Granulite-facies Overprinting of Ultrahigh-pressure Metamorphic Rocks, Northeastern Su-Lu Region, Eastern China

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 4 PAGES 563–582 2000 Granulite-facies Overprinting of Ultrahighpressure Metamorphic Rocks, Northeastern Su-Lu Region, Eastern China D. NAKAMURA∗ AND T. HIRAJIMA DEPARTMENT OF GEOLOGY AND MINERALOGY, GRADUATE SCHOOL OF SCIENCE, KYOTO UNIVERSITY, KYOTO 606-8502, JAPAN RECEIVED AUGUST 10, 1998; REVISED TYPESCRIPT ACCEPTED SEPTEMBER 27, 1999 Secondary enstatite is present in ultrahigh-pressure (UHP) metamorphic rocks at two localities of Rongcheng County, in the northeastern Su-Lu region, eastern China. An enstatite-bearing eclogitic rock consists mainly of large grains of garnet and clinopyroxene, and enstatite is present as fine-grained coronas around quartz in the matrix. The enstatite coronas tend to develop near garnet grains, and plagioclase forms between the two minerals. In the second enstatite-bearing eclogite, the assemblage of enstatite + diopside is present as coronas around quartz. Textural relationships indicate that enstatite formed under plagioclase-stable and Sisaturated conditions after the peak-P metamorphism. Several reaction curves constrain the enstatite-forming conditions to be 700–800°C and 0·7–1·2 GPa. Application of thermometers to enstatite coronas also indicates high-T (>700–800°C) conditions. In these rocks, kyanite has been partially replaced by spinel + anorthite symplectites. Similar development of spinel is common in kyanite eclogite from Rongcheng County, suggesting that all share a similar decompressional history. Granulite-facies overprinting of UHP eclogite is probably a common phenomenon in the northeastern Su-Lu region. Equilibrium temperature at the peak-P stage is approximately the same as that recorded at the granulite stage, implying nearly isothermal decompression. Although such an adiabatic path generally requires rapid exhumation of the UHP rocks, the scale of the exhuming body is also important. Calculation of the length scale for thermal conduction indicates that UHP rocks must be greater than >10 km in scale to avoid loss or gain of heat during the exhumation. Individual UHP eclogitic blocks are smaller than required for adiabatic exhumation in Rongcheng County, and hence they probably ascended together with the surrounding orthogneiss. ∗Corresponding author. Present address: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan. Tel.: +81-3-5734-2338. Fax: +81-3-5734-3538. e-mail: KEY WORDS: enstatite; granulite facies; ultrahigh-pressure (UHP) meta- morphism; Su-Lu region; Rongcheng County INTRODUCTION Coesite and diamond have been found in metamorphic rocks from several suture zones formed by continental collision (e.g. Chopin, 1984; Smith, 1984; Coleman & Wang, 1995). These rocks are termed ultrahigh-pressure (UHP) metamorphic rocks. Many of the UHP rocks are, however, eclogitic blocks surrounded by orthogneiss that lacks evidence for UHP metamorphism. This observation raises several questions: (1) did the eclogitic blocks ascend along with surrounding orthogneiss? (2) Why did the blocks ascend to crustal levels despite their higher density than mantle materials? (3) What is the main driving force for the ascent of UHP rocks? Previous studies have provided petrological data concerning the pressure– temperature (P–T ) histories of UHP rocks of several UHP provinces. For example, in the Dora Maira Massif of the Western Alps, detailed studies (e.g. Chopin et al., 1991; Schertl et al., 1991; Hirajima & Compagnoni, 1993) confirmed that the exhumation of the UHP rocks was accompanied by significant cooling. In contrast, the P–T paths of granulites with relict coesite found in Weihai of the Su-Lu region, eastern China (Wang et al., 1993), suggest that they ascended without significant cooling (Wang et al., 1993; Zhang et al., 1995b). Such granulitefacies overprinting of UHP rocks has, however, not been reported elsewhere in the Su-Lu region. We therefore  Oxford University Press 2000 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 4 APRIL 2000 Fig. 1. Geological map of Shangdong Peninsula. carried out a petrological study in Rongcheng County, which is about 50 km away from Weihai, and discovered secondary enstatite at two localities. This paper describes the enstatite-bearing eclogitic rocks and other UHP rocks from Rongcheng County and discusses their decompressional P–T path and its implication for exhumation processes. GEOLOGICAL SETTING The southeastern side of Shangdong Peninsula (the SuLu region; see Fig. 1) is the eastern extension of the Qinling–Dabie orogenic belt that developed between the Sino-Korean and the Yangtze cratons. In the last decade, coesite and its polycrystalline quartz pseudomorphs have been found as inclusions in eclogite minerals throughout the Dabie and Su-Lu region of this orogenic belt (see Coleman & Wang, 1995), which is thereby recognized as a UHP province formed by subduction of crustal materials during continental collision (Ernst & Liou, 1995). The metamorphic age of the UHP eclogites ranges from 210 to 240 Ma (e.g. Sm–Nd age by Li et al., 1993; U–Pb age by Ames et al., 1993). Shangdong Peninsula consists of two parts separated by the Yantai–Qingdao–Wulian (YQW) fault (Fig. 1); the southeastern half (the Su-Lu region) is made up of Mesozoic orthogneiss that contains UHP rocks, whereas the northwestern half is composed of middle Proterozoic gneiss that lacks evidence of UHP metamorphism (Enami et al., 1993a). Although coesite-bearing eclogite blocks are present in the Su-Lu region, the orthogneiss in which the blocks are found shows no evidence of UHP metamorphism. This raises the question of in situ vs exotic origins of the eclogite blocks. Recently, metagranitoid preserving UHP evidence was found on the Yangkou Beach in the Su-Lu region (Hirajima et al., 1993; Wallis et al., 1997) and hence at least some of the surrounding felsic rocks clearly have been metamorphosed together with the eclogite, supporting the in situ origin. Two enstatite-bearing eclogite localities [Yanggongtun (YGT) and Datuan (DX); see Fig. 2] are located in Rongcheng County of northeastern Su-Lu region (Fig. 1). At Yanggongtun, relationships between eclogite and country rocks are not clear because of poor exposures. Ye & Hirajima (1996) reported a marble lens about 1 km away to the south of this locality, but no peridotite 564 NAKAMURA AND HIRAJIMA GRANULITE-FACIES OVERPRINTING Fig. 2. Distribution of eclogitic blocks in Rongcheng County after Ye & Cong (1994), and the localities of the enstatite-bearing eclogitic rocks. YGT, Yanggongtun; DX, Datuan; CJ, Chijiadian; TJJ, Tengjiaji; X, Xianguling; LH, Linghou. has been found. At Datuan, enstatite-bearing eclogite is associated with peridotite. Coesite-bearing eclogite is likewise interbedded with peridotite in Chijiadian (CJ). Eclogites from Xianguling (X), Linghou (LH) and Tengjiaji (TJJ) probably occur as blocks surrounded by orthogneiss (Ishiwatari et al., 1992). SAM (...truncated)