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Geology of the Grove Mountains in East Antarctica Üünew Evidence for the Final Suture of Gondwana Land

Geology of the Grove Mountains in East Antarctica Üünew Evidence for the Final Suture of Gondwana Land

Vo l. 4 6 No . 4 SCIENCE IN CHINA (Series D) April 2003

Geology of the Grove Mountains in East üüNew evidence for the final of Land

LIU Xiaohan ()1,ZHAOYue(Ф)2, LIU Xiaochun ()2 & YU Liangjun (Ђυ )1

1. Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China Correspondence should be addressed to Liu Xiaohan (email: [email protected])

Received July 17, 2002 Abstract Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granu- lite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic and post-tectonic granodioritic veins. Geo- chemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D1 is related to the event before Pan-African age, D2 corresponds to the regional granulite peak , whereas D3 reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single 5 granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800k,9.3D10 5 Pa; M2 800810k,6.4D10 Pa ; and M3 650k have been recognized. The U-Pb age data from representative granitic gneiss indicate (529B14) Ma of peak metamorphism, (534B5) Ma of gran- ite emplacement, and (501B7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East extending from Prydz Bay via Grove Mountains to the southern . This orogenic belt could be the final suture during the Gondwana Land assemblage.

Keywords: East Antarctic Shield, Grove Mountains, Pan-African orogen, Gondwana Land, final suture.

As the core block of the East Gondwana Land, the East Antarctic Shield was traditionally thought, before 1992, as an amalgamation of a number of Archaean- nuclei, being welded by Grenville aged mobile belts during 1400900 Ma, while the Pan-African tectonism ü was demonstrated only as local magmatism with low-grade metamorphism[1 9]. However, some early thermo-tectonic event records have been reported from the Larsemann Hills (LHs) ü in 1992[10 12], and the hypotheses that Pan-African tectonism should be the key event for the achievement of Gondwana aggregation, has been proposed by Chinese geologists[13,14]. The re- cords of Pan-African age were subsequently recognized in Prydz Bay (PB), Lützow-Holm Bay, ü Dronning Maud Land, and Denman Glacier (Obruchev Hills)[15 17].Incomparisonwith 306 SCIENCE IN CHINA (Series D) Vol. 46 the Ross orogenic belt in the Trans-Antarctic Mountains, and equivalent aged orogenic belts in east Ghats (), Sri Lanka, and Darling mobile belt (west ), the concept that Pan-African tectonism played the key role for the achievement of Gondwana Land is widely ü accepted[2,4,7,9,18 22]. The outline of other super  paleocontinent, which had existed before the Gondwana Land, was gradually described in the same period. Regarding the East Antarctic Shield, some geologists thought that it has remained as a coherent block during ü Rodinia breakup and the later assembly of the East Gondwana[18 22]. This hypothesis is mainly based on the so-called Grenville aged Circum-East Antarctic mobile belt (fig. 1(a)), while the margin between the West and East Gondwana blocks is the Mozambique suture, extending be- tween eastern and the Great India southwards to the , connecting Dronning Maud Land via Lützow-Holm Bay (fig. 1(b)). In this view, there should not be any mobile belt younger than Grenville age inside the East Antarctic Shield. The discovery of Pan-African aged belt in PB-LHs makes a new blue print, which shows an offshoot belt branching from Mozam- bique belt at Lützow-Holm Bay, extending eastwards, via PB-LHs, to the Leewin Complex in ü western Australia[15 17] (fig. 1(c)). Some new challenges to the tectonic significance of Mozambique suture were recently pro- posed. In contrast to that East Antarctica Shield remained as a coherent block, Fitzsimons points out that it was amalgamated during Pan-African orogen with 3 different Grenville aged continen- tal blocks[23]. Based on the preliminary results from the first investigation in the Grove Mountains (GMs) during 19981999, Zhao and Liu have proposed that the geochronological data of GMs indicate that the PB belt may have extended inland to GMs rather than along the coast, spanning over the northern Prince Charles Mountains (nPCM) linked with Lützow-Holm Bay. Also the East Antarctic Shield could have been assembled by several continental blocks in collage, long after Rodinia super continent broke up1—3). Zhao et al. reported the U-Pb SHRIMP ages of Pan-African from GMs at the 31st International Geological Congress in August, 2000, then Boger et al[24] re- ported a new Pan-African record in southern Prince Charles Mountains (sPCM), and proposed that the PB-LHs belt extended southward inland, being linked with sPCM, bisecting the coherent East Antarctic Shield (fig. 1(d)). Further key information needed to reconstruct this global scale tec- tonic framework should be the geometric expansion of Pan-African orogenic belt, and its succes- sive relationship with Grenville aged belt. Being located on the right coast of the Lambert , within 72°20h73°10hS and 73°50h 75°40hE, 450 km to the south of the Chinese Zhongshan Station (LHs), GMs expose a group

1) Zhao Yue, Liu Xiaochun, Fanning, C. M. et al., The Grove Mountains, a segment of a Pan-African orogenic belt in East Antarctica, in Abstract Volume of 31st I.G.C., 9-7 session (CD), Rio de Janerro, Brazil, 2000. 2) Liu Xiaohan, Zhao Yue, Liu Xiaochun et al., Grove Mountains: A segment in the collage during East Antarctic Shield forming? Journal of Conference Abstracts, Strasbourg, France: E.U.G. XI, 2001, 374. 3) Liu Xiaochun, Zhao Yue, Liu Xiaohan, The Pan-African granulite facies metamorphism and syn-tectonic magmatism in the Grove Mountains, East Antarctica, Journal of Conference Abstracts, Strasbourg, France: E.U.G. XI, 2001, 379. No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 307

Fig. 1. (a) Reconstruction of Rodinia (after Hoffman, 1991)[18]. AMAZ, Amazonia; AUSTRA, Australia; BALT, Bal- tica; CEAMB, Circum-East Antarctic mobile belt; CON, Congo; EANT, East Antarctica; IND, India; KAL, Kalahari (Kaapvaal Zimbabwe ); NAME, ; SIBE, ; WAFR, west Africa. ((b)ü(d)) locality of the Mozambique suture and PB Pan-African belt proposed by different scientists (after Boger, 2001)[24].AFR,Africa;AUST, Australia; EANT, East Antarctica; GIND, Great India; GMs, Grove Mountains; LE, Leewin Complex (west Australia); LHB, Lützow-Holm Bay; PB, Prydz Bay; SAME, ; sPCMs, southern Prince Charles Mountains. of isolated nunataks in the inland of Antarctic ice sheet. Many geological contributions in adjacent regions have been made by Australia, Russia and Chinese scientists in recent 20 years, i.e. the Ar- chean nuclei discrete in Vestfold Hills[2], Rauer Islands[26] and sPCM[1,2,24]; the early Neoprotoro- zoic aged belts distribute in nPCM[1,25], Rauer Islands[3,27] and LHs[6]; the Pan-African belt crops ü out in PB, LHs[10 14],andsPCM[24]. Only the GMs rests as geologically unknown between them, which has therefore received much attention. The first geological field investigation on 53 nunataks in GMs was made by 15th and 16th Chinese Antarctic Research Expeditions (CHINARE) during 19982000. This paper presents the preliminary results (fig. 2).

1 Regional geologic setting

Totally 64 nunataks outcrop on the blue-ice and snowfield over an area of ~3200 km2,which are divided in 5 groups, forming a longitudinal valley-ridge landform in NEN trending, as a spiccato of island-chain shape. The general relative height of main nunataks is between 600 m and 800 m above blue ice-snow surface. Due to the north-westward flowing of glaciers, the snow lines on the south-east slope of nunataks are asymmetrically higher. The other sides are generally sharp bluffs, resulting from ice-sheet flowing scrape and normal faulting. The GMs is composed mainly of upper amphibolite to granulite facies metamorphic rocks, 308 SCIENCE IN CHINA (Series D) Vol. 46

Fig. 2. Locality map of Grove Mountains, showing the study area and distribution of nunataks. A, Area where the main granitic gneiss, granite and mafic granulite outcrop; B, area where the main felsic granulite, charnokite, granite and mafic granulite outcrop. 1, -Early Paleozoic mobile belt; 2, nuclei. syn-orogenic to late orogenic granite, post tectonic granodioritic aplite and pegmatite. The high grade complex includes light colored felsic granulite, dark colored mafic and ultramafic granulite, charnockite and gneissic granite. Whole area can be divided into two parts along NEN-SWS di- rection. The felsic granulite and charnockite expose dominantly in the east part, and the granitic gneiss in west part (A and B in fig. 2, and table 1).

Table 1 Metamorphic rock types in Grove Mountains Protolith Rock types Occurrence felsic granulite, thick to medium layer, regional low angle gneissosity mafic granulite, (Pl-Hbl granulite, Hbl-2 Py Thin interlayers and lens of a few centimeters to 5 m occur granulite, Bio-Hbl- 2 Py granulite) ultramafic in both of felsic granulite and granitic gneiss. Ultramafic granulite, (Grt clinopyroxenite, Hbl and Pl granulite outcrops in the northern part of Wilson Ridge and pyroxenite) Supracrust in Lamberts Nunataks. Pl-Hbl gneiss metapelite, (Opx bearing Grt-Bio-Crd-Pl Thin interlayers and lens often consort with mafic and ul- gneiss, Spl bearing Grt-Bio-Pl gneiss) tramafic granulite bodies. Metaquatzite occurs mostly in the metaquartzite (Mt bearing quartzite, Sil or southern part of Mt. Harding Py bearing quartzite) charnockite Massif to thick layer granitic gneiss Thick to medium layer with banded or ribbon structure, granodioritic gneiss regional low angle geissosity Plutonic Massif, stocks and net-veins, gneissic structure defined by orientation of K-feldspar, parallel to the gneissosity of host 2-feldspar granite, K-feldspar granite rocks, rare slightly oblique. Xenolith of host rocks is com- mon. granitic and granodioritic aplite, pegmatite Cut steeply the foliations of both metamorphic rocks and Magmatic dikes gneissic granite

The granulite and charnockite are about 55% of rock volume in the east part, outcropping in No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 309

Mount Harding, Zakharoff Ridge, Davey Nunataks, Wilson Ridge and Gale Escarpment. A few interlayers of grey colored granitic gneiss and plagio-amphibolite gneiss (~15%) appearing in the granulite are rather common. The light colored medium to coarse-grained granitic gneiss (~60%) dominates in the west part, outcropping in the Melvold Nunataks, Mason Peaks, Truman Nunataks and Black Nunataks. A few thin interlayers of felsic granulite appear within the granitic gneiss in Black Nunataks near the boundary between regions A and B. Dark mafic granulite occurs as interlayers and lens 0.10.3 m, with the maximum of 20 m, in both felsic granulite and granitic gneiss of whole region. Light yellow to brown cuticolored coarse-grained K-feldspar granite (~20%) intruded in var- ied metamorphic rock units of whole region, as small massif, stocks and net-veins of a few centi- meters to more than 100 m thick. Two larger granite massifs outcrop in northern part of the Gale Escarpment, and in the southern part of Davey Nunataks. The granite intrusive, with typical inner gneissic structure defined by the orientation of K-feldspar phenocryst, usually parallels the gneissosity of host rocks, but the minority is slightly oblique. Xenolith of host rocks, felsic, mafic granulite and gneiss exist often in the granite bodies. All these features suggest a syn-tectonic to late-orogenic relationship. The unique metapelite (Opx bearing Grt-Bio-Crd-Pl gneiss, Spl bearing Grt-Bio-Pl gneiss) appears in northern part of Wilson Ridge and Bryse Peaks. Some ultramafic granulite (Grt clino- pyroxenite, Hbl pyroxenite and Pl pyroxenite) thin layers of 12m,intheshapeofbandorlens, have been found out in this region. Fine-grained granitic and granodioritic alpite dikes, and pegmatite veins, generally 0.520 m wide, are widespread and cut steeply the foliations of host rocks.

2 Petrography

Light brown-gray colored felsic granulite is composed of hypersthene (~15%), hornblende (~10%), biotite (5%10%), plagioclase (~30%), perthite (~10%) and quartz (~20%), with accessory minerals of zircon, apatite and magnetite. The rocks present gneissic or week gneissic structure, with lepidoblastic and granoblastic texture. Light brown-red to grey colored charnockite contains melanocratic plagioclase (30%50%), K-feldspar (5%30%), melanocratic quartz (15%30%), hypersthene (<5%), biotite (~1%), rare clinopyroxene and hornblende. Accessory minerals are zircon, apatite and magnetite. It is regionally homogenious massif to weak gneissic structure with medium-grained granoblastic texture. K-feldspar attributes mainly perthite. The appearance of orientated milky drip-shaped plagioclase suggests exsolution texture. Some phenocrystal plagioclase includes vermicular quartz, presenting a myrmekitic-graphic texture. Hypersthene demonstrates as irregular granular, with local reaction rim of fine-grained magnetite, brown biotite or hornblende encircles, reflecting retrograde metamorphism. A little hypersthene pseudomorph composed of biotite, calcium and 310 SCIENCE IN CHINA (Series D) Vol. 46 opaque minerals can even be observed. Triple junction texture of plagioclase, hypersthene and diopside suggests an equilibrium paragenetic relationship. Syn-tectonic to post tectonic anatectite bands stretch along the gneissosities. Following the formation of felsic granulite, this homogene- ous coarse-grained charnockite, with weak gneissosity, presents an intrusive feature. Light yellow-grey colored granitic gneiss is 2-feldspar and K-feldspar gneiss. Major minerals are K-feldspar, a little microcline, plagioclase and quartz, with accessory minerals of biotite, horn- blende and opaque oxides. Garnet is rare, in which a few inclusions exist. The gneiss shows itself to be a massif to weak gneissic structure with coarse-medium granoblastic texture. Dark-grey colored mafic granulite consists mainly of Pl-Hbl granulite, associated with Hbl-2 Py granulite and Bio-Hbl-2 Py granulite. The major minerals include hypersthene (15%40%), hornblende (15%65%), plagioclase (~20%), and a little clinopyroxene, biotite and quartz. Ac- cessory minerals are zircon, apatite and magnetite. They demonstrate a massif to weak gneissic structure with fine-medium grained granoblastic texture and prismatic blastic texture, also some banded texture caused by inhomogeneous distribution of minerals. The triple junction texture means an equilibrium paragenetic relationship, and the peak-metamorphism mineral associations are observed. The hypersthene has somewhat reaction rim of hornblende. Appearance of garnet in some samples demonstrates metamorphic mineral association of 3 steps. The plagio-hornblende gneiss consists mainly of hornblende and plagioclase, showing itself to be a massif structure with prismatic and granoblastic texture. The garnet-clinopyroxenite crops out as layers of 1 to 2 m thick in felsic granulite at two nunataks near Wilson Ridge. The major minerals include garnet, clinopyroxene, and a little scapolite (meionite), fine-grained plagioclase, also trace amount of wollastonite and fluorite, and local retrogression of scapolite to zoisite + calcite. Pale-grey colored and thin layered metapelite consists essentially of Grt-Bio-Pl gneiss, in which some Opx-Crd-Grt-Bio gneiss exists. Porphyritic crystals in the porphyroblast texture are garnet and cordierite, while the matrix minerals have fine-grained cordierite, biotite, K-feldspar, plagioclase, quartz, orthopyroxene and spinel, with accessory zircon and apatite in a prismatic, granular and scale-like blastic texture. Garnet (~15%) has generally been elongated as (3.54) mmD1.7 mm to the maximum, in which the inclusions of quartz and biotite are observed. Pris- matic and granular cordierite (~15%) can be divided into 2 groups in grain size, the larger is 1.5 mm (porphyritic crystal as garnet) with biotite and zircon inclusion inside, the smaller is 0.75 mm forming partially thin bands. The zircon inclusion in porphyroblast cordierite has typical lemon yellow pleochroic halo. Oriented tabular-sheet biotite (~20%) in matrix, embodying small quartz inclusion, forms rock geissosity. This biotite is different from that enclosed in the garnet, which is round-sheeted. The fine-grained irregular hypersthene crystals (~2%) are often encircled by biotite resulting from retrograde metamorphism. Enclosed in cordierite porphyroblast, the small irregular vermiform ferromagnesian spinel (~1%) includes small biotite inclusions, and magnetite band exists in spinel crystals. No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 311

Light yellow-pink colored 2-feldspar granite consists of K-feldspar (orthoclase, microcline or perthite), plagioclase, quartz, biotite, and some hornblende. The fine-grained granitic and grano- dioritic dikes and veins consist of biotite, hornblende, plagioclase, perthite and quartz. The felsic pegmatite consist of K-feldspar, quartz, a little plagioclase and biotite. Based on the field structural relationship observation, the possible sequence of rock unit generation is as follows: mafic-ultramafic granulite, metapelite, felsic granulite, charnockite, granitic gneiss, 2-feldspar granite, granodioritic aplite and pegmatite.

3 Structure and deformation

The 53 nunataks investigated show a regional stable pervasive sub-horizontal gneissosity formed by oriented rock-forming minerals, but some of ductile high-strain zones occur spatially. In the background of intense co-axial flattening deformation, some low-angle isoclinal folds and recumbent folds of mafic bands and lens suggest also a “bedding” shearing. Gneissosity in syon- rogenic to post orogenic granite massif is relatively weak (fig. 3).

Fig. 3. Topographical features of major nunataks in GM and their schematic cross section. 312 SCIENCE IN CHINA (Series D) Vol. 46

Three generations of ductile deformation were distinguished. The first stage (D1) was recog- nized from some lens and thin interlayers of mafic grnulite and biotite gneiss included in felsic granulite and granitic gneiss. The relic mineral fabric (foliation S1 and lineation L1) was formed by oriented biotite, fine-grained plagioclase and perthite. Such mineral assemblage is often en- closed by the larger quartz crystals elsewhere. The kinematics of D1 fabric is not stable, because of recrystallization during a later thermo-tectonic event. These original D1 fabrics could present an early orogenic record before regional peak metamorphism.

To produce regional flat-pitching foliation (S2), the second stage of deformation (D2)is dominant structural event corresponding to the granulite facies metamorphism in GMs. This folia- tion is built up by oriented minerals of top-metamorphism mineral assemblage. A series of mafic veins of 10to 50 cm wide, 20 to 50 m in length, sub-vertically prolongated in host granitic gneiss has been observed in Melvold Nunataks. These veins have been folded in tight snaking shape, thining limbs, being thicker in hinges with fold axis surface parallel to the flat gneissosity of host rock. These syn-matamorphic folded veins indicate an intense vertical shortening due to coaxial flattening deformation. Its origin could be previous folds overprinted during late granulite meta- morphism. No clear lineation was observed in this deformation, indicating that it could be related with crust vertical cumulating caused by underplating. Contrasting with the regional stable flat foliation, local high-strained shear zones have been overprinted in northern Zakharoff Ridge, western Mason Peaks, northern Lambert Nunataks and southern Gale Escarpment. These zones of 10 to 100 m wide gradually transit to the country rocks. The original flat foliation within these zones has been rebuilt up as tight isoclinal asymmetric folds, even recumbent folds. Superposed relic folds can be observed locally. High-angle S3 and L3 penetrate previous fold hinges. The S-C structure and asymmetric micro-folds are common. The migmatization is generally evident within high-strained zones, because of felsic pegmatite veins intruded parallel to the foliation, oblique and net veins for somewhere, suggesting high tempera- ture and partial melting during shearing. These high-strained zones show a non coaxial simple shearing mechanism. The directions of S3 and L3 are stable in one zone, but it is difficult to recon- struct the regional-scale tectonic configuration due to the isolation of exposures. Considering gradual transition relationship between high-strained zones and country rocks and their similar mineral assemblage, it is suggested that D3 occurs shortly after D2, with similar pres- sure-temperature conditions. The transition of mechanism from pure shearing to simple shearing could be made due to the change of regional stress field and limit condition of geological units. Because almost all kinematic marks of microfolds within high-strained zones indicate normal faulting, D3 could however imply a large-scale extensional shearing movement during a late extension period of orogen. A series of steep normal fault extending in NEN direction developed along the NW slopes of most nunataks, forming regional-scale basin and range landscape. A few fault striae and steps on sub-vertical fault wall (dipping WNW) indicate that the hanging wall is going down. The fault No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 313 surfaces are generally straight, parallel to each other, and arrayed in echelon in regional scale. This normal fault system could be related with the activity of abortive Lambert Rift in Meso-Cenozoic.

4 Metamorphism

The representative mineral assemblages (table 2) and reaction textures of different metamor- phic rock types exposing in the GMs display generally a regional single granulite facies meta- morphism. The temperature calculated by garnet-orthopyroxene geothermometer from a garnet- bearing gneiss is 780B50k, and the pressure is (5.56.8)D105Pa1). Detailed studies on the ma- fic granulite display an isothermal decompression (ITD) path followed by a near-isobaric cooling path (IBC), in which 3 steps are shown as follows: M1 is represented by the mineral assemblage of Opx + Pl + Qtz, included in a garnet porphyroblast, with the P-T condition of 800k, 9.3D105 Pa;

M2 demonstrates as Opx + Cpx + Hbl mineral assemblage and the capped plagioclase round gar- 5 net crystal, with the P-T condition of 800810k, 6.4D10 Pa; M3 shows a low temperature of 650k, resulting from the green hornblende surrounding pyroxene crystal. In addition, a phe- nomenon of exsoluted fine-grained garnet and ribbon hypersthene in garnet porphyroblast has been recognized, which could imply an earlier isobaric cooling metamorphism[29].

Table 2 The typical mineral assemblages of different rock types in the Grove Mountains Rock types Mineral assemblage Felsic granulite Opx + Hbl + Bio + Pl + Qtz ± Kfs Charnockite Opx + Bio + Kfs + (dark) Pl + Hbl + (dark) Qtz Granitic gneiss Bio + Kfs + Pl + Qtz ± Hbl ± Grt Mafic granulite Opx + Cpx + Hbl + Bio + Pl ± Grt ± Qtz Ultramafic granulite Cpx + Hbl + Pl + Sc ± Grt ± Wol ± Fluor Metapelite Grt + Crd + Bio +Spl + Pl + Qtz ± Opx ± Sil Granite Kfs + Pl + Bio + Qtz ± Hbl Granodiorite Bio + Hbl + Pl + Kfs + Qtz

5 Geochronology

Sensitive High Resolution Ion Microprobe ( SHRIMP II ) dating of zircon from representa- tive granitic gneiss, 2-feldspar granite, and fine-grained granodioritic aplite was conducted in the College of Sciences, State University of Australia (Canberra). The zircons of granitic gneiss collected from the Melvold Nunataks generally possess core-zonal structures. The U-Pb age data of cores are scattered between 870 Ma and 953 Ma, referring to the distinct crystallization ages of inherited zircons. But a series of clustered rim age of (529B14) Ma illuminates the peak meta- morphism age of granulite facies (fig. 4(a)). An integrative zircon U-Pb SHRIMP age collected from syn-to late-tectonic 2-feldspar granite concentrated in (534B5) Ma (fig. 4(b)), suggesting an emplacement of regional magmatism accompanying peak metamorphism during Pan-African tec-

1) See footnote 1) on page 306. 314 SCIENCE IN CHINA (Series D) Vol. 46

tonism. The zircon U-Pb SHRIMP age of post tectonic granodioritic aplite was (501B7)Ma (fig. 4(c)). In addition, a biotite 40Ar/39Ar age from granodioritic vein is 498.2Ma, showing a quick cooling history, similar to that in the LHs. All these geochronological data indicate that the GMs has suffered the Pan-African aged intense tectonism, forming therefore a part of Pan-African mobile belt in the East Antarctic Fig. 4. Histograms of U-Pb zircon ages of representative gran- 1) itic gneiss (a), syn-tectonic granite (b) and granodioritic aplite (c) Shield . f in Grove Mountains. (a) Mean = 529 14, 95% conf., Wtd. by The age of peak metamorphism in GMs is data-point errors only, MSWD = 0.12, (error bars are 2σ); (b) mean = 534f5, 95% conf., Wtd. by data-point errors only, equivalent to that in LHs. A number of MSWD = 2.0, (error bars are 2σ); (c) mean = 501f7, 95% conf., Wtd. by data-point errors only, MSWD = 0.71, (error bars Pan-African age records has also been found are 2σ). out in nPCM. Despite the dominant Neopro- terozoic U-Pb SHRIMP age data from granulite facies complex in nPCM, a few zircon and mona- zite U-Pb data between 500 Ma and 550 Ma from granite and pegmatite appeared in east margin of nPCM[25]. In spite of the possibility of lead loss, the Pan-African age record could exist there. On the other hand, there are also two series of Sm/Nd ages from garnet/whole rock and gar- net/matrix of 800 Ma and 630550 Ma, collected from Beaver Lake and Jetty Peninsula[28]. Fur- ther, Boger has recently reported the early Paleozoic thermo-tectonic age from Lambert in southern Mawson Escarpment (73°30hS, 68°30hE) of sPCM. The clustered zircon U-Pb SHRIMP ages concentrated on 550490 Ma from garnet and biotite bearing granitic gneiss. These isotopic results disjoin the sPCM into two parts. The southern part preserves a Middle Archaean granitite series (3160 Ma and 2650 Ma), and the northern part (nPCM) is Neoproterozoic gneiss. This fact indicates that the PB-LHs Pan-African mobile belt extends southward through GMs to sPCM.

6 Geochemical and protolith analysis

Based on the systematic petro-geochemical analysis (major and trace element, REE compo- nents), the major rock types in the GMs can be divided into 3 lithological divisions. The su- pracrust original rocks include felsic granutite, mafic-ultramafic granulite, plagioclase-hornblende gneiss, metapelite, magnetite-bearing quartzite and (garnet) plagio-clinopyroxenite. Their protolith is island-arc grano-diorite series, oceanic island basalt (OIB), and middle oceanic ridge basalt (MORB). While plutonic original rocks include charnockite, granitic gneiss and 2-feldspar granite, belonging to the aluminous A-type granite. The partial melting original rocks include granitic and granodioritic aplite, and pegmatite veins. The major element composition of all mafic granulite and plagio-hornblende gneiss is com-

1) See footnote 1) on page 306. No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 315

parable with that of basalt, because SiO2 is 46.37%49.90%. The detailed analysis based on their major compositional feature, rare earth and trace element indicates 2 types of original basalts.

First type is enriched with Ti (TiO2 = 2.68%), LREE [(La/Yb)N = 4.77], Ti/Y(=343) and Zr/Y (=3.1), showing the character of the OIB. The magma source could be related with activity of en- riched mantle (EMI). The second basalt has lower content of Ti (TiO2 = 1.1%1.31%), lower content of P (P2O5 = 0.1%0.2%), being much less than that of OIB, and lower REE (4793 ppm), LREE/HREE(2.272.54), and LaN/YbN(1.301.62). The second basalt possesses there- fore the characteristics of MORB. This combination of OIB and MORB implies the existence of oceanic basin and continental margin. There is another series of volcanic felsic rocks, rooting in mature island-arc environment, developing in GMs (to be published in other paper), indicating a continental margin, back-arc basin and oceanic basin series[29].

7 Conclusion and discussions

The GMs consists of a series of upper amphibolite to granulite facies metamorphic rocks. The charnockite and syn- to late-tectonic granite belong to the aluminous A type plutonic rocks, whereas felsic and mafic granulite was from supracrust including island-arc basalt, OIB and MORB. A few high-strained shear zones disperse in regional sub-horizontal foliation of rocks. Three generations of ductile deformation were identified. The metamorphic feature from the gran- itic gneiss indicates a single granutite facies event, but 3 steps from mafic granulite were distin- guished, and its ordinal isothermal decompression (ITD) path, followed by a near-isobaric cooling path (IBC), is concordant with a typical tectonic model, in which the crust thickening event re- sulted from collisional orogeny, followed by consequent crust thinning by uplifting and erosion. The zircon U-Pb SHRIMP age of representative granitic gneiss indicates that the peak metamor- phic age of granulite facies is (529B14) Ma, the granite emplacement is (534B5) Ma, while the age of granodioritic aplite is (501B7) Ma. The geological feature and thermo-tectonic age indicate that the GMs has suffered an intense tectonism in early Paleozoic. The GMs links therefore with the PB, LHs in north, and with the sPCM in south, consisting of one whole Pan-African orogenic belt. This belt should not be sud- denly ended because of its large dimension, but must continue to outspread southward into inland. The structural trend of this belt in Lambert Terrane of sPCM is near E-W, implying that it con- nects with equivalent aged belt in Dronning Maud Land[24]. But the structural trends in both GMs and LHs are mainly NEN, suggesting that belt should link in northeast with Obruchev Hills near Denman Glacier, and further with Leewin Complex in west Australia. Both the nPCM and (India) record minor deformation, intrusive activity and some evidence of isotopic distur- bance during the . This belt could thus outspread northward via eastern margin of nPCM and northeast margin of eastern Ghats, into the west Yangtze block and northeast Tibet block at that time, forming, as a consequence, Gondwana aged metamorphic basements of north Tibet Pla- 316 SCIENCE IN CHINA (Series D) Vol. 46 teau (Qiangtang block) and east Yunnan Province. Some studies argue whether this mobile belt could be an intra-continental belt, but many evi- dences show that this belt has disjoined the Archaean nuclei and Neoproterozoic belt within East Antarctic Shield. In PB region, for example, this belt separates the Vestfold Archaean terrane (2480 Ma)[30] and Rauer Islands Archaean terrane (28002550 Ma)[31] on east side from the nPCM Neoproterozoic terrane and Lützow-Holm Bay Archaean terrane in west. In sPCM this belt has also detached the nPCM’s Neoproterozoic belt in north from the Archaean terrane (3160 2650 Ma)[24] in south. Furthermore, the metamorphic age and isotopic features of Grenville belts in nPCM, Lützow-Holm Bay, and eastern Ghats (India) are rather different from those in Dron- ning Maud Land[32]. They must belong to different lithospheric units before Pan-African tectonism. On the other hand, the inherited zircon ages obtained from both sides of Pan-African belt in sPCM are completely different. The Lambert terrane did not derive its sediments from the Ruker terrane in south, nor did they incidentally draw sedimentary material from the Archaean blocks exposed in the Vestfold Hills or Rauer Islands. Instead, it demonstrates, like in GMs, as Pan-African aged juvenile crust material. Thus the PB-GMs-sPCM Early Paleozoic aged belt could not be an in- tra-continental belt, but a tremendous suture during the aggregation of Gondwana Land (fig. 5). If this hypothesis is correct, the west part of East Antarctic Shield from PB-GMs-sPCM belt and Great India should no longer belong to the so-called East Gondwana realm. A few reliable ü paleomagnetic data do not rule out this possibility[33 35], and these blocks have assembled with Africa along Mozambique suture in about 570520 Ma, a little earlier than the PB-GMs-sPCM mobile belt (550490 Ma). This difference in age is consistent with the eastward Pan-African orogenesis getting younger, recognized within the West Gondwana Land[36]. As a collisional orogenic belt, the tectonic configuration of PB-GMs-sPCM is not yet clear owning to lack of outcrops and tough environment in Antarctic inland. Both tectonic polarity and direction of subduction remain still in the dark. The common features of almost all outcrops along PB-GMs-sPCM belt are high temperature metamorphism and partial melting, the intrusion of post-orogenic granite, and quick cooling history with uplifting. Comparing with LHs region, the peak metamorphic pressure is higher in GMs, and dominant rock is felsic one, like that in west part of LHs. There is rare metapelite observed in GMs, but it widely outcrops in Mirror Peninsula of LHs, showing a feature of tectonic mélange of island-arc flysch accretionary wedge. The is- land-arc originated gneiss outcrops in north-west part of GMs, also in west part of LHs. In south- east part of GMs are mainly plutonic rocks, whereas in southeast part of Mirror Peninsula of LHs is paragneiss. There are also some ophiolitic mélange pebbles in east part of GMs, such as cumu- lus gabbro, dunite and siliceous rocks carried out by ice sheet shear motion, implying the exis- tence of belt under the ice sheet nearby on east side of GMs. Abundant charnockite out- crops in GMs in a larger scale, corresponding to that in sPCM, Dronning Maud Land, India and Sri Lanca, but not in LHs. The inherited zircon ages of late Archean and ob- No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 317

Fig. 5. Reconstruction of Pan-African aged belts during assemblage of Gondwana Land (modified after Zhao et al. 2000). AND, Andes Belt; ANPEN, ; ARA, Arabia Peninsula; CON, Congo block; DML, Dronning Maud Land; GB, Guiana shield; GM, Grove Mountains; IND, India craton; KAL, Kalahari (Kaapvaal Zimbabwe cra- ton); LPL, Laplata shield; MBL, Marie Byrd Land; MC, Mawson craton; NAS, north Australia; NC, North China cra- ton; nPCM, northern Prince Charles Mountains; NP, Napier complex; NT, North Tibet; NZB, Newzealand block; OH, Obruchev Hills; SC, South China plate; SFR, Sao Francisco; sPCM, southern Prince Charles Mountains; TFB, Tasman mobile belt; TIB, ; TK, Tarim-Karakumy block; WAF, West Africa; WAS, west Australia; WK, ; VH, Vestfold Hills. tained from Lambert terrane in sPCM show that old block reworked during Pan-African orogen, then it should be located near the margin of the orogenic belt. However, the lack of old inherited zircon ages in GMs indicates that it consists of juvenile crust material during Pan-African tecton- ism. Thus PB-GMs-sPCM belt is a Pan-African aged mobile belt with rather complicated structure. Identifying the continuation of this belt beyond the sPCM in south and PB in north is difficult. Whether it links with Ross-Delamarian belt in the Trans-Antarctic Mountains, or with the Dron- ning Maud Land in south, with the eastern Ghats, west Yangtze and Tibet Plateau (Qiangtang block) in north needs further study. This work not only draws the outline of Pan-African aged belt within East Antarctic Shield, but also implies another final main Gondwana suture, equivalent to the Mozambique suture, connecting with the Yangtze block, Tibet Plateau (Qiangtang block) and Tarim-Karakumy block.

Acknowledgements We would like to thank the Chinese Antarctic and Arctic Research Administration, National Bureau of Oceanography for the special financial and logistical support to the field investigation in 1998ü2000. Financial support is given by Chinese Academy of Sciences (Grant No. KZCX2-303), and the Ministry of Science and Technology of China (Grant No. 98-927-01-06). 318 SCIENCE IN CHINA (Series D) Vol. 46

References

1. Tingey, R. J., The geologic evolution of the Prince Charles Mountainsü$n Antarctic Archaean cratonic block, in Antarctic Geoscience (ed. Craddock, C.), Madison WI: University of Wisconsin Press, 1982, 455ü464. 2. Tingey, R. J., The regional geology of Archaean and rocks in Antarctica, in The (ed. Tingey, R. J.), Oxford: Clarendon Press, 1991, 1ü78. 3. Sheraton, J. W., Black, L. P., Mc Culloch, M. T., Regional geochemical and isotopic characteristics of high-grade meta- morphics of the Prydz Bay area: The extent of Proterozoic reworking of Archaean continental crust in East Antarctica, Res., 1984, 26: 169ü198. 4. Grew, E. S., Manton, W. I., A new correlation of sapphirine granulites in the Indo-Antarctic metamorphic terrain, late Pro- terozoic dates from the Eastern Ghats Province of India, Precambrian Res., 1986, 33: 123ü137. 5. Black, L. P., Harley, S. L., Sun, S. S. et al., The Rayner complex of East Antarctica: Complex isotopic systematics within a Proterozoic mobile belt, J. Metamorphic Geol., 1987, 5: 1ü26. 6. Stüwe, K., Powell, R., Low pressure granulite facies metamorphism in the Larsemann Hills, East Antarctica: Petrology and tectonic implications for the evolution of the Prydz Bay area, J. Metamorphic Geol., 1989, 7: 465ü483. 7. Katz, M. B., Sri Lanka-Indian eastern Ghats-east Antarctica and the Australian Albany Fraser mobile belt: Gross geometry, age relationships, and in Precambrian Gondwanaland, J. Geology, 1989, 97: 646ü648. 8. Fitzsimons, I. C. W., Harley, S. L., Geological relationships in high-grade gneiss of the Brattstrand Bluffs coastline, Prydz Bay, east Antarctica, Austral. J. Earth Sci., 1991, 38: 497ü519. 9. Moores, E. M., Southwest U.S.üEast Antarctica (SWEAT) connection: A hypothesis, Geology, 1991, 19: 425ü428. 10. Ren, L., Zhao, Y., Liu, S. et al., Re-examination of the metamorphic evolution of the Larsemann Hills, East Antarctica, in Recent Progress in Antarctic Earth Science (eds. Yoshida, Y. et al.), Tokyo: Terra Sci. Pub. Com., 1992, 145ü153. 11. Zhao, Y., Song, B., Wang, Y. et al., Geochronology of the late granite in the Larsemann Hills, East Antarctica, in Recent Progress in Antarctic Earth Science (eds. Yoshida, Y. et al.), Tokyo: Terra Sci. Pub. Com., 1992, 155ü161. 12. Ren, L., Liu, X., The metamorphic age of garnet-pyroxene granulite in Sostrene Islands, Prydz Bay, East Antarctica, and its relationship with neighbor region, Antarctica Research (in Chinese), 1995, 7(3): 13ü17. 13. Zhao, Y., Song, B., Zhang, Z. et al., The Pan-African thermo-tectonic event in the Larsemann Hills, East Antarctica and its neighborhood region, Science in China (in Chinese), Ser. B, 1993, 23(9): 1001ü1008. 14. Liu, X., Tong, L., Li, J. et al., Mesoproterozoic to Early Paleozoic tectonic evolution of the East Antarctic Shield, in Ex- tended Abstract Bulletin of Symposium of Earth Science of Taiwan Strait (2) (in Chinese), Taibei: Press of the Chinese Association of Geology, 1995, 165ü169. 15. Carson, C. J., Fanning, C. M., Wilson, C. J. L., Timing of the progress granite, Larsemann Hills, evidence for Early Pa- laeozoic orogeniesis within the East Antarctic Shield and implications for Gondwana assembly, Australian Journal of Earth Sciences, 1996, 43: 539ü553. 16. Hensen, B. J., Zhou, B., East Gondwana amalgamation by Pan-African collision? Evidence from Prydz Bay, Eastern Ant- , in The Antarctic Region, Geological Evolution and Processes (ed. Ricci, C. A.), Siena: Terra Antartica Pub., 1997, 115ü119. 17. Fitzsimons, I. C. W., The Brattstrand paragneiss and the Sostrene orthogneiss: A review of Pan-African metamorphism and Greenvill relics in southern Prydz Bay, in The Antarctic Region, Geological Evolution and Processes (ed. Ricci, C. A.), Siena: Terra Antartica Pub., 1997, 121ü130. 18. Hoffman, P. F., Did the breakout of turn Gondwanaland inside out? Science, 1991, 252: 1409ü1412. 19. Clarke, G. L., Sun, S. S., White, R. W., Grenville-age belts and associated older terrains in Australia and Antarctica, AGSO Jour. Aust. Geol. Geophys., 1995, 16: 25ü39. 20. Yoshida, M., Assembly of East Gondwanaland during the Mesoproterozoic and its rejuvenation during the Pan-African period, in India and Antarctica during the Precambrian, Geological Society of India Memoir, 1995, 34: 22ü45. 21. Shackleton, R. M., The final collision zone between east and west Gondwana: Where is it? Journal of African Earth Sci- ences, 1996, 23: 271ü287. 22. Kriegsman, L. M., The Pan-African event in East Antarctica: A view from Sri Lanka and the Mozambique belt, Precam- brian Res., 1995, 75: 263ü277. 23. Fitzsimons, I. C. W., Grenville-age provinces in East Antarctica: Evidence for three separate collisional orogens, Geology, 2000, 28(10): 879ü882. No. 4 GROVE MOUNTAINSüüFINAL SUTURE OF GONDWANA 319

24. Boger, S. D., Wilson, C. J. L., Fanning, C. M., Early Paleozoic tectonism within the East Antarctic craton: The final suture between east and west Gondwana? Geology, 2001, 29(5): 463ü466. 25. Boger, S. D., Carson, C. J., Wilson, C. J. L. et al., Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, east Antarctica: Evidence for a single protracted orogenic event, Precambrian Res., 2000, 104: 1ü24. 26. Kinny, P. D., Black, L. P., Sheraton, J. W., Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands, Antarctic Sci., 1993, 5: 193ü206. 27. Harley, S. L., Proterozoic granulites from the Rauer Group, East Antarctica, I. Decompressional pressure-temperature paths deduced from mafic and felsic gneisses, J. Petrol., 1988, 29: 1059ü1095. 28. Manton, W. I., Grew, E. S., Hoffman, J. et al., Granitic rocks of the Jetty Peninsula, Amery Ice Shelf area, East Antarctica, in Recent Progress in Antarctic Earth Science (eds. Yoshida, Y. et al.), Tokyo: Terra Sci. Pub. Com., 1992, 179ü189. 29. Yu, L., Liu, X., Liu, X. C. et al., Geochemical characteristics of meta-mafic rocks of the Grove Mountains, East Antarctica, Petrography Bulletin (in Chinese), 2002, 18(1): 91ü99. 30. Snape, I., Black, L. P., Harley, S. L., Refinement of the timing of magmatism, high temperature metamorphism and deformation in the Vestfold Hills, East Antarctica, from new U-Pb zircon geochronology, in The Antarctic region, Geological Evolution and Processes (ed. Ricci, C. A.), Siena: Terra Antartica Pub., 1997, 139ü148. 31. Harley, S. L., Snape, I., Black, L. P., The evolution of a layered metaigneous complex in the Rauer Group, east Antarctica: Evidence for a distinct Archean terrane, Precambrian Res., 1998, 89: 175ü205. 32. Jacobs, J., Fanning, C. M., Henjes-Kunst, F. et al., Continuation of the Mozambique belt into East Antarctica: Gren- ville-ane metamorphism and polyphase Pan-African high-grade events in central Dronning Maud Land, J. Geology, 1998, 106: 385ü406. 33. Powell, C. M., Li, Z. X., McElhinny, M. et al., Paleomagnetic constraints on the timing of the Neoproterozoic break-up of Rodinia and the Cambrian formation of Gondwana, Geology, 1993, 21: 889ü892. 34. Gose, W. A., Helper, M. A., Connelly, I. N. et al., Paleomagnetic data and U-Pb isotopic age determinations from Coats Land, Antarctica: Implication for late Proterozoic plate reconstructions, J. Geophysical Res., 1997, 102: 7887ü7902. 35. Zhang Shihong, Li Zhengxiang, Wu Huaichun et al., New paleomagnetic results from the Neoproterozoic successions in soutnern North China Block and paleogeographic implications, Science in China, Ser. D, 2000, 43(supp.): 233ü244. 36. Grunow, A., Hansen, R., Wilson, T., Were aspects of Pan-African deformation linked to Lapeyus opening? Geology, 1996, 24: 1063ü1066.

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