ARTICLES Chinese Science Bulletin 2005 Vol. 50 No. 13 1366—1372

Breakdown of lawsonite subsequent to peak UHP metamorphism in the Dabie terrane and its implication for fluid activity LI Xuping1,2, LI Yiliang2 & SHU Guiming3 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; 2. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 3. School of Earth and Space Sciences, Peking University, Beijing 100871, China Correspondence should be addressed to Li Xuping (email: lixuping@ cugb.edu.cn)

Abstract Abundant occurrences of quartz vein within eclogites in the Dabie-Sulu orogen provide us an opportunity to study metamorphic fluid flow during subduction and exhumation of continental crust. It is, however, usually short of petrological constraints on pressure and temperature of vein formation. This study focuses on kyanite-quartz veins within low-T/high-P eclogite in the Dabie terrane that contain unique polycrystalline aggregates, interpreted as pseudomorphs after porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in garnet from eclogite, resulting in a revised estimate of peak P-T conditions at 670℃ and 3.3 GPa for the eclogite. This indicates a stability field at graphite/diamond transition, thus upgrading the HP unit to a UHP unit. Neither foliation texture, undulose extinction of quartz nor coesite were observed in quartz veins, although the peak P-T conditions were estimated the same as that in host eclogite in light of thermodynamic calculation based on mineral assemblage in kyanite-quartz veins. Therefore, the formation of the kyanite-quartz veins as well as the breakdown of lawsonite into the kyanite-quartz-zoisite assemblage took place during exhumation subsequent to the peak UHP conditions. In this regard, the continental subduction not only brought the water of water-bearing mineral such as lawsonite into the deep mantle, but also released the water from the mineral during exhumation. Keywords: Dabie, eclogite, kyanite-quartz vein, lawsonite pseudomorph, fluid flow. DOI: 10.1360/982004-407

Subduction of oceanic crusts releases abundant fluids, resulting in arc magmatism that is well accepted worldwide, whereas the processes of continental subduction are characterized by the relative lack of fluid with very limited mobility[1]. As a result, less attention has been paid to fluid activity during continental subduction and exhumation. While fluids are well known to play key roles in geochemical recycling during subduction of oceanic crusts, 1366

it is less known whether devolatilization and fluid mobility are also active during continental subduction and exhumation. Abundant quartz veins occur within HP-UHP eclogites in the Dabie-Sulu orogen[1,2], witnessing significant fluid flow in the processes of continental subduction and exhumation in relevant to UHP metamorphism. Determination of temporal sequence for the formation of quartz vein and other metamorphic events overprinting minerals or petrologic textures will help us to understand the characteristics of fluid activity during the high-grade metamorphism. A plenty of quartz, kyanite and zoisite/clinozoisite aggregates were observed within kyanite-quartz veins of HP eclogites in the Dabie-Sulu orogenic belt, which were considered resulting from the breakdown of lawsonite during the exhumation of deeply subducted continental slab. Lawsonite is a low-T mineral, which occurs mostly in blueschist facies and could be stable under UHP metamorphic conditions. There are only two examples for lawsonite associated with low-T eclogite: A lawsonite-bearing eclogite found in the well-known low-T/ high-P metamorphic rocks from eastern Corsican along the southern extension of the Alpine chain[3], and a newly discovered tourmaline-bearing lawsonite eclogite from the Elekdag area[4]. However, several occurrences of pseudomorphs after lawsonite have been reported from Alps and Southern Urals[5―7]. Pseudomorphs after lawsonite also have been found from the Zhujiachong eclogite in the South Dabie LT/HP unit[8] and from eclogite in western Tianshan[9]. But the mineral assemblage of Ky+Zo+Qtz as pseudomorph after lawsonite in response to initial exhumation is firstly reported here. 1

Geological setting

The Dabie-Sulu orogen in east-central China was formed by the continental collision between the Yangtze and the Sino-Korean plates in the Triassic[1,2]. The Dabie terrane mainly consists of five fault-bounded metamorphic units in the western part of the orogenic belt (Fig. 1). From north to south, they can be divided further as follows: (1) the Beihuaiyang low-grade metamorphic unit, (2) the North Dabie high-T/UHP granulite-facies unit, (3) the Central Dabie medium-T/UHP eclogite-facies unit, (4) the South Dabie low-T/HP eclogite-facies unit, and (5) the Susong low-T/HP blueschist to amphibolite-facies unit. On the basis of petrological studies, Okay[10] divided the eclogite zones in the eastern part of the Dabie terrane into two units: The northern “hot” eclogite characterized by higher metamorphic temperatures and the southern coesite- and diamond-free “cold” eclogite metamorphosed at lower temperatures. The boundary between the two units is located south of the Hualiangting Reservoir (Fig. 1). Wang et al.[11] and Carswell et al.[12] also subdivided eclogites into two units with distinctly different tectono-metamorphic histories, and termed them the South Dabie HP unit and the Central Dabie UHP unit, respecChinese Science Bulletin

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Fig. 1. Simplified geological map in the southeastern part of the Dabie terrane (revised after Carswell et al., 1997). Insert indicates the sampling area at the eastern end of the Qinling-Dabie orogenic belt.

tively. Zhai et al.[13] pointed out that the country rocks of the “hot” eclogites were generally garnet-bearing granitic orthogneiss, paragneiss, marble, or ultramafics, whereas the country rocks of the “cold” eclogite were principally magnetite-bearing metasandstone and typical parametamorphic rocks. The most important evidence for such a subdivision is that so far no UHP index mineral (e.g., coesite or micro-diamond) has been identified in the cold eclogite. However, Liu et al.[14] found coesite inclusions within zircons from the gneisses in the “cold” eclogite zone when they investigated the mineral inclusions in the UHP zone of the Dabie terrane, thus suggesting that the “cold” eclogite unit might also undergo UHP metamorphism. But yet there was no UHP evidence found directly from the “cold” eclogite before this report. Samples used in this study were collected from the Huangzhen-Zhujiachong area in the South Dabie LT/HP unit, i.e. the so-called the “cold” eclogite unit (Fig. 1). The terminology, LT eclogite, was commonly used because of the low apparent temperatures by cation partitioning geothermometry for the eclogites in this area. A prominent feature of the eclogites is significant retrogression, resulting in considerable disequilibrium oxygen isotope fractionations between the eclogite minerals[15,16]. Kyanitequartz veins frequently intercalate with eclogites in this area[8,17,18], and sometimes show a intergrowth with eclogite in microscope scale. Veins are more abundant in eclogite from Huangzhen-Zhujiachong area compared to Chinese Science Bulletin

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those at other UHP terranes. Sampled eclogites were interbedded with garnet amphibolite, and both were hosted by biotite paragneiss. Reaction zone, mono-mineral zone, or alteration zone are absent between the vein and the host eclogite, suggesting a chemical equilibrium between them[6]. The occurrence of quartz veins across the eclogites in this area witnesses the channellized flow of metamorphic fluid that may be caused by structural shearing and fracturing during initial exhumation. 2

Lithologies

Lithologies are mainly eclogite and associated kyanitequartz veins. The represented samples used for this study are described in the following three groups. 2.1

Kyanite-quartz vein

The investigated samples contain both host eclogite and quartz-kyanite vein, which show a clear contact in microscopic scale. The veins, intercalated with host eclogite, are of 2 to 10 mm width in the thin sections. The quartzkyanite veins are made up of over half volume of quartz, garnets (10%―15%), zoisite/clinozoisite (5%―10%), omphacite (<5%), and kyanite (5%―10%), and other minor minerals such as rutile and titanite. They are locally characterized by the occurrence of abundant millimeter-size mineral aggregations, which appear to be pseudomorphs after earlier porphyroblastic mineral with a prismatic habit (Fig. 2(a)―(c), (e) and (f)). The pseudo1367

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Fig. 2. Micrographs of textures from quartz-kyanite vein and associated eclogite from the Huangzhen-Zhujiachong in South Dabie. Mineral aggregates of Ky+Zo+Qtz as pseudomorph after lawsonite with a prismatic habit. (a) Pseudomorph under cross-polarized light consisting of two grains of zoisite, two grains of kyanite and many fine grains quartz; (b) pseudomorph under cross-polarized light consisting of single grain kyanite and multi, fine-grain zoisite and quartz; (c) pseudomorph under plane-polarized light consisting of multi, fine-grain kyanite, zoisite and quartz; (d) Coesite/quartz radiated texture under plane-polarized light from eclogite; (e) aggregate of presumed pseudomorph under cross-polarized light; (f) symplectitic rim of kyanite and aggregate of presumed pseudomorph under cross-polarized light.

morphs usually consist of one to two grains of kyanite, fine multigrain zoisite/clinozoisite and very fine-grained quartz, which is mostly included in the kyanite with a random orientation. In the thin sections, such three minerals as kyanite, zoisite/clinozoisite and quartz occur in proximity and are assumed to be in texture equilibrium. 1368

Based on the texture and mineral assemblages, the Ky-Zo-Qtz aggregates are considered to be breakdown products after lawsonite, which usually showed a typical prismatic habit. Lawsonite pseudomorphs or Ky-Zo-Qtz aggregates frequently occur in the veins and take about 7 vol%. Garnet in the veins shows grain size of 0.5―1 mm, Chinese Science Bulletin

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ARTICLES which is smaller than that (1―3 mm) in the host eclogite. Foliation and sutured boundaries of quartz are not well developed in both host eclogite and quartz veins. No undulose extinction, kink-bands and fractures are clearly observed in both eclogite and vein. In addition, there are also no coesite and its pseudomorph observed in veins. Veins occurring separately from eclogite macroscopically are heterogeneous in texture, variable from 2 mm to 1 m on the outcrops so that they consist of single vein samples. Mineral assemblage of veins consists of granoblastic quartz grains, randomly oriented needle-like white paragonite flakes, anhedral omphacite, zoisite, kyanite, garnet and rutile. Quartzs show little undulose extinction and kink-bands, and rare fractures. 2.2

Host eclogite

The eclogite consists of anhedral omphacite, euhedral porphyroblastic garnet, zoisite/clinozoisite, paragonite and retrogressive amphibole. Accessory minerals are rutile, apatite and retrograde titanite. Coesite was observed within garnets, and recognized by the occurrence of radial fractures in host garnet around the pseudomorph (Fig. 2(d)). Relict coesite could be distinguished from quartz by its higher birefringence, consisting of multi-quartz grains under crossed nicols. Most garnets are rimmed by retrogressive corona of sodic amphibole. Kyanite was observed in contact with garnet and omphacite or within the omphacite as inclusion, and associated with coesite-bearing garnet. Paragonite occurs in both the host eclogite and vein. In the host eclogite, kyanite sometimes consists of core and is rimed by omphacite, which suggests a reaction of paragonite = kyanite + omphacite + H2O. The eclogite minerals show a remarkable variability of pervasively retrograded texture. Omphacite is usually replaced by very fine-grained symplectite consisting of retrogressive sodic amphibole and feldspar. Some samples show a stronger retrograde texture than others so that amphibole and sometimes subsequent biotite are crystallized from symplectites. 2.3

Amphibolite and gneiss

Amphibolite results retrogressively from previous eclogites, and is made up of elongated garnet, aligned hornblende, plagioclase and quartz. Accessories are muscovite, biotite, clinozoisite, and rutile. Gneiss represents the country rock of host eclogite and veins, and is composed of quartz, biotite, plagioclase, sometimes garnet, epidote group minerals, apatite and rare magnetite. The foliation is defined by oriented mica flakes and elongated plagioclase. 3

grain in the eclogite shows a zonation from core to margin (Fig. 3). Pyrope component increases (Pyp26.73―Pyp36.86), while grossular component shows a slight decrease from core to margin (Grs14.80―Grs8.28), corresponding to the process of progressive metamorphism. Garnet in the vein exhibits no zonation, but is consistent chemically with the margin of garnets in host eclogite. Most garnets in both eclogites and veins contain a pronounced retrograde rim of amphibole in chemical composition.

Mineral chemistry

The microprobe analysis was carried out systematically on minerals presented in host eclogites and veins. Garnet Chinese Science Bulletin

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Fig. 3. Garnet compositional profile in the eclogite from the Huangzhen-Zhujiachong area in South Dabie. The zonation indicates a prograde metamorphic process.

Omphacite mostly shows a strong retrogressive symplectite texture of mixing amphibole and plagioclase, and usually has homogeneous chemical composition from core to rim. The jadeite content in omphacites reaches up to about 54%. Amphibole is present in both the eclogite and vein, and has variable composition. Majority of amphiboles are from retrogressive symplectite of omphacite, and the rim of garnet, containing MgO (10.70% to 19.15%) and Na2O (3.46 wt% to 5.37 wt%). Occasionally, amphibole forms inclusion within the garnet of eclogite, pointing to its formation before peak metamorphic conditions. There is no obvious difference for the amphiboles in chemical composition between host eclogite and veins. Zoisite/clinozoisite has two generations. The earlier generation with coarse-grain size probably formed prior to the peak metamorphic conditions, while the later one with fine-grain size may partially result from the breakdown of lawsonite into the mineral assemblage of kyanite, zoisite/clinozoisite and quartz. These two generations, however, have relative homogeneous chemical composition with pistacite components 12% to 14% and Fe3+ ranging 1369

ARTICLES from 0.35 to 0.42 atoms pfu. Kyanite occurs not only in the quartz-kyanite vein, but also in omphacite of host eclogite, which indicates two generations of kyanite resulting mostly from retrograde and occasionally from prograde process, respectively. Analyzed kyanites contain up to 0.08%―0.29% Cr2O3 and 0.32% ― 0.43% FeO in composition. Plagioclase forms during retrograde process and is absent from the peak eclogite assemblage. It occurs with amphibole together in symplectites after omphacite and in fine-grained symplectites after kyanite. The chemical composition varies greatly from An11 to An36. Paragonite exists pervasively in both eclogite and quartz-kyanite vein, with Na content from 1.63 to 1.90 and K content from 0.05 to 0.13 atoms p.f.u. 4

Estimated P-T conditions

Petrological studies on texture and minerals indicate that the initial UHP, plagioclase–free mineral assemblage was garnet + omphacite + kyanite + lawsonite + clinozoisite + rutile + coesite. The well-developed prograde zonation pattern of garnet with an increase in pyrope component and decrease in grossular and spessartine components from core to rim implies that garnet core formed under the relatively low P-T condition compared to the rim. A possible reaction involved in the prograde processes is paragonite = omphacite + kyanite + H2O although kyanite could also form after plagioclase breakdown during prograde metamorphism. Paragonite replacement by omphacite + kyanite assemblage at HP conditions as the stable assemblage in the eclogites is clearly observed from petrological investigation. Nevertheless, the preservation of relict paragonite in kyanite and omphacite means that the eclogite would at least pass over the reaction paragonite = kyanite + omphacite + H2O during subduction. Since the pseudomorphs of coesite are found in the investigated samples (Fig. 2), peak P-T conditions are constrained by the occurrence of coesite and by the coexistence of unzoned omphacite in equilibrium with end-member kyanite as well as by the rim chemical composition of garnet and three net-transfer equilibria[19]. Following the THERMOCALC method of Powell and Holland with the internally-consistent database of Holland and Powell[20], and the garnet and clinopyroxene activity model of Holland and Powell[21], peak metamorphic conditions are estimated to be 670℃ and 3.3 GPa (Fig. 4), reaching up to graphite-diamond transitional boundary. As illustrated in point A of Fig. 5, it is significantly higher compared to other study[11]. Since the composition of garnet from the vein is similar to that in the rim of garnet from the host eclogite, no significant difference in peak P-T values is calculated with respect to the vein and host eclogite. Thus peak P-T conditions for vein formation are also at about 670℃ and 3.3 GPa. During isothermal decompression subsequent to 1370

Fig. 4. Estimates of peak metamorphic condition for the eclogite and vein samples from the Huangzhen-Zhujiachong area in South Dabie using THERMOCALC and calibrated reactions.

peak UHP conditions (Fig. 5-B), lawsonite is expected to decompose at about 2.5 GPa to a kyanite + clinozoisite + quartz assemblage with the release of H2O. The ideal activity model for clinozoisite is used to calculate the reaction proposed by Holland and Powell[21]. Upon decomposition, garnet and omphacite react to produce amphibole and plagioclase, releasing Na2O and CaO, which then diffuse into the domains of kyanite and quartz grains, and form plagioclase between them[23] (Fig. 2(f)). Point C in Fig. 5 denotes the P-T conditions under which garnet was overgrowth by retrograde sodic amphibole rim, omphacite broke down into amphibole and plagioclase, and kyanite was also rimmed by plagioclase between the contact of quartz and kyanite (Fig. 2(f)). The mineral assemblage of Grt-Omp-Prg-Fsp-Ky-Qtz, therefore, well represents the retrograde stage from epidote eclogite to amphibole eclogite. The average P-T conditions of point C were also calculated using the THERMOCALC method from all linearly independent reactions for coexisted real mineral compositions of garnet, omphacite, pargasite, feldspar, end-member kyanite and quartz and water, which represent the reactions with retrograded mineral assemblage. The estimated P-T values of point C are 680℃ and 1.84 GPa. Zhai et al.[15] and Castelli et al.[8] estimated the P-T conditions of 650℃ to 700℃ and 1.8 GPa for peak metamorphism of the “cold” eclogite without coesite, similar to the amphibole eclogite from this study. By means of mineral Mossbauer analysis for the purpose of correcting Fe3+ contents in omphacite and garnet, Li et al.[24] calculated a maximum temperature of 647℃ from Fe-Mg partition thermometer. This temperature is still lower than the thermodynamic equilibrium temperature of 670℃ estimated in this study on the basis Chinese Science Bulletin

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Fig. 5. P-T constraint on eclogite and kyanite-quartz vein from the Huangzhen-Zhujiachong area in South Dabie. P-T boundaries of various subdivisions of the eclogite field including blueschist, amphibolite eclogite, epidote eclogite, lawsonite eclogite and dry eclogite are from Okamoto and Maruyama[22].

of the mineral parageneses. This difference may indicate retrograde reequilibration of cation exchange between the minerals during the exhumation of deeply subducted continental slab. Since no albite was found in any investigated samples and omphacite contains about 49 to 54 jadeite end-member, retrogressive P-T path is frozen above the reaction Jd50 + Qtz = Ab (calibration based on Holland 1980[25], point C in Fig. 5). 5

Discussion

It is well known that a great deal of fluid was released during subduction of oceanic crust, resulting in arc magmatism, quartz veining and metamorphism of syn-subduction[26]. While fluids play key roles in geochemical recycling during subduction of oceanic crusts, the processes of continental subduction by contrast is characterized by the relative lack of fluid with the very limited mobolity[1,26-28]. Thus, it was not possible to release significant amounts of aqueous fluid during subduction of continental slabs to metasomatize the overlying mantle wedge to form arc Chinese Science Bulletin

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magmatism. In contrast, it has been recognized recently that the fluid flow by hydroxyl exsolution from nominally anhydrous minerals is significant during exhumation of deeply subducted continental slabs[16], resulting in pervasive amphibolite-facies retrogression, quartz veining and syn-exhumation magmatism[1,26]. Coesite pseudomorphs were found for the first time in the eclogite garnet of the Huangzheng area in the Dabie terrane, resulting in a revised peak P-T condition of “cold eclogite” from 650℃―700℃/1.8―2.5 GPa[10,11,12,14,15] to 670℃ and 3.3 GPa, thus upgrading the HP unit to an UHP unit. The eclogites at Huangzhen are commonly cut by veins usually dominated by quartz, garnet, zoisite and kyanite[8,13,18,29]. Detailed petrological and mineralogical studies indicate that quartzs show little undulose extinction and kink-bands, and rare fractures. There was also no coesite pseudomorph observed in kyanite-quartz veins. All of the observations indicate that the veins at Huangzhen formed at an early stage of exhumation. The peak P-T conditions for the formation of quartz veins are estimated 1371

ARTICLES also to be 670℃ and 3.3 GPa, significantly higher than the breakdown reaction curve of lawsonite to zoisite, kyanite and quartz/coesite (Fig. 5). Lawsonite dehydration starts from about 2.7 to 22 GPa under isothermal decompression (Fig. 5-B), suggesting that the formation of kyanite-quartz veins is also later than peak metamorphism. We, therefore, conclude that both the formation of kyanite-quartz veins and the breakdown of lawsonite into kyanite-quartz-zoisite assemblage took place during the exhumation subsequent to peak pressure. Lawsonite is a major rock-forming mineral that can store more than 10 wt% water and is stable under UHP metamorphic conditions[20―32]. The released water from the breakdown of lawsonite becomes a possible fluid source for forming quartz veins during the exhumation of deeply subducted slab. It is important to note that lawsonite is able not only to transport water into the deep mantle during continental subduction, but also to possibly release water during subsequent exhumation. Although the retrograde metamorphism caused O and H isotope disequilibria between some of the eclogite minerals, the fluid for retrograde reactions was internally buffered in geochemical compositions[1,16]. As demonstrated in this study, fluid flow by lawsonite decomposition is also very active during the exhumation of subducted continental crust, and may together with the hydroxyl exsolution[1] play an important role in geochemical recycling. Acknowledgements We thank Drs. Wei Chunjin and Liu Fulai for their critical reviews and beneficial comments. This work was supported by the National Natural Science Foundation of China (Grant No. 40372034) and the Ministry of Science and Technology of China (Grant No. 1999075503).

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