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Earth and Planetary Science L
Geochronology of diamond-bearing zircons from garnet peridotite
in the North Qaidam UHPM belt, Northern Tibetan Plateau:
A record of complex histories from oceanic lithosphere
subduction to continental collision
Shuguang Songa,b,T, Lifei Zhanga,b, Yaoling Niuc, Li Sud, Ping Jianb, Dunyi Liub
aMOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, ChinabSHRIMP Laboratory, Chinese Academy of Geoscience, Beijing 100037, ChinacDepartment of Earth Sciences, University of Durham, Durham DH1 3LE, UK
dGeological Lab Center, China University of Geosciences, Beijing 100083, China
Received 24 July 2004; received in revised form 8 December 2004; accepted 23 February 2005
Available online 26 April 2005
Editor: B. Wood
Abstract
We report the results of a comprehensive study of zircons separated from the Lqliangshan peridotite massif within the 400-
km-long North Qaidam UHPM belt, northern Tibetan Plateau, NW China. The peridotite massif is dominated by garnet
lherzolite with minor amounts of interlayered garnet-bearing dunite and cross-cutting garnet pyroxenite dikes. Most zircons
from the garnet lherzolite show rather complex zoning. One diamond and a few graphite inclusions are identified in some
zircons by Raman spectroscopy. SHRIMP dating on these zircons show four major age groups: (a) 484–444 Ma (weighted
mean age, 457F22 Ma) for cores of most crystals, whose morphology and rare earth element (REE) systematics (i.e., very high
[Lu /Sm]CN=88–230) suggest a magmatic origin, consistent with the protolith being magmatic cumulate; (b) 435–414 Ma with
a mean of 423F5 Ma, which, given by mantle portions of zircon crystals, is interpreted to record the event of ultrahigh-pressure
metamorphism (UHPM) at depths greater than 200 km in an Andean-type subduction zone; (c) 402–384 Ma (mean age 397F6
Ma) for near-rim portions of zircon crystals; and (d) 368–349 Ma for outermost rims, which is interpreted as representing some
post-orogenic thermal events. Inherited cores in two zircon crystals were identified using CL and found to be Proterozoic.
Morphology and CL images show that zircons from dunite and garnet pyroxenite are of metamorphic origin. The mean age of
dunite zircons is 420F5 Ma, which overlaps the mantle age of the garnet lherzolite zircon (see (b) above). The mean age of
garnet pyroxenite zircons is 399F8 Ma, which overlaps ages of near-rim domains in garnet lherzolite zircons (see (c) above).
Some garnet pyroxenite zircons also recorded a retrograde event at 358F7 Ma. All these data suggest that the Lqliangshangarnet peridotite massif is not a fragment of ancient lithospheric mantle, but a peridotite body with long and complex histories
from Early Ordovician to Late Devonian, including the emplacement of ultramafic cumulate probably in the shallow section of
0012-821X/$ - s
doi:10.1016/j.ep
T Correspondi
University, Beiji
E-mail addr
etters 234 (2005) 99–118
ee front matter D 2005 Elsevier B.V. All rights reserved.
sl.2005.02.036
ng author. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking
ng 100871, China. Tel.: +86 1 6275 1145.
ess: sgsong@pku.edu.cn (S. Song).
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118100
a mantle wedge, deep (N200 km) subduction of the cumulate induced by the subducting slab, and ultimate exhumation
genetically associated with continental collision.
D 2005 Elsevier B.V. All rights reserved.
Keywords: diamond; mineral inclusions; SHRIMP data; zircon; garnet peridotite; UHPM belt; North Qaidam; NW China
1. Introduction
Garnet peridotite outcrops, although volumetrically
small, are widely exposed in every known ultrahigh-
pressure metamorphic (UHPM) terrane on Earth.
These terranes result from continental collision, e.g.,
Dabie-Sulu of eastern China [1–3], the Western
Gneiss Region of Norway, Variscan and Alpine
orogens in Europe [4–7], and Sulawesi of Indonesia
[8]. These garnet peridotites are thought to have three
possible origins prior to their tectonic exhumation
[4,9]: they are (1) peridotite derived from subducted
Fig. 1. (A) Geological Map of the North Qaidam—Altun eclogite belt in no
cratons or massifs covered with Mesozoic sediments. The N. Qilian Suture
(the west part of North China Platform) and Qilian Massif. (B) Geologica
showing location of the Luliangshan garnet peridotite massif.
lithospheric mantle; (2) originated from the mantle
wedge above subduction zones; and (3) products of
subduction-zone metamorphism of peridotites previ-
ously emplaced into the crust. However, deciphering
the exact origin of a particular garnet peridotite
occurrence is not straightforward, but requires a
detailed petrological, geochemical and geochronolog-
ical study of the rock suite in the context of the
regional tectonics.
Dating the orogenic garnet peridotites is a challenge
because of the complex histories they may have
experienced. For example, Sm–Nd [10,11] and Re–
rthern Tibet. The Tarim and Qaidam Basins are actually Precambrian
Zone is a typical Pacific-type subduction zone between Alax Massif
l map of eclogite-bearing metamorphic terrane in Da Qaidam area,
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 101
Os [12] radiometric methods have been used to date
garnet peridotites from the Western Gneiss Region,
Norway, and given fairly complex results. Zircon,
because of its chemical resistance and physical stability
over a wide range of P–T conditions [13,14], is the best
host mineral that may protect UHP phases (e.g. coesite
and diamond) entrapped during its crystallization and
may preserve zircon overgrowths from subsequent
alteration or retrograde events. Zircon is also the best
mineral for recording the metamorphic histories of the
host rocks as well as of the garnet peridotite protoliths.
It has been well established that SHRIMP (sensitive
high resolution ion microprobe) is the instrument of
choice for U–Pb in situ dating of zircons. The SHRIMP
techniques, in combination with CL (cathodolumines-
cence) image analysis, are ideal for dating rocks with
complex histories [15–18].
Discoveries of coesite inclusions in zircons sepa-
rated from pelitic gneisses and coesite pseudomorphs
in eclogitic garnet and omphacite in Dulan [19–21]
provide clear evidence for UHP metamorphism in the
North Qaidam (NQ) orogenic belt, northern Tibetan
Plateau, NW China. Zhang et al. [22] reported a single
zircon TIMS U–Pb age of 495F7 Ma from the Yuka
eclogite (the west part of the NQ belt, see Fig. 1A).
Recently, Song and co-workers reported zircon
SHRIMP U–Pb ages from 440 to 482 Ma with a
mean at 458F7 Ma (MSWD=0.86) (Song et al. in
review) and whole-rock-mineral Sm–Nd internal
isochron ages of 459F2.6 Ma and 458F10 Ma
[23], respectively, of eclogite samples from the Dulan
UHP terrane, the east part of the NQ UHP belt.
In this paper, we present detailed studies on
morphology, zoning and mineral inclusions in zircons
separated from garnet–lherzolite, garnet-bearing dun-
ite and garnet pyroxenite of the Lqliangshan garnet
peridotite massif using combined techniques of CL
image analysis, laser Raman spectroscopy, SHRIMP
in situ dating, and LA-ICP-MS analysis. The results
allow us to infer the origin and metamorphic histories
of the garnet peridotite body in the context of regional
tectonic evolution.
2. Geological setting and sample petrography
The garnet peridotite under study occurs as a large
(~500�800 m in size) massif within the North
Qaidam UHPM belt (Fig. 1). This belt is the east part
of the North Qaidam–Altun super-belt, which extends
for about 1000 km to the northwest and was offset by
the sinistral Altyn Tagh Fault in the northern Tibet
Plateau, NW China. Peak P–T conditions of eclogite
facies metamorphism in this super-belt are estimated
to be ~T=820–850 8C and P=2–3 GPa in the Altun
terrane [24], and T=630–770 8C and P=2.8–3.3 GPa
in the Dulan terrane [21] respectively. The garnet
peridotite massif, first reported by Yang and others
[25,26], is located in Lqliangshan area, 20 km south
of Da Qaidam town, and is hosted within an eclogite-
bearing quartzofeldspathic gneiss terrane (Fig. 1B).
The garnet peridotite massif is dominated by garnet
lherzolite with minor amounts of inter-layered dunite
(with or without garnet) and cross-cutting garnet
pyroxenite dikes.
2.1. Garnet lherzolite
The garnet lherzolite constitutes ~80 vol.% of the
garnet peridotite massif. It is massive and coarse-
grained without obvious foliations. The main constit-
uent minerals are garnet (Grt), olivine (Ol), orthopyr-
oxene (Opx), clinopyroxene (Cpx) and Cr-rich spinel
(Spl). Most garnets occur as porphyroblasts of varying
size (3–10 mm). All the garnet crystals are Mg-rich
with 64–73 mol% pyrope, 13–23% almandine, 4–10%
grossular, 0.9–1.8% spessartine and 2–5% uvarovite
in different samples and they have quite homogeneous
composition from core to rim. Olivine shows a wide
range of Fo values (i.e., Mg#=0.84–0.91) and con-
stitutes 40–60 vol.% of the rock, orthopyroxene, about
10–30 vol.% with Mg# ranging from 0.88 to 0.93 and
Al2O3 from 0.38 to 0.64 wt.%, and clinopyroxene,
about 5–15 vol.% with Jd content up to 8.8 mol%,
respectively. Electron microprobe analysis shows that
all pyroxenes are quite homogeneous without compo-
sitional zoning. Some samples show apparent layering
because of varying olivine and pyroxene proportions
on local scales. Fine-grained Cr-rich spinel (Cr#, [Cr /
Cr+Al]=0.60–0.69) is scattered fairly uniformly both
in the matrix and as inclusions of major silicate
minerals.
High concentrations of decompression-induced
exsolution textures have been observed in some
porphyroblastic garnets and olivines in the lherzolite.
Exsolution lamellae in garnets include densely packed
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118102
rods of rutile, orthopyroxene, clinopyroxene and sodic
amphiboles [27,28]. Exsolutions in olivine are needles
of ilmenite and chromian spinel [27]. The pyroxene
exsolutions suggest that the garnet host crystals
originally possessed excess silicon, i.e., they were
majoritic garnets that are only stable at depths well in
Table 1
U, Th and Pb SHRIMP zircon data of the garnet lherzolite C305 from th
Spot name U
(ppm)
Th
(ppm)
Th /U Pb*
(ppm)
Common
Pb (%)
207
Inherited Core
C305-14.1 623 70 0.12 56.2 0.15 0.0
C305-11.1 628 47 0.08 61.3 0.54 0.0
Core with oscillatory bands
C305-1.1 725 54 0.08 44.6 0.32 0.0
C305-4.1 418 5+ 0.01 26.1 0.10 0.0
C305-5.2 462 82 0.18 29.9 0.27 0.0
C305-17.2 1211 34 0.03 74.1 0.06 0.0
C305-16.1 643 9 0.02 43.4 0.70 0.0
Mantle with stubby and sector zones
C305-4.2 774 24 0.03 45.2 0.08 0.0
C305-7.2 741 24 0.03 44.5 0.34 0.0
C305-9.1 343 78 0.24 20.0 0.98 0.0
C305-13.1 635 16 0.03 37.3 0.08 0.0
C305-14.2 419 9 0.02 24.3 0.29 0.0
C305-15.1 324 4 0.01 19.6 0.79 0.0
C305-15.2 617 10 0.02 36.4 0.15 0.0
C305-21.1 861 15 0.02 50.3 0.12 0.0
C305-17.1 588 11 0.02 33.6 0.11 0.0
C305-20.1 929 14 0.02 53.1 0.14 0.0
C305-20.2 660 9 0.01 37.9 0.54 0.0
C305-23.1 774 15 0.02 45.0 0.27 0.0
C305-24.1 892 19 0.02 52.0 0.25 0.0
Rim
C305-1.2 258 3 0.01 14.3 0.25 0.0
C305-3.1 484 6 0.01 26.3 0.16 0.0
C305-5.1 412 5 0.01 22.0 0.13 0.0
C305-7.1 657 17 0.03 35.5 0.06 0.0
C305-10.1 1277 34 0.03 67.4 0.12 0.0
C305-14.3 423 5 0.01 23.4 0.28 0.0
C305-15.4 601 15 0.03 32.5 0.29 0.0
C305-19.1 607 13 0.02 33.6 0.11 0.0
C305-21.2 582 21 0.04 31.9 0.32 0.0
C305-22.1 900 13 0.01 49.8 0.16 0.0
Outer rim
C305-6.1 754 14 0.02 36.1 0.15 0.0
C305-8.1 836 12 0.02 40.8 0.10 0.0
C305-12.1 1096 32 0.03 53.8 0.00 0.0
C305-18.1 695 13 0.02 35.2 0.43 0.0
The radiogenic lead Pb* corrected for common Pb using 204Pb. All error
excess of 150 km [29,30]. Similar textures have been
observed in garnets from the Norwegian peridotites
[31]. The exsolution of rutile and sodic amphiboles
[28] further suggest that the inferred majoritic garnets
also contain excess Ti, Na and hydroxyl that are only
soluble at very high pressures. The exsolution of
e Luliangshan garnet peridotite massif, North Qaidam UHP belt
Pb*/ 206Pb* 206Pb / 238U 206Pb / 238U
Age (Ma)
207Pb / 206Pb
Age (Ma)
637F0.0009 0.1049F0.0046 643F27 731F30
759F0.0008 0.1136F0.0029 694F17 1093F20
580F0.0005 0.0716F0.0018 446F11 529F19
541F0.0010 0.0725F0.0018 451F11 374F41
574F0.0010 0.0751F0.0019 467F12 505F37
545F0.0009 0.0712F0.0013 444F8 394F37
577F0.0018 0.0779F0.0016 484F10 519F69
537F0.0008 0.0679F0.0017 423F10 357F35
568F0.0005 0.0698F0.0018 435F11 484F20
622F0.0021 0.0679F0.0018 423F11 683F72
574F0.0010 0.0683F0.0030 426F18 508F40
554F0.0018 0.0673F0.0029 420F18 429F72
565F0.0039 0.0698F0.0015 435F9 473F151
562F0.0012 0.0685F0.0013 427F8 459F47
567F0.0008 0.0679F0.0013 424F8 478F32
571F0.0009 0.0664F0.0013 414F8 495F35
571F0.0013 0.0664F0.0013 415F8 497F51
532F0.0026 0.0665F0.0014 415F8 337F112
563F0.0014 0.0675F0.0013 421F8 465F56
547F0.0014 0.0677F0.0013 422F8 399F56
518F0.0017 0.0640F0.0017 400F10 278F77
540F0.0010 0.0632F0.0016 395F10 373F41
544F0.0012 0.0620F0.0016 388F10 386F49
542F0.0008 0.0628F0.0016 392F10 378F34
563F0.0005 0.0614F0.0016 384F9 465F21
565F0.0012 0.0641F0.0028 401F17 473F47
568F0.0017 0.0627F0.0013 392F8 484F65
595F0.0012 0.0643F0.0012 402F7 584F46
555F0.0011 0.0636F0.0012 397F8 431F44
568F0.0007 0.0643F0.0012 401F7 482F26
549F0.0009 0.0556F0.0014 349F9 406F39
545F0.0007 0.0568F0.0014 356F9 393F30
524F0.0010 0.0572F0.0011 359F7 304F43
543F0.0024 0.0587F0.0012 368F7 382F100
s are 1r.
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 103
ilmenite and Al-chromite needles from the olivine is
also consistent with the peridotite once being equili-
brated at very high pressures (N300 km) [32].
Estimation of Ti and Na contents in the inferred
majoritic garnets gives an equilibrium pressure of z7
GPa [27,28]. Al-in-Opx geobarometry [33] and Grt–
Ol geothermometry [34] yield P=5.0–6.5 GPa and
T=960–1040 8C for the garnet lherzolite samples
2C44, C305 and 2C42. This calculated pressure
range, if reliable, would only represent minimum
values because of the subsequent re-equilibration
during exhumation (e.g., exsolution effects) [27].
2.2. Dunite
The dunite occurs as layers varying in thickness
from tens of centimeters to 1–2 m within the garnet
lherzolite. It is medium-grained and has an equigra-
nular texture dominated by olivine (Fo92–93) (N90
vol.%), with or without garnet, plus variable amounts
of orthopyroxene (Mg#=0.90–0.92), clinopyroxene
Fig. 2. Photomicrographs of mineral inclusions in zircons from garnet lhe
from C305—note that the microdiamond is far beneath the surface, bene
mantle domain. (b) Graphite (Grp) inclusion in a zircon from 2C44. (c) CL
the mantle domain of the zircon very near the core–mantle boundary. (d) G
and (d) are plane polarized lights.
(Mg#=0.94–0.96) and fine-grained Cr-spinel (Cr#=
0.61–0.65). Garnet, if present, is porphyroblastic and
Mg-rich (69–75 mol% pyrope, 11–18% almandine,
3–8% grossular, 0.8–2.0% spessartine and 3–6%
uvarovite). Some garnet crystals, however, are com-
pletely replaced by secondary kelyphitic Opx+Cpx+
Spl assemblages. The Al-in-Opx geobarometry [33]
and Grt–Ol geothermometry [34] yield P=4.6–5.3
GPa and T=980–1130 8C [27] for the primary
mineral assemblage.
2.3. Garnet pyroxenite
The garnet pyroxenite is a minor component,
occurring as 2–5 m thick dikes cross-cutting the
apparent layering of the massif. Most samples are fresh
with pink garnet and pale-green pyroxene conspicuous
in the field. The constituent phases are garnet (20–30
vol.%), orthopyroxene (5–10%), clinopyroxene (40–
60%) and phlogopite (2–5%) with no olivine observed.
It shows a fairly uniform medium-grained granular
rzolite. (a) Diamond (Dia) and clinopyroxene inclusions in a zircon
ath the large clinopyroxene inclusion, and should be located in the
image of (b) showing that the graphite inclusion should be located in
raphite and clinopyroxene inclusions in a zircon from 2C12. (a), (b)
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118104
texture. The garnet is also Mg-rich (62–68 mol.%
pyrope, 21–24%, almandine, 9.5–11% grossular, b1%
spessartine, 0.8–1.5% uvarovite). Most garnets are
rimmed with kelyphitic Opx+Cpx+Spl assemblage
and some break down to granular-textured high-Al
orthopyroxene, clinopyroxene and Al-spinal, suggest-
ing that these rocks have been overprinted by decom-
pression events. The Al-in-Opx geobarometer and two
pyroxene thermometer [33] yield significantly lower P
(2.5–3 GPa) and T (800–900 8C) estimates than garnet
lherzolite and dunite.
Fig. 3. Raman spectroscopic analyses for diamond and graphite in
zircons. (a) Raman spectrum of diamond inclusion in Fig. 2a. (b)
Raman spectrum of graphite inclusion in Fig. 2b.
3. Analytical techniques
The samples were crushed and sieved to ~300 Amfor the first separation and then to ~100 Am for the
second separation. Zircons were separated by combin-
ing magnetic and heavy liquid methods before finally
hand-picking under a binocular microscope. The
zircon crystals were embedded in 25 mm epoxy discs
and then polished down to approximately half thick-
ness. The internal zoning was examined using CL
images at Peking University. The CL images were
obtained on a FEI PHILIPS XL30 SFEG SEM with 2-
min scanning time at conditions of 15 kVand 120 AA.Mineral inclusions in zircons were detected using
laser Raman microspectroscopy (Ranisow RM-1000)
with the 514.5 nm line of an Ar-ion laser at Peking
University.
The zircons were analyzed for U, Th and Pb using
SHRIMP II at Beijing SHRIMP Centre, Chinese
Academy of Geosciences. Instrumental conditions
and measurement procedures were the same as
described by Compston et al. [35]. The spot size of
the ion beam was about 25 Am in diameter, and the
data were collected in sets of five scans through the
masses with 2 nA primary O2� beams. The reference
zircon was analyzed first and then after every three
unknowns. The measured 206Pb / 238U ratios in the
samples were corrected using reference zircon stand-
ard SL13 from a pegmatite from Sri Lanka (572 Ma)
and zircon standard TEMORA (417 Ma) from
Australia [36]. The common-Pb correction used the206Pb / 204Pb ratio and assumed a two-stage evolution
model [37]. The analytical data are summarized in
Tables 1, 3, 4, and graphically presented on Tera–
Wasserburg (TW) diagrams with 1r errors (Figs. 4, 8
and Fig. 10). The ages are weighted means with 2rerrors calculated using isoplot at 95% confidence
levels [38].
Trace element analysis of the zircons was done on
a Perkin Elmer laser-ablation ICP-MS in the MOE
Key Laboratory at the Northwest University, Xi’an,
China [39]. Laser sampling was done with a pulsed
193 nm ArF Excimer laser with 170 mJ energy at a
repetition rate of 10 Hz and pit size of 40 Am. A
helium stream was used to effectively transport the
ablated particles and to reduce deposition at the
ablation site. The helium carrier gas inside the
ablation cell is mixed with argon as a makeup gas
before entering the ICP-MS to maintain stable and
optimum excitation conditions [40]. The SiO2 content
of zircon was used as an internal calibration standard,
and NIST612 was analyzed twice every 5 analyses as
the external standard. Detection limits for most REEs
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 105
were b0.05 ppm. Repeated analysis of standards
yielded precisions better than 10% for most elements.
4. Zircon internal structure, inclusions and
SHRIMP data
4.1. Garnet lherzolite
Two garnet lherzolite samples C305 and 2C44
were chosen for bhuntingQ zircons. Several dozens of
Fig. 4. CL images of analyzed zircons from garnet lherzolite (C305). SHRI
zircon with a core age of 446 Ma and a rim age of 400 Ma. (b) A zircon sho
and a rim age of 388F10 Ma—note oscillatory bands in the core domain. (
oscillatory bands, a weakly luminescent mantle with fir-tree sector zone (42
luminescent core of late Proterozoic age of 694F17 Ma (207Pb / 206Pb ag
mantle–rim texture with mantle ages of 415F8 and 424F8 Ma respective
typical fir-tree or radial sector zones.
zircon grains were recovered from C305, and only six
zircons from 2C44. Zircons from 2C44 were
unstudied by SHRIMP analysis. Zircon crystals from
the garnet lherzolite are colorless, euhedral to sub-
hedral and mostly have oval to elongated shapes.
Their long axes vary from 50 Am up to 250 Am. Most
zircon crystals display clear internal zoning with cores
and rims conspicuous microscopically under trans-
mitted light. Mineral inclusions, including garnet,
olivine, orthopyroxene and clinopyroxene, were
detected using a Raman microspectroscopy. Garnet
MP analysis spots of about 25 Am are seen. (a) The diamond-bearing
wing clear core–mantle–rim texture with a core age of 467F12 Ma
c) A zircon showing a strongly luminescent core (451F11 Ma) with
3F10 Ma) and a very thin rim. (d) Zircon with an inherited weakly
e, 1093F20 Ma, Table 1). (e) and (f) Zircons showing clear core–
ly—note that the mantle domains are weakly luminescent and show
Fig. 5. (a) TW diagrams and (b) histogram of apparent 206Pb / 238U
age of the different zircon domains from the garnet lherzolite
(C305). Five cores yielded a weighted mean age of 457F22 Ma
(MSWD=3.1), twelve mantles yielded a weighted mean age of
423F5 Ma (MSWD=0.56), and, ten rims, 397F6 Ma (MSWD=
0.31). The two age peaks in (b) are correspondent to the mean ages
of mantles and rims, respectively.
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118106
inclusions usually occur in the mantle domains of the
zircon crystals. One microdiamond inclusion was
identified using the Raman spectroscopy (Fig. 2a).
The ovoid single diamond crystal, which is buried
deeply beneath the surface of the host zircon, is about
5 Am in diameter. Careful observation under micro-
scope indicates to be located in the mantle domain
because it is under a N20 Am clinopyroxene inclusion
and beneath the core–mantle boundary. Raman micro-
spectroscopy analysis yields the characteristic dia-
mond band at 1333 cm�1 with zircon characteristic
bands (Fig. 3a). This is the very first microdiamond
recognized in rocks from the North Qaidam–Altun
UHPM belt. To our knowledge, this is perhaps the
second din situT microdiamond recognized in garnet
peridotites associated with UHPM terranes on Earth,
with the first being found in garnet pyroxenite of the
Western Gneiss Region, Norway [41]. Two graphite
inclusions were also identified using Raman micro-
spectroscopy (Fig. 2b) with the characteristic bands at
1366 and 1582 cm�1 (Fig. 3b). CL image shows that
the graphite inclusion is located within the inner part
of a mantle domain close to the magmatic core of a
zircon crystal (Fig. 2c).
CL images show that most zircons possess rather
complex internal structures; cores of relatively strong
luminescence are usually surrounded by weakly
luminescent mantle domains and then by relatively
strongly luminescent rims (Fig. 4). The binterfacesQbetween the core and mantle domains are usually
rounded, whereas the bboundary zonesQ between the
mantle and near-rim domains vary greatly in shape.
Some cores show zoning with oscillatory bands or
sectors (Fig. 4a–d). The weak luminescent mantle
domains show either planar growth banding (Fig. 4a–
d) or fir-tree or radial sector zoning (Fig. 4e–f), which
is similar to inner structure of some granulite-facies
zircons [42] and reveals metamorphic overgrowths.
No inherited older zircons are observed except for two
crystals whose small (b50 Am) rounded cores with a
weak luminescence give older SHRIMP ages (Table 1
and Fig. 4d). Some zircons also exhibit a thin outer
rim with relative strong luminescence in CL images.
Uranium content in zircons from the garnet lherzolite
varies significantly from 258 to 1277 ppm with the
overall level much higher than that in zircons from
garnet-bearing dunite and garnet pyroxenite (see below).
The cores with oscillatory bands yielded Ordovician
ages (444–483Ma, Table 1) with amean of 457F22Ma
(MSWD=3.4) (Fig. 5a). This age is similar to zircon
SHRIMP (Song et al., in review) and Sm–Nd isochron
ages [23] of eclogites from Dulan UHPM terrain in the
east part of the QaidamUHPMbelt, and is also similar to
the zircon SHRIMP age (465F12 Ma) of eclogites
belonging to the North Qilian subduction-zone complex
[43,44]. We interpret this age group (i.e.,~450–470 Ma)
as recording a bpreservedQ phase of oceanic lithospheresubduction and subduction-zone associated magmatism
(see below).
Thirteen analyses of the zircon mantle domains
(weak luminescence in CL images) give SHRIMP206Pb / 238U ages ranging from 435 to 414 Ma with a
mean of 423F5 Ma (MSWD=0.56) (Table 1, Fig.
5b). This age is interpreted to represent the mean age
Table 2
Trace element analyses of zircon from garnet lherzolite, garnet-bearing dunite and garnet pyroxenite
Label Ti Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/
U
LuN/
SmN
Eu /
Eu*
Garnet lherzolite C305
C305-1R 9.16 118 14.82 0.065 3.09 0.104 1.31 1.23 0.428 4.86 1.26 12.22 2.72 10.08 1.95 19.89 3.21 8336 0.618 11.17 602.61 0.02 15.72 0.47
C305-2R 8.67 113 14.13 0.008 1.08 0.011 0.232 1.01 0.244 4.73 1.39 11.37 2.78 9.18 1.72 13.99 1.9 8874 0.586 8.54 429.45 0.02 11.33 0.28
C305-3M 3.24 178 17.56 0.041 1.38 0.027 0.13 1.3 0.248 4.27 1.79 16.73 4.73 18.89 3.89 35.68 5 9701 1.135 10.88 565.08 0.02 23.17 0.29
C305-3R 5.3 170 16.13 0.022 2.53 0.08 0.83 1.94 0.9 7.36 2.13 18.94 4.63 14.09 2.24 19 2.59 8847 0.72 14.71 650.81 0.02 8.04 0.64
C305-4R 5.07 352 19.98 0.06 4.42 0.051 0.52 1.5 0.301 8.21 2.76 33 11.76 48.98 11.39 109.2 15.38 8817 1.71 40.77 566.69 0.07 61.76 0.21
C305-5R 4.67 140 15.37 0.010 1.66 0.006 0.26 0.91 0.322 3.98 1.283 13.32 3.21 13.23 2.76 25.02 3.79 9497 0.579 11.42 629.26 0.02 25.09 0.44
C305-5C 5.64 1456 17.04 0.043 2.13 0.102 1.6 5.08 0.399 23.86 9.61 134.76 52.5 238.98 54.38 520.3 74.48 10,214 0.614 59.06 362.01 0.16 88.31 0.09
C305-6R 7.48 442 16.96 0.011 1.57 0.007 0.25 1.72 0.297 7.97 3.64 42.24 14.44 58.42 12.43 124.85 19.38 10,927 1.375 20.59 584.37 0.04 67.87 0.20
C305-6C 5.43 2257 18.42 0.064 1.41 0.041 1.16 2.86 0.094 28.5 14.93 199.51 76.95 351.48 80.03 740.82 109.49 11,287 0.92 71.34 630.64 0.11 230.60 0.02
C305-7C 7.07 2193 16.83 0.038 2.29 0.133 2 6.34 0.28 37.59 16.1 206.99 77.33 330.48 75.82 712.71 98.78 8510 0.66 86.9 336.71 0.26 93.85 0.04
C305-8R 11 134 14.89 0.009 1.82 0.022 0.33 1.58 0.828 6.8 1.94 14.89 3.52 9.36 1.73 12.43 1.6 8450 0.434 10.49 510.8 0.02 6.10 0.66
Garnet-bearing dunite 2C39
2c39-1.1 6.69 30.93 16.57 0.06 1.52 0.042 0.093 0.077 0.094 0.083 0.043 0.29 0.043 0.275 0.062 0.35 0.0105 9128 0.357 1.95 25.72 0.08 0.82 3.57
2c39-7.1 5.41 31.48 14.37 0.056 3.35 0.045 0.23 0.191 0.183 0.78 0.119 0.93 0.142 0.64 0.09 0.82 0.12 5283 0.194 18.6 72.29 0.26 3.78 1.25
2c39-8.1 5.71 30.47 15.81 0.095 11.42 0.122 1.02 0.24 0.155 0.51 0.059 0.51 0.172 0.52 0.092 0.99 0.137 9268 0.402 25.1 89.7 0.28 3.44 1.32
2c39-3.1 3.5 30 14.67 0.044 3.4 0.05 0.43 0.46 0.106 0.75 0.15 0.53 0.158 0.445 0.028 0.52 0.033 6443 0.294 8.85 33.17 0.27 0.43 0.55
Garnet pyroxenite 2C12 and 2C78
2c12-1 4.83 29.81 14.23 0.05 3.98 0.058 0.285 0.444 0.21 0.46 0.066 0.758 0.14 0.429 0.066 0.308 0.039 13,360 0.166 8.31 19.95 0.42 0.53 1.41
2c12-2 3.27 27.96 13.52 0.0238 0.728 b0.00 0.076 0.063 0.057 0.069 0.0188 0.134 0.064 0.256 0.041 0.235 0.061 11,430 0.162 0.916 25.19 0.04 5.83 2.63
2c12-3 5.19 29.98 14.18 0.047 7.41 0.099 0.74 0.41 0.245 1.22 0.134 0.79 0.181 0.699 0.108 0.67 0.06 11,643 0.261 17.9 47.67 0.38 0.88 0.98
2c12-4 4.85 27.75 13.75 0.0144 0.967 b0.00 0.231 0.137 0.021 0.375 0.031 0.162 0.034 0.189 0.059 0.52 0.055 12,509 0.296 2.21 106.73 0.02 2.42 0.27
2c12-5 6.87 28.53 14.35 0.093 3.09 0.037 0.267 0.256 0.077 0.382 0.067 0.367 0.113 0.202 0.033 0.55 0.044 13,803 0.099 8.4 25.53 0.33 1.04 0.75
2C78-1 6.15 32.31 14.13 0.048 4.43 0.048 0.46 0.38 0.108 0.67 0.135 0.71 0.186 0.52 0.051 0.27 0.072 14,744 0.258 11.73 43.84 0.27 1.14 0.65
2C78-2 5.63 32.95 15.85 0.058 4.42 0.071 0.4 0.28 0.083 0.65 0.056 0.85 0.167 0.64 0.113 0.78 0.132 11,937 0.288 19.41 66.87 0.29 2.84 0.57
2C78-3 6.25 31.95 14.07 0.127 4.63 0.072 0.51 0.23 0.091 0.65 0.083 0.39 0.131 0.342 0.042 0.52 0.138 14,699 0.148 19.9 130.82 0.15 3.61 0.67
2C78-4 4.63 32.64 15.14 0.075 4.7 0.033 0.26 0.41 0.155 0.56 0.139 0.64 0.151 0.263 0.072 0.36 0.079 14,498 0.17 10.7 47.62 0.22 1.16 0.99
2C78-5 6.73 31.12 14.68 0.099 4.12 0.07 0.52 0.35 0.186 0.87 0.091 0.59 0.192 0.49 0.078 0.5 0.108 15,395 0.189 7.88 35.42 0.22 1.86 0.98
R=rim; M=mantle; C=core; LuN /SmN=normalized to chondrite after Sun and McDonough [45].
S.Songet
al./Earth
andPlaneta
ryScien
ceLetters
234(2005)99–118
107
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118108
of the UHPM event of the garnet lherzolite, which is
significantly (~30–40 m.y.) younger than the mean
zircon core age (457F22 Ma) and the eclogite ages in
the region [19,23,44] (Fig. 1B). Ten analyses on
zircon rims yield ages of early Devonian with a mean
of 397F6 Ma (MSWD=0.31). The latter is ~20 m.y.
younger than the age of the interpreted major
metamorphic event recorded in the zircon mantle
domains. Four analyses on zircon outer rims give even
younger ages, ~349–367 Ma with a mean of 358F8
Ma. Fig. 5c shows these age variations with two major
peaks at 400 and 423 Ma, which are consistent with
mean ages of rim (397F6 Ma) and mantle (423F5
Ma) domains, respectively.
The two inherited cores that yield much older
discordant ages (643 and 694 Ma) show large
scattered isotopic ratios (Fig. 5a), indicating that they
have been affected by several Pb-loss events. If it is
assumed that Pb loss from the cores occurred at the
time of the latest thermal event that affected the rocks
(ca. 400 Ma), the upper intercepts of presumed mixing
lines (discordias) indicate that the inherited cores may
have grown in the Proterozoic (ca. 850–1400 Ma).
Fig. 6. Chondrite normalized rare earth element (REE) patterns of (a) zirc
C305; (b) zircon from garnet-bearing dunite 2C39; (c) and (d) zircon fro
McDonough [45].
Three spots at cores, one at a mantle and eight at
rim regions (three are mantle-rim mixed) were
analyzed for trace elements by LA-ICP-MS after
SHRIMP dating. The core domains have elevated
abundances of heavy rare earth elements (HREE; e.g.,
Yb=520–740 ppm and Y=1456–2257 ppm) with
steep light-heavy REE slopes (e.g., LuN /SmN=88–
231) on chondrite-normalized [45] REE plots. The
positive Ce anomaly is consistent with Ce4+(vs.
La3+and Pr3+) being more compatible in zircon
(substitutes Zr4+, Hf4+, U4+, Th4+, etc.). The bexcessQCe4+(vs. Ce3+) is certainly produced under oxidizing
conditions, but could very well be inherited from
sources materials of these zircon crystals. The weak Eu
anomaly (Eu /Eu*=0.02–0.09) and a relatively high
Th /U ratio (0.11–0.26) (Table 2, Fig. 6a) is common
in magmatic zircons [46–50]. The large positive Ce
anomalies reflect an oxidizing environment with Ce4+
much more compatible (than La3+ and Pr3+) in
zircons. The mantle to rim domains, on the other
hand, show significant differences in composition
from the core domains. They are characterized by low
Th /U ratios (0.02–0.07), low Th, Y and REE
on cores and metamorphic mantles and rims from garnet lherzolite
m garnet pyroxenites 2C12 and 2C78. Normalization after Sun and
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 109
concentrations, and subdued enrichment in HREE
(LuN /SmN=6–68) and smaller negative Eu anomalies
(Eu /Eu*=0.20–0.66), which suggest metamorphic
overgrowths under sub-solidus conditions [48,49].
The lack of significant variations of Ti4+, Hf4+, Nb5+
and Ta5+ concentrations in various domains is
consistent with these elements being compatible in
zircon and determined by the distribution of Zr4+.
4.2. Garnet-bearing dunite
By contrast, zircons from the garnet-bearing dunite
(e.g., sample 2C39) are colorless, rounded to ovoid in
Fig. 7. CL images of zircons from the garnet-bearing dunite (2C39). (a)
strongly luminescent rims, the ages of cores (440–447 Ma) are the same
zircons with ages of 425F19 Ma and 408F10 Ma—note that their lumin
420F15 Ma with a thin, strongly luminescent rim.
shape (similar to those typical high-grade metamorphic
zircons [42]), and ~50–150 Am in diameter. CL studies
reveal that most zircons in the dunite are fairly
homogeneous in their internal structures (e.g., Fig.
7d–e), and only a few exhibit weakly luminescent CL
cores surrounded by thin and relatively strongly
luminescent CL rims that are as bright as homogeneous
ones (Fig. 7a–c). Some are further surrounded by even
stronger luminescent rims (Fig. 7f). Garnet and olivine
inclusions are observed in some zircons and confirmed
by Raman spectroscopy and electron microprobe.
Twenty-two zircon grains from the dunite were
analyzed using SHRIMP and four grains were
, (b) and (c) zircons with weakly luminescent cores and relatively
as cores from garnet lherzolite (Fig. 5). (d) and (e) homogeneous
escence is as bright as the zircon rims in images a–c. (f) Zircon of
Fig. 8. TW diagram of SHRIMP analyses of zircons from the
garnet-bearing dunite (2C39).
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118110
analyzed by LA-ICP-MS. They have low HREE (e.g.,
Yb [meanF1r]=0.67F0.29 ppm), low Y (30.72F0.63 ppm), low [Lu /Sm]CN ratios (2.11F1.74) (Table
2) with relatively flat REE patterns (Fig. 6b), and very
low U (51F31 ppm) (Table 3), all of these being
much lower yet more uniform than in zircons of the
garnet lherzolite. Th /U ratio (0.08–0.85 by SHRIMP)
shows no spatial systematics from core to rim (Table
3). The large positive Ce anomalies (15F6) can be
explained similarly (see above). Both morphologies
and trace element systematics of zircons in the dunite
are best interpreted them as being of metamorphic
origin.
The data from the cores are mostly disconcordant
in the Tera–Wasserburg (TW) diagram (Fig. 8),
suggesting that they may have been affected by
subsequent overgrowth or Pb loss. They give apparent206Pb / 238U ages ranging from 461 to 440 Ma with a
mean of 446F13 Ma (MSWD=0.67), similar to
zircon core ages of the garnet lherzolite. However, the
15 homogeneous zircons give younger ages of 430 to
Table 3
U, Th and Pb SHRIMP zircon data of the garnet-bearing dunite 2C39 from the Luliangshan garnet peridotite massif, North Qaidam UHP bel
Spot U
(ppm)
Th
(ppm)
Th/U Pb*
(ppm)
Common
Pb (%)
207Pb*/ 206Pb* 206Pb / 238U 206Pb / 238U
Age (Ma)
Zircon cores
39-3.1 28 7 0.27 1.8 2.74 0.0824F0.0093 0.0741F0.0020 461F12
39-4.1 41 8 0.20 2.6 2.96 0.0747F0.0079 0.0706F0.0018 440F11
39-17.1 23 5 0.21 1.4 0.88 0.1177F0.0086 0.0709F0.0019 441F11
39-19.1 6 0.5 0.08 0.5 15.04 0.0518F0.0302 0.0718F0.0056 447F34
Homogeneous zircons
39-1.1 22 2 0.08 1.3 6.85 0.0256F0.0198 0.0654F0.0023 409F14
39-2.1 45 13 0.31 2.6 3.59 0.0385F0.0092 0.0653F0.0016 408F10
39-5.1 19 2 0.11 1.2 9.04 0.0721F0.0207 0.0673F0.0024 420F15
39-6.1 16 5 0.31 1.1 13.80 0.0352F0.0311 0.0681F0.0031 425F19
39-7.1 68 15 0.23 4.1 1.55 0.0660F0.0072 0.0688F0.0016 429F10
39-8.1 91 28 0.32 5.1 1.85 0.0595F0.0049 0.0642F0.0014 401F8
39-9.1 33 6 0.18 1.9 3.09 0.0768F0.0105 0.0649F0.0017 405F10
39-10.1 58 34 0.61 3.4 1.94 0.0714F0.0056 0.0661F0.0015 413F9
39-11.1 26 8 0.31 1.6 4.46 0.0791F0.0121 0.0681F0.0019 425F12
39-12.1 70 35 0.51 4.1 1.74 0.0609F0.0047 0.0669F0.0015 418F9
39-13.1 96 12 0.13 5.6 0.42 0.0688F0.0024 0.0677F0.0014 423F9
39-14.1 46 8 0.17 2.8 2.73 0.0630F0.0081 0.0687F0.0017 428F10
39-15.1 66 34 0.53 3.9 2.97 0.0610F0.0083 0.0671F0.0016 419F10
39-16.1 134 88 0.68 7.3 1.06 0.0567F0.0026 0.0621F0.0013 391F8
39-20.1 61 6 0.11 3.6 1.98 0.0707F0.0069 0.0676F0.0016 422F9
39-21.1 31 3 0.09 1.9 3.53 0.0728F0.0119 0.0670F0.0018 418F11
39-22.1 72 59 0.85 4.3 2.26 0.0642F0.0078 0.0683F0.0015 426F9
39-23.1 66 13 0.21 3.9 1.57 0.0681F0.0059 0.0667F0.0015 416F9
The radiogenic lead Pb* corrected for common Pb using 204Pb. All errors are 1r.
408 Ma with a mean of 420F5 Ma (MSWD=0.39).
This latter mean age corresponds to zircon mantle
domain ages of the garnet lherzolite, which may
represent the timing of the UHPM event. This
t
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 111
interpretation is sensible because both morphologies
and trace element systematics are consistent with the
zircon being of metamorphic origin. The presence of
the older cores in the four zircon crystals suggests that
magmatic zircons may have existed sparsely in the
dunite prior to the metamorphism. It is possible that
these rounded zircons could be produced during the
metamorphism although nucleation of zircon crystals
from highly depleted harzburgite (the protolith) under
solid condition seems difficult, let alone to grow into
zircon crystals of appreciable size. Three fine-grained
zircons (b50 Am) yielded ages of ~400 Ma (405, 401
and 391 Ma, respectively), the same as zircon rim
Fig. 9. CL images of zircons from garnet pyroxenites (2C12, 2C78). (a) Zi
age identical to core ages from the former two types of rocks), a mantle and
of 416 and 375 Ma. (d) Zircon from 2C12 surrounded by a thin and discon
wide strongly luminescent rims of ages at 355–358 Ma.
ages of the garnet lherzolite. CL images show that
they are homogeneous with stronger luminescence
than zircons described above (e.g. Fig. 7d–e).
4.3. Garnet pyroxenite
Zircons from garnet pyroxenite samples 2C12 and
2C78 are colorless, rounded, and variable in size
(~50–100 Am in diameter). Their morphology and
internal structure of these zircons are similar to those
of zircons in the garnet-bearing dunite. Raman
spectroscopy identified garnet, clinopyroxene, ortho-
pyroxene and rare graphite inclusions (Fig. 2d). The
rcon from 2C12 with a weakly luminescent core (of late Ordovician
a bright rim. (b) and (c) homogeneous zircons from 2C12 with ages
tinuous brightly luminescent rim. (e) and (f) zircons from 2C78 with
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118112
CL images show that most zircons from the two
samples are quite uniform (Fig. 9b, c). Only one
shows a weak luminescent core that is surrounded by
a relatively stronger luminescent rim (Fig. 9a). The
CL images suggest that some zircons from the garnet
pyroxenite may have grown in two stages or perhaps
reflect subsequent equilibration/modifications.
Twenty-five zircon grains from the two samples
were studied using SHRIMP and ten grains from
2C78 were studied by LA-ICP-MS. Similar to zircons
from the garnet dunite, zircons in the garnet pyrox-
enite also have very low HREE (e.g., Yb [meanF1r]=0.47F0.18 ppm), low Y (30.5F1.97 ppm), low
Ta (0.20F0.07 ppm), low [Lu / Sm]CN ratios
(2.13F1.63) (Table 2) with relatively flat REE
patterns (Fig. 6c–d), low U (62F47 ppm), and high
Th /U (0.23F0.12) (Table 4). The Hf content
(13,402F1443 ppm) is significantly higher than that
Table 4
U, Th and Pb SHRIMP zircon data of the garnet pyroxenite (2C12 and 2
Spot U
(ppm)
Th
(ppm)
Th /U Pb*
(ppm)
Common
(%)
2C12
1.1 10 2 0.20 0.67 3.68
2.1 (C) 9 2 0.22 0.67 19.31
3.1 21 9 0.43 1.22 11.31
4.1 41 12 0.29 2.24 8.76
5.1 10 0.4 0.04 0.67 12.53
6.1 12 3 0.25 0.76 13.58
7.1 40 19 0.48 2.37 5.80
8.1 18 5 0.28 1.16 14.72
9.1 52 21 0.40 3.10 4.85
10.1 22 8 0.36 1.28 4.90
2C78
1.1 47 13 0.28 2.57 3.16
2.1 31 1 0.03 1.66 –
4.1 126 26 0.21 6.96 0.54
5.1 167 53 0.32 9.68 0.11
6.1 (R) 118 28 0.25 5.71 –
7.1 (R) 100 19 0.20 4.86 0.14
8.1 (R) 77 23 0.31 3.75 –
9.1 90 13 0.14 5.07 0.73
10.1 161 24 0.15 9.32 0.47
11.1 61 14 0.23 3.27 2.43
12.1 31 1 0.03 1.73 3.95
13.1 (R) 83 15 0.19 4.13 2.36
14.1 48 7 0.15 2.48 2.10
15.1 86 26 0.30 4.65 –
17.1 (R) 97 34 0.35 4.99 1.66
The radiogenic lead Pb* corrected for common Pb using 204Pb. All error
in zircons of the garnet lherzolite (Hf 9405F1020
ppm) and garnet-bearing dunite (7530F1984 ppm).
The morphology and trace element systematics of
zircons in the garnet pyroxenite suggest that they may
also be of metamorphic origin. In sample 2C12, the
weakly luminescent core in Fig. 9a yields an apparent206Pb / 238U age of 443F17 Ma, similar to the zircon
core ages of the garnet lherzolite and dunite; other
nine data points of homogeneous zircon grains in
sample 2C12 give apparent 206Pb / 238U ages of 416–
372 Ma with a mean of 397F13 Ma (MSWD=1.5)
(Fig. 10a). The strongly luminescent rims are too thin
to be analyzed. Some analyses show large errors
because of very low radiogenic lead. Fifteen data
points in sample 2C78 give two groups of 206Pb / 238U
ages. Ten analyses of homogeneous grains give ages
of 420–371 Ma with a mean of 400F12 Ma
(MSWD=3.7). Five analyses of the strong lumines-
C78) from the Luliangshan garnet peridotite massif, North Qaidam
Pb 207Pb*/ 206Pb* 206Pb / 238U 206Pb / 238U
Age (Ma)
0.0603F0.0115 0.0666F0.0024 415.8F20.8
0.0403F0.0153 0.0711F0.0028 443.0F16.9
0.0482F0.0095 0.0593F0.0022 371.6F17.5
0.0527F0.0056 0.0599F0.0016 374.7F11.3
0.0350F0.0096 0.0637F0.0022 398.0F18.4
0.0470F0.0150 0.0608F0.0020 380.3F17.9
0.0553F0.0032 0.0658F0.0016 410.8F11.2
0.0447F0.0067 0.0651F0.0021 406.8F16.3
0.0536F0.0026 0.0661F0.0015 412.8F10.4
0.0325F0.0051 0.0622F0.0017 389.3F13.2
0.0532F0.0090 0.0612F0.0018 383.0F11.0
0.0867F0.0049 0.0643F0.0019 402.0F12.0
0.0572F0.0042 0.0637F0.0012 398.3F7.2
0.0580F0.0022 0.0673F0.0011 419.6F7.0
0.0663F0.0025 0.0567F0.0011 355.3F6.6
0.0591F0.0051 0.0566F0.0011 354.6F7.0
0.0673F0.0029 0.0571F0.0016 357.9F9.8
0.0562F0.0025 0.0653F0.0012 407.8F7.3
0.0543F0.0024 0.0669F0.0011 417.7F7.0
0.0415F0.0066 0.0613F0.0013 383.3F7.9
0.0338F0.0068 0.0627F0.0021 392.0F13.0
0.0433F0.0043 0.0565F0.0012 354.3F7.1
0.0528F0.0084 0.0593F0.0015 371.4F8.9
0.0656F0.0092 0.0635F0.0014 396.9F8.5
0.0487F0.0090 0.0587F0.0018 367.7F7.5
s are 1r. C=core, R=rim.
Fig. 10. TW diagrams and histogram of apparent 206Pb / 238U age of the different zircon domains from garnet pyroxenites. (a) TW diagram of
zircons from 2C12 with a mean age of 397F13 Ma. (b) TW diagram of zircons from 2C78 with a mean age of 400F12 Ma and a retrograde
age of 358F7 Ma. (c) TW diagram of all zircons from 2C12 and 2C78. (d) Histogram of apparent 206Pb / 238U age of all SHRIMP analyses of
zircons from garnet pyroxenites with peaks at 358 and 400 Ma that are consistent with the mean ages.
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 113
cent rims give uniform ages of 354–368 Ma (358F7,
MSWD=0.58) (Fig. 10b and Table 4). Fig. 10d show
all these data together in the form of histogram. The
superimposed probability curve for 206Pb / 238U ages
exhibits two peaks at 358 and 400 Ma, respectively,
which are consistent with mean ages shown in Fig.
10b–c. However, the large range from 420 to 371 Ma,
the wide 400-Ma-peak and the large MSWD (=2.5)
for the mean age of 399F8 Ma out of 19 analyses
from the two samples (Fig. 10c) do not show
statistically a single population, but suggest that
zircons in the garnet pyroxenite probably formed in
a long period of time from the Late Silurian to
Devonian. This mean age corresponds to the zircon
rim age of garnet lherzolite, whereas the age value
358 Ma in 2C78 corresponds to ages of the outer rims
of zircons from the garnet lherzolite. Given the
likelihood of the metamorphic origin of zircons in
the garnet pyroxenite, we can infer that the protoliths
of the garnet pyroxenite were emplaced earlier than
these zircon ages.
5. Discussion
5.1. Protolith of garnet lherzolite and the significance
of the 457F22 Ma event
Zircons from the garnet lherzolite record 4 major
events during a ~110 Myr period: (a) the 457F22 Ma
event recorded in the zircon cores, (b) the 423F5 Ma
event indicated by the zircon mantle domains, (c) the
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118114
397F6 Ma event by the zircon rim regions, and (d)
the 368–349 Ma event suggested by the outermost
rims of zircon crystals. The data are mostly con-
cordant in the Tera–Wasserburg (TW) diagram (Fig.
5). Such a core-to-rim younger age trend in zircons is
commonly interpreted as resulting from multiple-stage
zircon growth in response to younger geological
events. The elevated HREE abundances and large
negative Eu-anomaly patterns of the core domains are
consistent with the cores being of magmatic origin-
crystallized from a melt at ~457F22 Ma at depths
where plagioclase or spinel is stable, i.e. before
ultrahigh-pressure metamorphism. Given the fact that
the garnet porphyroblasts are clearly of metamorphic
origin and that the olivine has variably low Fo
(Mg#=0.84–0.91) in spite of the fact that the garnet
lherzolite has recrystallized during UHP metamor-
phism, we can infer that the protolith of the lherzolite
is neither fertile asthenospheric mantle material, nor
mantle melting residues, but is most likely ultramafic
cumulate from a cooling basaltic melt (Mg#=0.56–
0.72) [51]. Assuming that these zircon cores were co-
precipitated from a melt with other cumulate phases
(i.e., olivine, orthopyroxene, clinopyroxene etc.) and
that the bulk-rock Mg# had not changed during the
subsequent ultrahigh-pressure metamorphism, the
estimated liquidus temperature would be ~1197F26
8C [51]. Such relatively low liquidus temperatures
(low Mg# of the minerals and the bulk rock
compositions) argue for basaltic (vs. ultramafic)
parental melts. Given that the liquidus phases are
dominated by orthopyroxene, clinopyroxene and
olivine without plagioclase (too low Al2O3 and CaO
in the bulk-rock compositions), we can infer that the
crystallization pressures are significantly higher than
0.8 GPa [52] or the melts are rich in water that
suppresses plagioclase but enhances clinopyroxene on
the liquidus [53]. Such conditions are hard to meet
beneath normal ocean ridges, but readily satisfied in a
mantle wedge overlying a subduction zone.
We can thus further infer that the protoliths of the
garnet lherzolite represent a cumulate assemblage from
a mantle wedge basaltic melt. The exact depth and/or
bspaceQ in which the melt crystallized is unknown, but
it is probably in the sub-arc lithospheric mantle. The
subduction zone could either be ocean–ocean type
(e.g., western Pacific) or ocean–continent (the Andean)
type, but the latter is favored because: (1) The sub-arc
continental (vs. oceanic) lithospheric mantle, overlying
the convective mantle wedge, would be ancient, and
may be the source of inherited cores of zircons (ca.
1400–850 Ma) (Fig. 5a). (2) The subcontinental
lithospheric mantle is mostly compositionally depleted
(e.g., Mg#N0.91), which may be represented by the
spinel harzburgite, the protolith of bgarnet duniteQ, andinto which the melt parental to the cumulate (protoliths
of the garnet lherzolite) may have intruded, perhaps a
sub-arc lithospheric magma chamber (?). (Note that
mantle wedge melts are basaltic, but they become
felsic/andesitic when they are contaminated by con-
tinental crust on their way to the surface at the Andean
type active continental margins).
All these inferences allow us to hypothesize an
Early Paleozoic subduction system of Andean type:
(1) the ~458 Ma (zircon SHRIMP and Sm/Nd bulk-
rock mineral isochron ages) eclogites of MORB/OIB
protoliths from the same UHPM belt [23] (Fig. 1), and
the 464F12 Ma (SHRIMP zircon age) eclogites of
North Qilian subduction-zone complex north of the
area [44] represent the bsnapshotQ age of subduction-
zone eclogite facies metamorphism of oceanic litho-
sphere at that time, whereas (2) the 457F22 Ma age
from the garnet lherzolite zircon cores represent
subduction-zone associated mantle wedge magma
genesis at the same time. We infer that the preserva-
tion of such coeval slab metamorphism and mantle
wedge magmatism may have resulted from magmatic
cumulate (the protolith of the garnet lherzolite) being
dragged to depth by the down-going subducting slab,
followed by continental subduction, continental colli-
sion, and the collision-induced tectonic exhumation.
5.2. From a magmatic cumulate to garnet lherzolite
as a result of UHPM
The magmatic cumulate, which is inferred to have
formed in a deep level of sub-arc lithosphere, could be
dragged down to great mantle depths by the subducting
continental lithosphere once the oceanic lithosphere
had been consumed. The subducting slab may still be
cold at great depths, but the dragged magmatic
cumulate overlying the upper interface of the subduct-
ing slab remains hot (N900 8C) and undergoes meta-
morphism at progressively elevated pressures, at which
conditions, garnet porphyroblasts formed and olivine,
orthopyroxene and clinopyroxene recrystallized in
Fig. 11. P–T–time path for three rock types in the Luliangshan
garnet peridotite massif from the North Qaidam UHP metamorphic
belt. The SHRIMP analyses distinguished 4 major stages: 440–484
Ma for the magmatic origin (stage 1), 414–435 Ma (mean at 423F5
Ma, garnet lherzolite) and 408–429 Ma (mean at 420F6 Ma
garnet-bearing dunite) for ultrahigh-pressure metamorphic even
(stage 2), ~400 Ma for retrograde overprinting event during
exhumation (stage 3), and 349–368 Ma for post-orogenic retrograde
event. Path from stage 1 to 2 is not firmly constrained. The
diamond–graphite and coesite–quartz transitions are from Bundy
[56] and Bohlen and Boettcher [57].
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 115
response to increased pressures. The entrapment of
graphite and diamond inclusions in the mantle domains
of zircons suggesting a progressive metamorphism as
the subduction continued. Formation of diamond
suggests a pressure N4.5 GPa for T N1000 8C,equivalent to N~150 km in depth. Exsolutions from
the garnet porphyroblasts of pyroxenes (rich in Si),
rutile (rich in Ti) and sodic amphiboles (rich in Na, Ti
and hydroxyl) suggest a majoritic garnet formation/
equilibration at pressure N7.0 GPa or N200 km in depth
[27,28]. Exsolutions of ilmenite rods from olivine
suggest even higher pressures [27]. These estimates are
roughly consistent with P–T conditions (P=5.0–6.5
GPa and T=960–1040 8C) estimated using Al-in-Opx
geobarometry [33] and Grt–Ol geothermometry [34].
The age of the mantle domains of the zircon crystals
with the diamond and garnet inclusions, from 414 to
435 Ma with a mean at 423F5 Ma, may represent the
UHP metamorphic stage associated with continental
lithosphere subduction. On the other hand, the ages of
397F6 Ma may represent the timing of garnet
lherzolite exhumation, whereas the ages of 368–349
Ma represent effects of post-orogenic thermal events at
conditions below the garnet-spinal peridotite transition
(e.g., P b1.5 GPa and T b750 8C [54]).
5.3. From a residual harzburgite to garnet-bearing
dunite as a result of UHPM
The garnet-bearing dunite recognized in the field
and by petrography resembles in composition a highly
depleted harzburgite, and would be typical of Proter-
ozoic subcontinental lithospheric mantle [55]. This
allows us to infer that basaltic melt, parental to the
protolith of the bgarnet lherzoliteQ, may have been
derived from the mantle wedge and intruded into this
harzburgitic lithospheric mantle. This ancient subcon-
tinental lithospheric mantle may be the source for the
ancient (Fig. 3e and Table 1) zircon relics recognized
in the cores of the two zircon grains in the garnet
lherzolite. The 446F12 Ma cores of some zoned
zircon grains (Fig. 7a–b; Table 3) may in turn come
from the infiltrating melt. Volumetrically small
amounts of this ancient lithospheric material may be
included with the enclosed magmatic cumulate and
then dragged deep into the mantle by the subducting
continental lithosphere. The porphyroblastic garnets
and the metamorphic zircons of 420F5 Ma, which
are essentially the same age as the mantle domains of
zircons (423F5 Ma) from garnet lherzolite, may have
formed during this UHPM deep in the mantle over-
lying the subducting continental crust atop the litho-
spheric slab. The P–T conditions (P=4.6–5.3 GPa
and T=980–1130 8C) are also within the same P–T
ranges derived from the garnet lherzolite above.
5.4. The origin of garnet pyroxenite
The cross-cutting relationship of the garnet pyrox-
enite dikes suggests that their emplacement is rather
late, and significantly later than the peak metamorphic
event of ~423F5 Ma. The rounded morphology of
their zircons (Fig. 9), their uniform composition with
rather flat REE patterns (Fig. 6c–d), and their rather
young average age (388F21 Ma; Table 4) suggest
that these zircons are of metamorphic origin and
formed in response to several events. The large age
range (443 to 354 Ma) reflects rather complex thermal
,
t
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118116
histories as suggested by the younger ages of the rim
and outer-rim regions of zircons from the garnet
lherzolite. These ages are as young as ~349F9 Ma
(Table 1). The exact timing of emplacement of the
garnet pyroxenite would likely be in between 423F5
Ma and 388F21 Ma. This would further suggest that
these dikes must also have experienced subduction-
zone metamorphism. Graphite inclusions in zircon
crystals records conditions below the diamond–graph-
ite transition [56] that are consistent with the P–T
regime (P=2.5–3 GPa and T=800–900 8C) describedin Section 2.3.
5.5. P–T–time path
The ages obtained from SHRIMP analyses of
different zircons and their different domains from
the three rock types suggest four major discrete stages
of zircon growth: Stage 1, 440–486 Ma for magmatic
cores; Stage 2, 423F6 Ma for mantling zircons of
garnet lherzolite and 420F5 Ma for zircons of garnet-
bearing dunite; Stage 3, 397F6 Ma for rims of garnet
lherzolite zircons and 399F8 Ma for garnet pyrox-
enite zircons, and Stage 4, 346–368 Ma for outer rims
of garnet peridotite zircons. Based on the P–T
conditions of the three rock types described above, a
reasonable P–T–time path for the garnet peridotite
massif can be outlined in Fig. 11.
6. Conclusions
1. The Lqliangshan garnet peridotite massif has a
complex history. The zircon core ages of the three
types of rocks, ranging from 440 to 483 Ma,
correspond to the UHP metamorphic ages of
eclogites from the North Qaidam UHP belt. REE
patterns and oscillatory zoning in zircon cores from
the garnet lherzolite suggest that the main body
represents an ultramafic cumulate precipitated from
mantle wedge derived Mg-rich basaltic melt that
formed during oceanic lithosphere subduction,
which may be genetically associated with dehy-
dration of the subducting oceanic lithosphere.
2. The garnet peridotites must have subducted to
depths of greater than 200 km at 423F5 Ma during
continental subduction, dragged down by the
former down-going oceanic lithosphere slab, as
indicated by various exsolution textures in por-
phyroblastic garnet and olivine. Presence of a
diamond inclusion in the mantle domain of a zircon
crystal corroborates the subducted depth of N150
km. The peak UHPM timing of rocks from the
Qaidam UHP belt is therefore inferred to be 423F5
Ma, i.e., the time of subduction cessation as a result
of continental collision (the breakoff of the oceanic
lithosphere). This age is also consistent with
SHRIMP U–Pb ages of coesite-bearing zircons
from the pelitic gneisses (Song et al. in preparation).
3. Morphology, CL images and REE patterns suggest
that zircons from garnet-bearing dunite and garnet
pyroxenite are of metamorphic origin. The former
with the age of 420F5 Ma coincides with inferred
peak UHPM stage of the subducted continental
materials of the North Qaidam orogenic belt. The
later with the age of 399F8 Ma maybe genetically
associated with the exhumation. It was further
overprinted by the 355F7 Ma post-orogenic
retrograde events.
Acknowledgements
We thank B. Song and Y.S. Wan for their
assistance in lab work on SHRIMP dating, X.M.
Liu for his assistance on trace element analysis,
G.P. Brey for providing his P–T calculation
program, S.-S. Sun, J.-J. Yang and M.J. O’Hara
for discussion and critical comments, which helped
improve the manuscript. We also thank David Root
and Laurra Webb for their detailed and rather
constructive official review comments, and M.J.
O’Hara for smoothing the prose, which led to a
better presentation of the final product. This work is
financially supported by National Natural Science
Foundation of China (grants # 40272031,
40372031, 40228003, 40325005), Major State Basic
Research Development Projects (G1999075508) and
Key Laboratory foundation of Northwest University.
References
[1] J.J. Yang, G. Godard, J.R. Kienast, Y. Lu, J. Sun, Ultrahigh-
pressure (60 kbar) magnesite-bearing garnet peridotites from
northeastern Jiangsu, China, J. Geol. 101 (1993) 541–554.
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118 117
[2] R.Y. Zhang, J.G. Liou, B.L. Cong, Petrogenesis of garnet-
bearing ultramafic rocks and associated eclogites in the Su–Lu
ultrahigh-P metamorphic terrane, eastern China, J. Meta-
morph. Geol. 12 (1994) 169–186.
[3] J.G. Liou, R.Y. Zhang, Petrogenesis of ultrahigh-P garnet-
bearing ultramafic body from Maowu, the Dabie Mountains,
Central China, Isl. Arc 7 (1998) 115–134.
[4] L.M. Medaris, D.A. Carswell, Petrogenesis of Mg–Cr garnet
peridotites in European metamorphic belts, in: D.A.
Carswell (Ed.), Eclogite Facies Rocks, Blackie, Glasgow,
1990, pp. 260–291.
[5] L.M. Medaris, Garnet peridotite in Eurasian HP and UHP
terranes: a diversity of origins and thermal histories, Int. Geol.
Rev. 41 (1999) 799–815.
[6] H.K. Brueckner, L.G. Medaris, A general model for the
intrusion and evolution of dmantleT garnet peridotites in high-
pressure and ultra-high-pressure metamorphic terranes, J.
Metamorph. Geol. 18 (2000) 123–133.
[7] H.W. Green, L. Dobrzhinetskaya, E.M. Riggs, Z.-M. Jin, Alpe
Arami: a peridotite massif from the mantle transition zone?
Tectonophysics 279 (1997) 1–21.
[8] A. Kadarusman, C.D. Parkinson, Petrology and P–T evolution
of garnet peridotite from central Sulawesi, Indonesia, J.
Metamorph. Geol. 18 (2000) 193–209.
[9] J.G. Liou, D.A. Carswell, Preface: garnet peridotites and
ultrahigh-pressure minerals, J. Metamorph. Geol. 18 (2000)
121.
[10] B. Jamtveit, D.A. Carswell, E.M. Mearns, Chronology of the
high-pressure metamorphism of Norwegian Garnet peridotites/
pyroxenites, J. Metamorph. Geol. 9 (1991) 1–15.
[11] H.K. Brueckner, L.G. Medaris, A tale of two orogens: the
contrasting T–P–t history and geochemical evolution of
mantle in high and ultrahigh pressure metamorphic terranes
of the Norwegian Caledonides and the Czech Variscides,
Schweiz. Mineral. Petrogr. Mitt. 78 (1998) 293–307.
[12] H.K. Brueckner, D.A. Carswell, W.L. Griffin, Paleozoic
diamonds within a Precambrian peridotite lens in UHP
gneisses of the Norwegian Caledonides, Earth Planet. Sci.
Lett. 203 (2002) 805–816.
[13] C. Chopin, N. Sobolev, Principal mineralogic indicators of
UHP in crustal rocks, in: R.G. Coleman, X. Wang (Eds.),
Ultrahigh-Pressure Metamorphism, Cambridge University
Press, Cambridge, 1995, pp. 96–131.
[14] J.G. Liou, R.Y. Zhang, W.G. Ernst, D. Rumble, S. Maruyama,
High-pressure minerals from deeply subducted metamorphic
rocks, Rev. Miner. 37 (1998) 33–96.
[15] D. Gebauer, H.P. Schertl, M. Brix, W. Schreyer, 35 Ma old
ultrahigh-pressure metamorphism and evidence for very rapid
exhumation in the Dora Maira Massif, Western Alps, Lithos 41
(1997) 5–24.
[16] D. Rubatto, D. Gebauer, R. Compagnoni, Dating of eclogite-
facies zircons: the age of Alpine metamorphism in the Sesia-
Lanzo Zone (Western Alps), Earth Planet. Sci. Lett. 167
(1999) 141–158.
[17] U. Schaltegger, C.M. Fanning, D. Gunther, J.C. Maurin, K.
Schulmann, D. Gebauer, Growth, annealing and recrystalliza-
tion of zircon and preservation of monazite in high-grade
metamorphism: conventional and in-situ U–Pb isotope,
cathodoluminescence and microchemical evidence, Contrib.
Mineral. Petrol. 134 (1999) 186–201.
[18] I. Katayama, S. Maruyama, C.D. Parkinson, K. Terada, Y.
Sano, Ion micro-probe U–Pb zircon geochronology of peak
and retrograde stages of ultrahigh-pressure metamorphic rocks
from the Kokchetav massif, northern Kazakhstan, Earth
Planet. Sci. Lett. 188 (2001) 185–198.
[19] S.G. Song, Petrology, Mineralogy and Metamorphic Evolution
of the Dulan UHPM terrane in North Qaidam, NW China, and
its tectonic implications, Doctoral thesis, Chinese Academy of
Geological Science, Beijing (2001) 96 pp. (in Chinese).
[20] J.S. Yang, Z.Q. Xu, S.G. Song, J.X. Zhang, C.L. Wu, R.D.
Shi, H.B. Li, M. Brunel, P. Tapponnier, Subduction of
continental crust in the early Paleozoic North Qaidam ultra-
high-pressure metamorphism belt, NW China: Evidence from
the discovery of coesite in the belt, Acta Geol. Sin. 76 (2002)
63–68.
[21] S.G. Song, J.S. Yang, Z.Q. Xu, J.G. Liou, R.D. Shi,
Metamorphic evolution of the coesite-bearing ultrahigh-
pressure terrane in the North Qaidam, northern Tibet, NW
China, J. Metamorph. Geol. 21 (2003) 631–644.
[22] J.X. Zhang, J.S. Yang, Z.Q. Xu, Z.M. Zhang, W. Chen, H.B.
Li, Peak and retrograde age of eclogites at the northern margin
of Qaidam basin, Northwestern China: evidences from U–Pb
and Ar–Ar dates, Geochemica 29 (2000) 217–222 (in Chinese
with English abstract).
[23] S.G. Song, J.S. Yang, J.G. Liou, C.L. Wu, R.D. Shi, Z.Q. Xu,
Petrology, geochemistry and isotopic ages of eclogites from
the Dulan UHPM terrane, the North Qaidam, NW China,
Lithos 70 (2003) 195–211.
[24] J.X. Zhang, Z.M. Zhang, Z.Q. Xu, J.S. Yang, J.W. Cui,
Petrology and geochronology of eclogites from the western
segment of the Altyn Tagh, Northwestern China, Lithos 56
(2001) 187–206.
[25] J.J. Yang, J.F. Deng, Garnet peridotites and eclogites in the
northern Qaidam Mountains, Tibetan plateau: a first record.
First Workshop on UHP Metamorphism and Tectonics, ILP
Task Group III-6, Stanford (1994) A-20.
[26] J.J. Yang, H. Zhu, J.F. Deng, T.Z. Zhou, S.C. Lai, Discovery of
garnet–peridotite at the northern margin of the Qaidam Basin
and its significance, Acta Petrol. Mineral. 13 (1994) 97–105
(in Chinese with English abstract).
[27] S.G. Song, L.F. Zhang, Y. Niu, Ultra-deep origin of garnet
peridotite from the North Qaidam ultrahigh-pressure belt,
Northern Tibetan Plateau, NW China, Am. Mineral. 89 (2004)
1330–1336.
[28] S.G. Song, L.F. Zhang, J. Chen, J.G. Liou, Y. Niu, Sodic
amphibole exsolutions in garnet from garnet-peridotite, North
Qaidam UHPM belt, NW China: implications for ultradeep-
origin and hydroxyl defects in mantle garnets, Am. Mineral.
90 (5) (in press).
[29] A.E. Ringwood, A. Major, Synthesis of majorite and other
high pressure garnets and perovskites, Earth Planet. Sci. Lett.
12 (1971) 411–418.
[30] T. Irifune, An experimental investigation of the pyroxene–
garnet transformation in a pyrolite composition and its bearing
S. Song et al. / Earth and Planetary Science Letters 234 (2005) 99–118118
on the constitution of the mantle, Phys. Earth Planet. Inter. 45
(1987) 324–336.
[31] H.L.M. van Roermund, M.R. Drury, Ultra-high pressure
( P N6 GPa) garnet peridotites in Western Norway: exhuma-
tion of mantle rocks from N185 km depth, Terra Nova 10
(1998) 295–301.
[32] L. Dobrzhinetskaya, H.W. Green, S. Wang, Alpe Arami: a
peridotite massif from depths of more than 300 kilometers,
Science 271 (1996) 1841–1845.
[33] G.P. Brey, T. Kfhler, Geothermobarometry in four-phase
lherzolites: Part II. New thermobarometers, and practical
assessment of existing thermobarometers, J. Petrol. 31
(1990) 1353–1378.
[34] H.St.-C. O’Neill, B.J. Wood, An experimental study of Fe–
Mg-partitioning between garnet and olivine and its calibra-
tion as a geothermometer, Contrib. Mineral. Petrol. 70 (1979)
59–70.
[35] W. Compston, I.S. Williams, J.L. Kirschvink, Z. Zhang, G.
Ma, Zircon U–Pb ages for the early Cambrian time-scale, J.
Geol. Soc. London 149 (1992) 171–184.
[36] L.P. Black, S.L. Kamo, C.M. Allen, J.N. Aleinikoff, D.W.
Davis, R.J. Korsch, C. Foudoulis, TEMORA 1: a new zircon
standard for Phanerozoic U–Pb geochronology, Chem. Geol.
200 (2003) 155–170.
[37] J.S. Stacey, J.D. Kramers, Approximation of terrestrial lead
isotope evolution by a two-stage model, Earth Planet. Sci. Lett.
26 (1975) 207–221.
[38] K.R. Ludwig, Isoplot: a plotting and regression program for
radiogenic isotope data, USGS Open-File Report, 1991, pp.
91–144.
[39] S. Gao, X.M. Liu, H.L. Yuan, B. Hattendorf, D. Gunther, L.
Chen, S.H. Hu, Determination of forty two major and trace
elements in USGS and NIST SRM glasses by laser ablation-
inductively coupled plasma-mass spectrometry, Geostand.
Newsl. 26 (2002) 181–196.
[40] D. Gqnther, C.A. Heinrich, Enhanced sensitivity in laser
ablation-ICP mass spectrometry using helium–argon mixtures
as aerosol carrier, J. Anal. At. Spectrom. 14 (1999) 1363–1368.
[41] H.L.M. van Roermund, D.A. Carswell, M.R. Drury, T.C.
Herjboer, Microdiamonds in a megacrystic garnet websterite
pod from Bardane on the island of Fjbrtoft, western Norway:
evidence for diamond formation in mantle rocks during deep
continental subduction, Geology 30 (2002) 959–962.
[42] G. Vavra, D. Gebauer, R. Schmid, W. Compston, Multiple
zircon growth and recrystallization during polyphase Late
Carboniferous to Triassic metamorphism in granulites of the
Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP)
study, Contrib. Mineral. Petrol. 122 (1996) 337–358.
[43] S.G. Song, Metamorphic geology of blueschist, eclogites and
ophiolites in the North Qilian Mountain, 30th IGC Fieldtrip
Guide T392, Geological Publishing House, Beijing, 1996,
(40 pp.).
[44] S.G. Song, L.F. Zhang, Y. Niu, B. Song, Q.J. Wang, Zircon
SHRIMP ages and tectonic significance of North Qilian
eclogites, Chin. Sci. Bull. 49 (2004) 592–595.
[45] S.-S. Sun, W.F. McDonough, Chemical and isotopic system-
atics of oceanic basalt: implications for mantle composition
and processes, in: A.D. Saunders, M.J. Norry (Eds.),
Magmatism in the Ocean Basins, Geol. Soc. Lon. Spec. Publ.,
vol. 42, 1989, pp. 313–345.
[46] R.W. Hinton, B.G.J. Upton, The chemistry of zircon:
variations within and between large crystals from syenite
and alkali basalt xenoliths, Geochim. Cosmochim. Acta 55
(1991) 3287–3302.
[47] P.W.O. Hoskin, Minor and trace element analysis of natural
zircon (ZrSiO4) by SIMS and laser ablation ICPMS: a
consideration and comparison of two broadly competitive
techniques, J. Trace Microprobe Tech. 16 (1998) 301–326.
[48] P.W.O. Hoskin, L.P. Black, Metamorphic zircon formation by
solid-state recrystallization of protolith igneous zircon, J.
Metamorph. Geol. 18 (2000) 423–439.
[49] P.W.O. Hoskin, T.R. Ireland, Rare earth element chemistry of
zircon and its use as a provenance indicator, Geology 28
(2000) 627–630.
[50] D. Rubatto, Zircon trace element geochemistry: partitioning
with garnet and the link between U–Pb ages and meta-
morphism, Chem. Geol. 184 (2002) 123–138.
[51] Y. Niu, T. Gilmore, S. Mackie, A. Greig, W. Bach, Mineral
chemistry, whole-rock compositions and petrogenesis of ODP
Leg 176 gabbros: data and discussion, Proc. Ocean Drill.
Program Sci. Results 176 (2002) (60 pp., http://www-odp.
tamu.edu/publications/176_SR/VOLUME/CHAPTERS/
SR176_08.PDF.).
[52] C.H. Langmuir, E.M. Klein, T. Plank, Petrological systematics
of mid-ocean ridge basalts: constraints on melt generation
beneath ocean ridges, in: J.P. Morgan, D.K. Blackman, J.M.
Sinton (Eds.), Mantle Flow and Melt Generation at Mid-Ocean
Ridges, AGU Geophys. Monogr., vol. 71, American Geo-
physical Union, Washington DC, 1992, pp. 183–280.
[53] G.A. Gaetani, T.L. Grove, W.B. Bryan, The influence of water
on the petrogenesis of subduction-related igneous rocks,
Nature 365 (1993) 332–334.
[54] M.J. O’Hara, S.W. Richardson, G. Wilson, Garnet–peridotite
stability and occurrence in crust and mantle, Contrib. Mineral.
Petrol. 32 (1971) 48–68.
[55] W.L. Griffin, S.Y. O’Reilley, C.G. Ryan, The composition and
origin of subcontinental lithosphere, Geochem. Soc. Spec.
Publ. 6 (1999) 241–258.
[56] F.P. Bundy, The P, T phase and reaction diagram for elemental
carbon, J. Geophys. Res. 85 (1980) 6930–6936.
[57] S.R. Bohlen, A.L. Boettcher, The quartz–coesite transforma-
tion: a pressure determination and effects of other components,
J. Geophys. Res. 87 (1982) 7073–7078.