Triassic Continental Subduction Research Articles

Petrogenesis of late Mesozoic granitoids from the southern segment of the Tan–Lu Fault, eastern China: implications for the tectonic affinity of the Zhangbaling Uplift

ABSTRACT Late Mesozoic magmatic rocks of the Zhangbaling Uplift (ZBU) in eastern China, which comprises the northern Zhangbaling area and the southern Feidong area, can provide insights into the deep crustal processes and tectonic affinity of the massif. These rocks are dominated by granitoids with formation ages of 132–120 Ma and a peak at 129 Ma. The Shankouling dyke (120.4 ± 1.1 Ma) and the Zhuyuanzhang quartz monzodiorite (129.0 ± 2.2 Ma) crop out sporadically in the Feidong area. These rocks show adakitic signatures of high Al2O3 and Sr contents, high Sr/Y, (La/Yb)N, and (Dy/Yb)N ratios, and low Y and YbN contents. They have enriched (87Sr/86Sr)i ratios, negative εNd(t) and εHf(t) values, and unradiogenic Pb isotopic compositions that resemble those of coeval high Sr/Y granitoids in the Dabie Orogen. These ZBU rocks can be divided into two groups: low-Mg granitoids with more enriched Sr–Nd isotopic signatures and low Mg# values (<50), and high-Mg granitoids with weakly enriched Sr–Nd isotopic signatures and high Mg# values (mostly >50). Combining their geochemical features with previous results of experimental melts, we conclude that the low-Mg granitoids were derived from fluid-absent partial melting of in situ thickened lower continental crust, resulted from Triassic continental subduction and collision in this area, whereas the high-Mg granitoids were derived from delaminated eclogitic lower crust followed by interaction with mantle peridotites during magma ascent. Isotopic compositions show that these granitoids could have been derived from ancient crust of the Yangtze Block via re-melting of Neoarchean and Paleoproterozoic lower continental crust. The great depth of the Tan–Lu Fault Zone enabled delamination of the thickened eclogitic mountain root beneath the ZBU, which resulted in the formation and emplacement of the high-Mg rocks, followed by uplift of the Zhangbaling massif associated with rollback of the subducting Paleo-Pacific (Izanagi) Plate.

Generation of leucogranites via fractional crystallization: A case study of the Jurassic Bengbu granite in the southeastern North China Craton

The magmatic processes that form leucogranites are closely related to plate collision. As the products of crustal anatexis, leucogranites provide clues to intracrustal differentiation and the regional tectonic evolution. The Bengbu area is located along the southeastern margin of the North China Craton (NCC), 150 km north of the Dabie Orogen, and is bounded by the Tancheng–Lujiang Fault Zone to the east. Late Jurassic leucogranites are widely distributed in the Bengbu area. We collected samples of the Bengbu leucogranite (BLG) and analyzed data on whole-rock geochemistry, zircon U–Pb geochronology, zircon in situ Lu–Hf isotopes, quartz in situ O isotopes, and total bulk Sr–Nd–Pb isotopes. Zircon U–Pb dating yielded formation ages for these leucogranites of ∼160 Ma. The samples are characterized by high SiO2 (72.4 wt.%−76.9 wt.%), alkali (7.3 wt.%−9.4 wt.%), and Al2O3 (13.4 wt.%−15.5 wt.%) contents. In addition to two samples exhibiting the rare-earth element tetrad effect, all rocks have high differentiation index values (95–99), extremely low iron–magnesium–titanium oxide contents (total Fe2O3 < 1.4 wt.%, MgO < 0.3 wt.%, TiO2 < 0.2 wt.%), and low Zr/Hf (20–50) and Nb/Ta (5–25) ratios. These data suggest that the BLG is a highly fractionated granite. Furthermore, the samples contain Neoproterozoic and Triassic zircons, have low Ti-in-zircon temperatures (682–697 °C), low Rb contents (<192 ppm), high Sr contents (48–791 ppm), low Rb/Sr ratios, low radiogenic Pb isotopic compositions (206Pb/204Pb(t) = 17.169–17.297), and highly enriched Sr–Nd–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7104, εNd(t) = −16.6 to −12.3, and zircon εHf(t) = −14.5 to −21.7). Taken together, the geochronological and geochemical results suggest that the BLG was formed by high-degree fractional crystallization after hydrous partial melting of the North Dabie granitic gneisses, which have been subducted beneath the Bengbu area by Triassic continental subduction. Following collision between the South China Block and the NCC, Late Jurassic granitoids in the eastern NCC, including the BLG, were generated in the resulting post-orogenic extensional environment and were also influenced by ongoing subduction and rollback of the Izanagi Plate.

BOR-MING JAHN (BMJ) biographical memoir and brief introduction to BMJ’s scientific contributions and academic services

This memoir highlights the scientific and academic contributions of Bor-ming Jahn who passed away in last December. He is best known for his ability to draw novel insights about the Earth’s tectonics, evolution, and geological processes from in-depth field and laboratory studies of isotopic and elemental geochemistry. He is recognized as a global leader in the applications of geochemical and isotope tracers (Sr-Nd-Hf-Pb-O) to regional tectonic evolution, and made outstanding contributions to the formation of dominant juvenile crust (>70%) for the Phanerozoic Central and Northeast Asian Orogenic Belts. He created the term “TTG” for tonalite-trondhjemite-granodiorite suites formed in ancient and present-day magmatic arc setting and is the first to date the oldest Chinese rocks and to constrain Triassic continental subduction for the Dabie-Sulu UHP terrane. He received numerous honors, and was the first Chinese Earth scientist to be the Editor-in-Chief of the celebrated Journal of Asian Earth Sciences, Elsevier. We are sad for his sudden passing; yet his scientific devotion and contributions will be remembered for decades to come.

Carbonatitic metasomatism in orogenic dunites from Lijiatun in the Sulu UHP terrane, eastern China

Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab–mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo=92.0–92.6, Ca=42–115ppm), porphyroblastic orthopyroxene (En=91.8–92.8), Cr-spinel (Cr#=50.4–73.0, TiO2<0.2wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31–47.07wt.%) and Mg# (91.5–91.9), and low Al2O3 (0.48–0.70wt.%), CaO (0.25–0.44wt.%) and TiO2 (<0.03wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re–Os isotopic data suggest that dunites formed in the early Proterozoic (~2.2Ga). These data indicate that the Lijiatun dunites were the residues of ~30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo=91.6–92.6, Ca=233–311ppm), clinopyroxene-bearing (Mg#=95.9–96.7, Ti/Eu=104–838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mass transfer from the subducted crust to the mantle wedge through orogenic peridotites.

Triassic tectonics of the southern margin of the South China Block

Middle Triassic orogens are widespread around and inside the South China Block (SCB). The southern peripheral belts that develop from northwest to southeast, namely Jinshajiang, Ailaoshan, NW Vietnam, NE Vietnam, Yunkai and Hainan exhibit striking similarities, with Permian–Early Triassic magmatic arc, ophiolitic mélange, northeast- to north-directed synmetamorphic ductile nappes, and fold-and-thrust belt. These collisional belts result from oceanic, then continental subduction of the SCB below Indochina. Eastward of Hainan Island, a Triassic suture is hypothesized offshore of the SCB. Within the SCB, the Xuefengshan is a Middle Triassic intracontinental orogen with northwest-directed folds and thrusts, and an intracrustal ductile décollement. This orogen accommodated the Middle Triassic continental subduction of the western part of the SCB below the eastern part. At variance to the generally accepted models, the inter- and intracontinental Triassic orogens of the SCB are interpreted here as the result of south-directed subductions of the SCB.

Common Pb isotope mapping of UHP metamorphic zones in Dabie orogen, Central China: Implication for Pb isotopic structure of subducted continental crust

We report Pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed UHPM slices representing the North Dabie zone, the Central Dabie zone and the South Dabie zone of the Dabie orogen, central-east China. The feldspars from the gneisses were recrystallized during Triassic continental subduction and UHP metamorphism. Precursors of the orthogneisses are products of Neoproterozoic bimodal magmatic events, those in north Dabie zone emplaced into the lower crust and those in central and south Dabie zones into middle or upper crust, respectively. On a 207Pb/204Pb vs. 206Pb/204Pb diagram, almost all orthogneisses data lie to the left of the 0.23Ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it. On a 208Pb/204Pb vs. 206Pb/204Pb diagram the most of data of north Dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model. This pattern demonstrates that the Pb isotopic evolution of the feldspars essentially ended at 0.23Ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the Neoproterozoic rifting tectonic zone. According to geological evolution history of the locally Dabie orogen, a four-stage Pb isotope evolution model including a long time evolution between 2.0 and 0.8Ga with a lower crust type U/Pb ratio (μ=5–6) suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the Dabie orogen at 0.8Ga also play an important role in the Pb evolution of the exhumed UHPM slices, corresponding to their respective Pb characters at ca. 0.8–0.23Ga. For example, north Dabie zone requires low μ values (3.4–9.6), while central and south Dabie zones require high μ values (10.9–17.2). On the other hand, Pb isotopic mixing between north and central or south Dabie zones during retro-grade metamorphism enhanced by the extensive magmatism in the Cretaceous has also been observed in the 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams. A combined study of common Pb isotopic compositions of Dabie orthogneisses and Sulu UHPM rocks from the Chinese Continental Scientific Drilling project demonstrates that a slab marked by extremely unradiogenic Pb observed in the main hole was absent in the Dabie orogen. However, occurrence of some Mesozoic granitoids with such unradiogenic character in the Dabie orogen suggests that their source may be a buried unradiogenic unit underlying below north Dabie zone. This case study clearly shows that whether the position of the Dabie data relative to the orogen curve of the plumbotectonic model is helpful in understanding the Pb isotopic structure and evolution of subducted continental crust.

The origin and evolution of low-δ18O magma recorded by multi-growth zircons in granite

We for the first time reported low-δ18O granites within craton in Bengbu uplift, southeast margin of North China Craton. Integrated ion microprobe study of δ18O and U–Pb age on single zircon grains and zircon populations, gives direct evidence for the origin and later magmatic evolution of the low-δ18O granites. Three types of zircon domains are recognized: inherited magmatic core (Type I), inherited metamorphic core/mantle (Type II) and overgrown magmatic rim (Type III). Type I zircons were formed in Neoproterozoic, and have the average δ18O values of 5.3±0.6‰ around mantle value. Triassic Type II zircons have extremely low and highly heterogeneous δ18O values ranging from −9.4‰ to 8.6‰, consistent with the values and variations exhibited by metamorphic zircons from Dabie–Sulu orogen. This feature argues for a Triassic metamorphic origin for the 18O-depletion signature of zircons from ultrahigh pressure metamorphic rocks in Dabie–Sulu orogen. Jurassic Type III zircons have isotopically homogeneous δ18O values of 3.3±0.5‰. Collectively, zircon populations recorded the δ18O variations from source to low-δ18O granite. The primary protolith of Jurassic granites in Bengbu uplift is a Neoproterozoic granitic intrusion in South China Block. Later high-temperature meteoric hydrothermal alteration has lowered the whole-rock δ18O values in various degrees. The low-δ18O protolith was then buried to the middle-lower crust by Triassic continental subduction and Jurassic anatexis produced the low-δ18O granitic magma. Moreover, δ18O values of Type III zircons are isotopically lighter than previously reported values of magmatic garnets in Jurassic granites, indicating later magmatic evolution of low-δ18O magma toward high δ18O values probably as a result of infiltration of high-δ18O fluid from wall-rocks.

Diverse P– T paths of the northern Dabie complex in central China and its reworking in the early Cretaceous

The northern Dabie complex (NDC) in the Dabie orogen, central China, is a high-temperature migmatite terrane. It is composed mainly of banded gneiss, migmatite, and minor amphibolite, granulite, retro-eclogite, marble and spinel peridotite, and intruded by the early Cretaceous massive granitoid plutons and subordinate mafic–ultramafic rocks. Whether or not the NDC experienced a high-pressure or ultrahigh-pressure (HP/UHP) metamorphic history has been a long standing issue. However, in the last 10 years, an increasing number of retrograde UHP eclogite relics and marbles and HP granulites have been identified throughout the NDC. This, together with the discovery of microdiamond preserved as inclusions in zircons within eclogite relics and gneisses and numerous age dates of 220–240 Ma and ∼130 Ma from the NDC, supports the notion that at least part of the NDC was subjected to Triassic continental subduction and HP/UHP metamorphism, the youngest ages being related to an intense overprinting of ultrahigh-temperature (UHT) granulite facies metamorphism in the early Cretaceous. Distinct peak UHP conditions (⩾2.5 GPa and ∼800 °C; ∼3.5 GPa and ⩾750–800 °C; ⩾4 GPa and ⩾900 °C or ∼750 °C) have been inferred from oriented Qtz or Ab + Kfs + Qtz needles in Ca–Na clinopyroxene of retrograde eclogite, Ti-Chu-bearing assemblage preserved in marble, rod-like Cpx–Rt–Ap inclusion in garnet of retrograde eclogite, and microdiamond inclusions in zircon of tonalitic gneiss and retrograde eclogite from the NDC. The overprinted UHT (>900 °C or ∼1000 °C) metamorphism is indicated by Osm growth in garnet, Pgt exsolution in clinopyroxene of retrograde eclogite, and Spl + Crd + Qtz symplectite in felsic granulites. Diverse P– T paths of an overall clockwise evolution are suggested with post-peak decompression or heating followed by decompression-cooling, and consist of two distinct paths. Opx + Pl inclusions in garnet and zircon in retrograde eclogite relics indicate that the lower continental crust in the NDC underwent Triassic subduction. The NDC shows overprinted UHT granulite facies metamorphism, thus consists of two metamorphic events in contrast to the single event of the central Dabie UHP eclogite belt (CDB). Distinct peak P– T conditions may be attributed to the lack of suitable mineral assemblages for geobarometry. The overall clockwise P– T paths consist of a peak HP/UHP event with overprinted UHT event suggesting that the NDC experienced Triassic subduction and intense intracontinental reworking in the early Cretaceous. The latter stage may be associated with a heat pulse due to asthenospheric upwelling and lithospheric thinning as a result of Cretaceous mantle superwelling and the westward subduction of the Pacific plate.

Common Pb of UHP metamorphic rocks from the CCSD project (100–5000 m) suggesting decoupling between the slices within subducting continental crust and multiple thin slab exhumation

In order to understand the vertical structure of the Dabie–Sulu ultrahigh-pressure metamorphic (UHPM) belt, common Pb isotopic compositions of omphacites in eclogites and feldspars in gneisses from the Chinese Continental Scientific Drilling (CCSD) project (100–5000 m) have been investigated in this study. Samples from 0 to 800 m (unit 1) in the drilling core have moderately high radiogenic Pb isotopes with small variations of 206Pb/ 204Pb (16.82–17.38), 207Pb/ 204Pb (15.37–15.49), and 208Pb/ 204Pb (37.21–37.72), indicating either high µ ( 238U/ 204Pb) or high initial Pb isotope ratios of their protoliths. In contrast, the samples from 1600 to 2040 m (unit 3) and most of samples from 3200 to 5000 m (unit 5) have moderately or very unradiogenic Pb (unit 3: 206Pb/ 204Pb from 16.05 to 16.46, 207Pb/ 204Pb from 15.22 to 15.29, and 208Pb/ 204Pb from 36.68 to 37.48; unit 5: 206Pb/ 204Pb from 15.52 to 15.69, 207Pb/ 204Pb from 15.15 to 15.27, and 208Pb/ 204Pb from 36.48 to 37.20), indicating either low µ or low initial Pb isotope ratios of their protoliths. Pb isotopes of samples from 800 to 1600 m (unit 2) and from 2040 to 3200 m (unit 4) in the drilling core with abundant ductile shear zones are intermediate between those of units 1 and 3 or 5 and display larger variations. Pb isotopes combined with the published oxygen isotope data of the CCSD samples reveal the original positions of the five units before the Triassic continental subduction. Units 1, 3, and 5 as three UHPM rock slabs could be derived from the subducted upper continental crust, upper–middle continental crust and lower–middle continental crust, respectively. The ductile shearing zones in units 2 and 4 could be the interfaces where the detachment and decoupling took place between the upper, upper–middle and lower–middle continental crusts. The detachment between the upper slab and subducting continental lithosphere probably occurred during continental subduction, and the upper slab (unit 1) was uplifted to a shallow depth along the detachment surface by thrusting. Units 3 and 5 may be detached later from the subducted middle and lower crust and uplifted to a shallow level underneath unit 1. The low δ 18O values (− 4.0 to − 7.4‰) [Xiao, Y.-L., Zhang, Z.-M., Hoefs, J., Kerkhof, A., 2006. Ultrahigh-pressure Metamorphic Rocks from the Chinese Continental Drilling Project-II Oxygen Isotope and Fluid Inclusion Distributions through Vertical Sections. Contribution Mineral Petrology 152, 443–458.; Zhang, Z.-M., Xiao, Y.-L., Zhao, X.-D., Shi, C., 2006. Fluid-rock interaction during the continental deep subduction: oxygen isotopic profile of the main hole of the CCSD project. Acta Petrologica Sinica 22 (7), 1941–1951.] in units 2 and 4 suggest that the detachment interfaces could be developed along an ancient fault zones which were the channels of meteoric water activity during the Neoproterozoic.

Triassic continental subduction in central Tibet and Mediterranean-style closure of the Paleo-Tethys Ocean

The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QMB are 244–223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a Paleo-Tethys arc terrane. This collision, and the coeval South China–North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm.

Neoproterozoic granitoid in northwest Sulu and its bearing on the North China‐South China Blocks boundary in east China

Middle Neoproterozoic granitoid rocks are found in the Wulian region that is located in the northwestern part of the Sulu orogen and is approximately bounded by the Wulian‐Yantai fault in southeast. Zircon U‐Pb dating yields concordant ages of 738 ± 10 to 758 ± 5 Ma for three granites and one gabbro; hornblende Ar‐Ar dating on the gabbro gives consistent plateau and isochron ages of 719.1 ± 3.7 and 717.9 ± 7.2 Ma, respectively. This suggests that the Wulian granitoids correspond to bimodal magmatism during the mid‐Neoproterozoic in the northern margin of the South China Block in response to the breakup of Rodinia supercontinent. They were tectonic slices scraped off from the upper part of the South China Block during the Triassic continental subduction. This demonstrates that the suture location between the North China Block and the South China Block lies north of the Wulian and Penglai Groups rather than along the Wulian‐Yantai fault that marks the northern margin of ultrahigh pressure metamorphic belt.

Was Triassic Continental Subduction Solely Responsible for the Generation of Mesozoic Mafic Magmas and Mantle Source Enrichment in the Dabie-Sulu Orogen?

Mesozoic mafic rocks in the Dabie-Sulu orogen (DSO) are enriched in large-ion lithophile elements (LILE, e.g., Rb, K, Sr, Ba), relatively depleted in high-field strength elements (HFSE, e.g., Nb, Ta, P, Ti), and have strongly fractionated rare-earth elements (REE) and highly "enriched" Sr-Nd isotope ratios. These "arc-type" geochemical signatures have been generally ascribed to mantle source enrichment beneath the DSO by fluids/melts released from deeply subducted Triassic continental crust, i.e., part of the South China block (SCB). However, contemporaneous mafic rocks are widespread in the North China block (NCB) and possess similar elemental and Sr-Nd isotopic characteristics. Given that most of the NCB mafic rocks were emplaced at a considerable distance from the DSO, their arc-type geochemical features are unlikely to have been caused by Triassic continental subduction. Therefore, we argue that subduction of the SCB is a potential, but not necessarily the only, mechanism responsible for mantle enrichment beneath the DSO. We propose instead that the lithospheric mantle beneath the DSO and NCB was enriched prior to Triassic subduction, and that generation of the mafic rocks in both regions was due to a shared, regional-scale geological event, which we attribute to post-collisional delamination of the mantle lithosphere.

Post-orogenic bimodal volcanism along the Sulu orogenic belt in Eastern China

The early Cretaceous volcanism occurring along the Sulu orogenic belt, east Shandong Province, demonstrates a bimodal characteristic. The volcanic rocks belong to high-K alkaline series, dominated by alkali basalt, basaltic trachyandesite, latite and trachyte with LILE (e.g. K, Sr and Ba) and LREE enrichment but HFSE depletion (especially for Nb and P) in the primitive mantle-normalized spidergrams and steeply right-declined REE patterns. The enriched initial Sr-Nd isotopic ratios (87Sr/86Sr = 0.70724 to 0.70750 and εNd(t) = −17.0 to −15.9) of the basaltic samples suggest their origin from an enriched lithospheric mantle, which might had undergone a fluid metasomatism or source mixing by the continental crust during or shortly after the Triassic continental subduction. Significantly negative Nb anomalies observed in the spidergrams and other “crustal” signatures of these rocks suggest an important role of continental material in their petrogenesis. The felsic rocks demonstrate geochemical and Sr-Nd isotopic features (initial 87Sr/86Sr = 0.70814 to 0.70961 and εNd (t) = −18.9 to −17.0) similar to those of the post-collisional granitic plutons, probably derived from the anatexis of lower/middle crust in response to basaltic magma underplating. The widespread melting of the metasomatized mantle was probably attributed to the thermal perturbation or lithospheric extension induced by the mega-large displacement along the Tan-Lu wrench fault system when northward strike-slipping movement of the Izanagi Plate occurred during the late Mesozoic.

Metamorphic zoning and thermal structure of the Dabie ultrahigh‐ pressure–high‐pressure terrane, central China

The ultrahigh‐ and high‐pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite‐bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.