Occurrence and Geochemistry of Khondalite Series in the Shandong Peninsula, China
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RESOURCE GEOLOGY, 44(1), 39•`49, 1994 Occurrence and Geochemistry of Khondalite Series in the Shandong Peninsula, China Haizhang JI*, Hidehiko SHIMAZAKI*, Shouxi Hu** and Yiying ZHAO** Abstract: The Jingshan Group of early Proterozoic era in the Shandong Peninsula of China, consists of highly metamor- phosed rock assemblages, and is firstly recognized as one of typical khondalite series. The Group is characterized by eco- nomic graphite, magnesite, phosphorus and iron deposits. The metamorphic grade reached amphibolite to granulite facies. The original rocks in the Jingshan Group can be divided into four types such as alumina-rich rock, quartzo-feldspathic rock, calc-silicate rock and carbonate rock. Major and rare earth element data of these rocks show that the original clastic sediments were mainly derived from nearby Jiaodong greenstone belt of Archean era. The rock assemblages suggest a shallow-water depositional environment on the margin of stable continent in early Proterozoic era. The thick accumulation of alumina-rich clastic sediments, large-scale biogenic graphite and magnesite deposits demonstrate that the paleoclimate was warm and damp in that time. located in the central part of the Peninsula. Early 1. Introduction Proterozoic metamorphic rocks of amphibolite to The Shandong Peninsula, Shandong Province, granulite facies, named the Jingshan Group, occur China, is widely occupied by Archean and Prot- surrounding the southern half of the Jiaodong erozoic rocks. The Archean rocks named the greenstone belt, as shown in Fig. 1. Those rocks Jiaodong Group occur as greenstone belt, and are are intruded by later granitic rocks of various Fig. 1 Geological map of the Shandong Peninsula. 1. khondalite series (Jingshan Group), 2. greenstone belt (Jiaodong Group), 3. granite, 4. late Proterozoic to Phanero- zoic sediments. Received on April 19, 1993, accepted on September 10, 1993 * Geological Institute, University of Tokyo, Hongo, Tokyo 113, Japan ** Department of Earth Sciences, Nanjing University, Nanjing 210008, China Keywords: Khondalite series, Jingshan group, Shandong Peninsula, China 39 40 H. JI, H. SHIMAZAKI, S. Hu and Y. ZHAO RESOURCE GEOLOGY: ages, mainly of Mesozoic. wet chemical method, and REE analysis by the The word "khondalite " was used for the first atomic absorption spectrometry. About two hun- time by WALKER(1902) to describe garnet-graph- dred thin sections were examined to establish ite-sillimanite schists in the southwestern part of mineral assemblages of the metamorphic rocks. the Kalahandi region in India. At present, most ge- ologists define khondalite as metamorphosed alu- 2. Occurrence of the Jingshan Group mina-rich metasediment series of early Protero- The Jingshan Group consists of highly meta- zoic age, along with marbles and quartzites. morphosed, well-bedded sedimentary rocks, and Khondalite represents special rock associations is mainly distributed at south of the Archean rather than a new rock type, thus a term "khondal- greenstone belt in the Shandong Peninsula (Fig. ite series" is commonly used. The diagnostic min- 1). The available radiometric ages (zircon U-Pb) eral assemblage of khondalite series is garnet - sil- of original rocks concentrated at 2.4 - 1.8 Ga (LIN limanite - graphite - quartz - K-feldspar (DISSA- and Yu, 1988). According to the data obtained by NAYAKEand MUNASINGHE,1984). Cordierite, JIN et al. (1987) using geothermometries and geo- kyanite, staurolite, plagioclase and biotite com- barometries of opx-cpx, opx-hb, cpx-hb, hb-pl, monly exist. In the case of calc-silicate rocks, scapolite-pi and Fe-Ti oxides, the temperature of mineral assemblage is diopside-scapolite-tremo- the principal metamorphic phase was estimated as lite-plagioclase-hornblende. Hypersthene, allanite 720 - 810•KC, the pressure as 0.5 GPa. Thus the and epidote frequently appear in the assemblage. metamorphic rock series belong to upper amphi- Chemical composition of khondalite series is bolite facies to lower granulite facies (JIN et al., characterized by high silica and alumina. The 1987). maximum content of Al2O3reaches above 30 wt According to LIN and Yu (1988), the Jingshan %. The content of alkali is low, and usually K20 Group is classified into three Subgroups: the >Na2O and MgO>CaO. Khondalite series are Lugezhuang, Yetou and Douya, each of which can markedly characterized by economic graphite, also be subclassified into two Members. The total magnesite, phosphorus and iron deposits (JIANG, thickness reaches 2940 m. Lithology and mineral 1990). In India and Sri Lanka, khondalite series assemblage of representative specimens of each contain manganese ore deposits (RADHAKRISHNAmember are examined in the present study, and and NAQVI,1986). The metamorphic grade gener- summarized in Table 1. The dominant metamor- ally reaches amphibolite to granulite facies. The phic rocks are: sillimanite - cordierite - graphite reason of high-grade metamorphism was con- gneisses, quartz - K-feldspar gneisses, diopside - nected with A-type subduction (continent-conti- hypersthene granulites, tremolite - diopside nent) by CHACKO et al. (1987) and BARBEY et granulites along with marbles and quartzites. al. (1982). Their protoliths were estimated from the min- The present study describes the occurrence and eral assemblages, and were grouped into four chemical characteristics of the Jingshan Group. types as follows: alumina-rich, quartzo-felds- Based on these data, the Group is for the first time pathic, calc-silicate and carbonate rocks. Mineral recognized as typical khondalite seires. Then ori- assemblages observed in each type are listed in gin and depositional environment of protolith of Table 2. The following is a brief description of the the Jingshan Group will be discussed. four protholiths and associated mineral deposits The studied area has been investigated in detail including quartzite. by some Chinese geological teams. Following the 2. 1 Alumina-rich rocks suggestions by the member of the 4th Geological Alumina-rich rocks occur as gneisses, schists Team of Shandong Province of the Ministry of and granulites with porphyroblasts of aluminous Metallurgy, the field survey in the present study minerals such as garnet, sillimanite, kyanite, was carried out intensely in several selected areas. cordierite and staurolite. Typical mineral assem- The chemical analyses of the collected samples blage is sillimanite - cordierite - graphite - garnet - were carried out at Nanjing University. Major quartz - K-feldspar. These rocks are mainly rec- component analysis was made by the traditional ognized in the lower part of H, and whole se- 44(1), 1994 Occurrence and geochemistry of khondalite series in the Shandong peninsula, China 41 Table 1 Stratigraphic sequence and lithologv of the Jingshan Group. Table 2 Mineral assemblages of rock types. quence of H3 Subgroup of the Jingshan Group (Table 1). Sillimanite shows flow texture around garnet crystals. Various amounts of granitic gneisses regularly occur, and are interlayered with alumina-rich rocks. The total thickness of the alu- mina-rich rocks is generally large. 2. 2 Quartzo-feldspathic rocks Granitic gneisses and biotite granulites com- prise this rock type. Quartz and feldspar are major minerals. The mineral assemblage is quartz - or- thoclase - microcline - plagioclase - biotite. Mus- covite is often recognized in thin sections. This rock type occurs as interlayers with alumina-rich rocks. 42 H. Ji, H. SHIMAZAKI,S. Flu and Y.ZHAO RESOURCE GEOLOGY: 2. 3 Calc-silicate rocks 2. 7 Phosphorus deposits These rocks mainly occur in the lower part of Three phosphorus deposits occur also in the H2 Subgroup of the Jingshan Group, and are char- lower part of H2 Subgroup near Laizhou City. The acterized by mineral assemblage of hornblende - wall rocks are metamorphosed calc-silicate rocks diopside - tremolite - plagioclase. Hypersthene, such as amphibolites and tremolite-diopside gran- allanite, epidote and scapolite often occur. The ulites. The mineral assemblage of ores is apatite - content of ferromagnesian minerals is high. The homblende - diopside - tremolite - plagioclase. major rock types include tremolite-diopside 2. 8 Magnesite deposits granulites, plagioclase amphibolites and hyper- Magnesite deposits of various sizes occur in the sthene - diopside granulites. Phosphorus and iron upper part of H2 Subgroup including the Laizhou deposits are associated with this rock type, and mine, which is the second largest magnesite pro- occur exclusively in the lower part of H2 Sub- ducer in China. The ore bodies are stratiform, and group. the relict of cross-bedding can be seen in the ores. 2. 4 Carbonate rocks As will be discussed later, magnesite deposits are Carbonate rocks in the Jingshan Group are all thought to be of evaporation-sedimentation origin metamorphosed to marble. Dolomitic marble is followed by hydrothermal enrichment. Besides major and calcitic marble is relatively rare. magnesitization, the associated alterations include Quartzites intercalated in marbles are always pure desilication, talcization and dolomitization. in composition. Typical mineral assemblage of 2. 9 Quartzite marbles is dolomite - calcite - diopside - forsterite Quartzite beds are observed as interlayers in - phlogopite. In thin sections, serpentine always marbles and alumina-rich gneisses. They contain replaces forsterite crystals with relict texture. The various amounts of feldspar, and vary from pure amount of magnesian minerals such as forsterite quartzite to feldspathic quartzite.