Geochronology of diamond-bearing zircons from garnet peridotite in the North Qaidam UHPM belt,...

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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: [email protected] (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


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.


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)


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


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.


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


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


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).


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


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


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.


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


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].


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


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.

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