Additional Rb-Sr and single-grain zircon datings of Caledonian granitoid rocks from Albert I Land, northwest Spitsbergen

Previous K-Ar and Rb-Sr datings of the metamorphic and granitic rocks from the northwestern basement region of Spitsbergen mainly show the cooling time of the rocks, except for a Rb-Sr isochron age of the Hornemantoppen granitoid. New samples were collected during several years of geological mapping in the area and the Rb-Sr whole rock isochron and single-grain zircon evaporation methods were applied to the Hornemantoppen granitoids and the grey granites. A dioritic dyke was also dated by the latter method. The bulk rock chemistry study shows that most of both granitic rocks are of the S-type and probably post orogenic, with distinctive incorporation of crustal materials. The isotopic data also support this interpretation. The results of the Rb-Sr isotope analyses, 412 ± 4.8 Ma and the zircon Pb evaporation age of 424 ± 56 Ma, confirm the previous age of the Hornemantoppen granitoid, 414 ± 10 Ma. An older zircon age of 547 ± 19 Ma is considered to be the minimum age of inherited zircon. Zircons from the grey granites suggest an age of ca. 420 Ma with a large error. Field relations demonstrate that the grey granites are older than the Hornemantoppen granitoids. A minimum inherited zircon age, 952 ± 20 Ma, has been obtained from the grey granites. Three multi-grain Pb ages, 423 ± 22 Ma (2 grains), 461 ± 42 Ma and 561 ± 93 Ma (the last two 3 grains) were considered to be mixed ages. Although no definitive evidence for the presence of Grenvillian granites in this area has been obtained in the present study, preliminary results from the multi-grain zircon evaporation method, carried out in the Russian laboratory at Apatity, infer Paleo- and Mesoproterozoic protoliths for the metamorphic rocks of northwestern Spitsbergen.     

Rb-Sr whole rock and U-Pb zircon datings of the granitic-gabbroic rocks from the Skålfjellet Subgroup, southwest Spitsbergen

Recent remapping and new age determinations has shed light on the understanding of Precambrian rocks northwest of Hornsund, southwest Spitsbergen. The Skålfjellet Subgroup has been regarded as the eastern equivalent of the Vimsodden Subgroup, and both of these occur within the Precambrian Eimfjellet Group of southwest Spitsbergen. Although the Eimfjellet Group is considered to be older than the oldest unconformity in the area, the age of the rocks has not been known. The granitic-gabbroic rocks in the Skålfjellet Subgroup have been considered to be the products of granitisation for many years, but recent observations show that they are exotic blocks incorporated into the basic eruptive rocks which are the main constituents of the subgroup. These plutonic rocks have a wide range of compositions, from syenite via granite to gabbroic cumulates, which suggests the existence of a well-differentiated plutonic body at depth.
U-Pb zircon and Pb evaporation datings yielded magmatic ages of ca. 1,100 to 1,200 Ma, and a conformable age has been obtained by Rb-Sr whole rock dating. Detrital zircons from the micaceous schists of the Isbjørnhamna Group, which underlies the Skålfjellet Subgroup, show a poorly defined discordia with an upper intercept age of ca. 2,200 Ma and a lower intercept age of ca. 360 Ma. These dating results define the magmatic age of the granitic-gabbroic rocks as late Mesoproterozoic, early Grenvillian. This age is in broad agreement with that of the metavolcanic rock clasts of the Pyttholmen meta-pyroclastic-conglomeratic unit at Vimsodden, which is considered to be the westernmost occurrence of the Skålfjellet Subgroup.
A Rb-Sr whole rock age determination of the shaly phyllites from the Deilegga Group was performed in order to place constraints on the age of younger Precambrian event; however, no good isochron was obtained.     

南极乔治王岛晚白垩世地幔源火山岩Nd、Sr同位素和微量元素证据

对乔治王岛上白垩统半三角组火山岩样品进行了 Rb- Sr同位素等时线年龄测定 ,所获结果为 71 .3 3± 0 .3 Ma。通过 εND(T) - 14 7Sm/14 4Nd,14 3 Nd/14 4Nd- 87Sr/86 Sr,87Sr/86 Sr- Sr和 87Sr/86 Sr- K2 O/(K2 O+Na2 O)的相关性研究表明 ,该火山岩物质主要来自亏损地幔 (DMM)源区 ,基本上未受到地壳物质混染。其中 6个样品的平均 Sm- Nd模式年龄 (TNd DM)为 443 .3± 2 0 .6 Ma,可能反映了在研究地区的岩浆源区化学分异事件的时代。微量元素的特点表明 ,半三角组样品属典型的钙碱性火山岩系列 ,与长城站地区第三系火山岩相类似 ,同属于岛弧火山活动的产物。     

Geology of Gjelsvikfjella and western Mühlig-Hofmannfjella, Dronning Maud Land, east Antarctica

As a part of the Norwegian Antarctic Research Expedition 1984/85, geological mapping was performed in Gjelsvikfjella and western Mühlig-Hofmannfjella, Dronning Maud Land. The northern part of Gjelsvikfjella is dominated by the Jutulsessen metasupracrustals which have been intruded by a major gabbroic body and several generations of dykes. To the south the metasupracrustals gradually transform into the Risemedet migmatites. In western Mühlig-Hofmannfjella the bedrock is dominated by the large Svarthamaren Charnockite batholith. The batholith is bordered by the Snøtoa metamorphic complex outcropping to the south and west in Mühlig-Hofmannfjella and it is characterized by a high content of partly assimilated country rock inclusions. Mineral paragenesis and geothermometry/geobarometry suggest a two-stage tectonothermal-igneous history with an initial intermediate pressure, upper amphibolite to granulite facies metamorphism followed by high temperature transformations related to the charnockite intrusion. The age of the initial tectonothermal event is probably about 1,100 Ma. Geochronological work in the present study (Rb/Sr whole rock) gave an age of 500 ± 24 Ma for the Svarthamaren Charnockite, interpreted to record the age of crystallization. Late brittle faulting and undeformed dolerite dykes outcropping in Jutulsessen are believed to be related to Mesozoic crustal stretching in the Jutulstraumen-Pencksøkket Rift Zone to the west.     

Grenvillian single-grain zircon Pb age of a granitic rock from the southern island of Hesteskoholmen, Liefdefjorden, northwestern Spitsbergen, Svalbard

A N-S trending, narrow zone of crystalline basement occurs from Biscayarhalvøya to Holtedahlfonna in northwestern Spitsbergen and is composed of various metasedimentary and igneous rocks, including granites. Previous isotopic age determinations on these rocks are by the K-Ar. Rb-Sr, ⁴⁰Ar/³⁹Ar and conventional zircon U-Pb method and yielded the Caledonian and Grenvillian ages. The single-grain zircon Pb evaporation method has recently been applied to solve complex problems and this is the first report by the method.
A granitic rock, syntectonically intruded into the phyllitic metasediments of the Biscayarhuken formation, which is the uppermost lithotectonic unit in the metamorphic rocks of the zone, was dated on four zircon grains, yielding a narrow age range from 955 ± 4 to 968 ± 9 Ma in average. This age of ca. 960 Ma is considered to be the age of intrusion, based on the occurrence and zircon morphology, which is roughly simultaneous with the formation of the phyllitic cleavages of the surrounding metasediments. The data obtained imply that the Caledonian events did not reset the Pb isotope system of zircon and major metamorphism occurred during the Grenvillian time in the Biscayarhuken formation, accordingly, the protolith age of the metasediments is Mesoproterozoic.     

Pb-Pb single-zircon ages of granitoid boulders from the Vendian tillite of Wahlenbergfjorden, Nordaustlandet, Svalbard

There are three areas in eastern Svalbard where Vendian tillites are exposed: eastern Ny Friesland, western Nordaustlandet (north and south of Murchisonfjorden) and further east in inner Wahlenbergfjorden, near Aldousbreen. Clasts within the massive unmetamorphosed clay-mica-carbonate matrix of the tillites include carbonates, sandstones, siltstones, metavolcanics, schists and different granitoids, the metamorphic and igneous rock types being more frequent in the upper levels of the formation. Large granite boulders, up to 1 m in diameter, are known from the easternmost outcrop at Aldousbreen. Three granitoid boulders from the Aldousbreen outcrop, differing in petrography and chemistry. have been dated by the Pb-Pb singlezircon method. They yield ages of 2830 ± 5 Ma, 1802 ± 4 Ma and 1497 ± 26 Ma. These clasts also differ in petrography, chemistry and age from all known granitic rocks on Nordaustlandet, which have recently yielded Grenvillian (950-960 Ma) and Caledonian (ca. 410 Ma) ages. The concentration of large granitic clasts in the easternmost known tillite outcrops suggests derivation from the east and/or south. Possible areas include those beneath the ice of Austfonna and below the Carboniferous strata of southeastem Nordaustlandet. The apparent lack of a significant Grenvillian overprint suggests the possibility of a more distant source.     

西南极乔治王岛菲尔德斯半岛火山岩同位素年代及地层对比

以K-Ar,(39)~Ar-(40)~Ar和Rb-Sr法对菲尔德斯半岛的火山岩进行了同位素测年。实验结果表明,半岛的火山作用从晚古新世开始,可能一直延续至中中新世的早期。其间,碧玉山段的岩石分别以Rb-Sr和(39)~Ar-(40)~Ar法测定了形成年龄,年龄值为54Ma和55Ma,属于晚古新世;岛上广泛分布的玛瑙滩段熔岩的K-Ar年龄区间为45～50Ma,为早始新世;化石山段从中始新世到渐新世;而最晚形成的岩块山段的时代为中中新世的早期。这些数据,与地质和古生物证据一起,对了解该区地层的相互关系和讨论火山作用提出了正确的认识。     

Grenvillian U-Pb zircon ages of quartz porphyry and rhyolite clasts in a metaconglomerate at Vimsodden, southwestern Spitsbergen

Proterozoic metasupracrustal rocks form a NNW-SSE trending basement zone along the western coast of Spitsbergen. The rocks show complex structures as a result of both Caledonian and Tertiary deformation, and most of the subordinate metaigneous rocks are not suitable for isotopic age determination. Some zircon-bearing rocks were found in the southwestern part of Spitsbergen and an attempt of U-Pb dating was performed.
U-Pb dating was carried out on zircon fractions from quartz porphyry and rhyolite clasts in a metaconglomerate unit of the Pyttholmen Formation northwest of Hornsund, southwestern Spitsbergen. The Pyttholmen Formation is considered to be a lateral equivalent of the upper part of the Gulliksenfjellet quartzite and in the same time as the upper part of the Skålfjellet metavolcanites. Therefore, the obtained ages are applicable to the age of the Skålfjellet igneous activities. Some of the dated samples are strongly schistose and their magmatic origin is difficult to confirm; the interpretation of the isotopic results is not well constrained; however, some explanations are possible which refer to the known geological conditions; an igneous age of siliceous volcanic rocks of ca. 1200 Ma, inherited zircon ages of ca. 2500 Ma and a regional metamorphic age of ca. 930 Ma. The last age belongs to the Grenvillian period and is conformable with the Rb/Sr whole rock age obtained from the garnet-biotite schists of the Isbjernhamna Group underlying the Skålfjellet metavolcanites.     

南极中山站地区正长花岗岩及花岗伟晶岩脉的~(40)Ar/~(39)Ar同位素年龄测定

本文详细报道了用 4 0 Ar/3 9Ar定年法测定的二个黑云母样品的数据结果。样品来自拉斯曼丘陵米洛岛北部中国南极中山站附近。一个取自正长花岗岩 ,另一个取自侵入正长花岗岩的花岗伟晶岩脉。正长花岗岩的 4 0 Ar/3 9Ar坪年龄为 494.4± 1 .4Ma,花岗伟晶岩脉的 4 0 Ar/3 9Ar坪年龄为 485 .8± 0 .6 Ma。两者为区域泛非热事件晚期阶段的产物。     

Geological and palaeomagnetic significance of the Kulgera Dyke Swarm, Musgrave Block, NT, Australia

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The Kulgera Dyke Swarm consists of olivine tholeiites which have intruded late Proterozoic transitional-granulite gneisses and granites of the eastern Musgrave Block, in central Australia. Preliminary Rb/Sr results suggest that the dolerites were emplaced at 1054 ± 14 Ma. In addition, a Rb/Sr age of 1060 ± 10 Ma on a biotite from a pegmatite indicates thermal resetting of the country rock minerals during dyke emplacement. Palaeomagnetic investigations of the dykes yield a primary thermoremanent magnetization direction corresponding to a palaeomagnetic pole at 17S, 266E ( A ₉₅= 12). In addition to this primary magnetization, an overprint component was present in many of the samples, providing a palaeomagnetic pole at 30S, 138E ( A ₉₅= 24), which is similar to previous results from other central Australian rocks affected by the Alice Springs Orogeny. The results extend the area of influence of the Carboniferous Alice Springs Orogeny southward into the Musgrave Block. Further, the results provide no evidence for an earlier, Late Proterozoic, Petermann Orogeny affecting the Musgrave Block in the Kulgera region. However, the possibility that a Petermann Orogeny thermal overprint has been erased by the Alice Springs Orogeny cannot be dismissed.     

Rb–Sr,Sm–Nd,and Ar–Ar isotopic systematics of lherzolitic shergottite Yamato 000097

Isotopic analysis of the Martian lherzolitic shergottite Yamato 000097 yields a Rb–Sr age of 147 ± 28 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.710490 ± 0.000072, a Sm–Nd age of 152 ± 13 Ma with an initial ɛ¹⁴³Nd-value of +11.7 ± 0.2, and a ³⁹Ar–⁴⁰Ar age of ∼260 Ma. The near concordance of these ages, in combination with the Rb–Sr and Sm–Nd initial isotopic signatures, suggests that Yamato 000097 crystallized from low Rb/Sr, light rare earth element depleted source materials ∼150 Ma ago. Although the obtained ³⁹Ar–⁴⁰Ar age is significantly higher than the Rb–Sr and Sm–Nd ages, Yamato 000097 shows little or no evidence of trapped Martian atmospheric ⁴⁰Ar. The trapped ⁴⁰Ar concentration of Yamato 000097 is similar to that of Zagami, suggesting that both basaltic and lherzolitic shergottites may have similarly inherited excess ⁴⁰Ar from their magmas.The Rb–Sr and Sm–Nd ages, and initial ⁸⁷Sr/⁸⁶Sr and ɛ¹⁴³Nd-values of Yamato 000097 and Yamato 793605 lie on the same isotopic ingrowth curves, suggesting that they came from very similar mantle sources. Allan Hills 77005 was also probably derived from the same source, but Lewis Cliff 88516 appears to be from a distinct but similar source. Yamato 000097 represents the most recent known magmatism from its source, and is the youngest Martian meteorite for which concordant Rb–Sr and Sm–Nd ages have been determined.     

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommersø Dolerites, eastern North Greenland

Summary. Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersø Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts.
After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane.
Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A ₉₅= 3.8°; dolerites: 6.9°S, 62.0°E with A ₉₅ = 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W.     

Thermal history of the Cretaceous Sindong Group, Gyeongsang Basin, Korea based on fission track analysis

ABSTRACT Apatite and zircon fission track (FT) analyses were carried out to reconstruct the thermal history of the Lower Cretaceous Sindong Group, which is the lowermost stratal unit of the Gyeongsang Basin, Korea. Zircon FT central ages show a wide range from 83 ± 5 to 157 ± 18 Ma, and single-grain age spectra have multiple age populations, whereas all apatites have very consistent FT ages of c . 60 Ma, suggesting a totally reset cooling age. Co-existence of both older and younger ages compared with the depositional age and relatively short mean track length indicate that the Sindong zircons were partially annealed. The Sindong Group was heated into the zircon partial annealing zone (ZPAZ) around 80 Ma, and cooled below the apatite closure temperature at c . 60 Ma. Based on the zircon FT results combined with vitrinite reflectance data, the maximum palaeotemperature to which the Sindong Group had been subjected can be inferred to be about 260 °C. Zircon FT data from a granite body that is in contact with the Sindong Group and sandstones close to the granite body indicate that thermal influence caused by Upper Cretaceous intrusive rocks was limited in close vicinity of the intrusion and that the major heat source of the Sindong Group was burial. The thickness of uplifted and eroded section is estimated to be about 7 km.     

Sm–Nd and Rb–Sr studies of lherzolitic shergottite Yamato 984028

The distribution pattern of the trace elements Rb, Sr, Nd and Sm for Yamato 984028 (Y984028) is consistent with its classification as a lherzolitic shergottite. The Sm–Nd mineral isochron of this lherzolitic shergottite defines its age to be 170 ± 10 Ma for an initial ?_Nd = +11.6 ± 0.2. The corresponding Rb–Sr mineral isochron yields an identical age of 170 ± 9 Ma and an initial ⁸⁷Sr/⁸⁶Sr = 0.710389 ± 0.000029. The concordant Sm–Nd and Rb–Sr isochron ages suggest that Y984028 crystallized 170 ± 7 Ma ago contemporaneously with five other lherzolitic shergottites and ten enriched basaltic and olivine-phyric shergottites. The age, Sr- and Nd- isotopic signatures further suggest that Y984028 and Y-793605, and also probably Y000097 could come from a single magmatic body. Using a two-stage evolution model, the time-averaged ⁸⁷Rb/⁸⁶Sr-ratio for the mantle source of the parent magma of Y984028 is ～0.182, within the range of 0.178–0.182 that has been reported for other lherzolitic shergottites. The corresponding time-averaged ¹⁴⁷Sm/¹⁴⁴Nd-ratio for the source mantle of its parent magma is super-chondritic at ～0.217, implying its source was a depleted mafic part of the Martian mantle similar to that of diabasic shergottite Northwest Africa (NWA) 1460. Rb, Sr, Sm and Nd distributions in Y984028 are likely produced by pyroxene and olivine accumulation, probably from a NWA 1460-like parental melt, in an intrusive magma body.     

南设得兰群岛利文斯顿岛鲍勒斯山组火山岩的~(40)Ar/~(39)Ar年龄及地质意义

采用４０Ａｒ／３９Ａｒ法和激光微区等时线方法，对南极南设得兰群岛利文斯顿岛东部产出的鲍勒斯山组火山岩重新进行了年龄测定。结果表明，４５Ｄ安山质熔岩和２６２玄武安山质熔岩的等时年龄分别为（１０５．６２±２．１１）Ｍａ和（１１１．４８±２．２３）Ｍａ，其４０Ａｒ／３９Ａｒ初始比与尼尔值相当，岩石中不含过剩氩，表明等时年龄代表岩石真正的形成年龄。脉状产出的Ａ９３５２细晶岩的４０Ａｒ／３９Ａｒ坪年龄为（９６．７±１．６）Ｍａ，计算得到等时年龄为９６．６Ｍａ，晚于熔岩喷发而形成。新的年龄数据证实了鲍勒斯山组火山岩形成于白垩纪。白垩纪火山活动曾影响了整个利文斯顿岛，在赫德半岛出露的鲍勒斯山组火山岩曾被较年轻火山岩所覆盖，构造抬升使其出露。     

CHARACTERISTICS OF Nd AND Sr ISOTOPES AND TRACE ELEMENTS FOR LATE CRETAEOUS VOLCANIC ROCKS IN KING GEORGE ISLAND, ANTARCTICA: IMPLICATIONS FOR SOURCE OF THE VOLCANICS FROM DEPLETED MANTLE

Rb-Sr isotopic isochron dating of the volcanic rock samples from the Upper Cretaceous Half Three Point Formation on the King George Island is 71.33±0.3 Ma. Correlative study of _(εND)(T)-~(147)Sm/~(144)Nd, ~(143)Nd/~(144)Nd-~(87)Sr/~(86)Sr, ~(87)Sr/~(86)Sr-Sr and ~(87)Sr/~(86)Sr-K_2O/(K_2O+Na_2O) indicated that the volcanic rocks were chiefly derived from the depleted mantle source and generally were not mixed crust materials. Of the samples 6 were given the mean Sm-Nd model age (T_(DM)~(Nd) of 443.3±20.6 Ma possibly indicating the age of chemical variation event in the magma source of the study area. Features of the trace elements indicated that the rocks from the Half Three Point Formation are of typical eale-alkaline volcanic suite and similar to those from the Tertiary volcanic rocks of the Fildes Peninsula, being the same products of the island-arc volcanic activity.     

东南极普里兹湾Sφstrene岛麻粒岩变质年代及其与邻区的关系

对东南极普里兹湾Ｓｓｔｒｅｎｅ岛石榴辉石麻粒岩的变质年龄通过Ｓｍ－Ｎｄ法做了重新测定，得出６０４±２８（２σ）Ｍａ的石榴石－辉石－斜长石－全岩矿物内部等时线年龄，并指出本数值代表了该区主期中压麻粒岩相变质年龄。至于与其北东部的Ｂｏｌｉｎｇｅｎ群岛、拉斯曼丘陵的关系，通过岩石组合，变质叠加及其空间变化特征并结合变质时代推断，自Ｓｓｔｒｅｎｅ岛至拉斯曼丘陵后期低压变质叠加逐渐增强，早期中压及晚期低压变质可能代表了同一变质旋回的不同阶段     

HOLOCENE ROCKWALL RETREAT AND THE ESTIMATION OF ROCK GLACIER AGE, PRINS KARLS FORLAND, SVALBARD

This paper presents a simple analytical model for estimating rock glacier age by coupling the ratio of frontal advance to total rock glacier length and the ratio of debris volume in the rock glacier to present debris flux in the talus cone–rock glacier transition zone. The model was applied to two rock glaciers at Prins Karls Forland, Svalbard. By assuming volumetric debris contents in the deforming layer of the rock glacier of between 0.3 and 0.4, we obtained age estimates for the rock glaciers of between 12 ka and 24 ka. The corresponding average rockwall retreat rates are between 0.30 and 0.62 mm a^‐1. Considering the limitations of the model, we suggest a minimum age of 13 ka for the initiation of rock glacier development. Using this age, rockwall retreat rates for the seven rock glaciers investigated at Prins Karls Forland are between 0.13 and 0.64 m ka^‐1 (assuming the volumetric debris content for the whole rock glacier/talus cone is 0.35). The model requires further testing on other datasets, better field estimates of the debris content and depth of the deforming layer, and could also benefit from the inclusion of an unsteady debris supply function in order to refine age estimates.     

Quaternary bedrock erosion and landscape evolution in the Sør Rondane Mountains, East Antarctica: Reevaluating rates and processes

Rates and processes of rock weathering, soil formation, and mountain erosion during the Quaternary were evaluated in an inland Antarctic cold desert. The fieldwork involved investigations of weathering features and soil profiles for different stages after deglaciation. Laboratory analyses addressed chemistry of rock coatings and soils, as well as ¹⁰Be and ²⁶Al exposure ages of the bedrock. Less resistant gneiss bedrock exposed over 1 Ma shows stone pavements underlain by in situ produced silty soils thinner than 40 cm and rich in sulfates, which reflect the active layer thickness, the absence of cryoturbation, and the predominance of salt weathering. During the same exposure period, more resistant granite bedrock has undergone long-lasting cavernous weathering that produces rootless mushroom-like boulders with a strongly Fe-oxidized coating. The red coating protects the upper surface from weathering while very slow microcracking progresses by the growth of sulfates. Geomorphological evidence and cosmogenic exposure ages combine to provide contrasting average erosion rates. No erosion during the Quaternary is suggested by a striated roche moutonnée exposed more than 2 Ma ago. Differential erosion between granite and gneiss suggests a significant lowering rate of desert pavements in excess of 10 m Ma^− 1. The landscape has been (on the whole) stable, but the erosion rate varies spatially according to microclimate, geology, and surface composition.     

~(40)Ar/~(39)Ar IsOTOPIC DATING THE BIOTITES FROM THE IGNEOUS AND METAMORPHIC ROCKS OF THE ZHONGSHAN STATION AREA

The Zhongshan Station of China is located in the Larsemann Hills, East Antarctica. Low pressure granulite facies gneisses together with late granites are outcroped in the region. Three biotite samples from a garnet segregation, a syenogranite and a granite-pegmetite were measured with ~(40)Ar/~(39)Ar incremental heating technique. Biotites from the garnet segregation give an ~(40)Ar/~(39)Ar plateau age of 504±1Ma. Biotites from the syenogranite yield an ~(40)Ar/~(39)Ar plateau age of 494±1 Ma. Biotites from the granite-pegmatite give an ~(40)Ar/~(39)Ar plateay age of 486±1Ma. They verify 500 Ma thermal event called "Pan African event" by previous K-Ar and Rb-Sr data. They are cooling ages of the biotites when the paleogeotherm of the area droped to the K-Ar closure temperature for biotite.