Geochemistry of Precambrian gneisses: relevance for the evolution of the East Antarctic Shield

Archaean granulite-facies orthogneisses of the Napier Complex in Enderby Land, metamorphosed 3070 Maago, comprise two chemically distinct suites. The more abundant, mainly of tonalitic to granodioritic composition, shows strong Y depletion, explicable by hydrous partial melting of a garnet-bearing source (garnet amphibolite or possibly eclogite); it apparently represents new continental crust. Other gneisses (predominantly of trondhjemitic to granitic (s.s) composition) do not show Y depletion, and have higher TiO₂, Zr, Nb, La, Ce and Ga/Al, and lower CaO, Sr and Mg/(Mg + total Fe); they probably originated by relatively dry melting of predominantly felsic crystal rocks. Both suites show evidence for loss of Rb (relative to K), Th, and U during metamorphism. Late Archaean (−2800 Ma) amphibolite-facies gneisses of MacRobertson Land are of ‘undepleted’ type and may be representative of a higher crustal level than those of Enderby land. Late Proterozoic (1000 Ma) granulite-facies gneisses of Enderby Land (Rayner Complex) are to a large extent remetamorphosed Napier Complex rocks of igneous derivation; in contrast, gneisses of similar age in MacRobertson Land include a much higher proportion derived, either directly or by partial melting, from sedimentary protoliths.     

Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Preliminary isotopic data for Late Proterozoic (～ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (～ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (～ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons.As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K₂O₂, Rb, Pb, and Th, and decreases in CaO, Na₂O₂ and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.     

Age and Geochemical Characteristics of a Mafic Dyke Swarm in the Archaean Vestfold Block, Antarctica: Inferences about Proterozoic Dyke Emplacement in Gondwana

Archaean gneisses in the Vestfold Block, Antarctica are cutby abundant tholeiite and rare alkaline dykes. At least fivegenerations of dykes have been recognized on the basis of intrusiverelationships, petrography and geochemistry. Rb-Sr isotopicdata indicate that intrusion of the tholeiites occurred overa period of c. 1000 Ma, during three clearly defined events(viz. c. 2400, 1800, and 1400 Ma). Dykes in the southwesternpart of the Vestfold Block were recrystallized during a lateProterozoic (c. 1000-1100 Ma) high-grade event. Mineral-wholerock Rb-Sr isotopic data show that the terrain was also variablyaffected by an even younger thermal event c. 500 Ma ago whichis correlated with the Pan-African Orogeny. Isotopic, major element and trace element data suggest thatthe tholeiite dyke suites were derived by varying degrees ofpartial melting of compositionally distinct, in some cases ratherheterogeneous subcontinental mantle source regions, combinedwith variations in the extent and nature of crystal fractionation.Extensive crustal contamination does not appear to have playeda significant role in determining compositional trends withinthe different suites, although minor contamination cannot bediscounted. The oldest dykes, a diverse group of high-Mg tholeiites,and the youngest, c. 1400 Ma tholeiites, appear to have beenderived from isotopically similar source regions, which werenevertheless characterized by quite different abundances ofhighly incompatible elements (Pb, Rb, Ba, Th, K, La, and Ce).1800 Ma tholeiites were derived from a more radiogenic mantlesource region, characterized by long-term enrichment in incompatibleelements. Such enrichment is interpreted to be a consequenceof metasomatism of their mantle source region. 2400 Ma high-Mg dykes in Enderby Land have virtually identicalchemical and isotopic compositions to those in the VestfoldBlock, indicating generation from a similar source region; however,1200 Ma tholeiites were apparently derived from a more radiogenicsource region like that of the c. 1800 Ma Vestfold Block dykes.Hence, both Sr isotopic and geochemical data indicate that theancient subcontinental lithospheric mantle beneath Gondwanawas extremely heterogeneous.     

Hydrothermal alteration in volcanic rocks,eastern part of the Lukavice Group, Železné Hory Mountains,Czech Republic

Many rocks mapped as felsic metavolcanics in the eastern part of the Lukavice Group are shown to be altered mafic metavolcanics, similar to those in the Noranda and Flin Flon-Snow Lake mining districts, Canada. The relatively fresh rocks of the Lukavice Group are rhyolite, dacite-andesite, and andesite-basalt of calcalka-line character. Assuming no substantial volume change during alteration, Ti, P, La, Ce, Yb, Lu, Th (partly), Sc and V contents remained unchanged. Altered rocks are enriched in (Fe + Mg), K and Si and depleted in Na, Ca and Zr. Some elements show both increased and decreased contents in altered rocks (Mg, Ba, Sm, eu, Tb and Hf). Although hydrothermal alteration in the Lukavice Group is of large extent, it is of the proximal Kuroko style and not of regional Amulet Rhyolite style. Implications for a large hydrothermal system within a volcanic pile are discussed in relation to the Ordovician Lukavice Group and its mineral deposits and to some other parts of the Bohemian Massif with volcanosedimentary sequences of the same age.     

Metaterrigenous rocks of the Irkut granulite-gneiss Block as indicators of the evolution of the Early Precambrian crust

Major, trace element, and Sm-Nd isotope data are presented for the garnet-biotite and cordierite-garnet-biotite gneisses from the Early Precambrian granulite complex of the Irkut Block (Sharyzhalgai Uplift, Siberian Craton). The garnet-biotite and cordierite-bearing gneisses of the Irkut Block were formed owing to the granulite metamorphism of metaterrigenous rocks. The chemical index of weathering and the content of clayey (pelitic) components in the normative mineral composition increase from the garnet-biotite gneisses to the cordierite-bearing gneisses, thus reflecting the maturation degree of initial sediments. Protoliths of the studied paragneisses correspond to a rock series ranging from the graywacke siltstones to clayey rocks. The trace and rare-earth element distribution indicates that the terrigenous material of the paragneisses was derived from felsic and mafic provenance. Increase in contents of Fe, Ti, Cr, Ni, and Sc and the Cr/Th ratio and decrease in the La/Sc ratio from the garnet-biotite to the cordierite-bearing gneisses reflect growth of the abundance of mafic rocks in the provenance. Potential sources of the detrital material were intermediate-felsic and mafic volcanic rocks (orthogneisses and basic crystalline schists) of the Irkut Block. The paragneisses show a distinct negative Eu anomaly (Eu/Eu* = 0.38–0.85), which suggests the input of crustal melting products, such as the potassium granites. A wide range of model Nd age (T_Nd(DM) = 2.4–3.1 Ga) of the paragneisses indicates the Archean to Early Paleoproterozoic age of their protoliths. The complex of isotopic, geochemical, and geochronological data, as well as the character of association of metaterrigenous rocks (mature pelites and carbonate rocks included), implies that sedimentation was separated in time from volcanism. The sedimentation was preceded by metamorphism, granite formation, and tectonic stabilization of the Irkut Block crust.     

吉南天桥地区基性岩墙与其围岩TTG片麻岩的成因及构造意义

吉南地区太古宙基底中发育大量早前寒武纪基性岩墙群,是陆壳伸展的直接证据。对白山市东部天桥太古宙基底出露区内基性岩墙及其围岩进行了锆石U-Pb定年和地球化学分析,以确定该期伸展事件的形成机制及地质意义。天桥地区基性岩墙岩性为斜长角闪岩,侵位于TTG片麻岩中。英云闪长质片麻岩(TN1)中锆石具核-边结构,岩浆核的LA-ICP-MS测年结果为2500±6Ma,指示其形成于新太古代末期。天桥岩墙(TN3)中的锆石内部结构与TN1相同,酸性岩浆核的SHRIMP测年结果为2490±17Ma,与TN1在误差范围内一致,表明这些锆石不是基性岩墙原生锆石,而是岩墙侵位过程中在围岩中捕获的锆石,但根据岩墙仅侵位在太古宙基底中且变质程度高于周围古元古界老岭群,将其侵位年龄大致限制在新太古代末期-古元古代早期。地球化学特征显示,基性岩墙具有低SiO_2、Na_2O、K_2O含量,高CaO、MgO含量,A/CNK=0.56～0.59,属于准铝质的拉斑玄武岩系列岩石,∑REE低、配分曲线平坦,富集LILE(Rb、Ba和K),亏损HFSE(Th、U、Nb和Ta),具有与原始地幔相同的Nb/Ta、Zr/Hf比值及接近地壳的Nb/U、Ta/U比值,指示其岩浆可能来源于地幔且在上升过程中受到地壳混染,形成于板内伸展环境。TTG片麻岩具有中等的SiO_2和MgO含量,高Al_2O_3和Na_2O含量以及低CaO含量,A/CNK=1.00～1.14,属弱过铝质的钙碱性系列岩石,∑REE低、具有右倾的REE配分曲线,轻稀土富集、重稀土亏损,富集LILE(Rb、Ba、K和Sr),强烈亏损HFSE(U、Nb、Ta、Sm和Ti),其岩浆可能来源于变质玄武质岩石和极少量沉积岩的部分熔融,结合邻区TTG的研究成果,认为其形成于与俯冲相关的活动大陆边缘环境。前人研究表明,新太古代晚期板块构造体制可能已经启动,结合我们以往研究,认为新太古代晚期华北克拉通东北部可能发生了弧陆碰撞造山运动,天桥岩墙的侵位标志着新太古代末期至古元古代早期之间华北克拉通东北部进入造山后伸展环境,可能是对新太古代造山运动结束的响应。     

Diversity of Archean crust in the eastern Tula Mountains,Napier Complex,East Antarctica

The Napier Complex of Enderby and Kemp Lands forms the north-western part of the East Antarctic Shield and consists predominantly of gneisses and granulites metamorphosed during a ca. 2.8 Ga high-grade and a ca. 2.5 Ga ultra-high temperature event. The western segment of the Napier Complex includes coastal outcrops, islands and nunataks around Amundsen and Casey Bays, and the Tula Mountains. This region records some of the highest metamorphic temperatures measured on Earth, affecting a variety of gneisses as old as ca. 3.8 Ga. Five samples of orthogneiss from the less-studied eastern Tula Mountains, including three granitic, one trondhjemitic and one dioritic gneiss, were dated by zircon U-Pb Secondary Ion Mass Spectrometry (SIMS). The three orthogneisses yield protolith ages of 3750 ± 35 Ma (granitic), 3733 ± 21 (trondhjemitic) Ma and 3560 ± 42 Ma (dioritic), whereas the two other granitic orthogneisses record ages of 2903 ± 14 Ma and 2788 ± 24 Ma. Zircon growth during metamorphism occurred at 2826 ± 10 Ma, and also between 2530 Ma and 2480 Ma. Samples from the Tula Mountains can be geochemically subdivided into Y-HREE-Nb-Ta depleted and undepleted groups. Eoarchean granitoids are included in both geochemical groups, as are Meso- and Neoarchean granitoids. The Y-HREE-Nb-Ta depleted granitoids can be generated by medium- to high-pressure melting of mafic crust, whereas undepleted granitoids can be generated by low-pressure melting. However, relatively high potassium contents in most samples, and the presence of xenocrystic/inherited zircon in some, reflect the likely involvement of felsic crustal sources. This diversity in granitoid composition occurs across the Napier Complex. The lack of a simple correlation between protolith age and geochemical type is an indication that magmatism during the Eoarchean (and later) involved diverse sources and processes, including re-melting and recycling of various crustal components, rather than just the formation of juvenile crust.     

On the origin of early Archaean gneisses: A reply

The use of metamorphosed basic dykes as one of the most important single field criteria for subdividing gneisses in high grade areas into different chronological units is defended. The universal applicability of the lower greenstone-granite-upper greenstone model to all Archaean terrains is questioned using documented sequences of events in the North Atlantic craton. We prefer a chronology based on field criteria to one based on the application of theoretical crustal development models taken from one tectonic environment and used to explain the sequence of events in another.It is shown that the average K₂O and Rb content from the 3600 m.y. sialic rocks of the North Atlantic craton ranges between 2.26 (Amîtsoq grey gneisses, Greenland) to 2.66 (Uivak grey gneisses, Labrador). Average K/Rb ratios are respectively 200 and 177, Rb/Sr, 0.33 and 0.29 for the two areas. K and Rb values are thus markedly higher than those reported from most other Archaean gneiss suites. Secondary redistribution of K and Rb at about 3600 m.y. is demonstrated by the documentation of the massive addition of these elements to basic rocks included in the gneisses. Whole sale addition of alkalies during migmatisation to the level of crust now exposed is postulated as one explanation of the unusually high K and Rb contents. It is argued on statistical grounds that if Rb metasomatism occurred it is not possible to use low initial Sr ratios alone to preclude the possibility that part of the Archaean gneiss complexes consist of tonalitic gneiss which are much older than conventional Sr₀ interpretations allow.     

沂沭断裂带高桥盆地早白垩世火山岩的地球化学及岩石成因

沂沭断裂带内高桥盆地早白垩世火山岩SiO2含量为51.97%～68.94%;由玄武粗安岩、 粗面岩和流纹岩组成;都属于碱性岩。岩石富集Rb、 Ba、 K等大离子亲石元素和轻稀土元素;相对亏损Nb、 Ta、 Ti等高场强元素和重稀土元素;并具有富集的Sr-Nd-Pb同位素组成。钾质粗面岩具有高稀土Cr、 Ni含量、 La/Yb、Sr/Y和Th/U高比值;这类似于华北克拉通东南缘早白垩世富集岩石圈来源的基性岩（如方城玄武岩、 沂南辉长岩）;其可能主要来源于富集岩石圈地幔部分熔融。与之相比;钠质玄武粗安岩具有低Cr、 Ni含量 、 87Sr/86Sr（t）、 Th/U和高εNd（t）值;表明它可能由岩石圈地幔熔体与软流圈物质混合而成。沂沭断裂带高桥盆地火山岩形成于岩石圈伸展背景下;沂沭断裂带的活动可能诱使软流圈物质的上涌;导致岩石圈地幔升温发生部分熔融;并为软流圈物质的上升提供了通道。     

青藏高原西北缘第四纪火山岩的年龄、地球化学特征及意义

对青藏高原西北缘普鲁北钾质火山岩进行40Ar-39Ar定年得到的最新年龄为0.81Ma±0.21Ma。火山岩的SiO2含量为53.08%~55.12%,全碱含量为7.36%~8.04%,K2O/Na2O比值为1.17~1.2,属橄榄安粗岩系列。岩石以高度富集LREE、LILE(K、Rb、Sr、Ba、Th等)和高度亏损HREE、HFSE(Nb、Ta、Ti等)元素为特征。岩石具高的(87Sr/86Sr)i比值(0.7088~0.7089)和低的εNd值(-6.05~-5.54),表明火山岩源区为EMⅡ型的经古俯冲作用改造的大陆岩石圈富集地幔。同时,其Ti/Y、Zr/Y、La/Rb、Zr/Rb、K/La、Pb/La比值类似于板内洋岛玄武岩(OIB),可能暗示岩浆源区经历过软流圈流体或者深部流体对古俯冲地幔楔的叠加交代作用。普鲁北钾质火山岩为青藏高原西北缘晚新生代陆-陆碰撞造山深部岩石圈拆沉作用在地表响应的岩浆产物。     

Geochemistry of granulite‐facies granitic rocks from Battye Glacier,northern Prince Charles Mountains,East Antarctica

Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na₂O and K₂O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.     

Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen,West Greenland

In the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland reworked Archaean and juvenile Proterozoic orthogneisses occur side by side and are difficult to differentiate in the field. Archaean gneisses have tonalitic to trondhjemitic compositions with relatively low Al₂O₃ and Sr, and may have been derived from magmas formed by melting of basaltic or amphibolitic rocks at moderate pressures. The Proterozoic rocks are on average more mafic, and it is likely that they crystallised from mantle-derived magmas. Felsic varieties of the Proterozoic igneous suite probably formed from the original magma by fractional crystallisation, in which hornblende played an important role, and at SiO₂ > 65% Archaean and Proterozoic rocks have very similar major and trace element compositions (including REE), illustrating that different modes of origin may lead to very similar results.     

攀西茨达复式岩体年代学和地球化学:对峨眉山地幔柱活动时间的约束

茨达复式岩体位于中国西南扬子地台西缘的攀西裂谷内,其岩性从基性到酸性连续变化,SiO2含量为40.06％ ～68.54％,但以基性和酸性岩石为主,中性岩石较少,而且非常不均匀,通常具有斑杂构造特征.从基性岩到酸性岩,各岩石样品由轻稀土弱富集型变为较强富集型.微量元素表现为酸性岩中Rb、Th、K、La、Ce、Pb、Nd、Zr、Hf、Sm呈正异常和Ba、Nb、Ta、Sr、P、Ti的负异常;基性岩除Ti负异常和Pb正异常外,其它异常不明显;中性岩具有Ti、Sr负异常和Pb正异常,其它特征介于基性岩和酸性岩石之间.野外和岩相学特征明显指示出中性岩石具有混合特征.酸性端元岩浆准铝质的特征以及相对低的SiO2含量指示其起源于玄武质下地壳的部分熔融,而基性端元岩浆的地球化学特征以及高温特征暗示着其起源于地幔柱源区.锆石U-Pb年龄数据表明,该复式岩体中基性端元LA-MC-ICP-MS U-Pb锆石年龄为243.76±0.77Ma,酸性端元年龄为240.5±0.76Ma,可能代表了峨眉山大火成岩省岩浆活动的尾声阶段.     

Archaean crustal thickness of greenstone granite belts of South India

Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb?Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.     

Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.     

Geochemistry and petrogenesis of the post-orogenic bimodal dyke swarms in NW Sinai, Egypt: constraints on the magmatic–tectonic processes during the late Precambrian

The study area in the northwest Sinai represents one of the most significant regions in the Egyptian basement intensely invaded by post-orogenic calc-alkaline dyke swarms. Two post-orogenic dyke swarms have been recognized in NW Sinai namely: (1) mafic dykes of basalt, basaltic andesite and andesite composition and (2) felsic dykes of dacite, rhyodacite and rhyolite composition. These basaltic to rhyolitic dykes intruded contemporaneously and shortly after the intrusion of the post-orogenic leucogranite. The mafic and felsic dykes are enriched in incompatible elements, especially in the large ion lithophile elements (e.g. K, Rb, Ba) and depleted in high field strength elements with negative P, Ti and Nb anomalies. Major and trace element geochemistry indicates that investigated mafic and felsic magma types are not related via fractional crystallization. The protoliths of the mafic and felsic dykes appear to have evolved from different parental magmas. The incompatible trace element patterns favour a derivation of the mafic dykes from melting of a garnet peridotite source followed by fractional crystallization of olivine, clinopyroxene amphibole and zircon. The felsic dykes, on the other hand, could be generated by melting of garnet-free source modified subsequently by fractional crystallization of plagioclase, apatite and titanomagnetite. This implies variable source characteristics at the end of the Pan-African in the NW Sinai.The mafic and felsic dykes can be related to an intracontinental setting and that this was accompanied by a chemical evolution of the subcontinental lithosphere. Magma generation and ascent in the area was favoured by extensional movements, which is already known from other areas in NE Africa.     

辽南小黑山地区太古宙构造变形序列

采用地质调查和显微镜下观察方法,研究了辽南小黑山区太古宙岩石组成和构造变形特征。小黑山区太古宙岩石包括上壳岩、古老片麻岩和变基性岩脉,它们在小黑山变质岩体中呈包体出现。上壳岩由黑云变粒岩、条带状闪石磁铁石英岩组成;古老片麻岩为条带状角闪黑云斜长片麻岩、条带状角闪斜长片麻岩,原岩为英云闪长岩;变基性岩脉为斜长角闪岩和角闪石岩。上壳岩堆积之后有英云闪长岩侵位,基性脉侵位于上壳岩和英云闪长岩（古老片麻岩）。小黑山区太古宙岩石经历了2幕变形：D₁幕变形主要表现为褶皱构造（DF₁）、与褶皱轴面平行的面理（DS₁）、矿物线理（DL₁）;D₂幕变形在叠加褶皱作用下形成斜歪倾伏褶皱（DF₂）,面理和线理不发育。小黑山区太古宙变质岩中发育的变形序列、构造特征、变形特征、变质条件表明,这2幕构造形迹群属于中部构造相。D1幕变形形成逆冲推覆构造,D₂幕变形形成第Ⅲ型叠加褶皱,它们都是在同方向的水平挤压应力作用下的产物。     

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism

Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land. Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation. Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 m.y. ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous. Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies. These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce.Early Proterozoic (2,350±48 m.y.) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite. They tend to have higher ratios of highly to moderately incompatible elements (e.g., K/Zr, K/Ce), and larger Nb anomalies (i.e., higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions. Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement. Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite.     

Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland

Ph isotopic analyses are reported for 119 samples of late Archaean (ca. 3000-2800 Myr) calc alkaline orthogneisses and associated anorthosites from southern West Greenland. Over most of the area. $\text{Pb}\text{Pb}$ whole rock isotope systematics indicate derivation of the magmatic precursors of the gneisses and anorthosites from a source region with a typically mantle-type $\text{U}\text{Pb}$ ratio (μ₁ value of 7.5) at. or shortly before, ca. 3000-2800 Myr ago. In contrast, in the Godthaabsfjord region, late Archaean Nûk gneisses and associated anorthosites were emplaced into or through early Archaean (ca. 3700 Myr) Amîtsoq gneisses, and crystallised with variable proportions of two isotopically distinct types of Pb which commenced their respective crustal developments at ca. 3000-2800 Myr and at ca. 3700 Myr ago. Isotopic and other geochemical constraints demonstrate that Nûk gneisses and their temporal equivalents were not produced by reworking or melting of Amîtsoq gneisses. Mixing of early and late Archaean Pb results from contamination of the magmatic precursors of Nûk gneisses and anorthosites (characterised by mantle-type Pb at time of emplacement) with ancient, unradiogenic Pb derived from ca. 3700 Myr-old Amîtsoq-type continental crust invaded by the Nûk magmas. The contaminant is considered to be a trace-element enriched fluid phase released from dehydrating older continental crust during progressive burial and heating by emplacement of calc alkaline magmas in the late Archaean ‘accretion differentiation superevent’. This was followed by mixing of the released fluids with younger Nûk magmas.Pb isotopic compositions of late Archaean gneisses and anorthosites outside the Godthaabsfjord region provide no evidence for the presence of early Archaean Amîtsoq-type continental crust in southern West Greenland in areas more than a few tens of km outside the known outcrop of Amîtsoq gneisses. We suggest that early Archaean crust does not exist at depth in late Archaean areas with undisturbed Pb-isotope systematics, either in Greenland or elsewhere in the North Atlantic craton.Pb-isotope evidence for crust magma interaction, involving selective extraction of certain trace elements by a fluid phase from wall rock and subsequent mixing between magma and contaminant fluid, provides a powerful tool for detection, sub-surface ‘mapping’, and geochronological and geochemical characterisation of deep, ancient continental crust.     

Geochemistry of the Archean low- to high-grade transition zone,Southern India

The transition zone between Archean low- and high-grade rocks in southern India represents eroded crustal levels representative of 15–20 km. It is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite, both of which appear to have developed from the tonalitic gneisses.Tonalitic gneisses and charnockites are similar in major and trace elements composition while granitic gneisses are relatively enriched in Rb, K, Th, Ba and light rare earth element (REE) and depleted in Cr and Sc. All three rock types exhibit enriched light REE patterns with variable positive Eu anomalies. Total REE content decreases with increasing Eu/Eu and SiO₂ and with decreasing Fe₂O_3T and MgO in the tonalitic gneisses and charnockites.An internally consistent model for the production of the tonalitic gneisses involves partial melting of an enriched mafic source with variable ratios of hornblende to clinopyroxene. This source, in turn, is derived from an ultramafic mantle relatively enriched in incompatible elements. Granitic gneisses form from tonalitic gneisses by alkali metasomatism from chloride-bearing fluids with high H₂O/CO₂ ratios purged from the lower crust by CO₂, and charnockites are produced from tonalitic gneisses (and granitic gneisses) by ischochemical CO₂ metamorphism following the alkali metasomatism.