Sulu Ultrahigh-pressure Terrane Research Articles

Effects of melt fractional crystallization on Sr-Nd and Lu-Hf isotope systems: a case study of Triassic migmatite in the Sulu UHP terrane

The Weihai migmatite in the Sulu ultra-high-pressure (UHP) metamorphic terrane, eastern China, underwent partial melting in the Late Triassic during its exhumation. The primary partial melts experienced a decompressional fractional crystallization (DFC) process to produce plagioclase (Pl)-rich leucosome crystallized under eclogite to granulite facies conditions and K-feldspar (Kfs)-rich pegmatitic veins crystallized under amphibolite-facies conditions. In this study, our results demonstrate that the DFC process can cause decoupling between whole-rock Sr and Nd isotopes. The Pl-rich leucosome has εNd(t) values (–10.4 to −15.0) and initial (87Sr/86Sr) ratios (0.708173–0.712476) very similar to those of the melanosome, but the Kfs-rich pegmatitic veins have homogeneous εNd(t) values (−14.8 to −15.2) and significantly high initial (87Sr/86Sr) ratios (0.713882–0.716284). Our results also suggest that the DFC process can change zircon 176Yb/177Hf and 176Lu/177Hf isotopic ratios, with no effect on 176Hf/177Hf ratios or εHf (t) values. Zircon 176Yb/177Hf and 176Lu/177Hf ratios increase dramatically from the Pl-rich leucosome to the Kfs-rich pegmatitic veins, but zircon 176Hf/177Hf ratios (Pl-rich leucosomes = 0.282330 ± 0.000017; Kfs-rich pegmatitic veins = 0.282321 ± 0.000026) and εHf (t) values (Pl-rich leucosomes = −10.9 ± 0.6; Kfs-rich pegmatitic veins = −11.6 ± 0.8) remain almost unchanged. We propose that the isotopic decoupling between the Pl-rich leucosome and Kfs-rich pegmatitic vein might be caused by melt fractional crystallization occurring too rapidly to allow complete equilibrium between them.

The origin and geophysical implications of a weak C-type olivine fabric in the Xugou ultrahigh pressure garnet peridotite

We report here a comprehensive investigation of mineral chemistry, water content and deformation features (including the crystallographic preferred orientation and dislocation substructures of olivine) of the Xugou garnet peridotite from the southern Sulu ultrahigh pressure (UHP) terrane. Our results suggest: (1) the Xugou garnet peridotite was equilibrated and deformed at high pressure (4.8–6.6 GPa) and relatively low temperature (810–930 °C) conditions. (2) In contrast to the pronounced shape-preferred orientations of olivine, pyroxene and garnet, electron backscattered diffraction (EBSD) measurements show a weak C-type fabric in olivine, a strong (100)[001] fabric in clinopyroxene and orthopyroxene and a nearly random fabric in garnet, respectively. The deformation mechanism map suggests a dominant deformation mechanism of dislocation-accommodated grain boundary sliding (DisGBS) in olivine. (3) The trivial amount of water in olivine (15±10 ppm), orthopyroxene (21±17 ppm), clinopyroxene (64±48 ppm) and garnet (dry) correspond to a water-poor deformation condition. We proposed that the C-type olivine fabric was generated by a dominant DisGBS with some contribution from diffusion creep at elevated pressures. Calculated seismic properties of the peridotite show the fastest P-wave velocity and the fast polarization direction of S-waves are oriented normal to the foliation plane and perpendicular to the lineation in the foliation plane, indicating that the C-type olivine fabric could also be a candidate to explain the observed strike-parallel faster S-wave polarization near trenches and in collisional orogenic belts.

Prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane, eastern China

In order to decipher fluid/melt activity during subduction and subsequent exhumation of felsic continental crust, we carried out a combined study of zircon internal structure, zircon U–Pb age, zircon trace element composition and whole-rock geochemistry on migmatitic gneisses from the Weihai region in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Whole-rock geochemistry suggests that MgO, Fe2O3T, CaO, TiO2, P2O5, rare earth elements (REE), and large ion lithophile elements (HFSE) are nearly completely retained in the melanosome, whereas significant amounts of LILE (e.g., Rb, Ba, K, Sr, Pb) and U are partitioned into the Pl-rich leucosome and the Kfs-rich pegmatite veins. The Kfs-rich pegmatite vein has much higher K2O, U, Pb and Rb, but lower CaO and Na2O contents than those of the Pl-rich leucosome layer. Protolith magmatic zircon domains, with an upper intercept age of about 780Ma, are common in the cores of zircon grains from the melanosome and the Pl-rich leucosome layers, indicating that the protolith of the migmatitic gneiss is Mid-Neoproterozoic magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256Ma occur as overgrowth mantles on the protolith magmatic zircon cores. They are characterized by remarkably flat heavy rare earth element (HREE) patterns without obviously negative Eu anomalies, and have low Y and Th contents, low Th/U and high Hf/Y ratios, indicating a prograde metamorphism in the HP eclogite-facies during subduction. Recrystallized rims of zircons (228±2Ma) in the Pl-rich leucosome layer are similar to magmatic zircons in terms of their HREE enriched patterns with negative Eu anomalies, high Y and low Th contents, and low Hf/Y and Th/U ratios. This indicates that the gneiss has experienced decompressional partial melting and the Pl-rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation. Zircon grains (219±2Ma) in the Kfs-rich veins are characterized by aqueous fluid/melt-related signatures with strongly HREE enriched patterns, high U, Pb and low Th contents, and low Th/U and high Lu/Hf ratios, indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of Pl-rich leucosome layers during later stage cooling during exhumation. Our results suggest that the migmatitic gneiss successively recorded information concerning the protolith, prograde HP metamorphism during subduction, fractional crystallization of primary melt and crystallization of the residual melt during exhumation.

Origins of ultramafic rocks in the Sulu Ultrahigh-pressure Terrane, Eastern China

Ultramafic rocks in the Sulu belt are associated with high-pressure (HP) and ultra-high-pressure (UHP) rocks, a metamorphic product of the northern margin of the Yangtze Craton (YZC). These ultramafic rocks are important because they provide information relevant to the nature and evolution of the Mesozoic collisional belt. We selected ultramafic rocks from Yangkou Bay (YKB), Suoluoshu (SLS) and Hujialin (HJL) in the central region of Sulu belt. The ultramafic rocks in YKB and SLS are hydrated to form serpentinites and they contain low concentrations of moderately incompatible elements (Al, Ti, and V), high contents of Ir-group platinum-group elements (IPGE; Ir, Os, and Ru; 12.8–21.7ppb in total), and high ratios (1.2–5.5) of IPGE to Pd-group PGE (PPGE) in bulk rocks. Spinel contains high Cr (Cr#=atomic ratio of Cr/[Cr+Al], 0.57–0.79). The data suggest that they likely represent hydrated forearc mantle peridotites underlying the margin of the North China Craton (NCC). Dunite samples from HJL are not fully hydrated with loss on ignition (LOI) values ranging from 6.6 to 13.2wt.%, and contain olivine grains with high forsterite components (Fo=100*Mg/[Mg+Fe], 91.7–92.4) and NiO contents (0.36–0.41wt.%). Spinel grains show high Cr# (0.68–0.76). The bulk rock contains high IPGE (5.0–22.7ppb total) and show high ratios (up to 8.8) of IPGE to PPGE. The data suggest that they are also residual mantle peridotites after high degrees of influx partial melting in the subduction setting. The geochemical features of our serpentinite and dunite samples are different from peridotites of young Cenozoic subcontinental lithospheric mantle (SCLM) that have been brought to the surface as xenoliths of young volcanic rocks. Instead, our samples are similar but even more refractory than peridotites of relict Archean SCLM below the NCC that were enclosed as xenoliths in Paleozoic–Mesozoic igneous rocks. Thus, the studied ultramafic rocks likely represent the relic of old refractory lithospheric mantle peridotites that underwent partial melting in the margin of the NCC during the subduction of oceanic lithosphere before the collision with the YZC. The peridotites in the forearc mantle were dragged into the subduction channel during the Mesozoic subduction of the YZC below the NCC, and exhumed together with the buoyant granitic metamorphic rocks in the Sulu belt.

Temperature of prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane: Constraints from Ti-in-zircon thermometer

In order to constrain temperature during subduction and subsequent exhumation of felsic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai (威海) region in the Sulu (苏鲁) ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompressional partial melting during early stage exhumation and subsequent fractional crystallization of primary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the protolith of the migmatitic gneiss is Mid-Neoproterozoic (∼780 Ma) magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. The estimated growth temperatures (625–717 °C) of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eclogite-facies condition during subduction. Zircon recrystallized rims (228±2 Ma) in the Pl-rich leucosome layers provide the lower limit of the decompresssional partial melting time during exhumation. The ages from 228±2 to 219±2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 °C, and the temperatures of new growth zircon grains (219±2 Ma) in Kfs-rich vein are between 769 and 529 °C. The estimated temper atures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indicating that a process of fractional crystallization of the partial melt during exhumation.

Petrofabric and strength of SiO2 near the quartz‐coesite phase boundary

AbstractQuartz and coesite constitute an important portion of deep subducted continental upper crust. However, the rheology and petrofabric of quartz and coesite are not well constrained under high‐pressure (HP)/ultrahigh‐pressure (UHP) metamorphic conditions both in laboratory and in nature. We report here the first deformation microstructure and lattice preferred orientations of garnet, omphacite, interstitial coesite, quartz and rutile in UHP eclogites from the Yangkou Bay, Sulu UHP terrane, as well as fabric development of quartz and coesite in shear experiments at P–T conditions near the quartz‐coesite phase boundary. Our results show: (i) garnet and coesite develop weak to random fabric while omphacite, quartz and rutile develop pronounced fabrics in deformed natural eclogites; (ii) quartz develops fabrics in responding to a dominant c‐slip with increasing shear strain under HP/UHP conditions; (iii) coesite develops fabrics in responding to a dominant [100](010) slip with increasing shear strain under UHP conditions; and (iv) the relative strengths of major constituent minerals in continental deep subduction zones are quartz < omphacite < coesite. These results are consistent with the rheology determined in the laboratory for quartz, jadeite/omphacite and coesite. We propose that jadeite/omphacite dominates the rheology and seismic anisotropy of deeply subducted continental crust.

P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths

Research Article| July 01, 2012 P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths Shengsi Sun; Shengsi Sun 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada Search for other works by this author on: GSW Google Scholar Shaocheng Ji; Shaocheng Ji * 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada2State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Search for other works by this author on: GSW Google Scholar Qian Wang; Qian Wang * 3State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Search for other works by this author on: GSW Google Scholar Matthew Salisbury; Matthew Salisbury 4Geological Survey of Canada-Atlantic, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Search for other works by this author on: GSW Google Scholar Hartmut Kern Hartmut Kern 5Institut für Geowissenschaften, Universität Kiel, Kiel 24098, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Shengsi Sun 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada Shaocheng Ji * 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada2State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Qian Wang * 3State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Matthew Salisbury 4Geological Survey of Canada-Atlantic, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Hartmut Kern 5Institut für Geowissenschaften, Universität Kiel, Kiel 24098, Germany *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Publisher: Geological Society of America First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (7): 651–654. https://doi.org/10.1130/G33045.1 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shengsi Sun, Shaocheng Ji, Qian Wang, Matthew Salisbury, Hartmut Kern; P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths. Geology 2012;; 40 (7): 651–654. doi: https://doi.org/10.1130/G33045.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We compared the averages of P-wave velocities (Vp) for four lithologies (felsic gneiss, eclogite, retrograde eclogite, and amphibolite) collected from core material sampled by the Chinese Continental Scientific Drilling Project (CCSD) at depths to 5158 m, and from surface outcrops in the Dabie-Sulu (China) ultrahigh-pressure metamorphic terrane. The velocities were measured at pressures to 800 MPa for 69 CCSD samples and 145 surface samples. The Vp-pressure relationship is described by a four-parameter exponential equation, allowing the determination of the crack-closing pressure (Pc), the intrinsic pressure derivative of velocity (D), the intrinsic velocity of the crack-free compacted rock at zero pressure (V0), and the maximum velocity increase due to crack closure (B0). The shape of the nonlinear segment of the velocity-pressure curve is controlled by a decay parameter (k). The CCSD core samples are characterized by significantly higher V0, B0, and k values and lower D and Pc compared to their surface analogues. This suggests that extrapolation of the surface rock data to depth may result in substantial underestimates in seismic velocities, and that the intrinsic pressure derivatives obtained from the Vp-pressure relations for the core samples are more suitable for the determination of in situ velocities at great depth than those derived from measurements on rocks exposed at the surface. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fe3+ distribution between garnet and pyroxenes in mantle wedge carbonate-bearing garnet peridotites (Sulu, China) and implications for their oxidation state

This study presents new measurements of Fe3+ in garnet, olivine, clino- and orthopyroxene of a mantle-derived garnet peridotite from Donghai County, the southeastern end of the Sulu ultrahigh pressure terrane. These rocks correspond to a slice of supra-subduction lithospheric mantle wedge, tectonically emplaced into the crust. They record a multistage metasomatism by an alkali-rich silicate melt at high temperature, and a subsequent influx of a slab-derived incompatible element and silicate-rich fluid during the Triassic UHP metamorphism. We employed two “unconventional” techniques to measure the Fe3+/ΣFe content of mineral phases with high spatial resolution: (i) the flank method electron microprobe analyses for garnet, performing for the first time quantitative Fe2O3 map analyses on zoned garnets at the Dipartimento di Scienze della Terra, University of Milano, and (ii) the electron energy loss spectroscopy (EELS) for garnet, olivine and pyroxenes, at the Bayerisches Geoinstitut, University of Bayreuth.The results indicate that the pyrope-rich metasomatic garnets present a chemical zoning, with the complementary decrease in Al2O3 from ~23 to ~21wt.%, relative to the increase of Fe2O3 from ~0.8 to ~2.5wt.%. Such a trend is likely related to the Fe3+–Al substitution in the garnet octahedral site, which is sensitive to the garnet oxidation state. Clinopyroxenes are diopsidic in composition, whereas olivine and orthopyroxene have ~92mol% of forsterite and enstatite, respectively. The EELS measurements show that clinopyroxene contains relatively high Fe3+/ΣFe ratios and Na contents, ranging from 0.48 to 0.51 and from 0.13 to 0.17 a.p.f.u., respectively. Interestingly, also orthopyroxene may contain Fe3+/ΣFe up to 0.10 (±0.05), a percentage comparable to that of garnet, with important consequences in the study of redox processes in mantle rocks and in the application of many geothermometers.Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning indicates a minimum redistribution of Fe3+ from clinopyroxene to garnet. The enrichment in Fe3+ of Ca-clinopyroxene requires the incorporation of a NaFe3+Si2O6 (aegerine) component, particularly in garnet peridotites where the Al content of clinopyroxene is buffered by its coexistence with garnet. The coupled Na-Fe3+ enrichment of our clinopyroxene likely suggests a corresponding enrichment in the whole rock. The Fe3+ substitution mechanism into clinopyroxene as aegerine component could be therefore favoured by the influx of Fe2O3- alkali-rich metasomatic fluid phases. This suggests that a possible net bulk oxidation and the redistribution of Fe3+ between garnet and pyroxenes could depend on additional variables besides temperature and pressure. Such mechanisms open new possibilities to unravel the redox processes occurring in the portion of mantle wedge interfacing the subducting slab, which is a key location where the mantle redox reactions likely occur.

Seismic velocities and anisotropy of core samples from the Chinese Continental Scientific Drilling borehole in the Sulu UHP terrane, eastern China

A detailed study of seismic properties (P and S wave velocities, hysteresis, anisotropy and shear wave splitting) has been carried out on a unique suite of deep borehole core samples from the Chinese Continental Scientific Drilling (CCSD) project, which penetrated 5158 m into the Sulu ultrahigh‐pressure (UHP) metamorphic terrane (China). Seismic velocities of the deep core samples are more and less sensitive to pressure in the low pressure (<200–300 MPa) nonlinear and high pressure (>200–300 MPa) linear regimes, respectively, than samples from the surface. The comparison suggests that the high pressure data from the core samples are much more reliable for extrapolation to deeper crust than the data from surface analogs that have been subjected to long histories of weathering and alteration along intergranular and transgranular cracks. The significant increases in the pressure sensitivity of seismic velocities for the core samples in the nonlinear regime indicate that drilling‐induced and stress‐relief microcracks with small aspect ratios are fresh and clean without secondary mineral in‐fillings, and are thus easy to close completely under the applied hydrostatic pressure conditions of the laboratory. The data also elucidate that the velocity‐pressure data can successfully provide important hints about the preferred orientation of microcracks that causes P wave velocity anisotropy and shear wave splitting in cracked rocks, and that the effect of compression on the Vp/Vsratios is negligible for crack‐free compacted rocks. The seismic velocities of equivalent isotropic (fabric‐free) and crack‐free crystalline aggregates calculated from room pressure single crystal elastic constants using the Voigt average are in good agreement with the laboratory data at ∼200 MPa. Comparison of the seismic reflection image from the vicinity of the borehole with the normal‐incidence reflection coefficient profile computed from the laboratory‐measured velocities and densities infers that the seismic reflections originate from mafic (eclogite and retrograde eclogite) or ultramafic units within dominantly felsic rocks.

Significant cooling during exhumation of UHP eclogite from the Taohang area in the Sulu region of eastern China and its tectonic significance

AbstractThe decompressional pressure–temperature (P–T) path was estimated for ultrahigh‐pressure (UHP) eclogite from the Sulu region of eastern China by applying geo‐thermobarometers to well‐preserved equilibrium mineral pairs. The sample studied is a kyanite‐bearing eclogite that was collected from the Taohang area of the Sulu region. Garnet is relatively homogeneous in chemical composition, but omphacite has a clear chemical zoning with decreasing jadeite content from core to rim. Assuming that peak‐P equilibrium compositions are preserved in the cores of garnet (Grt) and omphacite (Omp), P–T conditions were calculated to be about 700°C and 3.4 GPa. On the other hand, the jadeite content of omphacite rims varies from 0.35 to 0.46 mol.%. Nevertheless, the variation in Fe/Mg ratios of omphacite rims is very small. Temperatures of 566 ± 54°C were obtained at 1.5 GPa for garnet rim and omphacite rim pairs. These petrological considerations indicate that temperatures should have significantly declined during the early decompression stage of this eclogite. In other areas of the Sulu region, isothermal decompression paths were proposed, and it was concluded that the UHP rocks were exhumed as a large mass tens of kilometers in thickness to avoid thermal effects from the surrounding materials. However, the newly identified decompression path accompanying the significant cooling may indicate that the Taohang outcrop was located at the margin of the Sulu UHP terrane. Thus, the decompressional P–T path is not unique in the Sulu region and varies depending on the location.

Melting-induced fluid flow during exhumation of gneisses of the Sulu ultrahigh-pressure terrane

Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average 206Pb/ 238U analyses reveal an age of 219 ± 3 Ma (2σ), which is younger than the UHP metamorphic age (236 ± 2 Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage. The formation age of the quartz vein (219 ± 2 Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219 ± 3 Ma, 2σ) of the host gneiss. The initial 176Hf/ 177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217 ± 5 Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane.

An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and garnet inclusions in clinopyroxene

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [\( 1\bar{1}00 \)]Ilm//[\( 0\bar{1}0 \)]Cpx (0001)Ilm//(100)Cpx; and [110]Spl//[\( 0\bar{1}0 \)]Cpx (\( \bar{1}11 \))Spl//(100)Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.

Garnet–spinel–corundum–quartz-bearing titanohematite veins in eclogite from the Sulu ultrahigh-pressure terrane: Imprint of a short-lived, high-temperature metamorphic stage

Eclogite-hosted garnet–spinel–corundum–quartz-bearing titanohematite veins and lenses (10–20 cm in width) are described for the first time in the Sulu ultrahigh-pressure metamorphic terrane. Some of the lenses were aligned parallel to the foliation of the host eclogite, suggesting that they were the product of ductile deformation of one titanohematite layer. A vein composed of titanohematite + ilmenite + hematite + spinel + garnet + corundum + quartz + K-feldspar + albite was studied in detail. Ti–Fe oxides account for up to >80% and Al-rich phases for ∼15% of the total volume of this vein. Electron microprobe analyses show that the titanohematite solid solution was made up of 0.75 hematite + 0.25 ilmenite. The unusual mineral assemblage of garnet + spinel + corundum + quartz implies that this vein could have experienced high temperatures (>900 °C). Although the garnets showed well-defined Mg and Mn diffusion zoning in the rim as a result of the high temperature event, slight Mg and Mn growth zoning was preserved in the core. Thus, we suggest that the Sulu UHP terrane could have experienced a short-lived, high-temperature (>900 °C) stage during exhumation. Garnets in the titanohematite vein were characterized by extremely low trace-element contents. Petrological and geochemical features of the veins suggest that they could be metamorphic products of igneous cumulates composed of magnetite + plagioclase ± clinopyroxene.

Zircon U–Pb ages, REE concentrations and Hf isotope compositions of granitic leucosome and pegmatite from the north Sulu UHP terrane in China: Constraints on the timing and nature of partial melting

Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz + Kfs + Pl + Ap inclusions, which record a Neoproterozoic protolith age of ∼ 790 Ma, (2) mantles with Coe + Phe + Ap inclusions that record Triassic UHP age at 227 ± 3 Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ∼ 210 ± 3 Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe + Phe + Ap inclusions record Triassic UHP age of 227 ± 3 Ma, and (2) outer magmatic areas with Qtz + Kfs + Ab + Ap inclusions that record partial melting time of 212 ± 2 Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz + Kfs + Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212 ± 2 Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201 ± 2 Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism. Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/ 177Hf of 0.28187 ± 0.00003 (2 SD; standard deviation) corresponding to εHf (790) and Hf model ages ( T DM2) of about − 16.3 and 2.41 Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (< 0.006), low Th/U (< 0.1) and considerably higher, 176Hf/ 177Hf (0.28233 ± 0.00002; 2 SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP ( εHf (227) = − 14.6 ± 0.8; 2 SD) and pre-metamorphic ( εHf (227) = − 27.7) domains indicates equilibration of the Lu–Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212 Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower 176Hf/ 177Hf (∼ 0.28216) and εHf(t) (− 17.6 ± 1.2; 2 SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression.

Unique coesite-bearing zircon from allanite-bearing gneisses: U-Pb, REE and Lu-Hf properties and implications for the evolution of the Sulu UHP terrane, China

Zircon from allanite-bearing para- and ortho-gneisses provides a unique insight into the geologic evolution of the Sulu ultrahigh-pressure (UH P ) terrane in eastern China. Laser Raman, cathodoluminescence (CL) imaging combined with trace-element, U-Pb, and Lu-Hf isotope data indicate that zircon grains consist of three distinct zones. Inherited magmatic core records a Neoproterozoic protolith age, the coesite-bearing mantle reveals a UH P metamorphic age of 230 ±7 Ma, and the low- P mineral-bearing rim retains an age of 210 ±3 Ma for the amphibolite-facies retrogression. Positive eHf(t) values for the inherited cores reflect involvement of juvenile materials in the protolith. Low 176 Lu/ 177 Hf ratios of coesite-bearing mantles indicate their formation during /after garnet crystallization. Their 176 Hf/ 177 Hf ratios are homogenous and more radiogenic than those of the inherited cores, implying fractionation of the Lu-Hf system during isotope equilibration within the metamorphic mineral assemblage. Moreover, coesite-bearing mantles show geochemical characteristics that are distinctly different from those of previous studies: (1) The chondrite-normalized pattern shows a steep slope from HREE (heavy rare earth elements) to LREE (light rare earth elements) with a moderately negative Eu anomaly (mean Eu/Eu* = 0.53). Such a pattern requires that phases enriched in LREE ( e.g. , allanite) and europium ( e.g. , K-feldspar) were stable during UH P conditions. (2) The positive Ce anomaly is very pronounced (Ce/Ce* of 612–657), e.g. , about seven to eight times higher than that of the inherited core; (3) The uranium content is very low (6–25 ppm) and the Th/U ratio elevated (0.52–2.83). These data for the coesite-bearing mantle are distinctly different from those of previous studies. We interpret the data to suggest high O fugacity during the UH P and retrograde metamorphic evolution, which also played a critical role in restraining diamond growth in Sulu-Dabie UH P rocks.

From emplacement to unroofing: thermal history of the Jiazishan gabbro, Sulu UHP terrane, China

On the eastern extremity of the Jiaodong penin- sula, China, shoshonitic magmas have been injected into the supracrustal rocks of the Sulu ultra-high pressure (UHP) terrane during the crustal exhumation phase. These gran- itoids (collectively termed the Shidao igneous complex or Jiazishan alkaline complex) show geochemical and isotopic signatures of an enriched subcontinental lithospheric mantle and intruded soon after the subducted Yangtze crust had reached peak metamorphic pressure conditions (240- 220 Ma). We have applied various geochronometers to an alkali-gabbro sample from the Jiazishan pluton and the results allow reconstruction of the Triassic-to-present thermal history. Initial rapid cooling of the gabbro at crustal depths is indicated by the close agreement between the Sm- Nd mineral isochron age (228±36 Ma) and the Rb-Sr

Petrology of low-temperature, ultrahigh-pressure marbles and interlayered coesite eclogites near Sanqingge, Sulu terrane, eastern China

AbstractMarbles and interlayered coesite-bearing eclogites near the village ofSanqingge in the Sulu ultrahighpressure (UHP) terrane ofeastern China were studied to estimate their P-T evolution. Using garnet, omphacite and phengite as geothermobarometers, the coesite eclogites are calculated to have experienced P-T conditions of3.4 –3.7 GPa and ~600ºC (stage I), followed by decompression and a slight temperature decrease to 2.7–3.2 GPa and 520–560ºC respectively (stage II) and later to 2.6–2.8 GPa and ~500ºC (stage III). No water influx affected the eclogites until reaching amphibolite facies conditions of 0.5–1.3 Gpa and 595–685ºC (stage IV). As we interpret the occasional appearance ofcalcite with magnesite relics in the core as a reaction ofUHP dolomite and magnesite with Ca-rich fluids at stage IV to form CaCO3, the calculated pressure for stage I could be the maximum pressure experienced by these rocks. Thus, the crustal material ofthe Sanqingge quarry, originally sedimentary carbonates (now marbles) and interstratified basic tuffs (now eclogites), has been buried to a depth of ≥ 120 km at ~600ºC. This burial occurred in a subduction setting along a very low geotherm of 5–6ºC/km. The exhumation possibly occurred in the environment ofa subduction channel.

Petrological investigation of the Ganyu peridotite in the Sulu ultrahigh-pressure terrane, eastern China

The ultramafic body sampled in the Chinese Continental Scientific Drilling (CCSD) Hole PP3 is located in the eastern part of the Dabie–Sulu UHP metamorphic belt near Donghai County. It is about 480 m thick, and consists chiefly of garnet peridotite, dunite and serpentinite. The principal minerals include olivine, chromium spinel, diopside, enstatite, garnet with minor secondary augite, phlogopite and amphibole. Both the olivine and orthopyroxene are highly magnesian, and the garnet is pyropic with 5.4–6.4% CaO and 0.3–3.3% Cr 2O 3. Two generations of clinopyroxene are present; an early diopside followed by augite. Chromium spinels are highly variable with Cr#s (100Cr / (Cr + Al)) between 51 and 89, and their compositions reflect different processes of formation, namely partial melting and eclogite, amphibole and greenschist facies metamorphism. The Mg#s (100 Mg / (Mg + Fe 2+))of the spinels correlate positively with the Cr#s but negatively with oxygen fugacity. Based on the spinel compositions the ultramafic rocks originated in the shallow mantle, then subducted to depths of more than 100 km and finally exhumed to the surface. They underwent partial melting at shallow depths, mostly in the spinel facies, and were later transformed into garnet peridotites during deep subduction. All of the rocks were weakly metasomatized during exhumation and were subjected to retrograde metamorphism.

Geochemistry, petrofabrics and seismic properties of eclogites from the Chinese Continental Scientific Drilling boreholes in the Sulu UHP terrane, eastern China

We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities ( V p) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 °C. At 500 MPa and room temperature, the mean V p reaches 8.50–8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7–2.7% to 5.5%, respectively. The pressure and temperature derivatives of V p are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest V p is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the V p distribution in eclogites (e.g., the fastest V p is at ~ 35° from the foliation in sample B295). Comparison between measured and calculated seismic properties indicates that the CPO of omphacite controls the seismic anisotropy of eclogites at high pressure, and compositional layering and retrograde minerals will increase the anisotropy. Calculated P-wave velocities agree well with velocities measured at 500 MPa and room temperature for fresh eclogites, but much higher than those of retrograde eclogite. As a case study, the laboratory-derived V p– P and V p– T relationships were used to estimate P-wave velocities of eclogites and peridotites beneath the Western Superior Province, Canada. The results indicate that besides the fabric-induced anisotropy, the direction dependence of pressure and temperature derivatives of V p can significantly increase seismic anisotropy of eclogites with depth, which results in eclogites being an important candidate for the seismic anisotropy in the upper mantle. Due to their very high density and velocity, garnet-rich eclogites within peridotite could be detected in seismic reflections in subduction zones.

Multistage metamorphism of orogenic garnet–lherzolite from Zhimafang, Sulu UHP terrane, E. China: Implications for mantle wedge convection during progressive oceanic and continental subduction

Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M 1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M 2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M 3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel lherzolite during continental subduction. The lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet lherzolite ( M 4) and UHP garnet lherzolite ( M 5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet lherzolite ( M 6) and chlorite-amphibole-bearing spinel lherzolite ( M 7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation.