Tectonic mélange records the Silurian–Devonian subductionmetamorphic process of the southern Dunhuang terrane,
southernmost Central Asian Orogenic Belt
Hao Y.C. Wang1,2, Hong-Xu Chen1, Qian W.L. Zhang1, Meng-Yan Shi1, Quan-Ren Yan1,2, Quan-Lin Hou1,2, Qing Zhang3,
Timothy Kusky4, and Chun-Ming Wu1,2*
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences,
P.O. Box 9825, Beijing 100029, China
3
Institute of Geomechanics, Chinese Academy of Geological Sciences, 11 South Minzudaxue Road, Beijing 100081, China
4
State Key Laboratory of Geological Processes and Mineral Resources, Center for Global Tectonics and School of Earth Sciences,
China University of Geosciences, Wuhan 430074, China
1
2
ABSTRACT
The Hongliuxia tectonic mélange of the southern Dunhuang terrane, northwestern China, southernmost Central Asian Orogenic
Belt (CAOB), consists of eclogite, mafic granulite, and amphibolite as
puddingstones within a matrix of metapelitic gneiss and marble; these
rocks are interpreted to be part of an ancient subduction zone setting. Secondary ion mass spectrometry U-Pb dating of metamorphic
zircons obtained from the puddingstones and matrix metapelite suggests that the metamorphism occurred at ca. 428–391 Ma. The metamorphic rocks all record similar clockwise metamorphic pressuretemperature-time (P-T-t) paths of the western Alpine type. However,
remarkable differences between metamorphic peak P-T conditions
ranging from 830 °C and 24.2 kbar for the eclogite puddingstone to
700 °C and 10.2 kbar for the metapelite matrix were found in the
mélange rocks. This indicates the mixing of rocks from significantly
different depths to create a tectonic mélange in a subduction channel, possibly juxtaposed during the uplift stage. These data suggest
that the southernmost CAOB underwent subduction and subsequent
exhumation caused by subduction of the Paleozoic Hongliuxia ocean
during the middle Silurian to middle Devonian.
INTRODUCTION
Mélange is defined as native or exotic rocks with different origins or
ages that are generally embedded in a highly internally disrupted metasedimentary or serpentinite matrix with block-in-matrix fabrics reflecting tectonic, sedimentary, and diapiric processes (e.g., Silver and Beutner, 1980;
Harlow et al., 2004; Festa et al., 2012). Tectonic mélange is composed
of a chaotic mixture of metamorphic rocks, and is an important type of
mélange that forms in a subduction channel (Festa et al., 2012). Moreover, tectonic mélange is a critical component in the tectonometamorphic
evolution of orogenic belts; the oldest of these rocks have been dated
as early Precambrian (e.g., Wang et al., 2013; Wan et al., 2015). In the
field, tectonic mélange can be identified by interruption of metamorphic
strata, chaotic gathering of metamorphic puddingstones (i.e., conglomerate containing rounded pebbles of colors that contrast with those of the
finer grained matrix) of discontinuous metamorphic facies, sharp contacts
of schistosity and/or gneissosity between different metamorphic rocks, or
a high angle of schistosity and/or gneissosity between the metamorphic
puddingstones and the matrix.
*E-mail: [email protected]
The Central Asian Orogenic Belt (CAOB) is a complex accretionary
orogen (Fig. DR1A in the GSA Data Repository1) that formed through
multiple convergence and collision events of many orogenic components
during multiple phases of amalgamation (Xiao et al., 2015). As depicted
in Figure DR1A, the CAOB hosts many accretionary complexes (Kusky
et al., 2013). However, the metamorphic evolution of the Dunhuang terrane, southernmost CAOB, has seldom been investigated (Zong et al.,
2012; Wang et al., 2016, 2017). We document here that the Hongliuxia
block in the southern part of the Dunhuang terrane is an amalgamated
tectonic mélange formed during the Silurian–Devonian, and report the
first finding of Paleozoic eclogite in the Dunhuang terrane. We integrate
this finding with field evidence, micropetrography, metamorphic geochronology, and the metamorphic pressure-temperature-time (P-T-t) paths of
this tectonic mélange to investigate the Paleozoic orogenic process of
the southernmost CAOB.
TYPICAL GEOLOGIC SETTING AND PETROGRAPHY
The Dunhuang terrane is located in the southern CAOB and is composed of several discrete blocks separated by Tertiary strike-slip faults.
The main part of the Hongliuxia block, exposed in the southern Dunhuang terrane (Figs. DR1B and DR1C), is a tectonic mélange of ~60 km
long and ~12 km wide. It is intruded by several late granitic bodies (Fig.
DR1C). Eclogite, mafic granulite, and amphibolite are preserved as puddingstones within a matrix of metapelitic schist and/or gneiss or marble
(Fig. DR2), indicating the block-in-matrix structure of a typical tectonic
mélange (e.g., Festa et al., 2012; Wang et al., 2013). This is further evidenced by the discontinuous inconsistent gneissosity or schistosity of the
metamorphic puddingstones and the matrix rocks (Fig. DR2). Garnet-free
amphibolite rinds surround the eclogite (Fig. DR2B) or mafic granulite
(Figs. DR2D–DR2F); however, no discontinuous boundaries have been
found between them. This suggests that the garnet-free amphibolite rinds
are possibly the retrograde products of either the eclogite or the mafic
granulite. Micropetrographic features of the metamorphic rocks (Fig. 1)
are concisely described herein (mineral abbreviations are after Whitney
and Evans, 2010).
The eclogite (sample D99) preserves three different mineral assemblages formed at the prograde (pre-eclogite facies, M1), peak (eclogite
facies, M2), and retrograde (post-eclogite facies, M3) metamorphic stages
1 GSA Data Repository item 2017123, documentation of faults, is available
online at http://www.geosociety.org/datarepository/2017/ or on request from
[email protected].
GEOLOGY, May 2017; v. 45; no. 5; p. 1–4 | Data Repository item 2017123 | doi:10.1130/G38834.1 | Published online XX Month 2017
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GEOLOGY 45 | ofNumber
5 For
1
A
B
C
D
E
F
Figure 1. Micropetrographs of the representative metamorphic rocks of the Hongliuxia tectonic mélange, China. Red arrows refer to electronic
microbe analytical profiles. Abbreviations of minerals: Act—actinolite; Bt—biotite; Chl—chlorite; Cpx—clinopyroxene; Di—diopside; Grt—garnet;
Hbl—hornblende; Ilm—ilmenite; Mag—magnetite; Omp—omphacite; Pl—plagioclase; Qz—quartz; Rt—rutile (after Whitney and Evans, 2010).
Oli is oligoclase; Sym. is symplectic. A: Eclogite sample D99. The prograde (pre-eclogite) metamorphic phases (M1; Di1 + Omp1 + Pl1 + Qz1)
occur as inclusions within the garnet porphyroblast (Grt2); the metamorphic peak assemblages (M2) include garnet porphyroblast (Grt2) plus
omphacite (Omp2); and the retrograde (post-eclogite) symplectic assemblages (M3) consist of Pl3 + Hbl3 + Bt3 + Qz3 rimming the Grt2 and Hbl3
rimming the Omp2. B: Backscattered electron (BSE) image of sample D99 showing decomposition of Omp2 into symplectite (Di3 + Oli3) during
the decompression stage. C–E: BSE images of the high-pressure mafic granulites, samples DE16 and DE33, and garnet-bearing amphibolite,
sample DE44. The prograde metamorphic assemblages (M1) appear as inclusions enclosed in the garnets; the peak assemblage (M2) appears
as matrix minerals; and the retrograde assemblages (M3) appear as symplectic intergrowths rimming the garnets. F: Photomicrograph of
kyanite-garnet–bearing metapelite showing prograde biotite (Bt1) enclosed by garnet. The metamorphic peak assemblages (M2) appear as
matrix minerals, and the retrograde assemblages (M3) appear as reaction selvages (Chl3 + Pl3 + Qz3) surrounding the garnet.
(Fig. 1A). The M1 assemblages are tiny (10–220 µm) diopside (Di1) +
plagioclase (Pl1) + omphacite (Omp1) + quartz (Qz1) and rare hornblende
(Hbl1) + biotite (Bt1) + K-feldspar (Kfs1) + epidote (Ep1) preserved in the
garnet porphyroblast (Grt2). The M2 assemblages are Grt2 + high-jadeitic
Omp2 + Qz2 + high-Ti Bt2 + ilmenite (Ilm2) + rutile (Rt2) + zircon (Zr2).
The M3 assemblages are the retrograde symplectite of Hbl3 + Pl3 + Qz3
± Bt3 rimming the garnet porphyroblast, and irregular Hbl3 as exsolved
blebs or short selvages within or rimming the Omp2 (Fig. 1A), as well as
the Di3 + oligoclase (Oli3) intergrowths (Fig. 1B) formed by the decomposition of Omp2.
The mafic granulites (samples DE16 and DE33) also preserve three
assemblages formed at the three different metamorphic stages (Figs.
1C and 1D). The prograde assemblages (M1) are inclusion minerals
(Pl1 + Hbl1 + Qz1 + Ilm1, 10–100 µm) enclosed in the garnet porphyroblasts. The metamorphic peak assemblages (M2) consist of Grt2 + matrix
minerals (Cpx2 + Hbl2 + Pl2 + Qz2 + Zr2 ± Ilm2), and the retrograde assemblages (M3) are represented by the symplectic intergrowth (Pl3 + Hbl3 +
Qz3 ± Mag3) rimming the garnet.
Three episodes of metamorphic mineral assemblages are also preserved in the garnet amphibolite (sample DE44; Fig. 1E). The prograde
assemblages (M1) are Pl1 + Hbl1 + Qz1 + Ilm1 (20–200 µm) enclosed
in the garnet porphyroblast. The metamorphic peak assemblages (M2)
consist of garnet porphyroblast (Grt2) plus matrix minerals (Hbl2 + Pl2
+ Qz2 + Zr2 ± Ilm2). The retrograde assemblages (M3) are symplectic Pl3
+ Hbl3 + Qz3 ± Ilm3 around the garnet.
The metapelitic matrix of the mélange (sample DE45; Fig. 1F) preserves only a few tiny (20–70 µm) biotites (Bt1) as prograde metamorphic
phases within the garnet owing possibly to a large consumption of the M1
assemblages during the transformation from M1 to M2 metamorphism.
The metamorphic peak assemblages (M2) are composed of garnet porphyroblast (Grt2) plus matrix minerals Pl2 + Bt2 + Ky2 + Qz2 + Ilm2 + Zr2,
and the retrograde assemblages (M3) are represented by the symplectic
intergrowth of chlorite (Chl3) + Pl3 + Qz3 around the garnet as an open
thin corona (Fig. 1F).
METAMORPHIC P-T PATHS
The chemical compositions of the representative metamorphic minerals (Tables DR1 and DR2) were determined at the Department of Geology, Hefei University of Technology, China. Both natural and synthetic
minerals were used as standards. All the garnets of the eclogite, mafic
granulite, amphibolite, and metapelite show negligible chemical zonations (Figs. DR7 and DR8). The XMn [= Mn/(Fe + Mg + Ca + Mn)] and
Fe# [= Fe/(Fe + Mg)] values slightly increase only at the extreme rim of
the garnets. This indicates (1) post-peak Fe-Mg re-exchange between the
garnet rim and the adjacent ferromagnesian minerals and (2) decomposition of the garnet rim (Kohn and Spear, 2000). The matrix omphacite
(Omp2) in the eclogite displays clear zoning (Figs. DR9 and DR10), and
the jadeite fraction (XJd) decreases from 0.42 at the core to 0.06 at the
rim. From the inclusion Omp1, matrix Omp2, to symplectic Cpx3, the XJd
fraction increases and then decreases (Fig. DR10), reflecting a pressure
| Volume 45 | Number 5 | GEOLOGY
2www.gsapubs.org increase from the prograde (M1) to the metamorphic peak (M2) followed
by a decrease from M2 to the retrograde (M3) stage. Amphiboles in the
eclogite, mafic granulite, and amphibolite are calcic and show meaningful systematic changes (Fig. DR11). From M1 to M2 and then to M3, the
albite component of the plagioclase systematically increases and then
decreases (Fig. DR12), similar to variations of the TiO2 content in the
biotite (Tables DR1 and DR2).
The available accurate geothermobarometers (Table DR3) were applied
to the eclogite, mafic granulite, and amphibolite puddingstones and
metapelite. The metamorphic P-T paths of the metamorphic puddingstones
and the matrix metapelite are quite similar, and are all of the clockwise
type, including the near isothermal decompression process (Fig. 2). This
suggests that they possibly underwent the same orogenic cycling (e.g.,
Harley, 1989) from subduction, corresponding to the prograde process
M1 → M2, to subsequent tectonic denudation and/or uplift, corresponding to the retrograde process M2 → M3. However, their metamorphic
peak P-T conditions differ considerably, ranging from 830 °C and 24.2
kbar for eclogite to 735–744 °C and 17.1–17.7 kbar for mafic granulites,
or 652 °C and 10.2 kbar for amphibolite. The M2 P-T conditions for the
matrix metapelite were estimated to be 700–708 °C and 10.2–10.7 kbar.
DATING METAMORPHISM
The zircons were separated by using standard heavy-mineral separation
processes, hand-picked for final purity, mounted, and polished with zircon
standard Qinghu (Li et al., 2009) for secondary ion mass spectrometry
(SIMS) analysis. Details of the instrument, a CAMECA 1280 microprobe
installed at the Institute of Geology and Geophysics, Chinese Academy of
Sciences, Beijing, and the analytical procedures can be found in Li et al.
(2009, 2010); the analytical data are given in Table DR4. Among the 94
total analytical points (Table DR4), 48 points show magmatic origins, and
46 points show metamorphic origins (Fig. DR5), as discriminated by the
criteria of Th/U ratios (e.g., Hoskin and Schaltegger, 2003; Rubatto, 2002).
Most of the zircons separated from the eclogite puddingstone (sample
D99) have irregular shapes (Fig. DR3A). No magmatic rhythmic zonations have been found, and some grains show core-rim structures. The
zircons contain U and Th contents of 95–283 ppm and 0.2–156 ppm,
respectively; 12 analytical points are concordant with low Th/U ratios of
0.001–0.143 and produce a young concordia age of 411.3 ± 1.7 Ma (Fig.
DR4A), possibly representing the metamorphic age. Such zircons might
be completely recrystallized from earlier zircons (Hoskin and Schaltegger, 2003). Scattered points (n = 22) suggest a heavy Pb loss but yield an
upper intercept age of 1316 ± 17 Ma (Fig. DR6A), possibly representing
the protolith age.
Zircons from the mafic granulite puddingstone, sample DE16 (Fig.
DR3B), contain U and Th contents of 67–431 ppm and 0.4–539 ppm,
respectively. Data points (n = 16) with low Th/U ratios of 0.006–0.428
yield a concordia age of 413.3 ± 1.5 Ma (Fig. DR4B), possibly representing the metamorphic age. The remaining four scattered points with higher
Th/U ratios and older ages cannot provide valid information due to heavy
Pb loss. However, they yield an isochron with the lower intercept age of
417 ± 14 Ma (Fig. DR6B), approaching the metamorphic age.
Zircons from the garnet amphibolite puddingstone, sample DE44 (Fig.
DR3C), contain U and Th contents of 35–177 ppm and 0.9–31 ppm,
respectively. Concentrated data points (n = 10) with low Th/U ratios of
0.009–0.082 yield a concordia age of 408.9 ± 2.0 Ma (Fig. DR4C), possibly representing the metamorphic age. The remaining four discordant data
points, with higher Th/U ratios and older ages, are informative, probably
due to heavy Pb loss. However, they yield a lower intercept age of 412
± 10 Ma, which possibly approaches the metamorphic age (Fig. DR6C).
Zircons from the metapelite sample, DE45 (Fig. DR3D), contain U
and Th contents of 147–596 ppm and 0.8–276 ppm, respectively. Data
points (n = 6) obtained from the zircon overgrowth rims, with very low
Th/U ratios of 0.005–0.006, yield a concordia metamorphic age of 413.7
GEOLOGY | Volume 45 | Number 5 | www.gsapubs.org
Figure 2. Metamorphic pressure-temperature (P-T) paths retrieved
from the eclogite, high-pressure (HP) mafic granulite, and amphibolite
puddingstones of the Hongliuxia tectonic mélange, China. Geothermobarometers can be found in Table DR3 (see footnote 1). Red dashed
line depicts the jadeite fraction (XJd) of the clinopyroxene varying from
8.6% to 26.8% in the inclusion assemblages. The boundaries of metamorphic facies and metamorphic facies series are from O’Brien and
Rötzler (2003) and Spear (1993), respectively.
± 2.5 Ma (Fig. DR4D). Zircons yielding significantly older 207Pb/206Pb
ages of ca. 1890.6–1809 Ma (Fig. DR6D) are possibly detrital in origin.
The SIMS U-Pb dating of the metamorphic zircons reveals that the
eclogite, mafic granulite, and amphibolite puddingstones as well as the
metapelite matrix all were affected by a middle Silurian–middle Devonian tectonometamorphic event. Thermodynamic forward modeling for
high-grade rocks (Kelsey and Powell, 2011) or high- and ultra-high-P
eclogites (Kohn et al., 2015) suggests that zircon grows mainly during
late-stage exhumation and cooling, thus postdating the metamorphic peak.
Therefore, the U-Pb ages of the metamorphic zircons of the eclogite, mafic
granulite, and amphibolite puddingstones as well as the metapelitic matrix
may record the metamorphic peak to the retrograde stage.
DISCUSSION
Eclogite is one of the most characteristic products of the subduction
factory. Our first discovery of eclogite in the Hongliuxia block supports
the existence of high-P metamorphism in a subduction zone, in which
the tectonic mélange was formed. The retrieved metamorphic P-T-t paths
of the eclogite, mafic granulite, and amphibolite puddingstones as well
as the matrix metapelite are all clockwise and are quite similar (Fig. 2);
these paths are comparable to the western Alpine–type P-T-t paths (Ernst,
1988). Specifically, these P-T-t paths resemble the uplift of blueschists of
the western Alps in which there are different units with different metamorphic peaks and different exhumation rates. However, the uplift rate
of the Hongliuxia metamorphic puddingstones cannot be quantitatively
estimated at present.
The huge differences in the metamorphic peak pressures of the
different metamorphic rocks are far beyond the errors of the related
3
geothermobarometers. Because most accretionary processes in accretionary wedges involve the addition of material to the overriding plate by
offscraping or underplating at similar metamorphic grades, our finding of
the remarkable mixing of rocks from tremendously different metamorphic
depths in a mélange suggests that these rocks have recorded the processes
in an ancient subduction channel along the plate boundary between the
overriding and downgoing plates (Cloos and Shreve, 1988). Therefore,
it is documented that the mixing of puddingstones from various depths
with strong deformation and subduction, metamorphism, and exhumation
occurred. It has been suggested that these features may be characteristic
of global subduction channels (e.g., Kusky et al., 1997).
The subduction-related metamorphism in the Hongliuxia tectonic
mélange, southern Dunhuang terrane, is possibly the result of subduction
of a Paleozoic ocean, referred to here as the Hongliuxia ocean. However,
at present we are unable to determine the direction of the subduction. Our
new findings of eclogite as well as high-P mafic granulite and amphibolite
puddingstones within the metapelite and marble matrix confirm that the
subduction and exhumation occurred at ca. 428–391 Ma. This finding,
combined with the northern distributed younger orogenic belts (Xiao et al.,
2015), indicates a younger orogeny age of the southern CAOB that continues northward, eventually joining the southern Mongolia active margin
to form the south Tianshan–Beishan–Solonker suture (Xiao et al., 2010).
CONCLUSIONS
Diverse metamorphic textures are preserved in the eclogite, mafic
granulite, and amphibolite puddingstones of the Hongliuxia block, southern Dunhuang terrane, southernmost CAOB. These rocks record quite
similar, clockwise, western Alpine–type P-T-t paths, which are indicative
of an orogenic process that occurred at ca. 428–391 Ma. The large gaps
of metamorphic peak pressures among these rocks indicate that the mixing of puddingstones from a wide depth range occurred in a subduction
channel formed during a subduction process of the Paleozoic Hongliuxia
ocean, possibly juxtaposed during the uplift stage. However, the subduction direction cannot be determined at present.
ACKNOWLEDGMENTS
Xian-Hua Li and Qiu-Li Li guided us in secondary ion mass spectrometry U-Pb
dating of zircons, and Yong-Hong Shi and Juan Wang helped us in electronic
microbe analyses. The quality of the original manuscript was substantially improved
through reviews by Wenjiao Xiao, Jacques Charvet, Juan Díaz-Alvarado, and an
anonymous reviewer, as well as Judith Totman Parrish. This research was financially
supported by the National Natural Science Foundation of China (grants 41225007,
41372199) and the State Key Laboratory of Lithospheric Evolution, Institute of
Geology and Geophysics, Chinese Academy of Sciences (grant KAI201605). This
paper is in honor of Yusheng Pan (Institute of Geology and Geophysics, Chinese
Academy of Sciences), dedicated to his contributions to Tibetan geology on his
80th birthday.
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Manuscript received 24 November 2016
Revised manuscript received 16 January 2017
Manuscript accepted 19 January 2017
Printed in USA
| Volume 45 | Number 5 | GEOLOGY
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