Juxtaposition of India and Madagascar: A Perspective

Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar (ca 2.6–3.0 Ga) in the north and the older Sargur (ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments.     

扬子块体南缘四堡群Sm－Nd同位素体系及其他壳演化意义

 详细的主元素和Sm-Nd同位素体系研究表明,扬子块体南缘元宝山地区四堡群中镁铁质-超镁铁质岩可能来源于Al亏损地幔;而宝坛地区镁铁质-超镁铁质岩的源区则可能包括了A1未亏损和A1亏损两种地幔端元组分,部分样品可能受到围岩混染。镁铁质-超镁铁质岩的Sm-Nd数据构成了一条无地质意义的假等时线,由其斜率获得的年龄约2.2Ga明显偏老。四堡群浅变质沉积岩的Nd模式年龄限定了镁铁质-超镁铁质岩和四堡群的地层年龄应小于1.8Ga.扬子南缘最老的基底四堡群(及相应地层)主要是由地壳存留年龄为1.8-1.9Ga的未成熟陆壳再循环物质组成,明显不同于华南块体(华夏古陆)的早-中元古代变质基底。迄今为止获得的沉积岩和花岗岩的Sm-Nd同位素资料都不支持扬子南缘存在早元古代-晚太古代基底。     

Three Palaeoproterozoic basins—Yerrida,Bryah and Padbury—Capricorn Orogen,Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Basins record periods of sedimentation and magmatism along the northern margin of the Archaean Yilgarn Craton. Each basin is characterised by distinct stratigraphy, igneous activity, structural and metamorphic history and mineral deposit types. The oldest of these basins, the Yerrida Basin (ca 2200 Ma) is floored by rocks of the Archaean Yilgarn Craton. Important features of this basin are the presence of evaporites and continental flood basalts. The ca 2000 Ma Bryah Basin developed on the northern margin of the Yilgarn Craton during backarc sea‐floor spreading and rifting, the result of which was the emplacement of voluminous mafic and ultramafic volcanic rocks. During the waning stages of the Bryah Basin this mafic to ultramafic volcanism gave way to deposition of clastic and chemical sedimentary rocks. At a later stage, the Padbury Basin developed as a retroarc foreland basin on top of the Bryah Basin in a fold‐and‐thrust belt. This resulted from either the collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny) or the ca 2000 Ma westward collision of the southern part of the Gascoyne Complex and the Yilgarn Craton (Glenburgh Orogeny). During the Capricorn Orogeny the Bryah Group was thrust to the southeast, over the Yerrida Group. Important mineral deposits are contained in the Yerrida, Bryah and Padbury Basins. In the Yerrida Basin a large Pb–carbonate deposit (Magellan) and black shale‐hosted gossans containing anomalous abundances of Ba, Cu, Zn and Pd are present. The Pb–carbonate deposit is hosted by the upper units of the Juderina Formation, and the lower unit of the unconformably overlying Earaheedy Group. The Bryah and Padbury Basins contain orogenic gold, copper‐gold volcanogenic massive sulfides, manganese and iron ore. The origin of the gold mineralisation is probably related to tectonothermal activity during the Capricorn Orogeny at ca 1800 Ma.     

Basement geology of the southern Thomson Orogen

Abstract

The diverse geological and geophysical data sets compiled, interrogated and interpreted for the largely undercover southern Thomson Orogen region reveal a Paleozoic terrane dominated by deformed metasedimentary rocks intruded by S- and I-type granites. An interpretive basement geology map and synthesis of geochronological constraints allow definition of several stratigraphic packages. The oldest and most widespread comprises upper Cambrian to Lower Ordovician metasedimentary rocks deposited during the vast extensional Larapinta Event with maximum depositional ages of ca 520 to ca 496 Ma. These units correlate with elements of the northern Thomson Orogen, Warburton Basin and Amadeus Basin. The degree of deformation and metamorphism of these rocks varies across the region. A second major package includes Lower to Middle Devonian volcanic and sedimentary units, some of which correlate with components of the Lachlan Orogen. The region also includes a Middle to Upper Ordovician package of metasedimentary rocks and a Devonian or younger package of intermediate volcaniclastic rocks of restricted extent. Intrusive units range from diatremes and relatively small layered mafic bodies to batholithic-scale suites of granite and granodiorite. S-type and I-type intrusions are both present, and ages range from Ordovician to Triassic, but late Silurian intrusions are the most abundant. Two broad belts of intrusions are recognised. In the east, the Scalby Belt comprises relatively young (Upper Devonian) intrusions, while in the west, the Ella Belt is dominated by intrusions of late Silurian age within a curvilinear, broadly east–west trend. The stratigraphic distributions, characteristics and constraints defined by this interpretive basement mapping provide a basic framework for ongoing research and mineral exploration.     

Coupled asteroid impacts and banded iron-formations,Fortescue and Hamersley Groups,Pilbara, Western Australia

Asteroid impact spherule layers and tsunami deposits underlying banded iron-formations in the Fortescue and Hamersley Groups have been further investigated to test their potential stratigraphic relationships. This work has included new observations related to the ca 2.63 Ga Jeerinah Impact Layer (JIL) and impact spherules associated with the 4th Shale-Macroband of the Dales Gorge Iron Member (DGS4) of the Brockman Iron Formation. A unit of impact spherules (microkrystite) correlated with the ca 2.63 Ga JIL is observed within a >100 m-thick fragmental-intraclast breccia pile in drill cores near Roy Hill. The sequence represents significant thickening of the impact/tsunami unit relative to the JIL type section at Hesta, as well as relative to the 20–30 m-thick ca 2.63 Ga Carawine Dolomite spherule-bearing mega-breccia. The ca 2.48 Ga-old Dales Gorge Member of the Brockman Iron Formation is underlain by an ?0.5 m-thick rip-up clast breccia located at the top of the ca 2.50 Ga Mt McRae Shale, and is interpreted as a tsunami deposit. We suggest that the presence of impact ejecta and tsunami units stratigraphically beneath a number of banded iron-formations, and units of ferruginous shale in the Pilbara and South Africa may result from a genetic relationship. For example, it could be that under Archean atmospheric conditions, mafic volcanism triggered by large asteroid impacts enriched the oceans in soluble FeO. If so, seasonal microbial and/or photolytic oxidation to ferric oxide could have caused precipitation of Fe₂O₃ and silica. In view of the possible occurrence of depositional gaps and paraconformities between impact ejecta units and overlying ferruginous sediments, these relationships require further testing by isotopic age studies.     

Beach sand of SE Australia traced by zircon ages through Ordovician turbidites and S-type granites of the Lachlan Orogen to Africa/Antarctica: a review

The Quaternary beach sand of SE Australia, driven northward by southern swell, contains zircons with dominant U–Pb ages of 700–500 Ma, model ages (T_DMc) of 2.2 Ga to 1.0 Ga, and ?_Hf of +12 to –30, indicating a host rock type of granitoids with alkaline affinity. These properties match those of detrital zircons in the Middle Triassic (ca 240 Ma) Hawkesbury Sandstone (T_DMc of 2.1 to 1.0 Ga, ?_Hf of +8 to –40, alkaline granitoids) and the Ordovician (ca 460 Ma) turbidites and ca 430 Ma S-type granitoids of the Lachlan Orogen (T_2DM of 2.0 to 1.0 Ga, ?_Hf of +5 to –30), all of which are identified as proximal provenances. Superimposed are the ca 400 Ma zircons in beaches in the south backed by the 420–375 Ma I-type Bega Batholith, and ca 350 Ma and ca 250 Ma zircons in the north backed by the New England Orogen. The Ordovician turbidites, part of a deep-sea super-fan, were fed by the detritus of the exhumed 700–500 Ma Transgondwanan Supermountains atop the East African–Antarctic Orogen. At the same time, the ancestral Gamburtsev Subglacial Mountains of East Antarctica probably contributed a subsidiary fan of 700–500 Ma sediment. Primary zircons aged 600–500 Ma in igneous and metamorphic rocks in Australia and the ancestral Transantarctic Mountains are minor contributors of the Australian sediments. The properties of the 700–500 Ma primary zircons in the East African–Antarctic Orogen are traceable through the first-cycle Ordovician turbidite and intruding second-cycle granite, and younger sediment, such as the third-cycle Triassic Hawkesbury Sandstone and the third-cycle beach sand. The sand at the northern terminus of the coastal system off Fraser Island spills over the shelf edge into the Tasman Abyssal Plain to reflect in miniature the deep-sea depositional environment of the Ordovician.     

Provenance,tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics,Tanami Region,Western Australia

This study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ?_Hf values (?_Hf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ?_Hf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth.     

Mafic and ultramafic magmatism and associated mineralization in the Dharwar craton,southern India

Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in nature, are preserved in the Sargur Group which is more than 3.3–3.4 Ga old, the youngest being manifested by 63–76 Ma old mafic dyke magmatism, possibly related to Deccan volcanism. In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties, the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga. Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8 and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga. From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids, derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures. The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous kimberlite/lamproite intrusions in the EDC.     

扬子块体南缘四堡群Sm－Nd同位素体系及其他壳演化意义

详细的主元素和Sm-Nd同位素体系研究表明,扬子块体南缘元宝山地区四堡群中镁铁质-超镁铁质岩可能来源于Al亏损地幔;而宝坛地区镁铁质-超镁铁质岩的源区则可能包括了A1未亏损和A1亏损两种地幔端元组分,部分样品可能受到围岩混染。镁铁质-超镁铁质岩的Sm-Nd数据构成了一条无地质意义的假等时线,由其斜率获得的年龄约2.2Ga明显偏老。四堡群浅变质沉积岩的Nd模式年龄限定了镁铁质-超镁铁质岩和四堡群的地层年龄应小于1.8Ga.扬子南缘最老的基底四堡群(及相应地层)主要是由地壳存留年龄为1.8-1.9Ga的未成熟陆壳再循环物质组成,明显不同于华南块体(华夏古陆)的早-中元古代变质基底。迄今为止获得的沉积岩和花岗岩的Sm-Nd同位素资料都不支持扬子南缘存在早元古代-晚太古代基底。     

Detrital zircon analysis from the Neoproterozoic–Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo,Argentina: Evidence of a rift and passive margin system?

Metamorphic basement and its Neoproterozoic to Cambrian cover exposed in the Sierra de Pie de Palo, a basement block of the Sierras Pampeanas in Argentina, lie within the Cuyania terrane. Detrital zircon analysis of the cover sequence which includes, in ascending order, the El Quemado, La Paz, El Desecho, and Angacos Formations of the Caucete Group indicate a Laurentian origin for the Cuyania terrane. The lower section represented by the El Quemado and La Paz Formations is interpreted as having an igneous source related to a rift setting similar to that envisioned for the southern and eastern margins of Laurentia at approximately 550 Ma. The younger strata of the El Desecho Formation are correlative with the Cerro Totora Formation of the Precordillera, and both are products of rift sedimentation. Finally, the Angacos Formation and the correlative La Laja Formation of the Precordillera were deposited on the passive margin developed on the Cuyania terrane. The maximum depositional ages for the Caucete Group include ca. 550 Ma for the El Quemado Formation and ca. 531 Ma for the El Desecho Formation. Four different sediment sources areas were interpreted in the provenance analysis. The main source is crystalline basement dominated by early Mesoproterozoic igneous rocks related to the Granite-Rhyolite province of central and eastern Laurentia. Possible source areas for 1600 Ma metamorphic detrital zircons of the Caucete Group include the Yavapai-Mazatzal province (ca. 1800–1600 Ma) of south-central to southwestern Laurentia. Younger Mesoproterozoic zircon is likely derived from Grenville-age medium- to high-grade metamorphic rocks and subordinate igneous rocks that form the basement of Cuyania as well as the southern Grenville province of Laurentia itself. Finally, Neoproterozoic igneous zircon in the Caucete Group records different magmatic pulses along the southern Laurentian margin during opening of Iapetus and break-up of Rodinia. Northwestern Cuyania terrane includes a small basement component derived from the Granite-Rhyolite province of Laurentia, which was the source for detrital zircons found in the middle Cambrian passive margin sediments of Cuyania.     

Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region,southern New England Orogen,New South Wales

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean ²⁰⁶Pb/²³⁸U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occurred after complete crystallization of the pluton and prior to emplacement of dykes and I‐type intrusives. K‐Ar biotite and hornblende ages show broadly synchronous intrusion of I‐type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of I‐type melts. Chemical similarities between the most mafic Moonbi Suite members and calc‐alkaline lamprophyres may also indicate a direct mantle contribution to some I‐type magmas.     

The ca 2.74 Ga Mopoke Member,Kylena Formation: a marine incursion into the northern Fortescue Group?

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ¹³C_carb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of ¹³C-depleted organic matter. δ¹³C_org values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less ¹³C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.     

Palaeoproterozoic tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt,North China Craton: a review

The Palaeoproterozoic Jiao‐Liao‐Ji Belt is located in the eastern margin of the Eastern Block of the North China Craton. In this paper, we synthesize the tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt based on recent information. A mantle plume‐related underplating from 2.53 to 2.36 Ga is envisaged which led to the emplacement of the 2.47–2.33 Ga alkali granite plutons and mafic dyke swarms, followed by the development of the Jiao‐Liao‐Ji Rift and bi‐modal volcanism. The underplating resulted not only in different sedimentary environments in the upper crust, but also in a differentiation of the initial thermal structure in the rift. This controlled the metamorphism and style of P‐T‐t paths in the different parts of the rift. Subsequent underplating resulted in the emplacement of the A‐type Liaoji granites of ca. 2.17 Ga in the lower crust, and the formation of associated pegmatites of 2.2 and 2.0 Ga, together with the development of a bedding‐parallel extension. However, the main orogeny occurred between 1.93 and 1.88 Ga with closing of the rift, compressional deformation and high‐pressure granulite metamorphism in the southern part of the orogen. Subsequently, lithospheric blocks were possibly delaminated at ∼1.85 Ga; anorogenic magmatic rocks such as rapakivi granite, alkaline granites and syenite were intruded, and pegmatite veins and mafic dyke swarms were emplaced cross‐cutting all the earlier structural traces. We identify that the underplating styles, collision processes and delamination types in the deep lithosphere controlled the tectonothermal evolution of the crust in the Jiao‐Liao‐Ji region. Copyright © 2011 John Wiley & Sons, Ltd.     

Deformation‐driven,regional‐scale metasomatism in the Hamersley Basin,Western Australia

Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km² Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc , in subsets of the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–Fe₂O₃, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional‐scale metasomatism in the Fortescue Group progressively produces a suite of prehnite‐bearing and pumpellyite–quartz/epidote–quartz‐dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional‐scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.     

Genesis of the Archean–Paleoproterozoic Tabletop Domain,Rudall Province,and its endemic relationship to the West Australian Craton

The Tabletop Domain of the Rudall Province has been long thought an exotic entity to the West Australian Craton. Recent re-evaluation of this interpretation suggests otherwise, but is founded on limited data. This study presents the first comprehensive, integrated U–Pb geochronology and Hf-isotope analysis of igneous and metasedimentary rocks from the Tabletop Domain of the eastern Rudall Province. Field observations, geochronology and isotope results confirm an endemic relationship between the Tabletop Domain and the West Australian Craton (WAC), and show that the Tabletop Domain underwent a similar Archean–Paleoproterozoic history to the western Rudall Province. The central Tabletop Domain comprises Archean–Paleoproterozoic gneissic rocks with three main age components. Paleo–Neoarchean (ca 3400–2800 Ma) detritus is observed in metasedimentary rocks and was likely sourced from the East Pilbara Craton. Protoliths to mafic gneiss and metasedimentary rocks are interpreted to have been emplaced and deposited during the early Paleoproterozoic (ca 2400–2300 Ma), and exhibit age and isotopic affinities to the Capricorn Orogen basement (Glenburgh Terrane). Mid–late Paleoproterozoic mafic and felsic magmatism (ca 1880–1750 Ma) is assigned to the Kalkan Supersuite, which is exposed in the western Rudall Province. The Kalkan Supersuite provided the main source of detritus for mid–late Paleoproterozoic metasedimentary rocks in the Tabletop Domain. Similarities in the age and Hf-isotope compositions of detrital zircon from these metasedimentary rocks and Capricorn Orogeny basin sediments suggests that a regionally extensive, linked basin system may have spanned the northern WAC at this time. The Tabletop Domain records evidence for two metamorphic events. Mid–late Paleoproterozoic deformation (ca 1770–1750 Ma) was high-grade, regional and involved the development of gneissic fabrics. In contrast, early Mesoproterozoic (ca 1580 Ma) high-grade deformation was localised and associated with more widespread, late-stage, greenschist facies alteration. These new findings highlight that the Tabletop Domain experienced a much higher grade of deformation than previously assumed, with a Paleoproterozoic metamorphic history similar to that of the western Rudall Province.     

The Belomorian eclogite province: sequence of events and age of the igneous and metamorphic rocks of the Gridino association

Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U–Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granitic leucosome; U–Pb dating of rutile from mafic dikes; ⁴⁰Ar/³⁹Ar dating of amphibole and mica; and Sm–Nd studies of rocks and minerals. The Sm–Nd model ages of felsic (2.9–3.1 Ga) and mafic (3.0–3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircon grains with an age of ca. 3.0 Ga indicate the Mesoarchean age of the metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the crystallization age of magmatic zircon from metagabbro. Superimposed amphibolitic metamorphism and the “final” exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of the metamorphic associations to 300 °C at 1.9–1.7 Ga is shown by U–Pb rutile dating and ⁴⁰Ar/³⁹Ar mica dating.     

西昆仑东段苏巴什洋向北俯冲:来自早—中二叠世火山岩的证据

西昆仑东段苏巴什蛇绿构造混杂岩带南侧卡拉勒塔什群以大面积分布的酸性和中基性火山岩为特征,本次对卡拉勒塔什群弧火山岩代表性的岩石组合进行了LA-ICP-MS锆石U-Pb年龄、地球化学及锆石Lu-Hf同位素研究。研究结果表明,LA-ICP-MS锆石U-Pb测年获得酸性晶屑凝灰岩、蚀变玄武岩²⁰⁶Pb/²³⁸U加权平均年龄为(284.2±1.6) Ma和(262.6±2.0) Ma,表明研究区卡拉勒塔什群火山岩形成于早—中二叠世。卡拉勒塔什群火山岩具有富铝、钠、铁,富集大离子亲石元素K、Rb、Ba和亏损高场强元素Sr、Ta、Nb、Ti的地球化学特征。其中,基性火山岩属钙碱性-拉斑玄武岩系列,岩石稀土元素配分模式接近大洋拉斑玄武岩,Nd/Th和La/Nb比值为8.91~13.76和0.39~2.28,Lu-Hf同位素ε_Hf(t)值为-0.15~4.95,表现为地幔物质来源,但加入了地壳组分。酸性火山岩属于钙碱性系列,相对亏损P和Zr元素,Nd/Th和La/Nb比值为1.92~4.10和2.52~3.39,Lu-Hf同位素ε_Hf(t)值分别为0.94~3.78和8.26~12.45,二阶段模式年龄分别为1.07~1.25 Ga、0.51~0.78 Ga,表明酸性火山岩物质来源为古老地壳和新生地壳物质重熔后的混合物。卡拉勒塔什群总体地球化学特征表现为岛弧环境。卡拉勒塔什群岛弧火山岩与北侧苏巴什蛇绿构造混杂岩带在形成时代、空间分布以及基性岩地球化学特征均表现成对关系,与苏巴什蛇绿构造混杂岩带内发育的硫磺达坂砂岩组深水复理石建造共同构成造山带沟-弧-盆体系,表明苏巴什洋盆由南向北的俯冲极性,说明苏巴什蛇绿构造混杂岩形成于岛弧偏向于海沟的弧前盆地构造背景。     

Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.     

Palaeoproterozoic arc magmatism and collision in Liaodong Peninsula (north-east China)

In the north‐eastern part of the North China Block, a mafic magmatic belt consisting of mafic–ultramafic rocks and marine sedimentary rocks crops out between the northern Archean Anshan Block and a southern Palaeoproterozoic Block. ⁴⁰Ar/³⁹Ar amphibole ages around 1.9 Ga from gabbros, and trace element analyses of gabbros, pyroxenite and shale show that these rocks formed along a Palaeoproterozoic active continental margin. The mafic magmatic belt is interpreted as an arc developed above a south‐directed subduction zone, which was subsequently overthrust to the north upon the Anshan Archean Block. This study provides a new example agreeing with increasing evidence supporting plate mobility and thrust tectonics during the Palaeoproterozoic. These new insights must be considered with regard to the formation of the North China Block by magmatic accretion and tectonic collision.     

Petrological,geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt

Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (～700 km²) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin.