Lawsonite-bearing chloritoid–glaucophane schist from SW Tianshan, China: Phase equilibria and P–T path

Du, Jinxue; Zhang, Lifei; Lü, Zeng; Chu, Xu

doi:10.1016/j.jseaes.2011.04.003

Cited by 40 publications

(27 citation statements)

References 55 publications

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“…Recently, Wei & Clarke (2011) have proposed that lawsonite cannot survive in most decompression processes of eclogite, even with subtle cooling, due to the dehydration reaction: o + law = gl + ep + H 2 O, as mentioned above. There are several lines of evidence to support the presence of lawsonite: (i) pristine lawsonite has been recently discovered in a glaucophane schist and an eclogite in the South Tianshan (Du et al. , 2011a,b); (ii) the aggregates of zo + pa ± q have been observed in the studied sample (Fig.…”

Section: Discussionmentioning

confidence: 83%

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P–T path

Zhang¹,

Wei²

2012

Journal Metamorphic Geology

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Eclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine‐grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits core–rim zoning with increasing grossular (Xgr) and pyrope (Xpy) contents, but a few porphyroblastic garnet grains in type I eclogite shows core–mantle zoning with increasing Xpy and a slight decrease in Xgr, and mantle–rim zoning with increases in both Xgr and Xpy. Garnet rims in type I eclogite have higher Xpy than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre‐peak temperature prograde heating to the pressure peak (Pmax) which was recognized by the garnet core–mantle zoning with increasing Xpy and decreasing Xgr. The P–T conditions at Pmax constrained from garnet mantle or core compositions with minimum Xgr content are 29–30 kbar at 526–540 °C for type I and 28.2 kbar at 518 °C for type II, suggesting an apparent thermal gradient of ∼5.5 °C km−1. Stage II is the post‐Pmax decompression and heating to the temperature peak (Tmax), which was modelled from the garnet zoning with increasing Xgr and Xpy contents. The P–T conditions at Tmax, defined using the garnet rim compositions with maximum Xpy content and the Si content in phengite, are 24–27 kbar at 590 °C for type I and 22 kbar at 540 °C for type II. Stage III is the post‐Tmax isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidote–amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P–T estimates obtained from conventional garnet–clinopyroxene–phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P–T conditions of stage II at Tmax, but with large uncertainties in temperature. On the basis of these metamorphic stages or P–T paths, we reinterpreted that the recently reported zircon U–Pb ages for eclogite may date the Tmax stage or the later decompression stage, and the widely d...

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Section: Discussionmentioning

confidence: 83%

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P–T path

Zhang¹,

Wei²

2012

Journal Metamorphic Geology

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“…We note that structurally coherent HP‐UHP ultramafic rocks coexist in this belt (e.g. Shen et al., ), and the Western Tianshan subduction zone reflects an extremely cold geotherm (~5.5°C/km; Du et al., ; Li, Klemd, Gao, & Meyer, ; Zhang, Ellis, Arculus, Jiang, & Wei, ) along the border of the “forbidden zone” (Liou, Zhang, Ernst, Rumble, & Maruyama, ; Schreyer, ). The peak metamorphic temperatures still lie within the stability field of antigorite (<650°C; Goffé, Bousquet, Henry, & Pichon, ; Yamato et al., ), which is why the hydrated mantle could provide positive buoyancy (Guillot, Schwartz, Reynard, Agard, & Prigent, ).…”

Section: Discussionmentioning

confidence: 90%

“…() subdivided the belt into a northern UHP zone and a southern HP zone (dashed line in Figure b). Lawsonite has been discovered in glaucophane schists and eclogites suggesting subduction along an extreme geotherm of ~5°C/km (Du, Zhang, Bader, Chen, & Lü, ; Du, Zhang, Lü, & Chu, ; Li, Gao, John, Klemd, & Su, ).…”

Section: Geological Settings and Sample Descriptionsmentioning

confidence: 99%

The early exhumation history of the Western TianshanUHPmetamorphic belt, China: New constraints from titanite U–Pb geochronology and thermobarometry

Zhang

Chu

et al. 2018

Journal Metamorphic Geology

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The Chinese Western Tianshan terrane is a well‐known ultrahigh‐pressure (UHP) metamorphic belt that mainly consists of subducted oceanic crustal materials. While two uplift stages have been recognized, the early exhumation history has not been fully constrained. We conducted U–Th–Pb and trace element microanalysis of titanite as well as zircon from UHP eclogites. Apart from several inclusions in garnet, three types of retrograde titanite have been identified based on their petrographic occurrences: (a) in matrix enclosing relict rutile; (b) as coarse‐grained coronas around garnet; and (c) in veins coexisting with epidote and/or apatite. The three types of titanite are homogeneous in chemical composition, low in Al, enriched in middle rare earth elements (REEs), and depleted in light REEs and heavy REEs, suggesting their growth during retrograde decompression. Sensitive high‐resolution ion microprobe (SHRIMP) and LA‐ICP‐MS U–Pb dating of titanite from four samples, regardless of their textural occurrences, gave consistent ages of c. 306 Ma. Phase equilibria modelling and Zr‐in‐titanite thermometry indicate that titanite replaced rutile at ~14 kbar and ~570°C, following a nearly isothermal decompression from the UHP peak conditions of 30 kbar and 550–570°C. SHRIMP zircon U–Pb dating yields two age groups: c. 320 Ma and c. 305 Ma. The former is generally interpreted to be the age of UHP stage. The latter, overlapping the titanite ages, represents the timing of titanite growth during exhumation. Thus, we estimate that the near‐isothermal exhumation from the UHP peak conditions to the epidote–amphibolite facies retrogression (14 kbar) lasted c. 15 Ma, corresponding to an average exhumation rate of 3.4 ± 2.5 mm/yr (2σ). This exhumation rate lies within the range of oceanic LT‐HP belts in the absence of continental subduction, suggesting a similar slab exhumation mechanism that operates in the UHP regime. The consistent retrograde metamorphic ages recorded by titanite from different localities, and the general agreement among P–T paths in the adjacent area, suggest that the UHP unit of Western Tianshan metamorphic belt was likely exhumed as a coherent unit.

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“…They have experienced pressures of 24e30 kbar and 500e600 C (Zhang et al, 2007b;Tian and Wei, 2013). Within the eclogites coesite (Lü et al, 2008), lawsonite (Du et al, 2011) and stilpnomelane (Du et al, 2014) inclusions have been found.…”

Section: Geological Settingmentioning

confidence: 99%

Northward subduction-related orogenesis of the southern Altaids: Constraints from structural and metamorphic analysis of the HP/UHP accretionary complex in Chinese southwestern Tianshan, NW China

Scheltens

Zhang

Xiao

et al. 2015

Geoscience Frontiers

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Northward subduction-related orogenesis of the southern Altaids: Constraints from structural and metamorphic analysis of the HP/UHP accretionary complex in Chinese southwestern Tianshan, NW China a b s t r a c tThe Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subduction polarity and the way of accretion are the main problems. Southward, northward subduction and multiple subduction models have been proposed. This study focuses on the structural geology of two of the main faults in the region, the South Tianshan Fault and the Nikolaev Line. The dip direction in the Muzhaerte valley is southward and lineations all point towards the NW. Two shear sense motions have been observed within both of these fault zones, a sinistral one, and a dextral one, the latter with an age of 236e251 Ma. Structural analyses on the fault zones show that subduction has been northward rather than southward. The two shear sense directions indicate that the Yili block was first dragged along towards the east due to the clockwise rotation of the Tarim block. After the Tarim block stopped rotating, the Yili block still kept going eastward, inducing the dextral shear senses within the fault zones.

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Lawsonite-bearing chloritoid–glaucophane schist from SW Tianshan, China: Phase equilibria and P–T path

Cited by 40 publications

References 55 publications

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P–T path

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P–T path

The early exhumation history of the Western TianshanUHPmetamorphic belt, China: New constraints from titanite U–Pb geochronology and thermobarometry

Northward subduction-related orogenesis of the southern Altaids: Constraints from structural and metamorphic analysis of the HP/UHP accretionary complex in Chinese southwestern Tianshan, NW China

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