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Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications
Accepted Manuscript Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications Xiaojing Zhang, Makoto Takeuchi, Masahiro Ohkawa, Nozomi Matsuzawa PII: DOI: Reference:
S1367-9120(18)30005-1 https://doi.org/10.1016/j.jseaes.2018.01.005 JAES 3377
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
17 February 2017 22 December 2017 11 January 2018
Please cite this article as: Zhang, X., Takeuchi, M., Ohkawa, M., Matsuzawa, N., Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes.2018.01.005
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1
Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi
2
Belt in Southwest Japan and Paleogeographic implications
3
Xiaojing Zhang1* Makoto Takeuchi1 Masahiro Ohkawa2 Nozomi Matsuzawa 1#
4
1. * Department of Earth and Planetary Sciences, Graduate School of Environmental Studies,
5
Nagoya University, Chikusa-Ku, Nagoya 464-8601, Japan; [email protected]
6
Current address: Institute of Earth Sciences, Academia Sinica, 128, Sec. 2, Academia Road,
7
Nangang, Taipei 11529, Taiwan
8
2. Mitsubishi Material Techno Co., 1-297, Kitabukuro-cho, Omiya-ku, Saitama-City, 330-0835,
9
Japan
10
11
# Current address: Sakai-City, Fukui Prefecture, Japan
Abstract
12
The Akiyoshi Belt in the Inner Zone of Southwest Japan is made up of a
13
Permian accretionary complex, the main component of which is the Late
14
Permian Nishiki Group that mainly consists of sandstone, mudstone, felsic tuff
15
and a minor amount of chert and conglomerate. We employ multiple methods,
16
which includes sandstone petrography, detrital garnet composition and detrital
17
zircon U-Pb dating to investigate the likely sources of these terrigenous deposits
18
and to reconstruct the paleogeographic link between the proto-Japan and the
19
East Asian continent. The highly immature Late Permian sandstones are
20
interpreted to derive from multi-type source rocks that include felsic igneous
21
rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks
22
in proximal locations. The detrital zircon U-Pb results show that all samples
23
contain a dominant Early to Late Permian zircon population (294–254 Ma) and
24
these zircons are interpreted to be derived from an active volcanic arc, which
25
was most likely caused by subduction of the Paleo-Pacific plate beneath the
26
South China Block. We found that spessartine-rich almandine garnets and
27
almandine garnets dominated assemblages in the lower unit changed to
28
grossular-andradite garnets dominated assemblage in the upper unit, which was
29
caused by a progressive uplifting and denudation of the Permian volcanic arc.
30
Keywords: Late Permian; petrography; garnet composition; detrital zircon;
31
provenance; South China
32
1. Introduction
33
The Japanese Islands are mainly composed of Upper Paleozoic to Cenozoic
34
accretionary complexes, and consists of trench-fill deposits, pelagic sediments,
35
and oceanic basalts. The Tanakura Tectonic Line (TTL) geologically divides
36
Japan into Northeast Japan and Southwest Japan. The pre-Triassic accretionary
37
complexes are mostly distributed in Southwest Japan, which can be subdivided
38
into the Inner (Japan Sea side) and Outer (Pacific side) Zones by the Median
39
Tectonic Line (MTL) (Fig. 1A).
40
The Japanese Islands have a complex geological history as they lie at the
41
junction of four tectonic plates which are the Eurasian, Pacific, North American,
42
and Philippine Sea plates. After the conversion from a passive continent margin
43
into a convergent one in the early Paleozoic, subduction of the Paleo-Pacific
44
plate generated the orogenic growth of Japan (Isozaki et al., 1990; Isozaki et al.,
45
2011; Maruyama et al., 1997; Taira, 2001). These subduction processes caused
46
accretion and tectonic erosion in the subduction zones from the late Paleozoic to
47
the present (Isozaki et al., 2011). The Permian to Triassic accretion is one of the
48
fundamental tectonic events of during the formation of proto-Japan. Therefore,
49
the Permian-Triassic tectonics of Japan is particularly important and will provide
50
the foundation in understanding the tectonic evolution of Japan (Isozaki, 1997).
51
The Akiyoshi Belt is occupied by a Permian accretionary complex, which
52
consists of unmetamorphosed Permo-Carboniferous sandstone, mudstone,
53
conglomerate, siliceous shale, felsic tuff, chert, limestone and basalt (Kanmera,
54
1990; Naka et al., 1986). There are some studies on investigating provenance,
55
deposition environment and tectonic settings of these sediments. Sano and
56
Kanmera (1988) described the lithological features of chert and associated
57
clastic limestone and interpreted these carbonates as sediments on and around
58
an oceanic seamount in the Paleo-Pacific Ocean (Panthalassan Ocean). A
59
sedimentary facies study on the siliciclastic component of the Nishiki Group in
60
the Akiyoshi Belt suggested that they were deposited in a trench and a
61
trench-slope basin at a crustal convergent margin (Hara and Kiminami, 1989).
62
Some provenance studies have been carried out on the sedimentary rocks of
63
the Akiyoshi Belt. Detrital garnet compositional analysis revealed that the Late
64
Permian
65
grossular-andradite garnets or none at all (Yokoyama, 1998). Detrital zircon
66
geochronology study of the trench-fill sandstone from the upper Nishiki Group
67
show a continuous age spectrum of 320–240 Ma, while the felsic tuff samples
68
from the middle part of the same unit contain two distinctive U-Pb age groups:
69
2700–2400 Ma and 320–260 Ma (Tsutsumi et al., 2000).
strata
in
the
Akiyoshi
Belt
either
contained
abundant
70
In spite of these studies, however, the paleogeography of the Akiyoshi Belt is
71
still not well established. It was proposed to initially develop along the
72
southeastern margin of the South China Block (e. g. Aoki et al., 2015; Isozaki et
73
al., 2010; Jahn, 2010; Maruyama et al., 1997; Sengör and Natal'In, 1996), while
74
some others argues for a North China Block origin (Takeuchi et al., 2008;
75
Tsutsumi et al., 2000). It is also not clear that what types of rocks existed in the
76
source region of the siliciclastic rocks in the Akiyoshi Belt. The petrographic,
77
detrital garnet compositional and detrital zircon U-Pb geochronological data
78
presented here provide new insight into the depositional setting and provenance
79
of the sedimentary rocks of the Akiyoshi Belt. Especially, zircon U-Pb ages
80
present
81
paleogeography because of the different age distributions of Neoproterozoic and
82
Phanerozoic rocks between the South China Block and the North China Block
83
together with their neighboring areas.
84
2. Geological Background and Stratigraphy
us
critical
information
to
reconstructing
the
late
Permian
85
The Akiyoshi Belt is distributed in several isolated areas of the Inner Zone of
86
Southwest Japan. It is structurally overlain by the Renge metamorphic rocks to
87
the north, which mainly consists of a suite of Carboniferous high-P/T schists
88
(Nishimura, 1990; Tsujimori, 2000, 2002), and is in fault contact to the the
89
Maizuru Belt in the south, which is occupied by a Permian island-arc and
90
back-arc basin system (Ishiwatari et al., 1990) (Fig. 1A).
91
The Nishiki area is located in the western tip of the main island of Japan,
92
where the Nishiki Group strata of the Akiyoshi Belt is exposed (Fig. 1). In the
93
north, the Nishiki Group is overlain uncomformably by the Cretaceous rhyolitic
94
and dacitic tuff, andesite lava and siliciclastic rocks; in the south it thrusts over
95
the Tsuno Group of the Suo metamorphic rocks (Nishimura and Nureki, 1966)
96
(Figs. 1B and 2). Middle to Late Permian radiolarian fossils reported from the
97
Nishiki Group include Follicuculus monacanthus Assemblage (Ishiga, 1990;
98
Nishimura et al., 1989), Fo. scholasticus Assemblage (Naka and Ishiga, 1985;
99
Nishimura et al., 1989) and fusulina Lepidolina multiseptata (Nishimura, 1971),
100
suggesting deposition in the Middle to Late Permian. The stratigraphy of the
101
Nishiki Group displays a sequence sandstone, mudstone, felsic tuff and minor
102
chert and conglomerate in an ascending order (Tanaka et al., 1987). These
103
rocks are mostly stacked by a series of thrusts (Nishimura et al., 1989).
104
Accordingly, the Nishiki Group was divided into Units I, II, III and IV with
105
younging stratigraphic ages (Figs. 2 and 3).
106
Unit I is distributed in the southeast of the study area, mainly consisting of
107
chert, siliceous shale, felsic tuff and mudstone with a total thickness of greater
108
than 100 m (Figs. 2 and 3). Resting on top of Unit I by a thrust-faulted contact
109
are coarse-grained clastic rocks with chert, siliceous shale, felsic tuff, mudstone
110
and conglomerate that are grouped as Unit II and are over 1 km thick. This unit is
111
further subdivided into subunits IIa, IIb, IIc, IId and IIe based on the lithology
112
(Figs. 2 and 3). Subunit IIa is felsic-tuff dominated; IIb and IId are mudstone and
113
fine-grained sandstone; lastly, IIc and IIe are dominated by coarse-grained
114
sandstone (Fig. 3). Unit III is exposed in the west, consisting of over 500 m thick
115
of tuffaceous mudstone, mudstone, sandstone and conglomerate. It is
116
subdivided into subunits IIIa, IIIb, IIIc and IIId (Fig. 3). Subunit IIIa consists of
117
tuffaceous mudstone; IIIb and IIId are dominated by mudstone; finally, IIIc is
118
mainly sandstone. The boundary between Units II and III is a thrust fault contact.
119
Unit IV has a normal fault contact with Unit III. It is exposed in the southwest and
120
east of the Nishiki area, and consists of weakly metamorphosed mudstone and
121
sandstone (Figs. 2 and 3).
122
3. Sampling and Methods
123
Units II and III hold the majority of siliciclastic rocks of the Nishiki Group and
124
cover most of the study region. In addition to this, Units II and III belong to a
125
coherent sequence, so that we are able to understand compositional changes in
126
different stratigraphic horizons. Therefore, these two units are our focus in this
127
study. We collected 17 sandstone samples at 17 localities, from Unit II (IIb to IIe)
128
and Unit III (IIIc and IIId). Sandstone petrography, garnet composition analysis
129
and detrital zircon U-Pb geochronology were carried out on these samples (Figs.
130
2 and 3, and Table 1).
131
3.1 Sandstone petrography
132
Twelve samples were used for sandstone petrography analysis. These
133
samples are medium-grained sandstones with less than 25% matrix. Thin
134
sections
135
hexanitrocobaltate solution to allow clear identification of K-feldspars during
136
point counting. At least 500 points were counted on each thin-section using the
137
Gazzi-Dickinson method (Dickinson, 1988; Ingersoll et al., 1984), with quartz,
138
feldspar and lithic fragments as the main counted framework grains.
139
3.2 Detrital garnet composition
of
these
samples
were
stained
with
saturated
sodium
140
Four sandstone samples (N1 from Unit IIb, N7 from Unit IId, N9 from Unit IIe
141
and N17 from Unit IIId) were crushed and sieved through a 250-μm mesh. The
142
initial volume of samples was reduced using a water-panning method (Hutton,
143
1950), and then dilute HCl 10% was used to dissolve any calcareous particles
144
and carbonate cement. Heavy mineral separation and preparation for chemical
145
analysis of garnets were performed following the procedures described by
146
Mange and Maurer (1992). Dried heavy mineral grains were mounted onto a
147
microscope slide under epoxy using the method by Leu and Druckman (1982).
148
Slides were ground with carborundum and then polished carefully with diamond
149
polishing compounds.
150
The chemical composition of detrital garnets were determined on a JEOL
151
JXA-8800R electron probe microanalyzer (EPMA) at Nagoya University with an
152
accelerating voltage of 15 kV, specimen current of 12 nA, beam diameter of 3
153
μm, and maximum count time of 20 s. Garnet grains were analyzed for Si, Ti, Al,
154
Cr, Fe, Mn, Mg, Ca, Na, and K. About 100 grains were analyzed per sample. The
155
ratio of Fe2+ to Fe3+ was calculated, assuming garnet stoichiometry. The
156
molecular end-members were calculated by the method of Deer et al. (1992).
157
The composition of detrital garnet can be expressed in terms of six
158
end-members: almandine, pyrope, spessartine, grossular, andradite, and
159
uvarovite.
160
3.3 Detrital zircon U-Pb geochronology
161
Detrital zircons from eight samples were dated using laser ablation
162
inductively-coupled plasma mass spectrometry (LA-ICP-MS). Zircons were
163
separated from about 2 kg of sample using conventional water table and heavy
164
liquid mineral separation methods. Around 300 grains were handpicked onto
165
double-sided adhesive tape, embedded into an epoxy resin disk and polished.
166
Cathodoluminescence (CL) images were prepared to study the internal
167
structures of zircons. U-Pb ages of detrital zircons were obtained from an Agilent
168
Technologies Agilent 7700 ICP-MS and ESI NWR213 Nd-YAG (λ=213 nm) laser
169
ablation system in Nagoya University following the analytical procedure
170
described by Orihashi et al. (2008). Pb/U calibration was performed relative to
171
the zircon standard 91500 with an age of 1065 Ma (Wiedenbeck et al., 1995).
172
The weighted mean ages of standard zircon 91500 for each sample are included
173
in the supplementary material. All probability density distribution plots were
174
made using ISOPLOT/Ex 4.15 (Ludwig, 2010). All errors are reported at the
175
2-sigma level.
176
4. Results
177
4.1 Sandstone petrography
178 179
Petrographic photos of sandstone samples are shown in Fig. 4 and the point
counting results of the framework grains are given in Table 1 and Fig. 5.
180
Sandstones of the Nishiki Group are immature, containing poorly sorted,
181
medium to coarse grains in angular to subangular shape (Fig. 4). Figure 5 shows
182
that one sample from the lower horizon of Unit II as feldspathic litharenite (Fig.
183
4A), ten samples from Units II and III are classified as lithic arkose (Fig. 4B),
184
and one sample from the upper horizon of Unit III as arkose (Fig. 4C). This
185
indicates that the Nishiki Group sandstones change from feldspathic litharenite,
186
through lithic arkose to arkose. Monocrystalline quartz (19–27%) prevails
187
polycrystalline quartz (1–5%). About half of them show slight corrosion and weak
188
undulose extinction, suggesting provenance from a volcanic source. Plagioclase
189
(27–46%) is more plentiful than K-feldspar (1–12%) and plagioclase grains have
190
undergone sericitic alteration or saussuritization (Fig. 4). Lithic fragments range
191
from 15% to 48% in abundance and are dominated by felsic to andesitic volcanic
192
lithics (Table 1 and Fig. 4). The felsic volcanic lithics have felsitic or vitric
193
groundmass with phenocrysts of quartz and plagioclase. Minor granitic and
194
dioritic lithics are also present. Sedimentary and metamorphic lithic fragments
195
include limestone, mudstone, sandstone, chert, mica schist and hornfels. On a
196
QmFLt plot (Dickinson, 1985), the samples are distributed in the “dissected arc”
197
and “transitional arc” fields (Fig. 5B).
198
4.2 Detrital garnet composition
199
Garnets are common heavy minerals in siliciclastic sediments. Due to its
200
large chemical variability, garnet chemical composition has been widely used as
201
a sediment provenance tool (e. g. Hartley and Otava, 2001; Morton et al., 2004;
202
Takeuchi, 1994; Takeuchi et al., 2008; von Eynatten and Gaupp, 1999 and many
203
others). Garnet is generally interpreted to derive from metamorphic rocks
204
(Mange and Maurer, 1992), but it is also present in plutonic igneous rocks,
205
pegmatites, ultramafic rocks and felsic volcanics (Deer et al., 1982; Mange and
206
Morton, 2007).
207
Sandstone samples from Units II and III contain various types of heavy
208
minerals, including abundant biotite, epidote, hornblende, titanite, garnet, rutile,
209
zircon, tourmaline and apatite. Detrital garnet grains from three Unit II samples
210
(N1, N7 and N9) and one Unit III sample (N17) are angular to rounded. Most
211
grains are colorless, while red, yellow and pink grains are occasionally
212
encountered. Some anisotropic garnets are present. Garnets are unetched,
213
reducing the possibility that garnet dissolution occurred in these rocks.
214
Appearance of epidote and titanite in the heavy mineral assemblage implies that
215
garnet assemblage of the samples was not altered by the burial diagenesis
216
because garnet is more stable than epidote and titanite during burial (Morton
217
and Hallsworth, 2007). Consequently, garnet composition results from Unit II and
218
Unit III of the Nishiki Group are able to convey true provenance information. The
219
compositional data are summarized in Figs. 6 and 7.
220
The analytical results of Unit II and Unit III samples are very distinct. Detrital
221
garnets from Unit II (samples N1, N7 and N9) are dominated by almandine
222
(54–77%) and spessartine-rich almandine (20-36%) with a minor amount of
223
pyrope-rich almandine (3-7%) and grossular-andradite garnet (0-7%). In contrast,
224
garnets from Unit III (sample N17) contain 90% grossular-andradite with 9%
225
almandine
226
compositional data were plotted in a Mg-(Fe+Mn)-Ca ternary plot (Fig. 7) in
227
order to discriminate the source rocks (Mange and Morton, 2007; Morton et al.,
228
2004). Most of the analyzed garnets of Unit II samples are categorized as Type
229
B garnets with minor as Type A and Type D garnets. Sample N1 from the lowest
230
stratigraphic level of Unit II contains no Type D garnet. In comparison, sample
231
N17 from Unit III is dominated by Type D garnet with a few grains as Type B
232
garnets (Fig. 7).
233
4.3 Detrital zircon U-Pb geochronology
and
1%
spessartine-rich
almandine
(Fig.
6).
The
garnet
234
Detrital zircons from five Unit II samples and three Unit III samples were
235
dated and results are summarized in Fig. 8. The complete dataset showing all
236
analysis spots can be found in the supplementary material.
237
Most zircons in the samples are colorless and have euhedral crystals with
238
subangular to angular terminations. Their CL images show oscillatory zoning,
239
which are typical features of igneous zircons (Corfu et al., 2003). Detrital zircon
240
age spectra of the eight samples do not display noticeable changes despite the
241
distinct garnet chemistry results from Units II and III. The majority of analyzed
242
detrital zircons in all samples fall into the age range of 245 to 350 Ma with a few
243
Cambrian to Devonian zircons (Fig. 8). The major age peaks vary from 254 Ma
244
to 294 Ma, re-confirming the previous results reported by Tsutsumi et al. (2000).
245
Sample N14 from the lower part of Unit II yield one Paleoproterozoic to Archean
246
zircon grain (2464 ± 70 Ma) (Supplementary material). The maximum
247
depositional ages of the Nishiki Group samples determined by the mean age of
248
the youngest two or three zircons that overlap in age at 1σ (Dickinson and
249
Gehrels, 2009) are in the range of 249 ± 9 Ma to 261 ± 3 Ma, which are
250
compatible with their stratigraphic ages determined by fossils (Ishiga, 1990;
251
Naka and Ishiga, 1985; Nishimura et al., 1989) (Fig. 8). For example, felsic tuff
252
and mudstone of the lower part of Unit II yielded radiolarian fossils (Follicuculus
253
sholasticus assemblage) that indicate deposition in the Late Permian (Ishiga,
254
1990), corresponding to Capitanian to Wuchiapingian (265–254 Ma) (Ogg et al.,
255
2016).
256
5. Discussion
257
5. 1 Depositional setting
258
The Nishiki Group sandstones are highly immature indicating a quick
259
deposition of first cycle detritus after short-distance transportation. The
260
petrographic composition suggests that sandstones of the Akiyoshi accretionary
261
complex are derived from a variety of source rocks, including felsic and mafic
262
volcanics, plutonics, sedimentary and metamorphic rocks. Abundant first-cycled
263
quartz and feldspar together with volcanic fragments reflect a direct derivation
264
from volcano-plutonic sources. Furthermore, the conglomerate facies of the
265
Nishiki Group contains 2-30 cm rounded rock clasts, including mafic to felsic
266
plutonic rocks, volcanic rocks, sandstone, shale, tuffaceous mudstone and
267
limestone. Kiminami et al. (2000) proposed that the Permian sandstones in the
268
Japanese Islands were supplied from an immature island arc and an evolved
269
island arc of these rocks based on whole-rock major element composition of
270
these rocks. These lines of evidence indicate that the sources of the Nishiki
271
Group are probably volcano-plutonic rocks from a volcanic arc.
272
Units II and III belong to a coherent sequence and Unit II thrust over Unit III.
273
Clastic rocks of the structurally-lower unit is usually younger than the upper unit,
274
suggesting Unit III was deposited after Unit II. Therefore, compositional changes
275
of sandstones could be obtained from the lower sample (N2) through the upper
276
one (N12) in Unit II and the lower one (N13) in Unit III to the upper one (N17) in
277
Unit III. The sandstone composition varied from lithic (volcanic fragment-rich) to
278
feldspathic through time, suggesting a progressive denudation of volcanic arc.
279
This is compatible with the detrital zircon age distributions of the Nishiki
280
sandstones, which has prevailing age peaks (294–254 Ma) plus minor older
281
ages (Fig. 8). Cawood et al. (2012) proposed that the detrital zircon spectra of
282
sediments in arc flanking basins along convergent margins are characterized by
283
dominant young ages from syn-depositional igneous activity with or without
284
some minor older ages from pre-existing rocks. Therefore, we conclude that the
285
Nishiki Group sediments of the Akiyoshi Belt were deposited in an arc-flanking
286
position and their sources are mainly from an Early to Late Permian magmatic
287
arc.
288
Permian magmatism has been reported in Hainan Island in the South China
289
(267–262 Ma) (Li et al., 2006), in southeastern Korea (257–250 Ma) (Cheong et
290
al., 2014; Yi et al., 2012), in the Hida Belt in the central Japan (256–250 Ma)
291
(Horie et al., 2010; Takahashi et al., 2010; Zhao et al., 2013) and in the
292
Jiamusi-Khanka Massif (296–255 Ma) (e. g. Wu et al., 2011; Yang et al., 2015).
293
These late Paleozoic rocks probably represent a magmatic arc produced during
294
westwards subduction (in present-day coordinates) of the Paleo-Pacific plate
295
along the east coastal region of a “Greater South China” block that extended
296
from South China Block through proto-Japan and covered South Korea further to
297
the Jiamusi-Khanka Massif in the Paleozoic (Isozaki et al., 2017) (Fig. 9A). The
298
age span of igneous rocks in this magmatic arc corresponds well with the
299
dominant detrital zircon ages of the Nishiki Group sandstones. Therefore, we
300
infer that the Permian detritus of the Akiyoshi Belt were derived from erosion of
301
uplifted magmatic arc materials along the South China and then these deposits
302
were accreted forming the Akiyoshi accretionary complex (Fig. 9B).
303
The Permian sandstones in southeastern China contain both the
304
syn-depositional Permian (300–260 Ma) and abundant much older zircon grains
305
(2500–370 Ma) (Hu et al., 2015; Hu et al., 2012; Li et al., 2012), whereas the
306
U-Pb age spectra of the Nishiki Group sandstones are dominated by
307
syn-depositional ages and have only a few grains with older ages. This is
308
because they were deposited in different positions relative to the volcanic arc.
309
Permian sandstones in southeastern China and the Nishiki Group sandstones
310
were deposited in the back-arc basin on the continental side and in the trench on
311
the oceanic side, respectively (Fig. 9). Such a provenance diversification within
312
an arc-trench system was also observed in Cretaceous intra-arc and fore-arc
313
basins in SW Japan (Aoki et al., 2014). Detritus from both sides of the back-arc
314
basin, i.e. the magmatic arc that yielded mainly syn-depositional zircons and the
315
interior of the block that provided old zircons, were shed into the basin, whereas
316
detritus from the volcanic arc were the dominant source for the trench sediments
317
(Fig. 9B).
318
5.2 Provenance interpretation on garnet composition
319
Compositional suites of detrital garnet display noticeable characteristics
320
between each unit. Most garnets from Unit II (samples N1, N7 and N9) cluster in
321
the field of Type B with increasing Type D garnets toward upper horizons. Most
322
garnets from Unit III (sample N17) cluster in the field of Type D (Fig. 7). Garnets
323
in Type B field have low-Mg, with variable Ca and Mn, and are thought to be
324
derived from low to medium grade metasedimentary rocks (type Bi) and acidic to
325
intermediate gneisses and granites (type Bii) (Morton et al., 2005). Garnets in
326
the Type D field (high-Ca, low-Mg) generally suggest derivation from
327
metasomatic rocks such as skarns, from very low-grade metabasic rocks, or
328
from ultra-high temperature metamorphosed calc-silicate granulites (Deer et al.,
329
1997; Mange and Morton, 2007). Type D garnets in Unit III sandstone samples
330
also contain variable abundances of Fe and some grains are anisotropic,
331
showing typical features of skarn garnets (Takeuchi, 1994, 2013). Therefore,
332
these garnets are sourced from skarn rocks. Minor Type A garnets (low Ca, low
333
Mn and relatively high Mg) are also present in Unit II samples. They must be
334
derived from high-grade metasediments and/or charnockites (Sabeen et al.,
335
2002). According to above, main sources of garnets of the Unit II and Unit III are
336
considered to change from low to medium grade metasedimentary rocks to skarn
337
rocks.
338
Such type B and D garnets are common in the Permian sandstones in Japan
339
(Takeuchi, 2013; Takeuchi et al., 2008). These Permian sandstones includes
340
Permian detrital zircons (Okawa et al., 2013) and Permian conglomerates
341
contain Permian granitic rock clasts (Shibata, 1973; Takeuchi and Suzuki, 2000).
342
Felsic hornfels fragments in sandstones (Yoshida and Machiyama, 2004) and
343
felsic, mafic and calcareous hornfels clasts in conglomerates (Kano, 1959) are
344
present in shelf deposits of the South Kitakami Belt in Northeast Japan. This
345
indicates that the low-grade metasedimentary rocks and skarn rocks were
346
formed by intrusions of Permian plutonic rocks. Consequently, we consider that
347
the provenance of the sandstones of the Nishiki Group were also derived from a
348
Permian volcanic arc and surrounding contact metamorphic and metasomatic
349
rocks. The change in garnet composition might be caused by change in
350
denudation level of metamorphic and metasomatic rocks surrounding plutonic
351
bodies by a progressive denudation. Sandstone composition also suggests
352
progressive denudation from volcanic rocks to plutonic rocks.
353
The South China Block was almost completely covered by carbonates
354
during the Carboniferous to earliest Permian (Liu and Xu, 1994) and the
355
Jiamusi-Khanka Massif also yields carbonates (Natal'in, 1993). It is likely that
356
skarn deposits were formed when plutonic bodies intruded these carbonates
357
rocks during the subduction of the Paleo-Pacific oceanic plate. The youngest
358
sample N1 includes the older zircons, ca. 400 and 500 Ma (Fig. 8), indicating
359
presence of pre-existed rocks before Permian igneous activities began. This is
360
very crucial for reconstructing the paleogeography of the Akiyoshi Belt, because
361
ca. 400-500 Ma zircons are common in the Greater South China which is
362
extended from the present South China Block to the Khanka Block (Isozaki et al.,
363
2017) (Fig. 9). A single subduction-related Paleozoic batholith belt is considered
364
to have existed along the Pacific rim of the Greater South China
365
2017). As mentioned above, Permian volcanic arc has also existed along the
(Isozaki et al.,
366
Greater South China. The Permian clastic rocks of the Akiyoshi Belt were
367
deposited at a trench along the Greater South China, and derived from the
368
uplifting volcanic arc associated skarn deposits, despite that the exact
369
deposition location has not been able to well constrain yet (Fig. 9).
370
7. Conclusion
371
An integrated petrographic, detrital garnet compositional and detrital zircon
372
U-Pb geochronological study of the Late Permian Nishiki Group sandstones
373
allow us to evaluate the provenance of the Akiyoshi accretionary complex in SW
374
Japan. When the results are combined with previous provenance studies of the
375
Permian strata in southeastern China, we are able to draw the following
376
conclusions:
377
1. The Nishiki Group sandstones are mostly highly immature and change from
378
feldspathic litharenite, through lithic arkose to arkose. Major source rock types
379
include felsic and mafic volcanics, plutonics, sedimentary and metamorphic
380
rocks.
381
2. Detrital zircons of the Nishiki Group are mostly Early to Late Permian ages
382
(294–254 Ma) with minor Cambrian to Devonian ages.
383
3. Garnet geochemical composition results suggest that their source rocks
384
changed from intermediate-acidic igneous rocks and low-medium grade
385
metasediments to metasomatic rocks such as skarns. These metamorphic and
386
metasomatic rocks were formed by Permian plutonic rocks
387
4. The detritus of the Nishiki Group of the Akiyoshi Permian accretionary
388
complex was derived from a contemporary volcanic arc along the eastern
389
margin of Greater South China which extended from South China Block to
390
Jiamusi-Khanka Block. The change in sandstone composition and garnet
391
chemical composition caused by a progressive uplifting and denudation of the
392
Permian volcanic arc.
393
Acknowledgements
394
We thank Prof. K. Yamamoto of Nagoya University and Mr. Y. Kouch of
395
University of Toyama for their help during U-Pb dating with Laser ICP-MS. We
396
also thank Ms. M. Nozaki of Nagoya University for her help in taking CL images.
397
The Japan Society for the Promotion of Science (JSPS) is thanked for its
398
financial support to X. Zhang. We thank R. Hansman from Stockholm University
399
for polishing the English. Prof. Isozaki, Prof. Tsutsumi and Prof. Kiminami gave
400
very constructive comments, from which this paper has benefited a lot.
401
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Figure Captions:
Fig. 1(A) Distribution of pre-Triassic system in Japan. South Kitakami (Sk),
Kurosegawa (Kg), Higo (Hi), and Hida-Gaien (Hg) Belts: shelf deposits; Akiyoshi
(Ak), Kurosegawa (Kg), and Ultra-Tamba (UT) Belts: accretionary complexes;
Renge
Belt
(Rn):
300Ma
high-P/T
schists
with
Oeyama
ophiolite
(Cambro-Ordovisian); Maizuru Belt (Mz): island-arc and back-arc system with
Yakuno ophiolite. MTL: Median Tectonic Line; TTL: Tanakura Tectonic Line. (B)
Geological map of the Nishiki area showing the location of study area.
Fig. 2 Geological map of the study area. Sampling locations are shown by red
spots with sample numbers.
Fig. 3 Stratigraphic column of the Nishiki Group in the study area showing
sampling level. Black spots are samples used for detrital zircon U-Pb dating.
Fig. 4 Representative photomicrographs of feldspathic litharenite (A), lithic
arkose (B) and arkose (C) of the Nishiki Group. Q, quartz; Pl, plagioclase; An,
andesite; Ry, Rhyolite; Ls, sedimentary; Lm: metamorphic fragments.
Fig. 5 (A) QFL sandstone classification plot (Folk, 1980); (B) QmFLt provenance
discrimination diagrams. Q = Qm + Qp; F = Pl + K; Lt = Lv + Ls + Qp.
Fig. 6 Composition plot of detrital garnet of four representative samples.
Fig. 7 Detrital garnet compositions of representative samples shown on the
garnet classification diagram of Morton et al. (2004). XMg, XFe, XMn, XCa = molecular proportions of Mg, Fe, Mn and Ca, with all Fe calculated as Fe 2+. Type
A:
high-grade
granulite-facies
metasediments
or
charnockites
or
intermediate-acidic igneous rocks sourced from deep in the crust. Type B:
amphibolite-facies metasediments. However, garnet populations that plot
exclusively in the Type Bi field suggest derivation from intermediate-acidic
igneous rocks. Type C：high-grade metabasic rocks；Type Cii: ultramafics such
as pyroxenites and peridotites. Type D: metasomatic rocks such as skarns, or
very low-grade metabasic rocks, or ultrahigh temperature metamorphosed
calc-silicate granulites.
Fig. 8 Relative probability diagrams of detrital zircon U-Pb ages of the Nishiki
Group sandstones. Note that the maximum depositional age is determined by the mean age of the youngest at least two grains that overlap in age at 1σ
(Dickinson and Gehrels, 2009) and n = concordant analyses/total analyses. The
single ca. 2.5 Ga age in sample N14 is not included in the plot.
Fig. 9 (A) Paleogeographic reconstruction of Akiyoshi Belt in the Late Permian
(modified after Isozaki et al., 2017) . Dash lines represent the nowadays
landmass boundaries in East Asia. Three shaded areas with question marks are
possible deposition sites of the Nishiki Group. (B) Cartoon illustrating a Permian
active continental margin in southeastern China (modified after Xu et al., 2016).
The locations of the Akiyoshi Belt and the Permian deposits in southeastern
China are shown in front and behind of the arc, respectively. The arrows
represent detritus transport directions.
Supplementary material:
Detrital zircon U-Pb LA-ICP-MS data of samples from
Units II and III of the Nishiki Group
Table 1 Modal composition of the Nishiki Group sandstones Sample
Qm
Qp
Pl
K
Lvf
Lvb
Ls
Lm
Total
N2
102
15
134
11
158
72
7
1
500
N3
113
16
226
46
106
14
12
4
537
N4
95
5
159
46
164
10
28
1
508
N5
114
18
202
52
110
9
21
0
526
N6
122
25
186
37
126
4
17
3
520
N8
111
16
192
18
97
9
26
0
469
N10
121
21
180
53
103
22
33
1
534
N11
140
8
236
24
37
69
19
0
533
N12
139
3
172
41
109
7
39
0
510
N13
134
9
246
64
58
2
19
2
534
N14
107
22
202
23
74
49
29
0
506
N15
121
22
227
7
58
52
20
0
507
Notes: Qm = monocrystalline quartz (>0.0625 mm); Qp = polycrystalline quartz; Pl = plagioclase grains; K = K-feldspar grains; F = Pl + K; Lvf = felsic volcanic lithic fragments; Lvb = basic volcanic lithic fragments; Ls = sedimentary lithic fragments; Lm = Lvf = metavolcanic lithic fragments; Lt = Lvf + Lvb + Ls + Lm + Qp
Graphical abstract
Research Highlight 1. 1162 zircons from sandstones in the Akiyoshi accretionary complex were analyzed for U-Pb age. 2. Felsic and mafic volcanics, plutonics, medium grade metasedimentary rocks and skarn rocks existed in the source regions of the Permian Nishiki sandstones in the Akiyoshi belt. 3. The Permian Nishiki Group sandstones were deposited in a trench in front of a volcanic arc along the Greater South China Block.
S1367-9120(18)30005-1 https://doi.org/10.1016/j.jseaes.2018.01.005 JAES 3377
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
17 February 2017 22 December 2017 11 January 2018
Please cite this article as: Zhang, X., Takeuchi, M., Ohkawa, M., Matsuzawa, N., Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes.2018.01.005
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1
Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi
2
Belt in Southwest Japan and Paleogeographic implications
3
Xiaojing Zhang1* Makoto Takeuchi1 Masahiro Ohkawa2 Nozomi Matsuzawa 1#
4
1. * Department of Earth and Planetary Sciences, Graduate School of Environmental Studies,
5
Nagoya University, Chikusa-Ku, Nagoya 464-8601, Japan; [email protected]
6
Current address: Institute of Earth Sciences, Academia Sinica, 128, Sec. 2, Academia Road,
7
Nangang, Taipei 11529, Taiwan
8
2. Mitsubishi Material Techno Co., 1-297, Kitabukuro-cho, Omiya-ku, Saitama-City, 330-0835,
9
Japan
10
11
# Current address: Sakai-City, Fukui Prefecture, Japan
Abstract
12
The Akiyoshi Belt in the Inner Zone of Southwest Japan is made up of a
13
Permian accretionary complex, the main component of which is the Late
14
Permian Nishiki Group that mainly consists of sandstone, mudstone, felsic tuff
15
and a minor amount of chert and conglomerate. We employ multiple methods,
16
which includes sandstone petrography, detrital garnet composition and detrital
17
zircon U-Pb dating to investigate the likely sources of these terrigenous deposits
18
and to reconstruct the paleogeographic link between the proto-Japan and the
19
East Asian continent. The highly immature Late Permian sandstones are
20
interpreted to derive from multi-type source rocks that include felsic igneous
21
rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks
22
in proximal locations. The detrital zircon U-Pb results show that all samples
23
contain a dominant Early to Late Permian zircon population (294–254 Ma) and
24
these zircons are interpreted to be derived from an active volcanic arc, which
25
was most likely caused by subduction of the Paleo-Pacific plate beneath the
26
South China Block. We found that spessartine-rich almandine garnets and
27
almandine garnets dominated assemblages in the lower unit changed to
28
grossular-andradite garnets dominated assemblage in the upper unit, which was
29
caused by a progressive uplifting and denudation of the Permian volcanic arc.
30
Keywords: Late Permian; petrography; garnet composition; detrital zircon;
31
provenance; South China
32
1. Introduction
33
The Japanese Islands are mainly composed of Upper Paleozoic to Cenozoic
34
accretionary complexes, and consists of trench-fill deposits, pelagic sediments,
35
and oceanic basalts. The Tanakura Tectonic Line (TTL) geologically divides
36
Japan into Northeast Japan and Southwest Japan. The pre-Triassic accretionary
37
complexes are mostly distributed in Southwest Japan, which can be subdivided
38
into the Inner (Japan Sea side) and Outer (Pacific side) Zones by the Median
39
Tectonic Line (MTL) (Fig. 1A).
40
The Japanese Islands have a complex geological history as they lie at the
41
junction of four tectonic plates which are the Eurasian, Pacific, North American,
42
and Philippine Sea plates. After the conversion from a passive continent margin
43
into a convergent one in the early Paleozoic, subduction of the Paleo-Pacific
44
plate generated the orogenic growth of Japan (Isozaki et al., 1990; Isozaki et al.,
45
2011; Maruyama et al., 1997; Taira, 2001). These subduction processes caused
46
accretion and tectonic erosion in the subduction zones from the late Paleozoic to
47
the present (Isozaki et al., 2011). The Permian to Triassic accretion is one of the
48
fundamental tectonic events of during the formation of proto-Japan. Therefore,
49
the Permian-Triassic tectonics of Japan is particularly important and will provide
50
the foundation in understanding the tectonic evolution of Japan (Isozaki, 1997).
51
The Akiyoshi Belt is occupied by a Permian accretionary complex, which
52
consists of unmetamorphosed Permo-Carboniferous sandstone, mudstone,
53
conglomerate, siliceous shale, felsic tuff, chert, limestone and basalt (Kanmera,
54
1990; Naka et al., 1986). There are some studies on investigating provenance,
55
deposition environment and tectonic settings of these sediments. Sano and
56
Kanmera (1988) described the lithological features of chert and associated
57
clastic limestone and interpreted these carbonates as sediments on and around
58
an oceanic seamount in the Paleo-Pacific Ocean (Panthalassan Ocean). A
59
sedimentary facies study on the siliciclastic component of the Nishiki Group in
60
the Akiyoshi Belt suggested that they were deposited in a trench and a
61
trench-slope basin at a crustal convergent margin (Hara and Kiminami, 1989).
62
Some provenance studies have been carried out on the sedimentary rocks of
63
the Akiyoshi Belt. Detrital garnet compositional analysis revealed that the Late
64
Permian
65
grossular-andradite garnets or none at all (Yokoyama, 1998). Detrital zircon
66
geochronology study of the trench-fill sandstone from the upper Nishiki Group
67
show a continuous age spectrum of 320–240 Ma, while the felsic tuff samples
68
from the middle part of the same unit contain two distinctive U-Pb age groups:
69
2700–2400 Ma and 320–260 Ma (Tsutsumi et al., 2000).
strata
in
the
Akiyoshi
Belt
either
contained
abundant
70
In spite of these studies, however, the paleogeography of the Akiyoshi Belt is
71
still not well established. It was proposed to initially develop along the
72
southeastern margin of the South China Block (e. g. Aoki et al., 2015; Isozaki et
73
al., 2010; Jahn, 2010; Maruyama et al., 1997; Sengör and Natal'In, 1996), while
74
some others argues for a North China Block origin (Takeuchi et al., 2008;
75
Tsutsumi et al., 2000). It is also not clear that what types of rocks existed in the
76
source region of the siliciclastic rocks in the Akiyoshi Belt. The petrographic,
77
detrital garnet compositional and detrital zircon U-Pb geochronological data
78
presented here provide new insight into the depositional setting and provenance
79
of the sedimentary rocks of the Akiyoshi Belt. Especially, zircon U-Pb ages
80
present
81
paleogeography because of the different age distributions of Neoproterozoic and
82
Phanerozoic rocks between the South China Block and the North China Block
83
together with their neighboring areas.
84
2. Geological Background and Stratigraphy
us
critical
information
to
reconstructing
the
late
Permian
85
The Akiyoshi Belt is distributed in several isolated areas of the Inner Zone of
86
Southwest Japan. It is structurally overlain by the Renge metamorphic rocks to
87
the north, which mainly consists of a suite of Carboniferous high-P/T schists
88
(Nishimura, 1990; Tsujimori, 2000, 2002), and is in fault contact to the the
89
Maizuru Belt in the south, which is occupied by a Permian island-arc and
90
back-arc basin system (Ishiwatari et al., 1990) (Fig. 1A).
91
The Nishiki area is located in the western tip of the main island of Japan,
92
where the Nishiki Group strata of the Akiyoshi Belt is exposed (Fig. 1). In the
93
north, the Nishiki Group is overlain uncomformably by the Cretaceous rhyolitic
94
and dacitic tuff, andesite lava and siliciclastic rocks; in the south it thrusts over
95
the Tsuno Group of the Suo metamorphic rocks (Nishimura and Nureki, 1966)
96
(Figs. 1B and 2). Middle to Late Permian radiolarian fossils reported from the
97
Nishiki Group include Follicuculus monacanthus Assemblage (Ishiga, 1990;
98
Nishimura et al., 1989), Fo. scholasticus Assemblage (Naka and Ishiga, 1985;
99
Nishimura et al., 1989) and fusulina Lepidolina multiseptata (Nishimura, 1971),
100
suggesting deposition in the Middle to Late Permian. The stratigraphy of the
101
Nishiki Group displays a sequence sandstone, mudstone, felsic tuff and minor
102
chert and conglomerate in an ascending order (Tanaka et al., 1987). These
103
rocks are mostly stacked by a series of thrusts (Nishimura et al., 1989).
104
Accordingly, the Nishiki Group was divided into Units I, II, III and IV with
105
younging stratigraphic ages (Figs. 2 and 3).
106
Unit I is distributed in the southeast of the study area, mainly consisting of
107
chert, siliceous shale, felsic tuff and mudstone with a total thickness of greater
108
than 100 m (Figs. 2 and 3). Resting on top of Unit I by a thrust-faulted contact
109
are coarse-grained clastic rocks with chert, siliceous shale, felsic tuff, mudstone
110
and conglomerate that are grouped as Unit II and are over 1 km thick. This unit is
111
further subdivided into subunits IIa, IIb, IIc, IId and IIe based on the lithology
112
(Figs. 2 and 3). Subunit IIa is felsic-tuff dominated; IIb and IId are mudstone and
113
fine-grained sandstone; lastly, IIc and IIe are dominated by coarse-grained
114
sandstone (Fig. 3). Unit III is exposed in the west, consisting of over 500 m thick
115
of tuffaceous mudstone, mudstone, sandstone and conglomerate. It is
116
subdivided into subunits IIIa, IIIb, IIIc and IIId (Fig. 3). Subunit IIIa consists of
117
tuffaceous mudstone; IIIb and IIId are dominated by mudstone; finally, IIIc is
118
mainly sandstone. The boundary between Units II and III is a thrust fault contact.
119
Unit IV has a normal fault contact with Unit III. It is exposed in the southwest and
120
east of the Nishiki area, and consists of weakly metamorphosed mudstone and
121
sandstone (Figs. 2 and 3).
122
3. Sampling and Methods
123
Units II and III hold the majority of siliciclastic rocks of the Nishiki Group and
124
cover most of the study region. In addition to this, Units II and III belong to a
125
coherent sequence, so that we are able to understand compositional changes in
126
different stratigraphic horizons. Therefore, these two units are our focus in this
127
study. We collected 17 sandstone samples at 17 localities, from Unit II (IIb to IIe)
128
and Unit III (IIIc and IIId). Sandstone petrography, garnet composition analysis
129
and detrital zircon U-Pb geochronology were carried out on these samples (Figs.
130
2 and 3, and Table 1).
131
3.1 Sandstone petrography
132
Twelve samples were used for sandstone petrography analysis. These
133
samples are medium-grained sandstones with less than 25% matrix. Thin
134
sections
135
hexanitrocobaltate solution to allow clear identification of K-feldspars during
136
point counting. At least 500 points were counted on each thin-section using the
137
Gazzi-Dickinson method (Dickinson, 1988; Ingersoll et al., 1984), with quartz,
138
feldspar and lithic fragments as the main counted framework grains.
139
3.2 Detrital garnet composition
of
these
samples
were
stained
with
saturated
sodium
140
Four sandstone samples (N1 from Unit IIb, N7 from Unit IId, N9 from Unit IIe
141
and N17 from Unit IIId) were crushed and sieved through a 250-μm mesh. The
142
initial volume of samples was reduced using a water-panning method (Hutton,
143
1950), and then dilute HCl 10% was used to dissolve any calcareous particles
144
and carbonate cement. Heavy mineral separation and preparation for chemical
145
analysis of garnets were performed following the procedures described by
146
Mange and Maurer (1992). Dried heavy mineral grains were mounted onto a
147
microscope slide under epoxy using the method by Leu and Druckman (1982).
148
Slides were ground with carborundum and then polished carefully with diamond
149
polishing compounds.
150
The chemical composition of detrital garnets were determined on a JEOL
151
JXA-8800R electron probe microanalyzer (EPMA) at Nagoya University with an
152
accelerating voltage of 15 kV, specimen current of 12 nA, beam diameter of 3
153
μm, and maximum count time of 20 s. Garnet grains were analyzed for Si, Ti, Al,
154
Cr, Fe, Mn, Mg, Ca, Na, and K. About 100 grains were analyzed per sample. The
155
ratio of Fe2+ to Fe3+ was calculated, assuming garnet stoichiometry. The
156
molecular end-members were calculated by the method of Deer et al. (1992).
157
The composition of detrital garnet can be expressed in terms of six
158
end-members: almandine, pyrope, spessartine, grossular, andradite, and
159
uvarovite.
160
3.3 Detrital zircon U-Pb geochronology
161
Detrital zircons from eight samples were dated using laser ablation
162
inductively-coupled plasma mass spectrometry (LA-ICP-MS). Zircons were
163
separated from about 2 kg of sample using conventional water table and heavy
164
liquid mineral separation methods. Around 300 grains were handpicked onto
165
double-sided adhesive tape, embedded into an epoxy resin disk and polished.
166
Cathodoluminescence (CL) images were prepared to study the internal
167
structures of zircons. U-Pb ages of detrital zircons were obtained from an Agilent
168
Technologies Agilent 7700 ICP-MS and ESI NWR213 Nd-YAG (λ=213 nm) laser
169
ablation system in Nagoya University following the analytical procedure
170
described by Orihashi et al. (2008). Pb/U calibration was performed relative to
171
the zircon standard 91500 with an age of 1065 Ma (Wiedenbeck et al., 1995).
172
The weighted mean ages of standard zircon 91500 for each sample are included
173
in the supplementary material. All probability density distribution plots were
174
made using ISOPLOT/Ex 4.15 (Ludwig, 2010). All errors are reported at the
175
2-sigma level.
176
4. Results
177
4.1 Sandstone petrography
178 179
Petrographic photos of sandstone samples are shown in Fig. 4 and the point
counting results of the framework grains are given in Table 1 and Fig. 5.
180
Sandstones of the Nishiki Group are immature, containing poorly sorted,
181
medium to coarse grains in angular to subangular shape (Fig. 4). Figure 5 shows
182
that one sample from the lower horizon of Unit II as feldspathic litharenite (Fig.
183
4A), ten samples from Units II and III are classified as lithic arkose (Fig. 4B),
184
and one sample from the upper horizon of Unit III as arkose (Fig. 4C). This
185
indicates that the Nishiki Group sandstones change from feldspathic litharenite,
186
through lithic arkose to arkose. Monocrystalline quartz (19–27%) prevails
187
polycrystalline quartz (1–5%). About half of them show slight corrosion and weak
188
undulose extinction, suggesting provenance from a volcanic source. Plagioclase
189
(27–46%) is more plentiful than K-feldspar (1–12%) and plagioclase grains have
190
undergone sericitic alteration or saussuritization (Fig. 4). Lithic fragments range
191
from 15% to 48% in abundance and are dominated by felsic to andesitic volcanic
192
lithics (Table 1 and Fig. 4). The felsic volcanic lithics have felsitic or vitric
193
groundmass with phenocrysts of quartz and plagioclase. Minor granitic and
194
dioritic lithics are also present. Sedimentary and metamorphic lithic fragments
195
include limestone, mudstone, sandstone, chert, mica schist and hornfels. On a
196
QmFLt plot (Dickinson, 1985), the samples are distributed in the “dissected arc”
197
and “transitional arc” fields (Fig. 5B).
198
4.2 Detrital garnet composition
199
Garnets are common heavy minerals in siliciclastic sediments. Due to its
200
large chemical variability, garnet chemical composition has been widely used as
201
a sediment provenance tool (e. g. Hartley and Otava, 2001; Morton et al., 2004;
202
Takeuchi, 1994; Takeuchi et al., 2008; von Eynatten and Gaupp, 1999 and many
203
others). Garnet is generally interpreted to derive from metamorphic rocks
204
(Mange and Maurer, 1992), but it is also present in plutonic igneous rocks,
205
pegmatites, ultramafic rocks and felsic volcanics (Deer et al., 1982; Mange and
206
Morton, 2007).
207
Sandstone samples from Units II and III contain various types of heavy
208
minerals, including abundant biotite, epidote, hornblende, titanite, garnet, rutile,
209
zircon, tourmaline and apatite. Detrital garnet grains from three Unit II samples
210
(N1, N7 and N9) and one Unit III sample (N17) are angular to rounded. Most
211
grains are colorless, while red, yellow and pink grains are occasionally
212
encountered. Some anisotropic garnets are present. Garnets are unetched,
213
reducing the possibility that garnet dissolution occurred in these rocks.
214
Appearance of epidote and titanite in the heavy mineral assemblage implies that
215
garnet assemblage of the samples was not altered by the burial diagenesis
216
because garnet is more stable than epidote and titanite during burial (Morton
217
and Hallsworth, 2007). Consequently, garnet composition results from Unit II and
218
Unit III of the Nishiki Group are able to convey true provenance information. The
219
compositional data are summarized in Figs. 6 and 7.
220
The analytical results of Unit II and Unit III samples are very distinct. Detrital
221
garnets from Unit II (samples N1, N7 and N9) are dominated by almandine
222
(54–77%) and spessartine-rich almandine (20-36%) with a minor amount of
223
pyrope-rich almandine (3-7%) and grossular-andradite garnet (0-7%). In contrast,
224
garnets from Unit III (sample N17) contain 90% grossular-andradite with 9%
225
almandine
226
compositional data were plotted in a Mg-(Fe+Mn)-Ca ternary plot (Fig. 7) in
227
order to discriminate the source rocks (Mange and Morton, 2007; Morton et al.,
228
2004). Most of the analyzed garnets of Unit II samples are categorized as Type
229
B garnets with minor as Type A and Type D garnets. Sample N1 from the lowest
230
stratigraphic level of Unit II contains no Type D garnet. In comparison, sample
231
N17 from Unit III is dominated by Type D garnet with a few grains as Type B
232
garnets (Fig. 7).
233
4.3 Detrital zircon U-Pb geochronology
and
1%
spessartine-rich
almandine
(Fig.
6).
The
garnet
234
Detrital zircons from five Unit II samples and three Unit III samples were
235
dated and results are summarized in Fig. 8. The complete dataset showing all
236
analysis spots can be found in the supplementary material.
237
Most zircons in the samples are colorless and have euhedral crystals with
238
subangular to angular terminations. Their CL images show oscillatory zoning,
239
which are typical features of igneous zircons (Corfu et al., 2003). Detrital zircon
240
age spectra of the eight samples do not display noticeable changes despite the
241
distinct garnet chemistry results from Units II and III. The majority of analyzed
242
detrital zircons in all samples fall into the age range of 245 to 350 Ma with a few
243
Cambrian to Devonian zircons (Fig. 8). The major age peaks vary from 254 Ma
244
to 294 Ma, re-confirming the previous results reported by Tsutsumi et al. (2000).
245
Sample N14 from the lower part of Unit II yield one Paleoproterozoic to Archean
246
zircon grain (2464 ± 70 Ma) (Supplementary material). The maximum
247
depositional ages of the Nishiki Group samples determined by the mean age of
248
the youngest two or three zircons that overlap in age at 1σ (Dickinson and
249
Gehrels, 2009) are in the range of 249 ± 9 Ma to 261 ± 3 Ma, which are
250
compatible with their stratigraphic ages determined by fossils (Ishiga, 1990;
251
Naka and Ishiga, 1985; Nishimura et al., 1989) (Fig. 8). For example, felsic tuff
252
and mudstone of the lower part of Unit II yielded radiolarian fossils (Follicuculus
253
sholasticus assemblage) that indicate deposition in the Late Permian (Ishiga,
254
1990), corresponding to Capitanian to Wuchiapingian (265–254 Ma) (Ogg et al.,
255
2016).
256
5. Discussion
257
5. 1 Depositional setting
258
The Nishiki Group sandstones are highly immature indicating a quick
259
deposition of first cycle detritus after short-distance transportation. The
260
petrographic composition suggests that sandstones of the Akiyoshi accretionary
261
complex are derived from a variety of source rocks, including felsic and mafic
262
volcanics, plutonics, sedimentary and metamorphic rocks. Abundant first-cycled
263
quartz and feldspar together with volcanic fragments reflect a direct derivation
264
from volcano-plutonic sources. Furthermore, the conglomerate facies of the
265
Nishiki Group contains 2-30 cm rounded rock clasts, including mafic to felsic
266
plutonic rocks, volcanic rocks, sandstone, shale, tuffaceous mudstone and
267
limestone. Kiminami et al. (2000) proposed that the Permian sandstones in the
268
Japanese Islands were supplied from an immature island arc and an evolved
269
island arc of these rocks based on whole-rock major element composition of
270
these rocks. These lines of evidence indicate that the sources of the Nishiki
271
Group are probably volcano-plutonic rocks from a volcanic arc.
272
Units II and III belong to a coherent sequence and Unit II thrust over Unit III.
273
Clastic rocks of the structurally-lower unit is usually younger than the upper unit,
274
suggesting Unit III was deposited after Unit II. Therefore, compositional changes
275
of sandstones could be obtained from the lower sample (N2) through the upper
276
one (N12) in Unit II and the lower one (N13) in Unit III to the upper one (N17) in
277
Unit III. The sandstone composition varied from lithic (volcanic fragment-rich) to
278
feldspathic through time, suggesting a progressive denudation of volcanic arc.
279
This is compatible with the detrital zircon age distributions of the Nishiki
280
sandstones, which has prevailing age peaks (294–254 Ma) plus minor older
281
ages (Fig. 8). Cawood et al. (2012) proposed that the detrital zircon spectra of
282
sediments in arc flanking basins along convergent margins are characterized by
283
dominant young ages from syn-depositional igneous activity with or without
284
some minor older ages from pre-existing rocks. Therefore, we conclude that the
285
Nishiki Group sediments of the Akiyoshi Belt were deposited in an arc-flanking
286
position and their sources are mainly from an Early to Late Permian magmatic
287
arc.
288
Permian magmatism has been reported in Hainan Island in the South China
289
(267–262 Ma) (Li et al., 2006), in southeastern Korea (257–250 Ma) (Cheong et
290
al., 2014; Yi et al., 2012), in the Hida Belt in the central Japan (256–250 Ma)
291
(Horie et al., 2010; Takahashi et al., 2010; Zhao et al., 2013) and in the
292
Jiamusi-Khanka Massif (296–255 Ma) (e. g. Wu et al., 2011; Yang et al., 2015).
293
These late Paleozoic rocks probably represent a magmatic arc produced during
294
westwards subduction (in present-day coordinates) of the Paleo-Pacific plate
295
along the east coastal region of a “Greater South China” block that extended
296
from South China Block through proto-Japan and covered South Korea further to
297
the Jiamusi-Khanka Massif in the Paleozoic (Isozaki et al., 2017) (Fig. 9A). The
298
age span of igneous rocks in this magmatic arc corresponds well with the
299
dominant detrital zircon ages of the Nishiki Group sandstones. Therefore, we
300
infer that the Permian detritus of the Akiyoshi Belt were derived from erosion of
301
uplifted magmatic arc materials along the South China and then these deposits
302
were accreted forming the Akiyoshi accretionary complex (Fig. 9B).
303
The Permian sandstones in southeastern China contain both the
304
syn-depositional Permian (300–260 Ma) and abundant much older zircon grains
305
(2500–370 Ma) (Hu et al., 2015; Hu et al., 2012; Li et al., 2012), whereas the
306
U-Pb age spectra of the Nishiki Group sandstones are dominated by
307
syn-depositional ages and have only a few grains with older ages. This is
308
because they were deposited in different positions relative to the volcanic arc.
309
Permian sandstones in southeastern China and the Nishiki Group sandstones
310
were deposited in the back-arc basin on the continental side and in the trench on
311
the oceanic side, respectively (Fig. 9). Such a provenance diversification within
312
an arc-trench system was also observed in Cretaceous intra-arc and fore-arc
313
basins in SW Japan (Aoki et al., 2014). Detritus from both sides of the back-arc
314
basin, i.e. the magmatic arc that yielded mainly syn-depositional zircons and the
315
interior of the block that provided old zircons, were shed into the basin, whereas
316
detritus from the volcanic arc were the dominant source for the trench sediments
317
(Fig. 9B).
318
5.2 Provenance interpretation on garnet composition
319
Compositional suites of detrital garnet display noticeable characteristics
320
between each unit. Most garnets from Unit II (samples N1, N7 and N9) cluster in
321
the field of Type B with increasing Type D garnets toward upper horizons. Most
322
garnets from Unit III (sample N17) cluster in the field of Type D (Fig. 7). Garnets
323
in Type B field have low-Mg, with variable Ca and Mn, and are thought to be
324
derived from low to medium grade metasedimentary rocks (type Bi) and acidic to
325
intermediate gneisses and granites (type Bii) (Morton et al., 2005). Garnets in
326
the Type D field (high-Ca, low-Mg) generally suggest derivation from
327
metasomatic rocks such as skarns, from very low-grade metabasic rocks, or
328
from ultra-high temperature metamorphosed calc-silicate granulites (Deer et al.,
329
1997; Mange and Morton, 2007). Type D garnets in Unit III sandstone samples
330
also contain variable abundances of Fe and some grains are anisotropic,
331
showing typical features of skarn garnets (Takeuchi, 1994, 2013). Therefore,
332
these garnets are sourced from skarn rocks. Minor Type A garnets (low Ca, low
333
Mn and relatively high Mg) are also present in Unit II samples. They must be
334
derived from high-grade metasediments and/or charnockites (Sabeen et al.,
335
2002). According to above, main sources of garnets of the Unit II and Unit III are
336
considered to change from low to medium grade metasedimentary rocks to skarn
337
rocks.
338
Such type B and D garnets are common in the Permian sandstones in Japan
339
(Takeuchi, 2013; Takeuchi et al., 2008). These Permian sandstones includes
340
Permian detrital zircons (Okawa et al., 2013) and Permian conglomerates
341
contain Permian granitic rock clasts (Shibata, 1973; Takeuchi and Suzuki, 2000).
342
Felsic hornfels fragments in sandstones (Yoshida and Machiyama, 2004) and
343
felsic, mafic and calcareous hornfels clasts in conglomerates (Kano, 1959) are
344
present in shelf deposits of the South Kitakami Belt in Northeast Japan. This
345
indicates that the low-grade metasedimentary rocks and skarn rocks were
346
formed by intrusions of Permian plutonic rocks. Consequently, we consider that
347
the provenance of the sandstones of the Nishiki Group were also derived from a
348
Permian volcanic arc and surrounding contact metamorphic and metasomatic
349
rocks. The change in garnet composition might be caused by change in
350
denudation level of metamorphic and metasomatic rocks surrounding plutonic
351
bodies by a progressive denudation. Sandstone composition also suggests
352
progressive denudation from volcanic rocks to plutonic rocks.
353
The South China Block was almost completely covered by carbonates
354
during the Carboniferous to earliest Permian (Liu and Xu, 1994) and the
355
Jiamusi-Khanka Massif also yields carbonates (Natal'in, 1993). It is likely that
356
skarn deposits were formed when plutonic bodies intruded these carbonates
357
rocks during the subduction of the Paleo-Pacific oceanic plate. The youngest
358
sample N1 includes the older zircons, ca. 400 and 500 Ma (Fig. 8), indicating
359
presence of pre-existed rocks before Permian igneous activities began. This is
360
very crucial for reconstructing the paleogeography of the Akiyoshi Belt, because
361
ca. 400-500 Ma zircons are common in the Greater South China which is
362
extended from the present South China Block to the Khanka Block (Isozaki et al.,
363
2017) (Fig. 9). A single subduction-related Paleozoic batholith belt is considered
364
to have existed along the Pacific rim of the Greater South China
365
2017). As mentioned above, Permian volcanic arc has also existed along the
(Isozaki et al.,
366
Greater South China. The Permian clastic rocks of the Akiyoshi Belt were
367
deposited at a trench along the Greater South China, and derived from the
368
uplifting volcanic arc associated skarn deposits, despite that the exact
369
deposition location has not been able to well constrain yet (Fig. 9).
370
7. Conclusion
371
An integrated petrographic, detrital garnet compositional and detrital zircon
372
U-Pb geochronological study of the Late Permian Nishiki Group sandstones
373
allow us to evaluate the provenance of the Akiyoshi accretionary complex in SW
374
Japan. When the results are combined with previous provenance studies of the
375
Permian strata in southeastern China, we are able to draw the following
376
conclusions:
377
1. The Nishiki Group sandstones are mostly highly immature and change from
378
feldspathic litharenite, through lithic arkose to arkose. Major source rock types
379
include felsic and mafic volcanics, plutonics, sedimentary and metamorphic
380
rocks.
381
2. Detrital zircons of the Nishiki Group are mostly Early to Late Permian ages
382
(294–254 Ma) with minor Cambrian to Devonian ages.
383
3. Garnet geochemical composition results suggest that their source rocks
384
changed from intermediate-acidic igneous rocks and low-medium grade
385
metasediments to metasomatic rocks such as skarns. These metamorphic and
386
metasomatic rocks were formed by Permian plutonic rocks
387
4. The detritus of the Nishiki Group of the Akiyoshi Permian accretionary
388
complex was derived from a contemporary volcanic arc along the eastern
389
margin of Greater South China which extended from South China Block to
390
Jiamusi-Khanka Block. The change in sandstone composition and garnet
391
chemical composition caused by a progressive uplifting and denudation of the
392
Permian volcanic arc.
393
Acknowledgements
394
We thank Prof. K. Yamamoto of Nagoya University and Mr. Y. Kouch of
395
University of Toyama for their help during U-Pb dating with Laser ICP-MS. We
396
also thank Ms. M. Nozaki of Nagoya University for her help in taking CL images.
397
The Japan Society for the Promotion of Science (JSPS) is thanked for its
398
financial support to X. Zhang. We thank R. Hansman from Stockholm University
399
for polishing the English. Prof. Isozaki, Prof. Tsutsumi and Prof. Kiminami gave
400
very constructive comments, from which this paper has benefited a lot.
401
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Figure Captions:
Fig. 1(A) Distribution of pre-Triassic system in Japan. South Kitakami (Sk),
Kurosegawa (Kg), Higo (Hi), and Hida-Gaien (Hg) Belts: shelf deposits; Akiyoshi
(Ak), Kurosegawa (Kg), and Ultra-Tamba (UT) Belts: accretionary complexes;
Renge
Belt
(Rn):
300Ma
high-P/T
schists
with
Oeyama
ophiolite
(Cambro-Ordovisian); Maizuru Belt (Mz): island-arc and back-arc system with
Yakuno ophiolite. MTL: Median Tectonic Line; TTL: Tanakura Tectonic Line. (B)
Geological map of the Nishiki area showing the location of study area.
Fig. 2 Geological map of the study area. Sampling locations are shown by red
spots with sample numbers.
Fig. 3 Stratigraphic column of the Nishiki Group in the study area showing
sampling level. Black spots are samples used for detrital zircon U-Pb dating.
Fig. 4 Representative photomicrographs of feldspathic litharenite (A), lithic
arkose (B) and arkose (C) of the Nishiki Group. Q, quartz; Pl, plagioclase; An,
andesite; Ry, Rhyolite; Ls, sedimentary; Lm: metamorphic fragments.
Fig. 5 (A) QFL sandstone classification plot (Folk, 1980); (B) QmFLt provenance
discrimination diagrams. Q = Qm + Qp; F = Pl + K; Lt = Lv + Ls + Qp.
Fig. 6 Composition plot of detrital garnet of four representative samples.
Fig. 7 Detrital garnet compositions of representative samples shown on the
garnet classification diagram of Morton et al. (2004). XMg, XFe, XMn, XCa = molecular proportions of Mg, Fe, Mn and Ca, with all Fe calculated as Fe 2+. Type
A:
high-grade
granulite-facies
metasediments
or
charnockites
or
intermediate-acidic igneous rocks sourced from deep in the crust. Type B:
amphibolite-facies metasediments. However, garnet populations that plot
exclusively in the Type Bi field suggest derivation from intermediate-acidic
igneous rocks. Type C：high-grade metabasic rocks；Type Cii: ultramafics such
as pyroxenites and peridotites. Type D: metasomatic rocks such as skarns, or
very low-grade metabasic rocks, or ultrahigh temperature metamorphosed
calc-silicate granulites.
Fig. 8 Relative probability diagrams of detrital zircon U-Pb ages of the Nishiki
Group sandstones. Note that the maximum depositional age is determined by the mean age of the youngest at least two grains that overlap in age at 1σ
(Dickinson and Gehrels, 2009) and n = concordant analyses/total analyses. The
single ca. 2.5 Ga age in sample N14 is not included in the plot.
Fig. 9 (A) Paleogeographic reconstruction of Akiyoshi Belt in the Late Permian
(modified after Isozaki et al., 2017) . Dash lines represent the nowadays
landmass boundaries in East Asia. Three shaded areas with question marks are
possible deposition sites of the Nishiki Group. (B) Cartoon illustrating a Permian
active continental margin in southeastern China (modified after Xu et al., 2016).
The locations of the Akiyoshi Belt and the Permian deposits in southeastern
China are shown in front and behind of the arc, respectively. The arrows
represent detritus transport directions.
Supplementary material:
Detrital zircon U-Pb LA-ICP-MS data of samples from
Units II and III of the Nishiki Group
Table 1 Modal composition of the Nishiki Group sandstones Sample
Qm
Qp
Pl
K
Lvf
Lvb
Ls
Lm
Total
N2
102
15
134
11
158
72
7
1
500
N3
113
16
226
46
106
14
12
4
537
N4
95
5
159
46
164
10
28
1
508
N5
114
18
202
52
110
9
21
0
526
N6
122
25
186
37
126
4
17
3
520
N8
111
16
192
18
97
9
26
0
469
N10
121
21
180
53
103
22
33
1
534
N11
140
8
236
24
37
69
19
0
533
N12
139
3
172
41
109
7
39
0
510
N13
134
9
246
64
58
2
19
2
534
N14
107
22
202
23
74
49
29
0
506
N15
121
22
227
7
58
52
20
0
507
Notes: Qm = monocrystalline quartz (>0.0625 mm); Qp = polycrystalline quartz; Pl = plagioclase grains; K = K-feldspar grains; F = Pl + K; Lvf = felsic volcanic lithic fragments; Lvb = basic volcanic lithic fragments; Ls = sedimentary lithic fragments; Lm = Lvf = metavolcanic lithic fragments; Lt = Lvf + Lvb + Ls + Lm + Qp
Graphical abstract
Research Highlight 1. 1162 zircons from sandstones in the Akiyoshi accretionary complex were analyzed for U-Pb age. 2. Felsic and mafic volcanics, plutonics, medium grade metasedimentary rocks and skarn rocks existed in the source regions of the Permian Nishiki sandstones in the Akiyoshi belt. 3. The Permian Nishiki Group sandstones were deposited in a trench in front of a volcanic arc along the Greater South China Block.