J. Cent. South Univ. Technol. (2008) 15: 488-492

DOI: 10.1007/s11771-008-0092-x

    Fluid flow on centimeter-scale in deep paleosubduction zones in 

western Tianshan, China: Evidence from high-pressure veins

ZHANG Jin-fu(张进富)¹, HUANG De-zhi(黄德志)¹, HUANG Shi-qi(黄始琪)¹,

LI Guo-ming(李国明)¹, GAO Jun(高 俊)², SHI Yong-hong(石永红)³

(1. School of Geosciences and Environmental Engineering, Central South University, Changsha 410083, China;

2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;

3. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230001, China)

Abstract: High-pressure(HP) veins were extensively developed in western Tianshan high-pressure(HP) metamorphic belt. The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids. Analytical data show that the immediately adjacent host-rocks of the studied HP vein are eclogites and gradually turned into blueschist as the distance from the veins increases, which indicates that the vein-forming fluid was derived from adjacent host-rocks; the boundaries between the vein and the host-rocks are sharp, which indicates that the fracture of the host-rocks is brittle during the vein-forming process. It is suggested that this type of HP veins is precipitated from the liquid formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies, which results in hydrofracturing of the rocks and provides the space for the vein to precipitate. The width of the eclogite-facies host-rocks is usually 1-2 cm, which provides the direct evidence that the fluid flow is on centimeter-scale.

Key words: subduction zone; high-pressure vein; scale of fluid flow; western Tianshan

1 Introduction

Identification of the origins of fluid, i.e. internal or external, can help us to evaluate the contribution of crust material to mantle during the process of subduction. Numerous studies on tracing the origins of fluids in the deep subduction zones have been made by the study of the geochemistry of island arc volcanic rocks^[1-2] or of HP metamorphic rocks within orogen^[3-4].

Some studies on island arc basalts found that the fluids in subduction zones transported remarkable slab components into the mantle, which shows that the fluid flow in the deep subduction zones was on large-scale. This is concordant with the experimental results that progressive metamorphism of rocks within subduction zones should generate large volumes of fluids by devolatilization^[5]. Study on oxygen isotope of HP metamorphic rocks in Catalina schist terrane obtained the result that the veins and host-rocks are equilibrium in oxygen isotope with aqueous fluids with δ¹⁸O_SMOW= (13±1)%^[6], which also shows that the fluid flow in the paleo-subduction zone is on large scale. Though it is suggested that the fluid flow is in channel manner around the pillow margins in the high-pressure Corsican and Zermatt-Saas ophiolites with limited fluid flow or diffusion into the inner of the pillows, but the fluid pathways were probably distributed throughout the pillow-basalt pile^[7]. All these studies came to the same conclusion that the fluid flow in the deep subduction zones was on a large-scale and the origin of the fluid was external.

However, it has been proved that the large-scale fluid flow was absent in some subduction zones by other studies. For example, studies on high-pressure metamorphic rocks in Alps came to the conclusion that few indications of large-scale fluid flow have been found, and oxygen isotope ratios of rocks vary between a microstructural domains and within a single microstructural domain, and regional homogeneity of oxygen composition is absent, which are attributed to disequilibria in oxygen isotope during HP metamorphism as a result of small-scale fluid flow at low water/rock ratios^[3]. It was suggested by BARNICOAT and CARTWRIGHT^[8] that the stable isotope and major element chemistry of high-pressure metabasic rocks from Western Alps reflecting seafloor alteration process    are not affected by significant pervasive fluid flow during subduction. This conclusion seems to be concurred with the widespread retention of pre- subduction oxygen isotope signature of HP metamorphic rocks.

Fluid flow in some subduction zones was possibly on small-scale and the origin of the fluid was internal. However, no direct evidence on sample scale has been presented. In this work, the direct evidence on centimeter-scale fluid flow in subduction zone was reported.

2 Geological setting and sample description

All samples were collected from Akeyazi River in the western Tianshan HP metamorphic belt, which extends to Kirghizia and Tajikistan in NEE-SWW direction. In China, it extends at least 200 km (Fig.1). The HP metamorphic belt is mainly composed of blueschist, eclogite and greenschist, with minor late Silurian marble lenses and slices of ultramafic rocks. It is a fault-bounded mélange zone, which is probably formed by subduction of Paleozoic South Tianshan Ocean between the Yili-central Tianshan and Tarim plates^[9]. Since the discovery of the eclogite^[10], studies on the age and the PT situations of the HP metamorphic rocks have been made^[11-12].

HP veins were extensively developed in this HP metamorphic belt, which provides us wonderful samples to study the fluids in the deep paleosubduction zone in western Tianshan. According to the host-rocks, two types of HP veins can be identified, hosted by blueschists and eclogites, respectively. Electronic microprobe analytical data show that the immediately adjacent host-rocks of the HP vein studied in this work are eclogites, which are gradually turned into blueschist as they are far away from the vein (see Fig.2).

Fig.1 Tectonic sketch showing distribution of HP metamorphic rocks in western Tianshan and localities of samples

Fig.2 Four domains at different distances from HP vein for electronic microprobe analysis

3 Analytical results of main minerals from HP vein and host-rocks

Four domains at different distances from the vein for electronic microprobe analysis are shown in Fig.2 and relative micrographs for each analytical domain are shown in Fig.3. The host-rocks farther away from the vein are blueschist-facies rocks (domain A in Fig.2 and Fig.3(a)), which are mainly composed of glaucophane, garnet, zoisite, paragite, epidote and some minor minerals without omphacite. The host-rocks nearer to the vein are shown in domains B and D in Fig.2 and Figs.3(b), (d), which have the similar mineral compositions to the above except occurred omphacite. The vein and the immediately adjacent host-rocks are shown in domain C in Fig.2 and Fig.3(c). The vein in domain C is mainly composed of omphacite and the immediately adjacent host-rocks contain more omphacites than those in domains B and D. This variation in mineral compositions among different domains can be seen from Table 1, which lists the electronic microprobe analytical results of minerals for each domain in Fig.3. Table 2 lists the electronic microprobe analytical results across single omphacite mineral grain in the HP vein. The chemical compositions of the omphacite at different localities inside the grain are almost the same, which indicates that the crystallization of the omphacite is rapid and the fracture for vein to fill is open. This may be the result of brittle fracture of the host-rocks during the vein- forming process.

4 Discussion

Fluids in the deep subduction zones play an important role in the process of crust-mantle interaction. This has been proved by a large number of studies on the geochemistry of island arc volcanic rocks^[13-15]. Some studies indicate that the fluids released during the process of eclogization of rocks are external^{[6, 16]}, which  originated from metasedimentary rocks. The fluid flow was on large scale (km-scale). In contrast, some other studies show that the fluids were internal^{[4, 17]}, which were released by devolatilization of subducted oceanic crust and the fluid flow was on small scale (centimeter- scale). However, direct evidence that fluids in the deep paleosubduction zones came from adjacent host-rock has not been reported. HP vein is a special kind of veins, which has HP mineral assemblages and occurs in orogen. Veins are generally taken as direct products of the precipitation of fluids. The adjacent host-rocks of the studied HP vein are eclogites, which gradually turn into blueschists as they are far away from the vein. This tendency of the lithological changes shows that the vein-forming fluid was derived from adjacent host-rocks. The sharp boundaries between the vein and the host-rocks indicate the brittle behavior of the host-rocks during the vein-forming process. Therefore, it is suggested that this type of HP veins was formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies, which resulted in hydrofracture of the rocks and provided the space for the vein to precipitate. The width of the eclogite-facies host-rocks is usually 1-2 cm, which provides the evidence that the fluid flow is on centimeter-scale. Another kind of eclogitic HP veins hosted by blueschist can be found in this HP belt. This kind of HP veins was precipitated from the fluid that flowed on large-scale (km-scale) and originated from the extern. It can be seen that the western Tianshan HP metamorphic belt is a wonderful natural laboratory for study the fluids in the deep paleosubduction zones.

Fig.3 Micrographs of four domains for electronic microprobe analysis: (a) Domain A; (b) Domain B; (c) Domain C; (d) Domain D; Omp—Omphacite; Grt—Garnet; Glau—Glaucophane; Epi—Epidote; Zoi—Zoisite; Pg—Paragite

Table 1 Electronic microprobe analytical results of representative minerals in Fig.3 (mass fraction, %)

Table 2 Electronic microprobe analytical results across separate omphacite grain in HP vein (mass fraction, %)

5 Conclusions

1) The HP vein studied in this work was precipitated from the fluid originated from immediately adjacent host-rocks. The fluid was formed by dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies.

2) The host-rocks immediately adjacent to the vein are eclogites with width of 1-2 cm and turn to blueschist-facies rocks as they are farther away from the vein. The fluid flow was on centimeter-scale.

3) The boundaries between the vein and the host- rocks are sharp and the dehydration of the host-rocks results in hydrofracture of the rocks and provides the space for the vein to precipitate.

Acknowledgements

This work was supported by the Major State Basic Research Development Program of China (Grant No.2001CB409803) and the electronic microprobe analysis was conducted by the electronic microprobe laboratory in Institute of Geology and Geophysics, Chinese Academy of Sciences.

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Foundation item: Project(2001CB409803) supported by the Major State Basic Research Development Program of China

Received date: 2008-01-25; Accepted date: 2008-04-05

Corresponding author: ZHANG Jin-fu, Doctoral candidate; Tel: +86-731-8830652; E-mail: Dzhuang@mail.csu.edu.cn

(Edited by YANG Hua)