Precambrian Research 102 (2000) 279 – 301
The roots of the Dabieshan ultrahigh-pressure metamorphic
terrane: constraints from geochemistry and Nd – Sr isotope
systematics
Changqian Ma
a,* , Carl Ehlers
b, Changhai Xu
a, Zhichang Li
c,
Kunguang Yang
aaFaculty of Earth Sciences,China Uni6ersity of Geosciences,Wuhan430074,PR China bDepartment of Geology and Mineralogy,A,bo Akademi Uni6ersity,A,bo20500,Finland
cYichang Institute of Geology and Mineral Resources,Yichang443003,PR China Received 13 January 1999; accepted 7 March 2000
Abstract
The Dabieshan area between the Sino – Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd – Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the
Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.390.2 Ga with
volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd – Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic – Cretaceous migmatization and magmatism. © 2000 Elsevier Science B.V. All rights reserved.
www.elsevier.com/locate/precamres
* Corresponding author. Tel.: +86-27-87692999; fax: +86-27-87801763.
E-mail address:wbh@wri.com.cn (C. Ma)
C.Ma et al./Precambrian Research102 (2000) 279 – 301 280
Keywords:Neodymium and strontium isotopes; Archean crust; Yangtze craton; Dabieshan; China
1. Introduction
The Qinling-Dabieshan orogen of east-central China (Fig. 1) is the suture zone formed by the Triassic collision of the Yangtze and Sino – Ko-rean cratons (e.g. Li et al., 1993; Ames et al., 1996). It is truncated at its eastern end by the sinistral Tan – Lu fault, one of the world’s largest continental strike-slip faults (Xu et al., 1987), which offsets the Dabieshan terrane by a mini-mum of 530 km northward to the Sulu area (Okay et al., 1989). Of particular interest is the extensive distribution of coesite and its pseudo-morphs as inclusions in garnet, omphacite, kyan-ite, epidote, dolomkyan-ite, zoiskyan-ite, and zircon in eclogites, ultramafic rocks, felsic gneisses, carbon-ate rocks and jadeite quartzite (e.g. Okay et al., 1989; Wang et al., 1989; Yang and Smith, 1989;
Hirajima et al., 1993; Wang and Liou, 1993; Zhang et al., 1996; Carswell et al., 1997; Liou et al., 1997). This ultrahigh-pressure (UHP) meta-morphism has affected the three easternmost ter-ranes of the collision belt, the Xinxian terrane (also called Hong’an), Dabieshan terrane and Sulu terrane (Fig. 1), and diamond inclusions have been discovered in eclogite of the Dabieshan terrane (Xu et al., 1992). The occurrence of co-esite- and diamond-bearing UHP metamorphic rocks indicates that the crustal rocks have been
buried to great depths of \100 km, significantly
exceeding the 75 km of the present mountain root in the Himalayas. Thus these UHP rocks and their enclosing country rocks may provide very important clues for unravelling the deep thermal and compositional structure and the mechanics of mountain belts.
C.Ma et al./Precambrian Research102 (2000) 279 – 301 281
In contrast to the extensively studied UHP eclogites, the Precambrian felsic orthogneisses in this orogen have received remarkably little atten-tion. A key issue, still unresolved, is the age and nature of the protoliths of the Dabieshan or-thogneiss and other country rocks enclosing the UHP eclogites. Many workers (Wang et al., 1990; Okay and Sengo¨r, 1992) have suggested that the
Dabieshan metamorphic complex represents
deeply subducted Archean continental basement of the Yangtze craton. However, recent Sm – Nd isotopic data have yielded Nd model ages ranging from Archean to Neoproterozoic for Precambrian felsic orthogneisses (e.g. Ames et al., 1996; Chav-agnac and Jahn, 1996; Liou et al., 1997; Chen and Jahn, 1998) and U – Pb and Pb – Pb zircon data on the same orthogneisses have given a wide spec-trum of ages, ranging from late Archean (Chen et al., 1996), Paleoproterozoic (Jian et al., 1999), Neoproterozoic (Ames et al., 1996; Rowley et al., 1997; Xue et al., 1997) to Cretaceous (Xue et al., 1997; Hacker et al., 1998). This indicates that the orogenic history of the Dabieshan is complex.
In this study we present new geochemical and
Nd – Sr isotope data for Precambrian felsic
gneisses, some amphibolites and eclogites, and Mesozoic intrusive rocks from the Dabieshan ter-rane. Our objective is (1) to delineate crustal age provinces within the Dabieshan terrane and North Yangtze craton, based on the new geo-chemical and Nd – Sr isotopic data and those al-ready published in the literature; (2) to identify
ancient crustal components in contaminated
mantle-derived rocks by recognizing mixing rela-tionships with the aid of Nd – Sr isotope systemat-ics; and (3) to provide evidence for involvement of the Archean Kongling gneisses of the Yangtze craton in continental subduction and the exhuma-tion of the UHP and high-pressure (HP) rocks.
2. Geologic setting
2.1. The Dabieshan terrane
The Dabieshan terrane, a fault-bounded Pre-cambrian metamorphic complex, is bounded to the south by a foreland fold-thrust belt along the
middle and lower reaches of the Yangtze River, and to the north in Beihuaiyang by a greenschist facies fold-thrust belt called the Fuziling Group (Fig. 1). It can be subdivided, from north to south, going upwards structurally, into units of different lithology, metamorphic facies and tec-tonic style (Wang et al., 1998), the Dabieshan
orthogneiss domes, the UHP/HP eclogite-bearing
units, and a blueschist-bearing fold-thrust belt (Fig. 2).
(1) The Dabieshan orthogneiss domes (hereafter referred as ‘the dome region’), including the Luo-tian and Yuexi domes, are the footwall unit of the Dabieshan terrane (Wang et al., 1998). Both domes form the western and northeastern parts of the Dabieshan complex and have similar tectonic and lithological features. The term ‘Dabieshan complex’ (DBC) has been used to refer to various metamorphic rocks in both the dome region and
the UHP/HP eclogite-bearing units. The DBC in
the dome region is composed of amphibolite- and granulite – facies felsic gneiss (75% of the total area), of a supracrustal sequence (24%), and of
metabasic – ultramafic rocks (B1%) (Sang et al.,
1997). A supracrustal sequence is mainly made up of metavolcanic amphibole – biotite gneiss, amphi-bolite and a small proportion of marble and magnetite quartzite (You et al., 1996). The gneisses underwent intense migmatization, which
mainly formed stromatic and ptygmatic
migmatites. The radial dips of foliation and the predominant NW, and, SE plunges of mineral lineations outline the structural character of both domes (Fig. 2) (Wang et al., 1998). The metamor-phic grade of the dome region decreases out-wards, from granulie – facies in the cores of the domes to upper amphibolite – facies conditions on the flanks of the domes. The core of the Luotian dome in western Dabieshan consists of tonalitic diatexites with granulite blocks and amphibolite enclaves. The intensity of migmatization decreases gradually towards the flanks of the dome where metatexites predominate (Wang et al., 1998). Very few eclogites have been found in this dome region. A few attempts have been made to date the granulites and migmatitic orthogneisses. Chen et al. (1996) obtained a U – Pb zircon upper-intercept
C.Ma et al./Precambrian Research102 (2000) 279 – 301 282
Fig. 2. Generalized geologic map of the Dabieshan (modified from Ma et al., 1998) showing sample localities, and distribution of Nd model ages for felsic gneisses and intrusive rocks,. and ofoNdvalues for amphibolites and eclogites. Fault zones: (1) Tan – Lu fault; (2) Xincheng – Xishui strike-slip fault; (3) Shangcheng – Macheng fault; and (4) Mozitan strike-slip fault.
sample from Huangtuling Primary School in the core of the Luotian dome (point M9 in Fig. 2), and Jian et al. (1999) reported a Pb – Pb zircon age
of 245697 Ma for the same granulite. Recently,
the protoliths of some orthogneisses in the dome region have been taken to be Neoproterozoic and Cretaceous in age from U – Pb zircon ages of
756.690.8 Ma on an orthogneiss from northern
Dabieshan (Xue et al., 1997), and of 133.792.3
Ma and 134.092.8 Ma on orthogneisses from
northern and western Dabieshan (Xue et al., 1997; Hacker et al., 1998). However, Ma (1999) prefers to interpret the Cretaceous ages as representing the time of intense migmatization and doming.
(2) The UHP/HP eclogite-bearing units
(‘eclog-ite units’ hereafter) comprise amphibol(‘eclog-ite facies felsic gneisses with minor amphibolite, eclogite, garnet-bearing peridotite, jadeite quartzite, and
marble (Liou et al., 1997). The Dabieshan eclog-ites have a predominant assemblage of
jadeite-bearing clinopyroxene (omphacite or chloro
melanite) and garnet, and may contain glau-cophane, kyanite, orthopyroxene, coesite and dia-mond (You et al., 1996). The eclogite units can be further differentiated into two subzones with dif-ferent P – T regimes, a coesite- and diamond-free HP unit in the south, and an UHP unit containing coesite- and diamond-bearing eclogites in the north (Fig. 2) (Okay, 1993; Carswell et al., 1997; Wang et al., 1998). It has been suggested that the
coesite-bearing UHP eclogites reached peak
meta-C.Ma et al./Precambrian Research102 (2000) 279 – 301 283
morphism in the Dabieshan terrane has been dated at 245 – 220 Ma by various geochronological meth-ods (e.g. Li et al., 1993; Ames et al., 1996; Chav-agnac and Jahn, 1996; Rowley et al., 1997), and these ages have been interpreted as representing the timing of north-directed underthrusting of conti-nental crust of the Yangtze craton (e.g. Li et al., 1993). The protolith ages of the Dabieshan eclog-ites are not well constrained though the possibility of their being Neoproterozoic has been discussed (Ames et al., 1996; Rowley et al., 1997; Jahn, 1998). Rowley et al. (1997) have obtained an upper
intercept age of 772.599.5 Ma on zircons from a
felsic gneiss that is a host rock of eclogites. Their result agrees well with an upper intercept age for a gneiss dated by Ames et al. (1996). Both papers interpreted the age as representing a protolith age for felsic gneisses in a rift environment along the northern margin of the Yangtze craton.
(3) The blueschist-bearing fold-thrust belt is composed of Meso – Neoproterozoic metasedimen-tary and metavolcanic rocks (‘Susong group’), lower Sinian metavolcanic rocks (‘Yaolinghe group’) and upper Sinian sedimentary rocks (You et al., 1996). The metamorphism of the Susong group attained conditions of the medium to high pressure greenschist facies. Epidote blueschists are extensively exposed in the belt, and the blueschist facies metamorphism has been dated at 230 – 195 Ma by Sm – Nd crossite-whole rock isochrons
(Yang et al., 1994) and40Ar
/39Ar phengite plateau
ages (Eide et al., 1994) on blueschists from near Hong’an.
The DBC is intruded by abundant middle Juras-sic- early Cretaceous granitic plutons, and minor late Triassic mafic monzodiorite and early Creta-ceous gabbroic rocks with shoshonitic and high-K calc-alkaline affinities (Ma et al., 1998; Jahn et al., 1999). Three groups of Mesozoic intrusive rocks have been identified (Ma et al., 1998). Group I
consists of late Triassic (210 Ma) mafic
monzo-diorites (the Liujiawa stock, Fig. 2), which could have been generated by partial melting of enriched subcontinental lithospheric mantle or by crustal assimilation of mantle-derived magma. Group II comprises middle Jurassic – early Cretaceous (160 – 120 Ma) quartz monzonites, monzogranites and syenogranites, and could have been produced by
crustal assimilation and fractional crystallization of mantle-derived magmas. Group III is represented by Cretaceous (125 – 95 Ma) granitic stocks and granitic porphyries, which could have been derived by anatexis of Dabieshan felsic gneisses and subse-quent fractional crystallization (Ma et al., 1998; Ma, 1999).
2.2. The Kongling complex
The oldest known basement of the Yangtze craton is the Kongling complex (KLC). It outcrops
in an area of about 150 km2 in the Three Gorges
region of the Yangtze River, Western Hubei (Fig. 1). It is composed of gneisses, including grey gneisses with amphibolites, and supracrustal rocks (Ma et al., 1997). The grey gneisses, composed of banded orthogneisses with medium to fine-grained high-Al trondhjemite, tonalite, and granodiorite (TTG) compositions, give U – Pb zircon upper
intercept ages of 2800 – 3000 Ma (e.g. 2936998
Ma, Ames et al., 1996; 2850915 Ma, Ma et al.,
1997). Ma et al. (1997) have interpreted these ages as the time either of TTG intrusion, or metamor-phism of the TTG to grey gneisses. Amphibolites occur as foliated enclaves in the grey gneisses, and they yield a Sm – Nd whole-rock errorchron age of
32909170 Ma (Ma et al., 1997). Supracrustal
rocks form a younger unit, which has been subdi-vided into a lower khondalite series and upper amphibole schists. Zircons from an amphibolite layer in the khondalite have yielded a U – Pb upper
intercept age of 203194 Ma, which is considered
an approximate estimate of the age of the basaltic protolith (Ma et al., 1997).
3. Analytical methods
Analyses of major, trace and rare earth element compositions were made at the Analytical Institute of the Hubei Bureau of Geology and Mineral
Resources. SiO2 and H2O+ were determined by
gravimetry; TiO2and P2O5by spectrophotometry;
Al2O3, Fe2O3, FeO and CO2 by volumetry; and
MnO, MgO, CaO, Na2O and K2O by atomic
stan-C.Ma et al./Precambrian Research102 (2000) 279 – 301 284
dard deviation) is usuallyB1% for major
ele-ments except H2O
+, B4% for REE and Y, and
5 – 10% for trace elements.
Sr and Nd isotopic analyses were performed at the Isotope Laboratory of the Yichang Institute of Geology and Mineral Resources, by procedures described by Ma et al. (1998). Repeated
measure-ments (n=6) of the NBS987 and La Jolla Nd2O3
standards were taken throughout the analytical period and yielded the following average ratios:
87Sr
147Sm/144Nd ratios were determined to a precision
of90.7% and90.2%, respectively. Total
proce-dural blanks are insignificant: B1 ng for Sr, and
86 pg for Nd.
Parameters used for calculations are: (143Nd
/
144
Nd)CHUR, today=0.512638, (
147
measurement, (143Nd/144Nd)
initialis the initial ratio
of the sample suite at the time of its formation (t)
and is calculated from the expression (143
Nd/
144Nd)
initial=(143Nd/144Nd)meas−(147Sm/144Nd)meas
(elt−1). (143Nd/144Nd)
CHUR, t is the isotope ratio
of CHUR at timetand is given by the expression:
(143Nd/144Nd)
CHUR,t=0.512638−0.2137×(e
lt−
1). Sm – Nd model ages (TDM) were calculated
using a linear isotopic ratio growth equation
(Peu-cat et al., 1989): TDM=l−1ln {1+
4. Results and discussion
VSB4.1.Geochemical characteristics of felsic and
grey gneisses
Representative analyses of major, trace and rare earth elements for felsic and grey gneisses are
presented in Tables 1 and 2. Among them, the Kongling grey gneisses were mainly analyzed by Li Fuxi and Ma Daquan (1991, unpublished data) and Ma et al. (1997), while a few data for the Dabieshan felsic gneisses are from You et al. (1996), Sang et al. (1997) and Wu et al. (1998).
The Kongling grey gneisses are high-Al TTG rocks, according to the criteria of Barker (1979)
and Drummond and Defant (1990), with Al2O3\
15 wt.% at the 70 wt.% SiO2level, high Sr (\300
ppm), and low Rb/Sr (B0.15), K/Rb (typically
B550), Y (B15 ppm), and Nb (B10 ppm,
ex-cept in sample H44). The average concentrations
of Rb (36914ppm), Sr (594963 ppm) and Ni
(1192 ppm) are also similar to the values
pro-posed by Condie (1981) for ‘high Al2O3’ Archean
gneisses. Leat et al. (1986) have argued that the
TiO2 versus Zr plot of Pearce (1980) can
effec-tively separate peralkaline from sub-alkaline felsic volcanics, and that the low-Zr group is
subalka-line (B350 – 700 ppm Zr) (Fig. 3). The Kongling
gneisses fall into the intermediate and silicic ‘vol-canic arc’ field below line A-B on Fig. 3, and are
all depleted in TiO2 and Zr relative to ‘within
plate’ rocks. Being low in Zr content (B350
ppm), rocks from the KLC are subalkaline (Leat et al., 1986), and fall into the calc-alkaline field of Fig. 3. Their REE patterns are highly fractionated
[(La/Yb)n=48.85−94.60], and may be divided
into two subgroups with negative and positive Eu anomalies (Fig. 4). In the chrondrite normalized
La/Yb versus Yb diagram (Fig. 5A), the Kongling
grey gneisses display typical Archean features (Jahn et al., 1981; Martin, 1986). Immobile trace elements are also used to test the tectonomag-matic affinity of the grey gneisses on the discrimi-nation diagram of Pearce et al. (1984). The data confirm the orogenic affinity of their protoliths, and all the grey gneisses from the KLC fall in the field of volcanic arc granitoids (Fig. 5B).
The Dabieshan felsic gneisses display a large variation in chemical composition. Compared with high-Al trondhjemitic gneisses fron the KLC,
they are richer in Zr and TiO2 (Fig. 3), and are
not high-Al granitoids. However, their low Zr
C
Chemical compositions of the Dabieshan felsic gneisses and granulitea
Dome region Eclogite units
Z083 94095 94084 Sh2-3 TP5-1 TS23-1
SG-2 LJW-1
Sample XXW-1 XXW-2 Htl-M4 109-1§
M9 M16 M16 M23 M24-1 M25 M27 M28 M29
Nos. in Fig. 2 M3 M4 M9
(2) (3) (3) (1) (4) (4)
(1) (1)
Refs. (1) (1) (1) (1)
SiO2(wt.%) 67.04 71.93 66.21 60.77 66.31 68.99 72.41 65.44 68.57 68.75 72.98 75.48
0.49 0.30 0.70 0.35 0.40 0.43
0.69 0.26
0.37 0.69
TiO2 0.82 0.49
14.75
14.66 13.48 16.14 13.37 14.97 12.88 15.65 15.89 14.96 13.47 11.71
Al2O3
2.07
1.58 0.71 0.70 0.77 1.17 1.32 1.69 0.81 1.18 1.17 0.87
Fe2O3
1.29 1.87 3.09 2.02 2.31 1.29
2.77 1.45
FeO 3.05 2.05 2.52 7.84
MnO 0.10 0.05 0.06 0.08 0.15 0.03 0.06 0.10 0.05 0.05 0.05 0.08
0.92 0.43 1.01 1.12 1.18 0.89
1.93 0.76
1.74
MgO 1.27 2.18 7.63
3.28
3.21 2.68 4.26 1.58 2.32 1.33 2.17 2.50 3.47 2.33 1.37
CaO
4.69
4.44 4.42 5.17 2.84 4.37 3.46 4.15 5.40 4.59 4.10 3.56
Na2O
4.02 4.55 4.38 2.11 2.06 1.42
2.15 3.62
K2O 1.93 1.79 1.22 2.17
0.21 0.07 0.17 0.24 0.12 0.10
P2O5 0.27 0.17 0.13 0.09 0.09 0.06
0.40 0.29 0.55 0.64 0.70 0.93
0.75 0.43
0.65 1.50
H2O+ 0.76 0.49
0.04
0.07 0.11 0.20 0.25 0.04 0.20 0.19 0.03 0.20 0.14 0.04
CO2
99.65
99.67 99.34 99.81 99.58 99.21 99.25 99.29 99.74 99.97 99.30 99.74
Total
69.9 132.2 110.4 45.4 40.6
83.0 49.7 34.9 91.7
36.1 50.9
75.4 Rb (ppm)
Sr 363 481 507 178 252 941 111 227 469 742 238 184
2780 682 1547 765 1563 534
1066 767
536 874
Ba 603 978
0.90
0.28 0.24 0.07 0.79 1.11 4.10 3.00 0.50 0.45 0.64 0.94
Ta
192 243 538.2 304 137 152
Zr 126 243 156 149 151 221
7.93 41.00 63.80 14.21 7.61 15.82
33.77 25.50
0.57 0.27 0.78 0.21 0.29 2.40 2.60 4.50 0.59 0.71 1.65
U
21.8 nd. nd. 8.9 22.9 B2
55.9 115.5
Cr 15.6 26.5 115.5 1014.9
11.0 8.9 12.4 4.9 14.9 3.9 4.1
16.15 9.41 11.28 26.51 58.50 54.90 66.70 97.11 55.58 36.07 31.38
La (ppm) 19.66
35.71 21.37 24.87 43.33 100.80 104.00 122.70 186.2 90.43 66.55 71.90
Ce 39.27
10.73 11.90 15.20 20.92 8.71 7.58
4.90 8.46
Pr 4.40 2.95 3.49 4.42
32.57 40.60 59.10 73.95 25.56 27.22
Nd 17.71 11.57 13.16 15.92 19.18 31.44
4.78 8.80 14.20 9.89 3.60 4.86
4.11 5.67
2.81 2.93
Sm 3.15 2.04
1.15
1.03 0.75 0.88 0.94 1.34 0.91 1.82 2.40 0.92 0.97 0.88
Eu
5.04
3.09 2.01 2.38 2.51 3.03 7.21 12.80 5.26 2.43 3.66 5.26
Gd
0.29 1.14 1.89 0.64 0.25 0.47
0.89 0.82
C
.
Ma
et
al
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/
Precambrian
Research
102
(2000)
279
–
301
286
Table 1 (Continued)
Dome region Eclogite units
Z083
Sample XXW-1 XXW-2 Htl-M4 109-1§ SG-2 94095 94084 Sh2-3 TP5-1 TS23-1 LJW-1
M16 M23 M24-1 M25 M27 M28
M16 M29
Nos. in Fig. 2 M3 M4 M9 M9
(1) (1) (2) (3) (3) (1) (4) (4) (1)
(1)
Refs. (1) (1)
1.64 7.02 11.30 2.59 1.60
2.49 2.86
Dy 2.13 1.50 2.32 5.47 4.34
0.44 0.30 0.45 1.25 0.31 1.57 2.46 0.59 0.32 0.57 0.89
Ho 0.51
0.71 4.50 6.95
Er 1.08 0.76 1.32 1.48 3.67 1.57 0.74 1.74 2.44
0.11 0.78 1.03 0.29 0.11 0.29
0.59 0.38
Tm 0.14 0.11 0.23 0.27
0.60 4.72 6.29 1.89 0.59 1.94
Yb 0.71 0.68 1.24 1.61 3.32 2.17
0.10 0.77 0.98 0.27 0.09 0.32
0.54 0.34
Lu 0.11 0.12 0.20 0.25
215.51
SREE 86.28 53.84 65.05 92.69 119.95 248.32 323.42 403.54 190.93 155.10 166.32
65.89 7.86 7.17 34.72 63.66
8.25 12.56
6.15 10.53 9.77
(La/Yb)n 15.37 9.35
Eu/Eu* 1.00 1.11 1.01 1.03 0.77 1.01 0.34 0.41 0.92 0.90 0.68 0.46
C.Ma et al./Precambrian Research102 (2000) 279 – 301 287
affinity, and may be classified further into high-K or calc-alkaline types (Fig. 3). Their REE patterns
are relatively flat (Fig. 4). Most felsic gneisses from the dome region do not show negative Eu
Table 2
Chemical compositions of the Kongling grey gneissesa
H45 H46
H34
Sample H36 H37 H44
(5) (5) (5) (5) (5)
Refs. (5)
71.44 67.84
70.82
SiO2(wt.%) 71.62 68.69 69.40
0.18 0.19 0.35 0.26 0.30
TiO2 0.58
1.21 1.55 1.85 2.11 3.06
FeO
0.05 0.02
MnO 0.02 0.03 0.04 0.03
0.85 0.51
MgO 0.45 0.51 0.85 0.73
2.36
0.07 0.08 0.11 0.08 0.21
P2O5
0.81 0.86
H2O+ 0.58 0.64 0.70 0.93
100.28
99.67 99.33 99.71 99.70 99.47
Total
28 53
53
Rb (ppm) 20 33 38
620 760 490 505 540
Sr 380
128 118 141 164 466
Zr
La (ppm) 12.00 24.50 19.10
21.00 35.20 27.30 78.90 49.90
Ce 339.00
19.50 5.23
1.98
Pr 3.65 2.77 8.23
101.00
Nd 6.98 13.10 9.94 30.70 21.00
3.48
1.10 2.17 1.57 5.21 13.50
Sm
0.69
0.56 0.66 0.54 0.81 0.83
Eu
0.13 0.19 0.16 0.37 0.78
Tb
0.47
Dy 0.80 0.63 1.76 3.60 1.09
0.20
0.09 0.16 0.12 0.26 0.51
Ho
0.03 0.05 0.04 0.08 0.17
Lu
63.53 626.05 117.32
SREE 45.40 82.32 177.44
48.85 53.06 56.12 81.57 64.82
(La/Yb)n 94.60
1.84 1.15 1.29 0.61 0.78
Eu/Eu* 0.26
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Fig. 3. Plot of Zr vs TiO2for the Dabieshan felsic gneisses and the Kongling grey gneisses. Fields of ‘volcanic arc’ and ‘within plate’ rocks from Pearce (1980). Rocks below line A-B, draw from co-ordinate TiO2=0.2%, Zr=10 ppm to TiO2=1.0%, Zr=100 ppm (Pearce, 1980), are interpreted as having inter-mediate or silicic compositions. It is suggested that Zr content ranging from 350 to 700 ppm can effectively divide intermedi-ate and silicic rocks into peralkaline and subalkaline types, and subalkaline rocks can be further divided into high-K and calc-alkaline (Leat et al., 1986).
In a plot of Nb versus Y, most of the Dabieshan felsic gneisses display volcanic arc affinity except for two gneisses of the UHP eclogite unit, which fall in the field of within-plate granites (Fig. 5B). Fig. 6 shows that chondrite-normalized REE patterns and primitive mantle-normalized incom-patible element patterns for two felsic gneiss sam-ples from the eclogite units match those of the Kongling grey gneisses well, implying that some of the felsic gneisses in the eclogite units may have been derived from the KLC perhaps during Meso-zoic exhumation of the UHP metamorphic rocks.
4.2. Rb–Sr and Sm–Nd isotopic characteristics
Twenty-four felsic gneiss, amphibolite, eclogite and other metamorphic rock samples, and twelve samples of Mesozoic intrusive rocks were col-lected from the Dabieshan terrane. Published Nd and Sr isotope data for the DBC, and for the KLC have been compiled from available literature sources. The isotopic data are given in Table 3 and Table 4, and sample locations for the Da-bieshan rocks are shown in Fig. 2.
anomalies, and they display Archean features (Fig. 5A). In contrast, the majority of felsic gneisses from the eclogite units show significant negative Eu anomalies (Fig. 4), and they straddle both Archean and post-Archean fields in Fig. 5A.
C
Rb–Sr and Sm–Nd isotopic data for metamorphic rocks and selected granitoid rocks from the Dabieshan
87Rb/86Sr
0.7092393 5.98 39.16 0.0923 0.51152598 −21.7
131.20 472.10 2.0 −11.6 (1)
Felsic gneiss 96-58
M1 0.8015
746.30 0.3408 0.7087693 7.01 52.81 0.0803 0.511821919 −15.9 1.5 −4.6 (1)
M2 96-53 Felsic gneiss 88.20
0.7084692 3.43 19.09 0.1088 0.512276912 −7.1 1.3
0.4100 1.5
XXW-1 (1)
M3 Felsic gneiss 75.59 363.66
X-1 Amphibolite 3.87 12.72 0.1838 0.512738913 2.0 2.1 3.2 (1)
M3(1)
0.7085197 4.35 27.40 0.0961
M4 X-2 Felsic gneiss 47.47 378.62 0.3610 0.51219897 −8.6 1.2 1.2 (1)
0.7077496 4.74 14.93 0.1920 0.51200091 −12.4 7.9
0.1270 −12.0
M4(1) XXW110 Amphibolite 15.38 347.87 (1)
0.7101791 9.03 34.55 0.1582 0.51238396 −5.0 2.1
M5 96-12 Amphibolite 8.64 287.50 0.0867 −1.2 (1)
0.7070591 4.70 22.48 0.1266 0.51189598 −14.5 2.2
0.2176 −7.7
M6 96-48-1 Felsic gneiss 36.00 276.90 (1)
0.8647
96-63-2 Felsic gneiss 139.40 464.60 0.71215924 7.29 40.48 0.1090 0.51161695 −19.9 2.2 −11.4 (1)
M7
0.71298910 5.08 29.45 0.1043 0.51136497 −24.9 2.5
0.3913 −15.9
96-110 641.50 (1)
M8 Felsic gneiss 87.02
213.80
Intermediate 1.1800 0.7464293 5.19 27.95 0.1123 0.511044913 −31.1 3.1 −22.9 (1)
109-1 87.13
M9
granulite
0.7418998 7.20 28.01 0.1555 0.51172198 −17.9 3.7
0.5815 −13.9
Felsic gneiss (1)
M9(1)a 96-M4 57.58 286.50
283.10
Amphibolite 0.3338 0.71243921 6.57 30.42 0.1305 0.51156999 −20.9 2.9 −14.4 (1)
96-M2 32.77
M9(2)a
xenolith
3.8 15.45 0.1489 0.51234798 −5.7
M10 d144 Felsic gneiss 1.9 −1.0 (2)
0.706594 5.96 24.21 0.1490 0.512511911 −2.5 1.5
0.1819 2.2
96-238 (1)
M11 Felsic gneiss 54.89 870.00
0.4368
96-111 Felsic gneiss 57.73 381.00 0.7072791 4.88 22.90 0.1290 0.512122912 −10.1 1.8 −3.5 (1)
M12
0.7059492 4.63 17.29 0.1620 0.51233497 −5.9
M13 96-212 Amphibolite 30.23 467.80 0.1863 2.4 −2.6 (1)
0.7063894 7.58 48.44 0.0947 0.51181099 −16.2 1.7
0.2738 −6.2
91.36 962.00 (1)
M14 LG2 Felsic gneiss
0.0590
SG-1 Felsic gneiss 17.42 849.53 0.7066195 10.04 50.38 0.1205 0.512032910 −11.8 1.8 −4.4 (1)
M15
0.7087494 4.05 19.51 0.1689 0.51233999 −5.8 2.7
M15(1) SG-2 Amphibolite 70.8 286.56 0.7120 −3.1 (1)
7.47 40.36 0.1118 0.51201197 −12.2 1.7 −4.0
d118 (2)
M16 Felsic gneiss
0.001
DB-91-34 Eclogite 0.01 47.4 0.7049492 4.74 20.08 0.143 0.512173910 −9.1 2.1 −3.9 (3)
M17
0.7088691 3.88 18.72 0.1255
M18 Bh-12 Felsic gneiss 76.57 529.60 0.4169 0.511895911 −14.5 2.2 −7.6 (1)
0.7064791 2.52 8.04 0.1894 0.512098910 −10.5 6.5
0.1473 −9.8
M18(1) Bh-9b Eclogite 6.01 117.50 (1)
0.70611911 5.94 26.13 0.1376 0.51202899 −11.9 2.2
M18(2)a Bh-7 Eclogite 11.76 570.70 0.0594 −6.2 (1)
0.7069892 10.94 79.47 0.0833 0.51141099 −24.0 2.0
0.1329 −13.0
M19 96-218 Felsic gneiss 85.71 1859.00 (1)
d143 Felsic gneiss 12.6 89.09 0.0855 0.51133597 −25.4 2.1 −14.6 (2)
M20
6.58 40.37 0.0984 0.51158999 −20.5 2.1 −10.9
d105 (2)
M21 Felsic gneiss
d123 Felsic gneiss 9.04 59.25 0.1088 0.51137996 −24.6 2.6 −16.0 (2)
M22
0.418
BJ93-01 Felsic gneiss 28.65 198.35 0.71084798 8.0 43.53 0.1111 0.51240795 −4.5 1.1 −3.8 (4)
M23
0.70375595 1.96 7.51 0.1578 0.51252094 −2.3 1.7
0.001 1.5
0.14 456.33 (4)
M23(1) BJ93-03 Eclogite
0.072
BJ93-04 Eclogite 5.81 232.31 0.70402698 1.63 5.55 0.1778 0.51260896 −0.6 2.3 1.3 (4)
M23(2)a
0.71076896 4.0 21.5 0.1125 0.51212998 −9.9 1.5
M24 BJ93-26 Felsic gneiss 49.61 361.23 0.398 −1.7 (5)
0.72403199 3.28 14.82 0.1338 0.511420918 −23.8 3.3
1.041 −17.7
BJ93-32 (6)
M25 Felsic gneiss 88.85 247.21
0.005
BJ93-31 Eclogite 0.19 112.12 0.71078196 4.57 16.89 0.1636 0.51169597 −18.4 4.4 −15.2 (6)
M25(1)
M25(2) BJ93-36 Eclogite 2.7 487.28 0.016 0.70488997 3.72 14.89 0.151 0.51222997 −8.0 2.2 −3.5 (6)
7.33 34.89 0.1272 0.51217699 −9.0 1.7 −2.3
T16-1 (7)
M26 Felsic gneiss
1.9370
LJW-1 Felsic gneiss 91.40 182.52 0.7194997 5.13 22.76 0.1363 0.511941938 −13.6 2.4 −7.7 (1)
M27
0.7132592 1.62 7.36 0.1330 0.512153915 −9.5 1.9
M28 D12 Felsic gneiss 1.71 39.30 1.1120 −3.3 (3)
7.14 37.70 0.1144 0.51200599 −12.3 1.8 −4.4
D132 (2)
M29 Felsic gneiss
D142 Susong schist 5.47 24.07 0.1374 0.512362912 −5.4 1.6 0.4 (2)
M30
0.4842 0.7096392 8.39 46.41 0.1093 0.51212797 −10.0 1.5
G1 XXW-N11 Quartz 80.84 481.43 (8)
monzonite
0.7234091 5.57 31.86 0.1058 0.51161297 −20.0
124.03 2.2
Granite 243.57 5.6703 (8)
G2 B11-2
B1-2 Granite 218.60 3.0430 0.7167594 9.80 58.58 0.1012 0.51152097 −21.8 2.2 (8)
G2 207.30
0.70598935 2.73 12.96 0.1275 0.511928910 −13.8 2.2
G3 109-2 Tonalite 73.13 606.90 0.3474 (1)
0.70841914 3.43 10.25 0.2022 0.51277397 2.6 4.9
0.398
Mafic enclave (1)
G3(1)a 96-M11 30.63 221.9
1.6030
96-63-1 Granite 147.80 266.10 0.7186192 7.39 50.73 0.0811 0.51140298 −24.1 2.0 (1)
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Table 3 (Continued)
87Sr/86Sr92s
No. in Fig. 2 No. of samples Rock types Rb (ppm) Sr (ppm) 87Rb/86Sr Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd oNd(0) TDM oNd (at 760 Sourcesb
Ma) (Ga) 92s
0.7080494 9.27 59.14 0.0949
G5 96-34 Granite 154.30 637.30 0.6981 0.51166598 −19.0 1.9 (1)
0.7085596 8.41 47.55 0.1070 0.511617910 −19.9 2.2
0.4378
96-116 (1)
G6 Granite 1110 731.20
0.5268
96-117 Granite 110.90 607.10 0.7087591 2.78 15.08 0.0914 0.511688919 −18.5 1.8 (1)
G7
1096.60 0.2140 0.7083892 14.54 94.08 0.0935 0.51160498 −20.2 2.0 (1)
Granite
G8 94-1 81.41
0.7144698 6.68 40.23 0.1004 0.511841940 −15.5 1.8
3.3160
Granite 215.42 187.43 (1)
G9 94-6
1123.50
Quartz 0.1951 0.7077191 5.18 32.52 0.0964 0.51144397 −23.3 2.2 (8)
089 76.04
G10
monzonite
D102 Granite 6.37 44.23 0.0870 0.51137595 −24.6 2.1 (2)
G11
0.7083295 12.54 76.52 0.0991 0.51172998 −17.7 1.8
0.3828
G12 YH61 Monzonite 94.73 713.50 (8)
0.7094693 12.38 82.55 0.0907 0.51169997 −18.3 1.8
G12(1)a A12 Quartz syenite 153.75 948.47 0.4674 (8)
10.08 65.53 0.0959 0.51169594 −18.4 1.9
Granite (1)
G13 D1211
302.80
Quartz 0.4577 0.7095294 3.90 17.81 0.1326 0.512341913 −5.8 1.5 (1)
Shiguan1 48.06
G14
monzonite
7.05 55.97 0.0800 0.51153093 −21.6 1.8
G15 D1351 Granite (1)
0.1238 0.70806912 9.86 44.13 0.1352 0.51219698 −8.6
523.90 1.9
96-217 (8)
G16 Quartz 22.47
monzonite
A1 Monzodiorite 24.91 0.0818 0.7080292 9.75 48.73 0.1211 0.51162999 −19.7 2.5 (8)
G17 878.36
G17(1)a A5 Monzodiorite 42.43 736.37 0.1660 0.7078094 6.26 33.17 0.1141 0.51157196 −20.8 2.4 (8)
0.332 0.70889911 5.16 33.77 0.0925 0.51135497 −25.0
700.07 2.3
Quartz 80.61 (1)
G17(2)a 103-2
monzonite
94-2 Granite 81.70 0.2721 0.7067792 2.64 16.62 0.0961 0.51164197 −19.4 2.0 (8)
G18 865.71
G19 MC-1 Monzonite 81.24 879.45 0.2663 0.7062294 4.68 29.31 0.0966 0.51177896 −16.8 1.8 (8)
10.12 67.34 0.0909 0.511338911 −25.4 2.2 (2)
G20 D1412 Granite
aThe sample has not been located in Fig. 2, but it has a sampling location similar to that before it.
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Table 4
Rb–Sr and Sm–Nd isotopic data for the Kongling complex from the Yangtze basementa
Rock typesb 87Rb/86Sr 87Sr/86Sr92s Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd92s oNd(0) T DM(Ga) Rb (ppm)
No. of Sr (ppm)
samples
9.08 70.70
H45 gg 60.56 460.87 0.3879 0.7191199 0.0776 0.510315917 −45.3 3.2
3.96 26.40 0.0906 0.51057916 −40.3
0.7155292 3.2
gg
H44 64.37 560.79 0.33166
0.7125295
gg 45.42 506.20 0.21608 1.40 8.63 0.0983 0.510511912 −41.5 3.5
H37
0.10096 0.7074494 7.07 0.09 0.0907 0.510548917 −40.8 3.2
H34 gg 24.29 693.47
6.07 0.15 0.1528 0.511789917 −16.6
0.7068893 3.4
363.96 0.08737 HvI-7 amph 11.03
amph 2.63 8.43 0.1888 0.512621912 −0.3 3.2
B17
amph 2.79 8.96 0.1888 0.512578926 −1.2 3.5
B19
4.27 15.28 0.1691 0.512142925 −9.7 3.4 B20 amph
amph 3.30 10.72 0.1860 0.5125397 −2.1 3.4
B21
5.35 24.94 0.1299 0.511289927 −26.3 3.4 amph
B22
amph 3.68 11.76 0.1894 0.5126599 0.2 3.1
B56
6.15 30.21 0.1231 0.51121198 −27.8
B57 amph 3.2
2.15 6.83 0.1896 0.512653913 0.3 3.1 amph
B61
amph 2.81 9.45 0.1800 0.51242797 −4.1 3.3
B62
4.66 17.54 0.1608 0.51207199 −11.1 3.1 B65 amph
4.80 15.47 0.1879 0.512531914 −2.0 3.6 B66 amph
amph 3.01 11.72 0.1553 0.511838915 −15.6 3.4
B70
3.15 24.19 0.0786 0.51031916 −45.4 3.2 B73 gg
C.Ma et al./Precambrian Research102 (2000) 279 – 301 292
Fig. 5. Plots of Ybnvs. (La/Yb)n(A) and Nb vs. Y (B) for the Dabieshan felsic gneisses and Kongling grey gneisses. For Ybn vs. (La/Yb)nplot: Chondrite normalizing values used are from Taylor and McLennan (1985), and fields for Archean and Post-Archean granitoids are indicated after Jahn et al. (1981) and Martin (1986). Data points from the KLC plot in the Archean field, whereas felsic gneisses from the Dabieshan straddle both Archean and post-Archean fields. The Nb vs. Y plot shows tectonomagmatic grid of Pearce et al. (1984). VAG=volcanic arc granitoids, WPG=within-plate grani-toids; ORG=ocean-ridge granitoids. Symbols as in Fig. 3.
amphibolite sample (HVI-7) from the KLC shows
similar 87Sr/86Sr and 143Nd/144Nd ratios to the
Dabieshan amphibolites.
Fig. 6. Chondrite-normalized REE patterns and primitive mantle-normalized incompatible element patterns, showing that some felsic gneisses in the eclogite units have geochemical features which match those of the Kongling grey gneisses. Percentages in brackets represent SiO2contents of the samples. Chondrite normalizing values used are from Taylor and McLennan (1985), and primitive mantle normalising factors are from Hofmann (1988).
Fig. 7. 143Nd/144Nd vs. 87Sr/86Sr isotopic variation for felsic gneisses, amphibolites and eclogites from the Dabieshan and Yangtze basement.
In a plot of 87Sr/86Sr versus 143Nd/144Nd (Fig.
7), the Dabieshan felsic gneisses have a higher
average 87Sr
/86Sr ratio of 0.712 than the
Da-bieshan eclogites and amphibolites (:0.707). The
felsic gneisses from the eclogite units show a wide
range, especially in 87Sr/86Sr ratio, probably
reflecting fractionation between Rb and Sr.
How-ever, the87
Sr/86
Sr ratios for the majority of meta-morphic rocks from the DBC and KLC are lower than an average ratio of continental crust (0.72; Hofmann, 1997). Felsic gneisses from the
Da-bieshan terrane have much higher 143Nd/144Nd
C.Ma et al./Precambrian Research102 (2000) 279 – 301 293
Fig. 8. Plot of 87Rb/86Sr vs. 147Sm/144Nd for felsic gneisses, amphibolites and eclogites from the Dabieshan and Yangtze basement. Also shown for comparison are the parent/daughter isotopic ratios for average continental crust (CC) and depleted mantle (DM). For Sm – Nd, CC value is taken from Taylor and McLennan (1985), DM value from Peucat et al. (1989). Rb – Sr ratios for CC and DM are taken from Mo¨ller et al. (1998). Symbols as in Fig. 7.
ing the formation of new continental crust from depleted mantle, that processes in the crust in-cluding partial melting and high-grade
metamor-phism did not affect the Sm/Nd systematics on
a whole-rock scale, and that all the material in the sample was derived from the upper mantle during a single event (Arndt and Goldstein, 1987; Milisenda et al., 1994). If this is the case,
measurement of 143Nd/144Nd and 147Sm/144Nd in
a crustal rock would yield a Nd model age
(TDM) which represents the latest time that Nd
was in isotopic equilibrium with depleted mantle (DM). A complication in interpreting the Nd model ages of orthogneisses is the possible effect of mixing between juvenile material and old crust. If that has occurred, the model ages would be intermediate between the actual times of derivation of crustal material from depleted mantle and the model residence age of the old crust, and would not correspond to crust-forma-tion events (Arndt and Goldstein, 1987).
Assuming that the Sm – Nd components of a crustal sample did not fractionate since addition of the sample to the continental crust from the mantle, calculated Nd model ages of felsic gneisses provide a powerful tool for insights into the premetamorphic crustal history of a high-grade terrane. However, the significance of Nd model ages for mafic rocks is ambiguous, as
many of them have mantle-like Sm/Nd ratios
(Fig. 8). It is clear that most amphibolites and
eclogites in Tables 3 and 4 have high 147Sm
/
144Nd ratios of \0.15, which would give
unre-alistically high Nd model ages with no
geological significance. We have thus limited ourselves to Nd model ages of the Dabieshan felsic gneisses in this paper (Fig. 9).
Grey gneisses from the KLC have a mean Nd
model age of 3.390.1 Ga (at 1 S.D.), which is
distinctly older than Nd model ages of the major-ity of the Dabieshan felsic gneisses (Fig. 9), most of which range from 1.0 to 2.9 Ga (Fig. 9). A Susong schist from the blueschist-bearing fold-thrust belt has a model age of 1.6 Ga. However, an intermediate granulite (sample 109-1) from the Luotian dome and a felsic gneiss (BJ93-32) from Thirty-six felsic gneiss samples from the
Da-bieshan yield a mean 147
Sm/144
Nd ratio of
0.1290.02, which is within the typical range
(0.09 – 0.13) for crustal rocks (Taylor and
McLennan, 1985). 147Sm
/144Nd ratios of the
Kongling amphibolites (0.12 – 0.19, see Table 4)
and most Dabieshan eclogites (\0.12, Fig. 8)
are higher than the typical range of crustal rocks, while grey gneisses from the KLC have
lower 147
Sm/144
Nd ratios (0.08 – 0.10) than the
mean 147Sm/144Nd ratio (0.12) of crustal rocks
(Fig. 8) (Milisenda et al., 1994; Mo¨ller et al., 1998). The Dabieshan felsic gneisses display 87
Rb/86
Sr ratios ranging from 0.059 to 1.937
with a mean value of 0.54690.102, which is
higher than average continental crust (0.4098, Faure, 1986), whereas the Dabieshan eclogites
show a relatively wide range in 87
Rb/86
Sr values (Fig. 8).
dur-C.Ma et al./Precambrian Research102 (2000) 279 – 301 294
Fig. 9.oNd(0) vs.TDM(Ga) variation for felsic gneisses from the Dabieshan and Yangtze basement. Symbols as in Fig. 3.
bieshan yield Proterozoic model ages (Table 3, Fig. 2), possibly suggesting that they came from, or were contaminated by, crustal sources and rocks of the Dabieshan and Yangtze basement (discussed blow).
Rowley et al. (1997) and Ames et al. (1996) have argued that an episode of rift-related mag-matism took place between 600 and 800 Ma. We
calculated initial oNd values of the Dabieshan
fel-sic gneisses, amphibolites and eclogites at 760 Ma
(Table 3), and also show the oNd (at 760 Ma)
values of a few samples of the Dabieshan
amphi-bolites and eclogites in Fig. 2. The oNd values of
the amphibolites and eclogites, ranging from
+3.2 to −15.2, are similar to those of the
Da-bieshan felsic gneisses (+3.8 to −17.7).
4.3. Geochronological significance
On a conventional Rb – Sr isochron diagram, most of the felsic gneisses, amphibolites and eclogites from the Dabieshan plot close to a 493 Ma reference line (Fig. 10). In contrast to the Qinling and Tongbai metamorphic rocks, which experienced high-temperature metamorphism be-tween 480 and 430 Ma (zircon Pb data by Kro¨ner et al., 1993; Ar – Ar hornblende data by Zhai et al., 1998), the geological significance of the Or-dovician chronological results from the Da-bieshan terrane has been a matter of some debate. Ames et al. (1996) suggested that the Dabieshan metamorphic rocks did not experience an early Paleozoic metamorphic event at all. However, You et al. (1996) obtained a U – Pb zircon lower
intercept age of 47192 Ma (MSWD=0.3) and a
Sm – Nd garnet-whole rock isochron age of 4819
25 Ma for the Dabieshan eclogites, and they considered these ages as another eclogite-facies metamorphic event in addition to the well-known Triassic HP and UHP metamorphism (Yang et al., 1994; You et al., 1996). Given the relative mobility of Rb and Sr, the Rb – Sr isotopic system might readily be disturbed either by influx of fluids or by a later thermal event. Thus it appears that the Rb – Sr errorchron of 493 Ma in Fig. 10 may indicate an early Paleozoic metamorphism in the Dabieshan terrane.
the eclogite units yield Archean model ages simi-lar to those of the KLC.
The regional distribution of the Nd model ages for the Dabieshan felsic gneisses and Mesozoic intrusive rocks is shown in Fig. 2. Most younger model ages of 1.0 – 1.6 Ga occur near the north-ern and southnorth-ern margins of the Luotian dome, and the northern margin of the eclogite units. Granulite facies rocks with a depleted-mantle model age of 3.1 Ga are exposed in the core of the Luotian dome, and the oldest model age of 3.3 Ga was found in a felsic gneiss that encloses UHP
diamond-bearing eclogite blocks from the
Shuanghe region (Xu et al., 1992; Liou et al., 1997). All Mesozoic intrusive rocks in the
C.Ma et al./Precambrian Research102 (2000) 279 – 301 295
Fig. 11. Sm – Nd isochron diagram for felsic gneisses, amphi-bolites and eclogites from the Dabieshan and Kongling com-plex. Ages and error are calculated with the Isoplot program (Version 2.95) of Ludwig (1997). Discussion see text. Symbols as in Fig. 7.
with oNd (T)= +2.8. The high MSWD value of
30.2 indicates that the variation along the regres-sion line is much larger than that caused by analytical error. This implies that the Sm – Nd isotopic system of these samples was not in equilibrium.
The scatter in the measured Sm – Nd ratios of the Dabieshan felsic gneisses indicates that either the Sm – Nd system was differentially mobilized or the rocks did not have the same initial ratios. The isotopic variation of gneiss samples on the mixing line 1 may be associated with mixing between (Neoproterozoic?) mantle-derived magmas and Kongling gneisses, because crustal assimilation by a primary mafic magma during ascent and em-placement combined with fractional crystalliza-tion (AFC), presumably generated the protolith of felsic gneisses (Ma, 1999). Three samples of Da-bieshan eclogites and amphibolite on mixing line 2 possibly represent a mixing relationship between a younger mafic magma and Kongling gneiss.
Among the Dabieshan metamorphic rocks, the Huangtuling intermediate granulite (sample 109-1, Table 3) has the oldest Nd model age of 3.1 Ga, and falls on line L2 in Fig. 11. In order to constrain the age of its protolith and the timing of granulite – facies metamorphism, Sm – Nd isotopic analyses were performed on a set of samples including intermediate granulite, host tonalite (fel-sic) gneiss and two mafic enclaves from the core of the Luotian dome (point M9 in Fig. 2). A few mineral separates from the granulite were also analyzed. The data are given in Table 5 and Table 3. Interestingly, the majority of data points in-cluding granulite, mafic enclaves, tonalite (felsic) gneiss, plagioclase and orthopyroxene define an The majority of Sm – Nd data for the DBC
displays rather a broad array on a Sm – Nd isochron diagram (Fig. 11). Five samples of
Da-bieshan eclogites and amphibolites yield a 21479
456 Ma reference line (L1) with an initial oNd of
+4.9 (age calculated using model 1 of Ludwig
(1997) in which scatter from a straight line is attributed to analytical errors alone), and indicat-ing derivation from a LREE-depleted mantle reservoir. Although the reference line does not meet the critical isochron prerequisites of White-house et al. (1996), we consider that the age provides an approximate estimate of the protolith age for some Dabieshan eclogites and amphibo-lites. In contrast to the Dabieshan eclogites, thir-teen data points for the Kongling amphibolites define a reference line (L2) whose slope
corre-sponds to an age of 32449145 Ma (95% conf)
Table 5
Sm–Nd isotopic data for the constituent minerals of the Huangtuling intermediate granulite and an amphibolite enclave
Mineral or rock
Samples Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd
92s 2.70
Garnet from 109-1a
Grt 4.59 0.3564 0.512475912
Biotite from 109-1
Bt 0.28 1.29 0.1302 0.511419922
Orthopyroxene from 109-1 1.67
Opx 8.70 0.1165 0.51112599
Plagioclase from 109-1 1.52 9.85 0.0934 0.510785921
Pl
Amphibolite enclave 3.43 10.25 0.2022 0.51277397
96-M11
C.Ma et al./Precambrian Research102 (2000) 279 – 301 296
Fig. 12. Sm – Nd isochron diagram for the Huangtuling inter-mediate granulite (sample 109-1), its constituent minerals, a host felsic gneiss of the granulite (sample 96-M4) and amphi-bolite xenolith and enclave (96-M2 and 96-M11) from the core of the Luotian dome. Data and abbreviations are given in Table 3 and Table 5.
4.4. Contribution of the Archean Kongling gneiss
to the Mesozoic Dabieshan mafic monzodiorite
The Mesozoic mafic monzodiorites of Group I (e.g. A1 and A5, Table 3) are characterised by
high Th/Yb ratios, relative enrichment in
incom-patible elements, negative Nb anomalies, low
oNd, high initial 87Sr/86Sr ratios and
Pale-oproterozoic Nd model ages, suggesting either interaction between crustal material and mafic magmas during their ascent through the crust, or enrichment of the mantle source by subduc-tion and recycling of crustal material (Ma et al., 1998). In order to examine the nature and ex-tent of possible crustal components which were involved in the petrogenesis of the Liujiawa mafic monzodiorites (Group-I), we have per-formed isotopic mixing calculations.
The element concentrations and isotope ratios of mixtures of two components are described by the following equations (Boger and Faure, 1976; Gray, 1984):
CI=f·CC+(1−f) ·CM (1)
and
CI· RI=f·CC·RC+(1−f) ·CMRM (2)
where CI, CC and CM are the concentrations of
element, ppm Nd, Sm, Sr, or Rb in isotopically mixed intrusion rocks (I), a crustal component
(C) and a mantle component (M), and RI, Rc,
and RM are their respective isotopic ratios
87Sr
/
86Sr or 143Nd/144Nd. The factor f is the
weight-ing factor of the crustal component in any given
mixture. For example, for the ratio 143Nd/144Nd
in a mixture, Eq. (2) becomes:
C(Nd)I(143Nd/144Nd)I
=f·C(Nd)C(143Nd/144Nd)C
+(1−f) ·C(Nd)M(
143
Nd/144Nd)M (3)
To examine the effects of mixing on the Nd
model ages (TDM), a parent/daughter isotopic
ratio (147Sm
/144Nd) in the mixture must be
cal-culated. An equation for the calculation can be given by finding the molar concentrations of
147Sm and 144Nd in the mixture, following the
procedure outlined by White et al. (1967) and using the atomic fractions (atom%) reported by Rosman and Taylor (1998):
errorchron whose slope corresponds to an age
of 27899650 Ma (95% conf.) (Fig. 12), but a
data point from garnet in the granulite does not lie on the line. Since biotite is most likely to be a non-primary phase, it was excluded from re-gression calculations. The very high MSWD value of 511 implies that the samples cannot be regarded as cogenetic or as equilibrium assem-blages. Thus line L1 may reflect only a mixing relationship. The inset in Fig. 12 shows a whole-rock (WR)- orthopyroxene-plagioclase isochron
age of 22389300 Ma with an initial oNd=
−6.6 for sample 109-1 (MSWD=1). The
non-linearity of data points from the granulite
sam-ple, orthopyroxene and plagioclase in a
143Nd/144Nd – 1/Nd plot (not shown) suggests
that the straight line L2 is a true isochron and not a mixing line. Consistent within the bounds of uncertainty with a Pb-Pb zircon age of
245697 Ma for the same granulite exposure
re-ported by Jian et al. (1999), the isochron age of
22389300 Ma probably represents the timing
of granulite – facies metamorphism. Therefore, the age of the protolith to the granulite must be
older than 22389300 Ma. Petrographical study
C.Ma et al./Precambrian Research102 (2000) 279 – 301 297
mixture can be calculated using the Eq. (1) and Eq. (3).
In order to numerically model isotopic mixing, an average isotopic composition for four Permian gabbro samples from the Xinxian block, ca. 70 km west of the Dabieshan terrane (Zhang et al., 1995) and three Cenozoic basalt samples from the Tan – Lu fault belt (Zhi and Chen, 1992) is used as the mantle component. The other crustal compo-nents that may contribute to the final composition of the Mesozoic Dabieshan mafic intrusive rocks
include those of the Yangtze basement repre-sented by the Kongling gneisses and Dabieshan
intermediate and felsic gneisses/granulite. Thus,
two felsic gneisses and one intermediate granulite with distinct Nd model ages are examined in turn as possible crustal end members (H44 from the Kongling complex; 96-110 and 109-1 from the Dabieshan dome region). Mixing calculations show that the addition of 27% Archean Kongling gneiss will yield a magma composition which closely matches the compositions of the Liujiawa mafic monzodiorites in Nd and Sr isotopes (Fig.
13), and that calculated TDM of the resulting
magma (ca. 2.5 Ga) agrees well with the Nd model ages of the samples A1 and A5 (Table 3), whereas mixing with components from the Da-bieshan terrane (samples 96-110 and 109-1) would not result in isotopic compositions of the Group I mafic monzodiorite stock. Fig. 13 shows that half the samples of the Group II lie on the trend line between the mantle-derived component and the
Kongling grey gneiss. It appears that the
Kongling grey gneisses have been involved in the Group II magmatism, and that the Dabieshan complex may have played a significant role in the petrogenesis of the Group III intrusive rocks (Fig. 13).
Distinguishing between the effects of crustal contamination and subduction enrichment (i.e. source contamination) is critical for assessing the implications of our results for crustal structure. The most compelling evidence for crustal contam-ination would be a correlation between isotopic ratios of Sr and Nd and chemical compositions
with a fractionation indicator (e.g. SiO2, MgO, or
Mg-number), because such a correlation would imply that changes in isotopic composition were produced during differentiation, while the magma was ascending and emplaced in the crust. Accord-ing to the Nd model ages, rocks of the KonglAccord-ing complex are not a major rock type exposed in the Dabieshan terrane and therefore composition ef-fects of the Kongling grey gneiss on the mantle-derived mafic magmas may be indicative of recycling of the Yangtze basement into the mantle via continental subduction. However, an alterna-tive possibility is that the KLC underlies the Dabieshan terrane, so that interaction between Fig. 13. oNd(0) vs. 87Sr/86Sr diagram showing the isotopic
C.Ma et al./Precambrian Research102 (2000) 279 – 301 298
Kongling grey gneiss and the Group-I magma could occur at a lower crustal depth (Ma et al., 1998). By comparing the two Liujiawa mafic mon-zodiorite samples (Table 3), it becomes clear that a sample (A5) with more ‘primary’ chemical
com-position, e.g. a Mg-number of 0.56 and a SiO2/
Al2O3ratio of 3.5 (Ma et al., 1998), close to those
of primary mantle melts (Kempton et al., 1997), does have less contaminated isotopic ratios of Nd and Sr than the other sample (A1) with a lower Mg-number of 0.45. The possibility of subduction enrichment therefore seems to be more plausible. Regardless of which mixing mechanism is pre-ferred, the Archean KLC may have been present below the Dabieshan terrane.
5. Implications for the crustal structure of the Dabieshan terrane
Analysis of geochemical and Nd and Sr isotope data of the DBC and KLC indicates that the protoliths of the Dabieshan felsic gneisses and Kongling grey gneisses have diverse origins. The Kongling grey gneiss is an Archean high-Al TTG rock with an average Nd model age of ca. 3.3 Ga, whereas the felsic gneisses of the DBC, which display a large variation in chemical composition and Nd and Sr isotopic ratios, constitute a meta-morphic thrust-stack sheet of welded slices that have different tectonic and metamorphic histories, as suggested by Okay and Sengo¨r (1992). Re-cently, Ames et al. (1996) attributed the protolith formation of the Dabieshan orthogneisses and eclogites mainly to rift magmatism between ca. 600 and 800 Ma. However, we argue that Archean components are present in the Da-bieshan terrane using four lines of evidence: (1) Some felsic gneisses in Dabieshan display the geochemical signature of Archean granitoids on a
plot of chondrite-normalized Yb versus La/Yb
(Fig. 5). Among them, two gneiss samples from the eclogite units show REE and incompatible element patterns that are identical to those of an Archean grey gneiss in the KLC (Fig. 6). (2) The Huangtuling intermediate granulite with a Nd model age of 3.1 Ga yields a WR-mineral
isochron of 22389300 Ma, which probably
rep-resents the age of granulite – facies metamorphism. This implies that the protolith age of the granulite
must be older than 22389300 Ma. (3) Besides
these Archean Nd model ages, Archean U – Pb zircon ages have been noted, not only in the dome region (e.g. Chen et al., 1996) but also in the eclogite units (Chavagnac and Jahn, 1996). Cao and Zhu (1995) separated zircon from a single Bixiling eclogite sample (M23 in Fig. 2) and ob-tained a four-point discordia with an
upper-inter-cept age of 2774924 Ma, and a lower-intercept
age of 452978 Ma. (4) Isotopic mixing
calcula-tions also indicate that the Archean Kongling grey gneisses have been involved the petrogenesis of the Liujiawa mafic monzodiorites intruded into in the eclogite units, and of some of the Group II rocks, implying that the KLC probably extended beneath the Dabieshan terrane during early Meso-zoic continental collision. It follows that the felsic gneisses with KLC affinity within the eclogite units may have resulted from tectonic extrusion of Yangtze basement beneath the Dabieshan during Mesozoic exhumation of the UHP metamorphic rocks.
C.Ma et al./Precambrian Research102 (2000) 279 – 301 299
1987) and would have geochemical features inter-mediate between typically Archean and post-Archean granitoids.
Therefore, we consider that the Dabieshan dome region was originally part of the continental base-ment of the Yangtze craton that formed as a late Archean to Early Proterozoic continental mag-matic arc, and later was separated from the Yangtze craton by Neoproterozoic rifting. After north-directed underthrusting of continental crust of the Yangtze craton and the Dabieshan micro-continent beneath the Sino – Korean craton,
indi-vidual terranes with distinct tectonic and
metamorphic histories amalgamated during early Mesozoic time. The orogen was then modified by intensive migmatization and magmatism, especially during Jurassic and Cretaceous time (Wang et al., 1998; Ma et al., 1998). Such a model can account for the wide range of geochemical features and isotope ages of the DBC, reflecting a variety of thermal and tectonic events and distinct crustal components.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Numbers 49572100 and 49972022), with addi-tional support from a grant (94-96) from the Fok Yingtung Education Foundation. MCQ is grateful to the China University of Geosciences at Wuhan
and A, bo Akademi University for having supported
his research effort at A,bo. We are grateful to Ma
Daquan for providing us with unpublished data, to Chris Gray for helpful suggestions on the isotopic mixing calculations, to Roger Mason for checking the manuscript, and to Tao Jidong and Ai Xiaoling for their help during the study. We appreciate the careful and constructive reviews by Michael Raith and Andreas Mo¨ller, which led to substantial improvement of the manuscript.
References
Ames, L., Zhou, G., Xiong, B., 1996. Geochronology and geochemistry of ultrahigh-pressure metamorphism with
im-plications for collision of the Sino – Korean and Yangtze cratons, central China. Tectonics 15, 472 – 489.
Arndt, N.T., Goldstein, S.L., 1987. Use and abuse of crust-formation ages. Geology 15, 893 – 895.
Barker, F., 1979. Trondhjemitic, definition, environment, and hypotheses of origin. In: Barker, F. (Ed.), Trondhjemites, Dacites, and Related Rocks. Elsevier, New York, pp. 1 – 12.
Boger, P.D., Faure, G., 1976. Systematic variations of sialic and volcanic detritus in piston cores from the Red Sea. Geochim. Cosmochim. Acta 40, 731 – 742.
Cao, R., Zhu, S., 1995. The Dabieshan coesite-bearing eclogite terrain — a NeoArchean ultra-high pressure metamorphic belt (in Chinese with English abstract). Acta Geol. Sinica 69, 232 – 242.
Carswell, D.A., O’Brien, P.J., Wilson, R.N., Zhai, M., 1997. Thermobarometry of phengite-bearing eclogites in the Da-bie Mountains of central China. J. Metamorph. Geol. 15, 239 – 252.
Chavagnac, V., Jahn, B.-M., 1996. Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China, Sm – Nd ages, geochemical characteristics and tectonic implica-tions. Chem. Geol. 133, 29 – 51.
Chen, J., Jahn, B.-M., 1998. Crustal evolution of southeastern China: Nd and Sr isotopic evidence. Tectonophysics 284, 101 – 133.
Chen, N., You, Z., Suo, S., Yang, Y., Li, H., 1996. U – Pb zircon ages of Dabieshan intermediate granulite and de-formed granite (in Chinese). Chin. Sci. Bull. 41, 1009 – 1012.
Condie, K.C., 1981. Archean Greenston Belts. Elsevier, Am-sterdam, p. 434.
DePaolo, D.J., 1981. Nd isotopes in the Colorado Front Range and crust – mantle evolution in the Proterozoic. Nature 291, 193 – 196.
Dickin, A.P., McNutt, R.H., Clifford, P.M., 1990. A neodymium isotope study of plutons near the Grenville Front in Ontario, Canada. Chem. Geol. 83, 315 – 324. Drummond, M.S., Defant, M.J., 1990. A model for
trond-hjemite – tonalite – dacite genesis and crustal growth via slab melting: archean to modern comparison. J. Geophys. Res. 95, 21 503 – 21 521.
Eide, E.A., McWilliams, M.O., Liou, J.G., 1994. 40Ar/39Ar geochronology and exhumation of high-pressure metamor-phic rocks in east-central China. Geology 22, 601 – 604. Faure, G., 1986. Principles of Isotope Geology, 2nd ed. Wiley,
New York, p. 589.
Gray, C.M., 1984. An isotopic mixing model for the origin of granitic rocks in southeastern Australia. Earth Planet. Sci. Lett. 70, 47 – 60.
Hacker, B.R., Ratschbacher, L., Webb, L., Dong, S., 1995. What brought them up? Exhumation of the Dabie Shan ultrahigh-pressure rocks. Geology 23, 743 – 746.
C.Ma et al./Precambrian Research102 (2000) 279 – 301 300
Hirajima, T., Wallis, S.R., Zhai, M., Ye, K., 1993. Eclogitized metagranitoid from the Su – Lu ultrahigh-pressure (UHP) province, eastern China. Proceedings of the Japanese Academy B69, 249 – 254.
Hofmann, A.W., 1988. Chemical differentiation of the Earth, the relationship between mantle, continental crust and oceanic crust. Earth Planet. Sci. Lett. 90, 297 – 314. Hofmann, A.W., 1997. Mantle geochemistry, the message
from oceanic volcanism. Nature 385, 219 – 229.
Jahn, B.-M., 1998. Geochemical and isotopic characteristics of UHP eclogites and ultramafic rocks of the Dabie orogen: implications for continental suduction and collisional tec-tonics. In: Hacker, B.R., Liou, J.G. (Eds.), When Conti-nents Collide: Geodynamics and Geochemistry of Ultrahigh-pressure Rocks. Kluwer, Dordrecht, pp. 203 – 239.
Jahn, B.-M., Glickson, A.Y., Peucat, J.J., Hickman, A.H., 1981. REE geochemistry and isotopic data of Archaean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolu-tion. Geochim. Cosmochim. Acta 45, 1633 – 1652. Jahn, B.-M., Wu, F., Lo, Ch.H., Tsai, Ch.H., 1999. Crust –
mantle interaction induced by deep subduction of the continental crust: geochemical and Sr – Nd isotopic evi-dence from post-collisional mafic-ultramafic intrusions of the northern Dabie complex, central China. Chem. Geol. 157, 119 – 146.
Jian, P., Yang, W., Zhang, Z., 1999.207Pb/206Pb zircon dating of the Huangtuling hypersthene-garnet-biotite gneiss from the Dabie Mountains, Hubei Province, China: new evi-dence for early Precambrian evolution. Acta Geol. Sinica 73, 78 – 83.
Kempton, P.D., Downes, H., Embey-Isztin, A., 1997. Mafic granulite xenoliths in Neogene alkali basalts from the Western Pannonian basin, insights into the lower crust of a collapsed orogen. J. Petrol. 38, 941 – 970.
Kro¨ner, A., Zhang, G.W., Sun, Y., 1993. Granulites in the Tongbai area, Qinling belt, China: geochemistry, petrol-ogy, single zircon geochronolpetrol-ogy, and implications for the tectonic evolution of eastern Asia. Tectonics 12, 245 – 255. Leat, P.T., Jackson, S.E., Thorpe, R.S., Stillman, C.J., 1986. Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the southern British Caledonides. J. Geol. Soc. Lond. 143, 259 – 273.
Li Fuxi, Ma Daquan, 1991. Huangling granitic batholith: the emplacement sequence, timing and petrogenesis. Yichang Institute of Geology and Mineral Resources Report, pp. 90 – 101.
Li, S.G., Xiao, Y., Liou, D., Chen, Y., Ge, N., Zhang, Z., Sun, S.-S., Cong, B., Zhang, R., Hart, S.T., Wang, S., 1993. Collision of the North China and Yangtse blocks and formation of coesite-bearing eclogites, timing and processes. Chem. Geol. 109, 89 – 111.
Liou, J.G., Zhang, R.Y., Jahn, B.-M., 1997. Petrology, geo-chemistry and isotope data on an ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie Mountains, east-central China. Lithos 41, 59 – 78.
Ludwig, K.R., 1997. Isoplot: a plotting and regression pro-gram for radiogenic isotope data version 2.95. USGS Open-File Report 91 – 445.
Ma, C., 1999. Magmatic Plumbing as a Window to Crustal Dynamics: An Example from the Dabieshan Ultradeep Metamorphic Rock Region, Central China. Ph.D. disserta-tion, A,bo Akademi University, A,bo/Turku, p. 142. Ma, C., Li, Z., Ehlers, C., Yang, K., Wang, R., 1998. A
post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone east-central China. Lithos 45, 431 – 456.
Ma, D., Li, Z., Xiao, Z., 1997. The constitute, geochronology and geologic evolution of the Kongling complex western Hubei (in Chinese with English abstract). Acta Geosci. Sinica 18, 233 – 241.
Martin, H., 1986. Effect of steeper Archaean geothermal gra-dient on geochemistry of subduction-zone magmas. Geol-ogy 14, 753 – 756.
Milisenda, C.C., Liew, T.C., Hofmann, A.W., Ko¨hler, H., 1994. Nd isotopic mapping of the Sri Lanka basement: update, and additional constraints from Sr isotopes. Pre-cambrian Res. 66, 95 – 110.
Mo¨ller, A., Mezger, K., Schenk, V., 1998. Crustal age domains and the evolution of the continental crust in the Mozam-bique belt of Tanzania: combined Sm – Nd, Rb – Sr, and Pb – Pb isotopic evidence. J. Petrol. 39, 749 – 783. Okay, A.I., Xu, S., Sengo¨r, A.M.C., 1989. Coesite from the
Dabie Shan eclogites, central China. Eur. J. Mineral. 1, 595 – 598.
Okay, A.I., Sengo¨r, A.M.C., 1992. Evidence for intracontinen-tal thrust-related exhumation of the ultra-high-pressure rocks in China. Geology 20, 411 – 414.
Okay, A.I., 1993. Petrology of a diamond and coesite-bearing terrain: Dabie Shan, China. Eur. J. Mineral. 5, 659 – 675. Pearce, J.A., 1980. Geochemical evidence for the genesis and
eruptive setting of lavas from Tethyan ophiolites. Proced-ings of the International Ophiolite Symposium, Nicosia, Cyprus, pp.261 – 272.
Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace ele-ment discrimination diagrams for the tectonic interpreta-tion of granitic rocks. J. Petrol. 25, 956 – 983.
Peucat, J.J., Vidal, P., Bernard-Griffiths, J., Condie, K.C., 1989. Sr, Nd, and Pb isotopic systematics in the Archean low- to high-grade transition zone of southern India, syn-accretion vs. post-syn-accretion granulites. J. Geol. 97, 537 – 550.
Rowley, D.B., Xue, F., Tucker, R.D., Peng, Z.X., Baker, J., Davis, A., 1997. Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan, U/Pb zircon geochronology. Earth Planet. Sci. Lett. 151, 191 – 203.
