Sulu Ultrahigh-pressure Research Articles

Identification of UHP and Non-UHP Orthogneisses in the Sulu UHP Terrane, Eastern China: Evidence from SHRIMP U-Pb Dating of Mineral Inclusion-Bearing Zircons

Mineral inclusions in zoned zircons reveal that both ultrahigh-pressure (UHP) and non-UHP orthogneisses occur in cores from drill hole CCSD-PP1 at Donghai, southwestern Sulu terrane. Zircons from the UHP orthogneiss have inherited magmatic cores containing low-P mineral inclusions, narrow mantles with UHP mineral inclusions, and rims that contain low-P minerals or are inclusion free. In contrast, zircons from non-UHP orthogneiss have only magmatic cores and metamorphic rims, both of which are characterized by low-P mineral inclusions, or are inclusion free. SHRIMP U-Pb analyses of zircons from the UHP orthogneiss identify three discrete and meaningful age groups: Proterozoic magmatic protolith ages >753 Ma in the cores, a UHP metamorphic event at 227 ± 8 Ma in the coesite-bearing mantles, and an amphibolite-facies overprint at 213 ± 7 Ma in the overgrowth rims. Zircons from non-UHP orthogneiss record only two discrete age groups, inherited magmatic cores with Proterozoic protolith ages >748 Ma, and metamorphic rims with an age of about 211 ± 6 Ma. The metamorphic rims have the same Triassic age as the late amphibolite-facies overprint of the UHP orthogneiss within analytical uncertainty. These data, together with previous studies, suggest that Neoproterozoic supracrustal protoliths of the Yangtze craton experienced Late Triassic (227 ± 8 Ma) subduction to depths of at least 120 km, and were then rapidly exhumed to mid-crustal depths where both UHP rocks and a few non-UHP Neoproterozic granitic intrusives were subjected to coeval amphibolite-facies retrograde metamorphism at 211 ± 6 Ma. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P-T data is estimated to be 6.3-7.1 km/m.y. This rapid exhumation suggests that the Sulu UHP metamorphic rocks returned to crustal depths as part of a buoyant sliver, possibly produced by slab breakoff.

Geochemistry of eclogites from the Dabie–Sulu terrane, eastern China: New insights into protoliths and trace element behaviour during UHP metamorphism

The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ∼ 2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature ( PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism (∼ 2.7 Gpa) began at 242.2 ± 0.4 Ma ( n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (> ∼ 4 Gpa) at 227.3 ± 0.7 Ma ( n = 72), before exhumation (< ∼ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.

Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China

Majorand trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18Ma) volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic ( 220Ma) Sulu ultrahighpressure (UHP) terrane along its southern margin. P–T estimates for the Paleozoic and Neogene peridotite xenoliths reflect different model geotherms corresponding to surface heat flows of 40mW/m (Paleozoic) and 80mW/m (Neogene). Garnet peridotite xenoliths or xenocrysts from the Paleozoic kimberlites are strongly depleted, similar to peridotites from other areas of cratonic mantle, with magnesium olivine (mean Fo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet (and spinel) peridotite xenoliths hosted in Neogene basalts are derived from fertile mantle; they have high Al2O3 and TiO2 contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnet and diopside with flat rare earth element (REE) patterns. The differences between the Paleozoic and Neogene xenoliths suggest that a buoyant refractory lithospheric keel present beneath the eastern North China Craton in Paleozoic times was at least partly replaced by younger, hotter and more fertile lithospheric mantle during Mesozoic– Cenozoic times. Garnet peridotites from the Sulu UHP terrane have less magnesian olivine (Fo91.5), and lower-Cr garnet than the Paleozoic xenoliths. The diopsides have low heavy REE (HREE) contents and sinusoidal to light REE (LREE)-enriched REE patterns. These features, and their high Mg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith. Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts or fluids in Mesoproterozoic, and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during UHP metamorphism in the early Mesozoic. Tectonic extension of the subcontinental lithospheric mantle of the North China Craton and exhumation of the Sulu UHP rocks in the early Mesozoic induced upwelling of the asthenosphere. Peridotites sampled by the Neogene basalts represent newly formed lithosphere derived by cooling of the upwelling asthenospheric mantle in Jurassic–Cretaceous and Paleogene time. JOURNAL OF PETROLOGY VOLUME 47 NUMBER 11 PAGES 2233–2256 2006 doi:10.1093/petrology/egl042

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0–2,050 m)

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO2, total Fe, V, Co and Sr, and very low SiO2, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.

North China and South China collision: Insights from analogue modeling

Abstract The North China–South China collision during early Mesozoic time formed the Dabie–Sulu ultrahigh pressure (UHP) metamorphic belt in eastern China, the largest UHP metamorphic belt on Earth. However, no consensus exists about the dynamics of this continental collision. In order to get insights into this process, a series of four analogue models was performed. The experiments used layers of silicone putty and sand mixtures to reproduce the rheological characteristics of four-layer continental lithospheres and two-layer oceanic lithospheres. Based on existing constraints, the models reproduced a thick and cool North China block and a relatively thinner and warmer South China block. The shape of these different blocks was varied in different experiments in order to reproduce the main geometrical boundary conditions suggested in previous models. It is suggested here that, under present knowledge of North–South China lithosphere strengths prior to the collision, South China could not have penetrated into North China. Detachment of South China upper crust from lower crust was not observed throughout the experiments. A model that North China as a rectangular promontory penetrated South China approximately normal to the future Tanlu fault zone was advocated here.

Pargasite and ilmenite exsolution texture in clinopyroxenes from Hujialing garnet-pyroxenite, Su-Lu UHP terrane, Central China: a geodynamic implication

We report here the discovery of amphibole exsolution texture in the Hujialing garnet-pyroxenite, Su-Lu ultrahigh pressure (UHP) metamorphic terrane. These rocks together with coesite-bearing eclogites were exhumed from mantle depths. The garnet-pyroxenite consists mainly of garnet and diopside with minor pargasitic amphibole, ilmenite, Mg-Al spinel, magnetite, tremolite, orthopyroxene, and olivine. Diopside contains abundant garnet inclusions as well as pargasite and ilmenite lamellae. The present mineral assemblages observed in the Hujialing garnet-pyroxenite indicate that the parental mineral of the reported exsolution texture should have a composition of mixture of pargasite, ilmenite and diopside. Primary clinopyroxene in the Hujialing garnet-pyroxenite contains ∼2000 ppm H 2 O, suggesting a peak metamorphic pressure probably at 8.0 GPa. Prevailing exsolution of pargasite (stage I) and ilmenite (stage II) within clinopyroxenes suggests an at least two-stage exsolution process, which implies a complex exhumation history for the Hujialing garnet-pyroxenite. Texture relationships apparently show that the pargasite lamellae formed earlier than the ilmenite lamellae in diopsides. These observational data implies that the exhumation of these UHP metamorphic rocks were accomplished in two steps, one at depths ∼100 km, another at much shallower levels.

The Xiaojiao high-grade eclogite-type rutile deposit in Jiangsu, China: Geology, geochemistry and metallogenesis

The Xiaojiao rutile deposit with stratiform, banded and lens-shaped rutile-bearing eclogite in the Sulu ultra-high pressure metamorphic zone represents the largest and richest primary rutile mineralization in the zone. Four kinds of rutile can be recognized. They are: (1) inclusions in garnet and omphacite; (2) intergranular infilling between garnet and omphacite; (3) altered remnant; and (4) hydrothermal infilling in veinlets. Electron microprobe data show that sodic metasomatism probably from metamorphic fluid could play an important role in rutile mineralization during high and ultra-high pressure meta-morphism. The present paper concludes that according to geochemical characteristics, the eclogite originated from mafic and ultramafic rocks in the upper mantle and crust-mantle boundary and the Xiaojiao eclogite-type rutile deposit is genetically closely related to ultra-high pressure metamorphism.

Tectonic Division of the Sulu Ultrahigh-Pressure Region and the Nature of Its Boundary with the North China Block

Detailed geological, geochemical, and geochronological data suggest that the boundary between the North China Block and the Sulu UHP region in eastern Shandong Peninsula is not a single fault as previously considered. Instead, it is a complicated magmatic-metamorphic zone about 40-60 km wide bounded by the Muping (MP) and Mishan (MS) faults, and herein termed the Kuyushan boundary zone. The complicated zone consists of a granitoid complex containing abundant lenses and pods of metamorphic rock of different protoliths and sizes that are tectonically juxtaposed. The Sulu UHP region is subdivided into two tectonic units: the Haiyangsuo eclogitized granulite unit, and the Rongcheng-Weihai UHP unit bounded by a large-scale ductile shear zone. The Haiyangsuo unit is considered to be a tectonic slab of the lower crust of the Yangtze Block, in which granulites underwent an eclogite-facies overprint. The Rongcheng-Weihai unit represents an ancient subduction relict of the Yangtze crust, and consists of two subunits. The Rongcheng subunit is a typical UHP unit, with a P-T path recording rapid uplifting from mantle (coesite-eclogite facies) to upper lower-middle crustal (amphibolite-granulite facies) depths. The Weihai subunit is a tectonic slab that was detached from the UHP zone during exhumation, and was tectonically emplaced into the lower crust of the North China Block above the subduction zone. UHP rocks of this unit have been strongly modified by subsequent granulite-facies metamorphism. The North China Block, the Kunyushan boundary zone, and the Haiyangsuo and Rongcheng-Weihai units are imbricated slabs. The Rongcheng subunit may have been subducted northwestward under the North China Block, but a detached UHP slab of the Weihai subunit was overthrust onto the North China Block. This inference is supported by a recent geophysical study.

Plastic flow of coesite eclogite in a deep continent subduction regime: Microstructures, deformation mechanisms and rheologic implications

Ultrahigh pressure (UHP) metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts. Despite the long distance transport of the UHP rocks in a deep subduction channel and the fast strain rate required for rapid exhumation, little evidence for UHP deformation is found in the exhumed UHP terrains so far identified. Some UHP rocks have even preserved primary igneous and volcanic structures. These observations have led to the postulation that the stress level is low (a few MPa) in the deep subduction zones. Deformation microstructures of some eclogite mylonites from a ductile shear complex in the Sulu UHP metamorphic belt, eastern China, provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the UHP rocks in a highly strain-localized shear zone at the UHP conditions. Omphacite, phengite and kyanite are the main strain-accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration. Coesite is either fractured, with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure. Garnet does not show optically discernable plastic deformation. P–T conditions for the deformation are estimated to be P≈3.3 GPa and T≈624 °C using garnet–omphacite–kyanite–phengite–coesite/quartz thermobarometry, suggesting that deeply subducted continental crust remains “cold” at the upper mantle depths. Large part of the deeply subducted crust remains relatively undeformed. Strain is strongly localized into narrow shear zones. Stress level in the deep subduction channel is extrapolated on the order of tens of MPa, much higher than it was previously thought.

U-Pb SHRIMP ages recorded in the coesite-bearing zircon domains of paragneisses in the southwestern Sulu terrane, eastern China: New interpretation

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSDPP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 ± 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane, eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz+Phe+Ap+impurities and Qtz+Phe+impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe+Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe+Ap and Coe+Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP meta-morphic stage and overprinted by the late retrogressive metamorphism related to the Sulu ter-rane exhumation. SHRIMP U-Pb dating reveals that zircons separated from the paragneisses recorded complicated geochronological traces. Some zircons retain inherited cores with the 206Pb/238U ages of 284–754 Ma, indicating the complexity of the detrital zircons in protoliths. Coesite-bearing zircon domains recorded 238–266 Ma (with the average age of 245±14 Ma) for the UHP metamorphic event. The retrogressive rims of zircons recorded 213–223 Ma (with the average age of 217±15 Ma) for the late-stage amphibolite facies metamorphic event, indicating that the rapid exhumation time for the Sulu UHP terrane is about 220 Ma.

Reply to ‘Discussion of the paper: “High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation by Zhao et al.”’

Reply to ‘Discussion of the paper: “High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation by Zhao <i>et al.</i>”’

Discussion of the paper ’High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation’ by Zhao et al.

Discussion of the paper ’High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation’ by Zhao <i>et al.</i>

Role of partial melting in the evolution of the Sulu (eastern China) ultrahigh-pressure terrane

Strongly deformed potassium feldspar–rich dikes are widely distributed in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The fact that the crystallization ages of these dikes overlap with the age of peak UHP metamorphic conditions implies the presence of melt during metamorphism. Sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic Sulu gneiss. Partial melting may be the key to solving several unusual features of the Sulu and other UHP terranes, such as the almost complete lack of mineralogical evidence for UHP conditions and the limited growth of zircon during UHP conditions in the dominant felsic gneiss. In addition, because partial melting will cause a drastic reduction in the strength of the UHP gneisses, the most likely exhumation mechanism is diapiric rise of a low-viscosity, partially molten mass containing entrained blocks of eclogite, and not a thin sheet as usually proposed.

Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China: geochronological, geochemical and Nd–Sr isotopic evidence

The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon 206Pb/ 238U age of 215±5 Ma was obtained for the quartz syenite and mineral 40Ar/ 39Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K 2O≈4.4–6.4 wt.% and K 2O/Na 2O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al 2O 3 (∼0.76) and TiO 2 (∼1.12 wt.%). They also have high Sr ( 87Sr/ 86Sr∼0.7073), low Nd ( ɛ Nd=∼−16.5) isotopic ratios, enriched LILE (Ba/La=66–74), LREE [(La/Yb) N =28–33] and depleted HFSE (La/Nb=4–6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle.

Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance

Garnet olivine pyroxenite at Hujialin is situated in the Sulu ultrahigh pressure (UHP) metamorphic belt, Shandong Province. Most of the zircon separated from the rock is well crystallized, prismatic and granular with a length to width ratio of 1:1.3-1:2.5. CL and BSE images show the magmatic oscillatory zoning in the zircon. Th/U ratio ranges from 0.99 to 2.81. These suggest a magmatic origin for the zircon studied. SHRIMP dating yields206pb/238U ages of 207–223 Ma, with a weighted average of 216±Ma. This age corresponds to zircon growth during exhumation of UHP slab and thus the timing of amphibolite-facies retrogression. The garnet olivine pyroxenite was wrapped and brought to the crust by the UHP slab during exhumation, and then suffered from metasomatism by fluid from the UHP slab itself. The zircon U-Pb age records the timing of the crystallization of metasomatic melt. Therefore, fluid that was released during exhumation of deeply subducted continental slab may be the important source for zircon growth.

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P– T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.

Retrograded textures and associated mass transfer: evidence for aqueous fluid action during exhumation of the Qinglongshan eclogite, Southern Sulu ultrahigh pressure metamorphic terrane, eastern China

AbstractThe Qinglongshan eclogites in the Southern Sulu ultrahigh pressure metamorphic (UHPM) terrane show very different retrograded textures from their counterparts in the Northern Sulu terrane, implying a different thermal history. Scanning electron and optical microscope observations indicate that the peak assemblage of the Qinglongshan eclogite is anhydrous, composed of Grt + OmpI + Rt + (Ky + coesite). These primary minerals were replaced by second and third stage minerals, resulting in symplectite pseudomorphs or coronas. The following relationships are inferred: OmpI → OmpII + Ab + Fe‐oxide symplectite (type I) and Rt → Rt + Ilm intergrowth; and, Ky → Pg, OmpII (+Pl) → Amp (+Pl) symplectite (type II), and Grt → Prg (+Fe‐oxide). Mineral chemistry and mass‐balance demonstrate that the pseudomorphed textures were developed by metasomatism involving dissolution and precipitation intensified by fluids along grain boundaries. The formation of symplectite type I produced Fe, Mg and Na but consumed Ca and Si. The Mg and Fe diffused to garnet where exchange of (Mg, Fe) with Ca of the garnet resulted in compositional zonation with decreased Ca towards the edge of garnet grains where Ca was consumed during symplectite formation. The replacement of kyanite by paragonite consumed the extra Na. In the later stage, fluid infiltration partially transformed symplectite type I to type II, and narrow rims of pargasite resorbed garnet from their boundaries. Mass balance suggests that the transformation and resorption would have been coupled during fluid infiltration. In the latest stage, epidote and quartz were precipitated at very late stage as a result of fluid activity along microfractures. Tentative P–T conditions based on mineral reactions and thermocalc software suggest that the retrograded eclogite did not record the granulite facies retrograde evolution characteristic of eclogites from the Northern Sulu terrane. The difference in retrograde evolution between the Southern and Northern Sulu eclogites suggests a different exhumation history.

Pressure-Temperature Path Recorded in the Yangkou Garnet Peridotite, in Su-Lu Ultrahigh-pressure Metamorphic Belt, Eastern China

Chemical variations along with changes in microstructure of the principal constituent minerals make it possible to identify at least four equilibrium stages in the evolution of the Yangkou garnet peridotite in the Su-Lu ultrahigh-pressure metamorphic belt, eastern China: Stage I-----a primary garnet lherzolite stage represented by coarse-grained (a few millimeters size) porphyroclastic aluminous pyroxenes þ chromian spinel garnet; Stage II-----an ultrahighpressure (UHP) stage defined by fine-grained matrix phases (0 1---0 3 mm size) of garnet þ extremely low-Al orthopyroxene þ high-Na clinopyroxene þ chromite; Stage III-----a medium-pressure stage defined by fine-grained mineral aggregates (50 1---0 2 mm size) mainly composed of aluminous spinelþ high-Al orthopyroxene in the matrix; Stage IV-----an amphiboliteto greenschist-facies stage defined by poikiloblastic amphibole. Orthopyroxene---clinopyroxene thermometry and an empirical spinel barometer give temperatures of around 800---830 C and pressures of 1 2---2 9GPa for porphyroclasts of Stage I. Garnet---orthopyroxene, garnet---clinopyroxene and empirical spinel geothermobarometers give relatively uniform P---T conditions for the matrix garnet---orthopyroxene---clinopyroxene--chromite assemblage of Stage II ( 730---760 C and 3 6--4 1 GPa). Aluminous spinel---olivine pairs in the aggregates give 650---680 C at 1 0---1 5GPa for Stage III. The granulation of the studied rocks may have taken place during subduction (Stages I and II) of the host peridotite. The lack of kelyphitic rims around the matrix garnet and the preservation of incompatible assemblages in the matrix of the studied rocks is due to extremely low H2O activity during the early decompression stage. The peak temperature of associated eclogites in the Yangkou UHP unit is 700---800 C at 3 1---4 1GPa. These observations suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Yangkou UHP unit both experienced a common UHP metamorphic event and that the exhumation path obtained by these reference points supports nearly isothermal decompression of the unit. The Yangkou UHP unit is considered to have been exhumed as part of a larger crustal mass (c.410 km in size) along with the surrounding orthogneisses. Such isothermal decompression paths have also been reported from other areas of the Su-Lu belt and the Dabie Mountains. As the orthogneisses volumetrically account for 490% of these areas, buoyancy-controlled uplift probably played an important role in the exhumation of the UHP rocks, at least from the upper mantle to the lower crust.