Qingchen Wang

Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China : New implications from coesite and omphacite inclusions in zircon of granitic gneiss

Abstract Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.

Sm–Nd geochronology and petrography of garnet pyroxene granulites in the northern Sulu region of China and their geotectonic implication

Abstract Abundant garnet-bearing granulite lenses are widely distributed in the northern part of the Sulu region and adjacent areas. They are possibly re-metamorphosed high-pressure metamorphic rocks. On the basis of detailed petrographic study, samples WD01, WD04 and ML06 from Laixi and Wendeng were identified as high-pressure granulites, and WH1 from Weihai as an original coesite-bearing eclogite. Three high-pressure granulite samples give mineral-WR isochron ages of 1846±76, 1743±79 and 1752±30 Ma. TDM ages are 3.3, 3.0 and 2.8 Ga. The Sm–Nd mineral-WR isochron ages are interpreted to date as the metamorphic resetting within the medium-pressure granulite facies, representing an isotopic re-homogeneity during uplifting of the high-pressure granulites from deep continent crust. It is important that Sm–Nd chronological characteristics are the same as Archaean high-pressure granulites in the North China craton. However, sample WH1 from Weihai demonstrates abnormal Sm–Nd characteristics. Its whole rock eNd (0) value is +129. TDM age is 1.3 Ga, and constrains the minimum age of re-metamorphosed eclogite protolith formation to the mid-Proterozoic. This result is identical to those reported by Jahn (1994) , showing complicated processes of metamorphism and metasomatism. The data in this paper provide further evidence to define the boundary between the North China craton and UHPM belt in eastern Shandong and to understand the geotectonic nature of the boundary.

Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)

The Tian Shan Mountains constitute central Asias longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ∼1 Ma and ∼23.6 Ma, with some uncertainty below ∼21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ∼16–15 Ma and ∼11–10 Ma. Rock magnetic parameters also undergo important changes at ∼16–15 Ma and ∼11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ∼16–15 Ma and ∼11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<∼5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont.

Decompression P–T path of coesite eclogite to granulite from Weihai, eastern China

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

A transitional eclogite- to high pressure granulite-facies overprint on coesite–eclogite at Taohang in the Sulu ultrahigh-pressure terrane, Eastern China

Abstract A transitional eclogite- to high-pressure granulite-facies paragenesis (Omp+Pl+Qtz±Grt) after peak coesite–eclogite facies metamorphism and predating the later amphibolite-facies overprint is identified in coesite–eclogite from the Taohang area of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. These minerals were equilibrated at 17 kbar and 820°C. This reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the UHP rocks. The tectono-metamorphic significance of the unusually high pressure overprint is also discussed.

Stable isotope geochemistry of marbles from the coesite UHP terrains of Dabieshan and Sulu, China

Abstract Marbles from Dabieshan and Sulu, China, suffered ultra high pressure (UHP) metamorphism in the coesite–eclogite facies at approximately 700°C and 30 kbars during Triassic continental collision and subduction. The marbles range in isotopic composition from +7 to +25 δ 18 O VSMOW and from 0 to +6 δ 13 C VPDB . High δ 13 C values are representative of unmodified protoliths and are similar to those of 13 C -enriched Sinian carbonate rocks from the Yangtze craton. High oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low 18 O meteoric water during diagenesis and dolomitization, by fracture-controlled infiltration of water during subduction, by metamorphic mineral reactions, or by a combination of these processes. No evidence of regional isotopic transport during UHP metamorphism has been found. Sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes. Only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions. Limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered UHP minerals and indicates that the metamorphic fluid–rock system was rock-dominated during and following peak metamorphism. A freely flowing, pervasive fluid phase was not present during UHP metamorphism. There is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted. Correlation of geochemical similarities of UHP marbles with Sinian limestones implies that the subducted edge of the Yangtze craton extends at least as far north as the coesite–eclogite facies rocks of Dabieshan. Deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with Neoproterozoic glaciation.

A high precision U–Pb age of metamorphic rutile in coesite-bearing eclogite from the Dabie Mountains in central China: a new constraint on the cooling history

This paper first reports a high precision U–Pb age of 218±1.2 Ma for rutile in coesite-bearing eclogite from Jinheqiao in the Dabie Mounteins, east–central China. This work shows that the U–Pb mineral (rutile+omphacite) isochron age of 218±2.5 Ma and conventional rutile U–Pb concordia age of 218±1.2 Ma obtained by common Pb correction based on the Pb isotopic composition of omphacite in the same eclogite sample are consistent, proving that the omphacite with low U/Pb ratio (μ=2.8) can be used for common Pb correction in U–Pb dating of rutile. Oxygen isotope analysis of rutile aliquots gave the consistent δ18O values of −6.1±0.1%, demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix. Oxygen isotope thermometry yields temperatures of 695±35 and 460±15 °C for quartz–garnet and quartz–rutile pairs, respectively. These oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet. According to field-based thermochronological studies of rutile, an estimate of the Tc of about 460 °C for U–Pb system in rutile under rapid cooling conditions (∼20 °C/Ma) was advised. Based on this U–Pb age as well as the reported chronological data with their corresponding metamorphic and/or closure temperature, an improved T–t path has been constructed. The T–t path confirms that the UHPM rocks in South Dabie experienced a rapid cooling following the peak metamorphism before 220 Ma and a long isothermal stage from 213 to 180 Ma around 425 °C.

Slab‐like high velocity anomaly in the uppermost mantle beneath the Dabie‐Sulu Orogen

Seismic tomography is used to determine three-dimensional velocity structure of the Dabie-Sulu orogen in eastern China. The high resolution image of the P-waves velocity structure is obtained for the uppermost mantle to a depth of at least 150 km, along the latitudes of 30°–38°N and longitudes of 114°–122°E. Results of this inversion reveal a slab-like high velocity anomaly under this orogen in the depth from Moho discontinuity to at least 110 km. It may represent a remnant of the subducted Yangtze block during the period of Triassic continent-continent collision.

THE DABIE-SULU UHP ROCKS BELT : REVIEW AND PROSPECT

The new results in the studies of the Dabie-Sulu UHP rocks belt during the past 5 years were summarized and discussed. The discussion included the following key points: ( i ) UHP eclogite has two kinds of country rocks, with one being UHP eclogite facies rocks and the other non-UHP granitic gneiss. ( ii ) The FeTiO3 in olivine indicated exsolution at depth of 300–400 km. However, the key point is to prove the peridotite in which the FeTlO3 in olivine was found once had been subducted down that depth. ( iii ) UHP hydrous phase evidenced that fluids had taken part in the UHP metamorphism, while the meter-scale inhomogeneous distribution of O-, C-isotope indicated no fluid activity in the deep subduction environment. ( IV ) No agreement has been arrived on many problems related to the tectonic background of the UHP rocks, such as “whether or not ophiolitic rocks there exist now?”, “when did UHP metamorphism proceed?”, “what is the subdution polarity?”, etc. ( V ) How did the UHP rocks exhume from mantle depth?The future studies will focus on the following three subjects: ( i ) thermal dynamics of the UHP metamorphism, ( ii ) relationship between UHP metamorphism and collision orogeny, as well as their geodynamics, and ( iii ) interactions between crust and mantle, and between continental lithosphere and asthenosphere during the collision orogenic process, as well as their constraints to the evolution of continental lithosphere.

First seismic evidence for continental subduction beneath the Western Alps

The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins.