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, dolomite, zoisite, 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.
Fig. 1. Simplified tectonic map of the Qinling – Dabieshan – Sulu collisional belt showing the major structural units and distribution of UHP and HP metamorphic zones. Inset shows the location of the collisional belt in relation to the major structure in China.
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
