Opening and closure of an island arc

UNIVER SIDAD
DE
CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
OPENING AND CLOSURE OF AN ISLAND ARC-BACK ARC SYSTEM IN
THE EARLY PALAEOZOIC AT THE GONDWANA MARGIN:
FAMATINIAN SYSTEM AND LAS TERMAS BELT,
NW ARGENTINA
MILLER, H., HÖCKENREINER, M. & SÖLLNER, F.
Department für Geo- und Umweltwissenschaften der LMU, München, Sektion Geologie,
Luisenstr. 37, D-80333 München; e-mail: hubert.miller@iaag.geo.uni-muenchen.de
INTRODUCTION
In the area between 66°- 68° W and 27°-30° S in southern South America, two geological units
have been identified by Aceñolaza & Toselli (1976): the Pampean System in the eastern, and the
Famatinian System in the western part (Fig. 1). Geomorphologically the Sierra de Famatina
cannot be distinguished from other Pampean Ranges, - range means sierra, rising from the
pampa, but its geological development was quite different from that of the “Eastern Pampean
Ranges”. In the following, we therefore use the terms Famatinian and Pampean systems to
describe the different geological history.
North of Tinogasta, squeezed between these two units, a third but lithologically independent
series could be separated recently, the so called Las Termas belt (Söllner et al. 2001).
The aim of this paper is to focus on the significance of the Las Termas belt, its boundaries to the
adjacent units to the east and west and its geochronological history, mostly affected by attendant
huge ductile shear zones.
THE PAMPEAN SYSTEM (EASTERN PAMPEAN RANGES)
The regional extent of the Pampean system can be traced from the Argentine/Bolivian border
(22° S) to the Sierras de Córdoba and San Luis (33°S) at least, but may be developed as well, in
Patagonia and the Transantarctic Mountains (Aceñolaza & Miller, 1982, Söllner et al., 2000a).
Traditionally, the Eastern Pampean Ranges have been separated from the apparently “exotic”
terranes of the Precordillera and the Famatinian System to the west. Moreover, the Precordillera
and the westernmost parts of the Andean basement of Argentina have been merged to the
apparently exotic terrane “Cuyania”. However, we focus here on the more uniform eastern part of
the Pampean system.
The Pampean system is mainly composed of meta-sedimentary rocks with low to high grade
metamorphic overprint. Within very low grade metamorphic rocks a Vendian to Early Cambrian
age has been proved by trace fossils (Durand & Aceñolaza, 1990). This period of sedimentation
has been supported by age determinations on a syn-sedimentary metavolcanic rock (529 ± 12 Ma,
U-Pb on zircons; Söllner et al., 2000b). Locally, high grade metamorphic rocks are suggested to
be older than Vendian, which argues for a start of the sedimentation in Precambrian times, at
least in some parts.
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
Calalaste
Re
al
Laguna
Blanca
Ch
ang
o
Buenos Aires
Bariloche
Rivadavia
Capillitas
Belen
Las Termas
belt
Ushuaia
Zapata
Caschuil
Tinogasta
28°S
Catamarca
La Rioja
La
sC
Fiambalá
e
zon
Las
Planchadas
r
shea
TIPA
27°S
Tucumán
ue
va
s
Papachacra
Quilmes
TUCUMÁN
Pampean system
Vinquis
CATAMARCA
29°S
Toro
Negro
Ambato
Aimogasta
ti
cas
Cerro
Toro
Famatina
TI
PA
Velasco
Mazán
sh
ea
r
zo
ne
CA
T
LA AM
R ARC
IO
JA A
Shear zone
Basic and
ultrabasic rocks
post-tectonic granite
Sierra
Brava
50km
68°W
Albigasta
La Rioja
Sañogasta
0
Catamarca
Paimán
Ñuñorco
30°S
El Alto
An
Famatinian system
La Pampa
Copacabana
67°W
66°W
pre-tectonic meta-granitoid
(Famatinian system)
metasediments of
the Las Termas belt
metamorphic basement
(Pampean system)
Fig.1. Geographical and geological overview of the area considered in detail.
The most widespread lithology forms sandstone/claystone series and their metamorphic
equivalents, but sporadically completed by marbles and meta-volcanics. The provenance of the
sediments has been determined as recycled components from orogens, prevalently from the
cratonic area to the north-east. They were transported and deposited mostly by turbidity currents.
Folding is manifold, from a simple singural deformation up to three or four stages of refolding
(Willner, 1990). Sedimentation of the basement rocks ended before the Mid Cambrian. Again,
local post-metamorphic sedimentation started not until Late Carboniferous. Granitoid rocks,
intruding the metasediments, are widespread. The intrusion ages show three main phases of
magmatic activity in Mid Cambrian, Late Ordovician and Carboniferous. The first two phases
can be correlated with the tectono-metamorphic events of the Pampean and Famatinian
orogenies.
THE FAMATINIAN SYSTEM
The Famatinian system stretches across about 300 km (Fig. 2) and forms various mountain
ranges, with the Famatina Range as the typical, the most important and the highest one. Its
lithology contrasts with that of the Eastern Pampean Ranges. Sedimentation started in Vendian as
well with sediments, comparable to that of the Eastern Pampean Ranges.
Andesites
Fig. 3. Generalised sketch of sedimentation and
volcanism of the Vuelta de Las Tolas member
(Suri Formation, Arenigian), a typical situation
of rock forming during the Ordovician in the
Famatinian system (taken from Mangano &
Buatois, 1996).
Legend
Mylonites
Granitoids
Sedimentary and
volcanic rocks
Fig. 2. The distribution of granitoids as well as of
sedimentary and volcanic rocks within the Famatina
Range (Famatinian system) is taken from Clemens
and Miller (1996).
In contrast to the Pampean system, in the
Famatinian
system
sedimentation
continued after a hiatus in the Cambrian,
again in the Early and Mid Ordovician
(Fig. 3; Mangano & Buatois, 1996).
During this time, fossiliferous sediments of
craton interior and recycled orogene type
were deposited, similar to those of the
northern Puna and the Eastern Cordillera
regions (Fig. 4; Clemens & Miller, 1996).
Volcanic rocks are frequent. Granitoids are
widespread in the Famatinian system but
obviously limited to the Ordovician. The
Ordovician magmatism in the Famatina
Range is of calc-alcaline composition (Fig.
5; Mannheim & Miller, 1996), and thus
can be related to a subduction zone
(Toselli et al., 1996).
The Sr-initial ratio of the magmatites of about 0.708 (Mannheim & Miller, 1996) and their
deposition upon pre-existing metasedimentary rocks allocate for the Famatinian arc to have
developed on continental crust, described in detail by Rapela (2000). This arc may have been
separated from the Gondwana continent (“Pampia terrane”) by spreading in Late Cambrian and
thus, took its own development, independent from that of the Eastern Pampean Ranges.
Q
SiO2 %weight
Rhyolite
75
Craton Interior
70
Dacite
65
Transitional Continental
Trachyte
60
Recycled
Orogenic
Andesite
55
Phonolite
Dissected Arc
50
Basement Uplift
45
Transitional Arc
Alcaline
Basalt
Subalcaline
Basalt
Undissected Arc
F
L
Fig. 4. Provenance of sediments of the Famatina
Range (all observed localities and ages). All
samples fit in the marked field, representing
homogeneous continental provenance sites (taken
from Clemens and Miller, 1996). Q = quartzose
grains, F = feldspar grains, L = lithic fragments.
40
Zr/TiO2
0.001
0.01
0.1
1
Fig. 5. Subalcaline, bi-modal characteristics
of the volcanic and subvolcanic rocks of the
Famatina Range. Circles: Syn-sedimentary
volcanic rocks of the Lower and Mid
Ordovician. Crosses: dykes. Drawing taken
from Mannheim & Miller (1996).
LAS TERMAS BELT
If there is an island arc, a back-arc basin should also be present. As a result, the manifold gneiss
and calc-silicate rock series (Fig. 6) around the thermal springs of Fiambalá north of Tinogasta
(Fig. 1) were supposed to be non-subducted remains of such a back-arc basin (Mannheim &
Miller, 1996; Neugebauer & Miller, 1996), formed between the Pampean hinterland and a
separated span.
The unit which developed during back-arc spreading is now called Las Termas belt (Söllner et
al., 2001). The rock types forming this belt are of medium- to high-grade metamorphic overprint
indicated by sillimanite-garnet-biotite gneisses, migmatites and banded calc-silicate rocks. These
metasediments were intruded by numerous basic dykes, boudined to several bodies aligned in the
lineation. The most prominent and largest is the Fiambalá meta-gabbronorite (FMGN), Late
Cambrian in age. The lithology characterises the series best as a small and discontinuous back-arc
basin, which opened in Late Cambrian to Early Ordovician and closed contemporaneous with the
cessation of sedimentation and volcanism in the adjacent Famatinian arc.
Therefore, we consider the Las Termas belt as part
of the Famatinian system. Furthermore, the Las
Termas belt is characterised by a huge amount of
meta-granitoids of crustal origin, which intruded the
series in Late Ordovican. Igneous activity was
followed by ductile shear deformation in long
lasting, wide mylonite zones. Again in Carboniferous, this area of crustal weakness has been the
location of granite intrusions.
Fig. 6. Sketch of the manifold deformation in the Las
Termas belt (not to scale, taken from Neugebauer &
Miller, 1996).
THE DUCTILE SHEAR ZONES OF THE LAS TERMAS BELT
In consequence of the special evolution in the Las Termas belt (back-arc spreading, crustal
thinning, huge magmatic activity) crustal weakness resulted in the development of important
shear zones. Several, subparallel ductile mylonite zones, striking NNW - SSE can be traced in the
Sierras de Fiambala, Copacabana and Velasco (Fig. 1). Our investigations concentrated on the
TIPA (Tinogasta-Pituil-Antinaco) shear zone, the most prominent. Investigations were carried
out to date movements along the shear zone as well as to examine the geochronological
framework (formation of the protolith, cooling history). Movements have been proceeded mainly
by WSW directed reverse faults with a dextral transcurrent component. Thus, the north-eastern
block (Eastern Pampean Ranges) was thrust over the Las Termas belt and the Famatinian system.
GEOLOGICAL HISTORY ALONG THE TIPA SHEAR ZONE
Several isotopic systems were applied to clarify the geological history of the TIPA shear zone
and the rocks adjacent to it (see Fig. 7 and Höckenreiner et al., 2003). The metasedimentary
frame, intruded by the granitoids, only preserved in slices yields a Nd 1-stage model age of about
2.5 Ga, pointing to an early Proterozoic provenance age of the detrital input. Nd 2-stage model
ages and U-Pb upper intercept ages on zircons from the meta-granitoids themselves, as well as of
the mylonites yield ages of about 1.6 Ga and 1.8 Ga, respectively. On the one hand, these ages
may indicate variable mixtures of Proterozoic components forming the actual crust. On the other
hand, as U-Pb upper discordia intercept ages may refer to inherited zircons derived from uniform
molten crustal components, the deviating age of about 1.3 Ga may reflect a real Grenville
provenance age.
Sedimentation and volcanism started in the Sierra de Famatina (magmatic arc) in the late
Cambrian or in the early Ordovician, after a Cambrian hiatus. Simultaneously the back-arc basin
of the Las Termas belt opened, documented by the intrusion of basic and ultrabasic rocks (510 to
515 Ma, Grissom et al. 1999). Unequivocal data result from meta-granitoid formation at 467 ± 4
Ma and 488 ± 4 Ma (zircon U-Pb lower intercept and concordant ages), the precursor rocks of the
mylonites. These granitoid intrusions, definitely of crustal origin (mean of Sr-initial ratios:
0.7125) are prevalently related to the Las Termas belt, an area of pronounced weakness and
thinning, and the series adjacent to the east. This distribution of magmatites suggests block
movements and a thermal separation in the evolution (downward shift of the Las Termas belt).
The culmination of movements along wide (up to 2 km) and long shear zones (more than 200
km) has been dated by syntectonically grown garnet at 402 ± 2 Ma (Sm-Nd, Fig. 8; Höckenreiner
et al., 2003). The present outcrop level reflects a crystallisation temperature of 587 ± 50 °C. We
correlate this fundamental deformation with the final collision between the Famatinian magmatic
arc and the Pampean hinterland.
Time
not to scale
300 Ma ---
350 Ma ---
Age
Method
Rb - Sr on biotite
(mean value)
300 ± 4 Ma
uplift and regional cooling
below 300°C
U - Pb on apatite
(rehomogenisation and
new crystallisation age)
328 ± 3 Ma
342 ± 2 Ma
regional reheating accompanied
by granite intrusions
T ~ 350-550(?)°C
355 ± 1 Ma
357 ± 8 Ma
Ar - Ar on muscovite
360 - 370 Ma
392 ± 9 Ma
402 ± 2 Ma
TIPA shear zone
and western
Las Termas belt
2500 Ma ---
U - Pb on zircon
lower intersept and/or
concordant age
U - Pb on zircon
upper intercept ages
uplift and regional cooling
below 300-350°C
uplift and regional cooling
below 350-400°C
uplift and regional cooling below
~500°C
400 Ma ---
Rb - Sr on muscovite
1)
2)
Shear zone
Sm - Nd on garnet
(crystallisation age)
2)
390 ± 10 Ma
Rb - Sr on muscovite
and apatite
1500 Ma ---
1)
Ar - Ar on biotite
Rb - Sr on biotite
Rb - Sr on muscovite
(mean value)
500 Ma ---
Interpretation
mylonitisation in
metasediments
T ~ 530 - 500 °C
mylonitisation and influx of
melts and fluids in granitoids of
the TIPA shear zone
T = 587 ± 50 °C
eastern Las Termas
belt and Eastern
Pampean Ranges
425 ± 17 Ma
2)
uplift and regional cooling
below ~500°C
467 ± 4 Ma
488 ± 4 Ma
Intrusion of granitoids
1268 ± 46 Ma
real Grenville crustal component
(?) or see below
1828 ± 119 Ma
Nd 2-stage model ages
granitoids, mylonites
1,59 - 1,64 Ga
Nd 1-stage model age
metasediment
about 2,5 Ga
variable mixture of
early and late Proterozoic
components
prevalently early Proterozoic
crustal detrital components
Fig. 7. Time table, representing age data of tectono-thermal events from the transition zone of the Famatinian
and Pampean systems (Las Termas belt). All data, except those marked by 1) and 2) are from Höckenreiner et al.
(2003); 1) data from Grissom et al. (1999); 2) data from Söllner et al. (2001). Nd 2-stage model ages of granitoids
and mylonites indicate very homogeneous average crustal residence ages of 1.59 – 1.64 Ga (T2 = 470 Ma in
0.523
Garnet isochron
0.521
143
+
Age = 402 ± 2 Ma
Nd/144Nd initial = 0.511857 ± 0.000015
MSWD = 1.03
PUN1
0.517
0.5126
0.515
0.5124
PUN1
Whole rock
samples
143
Nd/144Nd
0.519
+
ga
PUN6
KAP
+
PUN4
0.5120
0.10
0.511
0
1
0.14
2
147
ro
ch
n
PUN4
KAP
PUN6
HUE1
FLO2
0.5122
0.513
e
rn
o
t is
0.18
3
0.22
0.26
0.30
4
144
Sm/ Nd
Fig. 8. Garnet of four samples from mylonites of the TIPA shear zone yield a Sm-Nd isochron age of 402 ± 2 Ma.
This age clearly demonstrates shearing in Early Devonian. Deformation is correlated with the final closure of the
back-arc basin opened in the Ordovician between the Famatinian magmatic arc and the Pampia terrane (Eastern
Pampian Ranges).
Mineral age determinations indicate, that post-tectonic uplift and regional cooling developed
separately in blocks east and west of the TIPA shear zone (Fig. 7). A rapid regional cooling is
documented in the rocks east of the TIPA shear zone and within those of the Pampean system
(eastern block) such, that the present outcrop level has passed the 500°C- and 300°C-isogrades at
about 392 Ma and about 355 Ma, respectively. In contrast, the TIPA shear zone and the area west
of it remained in deeper crustal level. This is documented by U-Pb apatite recrystallisation in
metamorphic rocks at 342 ± 2 Ma and 328 ± 3 Ma and by numerous Carboniferous granites
intruding the block at about 335 Ma (Grissom et al., 1999). The western block passed the 300°Cisograde not until 300 ± 4 Ma (Late Carboniferous; Fig. 7).
Block-wise uplift and cooling history coincide with block movements along the shear zones. In
consequence of these movements, the eastern Las Termas block and the Pampean series were
thrust over the Famatinian arc series (Fig. 9).
CONCLUSIONS
The crust whereon the Famatinian system developed, including the Las Termas belt, is composed
of Proterozoic detritus and prevalently of continental origin. The formation of the Famatinian
magmatic arc and the Las Termas back-arc basin are short events during the continuous
subduction of the Pacific crust below the Pampia terrane (Fig. 9). The Famatinian tectonothermal event ceased with the closure of the back-arc basin and the back docking of the
Famatinian system to the Pampia terrane, and can easily be understood as a local phenomenon.
The collision of an exotic terrane, such as Chilenia, is not a requirement to explain the
deformation features.
Erosion level
Litho
sphe
ric m
antle
Lithosph
eric man
As
the
nos
ph
eric
tle
ma
nt le
b
Early Famatinian orogeny ca. 495 - 460 Ma
Famatinian
granitoids and
volcanics
Famatinian
accretionary
wedge
Lith
osp
he
ri c
As
the
a
no
sph
eric
Pampean
granites
Las Termas belt
Ast
he
n
ma
ntle
osp
he
ric
Puncoviscana
formation
Lithospheric mantle
ma
n tl e
ma
ntle
Post-Devonian
sedimentary cover
Post-tectonic granite
(Carboniferous)
Syn-tectonic granitoids
(Pampean age)
Meta-volcanics of the
Famatinian sytem
Pre-tectonic meta-granitoid
(Famatinian system)
Puncoviscana metasediments
(Pampean system)
Metasediments of the
Famatinian system
Basic to ultrabasic rocks
of the Las Termas belt
Metamorphic basement
(Pampean system)
Fig. 9. Schematic model of the orogenic evolution of the Pampean and Famatinian system in a cross section north of
Tinogasta (not to scale) for Ordovician (a) and Early Devonian to Carboniferous times (b), taken from Höckenreiner
et al. (2003). Back-arc spreading leads to formation of the Las Termas belt sediment series on continental crust (a),
accompanied by intrusion of basic rocks and crust-derived granitoids. Compression of the orogenic belt led to
backthrust of the Pampean series over the Famatinian system (b). The Las Termas belt was squeezed between these
two units. In consequence, ductile shear zones developed, with the TIPA shear zone as the most prominent. The
influx of melt/fluid during deformation led to syn-kinematic garnet growth in mylonites at 402 ± 2 Ma.
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