Introduction Geological background Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

Precambrian Research 102 2000 251 – 262 Geochemical and Sm – Nd isotopic study of amphibolites in the Cathaysia Block, southeastern China: evidence for an extremely depleted mantle in the Paleoproterozoic X.-H. Li a,b, , M. Sun b , G.-J. Wei a , Y. Liu a , C.-Y. Lee c , J. Malpas b a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, PO Box 1131 , Guangzhou 510640 , Guangdong, PR China b Department of Earth Sciences, The Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong c Department of Geology, National Taiwan Uni6ersity, 245 Choushan Road, Taipei 106 - 17 , Taiwan Received 3 June 1999; accepted 29 February 2000 Abstract Geochemical and Sm – Nd isotopic results are reported for late Paleoproterozoic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysia Block of SE China. Two suites of contemporaneous amphibolites are distinct in their geochemical characteristics. Group 1 samples, from NW Fujian, have chemical compositions of transitional and alkali basalts, show LREE-enriched patterns and plot mainly in the field of within-plate basalt on a number of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang, have tholeiitic compositions and are characterized by flat to LREE-depleted patterns and fall into the MORB field. All the amphibolite samples have high oNdT values of + 5.6 to + 8.5 T = 1766 9 19 Ma. A positive correlation between oNdT and NbTh suggests possible mixing of a mantle-derived magma and a crustal component, with the least-contaminated samples having very high oNdT values + 8 + 8.5 and NbTh ratios of 13. The geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto-oceanic basin e.g. the Gulf of Tadjoura. The exceptionally high oNdT values of up to + 8.5 for some of the amphibolites suggest the presence of a time-integrated extremely depleted mantle source beneath Cathaysia during the Paleoproterozic. © 2000 Elsevier Science B.V. All rights reserved. Keywords : Amphibolite; Paleoproterozic; Cathaysia Block; Southeastern China; Sm – Nd isotopes; Depleted mantle www.elsevier.comlocateprecamres

1. Introduction

South China consists of two major tectonic blocks separated by the Jiangshan – Shaoxing Fault Fig. 1. Most isotope age studies have shown that the Cathaysia Block is of Pale- oproterozoic-Mesoproterozoic age e.g. Shui, Corresponding author. Fax: + 86-20-85514130. E-mail address : gzisolabpublic.guangzhou.gd.cn X.-H. Li 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 6 7 - X 1987; Jahn et al., 1990; Hu et al., 1991; Li et al., 1992; Gan et al., 1993, 1995; Chen and Jahn, 1998, although a few workers have suggested that it is as old as Archean, based on Sm – Nd ‘isochron’ and inherited zircon U – Pb ages Fu et al., 1991; Zhou, 1997. More recently, Li 1997a reported a SHRIMP U – Pb zircon age of 1766 9 19 Ma for the oldest amphibolites exposed in NW Fujian, confirming a Late Paleoproterozoic age for these rocks. The Precambrian tectonic evolu- tion of Cathaysia is, however, still poorly under- stood, largely due to the paucity of detailed geochemical, petrological and structural data for the metamorphic rocks. This paper presents the results of a detailed geochemical and Sm – Nd isotopic study of amphi- bolites from NW Fujian and SW Zhejiang, within the Cathaysia Block, and provides evidence for the existence of an extremely depleted mantle source and for the development of magmatism in a riftproto-ocean basin system in the Paleoproterozoic.

2. Geological background

Precambrian basement of the Cathaysia Block is mainly exposed in an area between the Jiang- shan – Shaoxing and Lishui – Haifeng Faults in NW Fujian and SW Zhejiang Provinces Fig. 1. On the basis of lithological, structural and meta- morphic features, the basement rocks are divided into two metamorphic sequences. The lower se- quence, termed the Mayuan Group in NW Fujian and the Badu Group in SW Zhejiang, consists mainly of leptynites, schists, granitic gneisses, and mafic and felsic meta-volcanics Hu et al., 1991; Jin et al., 1992. Granitic gneisses crop out mostly Fig. 1. Simplified geological map of Cathaysia Block, SE China, illustrating the distribution of Precambrian basement modified after Hu et al. 1992 and Gan et al. 1993. in the Tangyuan Formation the lowest part of the Badu Group, and display a transitional rela- tionship with the surrounding leptynites, with a few granitic gneisses showing intrusive contacts with the country rocks Hu et al., 1991. These rocks have undergone amphibolite facies meta- morphism. The upper sequence, i.e. the Mamian- shan Group in Fujian and the Longquan Group in Zhejiang, mainly comprise schists, meta-vol- canics, Fe-bearing quartzites and marbles. These rocks have been metamorphosed to lower green- schist facies Hu et al., 1991; Jin et al., 1992. Samples of amphibolite analyzed in this study were collected from two localities along road-cuts in Tianjingping, Jianning County, NW Fujian and Zhulu, Longquan County, SW Zhejiang Fig. 1. Samples from NW Fujian include LG24, LG28, LG29 and LG35 with chemical compositions of transitional and alkali basalts Group 1, while those from SW Zhejiang include LB258, LB259, LB2622, LB263, LB264 and LB265 with tholeiitic compositions Group 2, and one sample, LB261, with transitional basalt composition similar to Group 1 samples. A 50 kg amphibolite sample LG28, collected from NW Fujian yields a SHRIMP U – Pb zircon age of 1766 9 19 Ma which is interpreted as the crystallization age for the protolith of the amphibolites Li, 1997a. This zircon date provides the best estimate of the for- mation age of the Mayuan Group in NW Fujian. Amphibolites in SW Zhejiang have not been di- rectly dated, but U – Pb zircon dates of 1832 9 80 Ma, 1870 9 36 Ma, 1889 9 95 Ma and 1975 9 80 Ma obtained by conventional U – Pb techniques were reported for the granitic gneisses Hu et al., 1992; Gan et al., 1993, 1995. Because the granitic gneisses are intrusive into the surrounding lep- tynites and amphibolites, the lowest part of the Badu Group, their ages 1.8 – 1.98 Ga are sup- posed as minimum age for the Badu Group. It is noted, however, that all the zircons from the granitic gneisses are highly discordant, and the significance of the upper intercept ages of 1.8 – 1.98 Ga is equivocal. A precise 207 Pb 206 Pb age of 1743 9 8 Ma by evaporation techniques has been reported for a pegmatite which intrudes the Tangyaun Formation Gan et al., 1995. This age provides a minimum age for the amphibolites in SW Zhejiang. Geological and chronological data therefore suggest that the amphibolites from NW Fujian and SW Zhejiang are likely contemporane- ous, although the precise age of the SW Zhejiang amphibolites is not yet known.

3. Analytical methods