or post partial-melting metasomatic migration of
exotic Na-Cr-rich melts.
The two main mineralogical reaction schemes are: i) the
dissolution of pyroxenes and the segregation of new
olivine in olivine-rich peridotites, and ii) the replacement
of primary olivine by orthopyroxene±clinopyroxene
in orthopyroxene-rich peridotites. These were produced
by channelled and/or pervasive melt extraction/
migration. Enhanced pyroxene dissolution is
attributed to channelling of silica- undersaturated
melts, whereas the replacement of primary olivine by
orthopyroxene±clinopyroxene points to reaction with
silica-saturated melts.
Late disequilibrium reactions identified in the xenoliths
comprise: the breakdown of orthopyroxene in contact with
the host basalt, and (rarely) reaction coronae on orthopyroxene,
clinopyroxene and spinel linked to glassy veins.
Such features are apparently related to the injection of
melt, likely during entrainment into the host basalts and
ascent to the surface.
关键词：Keywordsmantle xenoliths alkaline basalts Paso de Indios
Patagonia
