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Journal of Petrology Volume 41 Number 4 Pages 563-582 2000
© Oxford University Press 2000
Granulite-facies Overprinting of Ultrahigh-pressure Metamorphic Rocks, Northeastern Su-Lu Region, Eastern China
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
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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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 Si-saturated 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. KEY WORDS: enstatite; granulite facies; ultrahigh-pressure (UHP) metamorphism; Su-Lu region; Rongcheng County
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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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 granulite-facies overprinting of UHP rocks has, however, not been reported elsewhere in the Su-Lu region. We therefore 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 |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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The southeastern side of Shangdong Peninsula (the Su-Lu 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).
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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 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).
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SAMPLE DESCRIPTIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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This paper focuses on kyanite-bearing eclogitic rocks, as they provide more constraints on P–T conditions than kyanite-free ones. We refer to high-P (eclogite facies) and low- or middle-P (amphibolite or granulite facies) metamorphic stages as ‘primary’ and ‘secondary’, respectively (Table 1). Eclogites associated with peridotite are generally very fresh (91CJ6, 6B, 89CJ1A and 89DX3). In these ‘fresh’ eclogites, garnet, omphacite, kyanite and quartz grains are generally in direct contact with each other, and secondary minerals are rarely observed. Relict coesite is found as an inclusion in garnet in sample 91CJ6 (Fig. 3a). Sample 91CJ6B, however, displays a zone in which fine-grained symplectites have replaced most of the primary minerals (Fig. 3b). The symplectitic zone consists mainly of diopside, tremolite, pargasite, plagioclase and epidote. Relict garnet and omphacite grains are also present. Although the eclogite that contains this symplectitic zone rarely displays any hydrous minerals, the symplectitic zone is rich in such minerals. Aqueous fluid was evidently introduced into the eclogite block, and this fluid played important roles as a catalyst (e.g. Rubie, 1986
) and/or a reactant to form secondary minerals. Such aqueous fluid was probably released by dehydration of gneiss that contains eclogite and peridotite blocks, as shown in the Central Alps (Heinrich, 1982). Primary minerals in the eclogite blocks that are associated with peridotite are well preserved except in symplectitic zones, probably because the surrounding garnet peridotite prevented the influx of large quantities of fluid. In contrast, eclogite blocks that are directly surrounded by gneiss commonly display symplectites, and most grain boundaries between primary minerals have been replaced by secondary ones. In such blocks, fine-grained symplectites of diopside ± calcic amphibole + plagioclase have partially or completely replaced omphacite. However, symplectites are uncommon between garnet and omphacite; instead, amphibole layers have formed. Small grains of amphibole and diopside also occur as coronas around quartz. Plagioclase and fine-grained symplectites of spinel + plagioclase have partially replaced kyanite.
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Sample 91YGT4A from Yanggongtun consists mostly of garnet and clinopyroxene grains 0·5–1·5 mm in diameter. Enstatite is found in fine-grained coronas around matrix quartz. Clinopyroxene (Cpx), quartz and kyanite are present both as inclusions in garnet and as matrix grains. These are, therefore, regarded as primary minerals formed under high-P conditions. The kyanite inclusions, which occur as 0·03 mm grains, are found only near garnet rims, suggesting that kyanite formed relatively late. Primary matrix minerals have been partially replaced by secondary ones; plagioclase and/or pargasite surround garnet, tremolite + plagioclase symplectites have replaced Cpx, and subhedral grains of colorless hornblende cut Cpx. The coronas of enstatite are most commonly found around quartz grains near garnet with plagioclase between enstatite and garnet grains (Fig. 4a). Thin films of talc are found between enstatite and quartz (Fig. 4a). Matrix kyanite, which occurs as 0·1–0·5 mm grains, is surrounded by spinel + plagioclase symplectites.
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Samples 91YGT1 and 94YGT5D, which are typical of eclogitic rocks from Yanggongtun, consist mostly of garnet porphyroclasts about 3 mm in diameter, along with matrix grains of kyanite, quartz and colorless hornblende, and symplectites of diopside ± tremolite ± pargasite + plagioclase. Matrix omphacite has been completely replaced by symplectites, but a few grains are preserved as inclusions in garnet in sample 91YGT1. Many needles of rutile are also included in garnet and show preferred orientations. Some garnet grains contain single crystal inclusions of quartz, and the host garnet exhibits radial fractures around such inclusions. Generally, garnet grains have been partially replaced by pargasite ± plagioclase along rims and some cracks. Kyanite grains have been partially replaced by spinel + plagioclase symplectites, and matrix quartz is surrounded by coronas of diopside and/or tremolite (Fig. 4b).
Sample 91YGT2 consists of alternating melanocratic and leucocratic layers, both of which contain garnet grains 0·5–2·5 mm in diameter that display acicular rutile inclusions. Pargasite + plagioclase symplectites and small grains of plagioclase are the main constituents of the melanocratic and leucocratic layers, respectively. Plagioclase in the leucocratic layers is granoblastic in texture. Other constituents of the leucocratic layers are kyanite, quartz and zoisite (or epidote). Quartz is surrounded by coronas of tremolite and/or cummingtonite, and kyanite is rimmed by symplectites of spinel + plagioclase ± corundum (Fig. 4c). Small grains of staurolite and corundum also occur near kyanite relics. Staurolite grains are subhedral to euhedral and in contact with plagioclase (Fig. 4c). Corundum grains are anhedral or subhedral. Some have been partially replaced by margarite (Fig. 4c and 4d), although the other grains are in contact with plagioclase.
Sample 89DX2 from Datuan contains garnet grains 0·5–1·5 mm in diameter, but most omphacite grains have been completely decomposed. Enstatite is found in coronas around quartz and in symplectites composed of enstatite + plagioclase ± tremolite (Fig. 5a). The enstatite + plagioclase symplectites are found far from any of the primary minerals, and we cannot identify the precursor of symplectitic enstatite. Symplectites of pargasite + plagioclase and of diopside + tremolite + plagioclase have partially replaced garnet and omphacite, respectively. Coronas around quartz consist of three assemblages: (1) enstatite + diopside (Fig. 5b), (2) enstatite + tremolite, and (3) tremolite + diopside. Kyanite grains have been partially replaced by symplectites of spinel + plagioclase ± corundum.
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Samples 91TJJ4 and 4B from Tengjiaji consist of a melanocratic layer with leucocratic lenses about 10–20 mm along their major axes. Both the layer and lenses contain garnet grains about 1 mm in diameter. The matrix of the melanocratic layer consists mainly of calcic amphibole ± diopside + plagioclase symplectites. Omphacite and phengite are present only as inclusions in garnet and zoisite, respectively. Kyanite grains occur in the leucocratic lenses, where they are commonly surrounded by symplectites of spinel + plagioclase or of corundum + plagioclase. Small grains of staurolite are also disposed around kyanite grains. The staurolite grains are subhedral to euhedral, and some contain inclusions of spinel (Fig. 5c). Both of these features indicate that the staurolite formed after or during the growth of the spinel + plagioclase symplectites. Garnet + plagioclase ± biotite symplectites surround some garnet grains in the leucocratic lenses (Fig. 5d).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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Mineral compositions were determined with an electron-probe microanalyzer, Hitachi scanning electron microscope, S550, with an energy-dispersive X-ray analytical system (Kevex 8000 + Kevex Quantum detector) at Kyoto University (Table 2). The analytical procedure follows that of Mori & Kanehira (1984)
and Hirajima & Banno (1991). A 20 kV accelerating potential and a 500 pA beam current were employed with 250 s livetime for counting. Constancy of the beam current was confirmed before and after each analysis. Choice of standards is as follows: albite for Na, periclase for Mg, corundum for Al, quartz for Si, adularia for K, wollastonite for Ca, rutile for Ti, Mn metal for Mn, and hematite for Fe.
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Garnet
Garnets from eclogite blocks associated with peridotite are generally richer in Mg [Xprp, i.e. Mg/(Fe + Mn + Mg + Ca), = 0·3–0·5] than those from eclogite blocks associated with gneiss (Fig. 6). Garnet from the Yanggongtun samples is very rich in Mg (Xprp = 0·4–0·6). Therefore, the Yanggongtun eclogitic block was possibly associated with peridotite, although no peridotite has been found around this locality.
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In most samples, the composition of garnet varies little. Each grain is either chemically homogeneous or shows a slight decrease of grossular content from core to rim, and no chemical zonation that might reflect the prograde history was observed. In contrast, garnet grains from samples YGT, 89DX2, 91TJJ4 and 4B vary widely in composition (Fig. 6).
Garnet grains from the Yanggongtun samples are similar in composition [Xprp = 0·4–0·6; Xgrs, i.e. Ca/(Fe + Mn + Mg + Ca) 0·2] and in their preserved chemical zonation. Their pyrope contents decrease gradually with increasing almandine contents from grain cores to rims (e.g. Xprp = 0·60–0·48 in sample 91YGT4A; Xprp = 0·54–0·40 in sample 91YGT1). This similarity suggests that the Yanggongtun samples had similar protoliths and experienced the same P–T history.
Garnet grains from samples 89DX2, 91TJJ4 and 4B are rich in Ca (up to Xgrs = 0·5). Garnet from sample 89DX2 is also rich in Mg (Xprp = 0·3–0·4). Plagioclase occurs as vermicular intergrowth with garnet grain rims in sample 91TJJ4, which has produced symplectites of garnet + plagioclase ± biotite (Fig. 5d). Although some of the vermicular garnet grains have compositions similar to garnet grain cores, most are clearly poorer in Ca (Xgrs = 0·1–0·2) than the latter (Xgrs = 0·4; Fig. 6). This suggests a decrease of grossular during the growth of vermicular plagioclase.
Clinopyroxene
Except for the Yanggongtun samples, primary Cpx grains from eclogite blocks associated with peridotite are richer in Mg/Fe than those associated with gneiss (Fig. 7a). In the enstatite-bearing sample 91YGT4A, the Cpx is the most magnesian of all investigated samples [XMg, i.e. Mg/(Fe2+ + Mg) 0·95]. A positive correlation between average Fe/Mg ratios of garnet and Cpx (Fig. 7b) suggests that differences in mineral compositions among samples are mainly due to rock bulk composition. In addition, a positive inter-sample correlation between Fe/Mg and Na/(Na + Ca) (Fig. 7a) suggests that Fe-rich samples also have Na-rich bulk compositions, consistent with increasing crystal fractionation in the protolith basaltic magma.
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In less symplectized samples, omphacite occurs both as inclusions in garnet and as matrix grains with no difference in composition between the two. Omphacite grains display nearly homogeneous cores, but many grains show decreasing jadeite contents toward rims. However, the compositions of even such homogeneous cores may have been modified after the peak-P stage. Most omphacite grains have been partially replaced by symplectites of diopside ± amphibole + plagioclase. Cores of such omphacite grains are generally poorer in jadeite content than those of omphacite grains that lack symplectites [see fig. 10 of Nakamura & Banno (1997)
]. In addition, Na/(Ca + Na) and Si positively correlate in each sample (Fig. 8). Generally, the Ca-Tschermak component of Cpx increases and the jadeite component decreases with decreasing pressure or rising temperature (e.g. White, 1964). An increase in Ca-Tschermak content of Cpx is reflected in a decrease of its Si content and the correlation observed in Fig. 8. Thus, most Cpx grains were modified in composition during decompression and possible thermal relaxation, becoming less jadeitic and richer in Ca-Tschermak.
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Minerals around quartz
Various minerals occur as coronas around quartz. Diopside is the most common of these minerals, but enstatite is found in samples 91YGT4A and 89DX2. The paragenetic relationships of the enstatite-bearing eclogitic rocks are shown by solid tie-lines in the Ca–Mg–Fe diagram (Fig. 9a). In sample 89DX2, mineral compositions depend on the coexisting phase: enstatite (XMg 0·70) + diopside (XMg 0·85), enstatite (XMg 0·75) + tremolite (XMg 0·9), and tremolite (XMg 0·9) + diopside (XMg 0·90). Enstatite in sample 91YGT4A is richer in Mg than that in sample 89DX2 and does not coexist with diopside. The phase diagram of Fig. 9b was calculated employing thermodynamic data of Holland & Powell (1990). Both our observations and the calculated diagram show that the Fe–Mg distribution coefficient between enstatite (En) and tremolite (Tr), (Fe/Mg)En/(Fe/Mg)Tr, is >1·0 and that between diopside (Di) and tremolite, (Fe/Mg)Di/(Fe/Mg)Tr, is 1·0. Therefore, in relatively Mg-poor compositions the En + Di assemblage is present, whereas in Mg-rich compositions En and Di do not coexist with each other because tremolite is stabilized. Thus, mineral assemblages were evidently controlled by local bulk composition, and local equilibrium was probably achieved during enstatite formation. However, why is orthopyroxene absent from Fe-rich samples (Fig. 7)? A possible reason is suggested by phase diagrams for the CaO–FeO–MgO–Al2O3–SiO2–H2O system with excess quartz and H2O (Fig. 10). In all samples, plagioclase is in contact with coronas around quartz. The local bulk compositions should, therefore, lie within the field of almandine + hedenbergite + anorthite on the Al–Ca–Fe diagram (Fig. 10a). The Ca–Mg–Fe phase diagrams projected from anorthite, quartz and H2O (Fig. 10b) indicate that orthopyroxene is stable only in Mg-rich bulk composition under high-P granulite-facies conditions.
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Minerals around kyanite
Most minerals around kyanite are rich in Al and poor in Si. Kyanite is generally surrounded by plagioclase ± other Al-rich minerals. Corundum and spinel are found around kyanite grains in spite of the presence of quartz in the same thin sections. Spinel grains are hercynite or spinel compositions [Mg/(Fe + Mg) = 0·2–0·7] with minor amounts of chromite component. In general, the coexistence of spinel + quartz indicates ultrahigh-T (>1100°C at 1·0 GPa) metamorphism (Spear, 1993
). The assemblage of corundum + quartz is likewise only stable under extremely high-T (>1000°C at 1·4 GPa) conditions (Guiraud et al., 1996). Corundum and spinel in the study samples are, however, not in contact with quartz, suggesting that those phases were not in equilibrium with quartz.
Staurolite occurs near kyanite in samples 91YGT2, 91TJJ4 and 4B. Mg/(Fetotal + Mg) ratios (XMg*) of staurolite from low-P/T (andalusite–sillimanite type) metamorphic terranes are <0·20 (e.g. Guidotti, 1970, 1974; Baltatzis, 1979; Hudson, 1980), but the staurolite grains we studied show XMg* of 0·20–0·40. In general, Mg-rich staurolite is stable in Si-undersaturated high-P and -T environments (Deer et al., 1992). According to the summary of synthetic experimental data (Schreyer, 1988), Mg-staurolite is stable above 1·4 GPa and between 700 and 1000°C. Natural Mg-staurolite (XMg* = 0·85–0·95) has been found as inclusions in garnet from the UHP Dora Maira Massif (Chopin & Sobolev, 1995). However, the Mg-rich staurolite described here probably formed under medium-P conditions, for the following reasons. The staurolite grains occur in matrices with plagioclase grains [XAn, i.e. Ca/(Ca + Na) = 0·90–0·95 in sample 91YGT2], and the former is euhedral to subhedral not replaced by the latter. This texture indicates that the staurolite formed contemporaneously with the plagioclase. Schumacher & Robinson (1987) reported Mg-rich staurolite (XMg* = 0·35–0·42) that is set in a matrix of plagioclase (XAn = 0·4–0·9) or cordierite from southwestern New Hampshire, USA, and Nicollet (1986) reported Mg-rich staurolite (XMg* = 0·52) in an anorthosite (XAn = 0·9) vein from the south Vohibory area, Madagascar. In both examples, Mg-rich staurolite evidently grew under anorthite-stable conditions, as well as in the case in this study.
In staurolite-bearing samples 91YGT2, 91TJJ4 and 4B, anhedral or subhedral corundum grains have formed around kyanite. In samples 91TJJ4 and 4B, corundum + plagioclase (XAn = 0·4–0·8) symplectites have partially replaced some kyanite grains. In sample 91YGT2, spinel + corundum + anorthite (XAn 0·9) symplectites are found around kyanite, but some corundum grains are not associated with kyanite and instead appear in small clusters with anorthite (Fig. 4d). The bulk chemical composition of one such corundum + anorthite domain was calculated using the chemical compositions of constituent minerals, a modal analysis, and molar volume data of Holland & Powell (1990). The result gives a formula of about Al2·1Si0·7Ca0·3H0·2O5 (the other cations aggregate <0·1) which is close to the ideal formula of kyanite, Al2·0Si1·0O5, suggesting that this domain is a pseudomorph after kyanite. Additional elements, Ca and H, may be due to a chemical interaction with zoisite during the kyanite breakdown.
The above observations suggest that these rocks were saturated in Si under the peak-P conditions, but Si-undersaturated and Al-saturated environments were locally produced around kyanite during the decompression. Relatively slow diffusion of Si and Al (e.g. Mongkoltip & Ashworth, 1983; Obata, 1994) probably caused such a phenomenon.
Plagioclase
The anorthite content of plagioclase varies in the range 0·3–1·0 in YGT samples, 0·2–0·9 in DX and CJ samples, 0·1–0·8 in TJJ samples, 0·05–0·4 in X samples, and 0·05–0·6 in LH samples. These variations mainly depend on grain locations. Most plagioclase grains near quartz have low anorthite contents, whereas most grains near kyanite display high anorthite contents (e.g. Fig. 4b). This compositional change can be considered to reflect the chemical potential differences in Si and Al between quartz and kyanite, which were probably caused by slow diffusion of these cations. However, some plagioclase grains around kyanite in samples 91X3, 89X10, 91LH8A and B are albite or oligoclase compositions (XAn = 0·05–0·30).
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ESTIMATION OF P–T PATH |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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To estimate the peak-P conditions, we apply the garnet–omphacite–kyanite–coesite (or quartz) geothermobarometer (Nakamura & Banno, 1997
) to a variety of eclogites (see Tables 3 and 4). Primary compositions of some Cpx grains were modified during decompression, and hence we selected samples and grains that most closely reflect peak-P conditions. Compositions of primary Cpx grains with Si = 1·99–2·01 and cores of nearby garnet grains were used for the calculation. The deduced P–T conditions are scattered, but the calculated pressures range from about 2·8 to 3·4 GPa, consistent with the former presence of coesite (Fig. 11 and Table 4).
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The estimation of Fe3+ contents of Cpx can cause variation in our P–T estimations. We therefore recalculated the temperatures using two estimates of Fe3+ content of Cpx: (a) Fe2+ = Fetotal (Fe3+ = 0) and (b) Fe3+ = Na - Altotal (Fig. 12 and Table 4). The temperatures were calculated with the garnet–clinopyroxene thermometer (Ellis & Green, 1979; Powell, 1985; Krogh, 1988). Use of a recent calibration (Berman et al., 1995) yields T = 570–710°C at 3·0 GPa, which is inconsistent with the primary assemblage of kyanite + zoisite + SiO2 phase, if H2O activity was close to unity at the peak-P stage (Fig. 11). Application of this calibration (Berman et al., 1995) to UHP Dora Maira eclogite [data from Nakamura & Banno (1997)] also yields low-T conditions of 440–560°C, which greatly differ from the oxygen isotope temperature (700–750°C: Sharp et al., 1993), estimation with the garnet–phengite thermometer (T 700°C: Kienast et al., 1991), and P–T conditions constrained by mineral parageneses (T 700–800°C: e.g. Schertl et al., 1991; Hirajima & Compagnoni, 1993). In contrast, use of the thermometer of Ellis & Green (1979), Powell (1985) and Krogh (1988) gives T = 730 ± 45°C, compatible with the above estimations.
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There is a clear positive correlation between XNa [= Na/(Ca + Na)] of Cpx and temperature calculated with Fe2+ = Fetotal (Fig. 12a). This is possibly because non-ideality of Cpx raises the calculated temperature with increasing XNa of Cpx (e.g. Koons, 1984; Hirajima, 1996). However, magnitude of excess interaction between diopside and jadeite is approximately the same as that between hedenbergite and jadeite (Holland, 1990). Activity coefficients for diopside and hedenbergite, therefore, cancel out in application of the Grt–Cpx thermometer, when symmetrical regular solution is assumed. Another possible reason is that Fe3+/Fetotal ratio increases with increasing XNa of Cpx (Rossi et al., 1983b). Therefore, we also recalculated the temperatures with a tentative method of Fe3+ estimation: (c) Fe3+/Fetotal = -0·56XNa2 + 1·25XNa. This equation is obtained from regression of independent compositional data of omphacite from the Nybö eclogite (Rossi et al., 1983a). The temperature variation is clearly reduced using this approximation of Fe3+ content (Fig. 12c), although the physical meaning of such a compositional dependence for increase of Fe3+/Fetotal is not clear. In either case, temperatures estimated with low-XNa Cpx are more reliable than those estimated with high-XNa Cpx, because most of the Fe–Mg exchange experiments have been performed with low-XNa Cpx.
The decompressional path of the Yanggongtun eclogitic rocks has been estimated. In these rocks, the SiO2 phase in garnet is quartz, but the quartz inclusions are associated with radial fractures in host garnet, suggesting inversion from coesite. Although such radial fractures are not, by themselves, reliable indicators of the former presence of coesite, the primary assemblage of garnet (Xprp 0·6; Xgrs 0·2) + Cpx (XNa 0·2) + quartz indicates that these rocks underwent at least high-P (>2·3 GPa) metamorphism (Table 4). Application of the Grt–Cpx thermometer (Ellis & Green, 1979; Powell, 1985; Krogh, 1988) to sample 91YGT4A yielded T = 690 ± 40°C, when Cpx grains with Si = 1·99 were used for the calculation. Use of all the analyzed Grt + Cpx pairs in samples 91YGT4A and 91YGT1 gives temperatures ranging from 600 to 840°C (Fig. 13), although most of the Cpx grains show Si contents < 1·99 (Fig. 8). Fe3+ contents of Cpx were estimated with the three proposed methods. In both samples, Cpx grains display low XNa (0·17 and 0·27).
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The P–T conditions during the post-peak metamorphism can be constrained using mineral parageneses around quartz in sample 91YGT4A in conjunction with a petrogenetic grid in the MgO–SiO2–H2O system (Fig. 13). The petrogenetic grid was obtained using THERMOCALC v2.5 (Powell & Holland, 1997). The obtained grid has no significant differences from that calculated with the old version of THERMOCALC (Powell & Holland, 1988) or from that calculated with another data set [see fig. 13-1 of Spear (1993)]. Because enstatite occurs in coronas around quartz in this sample, the enstatite probably developed in an Si-saturated environment. The coexistence of enstatite + quartz indicates high-T and/or low H2O activity [a(H2O)] conditions constrained by the reaction
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0·17) grains may have supplied Na to plagioclase, although the enstatite + plagioclase association is not found near the Cpx grains. In either case, at least garnet and quartz were reactants. Enstatite formed at the expense of quartz, and garnet supplied Mg to enstatite as the garnet reacted to produce plagioclase. Thus, enstatite and plagioclase formed under medium-P conditions, under which both were stable. The pressure conditions were constrained by reaction (2) using THERMOCALC v2.5. Activities of pyrope (aprp) and grossular (agrs) were corrected as
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), Xprp = 0·5 and Xgrs = 0·2. The activity of anorthite was assumed to be 0·5, which corresponds to XAn 0·35–0·40 at 700–900°C (Newton et al., 1980). The P–T curve of its reaction is located around 1·15 GPa at 700°C to 1·30 GPa at 1000°C (Fig. 13). Accordingly, the enstatite in sample 91YGT4A must have formed at P less than 1·2 GPa. Talc (XMg 0·90) occurring locally between quartz and enstatite in this sample (Fig. 4a) is probably a product during or after enstatite + plagioclase formation. H2O-bearing fluid must have migrated into the local reaction system to produce talc, because none of the inferred reactants are hydrous minerals. This fluid could have carried Na to form plagioclase, and hence H2O activity may have been significantly lower than unity, as suggested in the UHP Dora Maira Massif (e.g. Philippot, 1993; Chopin et al., 1997). Therefore, the association of plagioclase + enstatite ± talc + quartz represents high-T and medium-P conditions (e.g. T = 720°C at 1·0 GPa for a(H2O) = 0·50; Fig. 13) or extremely low a(H2O) [e.g. a(H2O) = 0·20 at 630°C and 1·0 GPa]. Application of garnet–orthopyroxene Fe–Mg exchange thermometers to enstatite (Fe/Mg = 0·20) and garnet rim compositions (Fe/Mg = 0·65; Xgrs = 0·19) yields 700°C (Harley, 1984) to 830°C (Lee & Ganguly, 1988) at 1·0 GPa, supporting the high-T overprinting conditions inferred with the assumption of a(H2O) = 0·50. Such application of thermometers can be justified by the concept of ‘partial (or partitioning) equilibrium’ (e.g. Loomis, 1976). The Al2O3 content of the enstatite is, however, very low (<1 wt %). Therefore, the garnet–orthopyroxene Al barometer (Harley & Green, 1982), applied to the compositions of the enstatite (Al = 0·03) and garnet rim, yields 2·1 GPa at 700°C to 2·6 GPa at 800°C, which is much higher than enstatite + plagioclase stable pressures (Fig. 13). This is probably because the chemical potential of Al drastically varies along the direction of reaction progress, as can be seen in the compositional variation of plagioclase (Fig. 4a). Similar development of Al-poor orthopyroxene (<1 wt %) around quartz was reported in eclogites overprinted under granulite-facies conditions (700°C and 0·8 GPa) from the Oberpfalz area of the Bohemian Massif, Germany (O’Brien, 1989), although secondary Al-rich orthopyroxene (5–9 wt %) was also reported from another area of the Bohemian Massif, Czech Republic (Kotková et al., 1997).
The subsequent P–T path is based on mineral parageneses after kyanite in sample 91YGT2. In this sample, corundum is generally in contact with anorthite (XAn 0·9), but margarite is locally present between the two minerals (Fig. 4d). This texture indicates that margarite formed by the following hydration reaction after the corundum + anorthite formation:
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We also applied geothermometers to samples 89DX2 and 91TJJ4 to estimate their P–T histories. Application of the two-pyroxene thermometer to En + Di coronas in sample 89DX2 (Fig. 5b) yielded T 670°C (Bertrand & Mercier, 1985; Brey & Köhler, 1990) to 790°C (Wood & Banno, 1973; Wells, 1977). Application of the garnet–biotite thermometer [Hodges & Spear, 1982; Dasgupta et al., 1991; and Ferry & Spear (1978) with 
WCa = 12 550 J/mol by Ganguly & Saxena (1984) and WMn = 1440 J/mol by Wood et al. (1994)] to symplectitic garnet + biotite pairs in sample 91TJJ4 (Fig. 5d) gave T = 680–780°C. The presence of ilmenite near the biotite grains suggests low Fe3+/Fetotal ratio of biotite (e.g. Williams & Grambling, 1990), and we assume Fe2+ = Fetotal. Biotite grains in this sample are isolated from garnet grains, and hence late Fe–Mg exchange (e.g. Spear, 1991) may not have modified the biotite compositions (e.g. Indares & Martignole, 1985; Spear, 1993).
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DISCUSSION AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLE DESCRIPTIONSMINERAL CHEMISTRYESTIMATION OF P-T PATHDISCUSSION AND CONCLUSIONSREFERENCES |
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Several studies of eclogitic rocks from the Dabie and Su-Lu region have attempted to define their P–T paths (e.g. Enami et al., 1993b
; Wang et al., 1993; Okay, 1995; Zhang et al., 1995a, 1995b; Castelli et al., 1998). Most secondary minerals in the eclogitic rocks, however, have developed as reaction zones with sharp zone boundaries, which is consistent with diffusion-controlled structures (e.g. Fisher, 1978). This implies that chemical potential varies along the direction of reaction progress, and hence application of thermobarometers to such secondary minerals does not necessarily provide accurate P–T conditions. In this paper, we therefore placed most importance on presence of index minerals when defining P–T conditions during the decompression (Fig. 13).
Secondary enstatite was found in only two localities (YGT and DX), but high-T overprinting was also recognized in other localities. Application of the garnet–biotite thermometer gave high-T (700°C) conditions at Tengjiaji (TJJ; see Fig. 2). The presence of albite around kyanite in some localities (X and LH) and absence of paragonite from all samples also suggest high-T (670–730°C at 1·0–1·5 GPa: THERMOCALC v2.5, with unit H2O activity) conditions, which are constrained by the reaction paragonite + quartz = kyanite + albite + H2O, as previously suggested by Enami et al. (1993b). Spinel + plagioclase intergrowths around kyanite are common in eclogite that has been overprinted under granulite-facies conditions (e.g. O’Brien et al., 1990). This type of intergrowth is observed in many localities of Rongcheng County. Application of thermometers to Weihai granulite has yielded overprinting temperatures higher than 700°C (Wang et al., 1993; Zhang et al., 1995b). All these data suggest that the granulite-facies overprint is of regional importance in the northeastern Su-Lu region. Taking account of thermometric uncertainties, temperatures at the peak-P stage of these UHP rocks are not significantly different from those recorded at the granulite stage [e.g. Fig. 13; see also fig. 6 of Zhang et al. (1995b)], and a simple interpretation of these observations leads to an isothermal decompressional path. The post-peak path is, however, still debatable, because the granulite-facies overprint could have been achieved by reheating under medium-P conditions (e.g. Harley & Carswell, 1995).
In contrast to our results, Kato et al. (1997) have proposed that marbles from Rongcheng County suffered isobaric cooling from 650 to 450°C at P > 2·4 GPa and then followed isothermal decompressional path around 450°C. Their P–T path is based on the observation of rims of talc + calcite (possibly after aragonite) between diopside and dolomite. Those workers concluded that talc and aragonite formed by the cooling to 450°C under the diopside + talc stable UHP conditions. However, the diopside and talc grains in their sample are clearly separated by a calcite layer [fig. 3a of Kato et al. (1997)], indicating that the UHP-index association of diopside + talc was not necessarily stable when the reaction zones formed. The above reaction texture can be formed by cooling at low-P conditions.
We conclude that at least some of the UHP rocks of the Su-Lu region were overprinted by a high-T and medium-P event, and that UHP rocks did not necessarily undergo significant cooling during early exhumation. Nearly isothermal decompression best explains our data for UHP rocks. If this represents adiabatic ascent to crustal levels, then it allows us to draw some conclusions about the exhumation process. Generally, very rapid ascent is needed to create an adiabatic P–T path. However, unless an exhuming mass of UHP rocks is large enough, they would easily lose or gain heat during the exhumation. In this case, it is important to evaluate how large the UHP mass needs to be to achieve adiabatic ascent. Geochronological data suggest that the exhumation rate of the UHP rocks was 1–6 mm/year (Ernst et al., 1997), but a very rapid exhumation (20–24 mm/year) was also reported for the UHP Dora Maira Massif (Gebauer et al., 1997). We therefore assume the exhumation rate of 1–20 mm/year, to evaluate the scale needed for the adiabatic ascent. When the UHP mass ascends from 100 km depth to crustal levels, the length scale for thermal conduction (L) increases as follows:
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is the thermal diffusivity (10-6 m2/s: Spear, 1993), t is the time, d (m) is the depth, and v (m/s) is the exhumation rate. Even in the case of v = 20 mm/year, the length scale for thermal conduction reaches 10 km when the UHP rocks have ascended to lower-crustal levels (Fig. 14). Accordingly, UHP blocks must be greater than 10 km in scale to create an adiabatic path. In the Su-Lu region, the sizes of individual eclogitic blocks are, however, considerably less than this scale (e.g. Fig. 2; Ishiwatari et al., 1992). Therefore, if UHP eclogite ascended as isolated small blocks (the exotic model), the UHP eclogite would easily lose or gain heat and would not follow an adiabatic decompression path. In other words, UHP rocks are required to ascend as a large mass, and the only feasible candidate for such a large mass is the orthogneiss containing the eclogite blocks (the in situ model). Our suggestion can also help to explain the mechanism by which the eclogite was exhumed at least to crustal levels. If the UHP mass consists of not only small eclogite blocks but also large amounts of the surrounding orthogneiss, then during most of its rise such a body would be less dense than its mantle surroundings (Irifune, 1994). Accordingly, buoyancy-driven crustal advection may be one of the principal mechanisms for exhuming the UHP eclogite (e.g. Ernst et al., 1997; Wallis et al., 1997). After the ascent to crustal levels, the buoyancy-driven force and the exhumation rate would have diminished, and hence the subsequent P–T path would have approached and followed the steady-state geotherm. This model also implies that large amounts of orthogneiss have also suffered UHP metamorphism, something that has yet to be proved.
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ACKNOWLEDGEMENTS |
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We are grateful to S. Banno, S. R. Wallis and K. Shirahata for their critical reading and fruitful discussion. We would also like to thank M. Obata, T. Nishiyama, T. Mori and M. Aoya for their useful advice. Critical comments from S. S. Sorensen, S. L. Harley, W. G. Ernst and R. J. Beane are gratefully acknowledged. Thanks are also due to H. Tsutumi and K. Yoshida for making of thin sections. Samples from Rongcheng County were collected as part of a co-operative project between Kyoto University and Academia Sinica supported by JSPS and CNNSF, and we would like to thank the members of the project, M. Enami, A. Ishiwatari, K. Ye and the other members. D. Nakamura acknowledges the financial support of a JSPS Research Fellowship for Young Scientists.
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FOOTNOTES |
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*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: daisuke{at}geo.titech.ac.jp
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