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Lithos 160-161 (2013) 268–282
Contents lists available at SciVerse ScienceDirect
Lithos
journal homepage: www.elsevier.com/locate/lithos
Is Myanmar jadeitite of Jurassic age? A result from incompletely recrystallized
inherited zircon
Tzen-Fu Yui a,⁎, Mayuko Fukoyama a, b, Yoshiyuki Iizuka a, Chao-Ming Wu c, Tsai-Way Wu d,
J.G. Liou e, Marty Grove e
a
Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan, ROC
Graduate School of Engineering and Resource Science, Akita University, Japan
Department of Applied Arts, Fu-jen Catholic University, Hsinchuang, Taiwan, ROC
d
Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
e
Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA
b
c
a r t i c l e
i n f o
Article history:
Received 3 September 2012
Accepted 19 December 2012
Available online 27 December 2012
Keywords:
Myanmar jadeitite
Zircon
SHRIMP U–Pb dating
a b s t r a c t
Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U–Pb dating and
trace element composition analyses. Three types of zircons, with respect to U–Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons
are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type
III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from
153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic
replacement processes.
Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures,
with low Th/U ratio (b 0.1), small Ce anomaly (Ce/Ce*=b 5) and low ΣREE content (40–115 ppm). Type III
zircons, however, commonly have the above geochemical signatures straddle in between Type I and Type II
zircons. It is shown that the resetting rates of various trace element compositions and U–Pb isotope system of
inherited zircons are not coupled “in phase” in response to zircon recrystallization during jadeitite formation.
The observed abnormally low Th/U ratio and small Ce anomaly of some Type I zircons, as well as the lack of
negative Eu anomaly of all Type I zircons, should be suspected to be of secondary origin. In extreme cases,
incompletely recrystallized zircons may show typical metasomatic/hydrothermal geochemical signatures, but
leave U–Pb isotope system partially reset or even largely unchanged. Such zircons easily lead to incorrect age
interpretation, and hence erroneous geological implication.
The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India–Asia continental collision at Paleocene. Previously proposed Late
Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely
recrystallized inherited zircons.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Jadeitite is an uncommon rock type often associated with eclogite
and blueschist in high-pressure metamorphic belts around the world.
It is a fluid–rock interaction product enclosed within serpentinite
formed under subduction environments (e.g., Harlow and Sorensen,
2005; Harlow et al., 2007). The formation mechanism(s) of jadeitites
would thus reveal important clues on element migration/cycling
during subduction and the age of jadeitite, theoretically, would provide useful time constraints for regional tectonics (see Tsujimori
⁎ Corresponding author at: Institute of Earth Sciences, Academia Sinica, Taipei, 11529,
Taiwan, ROC. Tel.: +886 2 27839910x621; fax: +886 2 27839871.
E-mail address: [email protected] (T.-F. Yui).
0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.lithos.2012.12.011
and Harlow, 2012 and the references therein; Flores et al., 2012; Yui
et al., 2012).
There are two possible formation mechanisms for jadeitite, which,
accordingly, can be divided into two types. The “vein precipitation
type” or the “P-type” jadeitite is formed through direct precipitation
from a Na–Al–Si-rich aqueous fluid infiltrating through serpentinites
and the “metasomatic replacement type” or the “R-type” jadeitite is
formed through whole-sale metasomatic replacement of (igneous)
tectonic blocks within serpentinites (Tsujimori and Harlow, 2012;
Yui et al., 2010). Zircons in jadeitite may therefore be either
recrystallized/newly-formed contemporaneous with jadeite formation,
or inherited from protoliths. As a result, U–Pb dating of such zircons
would theoretically yield the jadeitite age or the protolith igneous age,
respectively. However, since both types of jadeitite may share similar
characteristics, it may not be easy to confidently categorize a specific
T.-F. Yui et al. / Lithos 160-161 (2013) 268–282
jadeitite sample in question. Furthermore, the lack of clear-cut criteria to
discriminate igneous zircons from hydrothermal/metamorphic ones has
also been noted recently (e.g., Bulle et al., 2010; Harley et al., 2007), not
to mention the potential problems with respect to incomplete recrystallization of inherited zircons during jadeitite formation. Controversial suggestions on the geological meaning of the U–Pb dating results of zircons
from a specific jadeitite occurrence (i.e., protolith igneous age or metasomatic/hydrothermal jadeitite age) have therefore been proposed
even with the information on internal textures, mineral/fluid
inclusions, Th/U ratio, trace-element characteristics, and O-isotope
composition of zircons (Bröcker and Keasling, 2006; Bulle et al., 2010;
Fu et al., 2010, 2012; Tsujimori et al., 2005).
The Jade Mine Tract in northern Myanmar, located at the northern
extremity of the Sagaing fault in the Hpakan area of the Kachin State,
is the most famous jadeitite resource in Asia. Recent U–Pb zircon dating studies suggested that these jadeitites might have formed during
269
the Late Jurassic time (Qiu et al., 2009; Shi et al., 2008). In this study,
SHRIMP U–Pb dating results on zircons with complicated internal textures from two Myanmar jadeitite samples are presented. The data
clearly demonstrate that any interpretation on jadeitite U–Pb zircon
ages must be regarded with caution.
2. Geological background
Geologically, Myanmar (Burma) can be divided into the Western
province and the Eastern province, separated by the Sagaing fault
(Fig. 1). The Western province is also named as the Burma microplate,
whereas the Eastern province is part of the Shan-Thai block. The east
dipping Andaman subduction zone that continues onshore along the
western margin of the Western province (the Burma microplate)
marks the presently active boundary with Indian plate to the west.
The Sukhotai-Lao fold belt and the Nan-Uttaradit suture denote the
Fig. 1. Geologic overview of the Myanmar area (modified after Searle et al., 2007). Samples in the present study are from the Hpakan area of the Jade Mines belt. STD: South Tibet
Detachment.
270
Early or Late Triassic suturing between the Shan-Thai block and the
Indo-China block to the east (see Metcalfe, 2000; Mitchell, 1977;
Mitchell et al., 2007; Searle et al., 2007; and the references therein).
The Indo-Burman Ranges of the western Burma microplate are
composed mainly of Upper Cretaceous–Paleogene marine sedimentary
rocks unconformably overlying Upper Triassic flysch-type sediments
and associated Jurassic ophiolitic rocks, thought to be the southern
continuation of the Indus-Yarlung Zangbo suture zone (Mitchell,
1993). To the east of the Indo-Burman Ranges, there are a series of
mid-Cretaceous to Miocene sedimentary basins (Chindwin, Minbu
and Pathein basins). Along with the basins, a belt of calc-alkaline Late
Cretaceous plutons and Cenozoic volcanoes signifies the long-lived
Andaman subduction system (Mitchell, 1993).
The Eastern province of Myanmar is composed of the Paleozoic
Mogok metamorphic belt (MMB in Fig. 1) to the west and the Upper
Carboniferous–Lower Permian Mergui Group metasediments (not
shown in Fig. 1) to the east (Mitchell, 1992, 1993). Subduction-related
Jurassic–Miocene granites intruded the Mogok metamorphics (Barley
et al., 2003). The age of metamorphism, however, has been discussed
controversially. Mitchell et al. (2007) proposed two possibilities; either
(1) there are three stages of metamorphism during Early Permian, Early
Jurassic and Early Tertiary time, respectively, or (2) there are two stages
of metamorphism during late Cretaceous and Early Tertiary time,
respectively. On the other hand, Searle et al. (2007) suggested that
metamorphism occurred during Jurassic–Early Cretaceous, Paleocene–
Early Eocene and Late Eocene-Oligocene time.
The N–S trending Sagaing fault is a 1200 km long dextral fault. The
distance of the right-lateral displacement has been estimated from
less than 100–150 km (Bertrand and Rangin, 2003) to ~ 450 km
(Mitchell, 1993) or even more. In the south, the fault connects to
the Andaman back-arc spreading centre, and in the north, it splays
into three prominent metamorphic belts within the Western province
(Fig. 1). These three metamorphic belts, from west to east, are the Jade
Mines belt (JM in Fig. 1), the Katha-Gangaw ranges (KGR in Fig. 1) and
the Tagaung-Myitkyina belt (TMB in Fig. 1). The Jade Mines belt has
been subjected to high-pressure metamorphism, whereas low- to
medium-pressure/high-temperature metamorphic rocks prevail in the
Katha-Gangaw ranges and the Tagaung-Myitkyina belt (Mitchell, 1993).
In the Jade Mines belt, various kinds of jadeitites, in association
with eclogite, amphibolite, blueschist and chromitite, occur as boulders
in drainages, or as tectonic blocks/veins within serpentinite(-peridotite)
mélange (Chhibber, 1934; Goffé et al., 2002; Shi et al., 2001). Metasomatic amphiboles are commonly present in between jadeitite
and hosting serpentinized peridotite. These amphiboles are sodic to
sodic-calcic in nature, including eckermannite, magnesiokatophorite,
nyböite, glaucophane, richterite and winchite (Shi et al., 2003). Late
stage albite veins cross-cutting jadeitites are not uncommon. The
P–T conditions for jadeitite formation have been estimated at 1.0–
1.5 GPa/300–500 °C (Mével and Kiénast, 1986), >1.4 GPa/400–450 °C
(Goffé et al., 2002), >1.0 GPa/250–370 °C (Shi et al., 2003) or ~1.5 GPa/
~380 °C (Oberhänsli et al., 2007). Based on U–Pb dating of zircons from
jadeitites, Shi et al. (2008) and Qiu et al. (2009) suggested that Myanmar
jadeitite would have formed during the Late Jurassic time, i.e., 147±3 and
158±2 Ma, respectively. However, Goffé et al. (2002) reported 39Ar/40Ar
ages on phengites in eclogites, blueschists, jadeitites and amphibolites
from the Jade Mines belt. The results yielded much younger ages, including an ~80 Ma for eclogite-facies metamorphism and an~30 Ma
for blueschist-facies overprinting.
zircon and catapleiite ((Na2,Ca)ZrSi3O9 · 2H2O) (Supplementary Table
S1). Due to the high content of amphiboles, the rock could also be
named as jadeite-amphibole rock. Jadeite is mostly sub- to anhedral
in form, replaced by retrograde prismatic to fibrous Na-amphiboles
(Fig. 2a). Catapleiite is Ca-rich and is present along rims of some zircon
grains as a retrograde phase (see Figs. 2c–d and 3a–b).
Sample BUR Z2 mainly consists of jadeite (~ 85 vol.%), omphacite
(Ae5Jd49Quad46, ~14 vol.%) and minor amounts of zircon. There are
two types of jadeite, reflecting different growth stages. Jd-I (Jd99Quad1,
~15 vol.%) is high in Jd component and low in modal abundance,
whereas Jd-II (Jd89Quad11, ~70 vol.%) is slightly lower in Jd component
but high in modal amount (Supplementary Table S1). Textural relation
shows that Jd-I is replaced by Jd-II, which, in turn, is replaced by
omphacite (Fig. 2b). Residual Jd-I is generally anhedral and small in
grain size, mostly less than 0.2–0.5 mm. It is surrounded by subhedral
prisms of Jd-II with a grain size of 0.5 × 1–1 × 2.5 mm. Omphacite, replacing Jd-II, also has a large grain size (0.5–2 mm). Both samples
(BUR Z1 and BUR Z2) mainly show a granoblastic texture. Slightly preferred orientation of minerals is observed only in small domains.
3. Sample description
5.1. Mineral inclusion and internal texture of zircon
Two jadeitite samples from the Hpakan area of the Jade Mines belt,
provided by a local miner, were chosen in the present study. Both samples are white in color. Sample BUR Z1 is coarse grained (0.5–1.5 mm). It
contains mainly jadeite (Ae1Jd96Quad3, ~65 vol.%) and Na-amphiboles
(eckermannite±glaucophane, ~34 vol.%), with minor amounts of
Zircons from jadeitite sample BUR Z1 are sub- to anhedral and
150 × 250–200 × 350 μm in size. Some grains are slightly fractured.
A few zircons exhibit a reaction rim with jadeite/Na-amphiboles
and/or later infiltrated fluid, forming catapleiite (Figs. 2c–d and
3a–b). Mineral inclusions are common in zircon, including feldspars
4. Analytical methods
Zircons, concentrated by standard heavy mineral separation processes and hand picking for final purity, were mounted in an epoxy
disc with a 25-mm diameter and a 4-mm thickness. All grains were
imaged with transmitted light and reflected light under a petrographic
microscope. Cathodoluminescence (CL) and back-scattered electron
images were taken with a JEOL 5600 SEM to identify internal texture,
inclusions and physical defects. Zircon U–Th–Pb dating analyses and
trace element (Y, Hf and REEs) determinations were conducted on
the SHRIMP-RG (reverse geometry) ion microprobe co-operated by
U.S. Geological Survey and Stanford University in the SUMAC facility
at Stanford University. The primary ion beam size is about 25 μm. Analytical and data reduction procedures followed those given by Williams
(1998). For age standardization, concentrations of uranium from standard zircon CZ3 (550 ppm U) were used. U–Pb ratios were determined
through replicate analyses of standard zircons R33 (419 Ma) (Black et
al., 2004). Detailed procedures were described by Yui et al. (2010). Precisions for REE determinations were estimated in the range of ±5% for
HREE, ±10–15% for MREE, and up to ±40% for La (all values at 2σ).
A LABRAM HR confocal micro-Raman spectrometer equipped with
a Ar + laser with 514.5 nm excitation, housed in the Institute of Earth
Sciences, Academia Sinica, was employed to identify inclusion phases
in zircons. The laser beam size was about 2–5 μm and the laser power
on the sample surface was about 15 mW. Qualitative chemical analyses for identification of mineral inclusions were also carried out,
when inclusions are exposed on the zircon polished surface, with a
JEOL SEM JSM-6360LV coupled with an energy dispersive X-ray (EDX)
spectrometer housed in the Institute of Earth Sciences, Academia Sinica.
Chemical analysis for jadeitite was carried out at the Department
of Earth Sciences, University of Western Ontario. Major-element analyses were done by X-ray fluorescence spectrometry with the precision
for all major-element determinations better than ±5%. Trace element
analyses including REEs were carried out by the Fusion-ICP-MS method
using a Perkin Elmer Sciex ELAN 6000 ICP-MS. The precisions are about
±15–20% for Th and U, and better than ±10% for others.
5. Results
271
Fig. 2. (a) Optical micrograph of sample BUR Z1, showing jadeite (Jd) replaced by Na-amphibole (Na-Am), and (b) backscattered electron image of sample BUR Z2 showing different
stages of jadeite and late-stage omphacite (omp). Backscattered electron images of zircons showing some composite mineral inclusion in (c) zircon grain 13 and (d) 23 from sample
BUR Z1 and (e) zircon grain 4 from sample BUR Z2; as well as zircon fracturing in (f) grain 7 from sample BUR Z2. See text for details. Small white speckles on zircon polished surface
in c-f are relicts of gold coating after SHRIMP analysis.
(K-feldspar and albite), chlorite, jadeite, Na-amphiboles and catapleiite,
on the basis of Raman and EDX spectra (Figs. 2c–d, 3a, c and e). Among
them, K-feldspar, albite and chlorite were only observed as inclusions
in zircon but are not present in jadeitite matrix. Many jadeite, Naamphibole and catapleiite inclusions, not restricted to zircon rims and
being irregular/angular in form, are mostly related/connected to fractures,
although isolated ones on zircon polished surface are also present
(Figs. 2c–d, 3c and e). So are some chlorite inclusions. A few inclusions
contain composite minerals, in which catapleiite replaces chlorite, jadeite
and/or Na-amphiboles (Figs. 2c–d, 3a and e), and jadeite replaces chlorite
(not shown). Fluid inclusions were not observed. Zircon grains/domains
without mineral inclusions or with feldspars and chlorite inclusions
generally exhibit oscillatory zoning pattern under cathodoluminescence
(CL) (Fig. 3b and d). These zircons usually display medium to dark, and
occasionally light, CL. On the other hand, zircon grains/domains, which
are fractured or contain jadeite, Na-amphibole, and occasionally chlorite
inclusions, mostly show a heterogeneous patchy/cauliflower-like texture
(Fig. 3d and f). Note that catapleiite inclusions are present in both kinds of
zircon grains/domains, although they are less common and smaller in size
in zircon grains/domains with oscillatory zoning.
Zircons from jadeitite sample BUR Z2 are sub- to anhedral and
100 × 200–500 × 700 μm in size. Some zircon grains are highly fractured.
Jadeite veins invade into zircon crystals along such fractures and contain broken zircon chips (Figs. 2f, 4a and e). Irregular/angular-form
jadeite and omphacite inclusions, some of which can often be traced
to fractures, are common in some zircon grains/domains (Figs. 2e–f,
4a and e). Chlorite inclusions are also present (Fig. 4c), although rare.
In a few cases, jadeite replaces chlorite in composite inclusion pockets
(Fig. 2e). Fluid inclusions were not observed. Zircon grains/domains
without prominent fractures and mineral inclusions usually exhibit
oscillatory zoning pattern under cathodoluminescence (CL) (Fig. 4d
and f). Sector zoning is also observed in a few cases. By contrast, zircon
272
Fig. 3. Representative backscattered electron images and CL images of zircon grains 2 (a and b), 10 (c and d) and 15 (e and f) from sample BUR Z1. Zircons are slightly fractured,
containing inclusions of albite (Ab), K-feldspar (Kfs), chlorite (Chl), jadeite (Jd), Na-amphibole (Na-Am) and catapleiite (Cat). Raman and EDX spectra of some inclusions are also
given. Note that some inclusions contain composite minerals. Zircon grains/domains with oscillatory zonings under CL (Type I) yield old U–Pb ages, whereas zircon grains/domains
with a heterogeneous patchy/cauliflower-like texture under CL (Type II) yield younger U–Pb ages. Zircon Type I or II is labeled next to the SHRIMP analyzing spot.
grains/domains, with fractures or jadeite/omphacite inclusions, often
show a heterogeneous patchy/cauliflower-like texture (Fig. 4b and f).
5.2. U–Pb age determination of zircon
Nineteen SHRIMP-RG dating analyses were carried out on fifteen
zircon grains from sample BUR Z1 (Table 1). One analysis shows a
high U content of 2562 ppm and yields the oldest age of 166 Ma. It has
been reported that zircons with U content in excess of ~2500 ppm usually
show a progressive increase in apparent radiogenic 206Pb/238U ratios (and
therefore apparent ages) with increasing U content due to U-dependent
changes in sputtering and secondary ionization efficiency during
SHRIMP analysis (Butera et al., 2001; White and Ireland, 2012). This
analysis is therefore excluded from age calculation. The other fifteen
analyses from zircon domains with oscillatory zonings yield an age of
160±1 Ma (MSWD=3.0) (Fig. 5a). Th/U ratios for these zircon analyses
are higher than 0.1 (Table 1). The remaining three analyses, all from heterogeneous patchy domains, give an age of 77±3 Ma (MSWD=1.8)
with Th/U ratios lower than 0.1 (Fig. 5a, Table 1). Zircons yielding old
ages with oscillatory zonings are categorized as Type I and zircons giving
young ages with a heterogeneous patchy texture, Type II.
Sixteen SHRIMP-RG dating analyses were carried out on twelve
zircon grains from sample BUR Z2 (Table 1). Eight analyses from zircon domains with oscillatory zonings yield an age of 159 ± 1 Ma
(MSWD = 2.1) (Fig. 5b) and have Th/U ratios ranging from 0.04 to
0.26 (Table 1). The age is comparable to those of Type I zircons from
sample BUR Z1. These zircon grains/domains are thus also categorized
as Type I. The other eight analyses from heterogeneous patchy domains
are mostly discordant and do not yield a coherent age. These ages scatter from 153 to 105 Ma (Fig. 5b), older than that of Type II zircons from
sample BUR Z1. The corresponding Th/U ratios range from 0.07 to 0.29
(Table 1). These zircons are categorized as Type III.
273
Fig. 4. Representative backscattered electron images and CL images of zircon grains 7 (a and b), 9 (c and d) and 10 (e and f) from sample BUR Z2. Raman and EDX spectra of some
inclusions are also given. Zircon grains/domains without prominent fractures (Type I) generally do not contain jadeite inclusions, show oscillatory zonings, and give old U–Pb ages.
On the other hand, highly fractured zircon gains/domains (Tpye III) contain jadeite inclusions, show a heterogeneous patchy/cauliflower-like texture and yield younger U–Pb ages.
Zircon Type I or III is labeled next to the SHRIMP analyzing spot.
5.3. Trace element composition of zircon
The chondrite-normalized REE patterns of Type I zircons from sample
BUR Z1 show characteristic positive Ce anomalies (Ce/Ce* = 12–1260)
and enriched HREE, but no Eu anomalies (Table 2, Fig. 6a). On the other
hand, Type II zircons show a small positive Ce anomaly (Ce/Ce*= 2–3),
slightly enriched LREE, and a small but distinct positive Eu anomaly
(Eu/Eu* = 2.0–3.8) (Table 2, Fig. 6a). Total REE content of Type I zircons
(i.e., 530–1170 ppm) is much higher than that of Type II zircons
(i.e., 40–115 ppm) (Table 2).
The chondrite-normalized REE patterns of Type I zircons from
sample BUR Z2 are similar to those of Type I zircons from sample BUR
Z1, including characteristic positive Ce anomalies (Ce/Ce* = 9–161),
enriched HREE, and no Eu anomalies (Table 2, Fig. 6b). Type III zircons
in this sample, however, show comparable but less distinct features
as Type II zircons in sample BUR Z1. For example, Type III zircons also
274
Table 1
SHRIMP-RG U–Th–Pb analytical data of zircon.
Type
BUR Z1
1-1
2-2
3-1
3-2
4-1
5-1
5-2
6-1
7-1
8-1
8-2
9-1
10-1
10-2
11-1
12-1
13-1
14-1
15-1
BUR Z2
1-1
1-2
2-1
3-1
3-2
4-1
5-1
5-2
6-1
7-1
8-1
9-1
10-1
10-2
11-1
16-1
U
Th
(ppm)
(ppm)
I
I
I
I
I
I
I
I
I
I
I
I
I
II
I
I
II
I
II
294
1685
2562
212
295
1841
501
326
449
332
1448
1829
166
220
407
297
68
329
650
44
654
1001
25
40
627
70
39
67
42
371
704
17
10
57
39
1
41
39
III
I
I
I
III
I
III
III
I
III
I
I
I
III
III
III
156
1878
466
113
122
145
68
546
660
199
1323
116
312
329
477
264
15
445
47
5
17
15
5
157
42
37
350
5
23
46
72
31
1. Radiogenic 206Pb.
2. Common Pb component (%) of total
206
Th/U
206
Pb*1
206
Pbc2
Total
238
U/206Pb
(ppm)
(%)
0.15
0.39
0.39
0.12
0.14
0.34
0.14
0.12
0.15
0.13
0.26
0.38
0.10
0.05
0.14
0.13
0.01
0.12
0.06
6.3
36.8
57.4
4.7
6.2
39.6
10.8
7.2
9.7
7.3
31.3
40.1
3.6
2.2
8.6
6.5
0.7
7.0
6.8
–
0.05
0.02
0.56
–
0.12
–
0.17
–
–
0.04
0.14
0.36
–
–
0.40
–
0.32
–
40.05
39.38
38.36
39.11
40.97
39.96
40.01
39.06
39.77
39.26
39.81
39.18
39.59
85.21
40.58
39.07
85.65
40.14
82.40
1.0
0.4
0.4
1.2
1.0
0.4
0.8
1.0
0.8
1.0
0.5
0.4
1.4
1.6
0.9
1.0
2.9
1.0
0.9
0.10
0.24
0.10
0.04
0.14
0.10
0.07
0.29
0.06
0.19
0.26
0.04
0.07
0.14
0.15
0.12
3.1
40.4
9.9
2.4
2.1
3.0
1.3
11.2
14.4
2.8
28.8
2.5
6.7
6.4
9.2
5.5
1.14
0.06
–
0.95
9.51
0.41
0.93
0.10
–
0.86
0.17
–
0.18
5.64
0.24
0.65
43.39
39.91
40.61
40.48
51.02
40.93
44.69
41.81
39.50
60.43
39.48
39.90
40.13
44.53
44.37
41.51
1.4
0.4
0.8
1.7
1.7
1.5
2.2
0.7
0.7
1.4
0.5
1.6
1.0
1.1
0.8
1.1
Pb, determined by
207
Total
207
1σ (%)
Pb/206Pb
206
Pb/238U age
1σ (%)
(Ma)
1σ (Ma)
0.0482
0.0488
0.0496
0.0541
0.0524
0.0486
0.0511
0.0501
0.0522
0.0488
0.0499
0.0504
0.0527
0.0474
0.0505
0.0517
0.0373
0.0511
0.0481
3.5
1.5
1.2
3.9
3.4
1.5
2.6
3.2
2.7
3.2
2.2
1.4
4.5
6.0
2.9
3.4
12.1
3.2
3.4
159.2
161.7
165.8
161.8
154.8
159.5
158.8
162.8
159.5
162.3
159.8
162.3
160.1
75.2
156.7
162.5
75.8
158.3
77.7
1.7
0.7
0.6
2.0
1.6
0.7
1.3
1.6
1.4
1.6
0.8
0.7
2.2
1.2
1.4
1.7
2.3
1.6
0.7
0.0517
0.0499
0.0476
0.0550
0.0990
0.0546
0.0529
0.0505
0.0519
0.0553
0.0518
0.0513
0.0538
0.0750
0.0479
0.0510
4.7
1.4
2.9
5.3
4.4
4.6
9.1
2.5
2.2
4.8
1.6
5.4
3.2
2.7
2.7
3.6
146.4
159.4
157.1
156.2
117.4
154.6
141.9
152.1
160.7
104.9
160.8
159.1
157.8
138.6
143.9
153.1
2.1
0.7
1.3
2.6
2.1
2.3
3.2
1.1
1.1
1.5
0.8
2.6
1.6
1.6
1.1
1.7
Pb correction (Williams, 1998).
exhibit small positive Ce anomalies (Ce/Ce* = 4–14), slightly enriched
LREE, and small positive Eu anomalies (Eu/Eu*=1–1.8) (Table 2,
Fig. 6c), but these features mostly overlap with those of Type I zircons
in sample BUR Z2. Total REE content of Type III zircons (i.e., 53–
1064 ppm) largely overlaps with that of Type I zircons (i.e., 248–
1143 ppm) (Table 2).
5.4. Chemical characteristics of jadeitite
Both jadeitite samples are high in Al and Na, and low in K
(Table 3). Sample BUR Z1 is higher in Mg and LOI, whereas sample
BUR Z2 is slightly higher in Ca, reflecting their different mineral constituents. The two samples also show different REE characteristics.
Sample BUR Z1 is higher in LREE and MREE content, whereas sample
BUR Z2 is higher in HREE (Fig. 7a). The latter sample has a relatively
flat or concave REE pattern ((La/Lu)N = 1.1) with a small positive Eu
anomaly (Eu/Eu* = 1.3), similar to those Myanmar jadeitites reported
by Shi et al. (2008). On the other hand, the former sample has an
enriched LREE and flat HREE pattern ((La/Lu)N = 19.5) without Eu
anomaly, similar to Tone (Japan) jadeitite reported by Yui et al.
(2012). In the primitive mantle (Sun and McDonough, 1989) normalized spidergram, these two jadeitites are notably enriched in Zr and
Hf, but depleted in Ti, Ta and Nb (Fig. 7b). The enrichment of Zr and
Hf relative to other elements in the spidergram seems to be a common feature for all jadeitites that have been studied (Fig. 7b).
6. Discussion
6.1. Geological meaning of zircon U–Pb dates
Type I zircons from sample BUR Z1 show oscillatory zoning under
CL, contain K-feldspar and albite inclusions that are not present in
jadeitite matrix, exhibit high Th/U ratio in the range of 0.10–0.39
(Table 1), and have large Ce anomaly (Ce/Ce* = 12–1260) and high
ΣREE content (530–1170 ppm) with HREE enrichment (Figs. 8 and 9).
All these features are similar to those of inherited igneous zircons in
Tone jadeitites from Kyushu (Japan) described by Mori et al. (2011)
and Yui et al. (2012). The geochemical signatures of these zircons are
also comparable with those of igneous zircons from oceanic crust characterized by Th/U > 0.1, Ce anomaly > 10, and ΣREE content > 500 ppm
(Grimes et al., 2009). However, it is noted that Type I zircons in this
sample contain inclusions of chlorite and catapleiite too. Catapleiite is
a reaction product between zircon and jadeite/Na-amphibole and/or
between zircon and an infiltrated fluid phase subsequent to jadeiteamphibole formation. It should be a post-zircon phase. Catapleiite
“inclusions” in Type I zircons therefore would be best interpreted
as “pseudo-inclusions” noted by Zhang et al. (2009) and Bulle et al.
(2010) that later interaction with rock matrix/infiltrated fluid may
cause the formation of metamorphic/hydrothermal “young” minerals
in “old (igneous)” zircon domains. In this respect, chlorite inclusions
could also be regarded as a hydration product of mafic igneous minerals.
The dating result 160 ± 1 Ma of Type I zircons from sample BUR Z1 is
Fig. 5. Tera-Wasserburg concordia diagram showing zircon ages from (a) sample BUR
Z1 and (b) BUR Z2.
therefore interpreted as the igneous age of jadeitite protolith. It is worth
to point out that oscillatory zoning itself is not a conclusive signature
that such Type I zircons should be of igneous origin. Metasomatic/
hydrothermal zircons from Guatemala jadeitite studied by Yui et al.
(2010) yielded a jadeitite formation age younger than that of regional
eclogite-facies metamorphism. These metasomatic/hydrothermal zircons were also shown to display oscillatory zoning. However, they exhibit distinct metasomatic/hydrothermal geochemical signatures as
Type II zircons in this study discussed below.
Type II zircons in sample BUR Z1, compared with Type I zircons,
exhibit a heterogeneous patchy texture under CL, show lower Th/U
ratios (b0.1), have smaller Ce anomaly (Ce/Ce* = 2–3) and lower
abundance of ΣREE content (40–115 ppm). These geochemical features are similar to those of metasomatic/hydrothermal zircons in
jadeitites from north of Motagua fault, Guatemala (Yui et al., 2010)
and Tone, Kyushu, Japan (Mori et al., 2011; Yui et al., 2012) (Figs. 8
and 9). The small but distinct positive Eu anomaly of Type II zircons
is surely related to the decomposition of protolithic feldspars. The
frequent occurrence of jadeite/Na-amphiboles/catapleiite inclusions
in Type II zircons of this specific sample indicates that these Type II
zircons would be genetically related to the formation of these phases
as a result of fluid infiltration. The common irregular/angular form
with microfractures from acute angles of catapleiite (composite) inclusions (Fig. 2c–d) demonstrates that such inclusions should be related to zircon fracturing. Similar features were also observed for
most, if not all, jadeite and Na-amphibole inclusions (Figs. 2c–d and
275
3e). Coupled with the heterogeneous patchy texture under CL, these
Type II zircons should be a result of recrystallization of Type I
inherited zircons due to fluid infiltration along fractures (Corfu et
al., 2003). Their age, 77 ± 3 Ma, would therefore designate the formation time of jadeite, Na-amphibole and/or catapleiite. In studying the
Na-amphiboles associated with Myanmar jadeitites, Shi et al. (2003)
concluded that Na-amphiboles might have formed simultaneously
with and slightly later than jadeite. Unfortunately, it is difficult to determine the time gap between the formation of jadeite/Na-amphibole
and the formation of catapleiite, except that catapleiite must have
formed later. Nonetheless, Type II zircon grain 15 is less affected by
catapleiite formation (Fig. 3e–f) but still yields a similar young age
as other Type II zircons. Moreover, catapleiite pseudo-inclusions were
also observed in some Type I zircons, but did not cause significant
zircon recrystallization. These observations would indicate that the
fluid responsible for the formation of jadeite/Na-amphibole may have
played a determinant role in forming Type II zircons. The U–Pb age of
Type II zircons, 77 ± 3 Ma, is therefore interpreted as the (minimum)
formation age of jadeitite.
By contrast, Type I zircons in sample BUR Z2 are not as distinct
in their geochemical signatures, which straddle both igneous and
metasomatic/hydrothermal values shown by zircons in sample BUR
Z1 (Figs. 8a and 9). For example, some Type I zircons exhibit low
Th/U ratios (b0.1) and small Ce anomalies (Ce/Ce* = ~ 9), different
from the geochemical characteristics of igneous zircons. However,
these zircons occasionally contain chlorite pseudo-inclusions. They
show oscillatory/sector zoning and yield an age of 159 ± 1 Ma, comparable to those of Type I zircons from sample BUR Z1. They are therefore
also regarded to be inherited from an igneous protolith, at least with
respect to the U–Pb isotope system. Pooled together, the 23 Type I
zircon data from both BUR Z1 and BUR Z2 samples lead to an igneous
protolith age of 160 ± 1 Ma (MSWD = 2.8) for the Myanmar jadeitite.
Type III zircons in sample BUR Z2 are mostly fractured, contain
jadeite/omphacite inclusions, and show a heterogeneous patchy texture under CL, indicating that they would have been subjected to deformation and recrystallization during jadeite/omphacite formation.
Many jadeite/omphacite inclusions have irregular/angular form with
microfractures (Figs. 2e–f, 4a and e), demonstrating that they could
be pseudo-inclusions. The geochemical characteristics of these zircons, however, are not as distinct as those of Type II zircons and can
not be clearly distinguished from Type I zircons (Figs. 8 and 9). The
resulting ages are discordant and non-coherent. Apparent ages of
8 analyses scatter from 153 to 105 Ma. This age range may indicate
a protracted jadeite formation period. Alternatively, the age range
is a result of incomplete resetting of U and Pb isotope compositions
during zircon recrystallization. In view of the non-distinct geochemical signatures of these Type III zircons compared with those of Type II
zircons in sample BUR Z1, as well as that most jadeite/omphacite
inclusions in Type III zircons might be pseudo-inclusions, the latter
interpretation is more likely.
In summary, zircons from both jadeitite samples BUR Z1 and
BUR Z2 would be inherited igneous ones, which may probably explain
the large grain size of some zircons up to 500–700 μm. These zircons
were affected by different degrees of fracturing and recrystallization
during the subsequent metasomatic/hydrothermal event forming
jadeitite. The jadeitites studied would then belong to the metasomatic
replacement (R-) type (Tsujimori and Harlow, 2012; Yui et al., 2010).
Type I zircons contain albite, K-feldspar and chlorite inclusions that
were not observed in jadeitite matrix. While chlorite is regarded as a
hydration product of igneous mafic minerals and albite might also be
a result of Na-metasomatism of igneous plagioclase, K-feldspar would
be an igneous mineral. Together with the large grain size of zircons,
the observations would indicate that the protolith of these jadeitites
might be felsic igneous intrusions. K-feldspar of igneous origin was
also reported as inclusions in inherited zircons from Tone (Japan)
jadeitite, of which the possible protolith was proposed to be oceanic
276
Table 2
Trace element concentration (ppm) of zircon determined by SHRIMP-RG.
BUR Z1
1-1
2-2
3-1
3-2
4-1
5-1
5-2
6-1
7-1
8-1
8-2
9-1
10-1
10-2
11-1
12-1
13-1
14-1
15-1
BUR Z2
1-1
1-2
2-1
3-1
3-2
4-1
5-1
5-2
6-1
7-1
8-1
9-1
10-1
10-2
11-1
16-1
Type
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Y
Hf
Ce/Ce*1
Eu/Eu*1
(Sm/La)N1
(Yb/Gd)N1
I
I
I
I
I
I
I
I
I
I
I
I
I
II
I
I
II
I
II
0.0081
0.0049
0.013
0.0084
0.13
0.072
0.39
0.0012
0.022
0.0046
0.0013
0.18
0.0047
0.28
0.060
0.024
0.022
0.0035
3.5
8.0
22.6
33.7
3.8
4.2
28.9
13.7
5.6
9.5
6.4
26.4
25.2
2.9
0.81
8.2
5.5
0.14
6.3
5.2
0.19
0.61
1.5
0.13
0.22
0.78
1.1
0.24
0.31
0.20
0.62
1.1
0.14
0.052
0.26
0.46
0.024
0.17
0.28
0.80
2.2
3.9
0.71
0.60
2.5
1.6
0.95
1.5
0.99
2.4
2.8
0.61
0.0090
1.3
1.4
0.0031
0.91
0.03
0.96
2.1
3.9
0.94
0.83
2.3
2.0
1.3
2.1
1.1
2.5
3.1
0.69
0.030
1.4
1.6
0.013
1.1
0.11
12
22
34
11
10
25
25
16
27
16
29
30
9.1
0.21
20
18
0.12
16
0.25
68
86
107
68
62
106
131
99
139
98
132
108
57
2.5
120
92
3.0
95
2.0
137
135
155
164
145
173
255
237
305
229
233
168
136
14
268
208
23
219
7.7
313
262
306
423
357
336
545
571
682
544
455
322
356
60
614
494
88
521
24
842
910
1096
977
871
1161
1627
1416
1904
1376
1518
1157
806
87
1645
1249
139
1307
51
16583
15287
15021
12685
12767
15884
13083
12777
12953
13648
15337
13927
12705
12495
13169
12824
11296
13970
15528
167
446
266
87
13
88
12
395
86
193
1260
37
96
2
40
43
3
237
2
0.93
0.91
1.0
1.0
1.0
0.88
0.95
1.1
0.99
0.88
0.90
1.0
0.91
2.1
0.84
0.97
2.0
0.87
3.8
153
685
474
131
7.2
53
6.5
1253
104
330
2792
24
199
0.050
34
91
0.22
404
0.013
30
15
11
48
42
16
26
43
30
42
19
13
48
343
36
33
922
38
118
III
I
I
I
III
I
III
III
I
III
I
I
I
III
III
III
0.19
0.13
0.0083
0.041
0.091
0.033
0.23
0.18
0.0047
0.20
0.35
0.017
0.06
0.50
0.24
1.1
2.7
12.6
5.3
1.0
1.2
2.4
2.3
6.9
5.1
1.9
12.3
0.94
2.6
6.7
6.0
11.5
0.33
0.74
0.24
0.11
0.32
0.26
0.49
0.48
0.17
0.40
1.0
0.075
0.31
0.92
0.59
2.3
0.49
2.7
1.9
0.46
0.28
0.65
0.20
1.3
0.69
0.44
2.6
0.42
0.94
1.3
2.3
1.8
0.73
3.3
2.8
0.47
0.37
0.99
0.25
1.7
0.83
0.48
2.7
0.51
1.3
1.9
3.0
2.3
8.3
30
36
5.6
3.6
12
0.93
20
10
3.2
26
6.2
16
20
37
24
38
84
165
31
19
67
5
75
42
14
79
33
70
84
147
130
85
109
318
63
46
143
11
123
70
37
107
68
135
163
279
291
200
175
615
147
87
296
33
208
136
79
183
163
272
307
494
600
496
706
1827
368
246
816
64
781
441
192
723
404
806
964
1660
1658
11108
11436
9489
12713
4589
10302
9278
7966
14043
8839
11283
13281
10931
7208
6144
9870
6
26
100
9
4
18
14
4
161
4
12
17
12
5
9
4
1.1
1.1
1.0
0.90
1.1
1.1
1.0
1.8
1.0
1.2
1.0
1.0
1.0
1.2
1.0
1.1
4.0
33
364
17
4.8
30
12
1.3
231
3.4
12
39
23
3.9
15
2.6
29
7.0
21
32
29
30
12
43
16
30
8.7
32
21
19
16
30
1/3
2/3
× NdN
, and the subscript “N” indicates normalization to chondrite values of Sun and McDonough (1989).
1. Ce/Ce* = CeN/(LaN × PrN)1/2, Eu/Eu* = EuN/(SmN × GdN)1/2, PrN = LaN
plagiogranite or subduction-zone adakitic granite (Yui et al., 2012). The
same protolith suggestion can be applied to Myanmar jadeitite.
6.2. Geochemical dilemma due to incomplete zircon recrystallization
Typical inherited igneous zircons and completely recrystallized
metasomatic/hydrothermal zircons in jadeitite can be clearly distinguished from each other not only by their U–Pb ages, but also by the
type of mineral inclusions and geochemical signatures, as exemplified
by Type I and Type II zircons from sample BUR Z1 in the present study
(Fig. 9), as well as by those zircons from Tone (Japan) and Guatemala
jadeitites reported previously (Mori et al., 2011; Yui et al., 2010, 2012)
(Fig. 8). Metasomatic/hydrothermal zircons are shown to be characteristically low in Th/U ratio, Ce anomaly, and ΣREE content. Complications
in geochemical distinctions, however, may arise if zircons are not
completely recrystallized.
Despite the patchy texture under CL and the presence of jadeite/
omphacite inclusions, incompletely recrystallized zircons, such as Type
III zircons in sample BUR Z2 in the present study, yield non-coherent
ages. It is noted that some such incompletely recrystallized Type III
zircons, although exhibiting metasomatic/hydrothermal-like low Th/U
ratios or small positive Ce anomalies, have high ΣREE content (Table 4).
It is further noted that the resetting of various trace element contents
and U–Pb isotope systems during zircon recrystallization may not be
coupled “in phase”. The resetting of Ce concentration seems to proceed
faster than that of Th/U ratio or U–Pb isotope system for most Type III
zircons during jadeitite formation (Fig. 9). As an example, Type III zircon
grains/domains 5-2 and 16-1 in sample BUR Z2 show patchy texture
under CL, have low Ce anomaly (Ce/Ce*=~4), but yet give 206Pb/238U
apparent ages at 153–152 Ma, only slightly younger than those of
inherited igneous zircons. However, these zircons exhibit Th/U ratios
higher than 0.1 (see Tables 1 and 2). Such inconsistent geochemical
characteristics of zircons would imply incomplete zircon recrystallization and be a warning not to take apparent ages of such zircons as
the time of jadeitite formation. This suggestion may also be applied to
Type II zircons from Osayama jadeitite (Fu et al., 2010), which were
suggested to be of metasomatic/hydrothermal origin but have Th/U
ratios larger than 0.1 (see Fig. 8c). In this respect, some Type I zircons in
sample BUR Z2 have low Th/U ratios or small Ce anomalies (Table 4).
These uncommon geochemical signatures for igneous zircon should be
suspected to be of secondary origin. Furthermore, inherited zircons in
the present study, as well as those in Tone (Japan) jadeitite (Mori et al.,
2011; Yui et al., 2012), do not show prominent negative Eu anomaly
(Fig. 6). It is well documented that prominent negative Eu anomaly is
a common feature for igneous zircons, except for zircons from
carbonatite, kimberlite, and some syenites (Belousova et al., 2002;
Hoskin and Schaltegger, 2003). Zircons from oceanic crust usually
have negative Eu anomaly (Eu/Eu*= b0.6) (Grimes et al., 2009), too.
Whether the lack of negative Eu anomaly is an indigenous signature
of inherited igneous zircons in jadeitite would be an interesting issue
to explore. All these observations indicate that resetting of U–Pb isotope
system seems to commonly take place at a slower pace than alteration
of trace element compositions during zircon recrystallization upon
jadeitite formation.
The factors causing this decoupling of recrystallization rate of
chemical/isotope systems in zircon would include P–T conditions,
zircon fracturing, fluid/rock ratio, ionic radii, and redox-pH conditions. For example, it is well known that the larger Th 4+ would be
277
lattice (Hoskin and Schaltegger, 2003). Besides, the original compositions may also play a role during such kinetic non-equilibrium recrystallizations. However, how these factors work together and what the
resulting integrated effect on recrystallization rate would be for those
elements concerned during zircon recrystallization have not yet been
fully studied. The consequences for such zircon incomplete recrystallization could be easily recognized as shown in the present case, but
could also lead to serious problems in age interpretation. Extreme
cases could be envisioned that incompletely recrystallized zircons
may have their geochemical signatures altered but still keep U–Pb
age dates largely unchanged or only slightly modified. This would easily
lead to an incorrect age/geological interpretation as shown below.
6.3. Jurassic subduction for the Jade Mines belt?
Fig. 6. Chondrite (Sun and McDonough, 1989) normalized REE patterns of (a) Type I and
Type II zircons from sample BUR Z1, (b) Type I zircons from sample BUR Z2 and (c) Type III
zircons from Bur Z2. Pr content of zircons was not analyzed and its chondrite normalized
1/3
2/3
×NdN
. Note that Type II zircons have lower ΣREE
value was calculated by PrN =LaN
abundances and show smaller positive Ce anomalies than Type I zircons from sample
BUR Z1. They also show small but distinct positive Eu anomalies. Such differences are
not clearly distinguished between Type I and Type III zircons from sample BUR Z2.
preferentially expelled from zircon lattice relative to the smaller U 4+
during recrystallization (Hoskin and Schaltegger, 2003). Besides,
Ce 4+ may be reduced to Ce 3+ during jadeitite formation (Yui et al.,
2010). The latter has a larger ionic radius less compatible with zircon
The Myanmar jadeitites have been previously dated by Shi et al.
(2008) and Qiu et al. (2009) based on zircon U–Pb SHRIMP/laser-ICPMS
techniques, respectively. Shi et al. (2008) reported an inherited igneous
(oceanic crust)/hydrothermal age at 163± 3 Ma by 18 analyses on
Group I zircons, a jadeitite formation age at 147 ± 3 Ma by 10 analyses
on Group II zircons and an unknown thermal event at 122 ± 5 Ma
by one single analysis on Group III zircon. On the other hand, Qiu et al.
(2009) suggested a slightly older jadeitite formation age of 158±2 Ma
by 75 U–Pb dates on hydrothermal/metasomatic zircons. However,
Qiu et al. (2009) proposed that their result is actually indistinguishable
from that given by Shi et al. (2008) if the igneous/hydrothermal
and jadeitite ages from the latter were pooled together, which would
lead to an age of 157 ± 4 Ma. Both studies therefore suggested that
Myanmar jadeitites should have formed during the Late Jurassic (i.e.,
158–147 Ma) subduction between the Indian plate and the Burma
microplate. Shi et al. (2008) related this subduction with some Mesozoic
subduction-related calc-alkaline magmatism at 170–120 Ma along the
Mogok belt (Barley et al., 2003). An Andean type continental margin
along the southern border of Asian plate since at least Early Jurassic
was also proposed based on temporal correlation of these igneous
activities with those in the Karakoram and Hindu Kush areas (Searle
et al., 2007). However, Mitchell et al. (2012) preferred to relate the
intrusives along the Mogok belt to the subduction–collision with the
Shan-Thai block, rather than to the subduction of the Indian plate. Qiu
et al. (2009) also denied the connection between the inferred Jurassic
subduction and the Mogok belt, but suggested a genetic relation with
the Katha-Gangaw ranges (Fig. 1). It is noted that eclogite was recently
reported west of the Katha-Gangaw ranges (Enami et al., 2012), although
the details, including the time of high-pressure metamorphism, are not
yet clear. These different opinions clearly indicate inadequate geological
constraints of the area.
The ages and the characteristics of the Myanmar jadeitite zircons
studied by Shi et al. (2008) and Qiu et al. (2009) are compiled in
Table 3
Major and trace element concentrations of Myanmar jadeitites.
Major element (%)
SiO2
TiO2
Al2O3
Fe2O31
MnO
MgO
CaO
K2O
Na2O
P2O5
L.O.I.
Total
BUR Z1
BUR Z2
58.40
57.98
0.01
0.02
19.53
22.44
1.53
1.04
0.02
0.02
4.96
1.72
0.30
2.19
0.05
0.05
13.84
13.23
0.01
0.02
1.34
0.65
99.99
99.36
Trace element (ppm)
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Bur Z1
Bur Z2
5.46
0.80
13.20
1.30
1.47
0.16
5.77
0.60
0.96
0.13
0.23
0.06
0.54
0.15
0.05
0.03
0.19
0.24
0.03
0.06
0.09
0.25
0.02
0.06
0.14
0.39
0.03
0.08
Trace element (ppm)
Hf
Ta
Nb
Zr
Y
Th
U
Sr
Rb
Ba
Ni
Co
Cr
Eu/Eu*2
Bur Z1
Bur Z2
5.0
16.9
b0.01
b0.01
b0.2
b0.2
169
744
2
2
2.5
0.2
0.11
0.30
31
64
2.0
b0.2
b10
11
65
161
18
5
90
10
1.0
1.3
1. Total Fe as Fe2O3.
2. Eu/Eu* = EuN/(SmN × GdN)1/2, and the subscript “N” indicates normalization to chondrite values of Sun and McDonough (1989).
278
Fig. 7. (a) Chondrite normalized REE pattern and (b) Primitive Mantle normalized spidergram of Myanmar jadeitites in this study. Also shown are respective fields of Myanmar
jadeitites reported by Shi et al. (2008), Itoigawa-Ohmi omphacites in jadeitite reported by Morishita et al. (2007), and Guatemala jadeitites reported by Simons et al. (2010)
and Yui et al. (2010). Patterns for one Tone (Japan) jadeitite (Yui et al., 2012) and one Monviso jadeitite (Compagnoni et al., 2012) are also included for comparison.
Table 4, together with those of zircons from this work. Although the
ages, the geochemical features and the textures of zircons they studied
are similar with each other and are largely comparable to those of
zircons in the present study, particularly resembling those of zircons
from sample BUR Z2, the related interpretations differ. For example,
inherited igneous zircons were suggested to be present in jadeitites
studied by Shi et al. (2008) and the present work, but all zircons studied were considered to be of metasomatic/hydrothermal origin by Qiu
et al. (2009). Both Shi et al. (2008) and Qiu et al. (2009) proposed a
Late Jurassic time for the formation of Myanmar jadeitite, the present
study, however, indicates a lower limiting jadeitite age of 77 Ma.
6.3.1. Evaluation of protolith age
Shi et al. (2008) characterized their Group I zircons by showing
oscillatory zoning and the presence of a Na-free/Mg-rich inclusion
phase. The Na-free/Mg-rich phase is similar to chlorite included within
zircons in the present study. While chlorite is interpreted as a pseudoinclusion phase resulting from hydration reaction of igneous mafic
minerals through fractures, other phases of possible igneous constituents, such as feldspars (especially K-feldspar), are also identified as inclusions in Type I zircons in the present study. The two ages derived
from these zircons, i.e., 163 ± 3 Ma (Shi et al., 2008) and 160 ± 1 Ma
(this study), are consistent with each other within analytical uncertainty and would unambiguously signify the age of igneous protolith
of Myanmar jadeitites.
East of the Indo-Burma Range, there are two, eastern and western,
belts of dismembered ophiolites signifying the ancient plate boundary
(Fig. 1) (Acharyya, 2007; Chhibber, 1934). Serpentinites (±peridotites),
hosting the Myanmar jadeitite and HP rocks in the Jade Mines belt, are
considered to be part of these belts. The on-land emplacements of
these ophiolitic rocks were suggested to have taken place during Early
Cretaceous to mid-Eocene (Acharyya, 2007; Mitchell, 1993). A K–Ar
age of 158 ±20 Ma from hornblende-bearing pegmatite associated
with harzburgites in the southern part of the western ophiolite belt
(United Nations, 1978) demonstrates that parts of these ophiolites
are relicts of Jurassic oceanic crust. The age of the inferred igneous
protolith of jadeitites, i.e., 163 ± 3 Ma (Shi et al., 2008) or 160 ± 1 Ma
(this study), agrees with this K–Ar date and is more precisely
constrained. The jadeitite, as well as other high-pressure rocks in the
Jade Mines belt, would therefore be genetically related to Jurassic igneous activities in oceanic environment(s), which could well be correlated
with some ophiolites of Jurassic ages along the Indus-Yarlung Zangbo
suture (see Hébert et al., 2012; and the references therein), as proposed
by Mitchell (1993).
279
Fig. 9. (a) Th/U-age and (b) Ce/Ce*-age plots of zircons in the present study. I, II and III
means Type I (igneous), Type II (metasomatic/hydrothermal) and Type III (incompletely
recrystallized) zircons, respectively. Compositional and age ranges for zircons from
Myanmar jadeitites reported by Shi et al. (2008) and Qiu et al. (2009) are also shown for
comparison. Note that most Type III zircons show small Ce/Ce* ratios. See text for details.
Fig. 8. Ce/Ce*–Th/U plot for zircons from (a) Myanmar, (b) Tone (Japan), and (c) Osayama
(Japan) and Guatemala jadeitites. For Tone jadeitite, Type II zircon of Mori et al. (2011) is
equivalent to Type II plus Type III zircon in this study, whereas Type I+II zircon of Yui et
al. (2012) is equivalent to Type III zircon in this study. For others, I, II and III means Type I
(igneous), Type II (metasomatic/hydrothermal) and Type III (incompletely recrystallized)
zircons, respectively.
6.3.2. Evaluation of jadeitite age
Both Group II zircons studied by Shi et al. (2008) and all zircons
studied by Qiu et al. (2009) are characterized by the lack of oscillatory
zoning and the presence of jadeite or omphacite inclusions. These authors
suggested that such zircons would be of metasomatic/hydrothermal
origin, yielding the jadeitite formation age of 147 ± 3 Ma (Shi et al.,
2008) or 158 ± 2 Ma (Qiu et al., 2009). Since the inclusion textures
were not presented, it is also possible that these jadeite/omphacite inclusions in zircons in these two studies are actually pseudo-inclusions.
As a matter of fact, Shi et al. (2008) mentioned that zircon grains they
studied are fractured in different extent.
The geochemical features of these zircons presented by Shi et al.
(2008) and Qiu et al. (2009) are not as distinct as those metasomatic/
hydrothermal signatures shown by Type II zircons in the present
study (see Table 4). In fact, their various compositional ranges are
more comparable to Type III zircons in this study (Fig. 9). The latter
contain jadeite/omphacite pseudo-inclusions and are interpreted to
be incompletely recrystallized. It is noted that about half of the analyzed “metasomatic/hydrothermal” zircons presented by Shi et al.
(2008) and Qiu et al. (2009) have at least one geochemical signature
being non-dintinct (Table 4). Despite that these zircons yield respective
coherent ages in these two studies (Fig. 10a), the dates should be
regarded as incompletely reset ones, analogous to Type III zircons in
this study, and should not bear any geological meaning. By contrast, the
rest of the zircons in these two studies do show typical metasomatic/
hydrothermal geochemical signatures. However, ages from these apparently completely recrystallized zircons are similar to, or even slightly older than, those of incompletely recrystallized zircons in these two
studies, respectively, and the resulting ages also differ between the two
studies (Fig. 10). These features are more consistent with an interpretation that all such “metasomatic/hydrothermal” zircons studied by
Shi et al. (2008) and Qiu et al. (2009) might be incompletely reset
with respect to U and Pb isotope systems. The fact that the degree
of age discordance between 206Pb– 238U and 208Pb– 232Th systems for
zircons studied by Qiu et al. (2009) is skewed to negative values with
decreasing Th/U ratios or Th content (Fig. 11), although may be an
analytical artifact due to low Th concentrations, can be interpreted as
a corollary of incomplete resetting. The age coherence, as well as the
apparent ages of these zircons being similar to the inherited igneous
zircon age, certainly implies very low degrees of recrystallization in
terms of U–Pb system. This may also explain why the “metasomatic/
hydrothermal” zircons studied by Shi et al. (2008) and Qiu et al.
(2009) are mostly feature-less under CL or show less prominent patchy
texture compared with Type II and III zircons observed here. Different
extent of zircon recrystallization could theoretically result from different fluid/rock ratios, different degrees of zircon fracturing, and/or different redox-pH conditions. In view of these assessments, the claimed
“metasomatic/hydrothermal” zircons in Shi et al. (2008) and Qiu et
al. (2009) should be better regarded as incompletely recrystallized
inherited zircons. Only Type II zircons in the present study can be reasonably considered, in all aspects, as being completely recrystallized
during jadeitite formation.
Only one single analysis of Group III zircon was given by Shi et al.
(2008), which yielded an age of 122 ± 5 Ma. The zircon has a low Th/
U ratio (i.e., 0.07). No other chemical compositions are available for
evaluating the extent of zircon recrystallization. Since the age is
older than the Type II zircons and falls into the age range of Type III
zircons in the present study, this Group III zircon is also suggested
to be incompletely recrystallized.
On the bases of 39Ar/ 40Ar phengite dating for HP rocks, Goffé et al.
(2002) postulated that eclogite facies metamorphism of the Jade
280
Table 4
Characteristics of zircons from Myanmar jadeitite.
BUR Z1 (this study)
Inherited igneous zircon (Type I)
Metasomatic/hydrothermal zircon
(Type II)
BUR Z2 (this study)
Inherited igneous zircon (Type I)
Incompletely recrystallized zircon
(Type III)
Shi et al. (2008)
inherited igneous/hydrothermal
zircon (Group I)
Metasomatic/hydrothermal zircon
(Group II)
Unknown thermal event zircon
(Group III)
Qiu et al. (2009)
metasomatic/hydrothermal zircon
Age (Ma)
Th/U
Ce anomaly/Eu anomaly
ΣREE (ppm)
CL images
Inclusions
160 ± 1 (n = 15)
0.10–0.39 (n = 15)
12–1260/~1.0 (n = 15)
530–1170
Oscillatory zoning
77 ± 3 (n = 3)
0.01–0.06 (n = 3)
2–3/2.0–3.8 (n = 3)
40–115
Patchy structure
Albite, K-feldspar, chlorite,
catapleiite
Jadeite, Na-amphibole,
chlorite, catapleiite
159 ± 1 (n = 8)
153–105 (n = 8)
0.04–0.26 (n = 8)
0.07–0.29 (n = 8)
9–161/~ 1.0 (n = 8)
4–14/1.0–1.8 (n = 8)
248–1143
53–1064
Oscillatory zoning
Patchy structure
Chlorite
Jadeite, omphacite, chlorite
163 ± 3 (n = 18)
0.05–0.31 (n = 18)
–
–
Oscillatory zoning
147 ± 3 (n = 10)
0.07–0.18 (n = 10)
–
–
No zoning, bright L
Na-free/Mg-rich phase
(chlorite?)
Jadeite
122 ± 5 (n = 1)
0.07 (n = 1)
–
–
No zoning, bright L
–
157 ± 4 (n = 75)
0.04–0.19 (n = 16)
1–4/0.8–1.9 (n = 16)
52–286
Irregular or slightly
patchy structure
Jadeite, omphacite
Mines belt took place at around 80 Ma. The (minimum) age of
jadeitite formation, 77 ± 3 Ma, suggested in this study, is in accord
with this time frame and implies a Late Cretaceous subduction event.
This age is older than the age of Himalayan ultrahigh-pressure gneisses,
53–46 Ma (e.g., Kaneko et al., 2003; Leech et al., 2005). The latter signifies the minimum age of continental collision at the western syntaxis
of Himalaya. It can therefore be concluded that jadeitites and associated
high-pressure rocks from the Jade Mines belt in northern Myanmar
should be products during Late-Cretaceous, but not Jurassic, subduction
before India-Asia continental collision.
6.3.3. Implications for other jadeitite occurrences
In the Cycladic blueschist belt, jadeitites are present as minor blocks
within eclogite- to epidote blueschist-facies mélange sequences on the
islands of Tinos and Syros, Greece. Zircons from these jadeitites and
associated mélange blocks of eclogite, omphacitite, glaucophanite and
Fig. 10. Histogram of “metasomatic/hydrothermal” zircon ages with (a) non-typical
and (b) typical metasomatic/hydrothermal geochemical characteristics from Shi et al.
(2008) and Qiu et al. (2009). The ages of these two zircon categories are similar in
these two studies, respectively. The resulting ages, however, differ between the two
studies. See text for details.
chlorite-actinolite rock have been dated previously, yielding U–Pb
ages of ca. 80 Ma (Bröcker and Keasling, 2006). These ages are older
than the Eocene HP metamorphic ages (ca. 53–40 Ma) of the Cycladic
blueschist belt (e.g., Bröcker and Enders, 2001; Tomaschek et al.,
2003; and the references therein). Despite of the characteristic metasomatic mineral inclusions (such as omphacite, epidote, glaucophane, and
actinolite etc.), zircons from these rocks generally show oscillatory/
sector zoning, have high Th/U ratios and exhibit igneous-like O-isotope
compositions (Bröcker and Keasling, 2006; Fu et al., 2010, 2012).
Most mineral inclusions in these zircons were also suggested to be
pseudo-inclusions (Bulle et al., 2010). Zircons are therefore most probably inherited. Consequently, the zircon U–Pb dates should designate
Fig. 11. (a) Degree of age discordance — Th/U and (b) U (Th) — Th/U plots for zircons
from Myanmar jadeitites reported by Qiu et al. (2009). Degree of age discordance is
defined as 100×((206Pb/238U)age/(208Pb/232Th)age−1). The degree of age discordance
is skewed to negative values with decreasing Th content. The relation may indicate incomplete resetting of U–Th–Pb isotope system as a result of preferential expelling of Th during
zircon recrystallization.
the time of their protolith igneous activities, but not the age of jadeitite
formation or HP metamorphism (Bröcker and Keasling, 2006; Fu et al.,
2010, 2012).
Recently, there have been several reports dealing with SHRIMP
U–Pb dating on zircons from various jadeitite occurrences, including
Voikar-Syninsky, Polar Urals, Russia (Meng et al., 2011), Sierra del
Convento, Cuba (Cárdenas-Párraga et al., 2012), Rio San Juan Complex, Dominican Republic (Schertl et al., 2012) and Kanto Mountains,
Japan (Fukuyama et al., 2013). Most zircons in these jadeitites show
oscillatory zoning under CL. They were claimed to contain jadeite/
omphacite or fluid inclusions and therefore to be of metasomatic/
hydrothermal origin. However, geochemical characteristics of these
zircons are largely non-typical (e.g., Th/U > 0.1) compared with Type
II metasomatic/hydrothermal zircons in this study. Some of the age
results were also shown to be not in accord with regional geological information (Tsujimori and Harlow, 2012). Zircons from these jadeitite
occurrences therefore may also be inherited ones. Further evaluation/
examination is obviously required. Incomplete recrystallization of
inherited zircons may not be uncommon during jadeitite formation
through whole-sale metasomatic replacement processes, probably
due to the low temperature conditions (~100–450 °C) (Harlow, 1994;
Shigeno et al., 2005).
281
chondrite (Shi et al., 2008). In this regard, dehydrated fluid from
subducted sediments may have contributed to the LREE enrichment
in the infiltrated fluid during the formation of Na-amphiboles. The
observed different chemistries, and consequently mineral constituents,
between the two jadeitite samples in this study may therefore be related to their spatial positions with respect to hosting serpentinite. In the
absence of field occurrence information of the samples studied, this
proposition may be a speculation. However, a comparable case was
presented by Compagnoni et al. (2012), who reported a rock sample
with quartz-jadeite rock at the core and jadeitite at the rim from
serpentinite debris of the Monviso massif, western Alps. The rim
jadeitite was shown to be distinctly higher in Mg, Ca, Ni, Cr and Co
than quartz-jadeite rock. The feature is presumably related to the infiltrated serpentinization fluid during jadeitite formation.
Acknowledgements
Helpful comments/suggestions from H.-P. Schertl and an anonymous reviewer are highly appreciated. We would like to thank J.L.
Wooden for his help and suggestions during our SHRIMP work. This
study was financially supported by the National Science Council,
Taiwan.
7. Concluding remarks
The Myanmar jadeitite samples in the present work, as well as
those studied by Shi et al. (2008) and Qiu et al. (2009), should be
categorized as “metasomatic replacement type” or “R type” following
the above discussion. Three types of zircons, in terms of U–Pb isotope
system, were recognized in this study. Type I zircons are inherited
ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons
are metasomatic/hydrothermal ones, giving a (minimum) jadeitite
formation age of 77 ± 3 Ma; and Type III zircons are incompletely
recrystallized ones, with non-coherent and geologically meaningless
ages from 153 to 105 Ma. This study also shows that completely
recrystallized zircons during jadeitite formation, such as Type II
zircons, would exhibit typical metasomatic/hydrothermal geochemical characteristics and record U–Pb age of jadeitite formation. However, the resetting rates of various trace element compositions and
U–Pb isotope system during zircon recrystallization are not coupled
“in phase”. Incompletely recrystallized zircons may partially retain
igneous geochemical signatures, and in the worst case, such zircons
may show typical metasomatic/hydrothermal geochemical signatures
but give a coherent apparent age irrelevant to jadeitite formation.
A thumb rule based on geochemical criteria for evaluating U–Pb
SHRIMP dating on metasomatic recrystallized zircons from jadeitite,
or other metasomatic products, can thus be derived. With typical
metasomatic/hydrothermal geochemical signatures, zircons may or
may not be completely recrystallized with respect to U–Pb isotope
system. On the other hand, if zircons show non-typical metasomatic/
hydrothermal geochemical signatures, they are most probably incompletely recrystallized. Their U–Pb ages must be subjected to rigorous
examinations before conclusions.
Lastly, a few words regarding jadeitite chemistry seem warranted.
Given the presence of inherited zircons with the same age, both samples studied most likely have resulted from whole-sale metasomatic
replacement of a common protolith. Despite the distinctly high Al
and Na contents in both samples, sample BUR Z1 is higher in Mg
and LREE than sample BUR Z2 (Table 3). The latter sample is chemically similar to those jadeitites studied by Shi et al. (2008) (Fig. 7).
The chemical differences are probably due to additional metasomatic
reactions forming Na-amphiboles in sample BUR Z1. Shi et al. (2003)
noted that metasomatic Na-amphiboles commonly developed in between jadeitite and hosting serpentinized peridotite in the Jade
Mines belt. While nearby serpentinite may be the source for Mg, it
has a flat REE pattern with REE abundances less than 10 times of
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.lithos.2012.12.011.
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