Analytical methods Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:
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.