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

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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 Antarctica üüNew evidence for the final suture of Gondwana 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 granite 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 metamorphism, 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 Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. 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-Paleoproterozoic 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 Paleozoic 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 regions (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 (India), Sri Lanka, Madagascar and Darling mobile belt (west Australia), 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 continentRodinia 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 Africa and the Great India southwards to the East Antarctica, 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 Rift, 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 craton); NAME, North America; SIBE, Siberia; 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, South America; 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 region 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, Neoproterozoic-Early Paleozoic mobile belt; 2, Archean 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.
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