Pressure-Temperature-time paths from the Limón Verde

Pressure-Temperature-time paths from the Limón Verde
Metamorphic Complex, Chile.
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María Fernanda Soto , Francisco Hervé , Mauricio Calderón , Hans Massonne and C. Mark Fanning
Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 833, Santiago, Chile
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Escuela de Ciencias de la Tierra, Universidad Andrés Bello, Sazié 2212, piso 5, Santiago, Chile.
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Servicio Nacional de Geología y Minería, Av. Santa María 0104, Santiago, Chile,
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Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenberstraße 18, D-70174 Stuttgart, Germany
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Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
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*E-mail: mf.soto@ug.uchile.cl
Abstract. A comprehensive study of mica schist and
amphibolite samples from the Limon Verde Metamorphic
Complex (LVMC) involving petrography, geochemistry,
geochronology and geothermobarometry: has been
performed in order to create a pressure-temperature-time
history of the complex. The late Palaeozoic evolution of the
western margin of Gondwana where the LVMC is set
remains unclear. Nevertheless results from this study
indicate clockwise paths and temperature and pressure
ranges between 550-660 ºC and 7.5- 14.4 kbar for a main
metamorphic event at 280 ± 1.8 Ma (U-Pb zircon), but with
remnants of ultra-high-pressure mineralogy (17.3 kbar.
equivalents to 60 km depth) from a possible previous event
at 377 ± 12 Ma (Sm-Nd garnet).
essential for understanding the evolution of the LVMC
during the late Paleozoic.
2 Methods and results
2.1 Petrography
Thin section petrographic analyses were targeted on
metamorphic rocks containing garnet and white mica:
strongly foliated biotite amphibolite (FS-10-16C), and
garnet bearing mica schist (FS-10-09). These samples
contain at least two visible generations of syn-kinematic
large equant garnet porphyroblasts. Zoisite and biotite are
secondary phases in both samples.
Keywords: Garnet, P-T-t, pseudosection, Limón Verde
Sample FS-10-16C is a nematoblastic garnet bearing
amphibolite. Zoisite, hornblende and biotie are major
minerals in this rock making up to 20% of the sample, and
making a nematoblastic texture for the rock. The
paragenesis of this sample consists of Amphibole + Garnet
+ Biotite + Zoisite + White mica + Quartz, and
characterised by the lack or trace amounts of plagioclase.
1 Introduction
The Limon Verde Metamorphic Complex (LVMC) consists
of a series of interleaved sequences of mica schists,
amphibolites, gneisses and quartzites, located in the northwestern part of the Sierra of the same name. The complex
also has associated meta-conglomerates in the southeastern portion of the Sierra, but the foci of this study are
set on foliated metamorphic rocks containing garnet and
white mica.
Sample FS-10-09 is a strongly foliated garnet bearing mica
schist, containing biotite and white mica in the same
amount. This sample was formed in a dynamic
environment, close to a shear zone, as indicated by synkinematic porphyroblasts. Biotite and white mica
composes up to 45% of the sample. The paragenesis of this
sample consists of White mica + Garnet + Biotite + Quartz
± Plagioclase
The CMLV is part of the Cordillera de Domeyko (late
Palaeozoic magmatic arc) in the Antofagasta Region,
formed during an upper Permian tectonothermal event with
associated high pressure metamorphism (Lucassen, 1999).
The complex is surrounded by late Carboniferous
intrusives from the Limón Verde Igneous Complex (LVIC),
and with ignimbrites lying uncomformably over the
complex (Tomlinson and Blanco, 2007; Marinovic and
Lahsen, 1984: Baeza and Venegas, 1984.)
2.2 Major element geochemistry
Major element geochemistry analysis was done on bulk
rock and on selected minerals: the objective was to provide
a geochemical characterization of the bulk rock (to
determine protolith affinities), and to determine the major
element characteristics of minerals sensitive to
metamorphic changes such as garnet and white mica.
We report a pressure-temperature-time path (P-T-t path) of
this metamorphic complex based upon a combination of
methods of geochronology (U-Pb and Sm-Nd) and
geothermo-barometry (conventional geothermobarometry
and forward modeling of P-T pseudosections). These
studies were done on two samples representative of the
complex (amphibolite and mica schist) then the
significance of the resulting P-T-t path of these rocks is
The bulk chemistry was determined by XRF for the same
representative samples (garnet-amphibolite and garnetmica schist). The whole rock analyses were simplified for
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Arizona along with powdered whole rock. Core and rim
were sampled using a micromill, and analysed with TIMS
along with the powdered whole rock. These analyses gave
a Sm-Nd isochron age of 380±21 for sample FS-10-16C
and 375± 13 Ma for sample FS-10-09. Core and rim
Sm/Nd ratios are very similar indicating homogeneity in
these elements in the crystals. Although in both samples
the ages are very concordant with each other, these differ
to what has been previously obtained with a similar
technique, around 270 Ma (Lucassen, et al 1999).
the thermodynamic calculations of P-T pseudosections
using the PerpleX programs (Connolly, 1990), available for
download
at
http://www.perplex.ethz.ch/.
Mineral
chemistry was determined with a SX-100 Cameca Electron
Microprobe Analyser (EMPA) and then recalculated to get
mineral structural formulae with CALCMIN (Brandelik,
2009).
Structural formulae for garnets from the garnet bearing
amphibolite and garnet mica schist (mentioned earlier),
were recalculated to 12 oxygens per formula unit. The
garnets from both samples have a prominent almandine
component; garnets from mica schist sample show a coreto-rim enrichment in their pyrope component and a coreto-rim depletion in their grossular one. On the contrary, the
garnet from amphibolite samples has a grossular
component with almost the same pattern as pyrope,
indicating a prograde enrichment in both elements (Ca and
Mg).
2.4 Pressure-temperature determinations
Previous studies in the area have given temperature ranges
of 660ºC-720ºC and pressure estimates of 13 ± 1 kbar
(Lucassen, et al 1999). The chemical composition of
minerals from studied samples were used in a combination
of the garnet-biotite thermometer and the garnet-Al2SiO5quartz-plagioclase barometer (Ferry and Spear, 1978;
Ghent, 1976) to obtain the following temperature ranges:
525-610ºC for the garnet schist and 370-525ºC for the
garnet amphibolite, and pressures of 7.5-12.5 in the schist
and 5.6-9.4 in the amphibolite. These intercepts were
measured from core-to-rim analyses and in both cases, a
prograde path is visible.
White mica is present in both samples: the two samples
analysed with microprobe contained large laths of white
mica parallel to the main foliation. Structural formulae
were determined along with the number of Si atoms per
formula unit, by recalculating the elemental analyses to a
double formula unit on the basis of 42 valences (Brandelik,
2009). The minimum value for the number of Si is barely
the same for the amphibolite and the mica schist samples:
3.12, but the maximum for mica schist was in the range of
3.17 as opposed to the larger value 3.23 for white micas in
amphibolites.
2.3 Zircon
U-Pb
geochronology
and
garnet
The bulk chemistry of the above mentioned samples plus a
third one, FS-10-16A (a zoisite schist), bear garnet and
white mica in different amounts. These bulk chemistry
analyses were considered to create the P-T pseudosections.
Although these samples had a different whole rock
composition, the pseudosections were calculated on the
NCKFMASHTO system. Similarly, the garnet components
and Si content of white micas were input in these scripts to
plot isopleths. Matching the modelled and measured
compositions of minerals contoured by the isopleths
provide thermobarometric contraints. Taking into account
the parageneses of the samples and the isopleth intercepts,
we were able to construct pressure-temperature fields: 576660ºC and 7.5-11.5 kbar for sample FS-10-09. Sample FS10-16C had a field of 503-603ºC a lower pressure bound of
8.1 kbar, but white mica isopleths indicate a minimum
pressure of 17.3 kbar. P-T ranges are indicated as boxes in
Figure 1.
Sm-Nd
Zircon and garnet are minerals that grow under medium
grade to upper metamorphic conditions, with the
characteristic that their growth leaves marks a zoning
pattern in the crystal structure: each of these patterns
constraints the temperatures of metamorphic events.
SHRIMP U-Pb zircon ages were obtained from two mica
schist samples (FO-09-08 and FO-09-13) collected during
a 2009 field campaign: although the sampling location is
not the same, it is within 300m of distance from where the
rest of the samples were taken. Mineral separation was
done at Universidad de Chile and zircon analysis at the
Australian National University. In sample FO-09-08: 41
grains and 47 spots were analysed, mostly from the borders
of the grains, and in sample FO-09-13: 50 grains were
analysed. Rim analyses from zoned zircons with Th/U
ratios smaller than 0.1 were considered as true
metamorphic (65 spots; cf. Hoskin and Schaltegger, 2007)
and show a normal distribution between 260 and 300 Ma
with a peak age of metamorphism at 280 ± 1.8 Ma
3 Discussion and conclusions
3.1 Discussion of results
Geothermo-barometry considering core-to-rim analyses in
garnet indicate a clockwise path for prograde
metamorphism, shown in Figure 1. PerpleX calculations
yield temperature ranges between 550ºC and 660ºC, and
pressure between 7.5-14.4 kbar for the main metamorphic
event which is presumed to have formed the parageneses
these samples. This event is correlated with a metamorphic
peak of 280 ± 1.8 Ma based on U-Pb zircon ages as seen in
Garnets examined for Sm-Nd isochron analyses belong to
the samples mentioned in earlier (FS-10-16C and FS-1009). These were handpicked and sent to University of
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results have neither been incorporated into a tectonodynamic model involving regional geology. The
correlation with existing data of the area and the
development of a model is part of the future work, and the
culmination of this research.
Figure 1. On the other hand, the Sm-Nd results from this
work indicate an average age of metamorphism of 377±12
Ma between both samples. Although there is a strong
major element zoning in garnets from both studied samples
core and rim Sm/Nd ratios are similar thus indicating
homogenization of minor elements. Because chemical
homogenization occurs at temperatures of ca. 650ºC
(Ducea, et al 2003), whithin the temperature range
calculated for the examined rocks, the Sm-Nd ages may
represent “mixing” age what needs further interpretation of
its geological meaning.
Acknowledgements
FONDECYT project # 1095099 (F.H.) and the BMBFCONICYT collaboration program 175-2009 (F.H. and H.J.M.) funded this project. Thanks also to Thomas Theye
for instructing with microprobe analysis techniques in
Stuttgart. The authors would also like to thank Antofagasta
Minerals S.A. for providing lodge and logistics of the field
campaigns.
Finally, the garnet-amphibolite sample (FS-10-16C) has
remnant mineralogy that indicates that this rock was once
subjected to pressures over 17.3 kbar (equivalent to at least
60km depth).
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Figure 1. Tentative pressure-temperature-time path for samples
FS-10-16C (garnet amphibolite), FS-10-16A (zoisite schist) and
FS-10-09 (garnet mica schist). Data was obtained through
modelling of PT pseudosection with PerpleX and correlated with
classic geothermobarometry calculations. Boxes PerpleX 1 and 2
correspond to the fields determined by white mica isopleth
intersection, and the paragenesis seen in sample FS-10-16C.
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3.2 Preliminary conclusions and future work
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A prograde P-T-t path during the late Paleozoic is deduced
from this work. However, the two distinct metamorphic
ages obtained, one from zircon and the other from garnet
Nd-Sm isochrons, seem to be contradictory, as detrital
zircons of intermediate age are also observed in the mica
schist. An interpretation of these data is needed. These
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