High Pressure/ultrahigh Pressure Eclogites From The Hong/'an Block ...
High-pressure/ultrahigh-pressure Eclogites from
the Hong’an Block, East-Central China:
Geochemical
Characterization,
Isotope
Disequilibrium and Geochronological Controversy
Bor-Ming Jahn1*, Xiaochun Liu2, Tzen-Fu Yui1, N. Morin3, and M. Bouhnik-Le Coz3
Abstract
The collision between the two Chinese Precambrian cratons, Yangtze and Sino-Korean, led
to the formation of ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts. The
suture zone of the collision is represented by the Qinling-Dabie-Sulu orogen (Figure 1). The
occurrence of abundant UHP and HP metamorphic rocks has attracted the world's attention in
the last two decades. The recognition of UHP metamorphic rocks has profoundly changed our
views about the geodynamics of crustal plates. Continental crust, long considered unsubductible
due to its relatively low density, is now proven subductible after the discovery of UHP metamor-
phic rocks. Some crustal slices have been subducted to mantle depths, over 120 km, as a result
of collision between two continental plates. The collision between the two Chinese cratons (or
plates) is generally accepted to have taken place in the Triassic, between 220 and 240 Ma
(Ames et al.
1993; Li et al.
1993;
Chavagnac and
Jahn 1996;
Rowley et al.
1997; Hacker et
al. 1998, 2000;
Jahn et al. 2003).
However, the
published age
data for eclogites
from the Hong’an
Block show a
much larger
range from 450
Ma to 220 Ma,
hence have
caused much
controversy
about the timing
of continental col-
lision and tecton-
ic evolution of the
Figure 1. Simplified geological map of the Hong’an Block showing different tectonic units and five eclogite zones.
UHP terranes.
The inset shows the location of the Hong’an Block with respect to the Qinling-Dabie-Sulu orogen of China.
1Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
2Institute of Geomechanics, CAGS, Beijing, China
3Geosciences Rennes, Universite de Rennes 1, 35042 Rennes Cedex, France
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omphacite has contributed to isotope disequilibri-
um. (2) Sm-Nd isotope analyses yield no isochron
ages for the Hong'an eclogites. (3) Rb-Sr isotope
analyses have mixed results; in some cases, coexist-
ing minerals are completely out of isotope equilibri-
um, and in others, isochron relationship is estab-
lished, yielding ages from 210 Ma to 225 Ma. The
pattern of Rb-Sr isotope disequilibrium is independ-
ent of the petrological and O-isotope temperatures.
(4) In contrast to the Sm-Nd isotopic systems, oxy-
gen isotopes of the eclogite minerals seem to have
attained isotope equilibrium or near-equilibrium,
comparable with petrological data. This, however,
may be an apparent feature due to mass balance
constraints. (5) Whole-rock δ18O values show a
Figure 2. Metamorphic P-T paths of three UHP terranes in the
large variation from +10‰ to -8‰, suggesting that
Dabie-Sulu orogen (modified after Liu et al., 2004). Note that the
P-T conditions generally decrease from the Sulu through Dabie
their protoliths have undergone very different
to Hong'an.
processes of water-rock interaction. In view of the
The Hong’an Block, also known as western
overall geochronological information, we conclude
Dabieshan, exposes a series of HP to UHP meta-
that the HP/UHP metamorphism in the Hong’an
morphic rocks. The metamorphic temperatures of
Block took place in the Triassic at about 220-230
the Hong’an Block, ranging from 700 to 500˚C, are
Ma, similar to those established for the Dabie and
lower by 50-150˚C than those of the Dabie and Sulu
Sulu terranes. Finally, because isotopic disequilibri-
terranes (Figure 2). Contrary to the Dabie-Sulu
um is the major cause of the age controversy, we
UHP rocks, which show a restricted range of meta-
caution that the Sm-Nd isochron technique is not
morphic ages from 220 to 240 Ma, a variety of ages
suitable for age determination on UHP metamor-
ranging from 450 to 220 Ma have been reported for
phic rocks (eclogites) if their metamorphic tempera-
the Hong'an HP eclogites. These age data also led
tures are lower than 600˚C, or if their major con-
some workers to propose two periods of UHP meta-
morphism, despite that this scenario appears to be in
conflict with other observations. The motivation of
the present work is to resolve the apparent conflict
and to find a satisfactory explanation for the com-
plex age patterns. In order to better evaluate the iso-
tope behavior, we have also made comprehensive
geochemical and oxygen isotope analyses on
whole-rock and constituent minerals. Our results
show that: (1) Trace element distribution patterns
suggest that garnet and omphacite in many cases are
out of chemical equilibrium; and the presence of
high-temperature LREE (light rare earth)-rich min-
Figure 3. Chondrite-normalized REE distribution patterns for
eral inclusions, such as epidote, in garnet and
eclogite samples.
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stituent minerals (garnet and omphacite) contain a
significant amount of REE-rich inclusions.
Rare Earth Element (REE) Abundances
of Eclogites and Constituent Minerals
Eclogites are of basaltic or gabbroic composi-
tions, and their protoliths could be of oceanic or
continental origin. All the eclogite samples from the
Hong'an Block show REE patterns with enrichment
in light REE (Figure 3), thus indicating their conti-
nental origin. They were probably emplaced as dia-
basic dikes, or occurred as amphibolite enclaves
within granitic gneisses. The data argue that the
Figure 4. Chondrite-normalized REE distribution pattern of gar-
net in sample QLP08.
subducted rocks must be of a continental slice, but
not an oceanic plate.
Typical REE patterns for garnet should be low
in light REE and show an increasing abundance
towards heavy REE. In some cases, they may show
maxima at middle REE (e.g., Jahn et al., 2003).
This rule, however, does not hold for all Hong'an
samples. For example, garnet grains from sample
QLP08 exhibit two distinct REE patterns (Figure 4).
Two of them display typical garnet patterns, where-
as the four others have much higher LREE abun-
dances. The latter could be accounted for by the
presence of omphacite or epidote inclusions, as
shown by petrographic examination (Figure 5).
The literature data show that typical
omphacites have hump-shaped REE patterns with
Figure 5. Photomicrographs showing inclusions in major phases garnet and/or omphacite
Figure 6. Chondrite-normalized REE distribution pattern of
of eclogite samples.
omphacite in sample QLP08.
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Figure 7. Sm/Nd versus Nd plots for garnet and omphacite from sam-
ple XHD07.
maxima at the middle REE (e.g., Jahn et al., 2003).
Similar to garnet, this relation does not hold in the
present study. For example, omphacite grains from
sample QLP08 show a large variation in both LREE
and HREE distribution (Figure 6). The lowest abun-
dances with hump-shaped patterns are of inclusion-
free omphacite, whereas the others can be explained
by variable contributions from epidote inclusions.
In addition to REE patterns, Sm/Nd ratios of
garnet and omphacite from the Hong'an samples
also vary significantly even within a single sample.
Figure 8. Rb-Sr and Sm-Nd isochron diagrams for sample
TP-03.
This clearly indicates chemical disequilibrium.
Figure 7 shows the data of non-equilibrated compo-
sitions for sample XHD07. The compositional
ranges for Maowu garnet/omphacite, which are con-
sidered to have reached chemical and isotopic equi-
librium (Jahn et al., 2003), are shown in the same
figure for comparison.
Rb-Sr and Sm-Nd Isotope Analyses
Examples of the Rb-Sr and Sm-Nd isotopic
analyses on the Hong'an samples are shown in
Figure 8 and 9. Some samples show an apparently
undisturbed, phengite-based Rb-Sr isochrons, such
as 212 ± 7 (2 sigma) Ma for sample TP03, 225 ± 34
Ma for QLP08, 216 ± 16 Ma for glaucophane
schist, and 210 ± 4 Ma for marble. These ages can
be interpreted as cooling ages. The Rb-Sr isotopic
systems in other samples, including QJH01 and
GQ01, however, were disturbed. Most strikingly,
none of the samples yielded useful Sm-Nd isochron
age information. Clearly, the Sm-Nd isotope sys-
tems in the constituent minerals of all samples have
Figure 9. Rb-Sr and Sm-Nd isochron diagrams for sample
QLP-08.
not reached equilibrium.
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composition of its precursor mineral (plagioclase)
because the rate of Nd isotope exchange is more
sluggish than that of the chemical and mineral
phase reconstitution; (2) garnet contains high-tem-
perature refractory LREE-rich inclusions, which
have not been isotopically re-equilibrated with host
garnet, though the garnet may have attained equilib-
rium with coexisting clinopyroxene and other prin-
cipal phases. The above disequilibrium does not
include the open system behavior occurring during
retrograde metamorphism with strong influence of
hydrothermal activity.
The present study clearly shows that chemical
and isotopic disequilibrium exist between con-
Figure 10. Correlation in oxygen isotope composition between coexisting minerals of
stituent minerals of the Hong'an eclogites. In com-
Hong'an eclogites.
parison, chemical and isotopic equilibrium were
O-isotope Composition
mostly established for eclogitic minerals from the
O-isotope compositions of whole-rock sam-
Dabie and Sulu terranes. The major factor that pro-
ples for the Hong’an eclogites show a large varia-
duced such a contrast in isotopic equilibrium is
tion from +10‰ to -8‰, suggesting that their pro-
probably the metamorphic temperature. The meta-
toliths have undergone very different processes of
morphic temperatures of the Hong’an Block, rang-
water-rock interaction (Yui et al., 1995). O-isotopic
ing from 700 to 500˚C and being lower by 50-
fractionations among minerals are close to equilibri-
150˚C than those of Dabie and Sulu terranes (Figure
um (Figure 10). The calculated oxygen isotope tem-
2), could have hindered chemical and isotopic equi-
peratures (not shown) agree well with the petrologi-
librium under a dry UHP metamorphic condition.
cal temperatures.
We therefore cast doubt on the published
Carboniferous ages for the Hong’an HP eclogites
Discussion and Conclusions
and conclude that the HP/UHP metamorphism in
The failure of producing a meaningful Sm-Nd
the Hong'an Block should have taken place in the
isochron age is due to isotope disequilibrium
Triassic at about 220-230 Ma, similar to those
between garnet and its coexisting minerals, includ-
observed in the Dabie and Sulu terranes.
ing inclusions. The disequilibrium results from two
distinct processes: (1) garnet preserves the isotopic
The original paper was published in Contributions to Mineralogy and Petrology 149 (2005): 499-526.
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