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

Nakamura, Daisuke; Hirajima, Tsuyoshi

doi:10.1093/petrology/41.4.563

Cited by 126 publications

(93 citation statements)

References 69 publications

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“…The analyzed omphacite does not include significant amount of Ca-tschermak component (CaAl 2 SiO 6 ), and Si contents are within the range of 1.95-2.05. Generally, Catschermak contents in clinopyroxene increase with decreasing pressure (e.g., Nakamura and Hirajima, 2000). The high Si contents of the analyzed omphacite indicate that most grains of omphacite in the studied samples did not undergo chemical modification during retrograde metamorphism.…”

Section: Petrology and Mineralogymentioning

confidence: 83%

“…1) in order to avoid chemical modification during the decompression. Replacement of omphacite by symplectites may cause the modification of the original chemical compositions at UHP conditions (e.g., Nakamura andHirajima, 2000). Quantitative analyses of garnet and omphacite were performed with wavelength-dispersive X-ray spectrometry (JEOL JXA-8230) at Okayama University.…”

Section: Petrology and Mineralogymentioning

confidence: 99%

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Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

Nakamura

Okada

Hirajima

2015

Journal of Mineralogical and Petrological Sciences

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This study examines the effect of jadeite content (Xjd) in omphacite on Fe-Mg distribution coefficient between garnet and clinopyroxene (K D ) by using natural data obtained from ultrahigh-pressure eclogites of the Sulu region, China. A previous study has already pointed out that a negative correlation between lnK D and Xjd and a positive correlation between Fe# [= Fe 2+ /(Fe 2+ + Mg)] of omphacite and Xjd are present. On the other hand, this study newly recognized that K D decreases with decreasing Ca + Na + K contents in M2 site of omphacite, indicating that excess Fe 2+ may be incorporated into M2 site of omphacite with decreasing Ca + Na + K contents and hence total Fe# values of omphacite may become excessively high. Thus, application of garnetclinopyroxene geothermometer for omphacite with significant amount of jadeite and/or ferrosilite (and enstatite) components may overestimate the peak metamorphic temperatures in some cases. As a first approximation, it is better to avoid the use of omphacite with Ca + Na + K contents significantly lower than 1.0 a.p.f.u. for the application of garnet-clinopyroxene geothermometer.

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Section: Petrology and Mineralogymentioning

confidence: 83%

Section: Petrology and Mineralogymentioning

confidence: 99%

Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

Nakamura

Okada

Hirajima

2015

Journal of Mineralogical and Petrological Sciences

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“…This agrees with the facts that trace element zonations in garnets, omphacites and apatites are not present widely and only the outmost rims of apatites show remarkably LREE and MREE decreasing. On the other hand, overprinting of typical granulite-facies metamorphism on eclogites is only reported in the northeastern part of the Sulu UHP belt (Weihai and Rongcheng) [4][5][6] . Furthermore, the easternmost part of the Sulu UHP rocks suffered partial melting [3] .…”

Section: Geochemistrymentioning

confidence: 99%

“…These observations indicate that granulite-facies metamorphism overprinting on eclogites is gradually weakened from eastern to western Sulu UHP belt, which could imply that the Sulu UHP belt suffered different degrees of heating. Strong heating could have resulted in partial melting and granulite-facies petrologic records in the eastern part [3][4][5][6] , whereas weak heating imprinted only a short-lived heating record in terms of trace element compositions in the western part of the Sulu UHP terrane. Synexhumation magmatism in the eastmost part of the Sulu orogen is ascribed Figure 4 Core values-normalized trace element zonation profiles of apatites in SD006 and SD014.…”

Section: Geochemistrymentioning

confidence: 99%

“…Rocks drilled through by CCSD ever resided in the deep level of mantle beneath the convergent plate boundary, which is the best area to study the continental subduction and geodynamics [1,2] . Recent studies reveal that the eastmost Sulu UHP rocks suffered partial melting during exhumation [3] , and some eclogites in the eastern Shandong were overprinted by granulite-facies metamorphism [4][5][6][7] . Then, one important question is what causes the partial melting.…”

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confidence: 99%

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Trace elemental records of short-lived heating during exhumation of the CCSD eclogites

Zong

Liu

et al. 2007

CHINESE SCI BULL

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Trace element compositions of garnet, omphacite and apatite in ultrahigh-pressure eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD) project were in situ analyzed by the LA-ICP-MS method. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from core to rim in rare earth elements. In particular, middle rare earth elements in the garnet, heavy rare earth elements in the apatite and all rare earth elements in the omphacite increase from core to rim, respectively. Based on dependence of partition coefficients on temperature and pressure in these minerals, we suggest that the trace elemental zonations in these minerals may record a short-lived heating event during exhumation. eclogite, in situ analysis, trace element, mineral zoningThe Sulu ultrahigh pressure (UHP) metamorphic belt is an important part of the Qinling-Dabie Central Orogenic Belt in China. The main hole of the Chinese Continental Scientific Drilling (CCSD) is located at Donghai County, the southwestern part of the Sulu UHP terrain. Rocks drilled through by CCSD ever resided in the deep level of mantle beneath the convergent plate boundary, which is the best area to study the continental subduction and geodynamics [1,2] . Recent studies reveal that the eastmost Sulu UHP rocks suffered partial melting during exhumation [3] , and some eclogites in the eastern Shandong were overprinted by granulite-facies metamorphism [4][5][6][7] . Then, one important question is what causes the partial melting. To answer this question, decompression, increasing temperature and adding volatile were ever suggested. Yao et al. [7] attributed overprinting of granulite-facies metamorphism on eclogites to fluid infiltration during the exhumation. However, P-T path of coesite eclogite to granulite indicates that the overprinting of granulite-facies metamorphism was associated with heating, and the extent of temperature increasing was ~40℃ [4] . O'Brien and Vrána [8] suggested that a short-lived heating event during exhumation resulted in overprinting of granulite-facies metamorphism on the UHP eclogites in the Moldanubian Zone. The presence of corundumbearing mafic and ultramafic rock is an indicator of ultrahigh temperature [9][10][11][12] . In the Sulu UHP belt, corundum is found in eclogite enclosed in ultramafic rocks [13] and garnetite in garnet peridotite [14] . Especially, we recently found a unique mineral assemblage (garnet + corundum + spinel + quartz + feldspar) indicating (ultra-) high temperature nearby the CCSD. These observations imply that the eclogites in some parts of the Sulu UHP terrane could suffer a short-lived high-T stage during the exhumation.The thermodynamic theory of trace element partitioning between metamorphic minerals indicates that compared with major elements, trace elements are potentially more sensitive to chemical and thermobaromet-

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Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

Yang

Wang

et al. 2020

Geological Journal

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Migmatite in the Sulu ultra‐highpressure (UHP) terrane is an ideal sample to provide information on anatexis during the exhumation of the deeply subducted continental crust. The studied migmatite is composed of the leucosome and melanosome with strongly ductile shear deformation. Petrographic observations clearly show an anatexis process, for example, cuspate and triangular K‐feldspars in triple junction and along grain boundaries, felsic veinlets and pockets mainly consisting of K‐feldspar + quartz + plagioclase ± biotite. The zircon grains of the migmatite have a core‐rim structure: the inherited magmatic core and the anatectic overgrowth rim. The inherited cores exhibit oscillatory zonings in the cathodoluminescence (CL) images and display strong heavy rare earth element (HREE) enrichment with clearly positive Ce and negative Eu anomalies. The inherited cores have an upper intercept U‐Pb age of 814 ± 45 Ma with εHf(t) values of −5.6 to ‐10.6 (mean = −8.5 ± 0.8, n = 16). Thus, protolith of the migmatite is a mid‐Neoproterozoic granitic rock. The overgrowth rims exhibit a typical anatexis genesis, that is, a bright homogeneous luminescence in the CL images, very low Th/U ratios (<0.02) with low U (<100 ppm) and Th (<1 ppm) contents, and lower total REE and Y contents with lower (Gd/Lu)N ratios and higher Hf/Y ratios than the inherited cores. They are Late Triassic with ages ranging from 211 to 234 Ma (mean = 223 ± 8 Ma), which, combined with previous studies on the Sulu UHP terrane, suggests that the anatexis occurred during the early exhumation stage. They have higher Ti‐in‐zircon temperatures ranging from 760 to 813°C (mean = 791 ± 16°C) than the pre‐anatexis zircon (244 ± 5 Ma and 739°C), suggesting a ‘hot’ exhumation. They have εHf(t) values of −17.6 to −14.5 (mean = −15.5 ± 0.6) which are lower than the estimated value (~12.0) on the crustal evolution array, suggesting an addition of crustal fluid during the anatexis. The anatexis also dramatically affected the zircon Lu–Hf isotopic system, that is, 176Yb/177Hf and 176Lu/177Hf isotopic ratios are markedly decreased but initial 176Hf/177Hf ratios are increased from the inherited magmatic cores to the anatectic rims. Thus, we propose that the anatexis plays a critical role in the exhumation of the UHP metamorphic rocks. The anatexis could lubricate the subducted slices and further facilitate their buoyant rise from the mantle depth to the crustal level.

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

Cited by 126 publications

References 69 publications

Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

Trace elemental records of short-lived heating during exhumation of the CCSD eclogites

Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

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