aguirre1989

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Geological Society, London, Special Publications
The link between metamorphism, volcanism and
geotectonic setting during the evolution of the Andes
L. Aguirre, B. Levi and J. O. Nyström
Geological Society, London, Special Publications 1989, v.43;
p223-232.
doi: 10.1144/GSL.SP.1989.043.01.15
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Notes
© The Geological Society of London 2013
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The link between metamorphism, volcanism and geotectonic
setting during the evolution of the Andes
L. Aguirre, B. Levi & J. O. Nystr~im
S U M MARY: A low-grade, non-deformational, regional metamorphism ranging from
burial to ocean-floor type characterizes the Andean range. Longitudinal variations exist,
coinciding with changes in the chemistry of the volcanic suites involved, both trends
being controlled by the geotectonic setting. The study of Cretaceous volcanic rocks in
Colombia, Peru and Chile indicates that: (i) ocean-floor metamorphism (high-T gradients) correlates with tholeiitic volcanism in marginal basins where the continental crust
was eliminated or extremely attenuated; (ii) burial metamorphism (low- to moderate-T
gradients) correlates with calc-alkaline to shoshonitic volcanism in ensialic, aborted,
marginal basins with moderate thinning of the continental crust. The metamorphism in the
Andes, which is characterized by a counter-clockwise P - T - t path, started during the
initial stages of basin formation and preceded the orogenic activity, contrary to the
development in compressional geotectonic settings.
A relationship between the geochemistry of
volcanic rocks and their tectonic setting has
been demonstrated by numerous authors
(e.g. Pearce 1983). A link between nondeformational low-grade metamorphism and
geotectonics is implicit from the description
of many volcanic terrains. An obvious relationship exists between ocean-floor metamorphism
and the setting for ocean-floor lavas in Recent
and fossil mid-ocean ridges (Spooner & Fyfe
1973, Elthon & Stern 1978, Cann 1979, Stern &
Elthon 1979, Alt & Honnorez 1984, Alt et al,
1986). However, few investigations have been
concerned with the variation of regional nondeformational metamorphism in other settings.
Aberg et al. (1984) and Aguirre & Offler (1985)
suggested that the type of metamorphism,
ocean-floor (steep thermal gradient at an
oceanic spreading centre) versus burial (low to
moderate thermal gradient, essentially related
to load pressure and independent of younger
intrusions, in a continental setting), was related
to basin type (marginal basin proper versus
aborted, ensialic marginal basin) during the
Cretaceous evolution of the Andes. Here, we
give evidence for this suggestion, showing that
metamorphic patterns, palaeothermal gradients
and volcanic chemistry are directly linked to
geotectonic evolution.
The commonly used three-fold division of the
Andean Cordillera into the Northern, Central
and Patagonian Andes (Fig. 1) reflects essential
crustal differences. The continental crust,
thin and with exposure of oceanic crust in the
Northern and Patagonian Andes, attains a
thickness of 70 km in the Central Andes. The
boundaries between the three segments are
major geotectonic breaks: the GuayaquilRomeral Megashear system separates the
Northern and Central Andes, and the Chile
Rise (Nazca-Antarctica plate boundary) marks
the dividing line for the Central and Patagonian
Andes. The evidence for a link between nondeformational low-grade metamorphism and
geotectonic setting emerges from a comparison
of four Cretaceous volcanic sequences: (i) the
Diabase Group in Colombia; (ii) the Casma
Group in Peru; (iii) the Ocoite Group in central
Chile; and (iv) the Rocas Verdes Group in
southernmost Chile (Fig. 1). These units are
the best documented cases of low-grade metamorphism from different Andean tectonic
settings.
The Diabase Group
(Northern Andes)
The Western Cordillera of Colombia is largely
made up of fault-bounded blocks of basaltic
lavas (some pillowed), intrusions and minor
sedimentary intercalations, generally referred
to as the Diabase Group. The age of the Group
is probably Barremian-Turonian (Barrero
1979). The intrusions comprise numerous
dolerite dykes, and some gabbroic and ultrabasic bodies. The total thickness of the Group
probably exceeds 5000 m. However, since
thrusting is the dominating tectonic style
(Bourgois et al. 1982, Marriner & Millward
1984), the original thickness could have been
considerably less. The basalts are low-K
tholeiites with geochemical characteristics
From DALY,J. S., CLIFF,R. A. & YARDLEY,B. W. D. (eds) 1989, Evolution of Metamorphic Belts,
Geological Society Special Publication No. 43, pp. 223-232.
223
224
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L. Aguirre et al.
b
Peru ",
i
"" i
I
Bohwa
1,... ..... ,:--
Chile..;
iI
¢~
s ,'t
;
g
"
i
i Ocoife
', Group
Diobase
Group
J
i
i
Argen-
(a) laumontite _+ (heulandite) + quartz +
chlorite + albite
(b) laumontite + pumpellyite + chlorite +
quartz + albite
(c) laumontite + pumpellyite + prehnite +
chlorite + quartz + albite
tina
,
,,.,
f
tt""
whereas in the prehnite-pumpellyite facies the
following calcite-free association occurs in
amygdules:
Peru
(d) prehnite + pumpellyite +
quartz + chlorite + albite
-Io" CnsrnQ
~I
I
epidote
+
No assemblages diagnostic of the greenschist
facies were reported.
Group
I
among others). Martinet & Millward (1984)
and Millward et al. (1984) favour an oceanic
flood-basalt origin for the Group due to similarities with basalts from the West Pacific Nauru
Basin. An alternative setting of the Diabase
Group, according to these authors, is a marginal basin resembling the Gulf of California.
The rocks of the Diabase Group are affected
by low-grade regional metamorphism in
zeolite, prehnite-pumpellyite and greenschist
facies (Barrero 1979, Espinosa-Baquero 1980,
Rodriguez 1981). The secondary changes have
been ascribed to ocean-floor metamorphism by
Espinosa-Baquero (1980), based on the spilitization pattern and primary chemistry of the
rocks. Bastouil (1985) found that in the B u g a Buenaventura section (4°N; 54 samples studied)
the zeolite facies is represented by the following
calcite-free associations in amygdules:
Group
~
I
Marginal bosins, proper
Marginal bosins, oborted {heavy
hatching in tronsitionol zones)
Oceanic island orc
FIG. 1. Distribution of volcanic rocks in Cretaceous
marginal basins along the western part of the Andes.
The Northern Andes extends from Colombia to the
Guayaquil-Romeral fault system (GR-MS); the
Central Andes is from GR-MS to the Chile Rise
(CHR); the Patagonian Andes lies to the south of the
Chile Rise. Modified from Aguirre (1987).
approaching those of T-MORB (Barrero 1979,
Millward et al. 1984, Bastouil 1985).
The Diabase Group is regarded as an accreted ophiolitic complex by many authors
(Barrero
1979, Espinosa-Baquero
1980,
Bourgois et al. 1982, McCourt et al. 1984,
Based on the composition of the metamorphic minerals Bastouil (1985) concluded that:
(i) prehnite, pumpellyite and epidote are Ferich, indicating a high oxygen fugacity; (ii) the
Fe content is higher in the pumpellyite than in
coexisting epidote; (iii) the X~e values decrease
from pumpellyite through epidote to prehnite
contrary to the trend in most terrains of lowgrade metamorphism, where epidote has the
highest XFe.
The passage from zeolite to prehnitepumpellyite facies is illustrated in a schematic
pseudobinary T - - X F e diagram (Fig. 2) using
the compositions of prehnite, pumpellyite and
epidote present in assemblages (a) to (d). With
increasing metamorphic grade the XFe-COntent
of epidotes decreases from zeolite facies to the
transition to prehnite-pumpellyite facies, then
slightly increases in the latter. The facies transition in the Diabase Group appears to be
defined by the reaction:
laumontite + pumpellyite + quartz =
prehnite + epidote + chlorite + H20
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Andean metamorphism, volcanism and geotectonic setting
PP
144
139
T
225
/
II
134
ZEO
110
109
I
0
~0
\
I
~ l / l [
20
30
I
I
z,o
50
60
100 X Fe3,,
FXG.2. Schematic pseudobinary T - X F e diagram (after Cho et al. 1986) based on the compositions of prehnite
(open bars), pumpellyite (hatched bars) and epidote (filled bars) in calcite-free amygdule assemblages, Diabase
Group, Colombia. One hundred and fifteen microprobe analyses in five polished sections. The samples
(numbers at left) have been placed qualitatively on the T axis in order of increasing metamorphic grade, but the
separation between successive samples is arbitrary. Curves = mean values of Xw in each mineral; horizontal
line = zeolite-/prehnite-pumpellyite-faciesboundary.
The diagram suggests the following approximate
XFe values for the phases in this reaction:
prehnite (0.09), pumpellyite (0.34) and epidote
(0.29). They permit an estimation of the
P - T values for the transition from zeolite to
prehnite-pumpellyite facies, using the petrogenetic grid of Liou et al. (1985): T = 165 +
5°C and P --- 1.05 + 0.05 kbar.
Bastouil (1985) points out that a steep T
gradient typical of ocean-floor metamorphic
conditions can be inferred from the mineral
chemistry of the phases in the prehnitepumpellyite facies assemblages. For example,
the pumpellyites plot in an Fe-rich area of
the A I - F e - M g diagram close to the field of
pumpellyites reported by Liou (1979) from the
Taiwan Ophiolite, which is affected by oceanfloor metamorphism. The proportion of Si is
slightly higher than the ideal 6 atoms per formula
unit, and there is a simultaneous saturation of
the X and W sites. These features point to
metamorphism of a low-pressure type with participation of fluids enriched in Si, Ca and Fe.
Finally, high oxygen fugacity (indicated by the
Fe-rich pumpellyite) and low pressure are consistent with metamorphism taking place under a
shallow water column, a condition compatible
with a marginal basin environment.
The Casma Group (Central Andes)
An approximately 3000-m-thick marine volcanic
sequence of Albian age known as the Casma
Group crops out along the coast of Peru (Fig.
1). The sequence , constituted by several litho-
logical units separated by regional unconformities, occurs as elongated blocks delimited
by faults (Cobbing 1985, and references
therein). Casma rocks from two areas are
treated in this paper: one situated between
10° and 12°30'S (here referred to as the Northern
Casma) and another at c. 14°S (the Southern
Casma).
The Casma Group consists of pillow lavas,
tufts, hyaloclastites and subordinate amounts of
non-volcanogenic sedimentary rocks deposited
in a basin. The Northern Casma lavas are predominantly basalts and range from low-K
tholeiites to calc-alkaline compositions. The
lavas from the Southern Casma sequence are
basaltic andesites and andesites of calc-alkaline
to high-K calc-alkaline affinity (Atherton &
Aguirre, in preparation).
The Casma volcanics are closely associated in
the field with coeval intrusions of gabbro and a
large number of dolerite dykes (suggestive of a
sheeted dyke complex); ultrabasic rocks have
not been reported. Due to mineralogical and
geochemical similarities, a genetic link between the extrusive and intrusive rocks is likely
(Atherton et al. 1985, and references therein).
Gravity profiles have revealed the existence of
dense material (3.0 g cm -3) with a seismic
velocity of 6.66 km s -1 at a high crustal level
along the axis of the Casma basin (Jones 1981,
Couch et al. 1981). The profiles show that
the continental crust below the Casma basin
becomes thinner towards the north, suggesting
that upwelling of mantle material to shallow
levels took place here. The depositional environment of the Casma volcanics was controlled by deep-reaching faults during an
226
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L. Aguirre et al.
extensional regime (Cobbing 1985) which resuited in an extreme crustal thinning.
The 'quasi-ophiolitic' nature of the Casma
Group in its northern exposures, together with
the regional structure and geophysical data,
point to a marginal basin as the most probable
tectonic setting of the Group. However, the
extension did not result in the creation of
oceanic crust, as shown by the lack of a proper
ophiolitic complex and the relatively evolved
chemical character of many of the lavas. Thus,
the Casma basin should be considered as an
aborted marginal basin of ensialic character
since it is situated in continental crust (Atherton
et al. 1985). According to Atherton et al. (1983,
1985) the Casma volcanics are geochemically
similar to lavas from some ensialic back-arc basins erupted during the splitting of a calc-alkaline
volcanic arc (e.g. the basalts from the Sarmiento
complex, the Bransfield Strait, the inner part of
the Gulf of California, and the Scotia Sea).
The Northern Casma volcanics show a complex pattern of low-grade non-deformational
metamorphism, covering the zeolite, prehnitepumpellyite and greenschist facies. Several
facies series comprising the whole or part of this
range are displayed in lithological units separated by regional unconformities. The structural
breaks coincide with breaks in secondary mineralogy, where strata of higher grade overlie
strata of lower grade. This pattern has been
attributed to an episodic history of burial
metamorphism (Aguirre et al. 1978, Offler et al.
1980, Aguirre & Offler 1985).
Rapid changes in facies with stratigraphic
depth within the different facies series of the
Northern Casma sequence demonstrate that
thermal gradients were high to very high. For
example, there is a range from zeolite to greenschist facies in one unit c. 360 m thick,i m p lying
a gradient of more than 300°C km -a_ The
presence of wairakite (Offler et al. 1980) and
the absence of pumpellyite-actinolite assemblages and lawsonite confirm the leading
role of temperature during this metamorphism.
Moreover, the chemical features of actinolites
in greenschist facies assemblages are consistent
with low pressure and a high thermal gradient
(Offler & Aguirre 1984). Secondary assemblages of higher temperature have been recorded in the vicinity of volcanic centres
(Cardozo & Wauschkuhn 1984), and the thickness of the volcanic sequence and the ratio of
volcanic to sedimentary rocks decreases with
lateral distance away from the centres. Hydrothermal metamorphism transitional between
burial and ocean-floor type is in better agreement with these patterns than the previous
interpretation (burial metamorphism). In the
Southern Casma sequence the thermal gradients
were less steep, as shown by a monotonous
display of epidote-poor prehnite-pumpellyite
facies assemblages (Aguirre & Offler 1985,
Atherton & Aguirre, in preparation).
The Ocoite Group (Central Andes)
The 3-13-km-thick Ocoite Group, of Berriasian to Albian age, constitutes a c. 1000-kmlong belt of volcanic rocks in the Coast Range
of central Chile (Fig. 1). It is part of a synclinorium which exhibits an overall symmetry with
regard to structure, stratigraphy and pattern of
cross-cutting dykes (Aberg et al. 1984). The
following description refers to the section between 32030' and 33°30'S (latitude of Santiago).
The lower third of the 9-km-thick sequence
consists mainly of marine and continental volcanic sediments, limestones, dacitic ignimbrites
and interbedded basalts. The main (central)
part of the lava pile is made up of continental
basalts and basaltic andesites morphologically
similar to flood lavas, with conspicuous phenocrysts of plagioclase. In this paper we use the
local name for this characteristic rock type
('ocoite') as the name for the entire Group. The
uppermost part of the sequence consists of continental flow-breccias of basaltic andesite to
andesite composition.
Most of the basic lavas belong to the high-K
calc-alkaline and shoshonite series (Levi et al.
1987). In the northernmost part of the belt the
Group is less thick, ignimbrites are absent or
scarce, and the depositional environment was
predominantly marine. Here, the basic lavas
are calc-alkaline, though poorer in potassium
than in the south.
The lavas of the Ocoite Group were erupted
in an extensional ensialic setting, characterized
as an aborted marginal basin by Aberg et al.
(1984). The huge thickness of the volcanic
pile together with a low geothermal gradient
suggest that the Group was deposited during
the opening (spreading) of a rapidly subsiding
basin (see below). Oceanic crust was never
formed. The high potassium contents of the
basic lavas imply substantial contamination by
continental crust (Levi et al. 1987).
The Ocoite Group ranges in metamorphic
grade from zeolite facies at the top to greenschist facies at the very bottom of the pile (Levi
et al. 1982, Aberg et al. 1984). This implies
a moderate geothermal gradient of around
20-30°C km -~. No breaks have been observed
in the facies series, suggesting that the burial
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Andean metamorphism, volcanism and geotectonic setting
metamorphism giving rise to it took place as a
single, continuous episode. However, the uncomformities that define the top and bottom of
the Group coincide with breaks in secondary
mineralogy (Levi 1969) of the same type as
described above for the Casma Group. The
metamorphic pattern is quite persistent along
the belt through outcrop distances of hundreds
of kilometres. The metamorphism is nondeformational; primary structures and textures
are preserved. Its grade increases with stratigraphic depth, regional facies boundaries being
parallel or subparallel to bedding and not to
contacts with younger granitoids, demonstrating
that the metamorphism is unrelated to the intrusions. These characteristics conform to the
definition of burial metamorphism (Coombs
1961).
Calcite-free amygdules in rocks of the zeolite
facies at the top of the pile are characterized by
the assemblage:
laumontite + chlorite + pumpellyite +
prehnite + epidote
In the prehnite-pumpellyite facies,
characteristic associations include:
the
pumpellyite + epidote + chlorite + prehnite
The appearance of actinolite in the absence of
pumpellyite and prehnite indicates the passage
to greenschist facies at the bottom of the pile.
The pumpellyite and epidote in the zeoliteto prehnite-pumpellyite-facies assemblages
increase in XFe with increasing metamorphic
grade (= depth; Fig.3) as already pointed out
by Levi et al. (1982). In epidote coexisting with
pumpellyite in amygdules, F e 3+ is preferentially
concentrated in the epidote. The transition
from the zeolite to the prehnite-pumpellyite
facies would be defined by the reaction:
laumontite + pumpellyite + quartz =
prehnite + epidote + chlorite + H20
with approximate XFe values of 0.12 for the
pumpellyite and 0.24 for the epidote. Using the
petrogenetic grid of Liou et al. (1985) the following approximate P - T values are obtained
for the transition from zeolite to prehnitepumpellyite facies: T = 175 + 5°C and P =
1.06 + 0.05 kbar.
Geochronological data (Fig. 4) indicate that
the burial metamorphism took place relatively
soon after the volcanic rocks were extruded,
pointing to rapid subsidence. The prevalence of
metastable mineral assemblages and the fact
that greenschist facies conditions were attained
only at a stratigraphic depth of several kilometres indicate that the subsiding volcanic rocks
227
did not experience conditions of high temperature or pressure. This suggests lateral displacement away from the axial thermal high of the
subsiding basin (Levi et al. 1982).
The Rocas Verdes Group
(Patagonian Andes)
The Rocas Verdes Group is a c. 600-kmlong belt composed of basic igneous rocks of
Late Jurassic to 'Middle' Cretaceous age in the
southernmost Patagonian Andes (Fig. 1). It is
up to 3500 m thick and has been interpreted as
an ophiolite complex that formed in a marginal
basin (Dalziel et al. 1974, Stern 1980, Dalziel
1981, and references therein). The lavas and
dykes of the Group are tholeiites, displaying
affinities with ocean-ridge basalts rather than
with island-arc tholeiites. Calc-alkaline volcanism took place contemporaneously in a volcanic arc located west of the basin (Su~irez
1979).
According to Elthon & Stern (1978) and
Stern & Elthon (1979) the c. 2-km-thick upper
part of the Rocas Verdes Group is metamorphosed in zeolite to 'upper actinolite' facies (the
latter corresponding to amphibolite facies according to the mineral assemblages given by
these authors; amphibole with 5.0-8.0 wt %
A1203, calcic plagioclase and titanomagnetite).
This metamorphic pattern implies a steep thermal gradient (probably about 180°C km -l)
and has been interpreted as the result of hydrothermal ocean-floor metamorphism at a
spreading centre (Elthon & Stern 1978, Stern &
Elthon 1979). The metamorphic facies have
a rather heterogeneous distribution and are
typified by the abundant presence of 'retrograde' disequilibrium textures, which are the
result of a sequential series of metamorphic
reactions occurring with decreasing temperature. In spite of the heterogeneous facies distribution there is a clear tendency of increasing
grade downwards in a short vertical distance,
from zeolite to amphibolite facies (Stern &
Elthon 1979).
Concluding remarks
The four case studies show that rifting occurred
during the Cretaceous and marginal basins,
'proper' or aborted, were generated along the
western border of South America. Crustal
thinning took place with varying intensity at
different latitudes; moderate in central Chile
228
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L. Aguirre et al.
PP
669
II
/-.48
333
327
329
288
177
ZEO
282
I
I
I
10
20
30
4.0
XFo3.
100
FI6.3. Schematic pseudobinary T--XFe diagram (after Cho et al. 1986) based on the compositions of
pumpellyite (hatched bars) and epidote (filled bars) in amygdule assemblages, Ocoite Group, central Chile.
Forty-seven microprobe analyses in eight polished sections. The samples (numbers at left) have been placed
qualitatively on the T axis in order of stratigraphic depth (= metamorphic grade), but the separation between
successive samples is arbitrary. Curves and horizontal line as in Fig. 2.
I
l
I
I
I
I
!
I
,'<b
13
&.
t_9
£1_
..Q
Cl
I
0
I
I
I
100
I
200
I
300
I
I
400
Temp. °C
FIG. 4. Schematic P - T - t history for the Ocoite Group, central Chile. Orogenic phase 1 can be correlated with
the Oregonian (mid-Cretaceous) orogeny around 100 Ma (Aguirre et al. 1974). Ages (in Ma) are based on
palaeontological evidence for point a (extrusion of the basaltic flows), and Rb-Sr whole-rock data for point b
corresponding to the age of the prehnite-pumpellyite-facies metamorphism of the flows (Aberg et a11984).
Reaction curves based on Liou et al. (1985) modified to take account of the actual epidote compositions.
(Ocoite Group) to extreme in Peru (Northern
Casma Group). The crustal attenuation resulted
in rupture at both ends of the continent (Diabase Group to the north and Rocas Verdes
Group to the south). We envisage that marginal
basin formation was controlled by subduction
of an oceanic plate alternating with crustal
spreading in the overlying continental margin
(Fig. 5), as proposed by Aberg et al. (1984) for
central Chile. A cyclic repetition of this process
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Andean metamorphism, volcanism and geotectonic setting
A
B
229
J
_-_
::v~i:if
FIr. 5. Marginal basin generation in the Andes during the Cretaceous. A--rapid oceanic spreading and
'Chile type' plate convergence (low-angle subduction; Uyeda & Kanamori 1979). Generation of a convective
cell below the continental margin. B---establishment of an extensional regime in the continental margin
(doming, rifting) and diapiric rise of mantle material (the intensity of the shading reflects the temperature).
Melting of the continental crust leads to bimodal volcanism (ignimbrites and subordinate basalts).
C--increasing rate of intra-continental spreading, and blockage of the downgoing oceanic slab at a steepening
angle ('Mariana type' convergence; Uyeda & Kanamori 1979), resulting in (1) a marginal basin proper (rupture
of the continental crust and formation of an ophiolite suite), or (2) an aborted marginal basin (thinning of the
crust and generation of flood basalts; this case is illustrated in the figure). Spreading-subsidence, heat
dissipation and metamorphism (of ocean-floor and burial type, respectively). The convective cell comes to a
halt. D--decoupling of the oceanic plate results in a short period of compression in the continental margin,
manifested as an orogenic phase (see Mrgard 1978). Emplacement of batholith granitoids (not shown) takes
place at the end of this phase. An increasing rate of oceanic spreading inaugurates a new cycle.
could account for the geological evolution of
the western margin of South America during
the Mesozoic and Palaeogene.
Upwelling rate and volume of mantle-derived
material, thickness of the continental crust, rate
of opening and other factors controlled the
generation of the marginal basins (Aberg et al,
1984, Aguirre & Offler 1985, Aguirre 1987).
They also determined the nature of the volcanism and the different metamorphic patterns and
thermal gradients illustrated in the four case
studies. The evolution of non-deformational
metamorphism under an extensional regime in
the Andean marginal basins is well exemplified
by the P - T - t
history of the Ocoite Group
(Fig. 4). The various stages of basin generation
outlined in Fig. 5 are reflected in the shape of
the P - T - t path.
A high rate of intra-continental spreading
(Fig. 5, stages B and C) corresponds to an increase in T and P (a to b in Fig. 4) due to the
diapiric rise of mantle material, intense volcanic
activity and rapid burial. A second segment of
the P - T - t path (from b to 1 in Fig. 4) rep-
resents the terminal part of stage C (Fig. 5)
when extension in the continent comes to a halt
and heat dissipation is high. Decoupling and
compression during stage D (Fig. 5) is represented by the orogenic phase 1. Finally, a
third segment starting from 1, marks the uplift
and cooling of the Ocoite Group. The geotectonic model envisaged here leads to a P - T - t
path which emphasizes the special nature of the
Andean metamorphism. Here, metamorphism
starts during the initial stage of basin formation
and precedes the orogenic activity, contrary to
the development in compressional geotectonic settings. The resulting counter-clockwise
P - T - t path was suggested by Robinson (1987)
to characterize extensional settings.
Some peculiar features of Andean metamorphism, such as its episodic nature, coincidence
between mineralogical and structural unconformities, and prevalence of metastable metamorphic assemblages, can be explained by the
mechanisms for basin formation outlined above.
The rate of opening determines the residence
time of the volcanic rocks in the hot axial region
230
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L. Aguirre et al.
TABLE 1. Relationship between upwelling rate of mantle-derived material and the volcanic,
metamorphic and geotectonic features of Cretaceous Andean marginal basins (modified from
Aguirre & Offier 1985)
Rapid mantle upwelling
Moderately rapid mantle
upwelling
Slow mantle upwelling
Marginal basin proper.
Rupture of the continental crust
with generation of oceanic
crust. No contamination from
continental crust
Aborted marginal basin.
Pronounced to extreme
thinning of the continental crust.
Slight crustal contamination
Aborted marginal basin.
Moderate thinning of the
continental crust. Considerable
crustal contamination
Slightly evolved basalts and
basaltic andesites of the
calc-alkaline to tholeiitic series
Evolved andesitic belts and
basalts of the high-K calcalkaline and shoshonitic series
Burial metamorphism
transitional to ocean-floor
metamorphism (very high
thermal gradient)
Burial metamorphism
(moderate to low thermal
gradient)
Primitive basalts (ocean-floor
tholeiites) in ophiolite suites
Ocean-floor metamorphism
(high thermal gradient)
Examples: (1) the Colombian
Diabase Group: (2) The 'Rocas
Verdes' Group of Patagonia
Example: The Casma Group of
north-central Peru
of the basin, and influences the nature of the
metamorphic assemblages (maximum temperature recorded and extent of metastability). The
lateral displacement (opening) will have the
consequence that volcanic sequences deposited
and metamorphosed during different cycles
of the geological history will overlap each
other. The metamorphic patterns in older volcanic sequences, separated by regional unconformities, will be preserved during subsequent
episodes of burial metamorphism. This is due to
their position away from the axial region, and
because their low permeability, caused by infilling by secondary minerals, in effect seals
them from Circulating fluids.
The fact that the thermal gradients estimated
for the Northern Casma Group (an ensialic
aborted basin with extremely attenuated continental crust) are considerably higher than
those inferred for the Diabase Group and the
Rocas Verdes Group ('proper' marginal basins)
can be explained by the more rapid heat dissipation due to larger volumes of fluids circulating through the rocks of the latter basins.
Example: The Ocoite Group of
central Chile.
The upwelling rate of mantle material can be
chosen as a parameter for comparisons of the
volcanic, metamorphic and geotectonic features
of the Cretaceous Andean terrains discussed
above (Table 1). With regard to metamorphism,
such comparisons allow the conclusion that:
(1) ocean-floor metamorphism (including types
transitional to high-T gradient, burial metamorphism) correlates with tholeiitic volcanism
in marginal basins where the continental crust
was eliminated or extremely attenuated; (2)
burial metamorphism with low- to moderate-T
gradients correlates with calc-alkaline to shoshonitic volcanism in ensialic, aborted, marginal
basins with slight thinning of the continental
crust.
ACKNOWLEDGEMENTS: We thank J. S. Daly, F.
Henriquez (University of Santiago, Chile) and an
anonymous referee for constructive criticism of the
manuscript. Financial support came from a Guggenhelm Foundation Research Grant (L.A.), and from
the Swedish Agency for Research Co-operation with
Developing Countries (SAREC project grant 86/199;
B.L. and J.O.N.).
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