Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating
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Abstract The 5‐km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)‐metamorphic rocks consisting mainly of eclogite, gneiss and garnet‐peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low‐ P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite‐facies minerals such as phengite and jadeite. The zircon cores give highly variable 206 Pb/ 238 U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO 2 , Al 2 O 3 , K 2 O, FeO, MgO, Na 2 O and H 2 O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.Keywords:
Coesite
Metasomatism
Peridotite
Abstract Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh‐pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A‐subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong‐northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo‐Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240‐200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.
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Coesite inclusions are found in kyanite from the Lanshantoueclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastem Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.
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Coesite- and kyanite-bearing eclogites are abundant in the southern part of the Dabie Mountains (southern Dabie terrane-SDT). Two types of eclogites from the SDT (Types III and IV) were selected for detailed paragenetic study. Type III eclogites, most abundant in the northern part of the SDT, occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite + garnet + phengite + epidote + coesite + kyanite + carbonate + rutile + ilmenite. These minerals exhibit weak compositional zoning and contain few mineral inclusions. Type IV eclogites, mostly in the southern part of the SDT, occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite + garnet + glaucophane + kyanite + epidote + phengite + quartz + rutile + ilmenite. Garnets of Type IV eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite, phengite, epidote, quartz, and rutile in the core and omphacite, barroisite, and Mg-katophorite in the rim. Prograde blueschist facies (~400°C) assemblages were partially preserved in Type IV eclogites. The eclogitic assemblages of both types of eclogite have been partially or completely retrograded to amphibolite and greenschist facies assemblages. Parageneses and compositions of minerals from eclogites indicate that these rocks have undergone a clockwise P-T evolution path. Within the SDT, the temperatures, estimated according to $$K_{D(Cpx-Gt)}$$ and $$K_{D(Gt-Phen)}$$ for eclogites, indicate a systematic decrease from about 770°C in the north to 580°C in the south. Such variation is also evident for pressure estimates, as coesite occurs only in eclogite in the north, whereas the assemblage omphacite + kyanite + quartz (without coesite) is found in the south. This study, together with ultrahigh-pressure mineral assemblages identified in the gneiss-marble country rock, suggests that the continental crust of the SDT has been subjected to a regional ultrahigh-pressure metamorphism as part of a north-dipping subduction zone formed between the Sino-Korean and Yangtze cratons in Triassic time.
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Recent highway construction across the Qinglong Mountains in Donghai, eastern China (Figure 1) has exposed a 660 m section of gneiss, eclogite, and minor quartzite. The outcrop consists of steeply SE-dipping layers cut by several high-angle (normal?) faults (Figure 2). Eclogite layers are concordant with gneissic rocks and minor quartzites; a few discordant eclogite contacts (e.g., QL16) appear to be intrusive. Most eclogites are characterized by abundant hydrous phases including talc, phengite, and epidote; many also contain kyanite. The Qinglong Mountains contain a variety of UHP eclogites, and are the classic locality for negative δ18O values (-14 to -16‰) for UHP minerals in eclogites, quartzites and surrounding gneisses (Yui et al. 1994; Rumble and Yui 1998) indicating that fluid may have been absent during metamorphism. Inclusions of coesite and coesite pseudomorphs in UHP phases, including epidote and kyanite from both eclogite and quartzite, of this region were reported by Zhang et al. (1995; in review). This region lies about 22 km NE from the Chinese Continental Scientific Drilling site in Donghai. Therefore, petrotectonic information derived from this continuous exposure of eclogitic and gneissic rocks is applicable to the CCSD site.
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Kyanite-bearing eclogitic assemblages occur in the highest-grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite-eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca-rich/Mg-poor inner and Ca-poor/Mg-rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg-rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower-pressure conditions of subsequent epidote-amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3-2.4 GPa/675-740 °C. The metamorphic P-T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg-rich composition was in equilibrium with other eclogite facies phases. Therefore, P-T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re-examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.
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