New Zealand Journal of Geology and Geophysics abstracts
A low-grade metamorphic model for the Miocene volcanic sequences in the Andes
of central Chile
L. AGUIRRE1
D. ROBINSON2
R. E. BEVINS3
D. MORATA1
M. VERGARA1
E. FONSECA4
J. CARRASCO1
1Departamento de Geología
Universidad de Chile
Casilla 13518, Correo 21
Santiago, Chile
2Department of Geology
University of Bristol
Bristol, United Kingdom
3Department of Geology
National Museum of Wales
Cardiff, United Kingdom
4Servicio Nacional de Geología y Minería
(SERNAGEOMIN)
Casilla 10465
Santiago, Chile
Abstract Calc-alkaline basic volcanic rocks in a
c. 600 m thick sequence of Miocene age, the Valle Nevado stratified
sequence (VNSS), have been affected by very low grade metamorphism
characterised by mineral assemblages of the zeolite facies. Metastable
conditions prevailed, most of the igneous minerals being wholly or partially
preserved. The main metamorphic phases are mafic phyllosilicates and zeolites
of calco-sodic and calcic composition. The intensity of the metamorphism was
controlled by depth and by hydrothermal activity related to volcanic centres.
From top to bottom of the sequence, the zeolites vary from
heulandite-clinoptilolite through mordenite to laumontite, whereas the
phyllosilicates show a compositional transition from tri-smectite to
smectite/chlorite with up to 75% chlorite layers. Strong fluid/rock interaction
took place only at flow levels with a high primary porosity and permeability as
illustrated by: (a) the similarity of the REE patterns of the basaltic host
flows and the secondary zeolites; and (b) the contrast in composition observed
between phyllosilicates in vesicular tops as opposed to the massive (central)
parts of the same flow. The thermal gradients acting during the metamorphic
event were high and are estimated at 150-175deg.C/km. The metamorphic zonation
is interpreted as the result of a rapid accumulation of considerable volumes of
rock generated by volcanic centres connected with geothermal field activity and
characterised in places by caldera collapse.
Keywords Andes; Chile; very low grade metamorphism;
phyllosilicates; zeolites; calc-alkaline volcanism; high thermal gradients
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 83-93
0028-8306/00/4301-0083 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1004K); (scanned from paper original: notes about this process)
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