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 UHPHP 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 UHPHP 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 B 1 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 age of 2663 9 56 Ma on an intermediate granulite
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 of o
Nd
values 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 2456 9 7 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 756.6 9 0.8 Ma on an orthogneiss from northern
Dabieshan Xue et al., 1997, and of 133.7 9 2.3 Ma and 134.0 9 2.8 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 UHPHP eclogite-bearing units ‘eclog- ite units’ hereafter comprise amphibolite 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
metamorphic conditions of 680 – 850°C and 2.6 – 3.9 GPa Hacker et al., 1995; Carswell et al.,
1997, while the HP eclogites attained metamor- phic conditions of 600 – 700°C at about 2.2 GPa
Carswell et al., 1997. The eclogite facies meta-
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 772.5 9 9.5 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 and
40
Ar
39
Ar 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 km
2
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. 2936 9 98 Ma, Ames et al., 1996; 2850 9 15 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 3290 9 170 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 2031 9 4 Ma, which is considered an approximate estimate of the age of the basaltic
protolith Ma et al., 1997.
3. Analytical methods
