New structural and petrological data unveil a very complicated ductile deformation history of the Xiongdian-Suhe HP metamorphic unit, north-western Dabie Shan, central China. The fine-grained symplectic amphibolite-facies assemblage and coronal structure enveloping eclogite-facies garnet, omphacite and phengite etc., representing strain-free decompression and retrogressive metamorphism, are considered as the main criteria to distinguish between the early-stage deformation under HP metamorphic conditions related to the continental deep subduction and collision, and the late-stage deformation under amphibolite to greenschist-facies conditions occurred in the post-eclogite exhumation processes. Two late-stages of widely developed, sequential ductile deformations D3 and D4, are recognized on the basis of penetrative fabrics and mineral aggregates in the Xiongdian-Suhe HP metamorphic unit, which shows clear, regionally, consistent overprinting relationships. D3 fabrics are best preserved in the Suhe tract of low post-D3 deformation intensity and characterized by steeply dipping layered mylonitic amphibolites associated with doubly vergent folds. They are attributed to a phase of tectonism linked to the initial exhumation of the HP rocks and involved crustal shortening with the development of upright structures and the widespread emplacement of garnet-bearing granites and felsic dikes. D4 structures are attributed to the main episode of ductile extension (D41) with a gently dipping foliation to the north and common intrafolial, recumbent folds in the Xiongdian tract, followed by normal sense top-to-the north ductile shearing (D42) along an important tectonic boundary, the so-called Majiawa-Hexiwan fault (MHF), the westward continuation of the Balifan-Mozitan-Xiaotian fault (BMXF) of the northern Dabie Shan. It is indicated that the two stages of ductile deformation observed in the Xiongdian-Suhe HP metamorphic unit, reflecting the post-eclogite compressional or extrusion wedge formation, the subhorizontal ductile extension and crustal thinning as well as the top-to-the north shearing along the high-angle ductile shear zones responsible for exhumation of the HP unit as a coherent slab, are consistent with those recognized in the Dabie-Sulu UHP and HP metamorphic belts, suggesting that they were closely associated in time and space. The Xiongdian-Suhe HP metamorphic unit thus forms part of the Triassic (250-230 Ma) collision orogenic belt, and can not connect with the South Altun-North Qaidam-North Qinling UHP metamorphic belt formed during the Early Paleozoic (500-400 Ma).
In the Xiongdian-Suhe area, north-western Dabie Shan, central China, the HP ((18-24) ×108 Pa) metamorphic rocks are widely exposed. Many publications have focused on the petrology, chronology and tectonics of the rocks from this area (Ratschbacher et al., 2003, 2000; Suo et al., 2003a, b; Bell and Chen, 2002; Sun et al., 2002; Eide and Liou, 2000; Hacker et al., 2000; Jian et al., 2000; Xu et al., 2000; Webb et al., 1999; Liu et al., 1997; Ye et al., 1993). However, the age and overall tectonic setting of the Xiongdian-Suhe HP metamorphic unit and its relationships to both the South Altun-North Qaidam-North Qinling UHP belt and Dabie-Sulu UHP and HP belts in the Central Orogenic Belt of China are still the subjects of much debate (Suo et al., 2003a; Yang J S et al., 2002; Wang and Ling, 2002; Yang et al., 2001). Moreover, except for a few workers (e.g. Ratschbacher et al., 2003, 2000; Hacker et al., 2000; Webb et al., 1999), few structural analyses and little detailed mapping of the HP rocks have been undertaken to explain the features of deformation fabrics and mineral aggregates occurred in different tectonic regimes, and p-T conditions linked with the formation and exhumation of the HP metamorphic rocks in the north-western Dabie Shan.The structural analysis of the Xiongdian-Suhe HPmetamorphic unit was performed by a study of meso- and microfabrics in the rocks following detailed field mapping at a scale of 1∶10 000 over the last two years. New structural and petrological data indicate that the HP metamorphic rocks have a complicated tectonometamorphic evolution history and deformation sequence (D1 to D5). This paper deals mainly with late-stage ductile deformations (D3 and D4) responsible for the exhumation of the HP metamorphic rocks, and clarifies the relations between the Xiongdian-Suhe HP metamorphic unit and the whole Dabie-Sulu UHP and HP metamorphic belts. We also try to determine how observed meso- and microscale structures translate into regional scale features in the north-western Dabie Shan. A speculative model of polyphase exhumation of the HP metamorphic unit as a coherent slab bounded above and below by north dipping normal and thrust ductile shear zones, respectively, is proposed.
REGIONAL SETTING
The Qinling-Dabie Mountains between the Yangtze and Sino-Korean cratons form part of the Central Orogenic Belt (COB), China, in which two UHP metamorphic belts were discriminated mainly based on a detailed structural and petrological analysis at the scale of the orogenic scale (Suo et al., 2002; Yang W R et al., 2002). The first one is the South Altun-North Qaidam-North Qinling UHP metamorphic belt, the second one is the Dabie-Sulu UHP and HP metamorphic belts (Fig. 1a). In the eastern Qinling-Tongbai-western Dabie Mountains, the two UHP metamorphic belts are separated by a series of faults, including the Shangnan-Danfeng fault system (SDFS), or ductile shear zone-bounded lithotectonic slices with different structural and metamorphic features (Fig. 1). The Dabie-Sulu UHP and HP metamorphic belts, in general, are interpreted to form due to oblique collision between the Yangtze craton and Sino-Korean craton during the Triassic (250-230 MPa) (Ratchbacher et al., 2003, 2000; Hacker et al., 2000; Eide and Liou, 2000; Webb et al., 1999; Liou et al., 1997; Okay et al., 1993), and have a very complicated tectonometamorphic evolution and deformational sequence as given in Table 1 (Suo et al., 2003b). However, research over the last two decades revealed that the geometry presently observed in the Dabie-Sulu UHP and HP metamorphic belts was mainly formed by ductile extensional processes (200-170 Ma), following the Triassic collision and initial, compressional exhumation of the UHP and HP rocks (Suo et al., 2002, 2000). The Balifan-Mozitan-Xiaotian fault (BMXF) or the Balifan-Niangniangmiao fault (BNF) in the Dabie Mountains (DBM), which is considered to be a major tectonic boundary and metamorphic break, separating the Northern Huaiyang (NHY) tectonic belt to the north from the UHP and HP metamorphic belts to the south, has been interpreted to represent the reworked Triassic suture zone. The overall tectonic framework of the UHP and HP metamorphic belts is characterized by a Cordilleran-type metamorphic core complex (Suo et al., 2000; Ames et al., 1996), in which five superposed petrotectonic units can be distinguished, from bottom to top, including the core complex unit (CC), the ultrahigh-pressure unit (UHP), the high-pressure unit (HP), the epidote-blueschist unit (EB) and the sedimentary cover unit (SC). These petrotectonic units are generally bounded by shallow-dipping, extensional ductile shear zones that are named, from base to top, the lower detachment zone, the middle detachment zone, the upper detachment zone and the top detachment zone, respectively (Fig. 1b).
Figure
1.
(a) Tectonic sketch map of the Central Orogenic Belt, China, and (b) simplified geologic and tectonic map of the eastern Qinling-Tongbai-western Dabie Mountains. 1. core complex unit (CC) and middle-high-grade metamorphic rock group; 2. Liuling Group and Fozilling Group; 3. Danfeng Group; 4. main tectonic boundaries; 5. important geological boundaries; 6. boundary of the Nanyang-Xiangfan basin (SXB); 7. lower detachment zone; 8. middle detachment zone; 9. upper detachment zone; 10. top detachment zone. UHP. ultrahigh-pressure unit; HP. high-pressure unit; EB. epidote blueschist unit; SC. sedimentary cover unit; SDFS. Shangnan-Danfeng fault system; BNF. Balifan-Niangniangmiao fault; GMF. Guishan-Meishan fault; SKC. Sino-Korean craton; YC. Yangtze craton; COB. Central Orogenic Belt; NHY. Northern Huaiyang tectonic belt; DM. Dabie massif; SL. Sulu area; SAT. South Altun; NQD. North Qaidam; TB. Tarim block; QB. Qaidam block; NQ. northern Qinling orogenic belt; SQ. southern Qinling orogenic belt; EQM. eastern Qinling Mountains; DBM. Dabie Mountains; SXB. Nanyang-Xiangfan basin; ①. South Altun-North Qaidam-North Qinling UHP metamorphic belt; ②. Dabie-Sulu UHP and HP metamorphic belts (Figure 1a is based on our observations and Yang W R et al. (2002)).
The Xiongdian-Suhe area, which is situated within the north-western Dabie Shan, about 40 km south of Luoshan, Henan, is underlain by three petrotectonic units (Fig. 2), from north to south, the Nanwan unit (Nw) which is composed of rhythmic layers of slate, phyllite, mica-quartz-schist and mica-quartzite of low-grade metamorphism, and was tentatively regarded as a lateral facies of the Foziling Group exposed in the Anhui Province, the Dingyuan unit (Dy) which consists mainly of metatuff and metabasalt has undergone intense ductile shear deformation, especially in its southern parts and of upper greenschist-facies metamorphism, and the Xiongdian-Suhe HP metamorphic unit. They are separated by the Chunqiumiao-Xinjian fault (CXF) and Majiawa-Hexiwan fault (MHF), respectively, and intruded by large volumes of 134-100 Ma undeformed granite plutons and dikes (Fig. 2). Several lines of evidence indicate that the first two units never reached to the HP metamorphic facies. Comprehensive reviews of rocks and chronological data for the Nanwan and Dingyuan units in the north-western Dabie Shan have been provided by Ye et al. (1993) and Li et al. (1998).
Figure
2.
Schematic geological map of the Xiongdian-Suhe area and the adjacent Northern Huaiyang tectonic belt, north-western Dabie Shan. HP. high-pressure unit; Dy. Dingyuan unit; Nw. Nanwan unit; ν. gabbro; γ5. Late Mesozoic granites; γπ. granitic porphyry dykes; q. quartz dykes. Two rectangles depict map areas are shown, respectively, in Figs. 3 and 4.
LATE STAGES OF DEFORMATION OF XIONGDIAN-SUHE HP METAMORPHIC UNIT
The Xiongdian-Suhe HP metamorphic unit, probably corresponding to the Huwan detachment (Webb et al., 1999) or the Huwan shear zone (Sun et al., 2002; Xu et al., 2000) is dominantly composed of white mica albite-quartz schist, white mica-quartz-albite gneiss, white mica-graphite-quartz schist, quartzite, fossiliferous marble, epidote-amphibolite, garnet-bearing white mica-albite-quartz schist and foliated garnet-bearing granite (orthogneiss), in which sheared eclogites or retrograded eclogites occur as lenses ranging from tens of centimeters to tens of meters in thickness. The detailed field mapping at the 1∶10 000 scale and structural analysis of the rocks in two selected areas, the Xiongdian and Suhe tracts (Figs. 3 and 4), indicate that deformation of the HP metamorphic unit can generally be divided into five main stages (D1-D5). Symplectic amphibolite facies assemblage and coronal texture around the HP host minerals (Fig. 5a), representing strain-free decompression and retrogressive metamorphism (Carswell, 1986), are critical among the criteria available to distinguish between the early syn-HP eclogite facies deformations (D1 and D2) and the late post-HP eclogite facies ductile deformations (D3, D4). The early deformation fabrics (e.g. S1 and S2) preserved in the internal part of eclogite bodies formed in the deep subduction and collision stage under HP eclogite facies conditions of 450-592 ℃ and (18-24) ×108 Pa (Liu et al., 1997). They were strongly overprinted by the post-eclogite or late-stage deformation under amphibolite and greenschist facies conditions, and are very rare on a regional scale. However, the fabrics are well observed at the outcrop and thin-section scales (Fig. 5b and 5c). Detailed descriptions of the early deformation fabrics have been discussed elsewhere (Suo et al., 2003b). A detailed discussion of the latest stage of deformation (D5) which is characterized by a frictional or frictional-viscous transition deformation mechanism is beyond the scope of this paper. The new set of structural and petrographical data presented here (Figs. 3 to 9) deal mainly with the late stages of ductile deformation D3 and D4 responsible for the exhumation of the HP metamorphic unit in the north-western Dabie Shan. We will return to these problems below.
Figure
3.
Geological and structural map of the Suhe tract. 1. Late Mesozoic dyke; 2. Late Mesozoic granite; 3. foliated garnet-bearing granite; 4. Nanwan unit; 5. Dingyuan unit; 6. gabbro; 7. gneiss and schist with relict eclogitic minerals; 8. eclogite and retrograded eclogite lenses; 9. serpentinite; 10. bedding; 11. early stage foliation of eclogite; 12. mylonite foliation; 13. undivided lineation; 14. undivided S3/S4composite foliation; 15. ductile shear zone.
Figure
6.
(a) A small scale north-vergent fold with a steep south-dipping axial plane, SW of Suhe; (b) highly sheared isoclinal, north-vergent F3folds, NE of Hufan.
Figure
9.
Sketch of an outcrop located on the southern boundary of the Majiawa-Hexiwan shear belt, showing asymmetrical folds and crenulation cleavage as mesoscopic-scale kinematic indicators.
D3 deformation is particularly best developed and preserved within the HP metamorphic unit in the Suhe tract (Fig. 3). There the present-day geometry of the HP metamorphic rocks is dominantly resulted from the D3 deformation event. The HP eclogites were mostly transformed into layered mylonitic am-phibolites which are essentially comprised of albite, amphibole, biotite, muscovite (white mica), quartz, epidote/zoisite, with a smaller amount of relict eclogite-facies garnet and rutile assemblage (Fig. 5d and 5e). The main structures formed during D3 are a strongly developed foliation (S3) dipping 50°-80° to the NE or SW and an associated stretching lineation (L3) plunging 40°-50° to the NW or SE. Both the foliation and lineation are defined by amphibolite, biotite, white mica and albite porphyroblast shape-preferred orientation (Fig. 5d and 5e). Composite planar fabrics in the form of C and S planes are generally found in the layered amphibolites, in which amphibole, biotite and quartz often show plastic deformation with only minor recovery, leading to the development of biotite, amphibole and quartz ribbons as a result of dislocation creep and mechanical twinning (Wenk and Christie, 1991; Simpson, 1985). On the other hand, the S3 foliation can be classified as a continuous foliation, although it locally appears as a differentiated layering visible in hand specimens, suggesting that this layering has developed by some differentiation process during the D3 stage.
Other prominent D3 structures are mesoscopic D3 folds (F3) that are, in general, close to tight with north-verging or south-verging. A small-scale fold occurs just southwest of Suhe, which has a steep south-dipping axial plane with a hinge plunging to the SE of 40°-50° (Fig. 6a). A set of highly sheared isoclinal, north-vergent or south-vergent, sheath-like F3 folds can also be observed in some places (Fig. 6b). They are commonly characterized by strongly attenuated limbs and thickened hinges with subvertical axial planes, indicating that these folds were dueto north-south-directed subhorizontal compression during the D3 deformation. Furthermore, D3 structures were associated with the emplacement of granite bodies and quartz veins containing relict eclogite and retrograded eclogite mineral assemblages, and partial melting of the crustal rocks, indicating that the late stage of D3 crustal contraction is coeval with a marked change in the thermal structure.
D4 Deformation
Deformation/metamorphism relationships indicate that D4 deformation, in more detail, can be subdivided into an early extensional deformation (D41) and a younger more localized shear deformation (D42), both occurring before D5 which is characterized by brittle deformation accompanied by the emplacement of late Mesozoic plutons.
The D41 structures are well exposed in the Xiongdian tract constructing part of the northern limb of a UHP-cored antiform (Fig. 1), where rocks including the retrograded eclogite, the country rock gneiss and schist, together with the garnet-bearing granite (Fig. 5f), have a regionally shallow north-dipping S41 composite foliation or transposed layering (Figs. 4 and 7) and an associated stretching lineation (L41). The eclogites and retrograded eclogites form lens-shaped bodies parallel to the foliation. Both the foliation and the lineation are defined by oriented growth of amphibolite facies minerals and their assemblages. In some outcrops, rootless, small-scale intrafolial, recumbent folds can be recognized, and their hinges are parallel to the L41 stretching lineation. It is important to highlight that the country rock gneiss or schist is generally interlayered with mylonitic or strongly sheared rock, indicating that the D41 deformation is inhomogeneous, and the geometry observed in the HP metamorphic unit appears a regional scale boudin-matrix or clast-matrix pattern (Figs. 4 and 7) (Suo et al., 2002; Handy, 1990). Associated kinematic indicators for the D41 deformation, including white mica fish, porphyroblast-matrix relationships and S-C geometrical relationships, supply a top-to-the-north sense of shear in the Xiongdian tract. These geometric and kinematic features are probably regarded to be due to S-N-directed subhorizontal crustal extension and subvertical shortening during D41.
In the Xiongdian tract, the D3 structures in local places of low D41 deformation intensity are only recognizable at the scale of outcrop. A tight fold, for example, with N-dipping axial planar fabrics and WNW-plunging hinge of 3°, that preserves top-to-the-S kinematic indicators, is interpreted to be related to D3 (Fig. 7). In the Suhe tract, overturned north-vergent folds, as shown in Fig. 8, made up by the foliated garnet-bearing granite represent the D41 deformation and reflect consistent with top-to-the-N extensional flow. However, in some cases, D3 and D41 structures are hard to distinguish in the field, and then termed D3/4' sometimes.
The D42 structures can be observed in the HP metamorphic unit from cartographic to microscopic scale (Figs. 3, 4 and 7), and are dominated by numerous moderately to steeply N-dipping, discrete ductile shear belts or mylonitic rock belts at 10 to > 500 m scale. All the pre-existing deformation structures were dissected by the localized shear or mylonitic rock belts in which the rocks consist mainly of albite, quartz, white mica, chlorite, actinolite, calcite and opaques, indicating that D42 essentially occurred under upper greenschist-facies conditions. A regional scale, N-dipping ductile shear belt, the Majiawa-Hexiwan shear belt of 1 km wide, fringes the Xiongdian-Suhe HP metamorphic unit to the north (Figs. 2 and 3). It consists mainly of mylonitic white mica-albite-quartz schist and epidote-amphibole schist which appear clear, penetrative mylonitic fabrics with an NW-plunging lineation at angles of 35° to 50°. Kinematic indicators, such as low-temperature S-C structure (Fig. 5g), extension crenulation cleavage C ' (Fig. 5h), quartz shape fabric, white mica fish and porphyroclast system (Passchiver and Simpson, 1986) observed in thin sections show a normal sense of motion of the hanging-wall along the shear belt. The asymmetrical, mesoscopic structures within the shear belt are also consistent with top-to-the-N-directed, non-coaxial extensional flow, and involved a significant component of sinistral shear (Fig. 9).
DISCUSSION
Majiawa-Hexiwan Shear Belt
The Majiawa-Hexiwan shear belt is exposed at the contact between the Dingyuan unit to the north and the HP metamorphic unit to the south (Figs. 2 and 3). The discovery of Brachiods, Foraminifera and Crinoidea in marbles of the HP metamorphic unit at Xiongdian gives an Ordovician formation age for the protolith, whereas the available chronological data suggest that the age of the Dingyuan unit ranges from (446±23) Ma to (444±31) Ma (Li et al., 1998; Ye at al., 1993). However, in the north-western Dabie Shan, the Dingyuan unit consisting of greenschist facies rock lies at structurally higher levels than immediately eclogite facies rocks. Consequently, a metamorphic break, with a pressure difference of c. (14-20) ×108 Pa, separates the Dingyuan unit above from the HP metamorphic unit below, implying a decoupling between the two units. Early ductile structuresand fabrics within the Majiawa-Hexiwan shear belt have been substantially modified and reconstituted by a later normal fault, namely, the Majiawa-Hexiwan fault (MHF) as illustrated in Fig. 2, which is a segment of the BNF (Fig. 1) and the westward continuation of the Balifan-Mozitan-Xiaotian fault of the eastern Dabie Mountains. This shear or fault belt of 1 000 km long in the E-W direction at the scale of the orogenic domain has been regarded as a major tectonic boundary or the reworked Triassic suture (Suo et al., 1999), and operated over the entire tectonic evolution period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for exhumation of the HP metamorphic rocks.
The contact between the Xiongdian-Suhe HP metamorphic unit and the adjacent HP metamorphic unit located in the Hong'an area is a continuous gradation relationship, and no major tectonic boundary has been observed so far. The middle detachment zone separates the HP unit above from the UHP unit below. Both the HP metamorphic unit and the Balifan-Niangniangmiao fault (BNF) through the Tongbai Mountains, farther toward the west, across the Nanxiang basin, and extend into the eastern Qinling Mountains. They, westward extension, do not appear to connect with the South Altun-North Qaidam-North Qinling UHP metamorphic belt. The latter is situated in the northern Qinling orogenic belt in the Qinling Mountains. The northern Qinling orogenic belt and the southern Qinling orogenic belt are bounded by the Shangnan-Danfeng fault system (Zhang et al., 2001). Consequently, the Xiongdian-Suhe HP metamorphic unit, together with the Hong'an HP metamorphic unit, is believed to be part of the Dabie-Sulu UHP and HP metamorphic belts (Fig. 1), and was formed under tectonic setting and conditions also similar to those in the whole Dabie-Sulu UHP and HP metamorphic belts (Hacker et al., 2000; Liou et al., 1997).
Geological Significance of Chronological Data from HP Metamorphic Unit
The age of UHP and HP metamorphism, in the whole Dabie-Sulu UHP and HP metamorphic belts, has been reliably determined at 250 to 230 Ma by several chronological methods (Hacker et al., 2000; Webb et al., 1999; Ames et al., 1996, 1993; Chavagnac and Jahn, 1996; Li et al., 1993). These age data record the Triassic collision between the Yangtze and Sino-Korean cratons (Liou et al., 1997). However, thus far, the age data from the Xiongdian-Suhe HP metamorphic unit obtained by some previous workers have yielded an unusually wide range of metamorphic ages from Triassic (Eide and Liou, 2000; Hacker et al., 2000, 1998; Webb et al., 1999; Eide et al., 1994; ) to Paleozoic (Sun et al., 2002; Jian et al., 2000). All of the ages can be roughly divided into four age groups, > 424 Ma (Jian et al., 2000), c. 377 Ma (Li et al., 1998), c. 309 Ma (Sun et al., 2002) and 232 to 205 Ma (Sun et al., 2002; Hacker et al., 2000). We believe that each age group of them would have some special geological significances. A critical question is which of them is probably interpreted to date the peak of HP metamorphism. It is obvious that geochronology plays a crucial role in our understanding of the processes of subduction and exhumation of the continental crust. However, structural and petrographical data can be used together with the age data, all the better, to unravel the evolution of the HP and UHP metamorphic belts and hence the orogenic processes (Jahn and Liu, 2002). Detailed discussion of the published age data from the study area, that appears to show some complexities and are often difficult to examine, can be found in Webb et al. (1999), Sun et al. (2002) and Jahn and Liu (2002). The structural data presented here and the age data obtained by others (Jahn and Liu, 2002; Sun et al., 2002; Hacker et al., 2000, 1998; Webb et al., 1999) suggest that the peak HP metamorphism of the Xiongdian-Suhe HP metamorphic unit took place between 250 and 230 Ma, which, broadly speaking, also is consistent with that occurred in the whole Dabie-Sulu UHP and HP metamorphic belts. In our opinion, the 424 Ma age recently obtained by the SHRIMP method (Jian et al., 2000) probably reflects the age of the protoliths of HP metamorphic rocks, or the age of an early Paleozoic tectonic and low-grade metamorphic event which developed in the Suizhou area, south of the Tongbai Mountains. Protoliths of most UHP metamorphic rocks from the Dabie-Sulu area yield middle and late Proterozoic ages of formation (Maruyama et al., 1998; Liou et al., 1997). It is indicated that the age and character of the protoliths of HP rocks from the Xiongdian-Suhe HP metamorphic unit may be quite different from that in the main Dabie-Sulu UHP and HP metamorphic belts. In addition, the idea of two HP eclogite facies metamorphic events developed in the studied area (Sun et al., 2002; Yang W R et al., 2002) is not supported by the new structural and petrological data.
Exhumation of Xiongdian-Suhe HP Metamorphic Unit
Platt (1993) indicated that there is no single mechanism that can explain the exhumation of all high-pressure terrains. The "two-way street" model (Ernst and Liou, 1995; Ernst, 1970) implies extrusional mode with reverse and normal fault boundaries, which is viable for most UHP and HP metamorphic rocks. In the case of the Xiongdian-Suhe HP metamorphic unit, a speculative tectonic evolution model of the post-eclogite polyphase exhumation of the HP rocks similar to Western Alps (Bucher et al., 2003) (Fig. 10) is proposed based mainly on the new structural and petrological data, as mentioned above. In this model, the initial or first exhumation stage, spanning about 230-200 Ma, the Xiongdian-Suhe HP metamorphic unit, together with the adjacent HP metamorphic unit as a coherent slab, bounded above and below by north-dipping normal and thrust shear zones or detachment belts, respectively, in the Dabie-Sulu subducted wedge or channel was rapidly extruded from the mantle depth to middle-lower crustal levels, accompanied by partial melting of the crustal rocks under a compressional regime and peak amphibolite facies conditions. This tectonic, syncollisional exhumation involved crustal shortening and thickening which resulted in the development of upright structures (D3) and the widespread emplacement of garnet-bearing granites (Fig. 10c). The second exhumation stage, spanning about 200-170 (140) Ma, is attributed to the main episode of crustal subhorizontal ductile extension and thinning (D41), followed by down, normal-sense shearing (D42) along the north-dipping Majiawa-Hexiwan shear belt, a segment of the Balifan-Niangniangmiao fault, under upper greenschist facies conditions (Figs. 1 and 10b). Hence, in the north-western Dabie Shan, the Triassic suture between the Yangtze and Sino-Korean cratons lies at the northern margin of the Xiongdian-Suhe HP metamorphic unit (Figs. 2 and 10a), not at the northern boundary of the Nanwan unit (Hacker et al., 2000, 1998; Xu et al., 2000) or at the southern margin of the Xiongdian-Suhe HP metamorphic unit (Sun et al., 2002).
Figure
10.
Evolutionary model for the tectonic exhumation of the Xiongdian-Suhe HP metamorphic unit and the adjacent HP unit in the north-western Dabie Shan. (a). present situation; (b). exhumation by subhorizontal extension (D41) followed by shearing along down, normal north-dipping detachment (D42) with a top-to-the-north sense of shear; (c). exhumation by extrusion within and parallel to the subduction channel or wedge (D3). UHP. ultrahigh-pressure unit; HP. high-pressure unit; Dy. Dingyuan unit; Nw. Nanwan unit; MD. middle-detachment zone; BNF. Balifan-Niangniangmiao fault.
The new data presented here and our previous regional structural studies in the whole Dabie-Sulu UHP and HP metamorphic belts lead to the following conclusions.
(1) The Xiongdian-Suhe HP metamorphic unit in the north-western Dabie Shan is part of the Triassic Dabie-Sulu UHP and HP metamorphic belts, and westward extension, does not appear to connect with the Paleozoic (500-400 Ma) South Altun-North Qaidam-North Qinling UHP metamorphic belt. The Triassic suture, in the north-western Dabie Shan, lies at the northern margin of the Xiongdian-Suhe HP metamorphic unit.
(2) The deformation history of the Xiongdian-Suhe HP metamorphic unit can be divided into five main stages (D1 to D5), among which D3 and D4 structures represent the two late-stage ductile deformations formed under the compressional and extensional regimes, respectively.
(3) The late stages of ductile deformations coeval with the proposed model of polyphase exhumation of the Xiongdian-Suhe HP metamorphic unit, together with the adjacent HP unit, which involved the extruded wedge formation (D3) bounded above and below by north-dipping normal and thrust shear zones, respectively, the subhorizontal extension (D41) and the down, normal-sense shearing (D42) with a significant sinistral shear component along the major north-dipping shear or detachment zone.
ACKNOWLEDGMENTS:
This study is supported by the National Natural Science Foundation of China (Nos. 40372094 and 49972067) and the Major State Basic Research Development Program (No. G1999075506). We are very grateful to Professors Yang Weiran, Zhang Hongfei and Ma Changqian for many stimulating discussions. We thank Dr. Zhang Zongheng for helping with the field work and Ms Shi Zhangyan for the help with drafting of figures.
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Figure 1. (a) Tectonic sketch map of the Central Orogenic Belt, China, and (b) simplified geologic and tectonic map of the eastern Qinling-Tongbai-western Dabie Mountains. 1. core complex unit (CC) and middle-high-grade metamorphic rock group; 2. Liuling Group and Fozilling Group; 3. Danfeng Group; 4. main tectonic boundaries; 5. important geological boundaries; 6. boundary of the Nanyang-Xiangfan basin (SXB); 7. lower detachment zone; 8. middle detachment zone; 9. upper detachment zone; 10. top detachment zone. UHP. ultrahigh-pressure unit; HP. high-pressure unit; EB. epidote blueschist unit; SC. sedimentary cover unit; SDFS. Shangnan-Danfeng fault system; BNF. Balifan-Niangniangmiao fault; GMF. Guishan-Meishan fault; SKC. Sino-Korean craton; YC. Yangtze craton; COB. Central Orogenic Belt; NHY. Northern Huaiyang tectonic belt; DM. Dabie massif; SL. Sulu area; SAT. South Altun; NQD. North Qaidam; TB. Tarim block; QB. Qaidam block; NQ. northern Qinling orogenic belt; SQ. southern Qinling orogenic belt; EQM. eastern Qinling Mountains; DBM. Dabie Mountains; SXB. Nanyang-Xiangfan basin; ①. South Altun-North Qaidam-North Qinling UHP metamorphic belt; ②. Dabie-Sulu UHP and HP metamorphic belts (Figure 1a is based on our observations and Yang W R et al. (2002)).
Figure 2. Schematic geological map of the Xiongdian-Suhe area and the adjacent Northern Huaiyang tectonic belt, north-western Dabie Shan. HP. high-pressure unit; Dy. Dingyuan unit; Nw. Nanwan unit; ν. gabbro; γ5. Late Mesozoic granites; γπ. granitic porphyry dykes; q. quartz dykes. Two rectangles depict map areas are shown, respectively, in Figs. 3 and 4.
Figure 3. Geological and structural map of the Suhe tract. 1. Late Mesozoic dyke; 2. Late Mesozoic granite; 3. foliated garnet-bearing granite; 4. Nanwan unit; 5. Dingyuan unit; 6. gabbro; 7. gneiss and schist with relict eclogitic minerals; 8. eclogite and retrograded eclogite lenses; 9. serpentinite; 10. bedding; 11. early stage foliation of eclogite; 12. mylonite foliation; 13. undivided lineation; 14. undivided S3/S4composite foliation; 15. ductile shear zone.
Figure 4. Simplified geological and structural map of Xiongdian tract. Stereogram shows orientation data of poles to foliation (S41) and stretching lineation (L41) (lower-hemisphere equal-area projection). Contours (per 1 % area) : 1.5 %-7.6 %-15.2 %-22.7 %.
Figure 5. Photomicrographs of deformation fabrics in XZ sections (X≥Y≥Z principal finite-strain axes) from the Xiongdian-Suhe HP metamorphic rocks. Long axis, all 5 mm. (a). foliated eclogite with fine-grained symplectitic aggregates of amphibole and plagioclase (sample s-r-113, plane polarized light); (b). massive eclogite with little retrograde alteration (sample R-S-3, plane polarized light); (c). foliated eclogite with an HP eclogite facies foliation (S2) (sample X-Y-7, cross polarized light); (d). layered mylonitic amphibolite with an amphibolite facies foliation (S3) (sample R-36, cross polarized light); (e). layered mylonitic garnet-bearing amphibolite (sample S-r-6, cross polarized light); (f). foliated garnet-bearing granite, with a foliation S41 (sample S-r-42, cross polarized light). Garnets have a diablastic texture; (g). S-C mylonite; sense of shear is top-to-the north (sample S-r-3, cross polarized light); (h). shear band (C ') developed in S-C mylonite, indicating top-to-the-N sense of shear (sample S-r-3, cross polarized light). Mineral abbreviations: Grt. garnet; Omp. omphacite; Rt. rutile; Qtz. quartz; Phe. phengite; Ab. albite; Amp. amphibole; Ep. epidote; Ms. muscovite (white mica); Bt. biotite; Kfs. K-feldspar; Zo. zoisite; Sym. symplectite.
Figure 6. (a) A small scale north-vergent fold with a steep south-dipping axial plane, SW of Suhe; (b) highly sheared isoclinal, north-vergent F3folds, NE of Hufan.
Figure 7. Vertical cross-section of the Xiongdian tract. Ec. eclogite and retrograded eclogite; Gn. gneiss and schist; Qt. quartzite; Mb. marble; FG. foliated garnet-bearing granite; Sh. ductile shear belt; γπ. granitic porphyry; S3. foliation formed during D3.
Figure 8. Asymmetrical, N-directed overturned folds formed by the foliated garnet-bearing granite, south of Huchong, Suhe.
Figure 9. Sketch of an outcrop located on the southern boundary of the Majiawa-Hexiwan shear belt, showing asymmetrical folds and crenulation cleavage as mesoscopic-scale kinematic indicators.
Figure 10. Evolutionary model for the tectonic exhumation of the Xiongdian-Suhe HP metamorphic unit and the adjacent HP unit in the north-western Dabie Shan. (a). present situation; (b). exhumation by subhorizontal extension (D41) followed by shearing along down, normal north-dipping detachment (D42) with a top-to-the-north sense of shear; (c). exhumation by extrusion within and parallel to the subduction channel or wedge (D3). UHP. ultrahigh-pressure unit; HP. high-pressure unit; Dy. Dingyuan unit; Nw. Nanwan unit; MD. middle-detachment zone; BNF. Balifan-Niangniangmiao fault.
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