The structure of a small complexly‐deformed area within Ordovician rocks near Mansfield,Victoria

Two distinct groups of granitoids occur on the eastern side of the Kosciusko Batholith. Those considered to be derivatives of sedimentary source rocks (S‐types) are usually foliated and either contain cordierite or white‐mica secondary after cordierite. The granitoids produced from igneous source material (I‐types) are generally massive and frequently contain hornblende. Geochemical parameters provide the best discriminant between the two groups, I‐types have higher Ca, Al, Na₂O/K₂O, and Fe₂O₃/FeO, and lower Fe, Mg, Sc, V, Cr, Co, Ni, Cu, Zn, Ba, Rb, Th, La, Ce, and Y than S‐types of comparable SiO₂ values.

The differences between the two groups are not the result of differences in the melt‐forming process but reflect differences in the nature of the source material. Thus the geochemical features of the S‐type granitoids are indicative of their source rocks having been through a process of chemical weathering in a sedimentary cycle. Conversely, the I‐type granitoids were derived from fractionated rocks that had not been involved in weathering processes.     

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.     

崆岭杂岩中角闪岩类的年代学和地球化学

通过崆岭杂岩中角闪岩类的年代学和地球化学研究,以揭示黄陵结晶基底的形成及演化。崆岭杂岩主要由太古代的TTG片麻岩和角闪岩类岩石,以及早元古代孔兹岩系组成,角闪岩类以围岩或包体的形式存在于TTG片麻岩的周围和内部。角闪岩类围岩的全岩Sm-Nd等时线年龄为2 998.9 Ma,原生岩浆锆石的U-Pb年龄为3 013 Ma,均代表角闪岩类原岩的形成时间,且与包体状斜长角闪岩原岩的年龄(3.0 Ga)相同。说明以围岩或包体存在的角闪岩类,其原岩的形成年龄均为3.0 Ga。微量元素和Nd同位素地球化学特征指示,角闪岩类原岩形成的构造环境为大陆初始裂谷环境。角闪岩类岩石中变质新生锆石的U-Pb年龄为2 043 Ma,指示黄陵地区存在第Ⅲ期(2.1～1.9 Ga)角闪岩相热变质事件,且该期热变质作用将松散的陆源碎屑岩等变质为孔兹岩系,从而构成早元古代结晶基底,并与晚太古代稳定陆块焊结在一起,最终完成整个黄陵结晶基底的形成。     

The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites

Rocks of the Moruya Batholith range from gabbros and gabbroic diorites through quartz diorites and tonalites to granodiorites and rare adamellites. The gabbros and gabbroic diorites appear as small, early bodies intruded and enclosed by quartz diorites and tonalites. These early gabbroids are petrographically and chemically distinct from the granitoids. The latter occur as a meridionally‐oriented sequence of nine separate plutons. Mafic xenoliths are most abundant in the quartz diorites and tonalites; they are petrographically similar to their host granitoids and are chemically a more mafic extension of the variation in granitoid compositions. The various granitoid bodies are considered to be derived from similar source rocks, with the xenoliths representing modified material relict from partial melting of that source.

Comparison of chemical data from the Moruya granitoids with those of the I‐types of the Jindabyne Suite in the Kosciusko Batholith, shows that the potassium content is indistinguishable in the two suites from each side of the Moruya‐Kosciusko Province, although elsewhere it has been shown to vary systematically across an orogenic belt. The most outstanding difference is the higher Na and Ti and lower Ca in the Moruya Batholith compared with those in Kosciusko Batholith I‐type granitoids.     

Rb‐Sr geochronology and Sr isotopic composition of Devonian granitoids,eastern Tasmania

Biotite igneous ages and well‐defined isochron ages of plutons from the composite Blue Tier Batholith and the Coles Bay area in northeastern Tasmania range from 395 to 370 Ma. The older limit of this range, for the George River granodiorite, is considerably older than any age previously recorded for NE Tasmania. The ages of the youngest plutons (Mt Paris and Anchor granites), which host cassiterite ores, record pervasive hydrothermal alteration events. The initial ⁸⁷Sr/⁸⁰Sr ratios of the granitoids range from 0.7061 to 0.7136 and suggest different protolith compositions, consistent with mineralogical and geochemical characteristics of each pluton. The S‐type garnetbiotite granites (Ansons Bay and Booby alia granites) have initial ratios greater than 0.7119, indicative of enriched, high Rb/Sr ratio, crustal source‐rocks of Proterozoic age (1700–800 Ma). The S‐type biotite granites (Poimena and Pearson granites) have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7070, 0.7105) but overlap with those of the I‐type granodiorites (George River, Scamander Tier, Pyengana and Coles Bay granodiorites) which are in the range of 0.7061 to 0.7073. The initial ratios of the enriched altered plutons are poorly constrained, and on both hand‐specimen and thin‐section scales, reveal open‐system Sr isotopic patterns.

Isochron ages for the arenite‐lutite and lutite sedimentary associations of the Mathinna Beds, which are intruded by the granitoids, reflect an approach to Sr isotopic equilibrium during regional metamorphism. The metamorphic age (401 ± 7 Ma) of the early Pragian arenite‐lutite association indicates a relatively small time interval between deposition, regional metamorphism and granitoid intrusion. The isotopic age for the lutite sedimentary association (423 ± 22 Ma) is tentatively correlated with a Benambran‐age burial metamorphic event that has not previously been recorded in Tasmania.     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

The Late Triassic Kataev volcanoplutonic association in western Transbaikalia,a fragment of the active continental margin of the Mongol-Okhotsk Ocean

The Kataev volcanoplutonic association has been recognized in western Transbaikalia. It unites the volcanosedimentary rocks of the Kataev Formation and associated granites localized within the lower plates of the Buteel-Nuur and Zagan metamorphic-core complexes. The rocks of the Kataev association are dynamometamorphosed to different degrees, which is due to the tectonic exposure of metamorphic-core complexes in the Early Cretaceous. The U-Pb zircon dating of the Kataev Formation rhyolites yielded an age of 226 ± 3 Ma. The U-Pb zircon age of the granites intruding the Kataev Formation rocks is 223.4 ± 5.0 Ma. The volcanics of the Kataev Formation belong to the subalkalic basalt-andesite-dacite-rhyolite series. The trachybasalts and trachyandesite-basalts of the Kataev Formation have geochemical characteristics of igneous rocks formed as a result of subduction, e.g., they show distinct negative Nb and Ti and positive Ba and Sr anomalies on multielemental patterns. The specific composition of mafic volcanics points to their formation through the melting of a mantle source resulted from the mixing of depleted mantle and subduction components. Trachyandesites have higher Th and U contents than basaltoids. They can result from the contamination of a mantle source, similar in composition to the Kataev Formation basaltoids, with crustal material. The felsic volcanics of the Kataev Formation and granites intruding them show nearly identical geochemical characteristics corresponding to both A-and I-type granites. These rocks might have formed through the melting of a moderately water-saturated magmatic source of diorite-tonalite composition at 742–833°C. We have established that the rocks of the Kataev volcanoplutonic association in western Transbaikalia and Northern Mongolia formed in the Late Triassic synchronously with the calc-alkaline granitoids of the Henteyn–Daurian batholith and the alkali granites and bimodal volcanic associations of the Kharitonovo and Tsagaan-Hurtey volcanoplutonic associations. The synchronous formation of volcanoplutonic associations of normal and high alkalinity agrees with the geodynamic setting of the Andean-type active continental margin existing in the area of present-day western Transbaikalia and Northern Mongolia in the Early Mesozoic. This setting was the result of the subduction of the Mongol-Okhotsk oceanic plate beneath the Siberian continent.     

Does ternary feldspar constrain the metamorphic conditions of high‐grade meta‐igneous rocks? Evidence from orthopyroxene granulites,Bohemian Massif

The presence of ternary feldspar in high‐grade meta‐igneous rocks, and the recognition of the thermometric significance of this mineral, has led recent researchers to postulate peak metamorphic temperatures in excess of 1000 °C. However, it needs to be established that such ternary feldspar is not in fact a survivor of the original high‐temperature crystallization of the igneous protolith. After exsolution, the host and lamellae in the ternary feldspar grains may be stable throughout subsequent history as long as recrystallization does not occur. Such a history may involve rehydration and metamorphism, including H₂O‐saturated conditions, with the compositions and proportions of the host and lamellae being modified to reflect the P–T conditions experienced. In the case of the high‐grade meta‐igneous rocks from the Moldanubian of the Bohemian Massif, some samples that contain ternary feldspar preserve a substantial measure of their igneous heritage. Orthopyroxene‐bearing granulites not only include types that are barely affected by the metamorphism, but also others that have undergone hydration of the igneous protolith prior to the development of a metamorphic overprint. A key to establishing the igneous origin of the ternary feldspar grains is their preservation in garnet that is either itself igneous, or of a relatively low‐temperature metamorphic origin. Applying the logic to the other ternary feldspar‐bearing meta‐igneous rocks deprives the Moldanubian of its ultrahigh temperature (UHT) metamorphic status.     

Heterogeneity of the Lithosphere in Tibetan Plateau on the Constraints of Magmatism

HETEROGENEITY OF THE LITHOSPHERE IN TIBETAN PLATEAU ON THE CONSTRAINTS OF MAGMATISM     

Petrology, geochemistry and geodynamic setting of Proterozoic igneous suites of the Orós fold belt (Borborema Province, Northeast Brazil)

The Orós belt is a metamorphosed and deformed supracrustal sequence whose deposition started at ca. 1.8 Ga. The volcanic rocks form an essentially bimodal association with a predominance of felsic volcanics. Mafic volcanics show geochemical and Nd isotopic differences which point to separate origins for each type. The mafic rocks are either chemically similar to EMORBs or are transalkaline types, enriched in LILE and LREE and relatively depleted in HFSE but with different isotopic signatures. One mafic type is associated with dominant andesites in a suite which could have evolved by an AFC process involving a Transamazonian-age source rock which later produced some of the overlying felsic volcanics. The rhyolites have variable geochemical signatures, all typical of anatectic products derived from continental crustal rocks. Sedimentary rocks are dominated by pelitic types of different provenances, accompanied by minor arenites and lesser carbonate rocks and calc-silicates. The depositional environment of the supracrustal sequence was continental, and a number of lines of evidence suggest that a rift environment probably developed during relaxation following the Transamazonian orogeny. The most abundant igneous rocks are felsic plutonics, emplaced nearly 100 Ma after the volcanic activity, but whose geochemical signature is similar to that of most of the rhyolites. These are cut by more alkaline anorogenic intrusives. The volcano-sedimentary sequence was intruded at ca. 0.9 Ga by a mafic-ultramafic sill. All deformation and metamorphism occurred during the Brasiliano orogeny, which was accompanied by the intrusion of syn-tectonic granites.The Orós belt, therefore, represents an intracontinental ensialic late Paleoproterozoic volcano-sedimentary basin initially related to strain relaxation of the crust. With subsequent collapse and development of faults, the anorogenic alkaline granites intruded at ca. 1.7 Ga. Based on the data available on this province, the Orós belt forms a part of a series of supracrustal belts of different ages, which were consolidated and welded in the Brasiliano orogeny.Within the Borborema Province, supracrustal sequences with ages of ≈ 2.0-1.9 Ga, ≈ 1.8-1.7 Ga (Orós), ≈ 1.1-1.0 Ga, and ≈ 0.6 Ga are presently known. All were aglutinated or amalgamated to neighboring blocks during the Brasiliano orogenic cycle.     

浙江省中生代火成岩的Nd-Sr同位素研究

本文报道了浙江省21个中生代火成岩的Nd-Sr同位素组成,其中火山岩的εNd值为-12.6——4.9,ISr值为0.70613-0.71079,tDM年龄为1945-1296Ma;花岗岩类的εNd值为-12.9——5.8,ISr值为0.70533-0.71208,tDM年龄为1900-1230Ma,表明两者具有相似的同位素组成。这种相似性在同一火山-侵入杂岩体中表现更为明显,意味着两者在时、空、源方面具有同一性。与扬子地块的相比,华夏地块的中生代火成岩具有较低的εNd值,较高的ISr值和较古老的Nd模式年龄,这种差异可能主要同这两个区域内基底变质岩在形成时代和成分上的差异有关。通过Sm-Nd同位素组成的对比研究,笔者认为,浙江境内的中生代火成岩可能主要是由基底变质沉积岩衍生的。原始岩浆的形成可能同中、下地壳岩石的熔融有关。     

Granitoids from the Moonbi district,New England Batholith,Eastern Australia

The late Palaeozoic granitoids of the Moonbi district are derived both from igneous (I‐type) and sedimentary (S‐type) sources. Field and petrographic observations and chemical data on the I‐type granitoids show that they are derived from four separate and distinct source‐rock compositions and that, consequently, these granitoids may be grouped into four suites. Mafic xenoliths and microxenoliths are relatively more abundant in more mafic I‐type granitoids. Such xenoliths are interpreted as restite, or material residual from partial melting of the source rocks. Variation within the granitoids is ascribed to varying degrees of separation of restite from the melt produced during each fusion event. The source material of the I‐type granitoids is considered to have been material underplated beneath the crust during an earlier subduction event. Two suites of S‐type granitoids can be recognized. These are derivatives of pelitic materials that have undergone only a small amount of chemical weathering.     

Geochemistry of the Archean Yellowknife Supergroup

The Archean Yellowknife Supergroup (Slave Structural Province. Canada) is composed of a thick sequence of supracrustal rocks, which differs from most Archean greenstone belts in that it contains a large proportion ( ~ 80%) of sedimentary rocks. Felsic volcanics of the Banting Formation are characterized by HREE depletion without Eu-anomalies, indicating an origin by small degrees of partial melting of a mafic source, with minor garnet in the residua. Granitic rocks include synkinematic granites [HREE-depleted; low (${}^{87}\text{Sr}{}^{86}\text{Sr}$)_I], post-kinematic granites [negative Eu-anomalies, high (${}^{87}\text{Sr}{}^{86}\text{Sr}$)_I] and granitic gneisses with REE patterns similar to the post-kinematic granites. Sedimentary rocks (turbidites) of the Burwash and Walsh Formations have similar chemical compositions and were derived from 20% mafic-intermediate volcanics, 55% felsic volcanics and 25% granitic rocks. Jackson Lake Formation lithic wackes can be divided into two groups with Group A derived from 50% mafic-intermediate volcanics and 50% felsic volcanics and Group B, characterized by HREE depletion, derived almost exclusively from felsic volcanics.REE patterns of Yellowknife sedimentary rocks are similar to other Archean sedimentary REE patterns, although they have higher $\text{La}{}_{\mathrm{N}}\text{Yb}{}_{\mathrm{N}}$. These patterns differ significantly from typical post-Archean sedimentary REE patterns, supporting the idea that Archean exposed crust had a different composition than the present day exposed crust.     

Neoproterozoic bimodal volcanic rocks and granites in the western Dabie area,northern margin of Yangtze block,China: implications for extension during the break-up of Rodinia

ABSTRACT

To better understand the Neoproterozoic tectonic evolution along the northern margin of Yangtze Block, we have determined the geochronological and geochemical compositions of newly recognized bimodal volcanic suite and coeval granites from the western Dabie terrain. LA-ICP-MS zircon U-Pb dating reveals that the felsic and mafic volcanics from the Hong’an unit have crystallization ages of 730 ± 4Ma and 735 ± 5Ma, respectively, indicating that the bimodal suite was erupted during the Neoproterozoic. The Xuantan, Xiaoluoshan, and Wuchenhe granites yield U-Pb ages of 742 ± 4 Ma, 738 ± 4 Ma, and 736 ± 4 Ma, respectively. The felsic volcanic rocks show peraluminous characteristics, and have a close affinity to S-type granite. The mafic volcanic rocks are basalt in compositions, and are likely generated from a depleted mantle source. The granites belong to high-K calc-alkaline and calc-alkaline series, display metaluminous to peraluminous, and are mainly highly fractionated I-type and A-type granite. The granites and felsic volcanics have zircon εHf(t) values of ?16.4 to + 5.6 and two-stage Hf model ages (T_DM2) of 1.28 to 2.40 Ga, suggesting that they were partial melting of varying Mesoproterozoic–early-Neoproterozoic crust. The granites have εNd_(t) of -14.7 to -1.5, and the two-stage Nd model ages (T_DM2) values of 1.54 to 2.61 Ga, also implying the Yangtze crustal contribution. These Neoproterozoic bimodal suite and coeval granites were most likely generated in a rifting extensional setting, triggered by the mantle upwelling, associated with crust–mantle interaction. Intensive magmatic rocks are widespread throughout the South Qingling, Suizhao, western Dabie and eastern Dabie areas during 810–720 Ma, and show peak ages at ~ 740 Ma. Combining regional geology, we support a continental rifting extensional setting for the north margin of the Yangtze Block during the break-up of the supercontinent Rodinia.     

Petrogenetic evolution of the Early Miocene Ala?amda? volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Ala?amdağ volcano-plutonic complex. The petrogenesis of the Early Miocene Ala?amdağ granitic and volcanic rocks are here investigated by means of whole-rock Sr–Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Ala?amdağ granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Ala?amdağ have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (⁸⁷Sr/⁸⁶Sr)_i and low ε _Nd(t) ratios suggesting Ala?amdağ volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Ala?amdağ volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.     

Barry Granodiorite and Sunset Hills Granite: Wyangala‐style intrusion at the margin of a regional ductile shear zone,northern Lachlan Fold Belt,New South Wales

The Barry Granodiorite is a weakly deformed I‐type, and the Sunset Hills Granite is a moderately deformed S‐type, granite. Both granites were passively intruded into an already foliated greywacke and volcanic sequence. Emplacement may have been facilitated by faults related to the oblique opening of the late Early Silurian Hill End Trough. The granites display a dominant foliation which formed during the late Middle Devonian and subsequently was reoriented during the Early Carboniferous. The Barry Granodiorite and Sunset Hills Granite are on the margin of north‐south ductile shear zones related to the Wyangala Batholith. These granites and the adjacent Carcoar Granodiorite have undergone reorientation during movement on ductile shear zones either due to megakinking during late‐stage north‐south shortening, or southeastward movement of the southern margin of the west‐northwest‐trending Lachlan Transverse Zone.     

Response of U‐Pb Zircon and Rb‐Sr total‐rock and mineral systems to low‐grade regional metamorphism in proterozoic igneous rocks,mount Isa,Australia

A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 ⁺²⁷ _‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in ⁸⁷Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).     

川西新元古代双峰式火山岩成因的微量元素和Sm-Nd同位素制约及其大地构造意义

系统的微量元素和Sm-Nd同位素分析表明,川西地区早震旦世苏雄组双峰式火山岩中的大多数玄武岩具有高的正ε_Nd(T)值(+5～+6)、大离子亲石元素和LREE富集,与现代典型的洋岛玄武岩和大陆溢流玄武岩省中的碱性玄武岩有非常相似的地球化学和同位素组成特征。酸性火山岩的ε_Nd(T)值较低(+1.1～+2.6),地球化学特征总体上与A₂-型花岗岩相似,它们是受地壳混染的OIB型玄武质岩浆在地壳中部的一个“双扩散”岩浆房通过结晶分异形成的。苏雄组双峰式火山岩形成于典型的大陆裂谷环境,非常类似于现代与地幔柱活动有关的高火山活动型裂谷火山岩,扬子块体西缘 800Ma前的裂谷作用和火山活动应是约825Ma前的华南地幔柱活动引发的结果。     

Petrogenesis of Late Silurian volcanism in SW Yunnan (China) and implications for the tectonic reconstruction of northern Gondwana

ABSTRACT

Early Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO₂ ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO₂ content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive ε_Nd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high ε_Nd(t) values of ?1.7 to + 5.4. The Dawazi silicic rocks have high Na₂O/K₂O ratios and positive ε_Nd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China.     

Zircon U-Pb Geochronology and Geochemical Characteristics of the Volcanic Host Rocks from the Tongyu VHMS Copper Deposit in the Western North Qinling Orogen and Their Geological Significance

Precise in situ zircon U-Pb dating and Lu–Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE(e.g. Th, U, Pb and La) and depletion of HFSE(e.g. Nb, Ta, Ti, Zr and Hf). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr(0.703457 to 0.708218) and εNd(t)(-2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-εNd(t) diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon εHf(t) values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores(2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.