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Journal of Petrology | Volume 42 | Number 6 | Pages 1141-1170 | 2001
© Oxford University Press 2001
High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications
1TECTONICS SPECIAL RESEARCH CENTRE, SCHOOL OF APPLIED GEOLOGY, CURTIN UNIVERSITY OF TECHNOLOGY, GPO BOX U1987, PERTH, W.A. 6845, AUSTRALIA
2DEPARTMENT OF GEOLOGY, COLLEGE OF EARTH SCIENCES, CHANGCHUN UNIVERSITY OF SCIENCE AND TECHNOLOGY, CHANGCHUN, 130026, P.R. CHINA
Received May 10, 2000; Revised typescript accepted October 1, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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Both high- and medium-pressure granulites have been found as enclaves and boudins in tonalitic–trondhjemitic–granodioritic gneisses in the Hengshan Complex. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early prograde assemblage (M1) is preserved only in the high-pressure granulites and represented by quartz and rutile inclusions within the cores of garnet porphyroblasts, and omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. The peak assemblage (M2) consists of clinopyroxene + garnet + sodic plagioclase + quartz ± hornblende in the high-pressure granulites and orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the medium-pressure granulites. Peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding embayed garnet grains, and decompression-cooling (M4), represented by hornblende + plagioclase symplectites on garnet. The THERMOCALC program yielded peak (M2) P–T conditions of 13·4–15·5 kbar and 770–840°C for the high-pressure granulites and 9–11 kbar and 820–870°C for the medium-pressure granulites, based on the core compositions of garnet, matrix pyroxene and plagioclase. The P–T conditions of pyroxene + plagioclase symplectite and corona (M3) were estimated at
6·5–8·0 kbar and 750–830°C, and hornblende + plagioclase symplectite (M4) at 4·5–6·0 kbar and 680–790°C. The P–T conditions of the early prograde assemblage (M1) cannot be quantitatively estimated because of the absence of modal minerals. The combination of petrographic textures, mineral compositions, metamorphic reaction history, petrogenetic grids and thermobarometric data defines a near-isothermal decompressional clockwise P–T path for the Hengshan granulites, suggesting that the Hengshan Complex underwent initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record a major phase of collision between two continental blocks, which resulted in the final assembly of the North China Craton at 1·8 Ga. KEY WORDS: continental collision; high-pressure granulite; North China Craton; P–T path; symplectite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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The Hengshan Complex in conjunction with the adjoining Wutai and Fuping Complexes is considered to represent a classic Early Precambrian orogenic belt in the North China Craton (Li et al., 1990; Tian, 1991; Wang et al., 1996). Of particular significance is the presence of a low-grade terrane, the Wutai Complex, between two high-grade terranes—the Hengshan and Fuping Complexes (Fig. 1). Models for the evolution of these complexes range from those invoking an autochthonous relationship, with the Hengshan and Fuping Complexes developing on a common basement that underwent late Archaean rifting associated with formation of the Wutai sequence and closed upon itself in early Proterozoic time (Tian, 1991; Yuan & Zhang, 1993), to those proposing that the mountain belt is a late Archaean continent–arc–continent collision system in which the Fuping the Hengshan Complexes represent two exotic Archaean continental blocks and the Wutai Complex represents an intervening island arc (Li et al., 1990; Bai et al., 1992; Wang et al., 1996). These models were based primarily on regional lithological and structural studies, combined with limited geochemical and isotopic data; few metamorphic studies have been undertaken on the major lithotectonic units within these complexes.
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Much of the recent research in metamorphic petrology has shown that modern field- and thermodynamics-based metamorphic investigations, in combination with lithological, structural and geochronological considerations, can be directed towards understanding the tectonic setting and processes that were operative during metamorphic events (England & Thompson, 1984; Thompson & England, 1984; Sandiford & Powell, 1986; Essene, 1989; Harley, 1989; Bohlen, 1991; Brown, 1993; Vernon, 1996). Determination of P–T paths of metamorphism is of particular importance in this regard, because variations of pressure and temperature that characterize a metamorphic event are considered to be a function of the tectonic setting and of the heat-generating processes (England & Richardson, 1977; Oxburgh, 1989). Generally, clockwise, especially isothermal decompressional P–T–t paths are considered to develop in collisional environments (England & Thompson, 1984; Thompson & England, 1984; Harley, 1989; Brown, 1993), whereas anticlockwise, especially isobaric cooling P–T–t paths are related to the intrusion and underplating of mantle magma that can occur in continental magmatic arc regions (Wells, 1980; Bohlen, 1991), hotspots related to mantle plumes (Hill et al., 1992; Zhao et al., 1998, 1999b), or rift environments (Sandiford & Powell, 1986). Thus, investigations on the tectonothermal evolution of metamorphism can provide important constraints on the tectonic history of metamorphic complexes.
Mafic granulites and amphibolites form major lithologies within the Hengshan, Wutai and Fuping Complexes, and are particularly useful in determining metamorphic P–T paths because these rocks contain mineral assemblages that are well suited to estimating the pressure and temperature conditions of metamorphism and, in some cases, preserve textural evidence that can be used to infer metamorphic reaction relations and their relative timing. We have discussed the P–T path and tectonic implications of these rocks from the Wutai and Fuping Complexes (Zhao et al., 1999a, 2000). This paper reports the presence of high-pressure granulites (retrograded eclogites) in the Hengshan Complex, and presents petrological and thermobarometric data suggesting P–T evolution through eclogite facies into high-pressure granulite facies, and decompression through medium-pressure granulite facies and amphibolite facies. These P–T evolutionary processes, in combination with lithological, structural and geochronological data, place important constraints on the tectonic history of the North China Craton.
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GEOLOGICAL SETTING |
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Regional setting and lithologies
The Hengshan–Wutai–Fuping region is located in the central part of the North China Craton,
300 km SW of Beijing (Fig. 1). Three distinct tectonic complexes are recognized in the area: the high-grade gneiss complexes of the Hengshan in the NW and Fuping in the SE, separated by the low-grade Wutai Complex (Fig. 1). Both the Hengshan and Fuping Complexes consist of a variety of grey granitoid gneisses, amphibolites, mafic granulites and metasedimentary rocks at amphibolite- to granulite-facies metamorphic grade. The Wutai Complex is considered to be a typical granite–greenstone belt and to unconformably overlie the Fuping and Hengshan Complexes (Ma et al., 1987; Tian, 1991). The Hengshan Complex was originally considered as a series of supracrustal sequences, termed the ‘Sanggan Group’ or ‘Hengshan Group’. The complex is composed of four lithological units (Fig. 2): the Hengshan TTG gneisses, the Hengshan mafic granulites and amphibolites, the Zhujiafang supracrustal rocks and the Yixingzhai granitoid plutons (Tian, 1991). The Hengshan TTG gneisses form >80% of the complex and underwent high-grade metamorphism and intense polyphase deformation. Petrological and geochemical data suggest that the gneisses were derived from the partial melting of mantle-derived basaltic rocks (Tian, 1986). The Zhujiafang supracrustal rocks are dominated by amphibolite, felsic gneiss, mica schist, banded iron formation (BIF) and quartzite. These rocks are considered to be the equivalents of the Wutai greenstone in the Hengshan area (Tian, 1991). Unlike the granulite-facies TTG gneisses, the Zhujiafang supracrustal rocks only underwent lower-amphibolite-facies metamorphism. The Yixingzhai granitoid plutons are chemically similar to the Hengshan TTG gneisses, but are only metamorphosed to greenschist to lower-amphibolite facies and are less deformed, locally preserving igneous textures. The boundaries between the Zhujiafang supracrustal rocks and the Hengshan TTG gneisses and the Yixingzhai granitoid plutons are defined by ductile shear zones (Tian, 1991).
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The ages of the rock units within the Hengshan Complex as well as the timing of metamorphism have until recently been poorly constrained. Tian (1992) reported an Sm–Nd whole-rock isochron age of 2818 ± 86 Ma from mafic rock enclaves (amphibolites and medium-pressure granulites) in the Hengshan TTG gneisses and a zircon U–Pb age of 2520 ± 26 Ma from the Yixingzhai grey gneisses. The Sm–Nd age was interpreted as the age of granulite-facies metamorphism and the U–Pb age as the age of amphibolite-facies metamorphism of the Hengshan Complex (Tian, 1991, 1992; Wang et al., 1996). Recent U/Pb SHRIMP dating reveals two zircon age groups within the Hengshan Complex at 2520–2550 Ma and 1800 Ma (Wilde et al., 1998; A. Kröner, personal communication, 1999), similar to the ages of the Fuping Gneisses (Wilde et al., 1998). In combination with field data and new U–Pb data for the Wutai complex (Wilde et al., 1997), the 2520–2550 Ma ages are thought to represent protolith ages for the Hengshan Complex, which along with the Wutai and Fuping complexes, underwent a single metamorphic event at 1800 Ma (A. Kröner, personal communication, 1999).
High-pressure granulites
High-pressure granulites are generally considered to represent high-grade metabasites with a main (peak) mineral assemblage of clinopyroxene + plagioclase + garnet + quartz (Yardley, 1989). They are distinguished from eclogites by the presence of plagioclase and from medium-pressure granulites by the lack of orthopyroxene in the main assemblage, although orthopyroxene may occur in high-pressure granulites as symplectites or coronas formed during post-peak decompression. Both high-pressure and medium-pressure granulites have been found in the Hengshan Complex, in which they are restricted to enclaves, boudins and sheets, ranging from 0·1 to 2 m in width and from 0·1 to 50 m in length, within the heterogeneous, veined and deformed upper-amphibolite-facies TTG gneisses. In most cases, the high- and medium-pressure granulites crop out separately; however, in some places, the high-pressure and medium-pressure granulites can be observed within the same outcrop, but they are never in contact with each other. In the field, the high-pressure granulite enclaves or boudins can be distinguished from the medium-pressure granulites by their coarse-grained textures and the lack of brown orthopyroxenes, which are ubiquitous in the medium-pressure granulite boudins. Although there are sharp contacts between the mafic granulites and the TTG gneisses, no obvious intrusive relationships have been observed between them. The long axes of the granulite enclaves or boudins are always parallel to the regional strike of the foliations of TTG gneisses. They may have been derived from metamorphosed basalts or from metamorphosed basic intrusive rocks, including gabbro and dolerite dykes; the latter appears to be the most likely interpretation.
Recent data reveal that the high-pressure granulites from the Hengshan Complex are situated in a NE–SW-trending high-pressure granulite zone, several kilometres in width, in the central North China Craton. The zone extends from the Hengshan area, through the Huaian and Xuanhua, into the northern Hebei Province, a distance of 500 km. Along this zone, high-pressure granulites have been found in the Huaian (Zhai et al., 1992, 1995; Guo et al., 1993), Xuanhua (Wang et al., 1994) and Northern Hebei (Li et al., 1998) Complexes. A high-pressure granulite sample from the Huaian Complex yielded a garnet–clinopyroxene–orthopyroxene–whole-rock Sm–Nd isochron age of 1824 ± 18 Ma (Guo et al., 1993); the U–Pb zircon age of the same sample is 1833 ± 24 Ga (Guo et al., 1993). A high-grade pelitic gneiss sample associated with the high-pressure granulites gave the U–Pb zircon age of 1892 ± 23 Ga (Guo et al., 1993). These data are considered to represent the timing of the high-pressure metamorphic event, whereas the protolithic ages of the high-pressure granulites have not been well constrained.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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The present study focuses on the high-pressure granulites in the Hengshan Complex, with some comments on the medium-pressure granulites from the same complex. On the basis of microstructures and reaction relations between mineral phases, four mineral assemblages are recognized from the high- and medium-pressure granulites: prograde assemblage (M1), peak assemblage (M2), pyroxene + plagioclase symplectite or corona (M3) and hornblende + plagioclase symplectite (M4).
Prograde assemblage (M1)
Like most other granulite-facies terranes around the world, much of the chemical and textural information concerning the early prograde metamorphic history in the mafic granulites from the Hengshan Complex has been lost during the subsequent annealing at the peak stage. The only preserved early prograde textures are represented by quartz and rutile inclusions within the cores of garnet porphyroblasts, and omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase (An10–20) symplectic intergrowths of which the exsolution-like sodic plagioclases make up to 30–40 vol. % (Fig. 3a). Some clinopyroxene + sodic plagioclase symplectites were retrograded into fine-grained orthopyroxenes (Fig. 3b). Similar textures have also been observed in many other high-pressure granulites and retrograded eclogites, and are thought to indicate the replacement of omphacite by plagioclase and clinopyroxene formed during the transition from eclogite facies to high-pressure granulite facies (Heinrich, 1982; Rubie, 1990; Smelov & Beryozkin, 1993; Möller, 1998).
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Peak assemblage (M2)
The peak metamorphic stage (M2) preserved in the mafic granulites is represented by the growth of relatively coarse-grained pyroxene, plagioclase, quartz and garnet in the rocks. The characteristic peak mineral assemblage is clinopyroxene + plagioclase + garnet + quartz ± hornblende for the high-pressure granulites and orthopyroxene + clinopyroxene + plagioclase + garnet + quartz ± hornblende for the medium-pressure granulites. Ilmenite, apatite, rutile and magnetite are common accessory minerals in both high- and medium-pressure granulites. The minerals locally display granoblastic polygonal texture. Garnet occurs mainly as inclusion-free porphyroblasts, 1–10 mm in diameter, but is locally poikiloblastic, containing oriented inclusion tracks of rutile and elongated quartz which defines an early foliation (S1). The preferred orientation of the prismatic minerals (orthopyroxene, clinopyroxene and amphibole) in the matrix resulted from the development of the regional gneissosity (S2).
Pyroxene + plagioclase ± magnetite ± ilmenite symplectite and corona (M3)
Pyroxene + plagioclase ± magnetite ± ilmenite symplectites and symplectic coronas are widespread in both the high- and medium-pressure granulites of the Hengshan Complex. They represent the M3 metamorphic stage following peak metamorphism (M2). The symplectic texture consists of intergrowths of fine-grained, worm-like orthopyroxene + plagioclase ± magnetite or plagioclase ± ilmenite around embayed garnet grains (Fig. 3c). Locally, the orthopyroxene + plagioclase ± magnetite symplectites completely resorb garnet grains and exist as pseudomorphs after garnets (Fig. 3d). The corona textures consist of very elongate plagioclase, clinopyroxene and/or orthopyroxnene, separating garnet and quartz, where plagioclase is always present adjacent to garnet, and the clinopyroxene and/or orthopyroxene mantles quartz (Fig. 3e). In some places, garnet is only surrounded by plagioclase + ilmenite symplectites, without symplectic orthopyroxene (Fig. 3f). These symplectic or coronitic textures have been observed in many other granulite-facies terranes and are considered to indicate a decompressional process following peak granulite-facies metamorphism (e.g. Harley, 1989, 1992; Kumar & Chacko, 1991; Thost et al., 1991).
Hornblende + plagioclase symplectite and corona (M4)
In addition to pyroxene + plagioclase, hornblende + plagioclase symplectites and coronas are also present in the high- and medium-pressure granulites from the Hengshan Complex. Symplectic hornblende and plagioclase formed a worm-like intergrowth adjacent to the garnet (Fig. 4a), and coronitic hornblende and plagioclase occur as an incomplete concentric shell around garnet grains (Fig. 4b). In most cases, hornblende + plagioclase symplectites or coronas in the granulites occur around those garnet grains that lack pyroxene + plagioclase symplectites or coronas. In places, however, the two kinds of symplectic corona coexist around the same garnet grain. Recognition of this coexisting relationship in other granulites has led to the suggestion that the two kinds of symplectic corona developed at constant P and T conditions and their formation is largely dependent on grain-boundary fluid compositions around the garnet (Mengel & Rivers, 1991; Kumar & Chacko, 1994), or the mineral phases adjacent to the garnet (Thost et al., 1991). In the Hengshan mafic granulites, however, in domains where the two kinds of symplectic corona are present around a single garnet grain, the hornblende + plagioclase symplectic corona is invariably located adjacent to the garnet grain and is mantled by the pyroxene + plagioclase symplectite (Fig. 4b). Locally, the symplectic orthopyroxene and/or clinopyroxene grains were replaced by symplectic hornblende (Fig. 4c). In addition, matrix-type clinopyroxene and orthopyroxene grains were also replaced by hornblende retrograde rims (Fig. 4d). These textures suggest that the hornblende + plagioclase symplectic corona formed later than the pyroxene + plagioclase symplectic corona and represents an independent metamorphic episode (M4). The difference in P–T conditions obtained from the two types of symplectite and corona supports this observation.
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MINERAL COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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Selected minerals were analysed with a Link EDS system connected to a Jeol 6400 electron microprobe at the University of Western Australia, using Link’s software for ZAF correction and data processing. Analyses were performed with a 15 kV accelerating voltage,
5 nA beam current and counting time of 30–40 s. Natural and synthetic minerals were used as standards. A representative selection of the minerals used for P–T calculations is included in Tables 1–5.
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Garnet
Representative garnet analyses are given in Table 1. These are: (1) core and rim compositions of non-coronitic garnet surrounded by matrix-type plagioclase and quartz in the high-pressure granulites; (2) core and rim compositions of non-coronitic garnet surrounded by matrix-type plagioclase and quartz in the medium-pressure granulites; (3) rim compositions of garnet surrounded by pyroxene + plagioclase symplectites or coronas; (4) rim compositions of garnet surrounded by hornblende + plagioclase symplectites or coronas. All garnets are dominantly almandine (50–65%), with grossular (17–37%), pyrope (7–16%), and minor spessartine (1–7%) and andradite (0–5%) components. Microprobe analyses reveal three types of variations: (1) zoning patterns within individual grains; (2) variations in the chemistry of garnet cores and rims between the high- and medium-pressure granulites; (3) systematic variations between garnet rims in contact with different symplectites or coronas.
Zoning profiles of garnets surrounded by plagioclase and/or quartz in the high- and medium-pressure granulites are shown in Fig. 5. Garnet from the high-pressure granulites is characterized by a decrease in almandine and pyrope and an increase in grossular, spessartine and XFe [=Fe/(Fe + Mg)] from rim to core (Fig. 5a). The cores of large near-euhedral grains are compositionally homogeneous with relatively flat profiles (Fig. 5a), which we interpret as having developed during the peak metamorphism. The outermost rims, 0·5 mm wide, have extremely low grossular and high almandine contents (Fig. 5a), which reflects resetting by diffusion and/or net-transfer reactions during the post-peak decompression and cooling. Garnet from the medium-pressure granulites shows almandine and grossular variation trends similar to those of garnet in the high-pressure granulites, but its spessartine and pyrope contents are nearly uniform from rim to core (Fig. 5b).
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A pronounced compositional difference is present between garnets from the high-pressure and medium-pressure granulites. The cores of garnets from the high-pressure granulites have relatively higher grossular and lower almandine and pyrope contents than those of garnets from the medium-pressure granulites (Table 1). Most cores of garnets from the high-pressure granulites have >30% grossular component, with the maximum grossular content of 37·5% in sample B12 (Table 1), whereas garnets from the medium-pressure granulites contain <28% grossular component (Table 1).
There are slight variations between garnets in contact with pyroxene + plagioclase symplectites or coronas and those in contact with hornblende + plagioclase symplectites or coronas. The rim compositions of most garnet grains in contact with pyroxene + plagioclase symplectites or coronas are higher in grossular and pyrope, and lower in almandine and spessartine than those of garnet grains in contact with hornblende + plagioclase symplectites or coronas (Table 1). These compositional variations imply that the two textural garnets are likely to have re-equilibrated under different conditions.
Plagioclase
Table 2 lists representative analyses of: (1) matrix-type plagioclase in the high-pressure granulites; (2) matrix-type plagioclase in the medium-pressure granulites; (3) plagioclase from pyroxene + plagioclase symplectic coronas; (4) plagioclase from hornblende + plagioclase symplectic coronas. The principal compositional features of the different textural plagioclases include the following:
- plagioclase in the matrix of the high-pressure granulites shows a distinct compositional zoning, varying in composition from An12 to An46, with an oligoclase core and an andesine rim (Table 2 and Fig. 6a). The higher anorthite contents in matrix-type plagioclase are always present in the rims in contact with or close to garnet grains. Matrix-type plagioclase from the medium-pressure granulites shows core-to-rim compositional variation trends similar to those of matrix-type plagioclase from the high-pressure granulites, with an andesine core and a labradorite rim (Table 2 and Fig. 6b).
- Symplectic plagioclases are generally more calcic than matrix-type plagioclases and are labradorite to bytownite in composition. A symplectic plagioclase grain, occurring in association with symplectic orthopyroxenes as pseudomorph after garnet, has nearly uniform compositions of An82–86 across the whole grain (Fig. 6c). There is no pronounced compositional difference between plagioclases from pyroxene + plagioclase symplectite and those from hornblende + plagioclase symplectites (Table 2).
- A plagioclase corona, 0·25 mm across and located between a large garnet crystal and coronitic clinopyroxene, shows a compositional zoning from An33 to An52, with anorthite increasing from the contact with clinopyroxene to the garnet (Fig. 6d). Some coronitic plagioclase grains have nearly uniform compositions of labradorite.
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Clinopyroxene
Both matrix-type and coronitic clinopyroxenes are salite–augite, typical of mafic granulites, and contain a low jadeite component (Table 3). The matrix-type clinopyroxenes from the high- and medium-pressure granulites show irregular zoning patterns, especially with respect to Ca, Mg and Fe, and their Al contents slightly increase from rim to core, whereas Na contents are nearly uniform (Fig. 7a and b). A clinopyroxene corona of 0·2 mm width, between coronitic plagioclase and matrix-type quartz, is compositionally homogeneous with a relatively flat compositional profile; Ca, Al and XFe slightly increase towards the rim at coronitic plagioclase, and Mg remains almost constant (Fig. 7c).
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Orthopyroxene
Table 4 lists representative core and rim compositions of matrix-type orthopyroxene and the compositions of symplectic and coronitic orthopyroxene. Matrix-type orthopyroxenes occur only in the medium-pressure granulites. There is no significant difference in chemical composition between the matrix-type orthopyroxene and symplectic or coronitic orthopyroxene. Matrix-type orthopyroxenes show relatively flat profiles, with nearly uniform compositions in most parts of the grains. However, across the outermost rims (
0·25 mm wide), XFe and Fe contents decrease whereas Mg contents slightly increase and Al contents remain constant (Fig. 7d). An orthopyroxene corona, 0·2 mm across, separating matrix-type quartz and coronitic plagioclase surrounding a large garnet, shows a compositional profile essentially similar to that of matrix-type orthopyroxene (Fig. 7e). A worm-like symplectic orthopyroxene, 0·2 mm long, has nearly uniform XFe, Fe, Mg and Al contents (Fig. 7f).
Hornblende
Hornblende analyses were divided into three groups: (1) dark cores of matrix-type hornblendes; (2) light rims of matrix-type hornblendes; (3) symplectic hornblendes associated with symplectic plagioclases. Representative analyses are listed in Table 5. There is a marked difference in compositions between the core and rim of matrix hornblende. According to the nomenclature of Leake (1997), the core compositions of matrix hornblendes range from tschermakitic, through edenitic, to magnesio-hornblende, whereas the rim compositions range from ferro-edenitic hornblende, through ferropargasite, to magnesian hastingsite. The core of matrix-type hornblende is higher in SiO2 and MgO but lower in Al2O3 than the rim (Table 5 and Fig. 8a). Variations in core-to-rim compositions could reflect partial re-equilibration of the cores during the post-peak thermal events (i.e. M3 and/or M4). Symplectic hornblende has compositions between the core and rim composition of the matrix-type hornblende, ranging from pargasitic hornblende, through magnesio-hornblende, to magnesio-hastingsite. Small worm-like symplectic hornblende grains are unzoned towards the grain boundaries with embayed garnet grains (Fig. 8b).
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REACTION HISTORY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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In the high-pressure granulites, the Ca-rich garnet and sodic plagioclase + clinopyroxene symplectites, with up to 30–40 vol. % exsolution-like sodic plagioclase (Fig. 3a), provide evidence that eclogitic assemblages developed during an early stage of metamorphism, followed by high-pressure granulite-facies metamorphism. Sodic plagioclase + clinopyroxene symplectites can be explained by a breakdown of jadeite-rich clinopyroxene (omphacite) through the following solid–solid reactions during the transition from eclogite facies to high-pressure facies (Heinrich, 1982; Rubie, 1990; Smelov & Beryozkin, 1993; Möller, 1998):
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In the medium-pressure granulites, the actual metamorphic reactions for the formation of the peak mineral assemblage orthopyroxene + clinopyroxene + plagioclase + quartz + garnet cannot be determined because of the lack of the early prograde minerals. In a few cases, some relict hornblende inclusions have been found within garnet porphyroblasts and coarse-grained orthopyroxene and clinopyroxene grains. This suggests that the following general metamorphic reactions may have occurred during peak metamorphism:
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Reaction zones between garnet and quartz, with formation of the orthopyroxene + plagioclase ± magnetite symplectites and clinopyroxene + plagioclase ± orthopyroxene coronas in both the high- and medium-pressure granulites, suggest a combination of the following reactions:
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Plagioclase + ilmenite symplectites surrounding embayed garnet grains suggest the following reaction:
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Hornblende + plagioclase ± magnetite symplectites or coronas, although volumetrically subordinate to the orthopyroxene + plagioclase ± magnetite symplectites or coronas, are nevertheless widespread in both the high- and medium-pressure granulites of the Hengshan Complex. Their formation has been related to the following two reactions by Harley (1989) and Mengel & Rivers (1991):
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The phase relations of reaction (10) are shown in Fig. 9d. Both reactions (10) and (11) require the presence of some water, suggesting that their occurrence depends on fluid compositions as well as P–T conditions.
All of the observed reactions are typical of decompression and retrogression, suggesting a clockwise P–T history, progressing through eclogite facies, decompressing and cooling through high- and medium-granulite facies, to amphibolite facies.
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P–T EVOLUTION CONSTRAINED BY PETROGENETIC GRIDS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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The petrogenetic (or P–T) grids of metamafic rocks have been investigated using essentially two different approaches. Some workers calculate stable N2O–CaO–FeO–MgO–Al2O3–SiO2–H2O (NCFMASH) system reactions using experimentally based, internally consistent thermodynamic datasets for the mineral end-members, combined with estimated solution models (e.g. Hansen, 1981; Holland & Powell, 1998), whereas others establish reaction grids based on repeated occurrences of natural mineral assemblage sequences, combined with Schreinemakers’ rules (e.g. Mueller & Saxena, 1977). In this study, the P–T grid of Holland & Powell (1998) is used because it was constructed based on the updated thermodynamic data and solution models, and it agrees well with the sequence of mineral reactions deduced for the high- and medium-pressure mafic granulites in the Hengshan Complex.
Figure 10 is a P–T pseudosection of the NCFMAS grid of Holland & Powell (1998), but extension for low-temperature domains has been made to take into account the hornblende + plagioclase symplectite assemblage (M4) observed in the mafic granulites of the Hengshan Complex. This NCFMAS (+ quartz) grid was built up for a silica-saturated aluminous basalt bulk composition (in wt %, SiO2, 54·5; Al2O3, 19·2; CaO, 8·4; MgO, 6·0; FeO, 8·2; Na2O, 3·7) with <0·5% normative quartz. Compared with the chemical compositions of the mafic granulites from the Hengshan Complex, this model NCFMAS composition is higher in SiO2, Al2O3 and Na2O. Despite these compositional variations, the calculated P–T pseudosection of the NCFMAS grid shows a sequence of mineral assemblages similar to those observed in the Hengshan high- and medium-pressure granulites, except for the presence of kyanite, which is not present in the Hengshan mafic granulites, probably because of their lower Al2O3 contents than the model NCFMAS composition. For example, the fields garnet + omphacite + kyanite, garnet + clinopyroxene + plagioclase, garnet + clinopyroxene + orthopyroxene + plagioclase and clinopyroxene + orthopyroxene + plagioclase in the pseudosection, each of which is with quartz in excess, can be used to show the prograde assemblage (M1), the peak assemblages (M2) in the high-pressure granulites, the peak assemblages (M2) in the medium-pressure granulites, and the orthopyroxene + plagioclase + clinopyroxene corona and symplectite assemblage (M3) in the Hengshan mafic granulites, respectively (Fig. 10). As seen in Fig. 10, at the temperature range between 700 and 850°C, the equilibrium pressure conditions of the field garnet + omphacite (+ kyanite) are certainly higher than those of both high-pressure granulite field clinopyroxene + garnet + sodic plagioclase and medium-pressure granulite field orthopyroxene + clinopyroxene + garnet + plagioclase, and thus, the transition from the early prograde assemblage (M1) to the peak assemblage (M2) represents a decompressional process. Similarly, the transition from the peak assemblage (M2) to the orthopyroxene + clinopyroxene + plagioclase corona and symplectite assemblage (M3) also mainly reflects a decompressional process (Fig. 10).
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Figure 10 also shows the ‘hornblende-out’ (often corresponds to ‘orthopyroxene-in’) curve. The location of this curve was not well constrained by experimental data, but was generalized by Baker (1990) mainly on the basis of petrographic observation, geothermometry and geobarometry. The field showing the hornblende + plagioclase symplectite assemblage should be present on the left side of this curve, assuming that the location of the curve is correct, and therefore, the transition from M3 to M4 should be characterized by cooling, accompanied simultaneously with decompression. Taken together, a clockwise P–T path has been qualitatively inferred according to the sequence of different mineral assemblages in the Hengshan high- and medium-pressure granulites. The quantitative P–T trajectory cannot be defined from this P–T pseudosection because of the lack of sufficient reactions and the difference between the model and actual chemical compositions.
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QUANTITATIVE P–T ESTIMATES |
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Quantitative estimates of the early prograde metamorphic assemblage (M1) cannot be made because of the absence of modal minerals for this assemblage. The peak assemblage (M2), pyroxene + plagioclase corona and symplectite (M3) and hornblende + plagioclase symplectite (M4) have potential for quantitative P–T evaluation because of the possibility of equilibrium between core compositions of the peak minerals and local equilibrium between the re-equilibrated garnet rim and the newly formed symplectic or coronitic clinopyroxene, orthopyroxene, plagioclase and hornblende. The P–T estimates based on these mineral compositions should define a P–T path consistent with the above qualitative analysis of P–T evolution constrained by reaction history and petrogenetic grids.
Two significant factors may hinder quantitative P–T determinations. They are: (1) different-scale chemical zoning, which may be the product of growth processes and/or diffusion that make it impossible to infer with certainty the compositions that were, at any time, in mutual equilibrium; (2) lack of general agreement about the calibration of individual conventional geothermobarometers, which are often not internally consistent with respect to thermodynamic properties. To reduce the influence of the first factor, we estimate the P–T conditions of the M3 and M4 assemblages based on chemically unzoned symplectic or coronitic minerals and the compositions of garnet just inwards of the outermost diffusional rims, assuming that these compositions have been in equilibrium. The P–T conditions of the peak assemblages (M2) were estimated using the core compositions of garnet porphyroblasts and matrix-type plagioclase, clinopyroxene, orthopyroxene and hornblende.
To avoid the influence of the second factor, we calculate pressures and temperatures using P–T programs based on internally consistent thermodynamic databases, rather than traditional geothermobarometers. Of several P–T programs available, THERMOCALC (Powell & Holland, 1994; Holland & Powell, 1998) and TWQ (Berman, 1991) have been most widely used. Both programs calculate pressures and temperatures from the intersections of two or more end-member reactions in P–T space using one set of internally consistent thermodynamic data. A major difference between THERMOCALC and TWQ is that the former calculates average pressures and temperatures based on an independent set of equilibria, whereas the latter uses all possible equilibria to compute pressures and temperatures (Berman, 1991). In this study, the THERMOCALC program was used because it allows the likely uncertainties in the results of P–T calculations to be estimated, whereas the TWQ program does not allow the reliable calculation of uncertainties to be performed.
Average P–T calculations followed the method of Powell & Holland (1994), using an updated and expanded version of the internally consistent Holland & Powell (1998) thermodynamic dataset and the computer program THERMOCALC version 2·75. Mineral activities were calculated for pyroxenes following Holland & Powell (1990), using an ideal two-site mixing model; for garnet following Berman (1990), using the ternary mixing model; for plagioclase activities using Darken’s quadratic formulism (Holland & Powell, 1992); and for hornblende following Holland & Blundy (1994), using a non-ideal mixing model. Quartz was assumed to be pure.
To examine and reduce the effect of resetting of post-peak thermal events (M3 and M4), the P–T conditions of the peak assemblage (M2) are estimated for both the core and rim compositions of garnet porphyroblast and matrix-type clinopyroxene, orthopyroxene, hornblende and plagioclase from those samples that are devoid of symplectites and coronas around embayed garnet grains. The complete end-member phases used in the calculations for the high-pressure granulites include anorthite, albite, pyrope, almandine, grossular, quartz, diopside, hedenbergite, Ca-Tschermak’s pyroxene, tremolite, tschermakite, Fe-actinolite, pargasite, glaucophane and H2O, which constitute eight independent equilibria (Table 6a). Samples A60 and A03 are devoid of hornblende, and thus only three independent equilibria [reactions (1)–(3) in Table 6a] are used in the calculations. These independent reactions from eight samples gave average P–T estimates of 13·4–15·5 kbar and 770–840°C and 8·2–10·7 kbar and 638–835°C for the core and rim compositions, respectively, of the peak minerals of the high-pressure granulites (Table 7). The results overlap at relatively large uncertainty, and most of their 
fit values are over statistical limits (Powell & Holland, 1994). These may have resulted from resetting by post-peak thermal events (M3 and M4). Despite high uncertainties and large fit values, eight samples from the high-pressure granulites yielded similar results and recorded a nearly isothermal decompression from core to rim compositions, except for sample B61, which gave a lower temperature estimate (638°C), probably reflecting resetting during the M4 event.
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The end-member phases of the peak assemblage (M2) in the medium-pressure granulites are anorthite, pyrope, almandine, grossular, quartz, diopside, hedenbergite, Ca-Tschermak’s pyroxene, ferrosilite, enstatite and Mg-Tschermak’s pyroxene, which make up six independent equilibria (Table 6b). The P–T conditions estimated with these independent equilibria from seven medium-pressure granulites samples are 9–11 kbar and 820–870°C for the core compositions, and 7·0–8·0 kbar and 730–850°C for the rim compositions (Table 7). Compared with those of the high-pressure granulites, these results have relatively smaller uncertainties and
fit values, most of which are within statistical limits (Powell & Holland, 1994). As in the high-pressure granulites, the core-to-rim compositions of the peak minerals of medium-pressure granulites also recorded a near-isothermal decompression process.
To calculate the pressure and temperature of the orthopyroxene + plagioclase symplectites and clinopyroxene + orthopyroxene + plagioclase coronas (M3), mineral analyses were taken from the rims of garnet porphyroblasts and associated symplectic or coronitic plagioclase, orthopyroxene and clinopyroxene grains that were devoid of chemical zoning. The end-member phases and independent equilibria are the same as those used for the P–T estimates of the M2 assemblage in the medium-pressure granulites (Table 6b). For the orthopyroxene + plagioclase symplectite, only three independent equilibria [reactions (1)–(3) in Table 6b] could be used for their P–T estimates. The results of the average P–T calculations for 10 samples or microdomains are summarized in Table 8. These samples yielded P–T conditions of 6·5–7·5 kbar and 740–840°C (Table 9), with relatively large uncertainties and small fit values.
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Mineral analyses for the P–T estimates of the hornblende + plagioclase symplectite (M4) were also taken from the rims of garnet porphyroblasts and associated chemically unzoned hornblende and plagioclase symplectites. The end-member phases are anorthite, albite, pyrope, almandine, grossular, quartz, tremolite, tschermakite, Fe-actinolite, pargasite, glaucophane and H2O, and independent equilibria built up from these end-phases are listed in Table 6c. Nine samples or microdomains yielded similar results that fall in the range 4·0–6·0 kbar and 680–790°C (Table 9). These results overlap at smaller uncertainties and fit values are within statistical limits (Powell & Holland, 1994).
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P–T PATH |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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The combination of petrographic textures, mineral compositions, metamorphic reaction history, petrogenetic grids and thermobarometric data of the Hengshan granulites defines a near-isothermal decompressional clockwise P–T path as summarized in Fig. 11. Although the P–T conditions of the early prograde eclogite facies (M1) cannot be quantitatively determined because of the absence of the major minerals, the remaining key part of the P–T path is well constrained by quantitative THERMOCALC estimates for the M2, M3 and M4 assemblages. The establishment of this P–T path is based on the assumption that the reaction textures and mineral compositions used to determine the P–T path are related to a single metamorphic cycle rather than to two or more unrelated metamorphic events. A single-cycle granulite-facies model for the Hengshan granulites is favoured because there is no petrographic and chronological evidence suggesting the existence of a second regional metamorphic event. Furthermore, Kumar & Chacko (1994) suggested that even the minimum calculated dimensions of grain growth under regional metamorphic events of an
10–50 Ma time scale are larger than the observed dimensions of most orthopyroxene, clinopyroxene and plagioclase symplectite grains (<0·2 mm in their longest dimension). Therefore, it seems unlikely that the relatively slow prograde heating path of a second regional metamorphic event would be conducive to the formation of the fine-grained symplectite minerals found in the Hengshan granulites.
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Despite effects from resetting of Fe–Mg exchange equilibria during post-peak processes, the core compositions of the peak assemblage garnet + clinopyroxene + sodic plagioclase + quartz ± hornblende in the Hengshan high-pressure granulites preserve high-pressure granulite-facies conditions of 13·4–15·5 kbar and 770–840°C, and likewise, the core compositions of the peak assemblage of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the medium-pressure granulites preserve medium-pressure granulite-facies conditions of 9–11 kbar and 820–870°C. The obtained high-pressure conditions are supported by the presence of grossular-rich cores in garnet porphyroblasts and albite-rich cores in matrix plagioclases in high-pressure granulites. The THERMOCALC results also reveal that core-to-rim compositional variations of the peak minerals in high- and medium-pressure granulites reflect a nearly isothermal decompression process. Also, the rim compositions of the peak assemblage in the high-pressure granulites yielded P–T conditions similar to those estimated for the core compositions of the peak assemblage in the medium-pressure granulites. This suggests that the high-pressure granulites may have undergone medium-pressure granulite-facies metamorphism following the high-pressure event, and that the medium-pressure granulites in the Hengshan Complex may have resulted from retrograded high-pressure granulites.
Medium- to low-pressure granulite-facies conditions of 6·5–8·0 kbar and 750–830°C estimated for the pyroxene + plagioclase corona or symplectite assemblage (M3) suggest nearly isothermal decompression from the peak M2 assemblage (Fig. 11). Harley (1989) displayed various mineral reaction textures that could be expected in rocks that have experienced nearly isothermal decompression. He noted that orthopyroxene + plagioclase symplectites around embayed garnet grains are characteristic of mafic granulites that have undergone near-isothermal decompression. Near-isothermal decompression paths require that unroofing of deep-seated metamorphic rocks is rapid relative to the rate of thermal relaxation and cooling. This can typically be accomplished by rapid erosional exhumation or extensional faulting (England & Richardson, 1977; England & Thompson, 1984; Thompson & England, 1984; Oxburgh, 1989; Brown, 1993).
The P–T conditions of 4·5–6·0 kbar and 680–790°C estimated for hornblende + plagioclase symplectite (M4) indicate further decompression accompanied by cooling and retrogression that is represented by the presence of hydrous phase minerals in this assemblage. The M4 assemblage probably formed when the metamorphic terrane was being exhumed to a shallower level, and hence, the pressure and temperature were simultaneously decreasing.
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GEOTECTONIC IMPLICATIONS |
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The North China Craton is not well constrained in terms of its tectonic evolution. Traditionally, it has been considered to be composed of a uniform Archaean to Early Proterozoic basement, overlain by younger cover, and its tectonic history was explained using a pre-plate tectonic model (Huang, 1977). Terrane accretion and collision models have only recently been applied, including recognition of a Palaeoproterozoic orogen—the Trans-North China Orogen, which separates the craton into eastern and western blocks (Fig. 12; Zhao et al., 1999b, 1999c). The western block has a double-layered basement, with the late Archaean TTG gneisses and supracrustal rocks overlain by Early Proterozoic khondalites, which mainly crop out along the boundary with the Trans-North China Orogen. The Archaean TTG gneisses and supracrustal rocks experienced metamorphism at
2·5 Ga, with isobaric cooling (IBC) anticlockwise P–T–t paths, suggesting an origin related to the intrusion and underplating of mantle-derived magmas, whereas the Early Proterozoic khondalites underwent metamorphism at 1·8 Ga, with near-isothermal decompressional clockwise P–T–t paths, suggesting an origin related to continental collision (Zhao et al., 1999c). The eastern block is composed predominantly of Middle to Late Archaean TTG gneisses and syn-tectonic granitoids, with minor rafts or sheets of Early to Middle Archaean supracrustal rocks including ultramafic to felsic volcanic rocks and metasediments. The TTG gneisses make up >80% of the basement and the structural style is dominated by ovoid domes, separated by linear belts. All these rocks underwent regional metamorphism at 2·5 Ga, with anticlockwise P–T–t paths (Zhao et al., 1998). Intervening between the western and eastern blocks is the Trans-North China Orogen. The orogen consists of a series of low-grade and high-grade belts containing reworked Archaean components and juvenile Early Proterozoic igneous and sedimentary rocks. Geochemical and geochronological studies show that these rocks developed in magmatic arc and intra-arc basin environments (Bai et al., 1992; Wang et al., 1996), and experienced regional metamorphism at 1·9–1·8 Ga (Wilde et al., 1997, 1998; Zhao et al., 2000; A. Kröner, personal communication, 1999).
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This study reports the presence of 1·9–1·8 Ga high-pressure granulites from the Hengshan Complex in the Trans-North China Orogen. The high-pressure granulites (or retrograded eclogites) from the Hengshan Complex are petrologically and geochronologically similar to those found in the Huaian (Zhai et al., 1992, 1995; Guo et al., 1993), Xuanhua (Wang et al., 1994) and Northern Hebei (Li et al., 1998) Complexes elsewhere in the orogen. They constitute a NE–SW-trending high-pressure granulite belt, tens of kilometres wide and up to 500 km long, in the northern part of the Trans-North China Orogen. The southern part of the orogen is occupied by a high-pressure amphibolite belt along which 10–14 kbar garnet amphibolites and kyanite–staurolite–anthophyllite mafic schist occur (Wang et al., 1997; Zhao, 2001). This high-pressure amphibolite belt seems to be the tectonic counterpart of the high-pressure granulite belt discussed in this paper, and they together constitute a large-scale Early Proterozoic high-pressure belt that traverses the orogen and represents an important terrane boundary. The existence of this high-pressure belt is important because it suggests that Phanerozoic-style geodynamic processes operated far back in Early Proterozoic time in the North China Craton.
The P–T path established for the high-pressure granulites in the Hengshan Complex, along with data on the tectonothermal evolution of the other complexes in the Trans-North China Orogen, places important constraints on the tectonic setting of the orogen and in evaluating tectonic models for the evolution of the North China Craton. The estimated P–T path for the high- and medium-pressure granulites from the Hengshan Complex suggests a tectonothermal event that involved an initial phase of crustal thickening in M1, followed by nearly isothermal exhumation in M2 and M3, and cooling and retrogression during M4. This tectonothermal history was likewise shared by the Wutai and Fuping Complexes and other tectonic complexes in the Trans-North China Orogen (Zhai et al., 1992, 1995; Guo et al., 1993; Wang et al., 1994; Li et al., 1998; Zhao et al., 1999a, 2001). Thus, rather than metamorphism and deformation of the Hengshan Complex being related to localized interaction with the Fuping and Wutai Complexes, through closure of either an intracratonic rift (Tian, 1991; Yuan & Zhang, 1993) or a large ocean (Li et al., 1990; Bai et al., 1992; Wang et al., 1996), we consider the Hengshan Complex, along with the Fuping, Wutai and other complexes in the Trans-North China Orogen, to be part of a single system that was accreted to the North China Craton during the collision between the eastern and western blocks, resulting in the final assembly of the North China Craton at 1·8 Ga.
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ACKNOWLEDGEMENTS |
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We thank E. Essene and R. Powell for their helpful and critical comments on an earlier version of this paper, as well as A. Kröner, K. Y. Wang, M. G. Zhai and S. W. Liu for numerous discussions. The work was financially supported by an NSFC Grant to L.Z.L. (No. 49772144), an ARC Large Grant to S.A.W. and P.A.C. (No. A39532446), and Tectonics Special Research Centre Funds to P.A.C. This is Tectonics Special Research Centre Publication No. 74.
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FOOTNOTES |
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*Corresponding author. Present address: Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong. E-mail: gzhao{at}hkucc.hku.hk
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL COMPOSITIONSREACTION HISTORYP-T EVOLUTION CONSTRAINED BY...QUANTITATIVE P-T ESTIMATESP-T PATHGEOTECTONIC IMPLICATIONSREFERENCES |
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