| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology | Volume 44 | Number 2 | Pages 197-226 | 2003
© Oxford University Press 2003
Metamorphic and Tectonic Evolution of the Hwacheon Granulite Complex, Central Korea: Composite P–T Path Resulting from Two Distinct Crustal-Thickening Events
1GEOLOGY DIVISION, KOREA INSTITUTE OF GEOSCIENCE AND MINERAL RESOURCES, TAEJON, 305-350 SOUTH KOREA
2SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, SEOUL NATIONAL UNIVERSITY, SEOUL, 151-742 SOUTH KOREA
RECEIVED September 3, 2001; ACCEPTED July 29, 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Hwacheon granulite complex (HGC), occupying the northeastern margin of the Gyeonggi massif, consists mainly of garnetiferous leucocratic gneiss and leucogranite together with minor kyanite–garnet gneiss, aluminous gneiss, mafic granulite and garnet amphibolite. Mineral assemblages and reaction textures in various rock types of the HGC document five distinct metamorphic stages: pre- (M1) and peak (M2) granulite-facies metamorphism; lower temperature, high (M3) and low (M4) pressure upper amphibolite-facies metamorphism; and local retrogression (M5) producing andalusite-bearing assemblages. Each metamorphic stage can be integrated to give a composite P–T path consisting of two distinct trajectories, characterized by clockwise P–T loops at relatively high and low temperatures, respectively. The first P–T trajectory (M1–M3) corresponds to a Palaeoproterozoic tectonometamorphic event responsible for the formation of the granulite complex at
1·87 Ga. Rare inclusions of kyanite in M2 garnet from pelitic granulites suggest an episode of crustal thickening (M1) before M2. The peak granulite-facies metamorphism at 7·0–9·5 kbar and 790–830°C induced widespread partial melting in pelitic granulites and produced syn- to post-tectonic, (para-)autochthonous leucogranites. An episode of quasi-isobaric cooling (M3) following the M2 event is apparent from the occurrence of garnet coronas around orthopyroxene in mafic granulites and kyanite replacing sillimanite in pelitic granulites. The heat required for granulite formation is attributed to the burial of sedimentary protoliths rich in radiogenic elements during the Palaeoproterozoic crustal-thickening event. The second P–T trajectory (M4) is correlated with the final exhumation of the HGC. This decompressional process, probably initiated in the kyanite stability field, reached pressures of 3–6 kbar at 660–750°C. The clockwise P–T path may reflect the exhumation of a deep-seated crustal segment along discrete, ductile shear zones during the Permo-Triassic collisional orogeny prevalent in Far-East Asia. KEY WORDS: crustal thickening; granulite; Gyeonggi massif; Korea metamorphism
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Metamorphic rocks contain a variety of clues for understanding the ancient thermal structure and evolution of the continental crust, and the delineation of pressure–temperature histories of such rocks provides some constraints on the associated tectonic processes (e.g. Thompson & Ridley, 1987; Harley, 1989; Brown, 1993). Exposed granulite-facies terranes are commonly regarded as windows into the middle to lower continental crust (e.g. Fountain & Salisbury, 1981; Percival et al., 1992). Thus, investigations on the pressure–temperature paths of granulites help us to evaluate the deep crustal processes responsible for formation and stabilization of the continental crust.
In most granulite-facies terranes, chemical and textural evidence from the prograde stage of metamorphism is eradicated during subsequent recrystallization at peak metamorphic conditions. As a result, quantitative data are generally available only for the retrograde portion of the P–T path (e.g. Bohlen, 1987; Harley, 1989). In this respect, mineral inclusions and reaction textures, such as coronas or reaction rims, provide additional constraints for delineating the pressure–temperature–time (P–T–t) path, because they reflect textural readjustment to changes in pressure and temperature. On the basis of reaction textures supplemented by geothermobarometric data, near-isothermal decompression (ITD) or near-isobaric cooling (IBC) paths have been identified as two end-member situations for the thermo-tectonic evolution of granulite-facies terranes (e.g. Harley, 1989). These P–T–t paths are often assumed to be the product of single metamorphic episodes when inferring the tectonic setting of granulite formation. However, because dry rocks such as granulites are likely to undergo significant recrystallization only during deformation and/or fluid influx, mineral assemblages and reaction textures in such rocks may represent segments of P–T paths that are unrelated in time (e.g. Harley, 1992). As a consequence, these segments cannot be used to construct a realistic P–T–t path, unless reliable ages are available (Hensen et al., 1995; Vernon, 1996). Such composite P–T paths could be common for Precambrian granulite-facies terranes, especially in the context of the timing and nature of granulite exhumation with respect to granulite formation (Percival et al., 1992). Exhumation of granulite-facies terranes may occur through their involvement in a separate tectonic event sometime after their high-T formation (Ellis, 1987; Harley, 1989). Thus, reaction textures in granulites may not reflect a single metamorphic episode recording a single P–T–t path (see Hollister & Crawford, 1986).
The Hwacheon granulite complex (HGC), a coherent region of granulites, has been identified in the northeastern margin of the Gyeonggi massif, South Korea (Figs 1 and 2; Lee et al., 1997, 2000). This complex formed at 1·87 Ga (Lee et al., 2000), and represents the middle to lower crust of the Gyeonggi massif. The HGC preserves various mineral parageneses and reaction textures that allow the construction of a reliable P–T path. Accordingly, the Hwacheon granulites provide a good opportunity for improving our understanding of deep crustal processes with regard to granulite formation and its reactivation in central Korea. In this study, we have investigated the P–T evolution of the HGC, on the basis of mineral parageneses deduced from various reaction textures as well as geothermobarometric data on both mafic and pelitic granulites. These results are combined with available geochronologic data to unravel the timing and nature of granulite formation and subsequent reactivation. Finally, the elucidation of a P–T–t path for the HGC provides some insight into the geodynamic processes that governed the tectonometamorphism of basement rocks in not only the Korean Peninsula but also Far-East Asia as a whole.
|
|
|
GENERAL GEOLOGY |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Gyeonggi massif, situated between the Nangrim and Yeongnam massifs of the Korean Peninsula (Fig. 1), is a Precambrian terrane consisting primarily of Archaean to Palaeoproterozoic crystalline basement and Palaeo- to Mesoproterozoic supracrustal rocks (Fig. 1; Lee, 1987). The Gyeonggi massif has been regarded as an eastern promontory of the South China (or Yangtze) craton, and the Nangrim and Yeongnam massifs as parts of the Sino-Korean craton (Cluzel et al., 1991; Yin & Nie, 1993; Li, 1994; Ree et al., 1996; Chough et al., 2000; Kim et al., 2000; Lee et al., 2000). Hence, both northern and southern margins of the Gyeonggi massif are considered as tectonic boundaries. However, the geodynamic processes responsible for the amalgamation of Precambrian massifs in Korea are poorly understood.
We have previously reported the occurrence of a coherent granulite terrane in the Hwacheon area, which forms part of the northeastern Gyeonggi massif (Fig. 1; Lee et al., 1997, 2000). On the basis of lithology and field relationships, two lithotectonic units are distinguished: the Hwacheon granulite complex (HGC) and the marginal zone gneiss complex (MZGC) (Fig. 2). The HGC occurs as a tectonically exhumed block, separated from the MZGC by thrust faults and ductile shear zones. The southward-directed thrust emplacement of the HGC over the MZGC produced intense folding in the MZGC, and was followed by extensional shearing focused along the boundary between the HGC and the MZGC. The latter is interpreted to result from gravitational collapse of the thickened crust (Lee et al., 2000). It should be noted that the majority of structural fabrics observed are not associated with the formation of the granulites but rather with their exhumation.
The HGC is a composite migmatite complex that has experienced granulite-facies metamorphism and partial melting of pelitic and psammopelitic rocks. Thus, this complex belongs to a metatexite–diatexite terrane, following the definitions of Brown (1973). The HGC consists primarily of garnetiferous leucocratic gneiss and leucogranite together with subordinate kyanite–garnet gneiss, aluminous gneiss, mafic granulite, and garnet amphibolite. Pelitic granulites include leucocratic gneiss as well as kyanite–garnet and aluminous gneisses. The leucogranites or the leucosomes in migmatitic granulites contain grains or clusters of garnet, and are probably crystallized from in situ or para-autochthonous partial melts. They commonly occur as layers conformable with the foliation or as irregular patches filling tension gashes (Fig. 3a and b), together with garnet-rich rafts or layers in metatexitic regions. Metatexitic migmatite is locally transformed into diatexite containing rare patches of garnet-rich residue (Fig. 3c). The melt-enriched diatexite is considered to be the source of para-autochthonous granite occurring as large massive bodies in the HGC and MZGC (Figs 2 and 3d). Blocks or layers of kyanite–garnet gneiss are commonly enclosed by the diatexite and closely associated with leucosomes (Fig. 3e). Aluminous gneiss is equivalent in lithology to the garnet-rich residue observed in the metatexite, but occurs as a mappable body (Fig. 3f). This gneiss is considered to be the product of extensive partial melting, and to be complementary to the diatexite. The igneous protoliths of the mafic granulites were apparently emplaced before the granulite-facies metamorphism, although their temporal relationships are often ambiguous as a result of structural disturbance during partial melting. The occurrence of mafic granulite is restricted to the HGC, whereas garnet amphibolites are present in both the HGC and the MZGC. Widespread partial melting has apparently occurred after the major deformation, because the diatexites show no penetrative fabric.
|
The MZGC represents a northern extension of the Gyeonggi metamorphic complex (GMC) in the central Gyeonggi massif, and consists of banded biotite gneiss, quartzofeldspathic gneiss and migmatitic gneiss together with minor amphibolite (Lee, 1987; Lee & Cho, 1995). Because of its lithologic similarity, the migmatitic gneiss of the MZGC is tentatively interpreted as the lower-grade or retrogressive equivalent of leucocratic gneiss in the HGC. A conventional U–Pb zircon age of 2164 ± 18 Ma was reported from banded gneiss in the MZGC (Kim et al., 1999). On the other hand, the protoliths of garnet amphibolites that underwent near-isothermal decompression along a clockwise P–T–t path were emplaced during Neoproterozoic (852 ± 48 Ma) time (Lee & Cho, 1995). Thus, the maximum age for regional metamorphism in the MZGC should be younger than 850 Ma.
|
PETROGRAPHY |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Mineral parageneses in representative leucocratic and aluminous gneisses, kyanite–garnet gneiss, mafic granulites and amphibolites are summarized in Table 1. Although the granulite-facies assemblages are dominant in all lithologic units, each unit partially preserves textural evidence for mineral growth during pre- and post-peak metamorphic stages. Mineral parageneses recorded in various rock types suggest that the HGC has experienced five metamorphic stages, M1 to M5 (Table 1). Among them, M2 defines the peak granulite-facies metamorphism, whereas M1 minerals are present only as inclusions in M2 porphyroblasts. M3 and M4 represent high- and low-P upper amphibolite-facies stages, respectively. M5 represents a local thermal overprint associated with intrusion of Jurassic plutons.
|
In particular, three generations of garnet are distinguished during polymetamorphic evolution of the HGC. The majority of garnet grains are formed during M2 peak metamorphism, but texturally later garnet growths are characteristic for the M3 and M4 stages in mafic and pelitic granulites, respectively (Table 1). Moreover, M3 garnet grains in mafic granulites are thought to be a retrograde product, whereas texturally late garnet growths in pelitic granulites are attributed to an even later, prograde M4 event (see below for further discussion).
Leucocratic gneiss and leucogranite
The leucocratic gneisses are commonly layered, and occur as isolated blocks or rafts in the diatexitic region dominated by garnet-bearing leucogranite. Mafic layers of the leucocratic gneiss are generally rich in residual garnet and biotite, and are considered to represent a restitic portion, from which felsic melts have been segregated or extracted. The leucogranites contain lesser amounts of biotite than the leucocratic gneisses. Primary mineral assemblages of leucocratic gneisses and leucogranites are represented by garnet–sillimanite–biotite–K-feldspar–plagioclase–quartz. Primary M2 garnet occurs as sub- to euhedral porphyroblasts ranging in diameter up to 10 cm, which commonly enclose biotite, sillimanite, plagioclase, quartz and rare hercynite (Fig. 4a). Sillimanite is the dominant Al-silicate in the leucocratic gneiss and occurs as acicular crystals subparallel to the foliation primarily defined by the preferred orientation of biotite. K-feldspar forms medium-sized to large perthitic poikiloblasts, commonly enclosing biotite, sillimanite and quartz.
|
M4 minerals in leucocratic gneisses and leucogranites include cordierite, garnet and hercynite, which commonly coexist with residual biotite (Fig. 4b and c). Cordierite occurs dominantly as a xenoblastic phase that mantles the M2 garnet porphyroblasts (Fig. 4b), and uncommonly as isolated patches replacing residual biotite. M4 garnet and hercynite occur in close association with cordierite (Fig. 4c). In leucocratic gneisses (samples HC19-1B and HC85D), rare idioblasts of kyanite are observed in the matrix together with rutile and ilmenite. These M3 minerals are often mantled by cordierite.
Kyanite–garnet gneiss
The kyanite–garnet gneiss occurs as discrete layers and isolated blocks in the diatexitic region. It consists primarily of garnet, biotite and kyanite, and contains variable amounts of sillimanite, plagioclase, quartz, cordierite and rare staurolite and hercynite. K-feldspar is absent in all the examined kyanite–garnet gneisses. These gneisses commonly preserve textural evidence for mineral growth at various metamorphic stages. M1 kyanite, together with sillimanite, biotite and rare staurolite, occur as inclusions within M2 garnet porphyroblasts (Fig. 4d). On the other hand, texturally late M3 kyanite occurs together with prismatic staurolite and rutile, replacing M2 garnet porphyroblasts (Fig. 4e). Hercynite, pseudomorphic after staurolite, often coexists with M3 kyanite and rutile in the matrix. In contrast to the leucocratic gneiss, kyanite is the dominant Al-silicate in the kyanite–garnet gneiss. Both M2 and M3 minerals are commonly mantled by M4 cordierite (Fig. 4e), and are also locally replaced by M5 andalusite in some kyanite–garnet gneisses.
Aluminous gneiss
The mineral assemblages of the aluminous gneisses are equivalent to those of garnet-enriched layers of the leucocratic gneiss, except for the rare occurrence of biotite. Both lithologies probably represent the residua after the extraction of significant amount of granitic melt, and could be complementary in composition to the widespread stock-like bodies of leucogranite. Kyanite is ubiquitous in aluminous gneisses, occurring not only as inclusions within M2 garnet porphyroblasts but also as texturally later M3 grains in the matrix. The latter commonly replace prismatic sillimanite, and, together with rutile, uncommonly overgrow biotite aggregates around resorbed M2 garnet (Fig. 4f).
Mafic granulite
The majority of mafic granulites occur as boudinaged layers or isolated blocks within leucocratic gneisses and leucogranites. Mafic granulites also occur as remnants within amphibolites, and these metabasites apparently show the intrusion relationship with the host gneisses. M2 mineral assemblages of the mafic granulite comprise orthopyroxene, plagioclase and quartz with or without clinopyroxene, garnet, hornblende and biotite. Garnet and clinopyroxene are exclusive to each other with rare exceptions. Primary M2 hornblende is commonly present as discrete subhedral grains or minute inclusions within clinopyroxene of two-pyroxene granulites (Fig. 5a). Mafic granulites commonly show partial to complete retrogression to garnet amphibolites, where orthopyroxene and clinopyroxene are replaced by M3 aggregates of cummingtonite and hornblende, respectively. Secondary M3 garnet occurs as fine idioblastic crystals rimming pyroxene and plagioclase (Fig. 5b), but apparently does not mantle primary M2 garnet. The retrogression probably occurred under static conditions, because primary textures are well preserved even in significantly retrogressed mafic granulites.
|
Together with typical mafic granulites described above, there is an orthopyroxene-free mafic gneiss (sample YK5H3), consisting of garnet, clinopyroxene, plagioclase and quartz. Primary M2 garnet shows the overgrowth of secondary M3 garnet at the outer rim, and clinopyroxene is replaced by green hornblende (Fig. 5c).
Amphibolites
Two types of amphibolites are distinguished on the basis of field occurrences and mineral parageneses. Type 1 amphibolites occur as sills or dykes that intrude leucocratic gneiss, and are commonly boudinaged. These amphibolites are pervasively intruded by pegmatitic veinlets stemming from the leucogranite, and are also rarely intruded by garnet-bearing leucogranite. Mineral assemblages consist primarily of hornblende and plagioclase, together with lesser amounts of garnet, cummingtonite and quartz. These assemblages are consistent with the M3 assemblage of the mafic granulites. Some amphibolite bodies preserve remnants of mafic granulites, suggesting that type 1 amphibolites are the product of complete retrogression of mafic granulites.
Type 2 amphibolites occur as massive bodies, ranging up to a few tens of metres in width, and are not intruded by leucogranites or pegmatitic veins. These amphibolites consist primarily of garnet, hornblende and plagioclase together with minor quartz. In particular, they do not contain cummingtonite pseudomorphs after orthopyroxene. Thus, the protoliths of these massive amphibolites, similar in appearance to the MZGC amphibolite (Lee & Cho, 1995), are likely to have been emplaced after the granulite-facies metamorphism. Symplectites consisting of hornblende and plagioclase commonly mantle garnet porphyroblasts in both types of garnet amphibolite (Fig. 5d).
|
MINERAL CHEMISTRY |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Minerals were analyzed using a JEOL 733 JXA electron microprobe at Seoul National University, with an operating voltage of 15 kV and a beam current of 10 nA. Beam diameter was typically 5 µm, but a wider beam of 20 µm was used for analyzing pyroxene and K-feldspar showing exsolution textures. Natural and synthetic oxides as well as silicate minerals were used as standards. Data acquisition and reduction were performed using an automated ZAF correction program. Representative analyses of minerals are given in Tables 2–8.
|
|
|
|
|
|
|
Garnet
M2 garnet porphyroblasts from pelitic granulites are pyrope-rich almandines, containing minor amounts of grossular and spessartine components (Table 2; Fig. 6). Although original lithologies and bulk chemistries are significantly affected by granulite-facies metamorphism and associated partial melting, garnet compositions are rather uniform for all types of pelitic granulites: 2–6 mol % for grossular, and 1–4 mol % for spessartine components, respectively. The Fe/(Fe + Mg) value of the garnet core ranges from 0·61 to 0·75, and is generally high in relatively small grains. Most M2 garnet grains from pelitic granulites show a diffusional zoning pattern, characterized by increasing Fe/(Fe + Mg) and decreasing Mg towards the rim, and a slight enrichment of Mn at the outermost rim (Fig. 7a). This pattern is prominent especially in garnet grains that are corroded or mantled by other Fe–Mg minerals such as biotite and cordierite. In cordierite-bearing leucocratic gneisses, M4 garnet is similar in grossular (3–8 mol %) and spessartine (1–2 mol %) contents to M2 garnet, but significantly higher in the almandine content (74–79 mol %) (Fig. 6). In addition, the Fe/(Fe + Mg) value ranges from 0·74 to 0·82, and is generally similar to that of the rim of M2 garnet.
|
|
In mafic granulites, M2 garnet is more enriched in the grossular component than that in the pelitic granulites (Fig. 6). The almandine contents (66–71 mol %) are rather constant, but the grossular contents (6–18 mol %), generally complementary to pyrope contents, are variable. The Fe/(Fe + Mg) value ranges from 0·75 to 0·79, and compositional zoning is not prominent, in contrast to M2 garnet from the pelitic granulites. M3 garnet mainly occurs as coronas around pyroxene, and its grossular content ranges from 11 to 27 mol %. The compositional variations of M2 and M3 garnets are shown in Fig. 7b. The Fe/(Fe + Mg) value is more or less constant throughout the whole grain of garnet, but the grossular content increases abruptly in the M3 garnet.
Biotite
Biotite is one of the major constituents in pelitic granulites, and has apparently formed at various metamorphic stages. Biotite commonly coexists with Ti-oxide phases such as ilmenite and rutile. Primary M2 biotite, mainly occurring as the matrix phase, is generally higher in Ti content (0·32–0·55 atoms per formula unit, a.p.f.u.) than secondary M3 biotite (0·14–0·28 a.p.f.u.). Primary biotite is well preserved, especially in the cordierite-bearing leucocratic gneiss, and its Fe/(Fe + Mg) value varies from 0·25 to 0·55. In the cordierite-absent leucocratic gneiss, however, the composition of the Ti-rich, primary biotite is enriched in Mg. Biotite, occurring as inclusions in M2 phases such as garnet and K-feldspar, shows a similar composition to the Ti-rich, magnesian biotite (Table 3). The primary biotite is generally absent in kyanite–garnet and aluminous gneisses. The Cl and F contents of primary biotite are minor (<0·02 a.p.f.u.), except for one sample, HY7, where the Cl content reaches 0·29–0·37 a.p.f.u. (Table 3).
Secondary biotite occurs mainly as a part of the late-stage assemblages that replaces M2 minerals, and shows higher Fe/(Fe + Mg) and lower Ti content than primary biotite (Table 3). This biotite is common mainly in kyanite–garnet and aluminous gneisses, and rare in cordierite-absent leucocratic gneiss.
Primary M2 biotite is rare in mafic granulites, but often coexists with M2 hornblende in two-pyroxene granulites. However, post-M2 biotite is ubiquitous in the retrogressed mafic granulite. The Fe/(Fe + Mg) value of M2 biotite (0·40–0·41) is generally lower than that of post-M2 biotite (0·41–0·47). The F content (0·27–0·49 a.p.f.u.) of biotite from mafic granulites appears to be higher than that from pelitic granulites (Table 3).
Pyroxenes
Pyroxenes are major constituents of the mafic granulite, but are absent in the pelitic granulite. Most orthopyroxene grains are ferro-hypersthene, and less commonly hypersthene with Fe/(Fe + Mg) values of 0·48–0·67. The Al contents of orthopyroxene vary from 0·02 to 0·15 a.p.f.u. in clinopyroxene-bearing granulite, and from 0·04 to 0·13 in garnet-bearing granulites (Table 4). Some grains are zoned towards low Al at the rim, and the difference in Al contents between core and rim reaches 0·015 a.p.f.u. (Table 4). The variation in Fe and Mg contents is negligible, but Fe/(Fe + Mg) generally decreases towards the rim.
The clinopyroxene is augite with Fe/(Fe + Mg) values ranging from 0·40 to 0·66. The Al content varies from 0·06 to 0·08 a.p.f.u. in two-pyroxene granulites, but is <0·04 a.p.f.u. for clinopyroxene coexisting with garnet (Table 4). In the latter case, clinopyroxene is slightly zoned in Al with core-to-rim variations of 0·01 a.p.f.u.
Amphibole
Primary M2 amphiboles in two-pyroxene granulites are mostly magnesio-hornblende, and rarely edenitic and ferro-tschermakitic hornblende (Leake, 1978). The Si content ranges from 6·26 to 7·17 a.p.f.u. and (Na + K)A from 0·11 to 0·53 a.p.f.u. The Fe/(Fe + Mg) value varies between 0·30 and 0·65, and the Ti content ranges from 0·07 to 0·14 a.p.f.u. (Table 5). The Cl content (0·14–0·31 a.p.f.u.) of M2 hornblende is higher than the F content (<0·02 a.p.f.u.).
M3 amphiboles from the mafic granulite can be divided into calcic and iron–magnesium–manganese groups, respectively. The majority of calcic amphiboles are magnesio-hornblende, with Si of 6·59–6·71 a.p.f.u. and (Na + K)A of 0·12 to 0·18. The Fe/(Fe + Mg) value ranges from 0·32 to 0·35 and the Ti content from 0·05 to 0·08 a.p.f.u. In the garnet–clinopyroxene-bearing mafic granulites, however, the calcic amphiboles are higher in Fe/(Fe + Mg) (0·72–0·85) and (Na + K)A (0·69–0·80 a.p.f.u.). These amphiboles are ferro-pargasite with Si contents of 6·13–6·54 a.p.f.u. All the M3 calcic amphiboles are lower in Ti content than M2 amphiboles. Iron–magnesium–manganese amphiboles are cummingtonite with Si of 7·65–7·91 a.p.f.u. and Fe/(Fe + Mg) value of 0·43–0·47 (Table 5). The F content of cummingtonite (<0·11 a.p.f.u.) appears to be higher than that of M2 hornblende.
In the amphibolites, all the analyzed amphiboles are tschermakitic and magnesio-hornblende with Si of 6·33–6·69 a.p.f.u. and (Na + K)A of 0·20–0·39 a.p.f.u. The Fe/(Fe + Mg) value varies between 0·37 and 0·48, and the Ti content between 0·04 and 0·14 a.p.f.u.
Cordierite
The Fe/(Fe + Mg) value of cordierite ranges from 0·29 to 0·34 in leucocratic gneisses, and from 0·26 to 0·38 in kyanite–garnet gneisses. The channel cations consisting of Na and K in cordierite are generally higher in the leucocratic gneiss (0·02–0·09 a.p.f.u.) than in the kyanite–garnet gneiss (0·01–0·03 a.p.f.u.) (Table 6). Some cordierites are zoned with decreasing Fe/(Fe + Mg) toward rims, and the difference between core and rim reaches 0·09.
Feldspars
The compositional ranges of plagioclase are distinctly different between pelitic (An30–50) and mafic (An50–90) rocks, as can be inferred from the difference in their bulk chemistries (Table 7). In pelitic granulites, most plagioclase grains are andesine, and their orthoclase contents are low (0–3 mol %). The orthoclase component in some plagioclase grains exsolved from K-feldspar hosts in aluminous gneiss ranges up to 17 mol %. Plagioclase commonly shows asymmetric reverse zoning with increasing anorthite content toward the rim, particularly when it is in contact with garnet porphyroblasts.
In two-pyroxene mafic granulites, most plagioclase grains are anorthite-rich (An76–92). Plagioclase compositions in the garnet-bearing mafic granulite vary significantly even within individual specimens, and are An46–58 and An68–79, suggesting a compositional gap between An58 and An68. Reverse zoning of plagioclase is common in mafic granulites, but often shows irregular patterns.
In garnet-bearing amphibolites, plagioclase is andesine (An38–48). Most grains of plagioclase show asymmetric normal zoning, in contrast to mafic granulites.
Except for kyanite–garnet gneisses, perthitic K-feldspar is ubiquitous in all the pelitic granulites. Its orthoclase content ranges from 55 to 87 mol %, and the anorthite content from 0 to 5 mol % (Table 7). K-feldspar is absent in most mafic granulites, but rarely observed as discontinuous or patchy lamellae within the exsolved antiperthite.
Spinel
The spinel in all pelitic granulites belongs to the gahnite–hercynite–spinel solid solution, and its Fe/(Fe + Mg) value varies widely (0·60–0·90; Table 8; Fig. 8). The chromite content is generally low in most spinels, but rarely reaches 0·12 mol % in some cordierite-bearing leucocratic gneisses. Low values of Fe/(Fe + Mg) are recorded in spinel occurring as inclusions in M2 garnet, and vary from 0·60 to 0·76. In these spinel inclusions, the Zn content ranges from 0·06 to 0·08 a.p.f.u. On the other hand, M3 spinel grains in the kyanite–garnet gneiss range in Fe/(Fe + Mg) from 0·85 to 0·90, and in the Zn content from 0·14 to 0·18 a.p.f.u. In the cordierite-bearing leucocratic gneisses, Fe/(Fe + Mg) of M4 spinel varies from 0·71 to 0·85, and the Zn content is 0·06–0·29 a.p.f.u.
|
Staurolite
Staurolite occurs as either inclusions within M2 garnet or retrograde grains replacing this garnet. Both types of staurolite are similar in composition. The Fe/(Fe + Mg) value ranges from 0·70 to 0·72, and the Zn content from 0·22 to 0·37 a.p.f.u. (Table 8; Fig. 8).
Fe–Ti oxide
Ilmenite is the dominant Fe–Ti oxide in both pelitic and mafic granulites, and has <4 mol % hematite component (Table 8). Rutile occurs as a discrete grain or intergrows with ilmenite especially in kyanite-bearing rocks. Magnetite and hematite are absent in all types of granulites.
|
THE FIVE METAMORPHIC STAGES AND THEIR PHYSICAL CONDITIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Five metamorphic stages (M1–M5) were deduced on the basis of inclusion-mineral relationships, reaction textures, and the P–T stability of minerals in the different lithologies (Table 1). In this section we describe various reactions responsible for producing mineral assemblages at each metamorphic stage, and P–T conditions estimated from various geothermobarometers. These results are used to construct a composite P–T path, consisting of two distinct clockwise P–T trajectories whose temporal relationships are given in the next section.
M1 metamorphism
Prograde metamorphism occurred at medium to high pressures, as inferred from the inclusions of kyanite and rare staurolite in M2 garnet of kyanite–garnet and aluminous gneisses. These inclusions are absent in the leucocratic gneiss, where zincian hercynite occurs as inclusions within M2 garnet. This observation may indicate the former presence of staurolite, because hercynite is commonly produced by the breakdown of staurolite in high-grade rocks (Stoddard, 1979; Cesare, 1994). Although quantitative P–T conditions could not be obtained because of the lack of appropriate mineral pairs for geothermobarometry, all of the above observations indicate that prograde metamorphism was initiated within the kyanite stability field.
M2 metamorphism
The M2 stage, corresponding to peak granulite-facies metamorphism, is manifested by the occurrence of orthopyroxene in mafic granulites. Moreover, the lack of garnet–clinopyroxene–orthopyroxene assemblage indicates medium-pressure conditions for granulite-facies metamorphism (Green & Ringwood, 1967).
The P–T conditions of M2 were estimated using the two-pyroxene geothermometer and garnet–orthopyroxene–plagioclase–quartz geothermobarometer for mafic granulites. Except for the two-pyroxene geothermometer, the P–T conditions were calculated using the core–core pairs of minerals, based on the assumption that they preserve peak metamorphic compositions. Temperatures estimated from two-pyroxene geothermometry (Anderson & Lindsley, 1988), using the integrated compositions of pyroxenes, vary from 791 to 799°C at an assumed pressure of 7 kbar (Table 9; Fig. 9a). For garnet–orthopyroxene–plagioclase–quartz assemblages, P–T conditions were estimated using both conventional (Harley & Green, 1982; Moecher et al., 1988) and multi-equilibrium (TWQ; Berman, 1991) geothermobarometers. The former, using the calibrations of Harley & Green (1982) and Moecher et al. (1988), yielded the P–T condition of 6·4–8·8 kbar and 711–842°C (Table 9; Fig. 9a). To minimize the effect of post-metamorphic compositional adjustment between garnet and orthopyroxene (Fitzsimons & Harley, 1994), the calibration of Harley & Green (1982), based on the solubility of alumina in orthopyroxene coexisting with garnet, was adopted. The P–T conditions estimated from the TWQ software (Berman, 1991) with the internally consistent thermodynamic set updated by Aranovich & Berman (1997) are in the range of 5·5–7·6 kbar and 684–884°C for the garnet–orthopyroxene–plagioclase–quartz assemblage (Table 9; Fig. 9a). Low temperatures estimated for some samples (e.g. HC87 and HC87') may reflect the effect of retrograde Fe–Mg exchange. By excluding these samples, the P–T estimates are 7·8 ± 0·7 kbar and 827 ± 29°C, and 6·9 ± 0·9 kbar and 814 ± 61°C, respectively, for both calibrations. These results corroborate those of the two-pyroxene geothermometer.
|
|
, P–T estimates from the multi-equilibrium geothermobarometry (
, mafic granulite (HC14-1); 
, garnet amphibolite (HC130). Grey and dashed boxes represent the P–T conditions for garnet amphibolites and hornblende–plagioclase symplectites, respectively, reported in the HGC by
The M2 assemblage of pelitic granulites is represented by garnet + sillimanite ± biotite + K-feldspar + plagioclase + quartz, and their pressure conditions were estimated from the assemblage garnet–sillimanite–plagioclase–quartz, following the calibration of Koziol (1989). When the core compositions of primary garnet and plagioclase are used at an assumed temperature of 800°C, the calibration of Koziol (1989) yields pressure estimates varying from 7·1 to 9·1 kbar in leucocratic gneisses and from 8·4 to 10·7 kbar in kyanite–garnet and aluminous gneisses (Table 9; Fig. 9a). Thus, average pressures are 7·6 ± 1·0 kbar for the leucocratic gneisses and 9·5 ± 1·0 kbar for the kyanite–garnet and aluminous gneisses. When the occurrence of kyanite inclusions in kyanite–garnet and aluminous gneisses is taken into account, the higher pressure of the latter is consistent with the field occurrence and mineralogical evidence. In addition, the pressures estimated from the leucocratic gneiss are consistent with those from the mafic granulite.
Pressures and temperatures attending peak granulite-facies metamorphism coincide with the experimentally determined conditions for fluid-absent melting of pelitic rocks (e.g. LeBreton & Thompson, 1988; Vielzeuf & Holloway, 1988; Stevens et al., 1997), and are corroborated by the abundance of garnetiferous leucogranites in the HGC. Garnet grains enclosing biotite, sillimanite, rare kyanite, plagioclase and quartz in migmatitic leucosomes and leucogranites indicate that these granitic bodies are the product of dehydration-melting reactions consuming biotite. Thus, the following reactions may account for the occurrence of garnet-bearing anatectic melts in pelitic granulites:
 |
 |
 |
|
M3 metamorphism
The occurrence of M3 kyanite is characteristic for the retrogression of pelitic granulites. Kyanite commonly occurs in kyanite–garnet and aluminous gneisses, and rarely in leucocratic gneisses. Kyanite primarily occurs in the matrix as corroded grains, together with rare rutile and staurolite, replacing M2 garnet porphyroblasts. However, a few grains of M3 kyanite also occur along fractures in M2 garnet porphyroblasts and commonly replace acicular sillimanite. These textural features suggest that the following reactions were operative:
 |
 |
 |
The quasi-isobaric cooling path inferred from the pelitic granulites is compatible with the formation of garnet coronas in mafic granulites. The garnet coronas occur as thin, continuous bands between plagioclase and ortho- or clinopyroxene, and are commonly interspaced with cummingtonite–hornblende–quartz symplectites around pyroxenes. The formation of garnet coronas at the expense of pyroxene is attributed to the following reactions:
 |
 |
P–T conditions of the M3 stage were estimated from the garnet–hornblende–plagioclase–quartz assemblage of mafic granulite, and from the garnet–rutile–kyanite–ilmenite–quartz assemblage of the pelitic granulites. For the latter, rim compositions of fragmentary M2 garnet were used. The P–T conditions from coronitic garnet and hornblende in retrogressive mafic granulite (sample HC14-1) were estimated to be 7·8 kbar and 689°C, using the garnet–hornblende geothermometer (Graham & Powell, 1984) and garnet–hornblende–plagioclase–quartz geobarometer (Kohn & Spear, 1990) (Table 11; Fig. 9b). Pressures estimated from the garnet–rutile–kyanite–ilmenite–quartz geobarometer (Bohlen et al., 1983) are in the range of 7·2–8·4 kbar at 700°C (Table 11). The P–T conditions estimated for M3 lie in the kyanite stability field, corroborating the occurrence of M3 kyanite in the pelitic granulites.
|
M4 metamorphism
The M4 stage is characterized by the growth of cordierite that replaces the M2 and M3 assemblages in the pelitic granulites. Cordierite enclosing M2 garnet and sillimanite/kyanite is produced by the following reaction:
 |
 |
 |
 |
The P–T conditions for the onset of M4 are difficult to estimate in the absence of geochronologic information for discerning M3 and M4 assemblages. However, M4 is interpreted to be responsible for the development of garnet-bearing assemblages in the type 2 amphibolites (sample HC130) that were emplaced subsequent to M2 and M3. The P–T conditions of such a garnet amphibolite were estimated to be 9·2 kbar and 700°C, using the garnet–hornblende geothermometer (Graham & Powell, 1984) and garnet–hornblende–plagioclase–quartz geobarometer (Kohn & Spear, 1990) (Table 11; Fig. 9b). Overall, the M4 event is characterized by the formation of symplectites consisting of hornblende and plagioclase at garnet margins. This texture, together with the presence of quartz near the symplectite, suggests the growth of hornblende at the expense of garnet and quartz by the following continuous reaction:
 |
2–5 kbar and 580–700°C. This result is consistent with that calculated from cordierite-bearing pelitic granulites, suggesting a decompression of 4–7 kbar during the M4 metamorphism.
M5 metamorphism
After the M4 stage, further thermal disturbance is apparent because of the local growth of andalusite in some pelitic granulites. Andalusite mantles secondary garnet in the banded biotite gneiss of the MZGC adjacent to a Jurassic granitoid batholith. Thus, rare andalusite in the HGC is interpreted to result from the thermal effects of intrusion of this granitoid.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Pressure–temperature–time evolution
A composite P–T path for the metamorphic evolution of HGC was deduced on the basis of mineral inclusion relationships, reaction textures and geothermobarometry. Two clockwise P–T trajectories apparently occur at relatively high and low temperatures, respectively (Fig. 10). Moreover, available geochronologic data suggest that these P–T trajectories cannot be accounted for by a single tectonothermal event, but by multiple events unrelated to each other (Ellis, 1987; Harley, 1989, 1992; Bohlen, 1991).
|
The first trajectory (M1–M3), responsible for the formation of granulites, has been dated at 1872 ± 7 Ma by ion microprobe U–Pb analyses of zircon in a migmatitic leucocratic gneiss (Lee et al., 2000). In addition, U–Pb zircon ages are identical for leucosome and melanosome, suggesting that the granulite-facies metamorphism and partial melting are synchronous.
The second trajectory (M4) has not been precisely dated yet, but appears to be related to Permo-Triassic orogenesis because the CHIME (chemical U–Th–total Pb isochron method) monazite ages of 255–240 Ma are prevalent in a wide area covering both the HGC and the MZGC (Cho et al., 1996; Suzuki & Adachi, 1999). These ages were obtained largely from the overgrowth domains mantling Palaeoproterozoic monazite cores, suggesting that monazite has recrystallized at the upper amphibolite-facies condition during M4. On the other hand, a preliminary CHIME monazite age of 220 Ma, reported from a mylonitic granulite near the HGC–MZGC boundary (Yi et al., 2001), indicates that extensional shearing movement occurred during Triassic time. These Permo-Triassic growths of monazite are compatible with 40Ar/39Ar ages of hornblende in garnet amphibolite (226 ± 8 Ma), and muscovite in a deformed pegmatite (202 ± 4 Ma; Cho et al., 1999). Taken together, these geochronologic data attest to the superimposition of upper amphibolite-facies Permo-Triassic metamorphism on Palaeoproterozoic granulite-facies metamorphism in the HGC.
Following the M4 regional metamorphism and subsequent cooling, local thermal perturbation (M5) has occurred in association with the Jurassic granitoids. Porphyritic granite and hornblende gabbro in the MZGC yield U–Pb zircon ages of 164·7 ± 2·4 Ma and 166·2 ± 1·2 Ma, respectively (Kim et al., 1999). Thus, M5 thermal metamorphism is attributed to the Jurassic thermal event unrelated to M4.
Palaeoproterozoic evolution of the HGC
The Palaeoproterozoic metamorphic episode defines a clockwise P–T path, consisting of prograde heating, thermal peak, and subsequent quasi-isobaric cooling (Fig. 10). The rare occurrence of kyanite inclusions in M2 garnet from pelitic granulites shows that the onset of prograde (M1) metamorphism occurred within the kyanite stability field, and suggests crustal thickening before the granulite-facies metamorphism. However, the predominance of sillimanite in pelitic granulites indicates that sillimanite was the stable Al-silicate at the thermal peak (M2). Moreover, the general absence of garnet–clinopyroxene assemblages in the mafic granulites indicates that the peak temperatures were reached at medium-pressure conditions (Green & Ringwood, 1967). These observations suggest that the thermal peak postdates the major compressional thickening and has occurred during denudation of the thickened crust.
Peak (M2) metamorphism exceeded the conditions necessary for dehydration-melting of pelitic rocks, and granulite-facies metamorphism was accompanied by widespread partial melting that produced locally abundant leucogranite (Fig. 10; Stevens et al., 1997). Despite the voluminous production of granitic melts, significant intracrustal differentiation is lacking in the HGC, probably because the extraction and ascent of granitic magmas were hampered by the concomitant ascent of residual host gneisses (Sawyer, 1994). This ascending process resulted in a migmatitic complex comprising both metatexite and diatexite, but was terminated at mid-crustal depths (25–30 km).
The retrograde P–T path of the granulite-facies metamorphism is recorded by the development of M3 assemblages (e.g. secondary kyanite and garnet corona in pelitic and mafic granulites, respectively). Although no direct constraints on the timing of the M3 metamorphism are yet available, the lack of deformation-related growth of M3 minerals suggests that this metamorphic stage represents the final stage of the Palaeoproterozoic metamorphic event associated with adjustment to a steady-state geotherm during thermal relaxation and cooling. The M3 minerals occur only locally because of the sluggish kinetics of retrograde reaction in the absence of a fluid phase and/or deformation (Harley, 1989; Ellis & Maboko, 1992; Hensen et al., 1995). It is rather common to find granulites that have cooled isobarically into the kyanite stability field before final exhumation (Bohlen, 1987; Harley, 1989; Ellis & Maboko, 1992).
Heat source for the granulite formation
The HGC experienced a typical clockwise P–T path, which involves deep burial of supracrustal rocks and subsequent thermal relaxation, followed by quasi-isobaric cooling. Such a path is commonly assumed to be typical of the lower continental mass in a crustal section doubled in thickness by collision (England & Thompson, 1984; Ellis, 1987; Chapman & Furlong, 1992). Collisional thickening may therefore account for the formation of the HGC. However, thermal models using reasonable values for thermal conductivity, crustal heat production and basal heat flux predict that such P–T paths should lie within the kyanite stability field (England & Thompson, 1984; Thompson, 1990; De Yoreo et al., 1991; Chapman & Furlong, 1992). As a consequence, the attainment of the granulite-facies condition within the sillimanite stability field requires an additional heat source.
Advective heat transport from the mantle by underplating of mafic magmas has been commonly proposed as the mechanism to allow attainment of granulite-facies conditions within the sillimanite stability field in regions of thickened crust (Bohlen, 1987; Harley, 1989; De Yoreo et al., 1991; Thompson & Connolly, 1995). In addition, anomalously high basal heat flow may result from lithospheric extension before thickening, or large-scale infiltration of hot fluids (Chamberlain & Rumble, 1988). Pre-thickening input of heat is unlikely for the HGC, because kyanite and staurolite occur as relict minerals. On the other hand, large-scale fluid infiltration is not consistent with the widespread anatexis by fluid-absent dehydration melting observed in pelitic granulites.
Advective heating could be a consequence of either lithospheric delamination (Bird, 1979; Houseman et al., 1981) or convective thinning of thickened lithosphere (Loosveld & Etheridge, 1990; Sandiford & Powell, 1990, 1991). These processes may cause an increase in magmatism, variation in magma composition, rapid uplift, and a change in stress regime from compression to extension (Kay & Kay, 1993; Rudnick, 1995). In addition, large-scale granulite-facies metamorphism occurs only if the volume of accreted magma is similar to or greater than the volume of the pre-existing crust, and requires a major period of crustal growth (Wells, 1980; Bohlen, 1987; Huppert & Sparks, 1988; Bergantz, 1989; Oxburgh, 1990). In the northeastern Gyeonggi massif, however, no evidence for voluminous magmatism is apparent at 1·87 Ga. Furthermore, Nd-depleted mantle model (TDM) ages of pelitic granulites suggest a major accretion of juvenile materials at 2·8–2·6 Ga (Lee et al., 2000). Thus, in lieu of advective heating through mantle magmatism, widespread anatexis could be merely a reflection of the heating that caused the granulite-facies metamorphism (Wickham, 1987; Thompson, 1989).
Alternatively, the granulite-facies conditions may be attained by the burial of layers containing high concentrations of heat-producing elements (K, U and Th) during crustal thickening (De Yoreo et al., 1989; Chamberlain & Sonder, 1990; Patiño Douce et al., 1990; Buick et al., 1998; Gerdes et al., 2000). Chamberlain & Sonder (1990) showed that, in the absence of abnormal mantle heat flow or heat advection by magma emplacement, burial of high heat-producing layers could result in the attainment of upper amphibolite- to lower granulite-facies conditions by thermal relaxation during compressional orogenesis. The present-day concentrations of heat-producing elements in pelitic granulites of the HGC were estimated by Yi (1998) as 1·70 ± 0·92 ppm U, 31·0 ± 13·6 ppm Th and 2·63 ± 0·33 wt % K. Thus, the average radiogenic heat production is 3·0 ± 1·2 mW/m3. This value is higher than the radiogenic heat production of average upper continental crust [1·8 mW/m3; calculated from Taylor & McLennan (1985)]. On the other hand, it is comparable with 2–3 mW/m3 estimated from the metasedimentary protoliths of post-orogenic granites in the Variscan collision zone, characterized by high-temperature metamorphism associated with crustal magmatism (Gerdes et al., 2000). Therefore, although the variation of heat production with depth in sedimentary protoliths of granulites is poorly constrained, we tentatively propose that abnormally high internal heat production made an important thermal contribution to the granulite-facies metamorphism in the HGC.
Permo-Triassic exhumation of the HGC
Subsequent to M3, the HGC resided at mid- to upper-crustal depths for a prolonged time before the Permo-Triassic crustal-thickening event (M4). This reactivation of the HGC is characterized by a quasi-isothermal decompression (ITD) path, accounting for the growth of cordierite in pelitic granulites and hornblende–plagioclase symplectites in garnet amphibolites. Moreover, high-pressure conditions of type 2 amphibolites suggest that the decompression was initiated from the kyanite stability field. This inference is also supported by near-isothermal decompression along a clockwise P–T path reported from garnet amphibolites of the Chuncheon area, southern MZGC (Lee & Cho, 1995) (Fig. 9b). Although most rock types show little evidence for significant overprinting of M4, cordierite-bearing gneisses and garnet amphibolites suggest the wholesale denudation of the HGC at 700°C. This decompressional process is probably triggered by upward movement along ductile thrusts and shear zones that separate the HGC from the MZGC (Lee et al., 2000) (Fig. 2). The majority of compressional shear zones are severely overprinted by later extensional, ductile deformation, suggesting gravitational collapse of the thickened crust (Lee et al., 2000). Thus, these shear zones preserve structural and metamorphic records of exhumation from depths of 30 km to 10–15 km. Such decompressional paths caused by compressional exhumation along major thrusts or ductile shear zones have been also proposed for some reworked granulite terranes within collisional orogens (e.g. Schenk, 1984; Sandiford, 1985; Currie & Gittins, 1988; Percival et al., 1992).
Tectonic implications for crustal evolution of East Asia
The discovery of a coherent volume of granulite-facies rocks, characterized by a collision-related orogenic event of Palaeoproterozoic age, provides an important constraint on the tectonic evolution of the Gyeonggi massif. The basement rocks of the Gyeonggi massif are traditionally considered as the product of Archaean to Palaeoproterozoic low-P/high-T thermal regimes (Lee, 1987). However, relict grains of kyanite observed throughout the Gyeonggi massif (Lee & Cho, 1992; Cho & Kim, 1993; Cho et al., 1995; Ahn et al., 1998) suggest that high- to medium-pressure conditions were attained before the widespread low-P/high-T metamorphism. In light of our results, the occurrence of relict kyanite suggests that the collisional orogeny has affected not only supracrustal rocks but also basement rocks of the Gyeonggi massif.
Temporal and tectonic relationships for the exhumation of Palaeoproterozoic granulites indicate that thick-skinned crustal reworking has occurred in the Gyeonggi massif, possibly during the Permo-Triassic. The northern margin of the Gyeonggi massif is bounded by the east-trending Imjingang belt, which has also experienced the Permo-Triassic thermotectonic event (Cho et al., 1995; Ree et al., 1996). The Imjingang belt has been interpreted as an eastward extension to the Korean Peninsula of the ultrahigh-pressure continental collision belt between the Yangtze and Sino-Korean cratons (Yin & Nie, 1993; Li, 1994; Cho et al., 1995; Ree et al., 1996). Our results further suggest that the northern margin of the Gyeonggi massif has experienced compressional tectonic processes during the Permo-Triassic, coeval with the timing of continental collision in east–central China (Ames et al., 1993, 1996; Li et al., 1993; Eide et al., 1994). Therefore, it is likely that the northern Gyeonggi massif as well as the Imjingang belt have participated in the Permo-Triassic tectonic process correlative to the continental collision between the Yangtze and Sino-Korean cratons.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The granulite complex in the Hwacheon area, northeastern Gyeonggi massif, consists of migmatitic granulites enclosing minor mafic granulite and garnet amphibolites. The mineral parageneses and reaction textures indicate that the granulite complex has experienced two metamorphic events, in association with collisional crustal thickening, in Palaeoproterozoic (
1·87 Ga) and Permo-Triassic (255–240 Ma) times, respectively. The peak metamorphic conditions during the earlier collisional event were estimated to be in the range of 7·0–9·5 kbar and 790–830°C, whereas the second metamorphic event reached P–T conditions of 3–6 kbar and 660–750°C. The burial of sedimentary protoliths enriched in heat-producing elements may have resulted in granulite-facies metamorphism and widespread partial melting during the Palaeoproterozoic crustal-thickening orogeny. The occurrence of M3 kyanite and garnet coronas in pelitic and mafic granulites, respectively, indicate that the HGC cooled near-isobarically following granulite-facies metamorphism and resided at mid-crustal depths before final exhumation. The second tectonometamorphism defined by the ITD path is attributed to reactivational exhumation during the Permo-Triassic compressional orogeny which is prevalent in Far-East Asia.
|
ACKNOWLEDGEMENTS |
|---|
This paper represents a part of a Ph.D. thesis of the senior author at Seoul National University, and the field assistance of our colleagues, in particular Keewook Yi, is greatly appreciated. We thank Ian Fitzsimons and Simon Johnson for critically reviewing an early version of the manuscript. We also thank the journal reviewers Ian Buick, Chris Carson, Simon Harley and Michael Raith for their helpful and constructive comments that greatly improved the clarity of this paper. This study was supported by Korea Research Foundation Grant (KRF-2001-041-D00255) to M.C.
|
FOOTNOTES |
|---|
*Corresponding author. E-mail: moonsup{at}snu.ac.kr
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGENERAL GEOLOGYPETROGRAPHYMINERAL CHEMISTRYTHE FIVE METAMORPHIC STAGES...DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Ahn, K. S., Park, Y. S., Kim, J. B. & Chen, J. (1998). Metamorphism of gneiss complex in the Paju-Gimpo area, northwestern Gyeonggi massif, Korea. Journal of Petrological Society of Korea 7, 177–189.
Ames, L., Tilton, G. R. & Zhou, G. (1993). Timing of collision of the Sino-Korean and Yangtze cratons: U–Pb zircon dating of coesite-bearing eclogites. Geology 21, 339–342.
Ames, L., Zhou, G. Z. & Xiong, B. C. (1996). Geochronology and isotopic character of ultrahigh-pressure metamorphism with implications for collision of the Sino-Korean and Yangtze cratons, central China. Tectonics 15, 472–489.[CrossRef][Web of Science]
Anderson, D. J. & Lindsley, D. L. (1988). Internally consistent solution models for Fe–Mg–Mn–Ti oxides: Fe–Ti oxides. American Mineralogist 73, 714–726.[Abstract]
Aranovich, L. & Podlesskii, K. K. (1983). The cordierite–garnet–sillimanite–quartz equilibrium: experiments and applications. In: Saxena, S. K. (ed.) Advances in Physical Geochemistry, Kinetics and Equilibrium in Mineral Reactions, 3. New York: Springer, pp. 172–198.
Aranovich, L. Y. & Berman, R. G. (1997). Optimized standard state and solution properties of minerals. II. Comparisons, predictions, and applications. Contributions to Mineralogy and Petrology 126, 25–37.
Bergantz, G. W. (1989). Underplating and partial melting: implications for melt generation and extraction. Science 245, 1093–1094.
Berman, R. G. (1991). Thermobarometry using multi-equilibrium calculations: a new technique, with petrological applications. Canadian Mineralogist 29, 833–855.[Web of Science]
Bird, P. (1979). Continental delamination and the Colorado Plateau. Journal of Geophysical Research 84, 7561–7571.
Bohlen, S. R. (1987). Pressure–temperature–time paths and a tectonic model for the evolution of granulites. Journal of Geology 95, 617–632.[Web of Science]
Bohlen, S. R. (1991). On the formation of granulites. Journal of Metamorphic Geology 9, 223–229.[Web of Science]
Bohlen, S. R., Wall, V. J. & Boettcher, A. L. (1983). Experimental investigations and geological applications of equilibria in the system FeO–TiO2–Al2O3–SiO2–H2O. American Mineralogist 68, 1049–1058.[Abstract]
Bohlen, S. R., Dollase, W. A. & Wall, V. J. (1986). Calibration and application of spinel equilibria in the system FeO–Al2O3–SiO2. Journal of Petrology 27, 1143–1156.
Braun, I., Raith, M. & Kumar, R. R. (1996). Dehydration-melting phenomena in leptynitic gneisses and the generation of leucogranites: a case study from the Kerala Khondalite Belt, southern India. Journal of Petrology 37, 1285–1305.
Brown, M. (1973). The definition of metatexis, diatexis and migmatite. Proceedings of the Geologists’ Association 84, 371–382.
Brown, M. (1993). P–T–t evolution of orogenic belts and the causes of regional metamorphism. Journal of the Geological Society, London 150, 227–241.
Buick, I. S., Cartwright, I. & Harley, S. L. (1998). The retrograde P–T–t path for low-pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low-P/high-T metamorphism. Journal of Metamorphic Geology 16, 511–529.[CrossRef][Web of Science]
Cesare, B. (1994). Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette-di-Ries, Eastern Alps, Italy. Contributions to Mineralogy and Petrology 116, 239–246.[CrossRef][Web of Science]
Chamberlain, C. P. & Rumble, D., III (1988). Thermal anomalies in a regional metamorphic terrane: an isotopic study of the role of fluids. Journal of Petrology 29, 1215–1232.
Chamberlain, C. P. & Sonder, L. J. (1990). Heat-producing elements and the thermal and baric patterns of metamorphic belts. Science 250, 763–769.
Chapman, D. S. & Furlong, K. P. (1992). Thermal state of the continental crust. In: Fountain, D. M., Arculus, R. & Kay, R. W. (eds) Continental Lower Crust, Developments in Geotectonics, 23. Amsterdam: Elsevier, pp. 179–199.
Cho, D.-L., Suzuki, K., Adachi, M. & Chwae, U. (1996). A preliminary CHIME age determination of monazite from metamorphic and granitic rocks in the Gyeonggi massif, Korea. Journal of Earth and Planetary Sciences, Nagoya University 43, 49–65.
Cho, M. & Kim, J. (1993). Occurrence of kyanite in the Yongduri gneiss complex of the Chuncheon–Hongcheon area: its tectonometamorphic implication. Journal of the Petrological Society of Korea 2, 1–8.
Cho, M., Kwon, S.-T., Ree, J.-H. & Nakamura, E. (1995). High-pressure amphibolite of the Imjingang Belt in the Yeoncheon–Cheongok area. Journal of the Petrological Society of Korea 4, 1–19.
Cho, M., Lee, S. R., Yi, K. & Stern, R. (1999). Metamorphic and tectonic evolution of the Palaeoproterozoic granulites in the northeastern Gyeonggi massif, South Korea. In: Miller, J. A. & Buick, I. S. (eds) Orogenesis in the Outback, a Look at Cyclicity and Reactivation in Orogenic Belts. Geological Society of Australia Abstracts 54, 17.
Chough, S. K., Kwon, S.-T., Ree, J.-H. & Choi, D. K. (2000). Tectonic and sedimentary evolution of the Korean peninsula: a review and new view. Earth-Science Reviews 52, 175–235.
Cluzel, D., Lee, B. J. & Cadet, J. P. (1991). Indosinian dextral ductile fault system and synkinematic plutonism in the southwest of the Ogcheon belt (South Korea). Tectonophysics 194, 131–151.[CrossRef][Web of Science]
Currie, K. L. & Gittins, J. (1988). Contrasting sapphirine parageneses from Wilson Lake, Labrador and their tectonic implications. Journal of Metamorphic Geology 6, 603–622.[Web of Science]
De Yoreo, J. J., Lux, D. R. & Guidotti, C. V. (1989). The role of crustal anatexis and magma migration in the thermal evolution of regions of thickened continental crust. In: Daly, J. S., Cliff, R. A. & Yardley, B. W. D. (eds) Evolution of Metamorphic Belts. Geological Society, London, Special Publications 43, 187–202.
De Yoreo, J. J., Lux, D. R. & Guidotti, C. V. (1991). Thermal modeling of low-pressure/high-temperature metamorphic belts. Tectonophysics 188, 209–238.[CrossRef][Web of Science]
Eide, E. A., McWilliams, M. O. & Liou, J. G. (1994). 40Ar/39Ar geochronology and exhumation of high-pressure to ultra high-pressure metamorphic rocks in east–central China. Geology 22, 601–604.
Ellis, D. J. (1987). Origin and evolution of granulites in normal and thickened crust. Geology 15, 167–170.
Ellis, D. J. & Maboko, M. A. H. (1992). Precambrian tectonics and the physicochemical evolution of the continental crust. I. The gabbro–eclogite transition revisited. Precambrian Research 55, 491–506.[CrossRef][Web of Science]
England, P. C. & Thompson, A. B. (1984). Pressure–temperature–time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. Journal of Petrology 25, 894–928.
Fitzsimons, I. C. W. & Harley, S. L. (1994). The influence of retrograde cation exchange on granulite P–T estimates and a convergence technique for the recovery of peak metamorphic conditions. Journal of Petrology 35, 543–576.
Fountain, D. M. & Salisbury, M. H. (1981). Exposed cross-sections through the continental crust: implications for crustal structure, petrology and evolution. Earth and Planetary Science Letters 56, 263–277.[CrossRef][Web of Science]
Gerdes, A., Wörner, G. & Henk, A. (2000). Post-collisional granite generation and HT–LP metamorphism by radiogenic heating: the Variscan South Bohemian Batholith. Journal of the Geological Society, London 157, 577–587.
Graham, C. M. & Powell, R. (1984). A garnet–hornblende geothermometer and application to the Pelona Schist, southern California. Journal of Metamorphic Geology 2, 13–32.[Web of Science]
Green, D. H. & Ringwood, A. E. (1967). An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochimica et Cosmochimica Acta 31, 767–833.[Web of Science]
Harley, S. L. (1989). The origins of granulites: a metamorphic perspective. Geological Magazine 126, 215–247.[Abstract]
Harley, S. L. (1992). Proterozoic granulite terranes. In: Condie, K. C. (ed.) Proterozoic Crustal Evolution, Developments in Precambrian Geology, 10. Amsterdam: Elsevier, pp. 301–359.
Harley, S. L. & Green, D. H. (1982). Garnet–orthopyroxene barometry for granulite and peridotites. Nature 300, 697–701.[CrossRef]
Hensen, B. J., Zhou, B. & Thost, D. E. (1995). Are reaction textures reliable guide to metamorphic histories? Timing constraints from garnet Sm–Nd chronology for ‘decompression’ textures in granulites from Sostrene Island, Prydz Bay, Antarctica. Geological Journal 30, 261–271.[Web of Science]
Holdaway, M. J. (1971). Stability of andalusite and the aluminum silicate phase diagrams. American Journal of Science 271, 97–131.[Abstract]
Holdaway, M. J. & Lee, S. M. (1977). Fe–Mg cordierite in high grade pelitic rocks based on experimental and natural observations. Contributions to Mineralogy and Petrology 63, 175–198.[CrossRef][Web of Science]
Hollister, L. S. & Crawford, M. L. (1986). Melt-enhanced deformation: a major tectonic process. Geology 14, 558–561.
Houseman, G. A., McKenzie, D. P. & Molnar, P. (1981). Convective instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts. Journal of Geophysical Research 86, 6115–6132.
Huppert, H. E. & Sparks, S. J. (1988). The generation of granitic magmas by intrusion of basalt into continental crust. Journal of Petrology 29, 599–624.
Ito, K. & Kennedy, C. (1971). An experimental study of the basalt–garnet granulite–eclogite transition. In: Heacock, J. G. (ed.) The Structural and Physical Properties of the Earth’s Crust. Geophysical Monograph Series, American Geophysical Union 14, 303–314.
Kay, R. W. & Kay, S. (1993). Delamination and delamination magmatism. Tectonophysics 219, 177–189.[CrossRef][Web of Science]
Kim, C.-B., Turek, A., Chang, H.-W., Park, Y.-S. & Ahn, K.-S. (1999). U–Pb zircon ages for Precambrian and Mesozoic plutonic rocks in the Seoul–Cheongju–Chooncheon area, Gyeonggi massif, Korea. Geochemical Journal 33, 379–397.[Web of Science]
Kim, J. N., Ree, J.-H., Kwon, S.-T., Park, Y., Choi, S. J. & Cheong, C.-S. (2000). The Kyonggi shear zone of the central Korean peninsula: late orogenic imprint of the North and South China collision. Journal of Geology 108, 469–478.[CrossRef][Web of Science][Medline]
Kohn, M. & Spear, F. S. (1990). Two new geobarometers for garnet amphibolites, with applications to southeastern Vermont. American Mineralogist 75, 89–96.[Abstract]
Koziol, A. M. (1989). Recalibration of the garnet–plagioclase–Al2SiO5–quartz (GASP) geobarometer and application to natural parageneses. EOS Transactions, American Geophysical Union 70, 493.
Kretz, R. (1983). Symbols for rock-forming minerals. American Mineralogist 68, 277–279.[Abstract]
Leake, B. E. (1978). Nomenclature of amphiboles. Mineralogical Magazine 42, 533–565.[Web of Science]
LeBreton, N. & Thompson, A.B. (1988). Fluid-absent (dehydration) melting of biotite in metapelites in the early stages of crustal anatexis. Contributions to Mineralogy and Petrology 99, 226–237.[CrossRef][Web of Science]
Lee, D.-S. (ed.) (1987). Geology of Korea. Kyohaksa, S. Korea: Geological Society of Korea, 514 pp.
Lee, K. J. & Cho, M. (1992). Metamorphism of the Gyeonggi Massif in the Gapyeong–Cheongpyeong area. Journal of the Petrological Society of Korea 1, 1–24.
Lee, S. R. & Cho, M. (1995). Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea. Journal of Metamorphic Geology 13, 315–328.[Web of Science]
Lee, S.R., Cho, M., Cheong, C.-S. & Park, K.-H. (1997). An early Proterozoic Sm–Nd age of mafic granulite from the Hwacheon area, South Korea. Geosciences Journal 1, 136–142.
Lee, S. R., Cho, M., Yi, K. & Stern, R. (2000). Early Proterozoic granulites in central Korea: tectonic correlation with Chinese cratons. Journal of Geology 108, 729–738.[CrossRef][Web of Science]
Li, S. G., Chen, Y. Z., Cong, B. L., Zhang, Z. Q., Zhang, R. Y., Liu, D. L., Hart, S. R. & Ge, N. J. (1993). Collision of the North China and Yangtze blocks and formation of coesite-bearing eclogite: timing and processes. Chemical Geology 109, 70–89.
Li, Z. X. (1994). Collision between the North and South China Blocks: a crustal-detachment model for suturing in the region east of the Tanlu fault. Geology 22, 739–742.
Loosveld, R. J. H. & Etheridge, M. A. (1990). A model for low-pressure facies metamorphism during crustal thickening. Journal of Metamorphic Geology 8, 257–267.[Web of Science]
Moecher, D. P., Essene, E. J. & Anovitz, L. M. (1988). Calculation and application of clinopyroxene–garnet–plagioclase–quartz geobarometers. Contributions to Mineralogy and Petrology 100, 92–106.[CrossRef][Web of Science]
Mukhopadhyay, B. & Holdaway, M. J. (1993). Cordierite–garnet–sillimanite–quartz equilibrium: I. New experimental calibration in the system FeO–Al2O3–SiO2–H2O and certain P–T–XH2O relations. Contributions to Mineralogy and Petrology 116, 462–472.
Oxburgh, E. R. (1990). Some thermal aspects of granulite history. In: Vielzeuf, D. & Vidal, Ph. (eds) Granulites and Crustal Evolution. NATO ASI Series 311, 569–580.
Patiño Douce, A. E., Humphreys, D. & Johnston, A. D. (1990). Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America. Earth and Planetary Science Letters 97, 290–315.[CrossRef][Web of Science]
Perchuk, L. L. & Lavren’teva, I. V. (1983). Experimental investigation of exchange equilibria in the system cordierite–garnet–biotite. In: Saxena, S. K. (ed.) Advances in Physical Geochemistry, Kinetics and Equilibrium in Mineral Reactions, 3. New York: Springer, pp. 199–239.
Percival, J. A., Fountain, D. M. & Salisbury, M. H. (1992). Exposed crustal cross sections as windows on the lower crust. In: Fountain, D. M., Arculus, R. & Kay, R. W. (eds) Continental Lower Crust, Developments in Geotectonics, 23. Amsterdam: Elsevier, pp. 317–362.
Phillips, G. N. (1980). Water activity changes across an amphibolite–granulite-facies transition, Broken Hill, Australia. Contributions to Mineralogy and Petrology 75, 377–386.[CrossRef]
Ree, J.-H., Cho, M., Kwon, S.-T. & Nakamura, E. (1996). Possible eastward extension of Chinese collision belt in South Korea: the Imjingang Belt. Geology 24, 1071–1074.
Rudnick, R. L. (1995). Making continental crust. Nature 378, 571–578.[CrossRef]
Sandiford, M. (1985). Metamorphic evolution of granulites at Fyfe Hills: implications for Archean crustal thickness. Journal of Metamorphic Geology 3, 155–178.[Web of Science]
Sandiford, M. & Powell, R. (1990). Some isostatic and thermal consequences of the vertical strain geometry in convergent orogens. Earth and Planetary Science Letters 98, 154–165.[CrossRef][Web of Science]
Sandiford, M. & Powell, R. (1991). Some remarks on high-temperature–low-pressure metamorphism in convergent orogens. Journal of Metamorphic Geology 9, 333–340.[Web of Science]
Sawyer, E. W. (1994). Melt segregation in the continental crust. Geology 22, 1019–1022.
Schenk, V. (1984). Petrology of felsic granulites, metapelites, metabasics, ultramafics and metacarbonates from southern Calabria (Italy): prograde metamorphism, uplift and cooling of a former lower crust. Journal of Petrology 25, 255–298.
Stevens, G., Clemens, J. D. & Droop, G. T. R. (1997). Melt production during granulite-facies anatexis—experimental data from primitive metasedimentary protoliths. Contributions to Mineralogy and Petrology 128, 352–370.[CrossRef][Web of Science]
Stoddard, E. F. (1979). Zinc-rich hercynite in high-grade metamorphic rocks: a product of the dehydration of staurolite. American Mineralogist 64, 736–741.[Abstract]
Suzuki, K. & Adachi, M. (1999). CHIME ages of orthogneisses and paragneisses from the Gyeonggi Massif, Korean Peninsula. Japanese Association of Mineralogical, Petrological and Economic Geologists, Abstract Volume, D14.
Taylor, S. R. & McLennan, S. M. (1985). The Continental Crust: its Composition and Evolution. Oxford: Blackwell.
Thompson, A. B. (1990). Heat, fluids and melting in the granulite facies. In: Vielzeuf, D. & Vidal, Ph. (eds) Granulites and Crustal Evolution. NATO ASI Series 311, 37–57.
Thompson, A. B. & Connolly, A. D. (1995). Melting of the continental crust: some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings. Journal of Geophysical Research 100, 15565–15579.[CrossRef]
Thompson, A. B. & Ridley, J. R. (1987). Pressure–temperature–time (P–T–t) histories of orogenic belts. Philosophical Transactions of the Royal Society of London, Series A 321, 27–45.[CrossRef]
Thompson, P. H. (1989). Moderate overthickening of thinned sialic crust and the origin of granitic magmatism and regional metamorphism in low P–high T terranes. Geology 17, 520–523.
Vernon, R. H. (1996). Problems with inferring P–T–t paths in low-P granulite facies rocks. Journal of Metamorphic Geology 14, 143–153.[CrossRef][Web of Science]
Vielzeuf, D. & Holloway, J. R. (1988). Experimental determination of the fluid-absent melting relations in the pelitic system—consequences for crustal differentiation. Contributions to Mineralogy and Petrology 98, 257–276.[CrossRef][Web of Science]
Wells, P. R. A. (1980). Thermal models for the magmatic accretion and subsequent metamorphism of continental crust. Earth and Planetary Science Letters 46, 253–265.[CrossRef][Web of Science]
Wickham, S. M. (1987). The segregation and emplacement of granitic magmas. Journal of the Geological Society, London 144, 281–297.
Yi, K. (1998). Reaction textures and P–T paths in the Hwacheon granulite complex, South Korea. M.Sc. Thesis, Seoul National University, 87 pp.
Yi, K., Cho, M., Lee, S. R. & Wintsch, R. P. (2001). Dating polymetamorphism in a granulite–amphibolite: Early Proterozoic and Permo-Triassic chemical ages of monazite. Northeastern Regional Meeting, Geological Society of America, Abstract Volume, A-82.
Yin, A. & Nie, S. (1993). An indentation model for the North and South China collision and the development of the Tan-Lu and Honam fault systems, eastern Asia. Tectonics 12, 801–813.[Web of Science]
Young, E. D., Anderson, J. L., Clarke, H. S. & Thomas, W. M. (1989). Petrology of biotite–cordierite–garnet gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic low-pressure fluid-absent granulite-grade metamorphism in the southern Cordillera. Journal of Petrology 30, 39–60.
Zhao, G., Wilde, S. A., Cawood, P. A. & Sun, M. (2001). Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P–T path constraints and tectonic evolution. Precambrian Research 107, 45–73.[CrossRef][Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  M. ZHAI, J. GUO, P. PENG, and B. HU U Pb zircon age dating of a rapakivi granite batholith in Rangnim massif, North Korea Geological Magazine, May 1, 2007; 144(3): 547 - 552. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. ZHAI, Z. NI, C. W. OH, J. GUO, and S. G. CHOI SHRIMP zircon age of a Proterozoic rapakivi granite batholith in the Gyeonggi massif (South Korea) and its geological implications Geological Magazine, January 1, 2005; 142(1): 23 - 30. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||