Lawsonite blueschist in Northern Qilian, NW China: P–T pseudosections and petrologic implications

Zhang, Lifei; Wang, Qianjie; Song, Shuguang

doi:10.1016/j.jseaes.2008.11.007

Cited by 50 publications

(36 citation statements)

References 51 publications

(64 reference statements)

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“…It should be noted that island-arc volcanic complex and high-pressure metamorphic rocks including lawsonite-bearing blueschist and eclogite are well-developed in the middle part of North Qilian Orogen [33,48,49,51,67], but absent in the studied area. Low subdution angle would be responsible for this absence, similar to the case in the southern island arc of Japan [68,69].…”

Section: Heat Sourcementioning

confidence: 91%

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab

2012

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A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian orogen. It is geochemically characterized by high SiO 2 , Na 2 O (high Na/K), elevated Sr/Y and (La/Yb) N , positive Sr anomaly, relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted Nb, Ta, Ti, resembling the high-silica adakite. Zircon U-Pb SHRIMP dating yields a weighted mean age of 438±3 Ma. This age is significantly younger than eclogitization ages of 460-490 Ma in the North Qilian orogen, suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean. Whole-rock Sr and Nd isotopic analyses give initial ratios of I Sr =0.70440.7047 and  Nd (t)=3.04.1, indicating that they are derived from partial melting of the juvenile oceanic crust. In view of the tectonic evolution of the North Qilian orogen, the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of ~60 km. The terminology "adakite" is referred to a series of igneous rocks that are characterized by low Y, elevated Sr, Sr/Y and (La/Yb) N [1]. The petrogenesis of adakite is of great significance and has been hotly discussed, not only for the tectonic reconstruction of the ancient ocean and plate [1-6], but also for the origin of the continent crust dominated by Archaean tonalite-trondhjemite-granodiorite (TTG) [7][8][9][10][11][12]. Adakite is originally linked to melting of young and hot slab, and mainly distributes in the circum-Pacific margins [7,8,13,14]. However, recent studies have revealed that partial melting of thickened lower crust and differentiation of parental basaltic magma at high pressure are also possible mechanisms [15][16][17][18][19][20][21][22][23][24][25]. This implies that adakite is of diverse derivation and the slab melting model, to some extent, has defect as extremely limited conditions for melting, thus prerequisites such as high geothermal gradient, relatively hot and juvenile slab, flat subduction and slab window have been proposed [1,6,7,26]. Heat source plays a key role in the formation of magma, but absolutely not the unique one [27]. Decompression melting has been confirmed to be one of the most important mechanisms for magma generation. For instance, it is common that when metamorphic rocks exhume to the Earth's surface, break-down of hydrous minerals will decrease the solidus line of surrounding rocks, inducing melting due to pressure-decreasing [28][29][30][31][32]. This paper presents geochemical, Sr-Nd isotopic and geochronological data for an Early Silurian-aged high-silica adakite in the west segment of the North Qilian orogen. Combining the temporal relation with formation of eclogite and tectonics of the North Qilian Orogen, we suggest that the adakitic magma is generated by decompression melting of eclogite during exhumation. This model provides a new explanation for the origin of adakite, as well as tectonic evolution of the North Qilian oro...

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Section: Heat Sourcementioning

confidence: 91%

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab

2012

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“…To its south is the lawsonite-bearing blueschist belt, and to the further south, they thrust southwestwards onto the Devonian molasses [37]. New 40 Ar/ 39 Ar dating revealed that the blueschist facies metamorphism happened at ~413-415 Ma [38].…”

Section: Geological Background and Samplingmentioning

confidence: 99%

“…Consequently, the Jiugequan ophiolite is more likely to be formed in a back-arc basin spreading center when the ancient Qilian oceanic plate subducted northwards; the cold subduction led to the island arc magmatic activity and back-arc basin spreading. In the Late Silurian, this back-arc basin started to subduct southwards and form HP/LT lawsonite-bearing blueschist [16,28,37], which led to final close of the Qilian Ocean. …”

Section: Tectonic Implications Of Zircon U-pb Agementioning

confidence: 99%

Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain, NW China

Xia

Song

2010

Chin. Sci. Bull.

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The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain. It has been drawn much attention and extensively studied in recent years. In this study, ion microprobe (SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite. Eighteen analyses yield a relatively consistent apparent 206 Pb/ 238 U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma (MSWD=1.06), which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite. Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures. The petrological, geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician, while the ancient Qilian oceanic plate subducted northwards. The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era. North Qilian Mountain, Jiugequan ophiolite, SHRIMP U-Pb dating, spreading of back-arc basin Citation:Xia X H, Song S G. Forming age and tectono-petrogenises of the

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“…Phengite has a Si content between 3.43 and 3.48 p.f.u, and the composition of sodic amphiboles in the blueschist are glaucophane and crosstie [13]. Using internally consistent mineral thermodynamic database and Domino/Theriak calculation software [14], the calculated P-T conditions of these two types of blueschist are 335-355°C and 0.8-0.95 GPa for lawsonite blueschist and 345-375°C and 0.75-0.85 GPa for epidote blueschist [5].…”

Section: Geological Settingmentioning

confidence: 99%

“…And then, Wu et al [3] and Song [4] reported its field occurrences and petrological characteristics in detail. Recently, the accurate peak P-T conditions of the lawsonite blueschist have been calculated by Zhang et al [5] using phase equilibrium method. However, the metamorphic ages of this lawsonite blueschist have not been dated precisely yet [6][7][8].…”

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confidence: 99%

40Ar/39Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain, NW China, and their tectonic implications

Lin

Zhang

et al. 2010

Chin. Sci. Bull.

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Using laser 40 Ar/ 39 Ar dating method, we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan, northern Qilian Mountain, NW China. The high quality laser 40 Ar/ 39 Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma. The isochron age obtained from phengite in epidote blueschist is 415±7 Ma. These data, combining with peak metamorphic P-T conditions and regional geological setting, allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca. 413-415 Ma, which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone. This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma, which also represents the convergent age between the North China Craton and the Qaidam block. northern Qilian Mountain, lawsonite-blueschist, 40 Ar/ 39 Ar isochron age, corner flow Citation: Lin Y H, Zhang L F, Ji J Q, et al. 40 Ar/ 39 Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain, NW China, and their tectonic implications.

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Lawsonite blueschist in Northern Qilian, NW China: P–T pseudosections and petrologic implications

Cited by 50 publications

References 51 publications

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab

Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain, NW China

40Ar/39Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain, NW China, and their tectonic implications

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