Geochemistry and geotectonic setting of Late Proterozoic Katangan basic rocks from Kibambale in central Shaba (Zaire)

The Late Proterozoic Katangan basic rocks (dolerites and basalts) outcropping at Kibambale in central Shaba (Zaire) are classified as extension-related tholeiites. The concentrations of immobile major and trace elements resemble those encountered in Phanerozoic enriched oceanic ridge basalts (E-MORB) emplaced in incipient oceanic rifts such as Red Sea and Tadjura Gulf troughs. This conclusion is also sustained by: elemental ratios which are MORB-like; multielement chondrite- and MORB-normalized patterns; sedimentological data, especially the shallow water conditions which prevailed during the deposition of lower Kundelungu tillite in which these basic rocks are interstratified; and the geological and structural framework, notably the emplacement of these igneous rocks in a basin not distant from the Kibaran continental land mass.In this area the geological units containing the igneous rocks are not affected by significant tangential tectonism and so the present structural relationships are relatively undisturbed compared with those which prevailed during the emplacement of the igneous rocks.     

Geochemistry of Precambrian mafic dikes,central Bighorn Mountains,Wyoming, U.S.A.

Precambrian quartz dolerites and metadolerites of the central Bighorn Mountains form dikes that intrude a Precambrian metamorphic and igneous terrane typical of the Laramide uplifts of the middle Rocky Mountains. They have a restricted range of major- and trace-element compositions and are typical of basalts in the middle stages of tholeiitic fractionation. Fractionation in the direction of iron enrichment occurred by removal of plagioclase. Average element concentrations of the two groups are nearly identical to one another, are comparable to those in Archean metabasalts from numerous shield areas, and are intermediate between those of modern oceanic tholeiites and continental tholeiites. These average concentrations suggest a depth of magma generation and thickness of crust intermediate between those for the oceanic and continental environments.     

First report of Lower Permian basalts in South Tibet: tholeiitic magmatism during break-up and incipient opening of Neotethys

The Neo-Tethys Ocean began to form at Early Permian times, when continental flood basalts were emplaced in various areas of the newly-formed Indian passive margin, exposed today in the so-called Tibetan Sedimentary Zone of the Himalaya. Lower Permian mafic volcanic rocks, which have long been known from various Himalayan localities from Kashmir to Arunachal Pradesh, are here for the first time reported to occur also in South Tibet (Bhote Kosi Basalts of the Gyirong County). The basalts unconformably overlie lowermost Permian diamictites, with locally intervening black shales and debris flow deposits, and are followed in turn by chert-bearing quartzarenites and silty to phosphatic marls yielding brachiopods of Roadian–Wordian age. The age of the lavas can thus be bracketed as late Early Permian (post-Sakmarian and pre-Roadian).The geochemistry of these subalkalic tholeiites, akin to MORBs, testifies to their similarity not only with the adjacent Nar-Tsum Spilites of central Nepal, but also with the Panjal Traps and Abor Volcanics of the western and eastern Himalayas respectively. The geochemical signature of Lower Permian volcanic rocks is in fact uniform all along the Himalayan Range, and markedly different from that of basaltic–rhyolitic alkalic products sporadically emplaced during the previous rifting stage. Rift volcanism in the Tethys Himalaya began in the Early Carboniferous and came to an end in Sakmarian times. In the Early Permian, initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism, which is therefore interpreted as following break-up and incipient sea-floor spreading in the Neotethys Ocean. Roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break-up and opening of the Neotethys between Northern Gondwana and the Peri-Gondwanian blocks.     

Mafic rocks spatially associated with Devonian felsic intrusions of the southern Lachlan Fold Belt: A possible mantle contribution to crustal evolution processes

Devonian basaltic to andesitic dykes and compositionally similar plutons of the southern Lachlan Fold Belt are often temporally and spatially closely associated with large granitic complexes. Mafic intrusions play a major role in the transfer of heat into the continental crust, providing a thermal ‘engine’ which leads to crustal melting, and geochemical/isotopic evidence indicates that they contribute chemical constituents to the products of this melting. Studied mafic‐intermediate dykes in the southern Lachlan Fold Belt have tholeiitic to alkaline affinities and include groups with both high and low Ti and K. Several dyke generations may be associated with a single felsic complex. Primitive mantle‐normalised trace‐element abundance patterns with negative Nb and Ti anomalies for basaltic/andesitic and gabbroic/dioritic rocks as young as Early Devonian most resemble those of modern island arcs and suggest an influence of subduction on mantle magma sources. However, some Middle and Late Devonian mafic rocks are enriched in light rare‐earth elements and other incompatible elements, lack significant Nb anomalies, and confirm the change to continental‐rift extensional settings clearly indicated by Lachlan Fold Belt geology.     

Géochimie et signification géotectonique des volcanites du Cryogénien inférieur du Saghro (Anti-Atlas oriental,Maroc)

The Early Cryogenian groups of Sidi Flah, Kelaat Mgouna and Boumalne (Saghro, Anti-Atlas, Morocco) are constituted by turbiditic deposits and interbedded lavas, accumulated in tectonic basins. At Sidi Flah, volcanics are transitional showing initial rift tholeiites (IRT) fingerprint and alkali basalts of oceanic island basalt (OIB) compositions. At Kelaat Mgouna, volcanics consist of low-Nb continental tholeiites. At Boumalne, basalts are of IRT composition. The volcanic and sedimentary formations belong to a nascent rift caused by thermal doming along a SW-NE axis, the Saghro rift. The continental break-up occurred in the early Neoproterozoic and during the Rodinia supercontinent dislocation, within a continent called “Ibero-saharian Craton” which was in front of the West-African Craton located near the South Pole. The Saghro rift is contemporaneous to the opening of an oceanic domain represented by Central Anti-Atlas ophiolites and related to the extension of the Brazialiano Ocean.     

Isotopic constraints on crustal architecture and Permo‐Triassic tectonics in New Guinea: Possible links with eastern Australia

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high‐grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle‐like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian‐type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post‐breakup margin comprised promontories of extended Proterozoic‐Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south‐southwest beneath northeast Australia generating arc‐related magmatism in New Guinea and the New England Fold Belt.     

西秦岭月照 琵琶寺地区火山岩地球化学特征及地质意义

月照琵琶寺地区的变质火山岩出露于甘肃武都县境内,夹在泥盆系和前震旦碧口群地层之间。按地球化学分类,火山岩可划分为双峰式和碱性两个系列。双峰式火山岩系由低K富Na拉斑玄武岩和酸性英安岩、流纹岩组成。该玄武岩具有类似MORB的微量元素特征,平坦的稀土配分模式,但富Th贫Nb显示其受到陆壳混染的影响。该套火山岩总体特征指示其为大陆裂谷向成熟洋盆转化阶段的产物,从而推测本区火山岩可能为勉略古洋盆西延的分支产物。     

西秦岭月照-琵琶寺地区火山岩地球化学特征及地质意义

月照-琵琶寺地区的变质火山岩出露于甘肃武都县境内,夹在泥盆系和前震旦碧口群地层之间.按地球化学分类,火山岩可划分为双峰式和碱性两个系列.双峰式火山岩系由低K富Na拉斑玄武岩和酸性英安岩、流纹岩组成.该玄武岩具有类似MORB的微量元素特征,平坦的稀土配分模式,但富Th贫Nb显示其受到陆壳混染的影响.该套火山岩总体特征指示其为大陆裂谷向成熟洋盆转化阶段的产物,从而推测本区火山岩可能为勉略古洋盆西延的分支产物.     

Géochimie et signification géotectonique des volcanites du Cryogénien inférieur du Saghro (Anti-Atlas oriental,Maroc) / Geochemistry and geotectonic significance of Early Cryogenian volcanics of Saghro (Eastern Anti-Atlas,Morocco)

Abstract

The Early Cryogenian groups of Sidi Flah, Kelaat Mgouna and Boumalne (Saghro, Anti-Atlas, Morocco) are constituted by turbiditic deposits and interbedded lavas, accumulated in tectonic basins. At Sidi Flah, volcanics are transitional showing initial rift tholeiites (IRT) fingerprint and alkali basalts of oceanic island basalt (OIB) compositions. At Kelaat Mgouna, volcanics consist of low-Nb continental tholeiites. At Boumalne, basalts are of IRT composition. The volcanic and sedimentary formations belong to a nascent rift caused by thermal doming along a SW-NE axis, the Saghro rift. The continental break-up occurred in the early Neopro- terozoic and during the Rodinia supercontinent dislocation, within a continent called “Ibero-saharian Craton” which was in front of the West-African Craton located near the South Pole. The Saghro rift is contemporaneous to the opening of an oceanic domain represented by Central Anti-Atlas ophiolites and related to the extension of the Brazialiano Ocean. © 2001 Éditions scientifiques et médicales Elsevier SAS     

A Study on the Kunyang Aulacogen

The Kunyang Aulacogen is a continental rift which initiated at the beginning of the Middle Proterozoic. Itwas generated on the rigid continental crust represented by the crystalline Kangding Complex, and is boundedby two N-S-trending deep faults--Anninghe-Luzhijiang Fault and Xiaojiang Fault. Its early stage of rupturewas characterized by alkalinc mafic igneous activities in the Dahongshan Group and Hekou Group. Duringthe later tensile-depression stage the aulacogen evolved into a graben where scdimentation of the KunyangGroup took place. The sedimentary sequence comprises terrestrial red beds, algal dolomite, marine black shaleand carbonate rocks which are quite typical of rift sedimentary association, and contains stratabound copperdeposits of the well-known Dongchuan type.     

柴北缘斜长角闪岩的地球化学特征及其构造背景

分布在柴北缘超高压变质带中的斜长角闪岩主要有两种类型,一种由榴辉岩退变而成,一种只经历角闪岩相变质作用。它们的原岩属于拉斑玄武质岩石,轻稀土富集,Nd同位素组成亏损,这些玄武岩浆分别来自不同的地幔源区,地壳混染不明显,形成的环境可能为大陆裂谷或初始洋盆,这可能是柴北缘早古生代洋盆打开的前兆。随着早古生代洋盆的关闭,这些基性火成岩部分经历了超高压变质作用,即发生了深俯冲,部分只经历角闪岩相变质。无论哪种情况,它们在遭受变质作用之前,就与陆壳岩石共生在一起,支持柴北缘榴辉岩、斜长角闪岩与片麻岩的关系为原地关系(in situ)?     

Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood.We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite).Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties.The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.     

An alternative model for the Damara Mobile Belt: Ocean crust subduction and continental convergence

The Pan-African Damara Mobile Belt has previously been described as ensialic, possibly resulting from a modified aulacogen.Three features of the Damara Mobile Belt are difficult to reconcile with ensialic models. Firstly, the complex asymmetrical structural pattern of linear zones, with up to 80% shortening across the belt. Secondly, the markedly asymmetric metamorphic pattern broadly follows the structural pattern forming two distinct, parallel metamorphic belts of relatively high (northern belt) and low (southern belt) geothermal gradients, respectively. Abundant granitic intrusions occur in the high-grade metamorphic belt. Thirdly, the evolution of the Damara igneous rocks; the early (Nosib) igneous rocks are alkali; mid-Damara (Matchless Member) amphibolites resemble oceanic-floor basalts. Depleted upper-mantle material representing oceanic lithosphere was tectonically emplaced into the Damara metasediments during early tectonism. An extensive calc-alkali suite (the Salem Suite) intruded the high-grade metamorphic belt during a long period spanning most of the Damara tectonism.A model invoking the formation of alkali rocks, followed by the development of oceanic crust, initiation of northwestward subduction and ocean closure terminating in continental collision is considered to explain the major features.     

东天山博格达造山带石炭纪火山岩及其形成地质环境

东天山博格达造山带早、中石炭世海相火山地具有双峰式特征,主要岩性为富钠的玄武岩和流纹岩,其次是英安岩,安山质岩石极少出现。玄武岩的特征是：少数岩石含有实际矿物石英,个别岩石含橄榄石斑晶;辉石主要是透辉石和次透辉石,其成分富含铝（Ａｌ２Ｏ３＝４．１７～５．９９）和钛（ＴｉＯ２＝２．８０～４．７８）;基质中的长石主要是钠－更长石,斑晶中有相当数量的中长石和拉长石;全岩化学成分ＣＩＰＷ计算结果绝大部分含     

Petrology and geochemistry of post–kinematic mafic rocks from the Paleoproterozoic Ubendian belt, NE Katanga (Democratic Republic of Congo)

The Paleoproterozoic post-kinematic Ubendian mafic rocks from northeastern Katanga (Democratic Republic of Congo) are olivine-and-quartz tholeiites which in many respects resemble Phanerozoic continental tholeiites. The analogies are suggested by the petrographic features and the major element diagrams classically used to infer magmatic affinity. The clinopyroxene compositions straddle the boundary between clinopyroxenes from orogenic and extensional tectonic settings. In addition, the whole-rock compositions are mostly Ti- and P-poor as in low Ti–P continental flood basalts and in subduction-related mafic magmas. The same conclusion is sustained by the trace-element compositions (e.g., occurrence of mafic magmas with high Th/Ta and La/Ta values; low Sr/Ce ratios, etc). These geochemical features indicate involvement of a subduction component at the source of these extensional igneous rocks. Convective mixing of asthenospheric mantle with the overlying lithospheric mantle enriched during the Ubendian subduction or mixing of melts from both mantle components can account for the composition of the post-orogenic Ubendian mafic rocks.     

中国东部新生代玄武岩记录古太平洋俯冲带壳幔相互作用

大陆玄武岩通常具有与洋岛玄武岩相似的地球化学成分,其中含有显著的壳源组分.对于洋岛玄武岩来说,虽然其中的壳源组分归咎于深俯冲大洋板片的再循环,但是对板片俯冲过程中的壳幔相互作用缺乏研究.对于大陆玄武岩来说,由于其形成与特定大洋板片在大陆边缘之下的俯冲有关,可以用来确定古大洋板片俯冲的地壳物质再循环.本文总结了我们对中国东部新生代玄武岩所进行的一系列地球化学研究,结果记录了古太平洋板片俯冲析出流体对地幔楔的化学交代作用.这些大陆玄武岩普遍具有与洋岛玄武岩类似的地球化学成分,在微量元素组成上表现为富集LILE和LREE、亏损HREE,但是不亏损HFSE的分布特点,在放射成因同位素组成上表现为亏损至弱富集的Sr-Nd同位素组成.在排除地壳混染效应之后,这些玄武岩的地球化学特征可以由其地幔源区中壳源组分的性质来解释.俯冲大洋地壳部分熔融产生的熔体提供了地幔源区中的壳源组分,其中包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分.华北和华南新生代大陆玄武岩在Pb同位素组成上存在显著差异,反映它们地幔源区中的壳源组分有所区别.中国东部新生代玄武岩的地幔源区是古太平洋板片于中生代俯冲至亚欧大陆东部之下时,在>200 km的俯冲带深度发生壳幔相互作用的产物.在新生代期间,随着俯冲太平洋板片的回卷引起的中国东部大陆岩石圈拉张和软流圈地幔上涌,那些交代成因的地幔源区发生部分熔融,形成了现今所见的新生代玄武岩.      

Petroi Metabasalt: Alkaline within‐plate mafic rocks from the Nambucca Slate Belt,northeastern New South Wales

The Petroi Metabasalt comprises approximately 2000 m of massive and pillowed metabasalt flows, breccias, and metadolerite sills that overlie and are intercalated with Early Permian epiclastic rocks of the Nambucca Slate Belt. Both the basaltic rocks and associated sedimentary material were multiply deformed and metamorphosed to pumpellyite‐actinolite facies grade at about 255 Ma. Metamorphism and earlier sea‐floor alteration of these mafic rocks have led to hydration, carbonation and oxidation and considerable redistribution of the major elements and the more labile traces, notably Rb, Ba and Sr. However, abundances of TiO₂, the high field strength trace elements, Ni, Cr and V, the light rare earths and yttrium are interpretable as being the largely unmodified magmatic abundances of mildly alkaline within‐plate basalts. This interpretation is supported by the composition of relic Ca‐rich pyroxenes in the metadolerites which fall in the fields of mildly alkaline basalts. The field relationships, age and composition of these rocks suggest either eruption on oceanic crust covered by a thick sequence of epiclastic rocks and subsequent incorporation into an accretionary subduction complex, or generation during rifting of the eastern part of the New England Fold Belt and accumulation, together with the associated sedimentary rocks, in a graben. The chemical and mineral characteristics of the igneous rocks indicate that the volcanism was not related to magmatic arc activity, and their presence demonstrates the rocks of the Nambucca Slate Belt are neither fore‐arc basin nor slope‐basin deposits.     

Revisiting Early–Middle Jurassic igneous activity in the Nanling Mountains,South China: Geochemistry and implications for regional geodynamics

Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.     

Intra-oceanic settings of the western Mexico Late Jurassic-Early Cretaceous arc sequences. Implications for the Pacific-Tethys geodynamic relationships during the Cretaceous

Abstract

The Guerrero suspect terrane composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California up to Acapulco and is considered to be coeval with the Late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, Venezuela and Western Cordillera of Colombia. New geological, petrological and geochemical data from central and southern Mexico, led us to propose a new model for the building of the Alisitos-Teloloapan arc. This arc, partly built on the Pacific oceanic lithosphere and partly on continental fragments, could be related to the subduction of an oceanic basin - the Arperos basin - under the Paleo-Pacific plate. This subduction was dipping southwest.

At the beginning of the magmatic activity of the oceanic segment of this arc, depleted tholeiitic basalts were emitted in a submarine environnement below the CCD. While subduction was going on, the arc magmas evolved from LREE depleted tholeiites to slightly LREE enriched tholeiites and then, to calc-alkaline basalts and andesites enriched in LREE and HFSE. Concurrently, the arc sedimentary environment changed from deep oceanic to neritic with the deposition of Aptian-Albian reefal limestones, at the end of the arc building. In the continent-based segment, the arc magmas are exclusively differentiated calc-alkaline suites depleted in HREE and Y, formed of predominantly siliceous lavas and pyroclastic rocks, emitted in a sub-aerial or shallow marine environment.

Thus, taking into account this above mentioned model, the Cretaceous volcanic series, accreted to the margins of cratonal America, in Colombia, Venezuela, Greater Antilles and Mexico, could be related to the same west-south-west dipping subduction of oceanic basins, fringing the North and South American continental cratons and connected directly with the inter-American Tethys. While the subduction was proceeding, this magmatic arc drifted towards the North and South American cratons and finally, collided with the continental margins at different periods during the Cretaceous.     

北祁连山元古宙末－寒武纪主动大陆裂谷火山作用

北祁连山元古宙末－寒武纪大陆裂谷火山岩系为双峰式火山岩套，主要由基性与酸性火山岩组成。基性火山岩有磁性玄武岩与拉斑玄武岩两个岩浆系列，且富集ＬＲＥＥ与ＬＩＬ，其岩浆源区为与洋岛玄武岩源相似的富集地幔柱源。软流圈地幔柱上涌导致岩石圈地慢部分熔融，其熔体与地幔柱衍生熔浆混合，形成本区具有中等钕，锶同位素比值特点的基性岩浆。基性岩浆上侵至陆壳，引起下部陆壳深熔，产生长英质岩浆。地幔柱上隆促使大陆扩张，及至形成北祁连山元古宙末－寒武纪大陆裂谷。