Geochemical evolution of intraplate magmatism in the Paleo-Asian Ocean from the Late Neoproterozoic to the Early Cambrian

A group of oceanic islands and/or seamounts (hereafter, paleoseamounts) was produced by oceanic hot-spot magmatism in the Late Proterozoic-Early Cambrian in the southwestern margin of the Paleo-Asian Ocean. They were accreted to the Kuznetsk-Altai island arc in the Late Cambrian and were subsequently incorporated during the closing of the paleocean into the accretionary complexes of the western part of the Altai-Sayan area (southwestern Siberia, Russia). The major-and trace-element compositions and Sr and Nd isotopic systematics of pillow lavas and basalt flows from the Kurai (600 Ma) and Katun’ (550–530 Ma) paleoseamounts of Gorny Altai characterize the evolution of Hawaiian-type magmatism in the Paleo-Asian Ocean during that period. The obtained data show a significant change in lava composition between 600 and 550–530 Ma. The tholeiitic basalts of the Kurai Paleoseamount (600 Ma) from the southern part of Gorny Altai have lower incompatible element contents and higher ¹⁴⁷Sm/¹⁴⁴Nd values compared with the younger tholeiitic and alkali basalts of the Katun’ Paleoseamount (550–530 Ma), whose rocks are exposed in northern Gorny Altai. The trace-element compositions of the Katun’ lavas are similar to those of the Hawaiian tholeiites, and their ¹⁴⁷Sm/¹⁴⁴Nd ratios are lower than those of the Kurai basalts. It was suggested that the older Kurai Paleoseamount was formed above a thinner oceanic lithosphere, i.e., closer to a paleospreading axis compared with the younger Katun’ Paleoseamount. The observed temporal variations in the chemical and isotopic characteristics of lavas are probably related to differences in the degree of melting of the heterogeneous mantle owing to the different thickness of the oceanic lithosphere above which the Kurai and Katun’ paleoseamounts were formed. During the Ediacaran, a plume developed beneath the younger and, consequently, thinner lithosphere of the Paleo-Asian Ocean. The higher degree of melting in the mantle column resulted in a more considerable contribution from the refractory depleted material of the upper mantle. After 50–70 Ma, i.e., in the Early Cambrian, the plume affected a thicker lithosphere, its mantle column became shorter, and the degree of melting was lower. Owing to this, the basaltic melt was more contributed by incompatible element enriched less refractory material of the lower mantle.     

Early Cambrian magmatism in the northeastern Siberian Craton (Olenek Uplift)

The Vendian–Lower Cambrian tectonomagmatic activation took place in the northeastern Siberian Craton, within the Olenek Uplift and in the Kharaulakh segment of the Verkhoyansk fold-and-thrust belt (the lower reaches of the Lena River). The Early Paleozoic volcanic activity in the Olenek Uplift is expressed in the form of basitic diatremes, small basaltic covers, and doleritic dikes and sills intruding and covering the Upper Vendian carbonate deposits. The material specificity of the Lower Cambrian basites and their mantle sources, jointly with the Vendian–Cambrian sedimentation history, gives reason to consider the Lower Cambrian riftogenesis and the associated magmatism as a consequence of the plume–lithosphere interaction in the northeastern Siberian Craton.     

Neoproterozoic to Early Ordovician Evolution of the Paleo-Asian Ocean: Implications to the Break-up of Rodinia

The paper reviews and integrates the recent geological and geochronological data, which allow us to recognize three stages of the evolution of the Paleo-Asian Ocean.The opening of the Paleo-Asian Ocean at 970-850 Ma is dated by the Nersin Complex in the Aldan shield, plagiogranites of the Sunuekit massif, enderbites of the Sludinsk Lake area, and passive margin sediments of the Patoma or Baikal series. The initial subduction (850-700 Ma) is marked by volcanic rocks, trondjemite and gabbro of the Sarkhoy island arc series. Collisions of microcontinents with Siberia at 660 to 620 Ma are evidenced by the exhumation of Muya eclogites (650 Ma), formation of migmatites and amphibolites of the Njurundukan belt (635 and 590 Ma), metamorphic units of the Near-Yenisei belt (640-600 Ma), and orogenic molasse (640-620 Ma). The Paleo-Asian Ocean maximally opened at 620-550 Ma, because at that time a long island arc composed of boninite volcanic rocks was formed. Primitive island arcs of that age have been reconstructed in Kazakhstan, Gorny Altai, West and East Sayan, and North Mongolia. HP and UHP rocks formed in two stages at 550-520 and 520-490 Ma. At 550-490 Ma oceanic islands and Gondwana-derived microcontinents (Kokchetav, Tuva-Mongolian, Central Mongolian and others) collided with the Cambrian-early Ordovician island arc of the Siberian continent. As a result, the island-arc system was extensively modified. Collision occurred twice at 550-520 and 520-490 Ma during which many HP and UHP rocks formed. At that time, the new oceans - the Junggar, Kazakhstan and Uralian - with an Ordovician island arc were formed.     

Evolution of the Karoo-Maud mantle plume in antarctica and its influence on the magmatism of the early stages of Indian ocean opening

New results on the petrochemistry and geochemistry of dolerites from the Schirmacher Oasis shed light on the development of the Karoo-Maud plume in Antarctica. The basalts and dolerites are petrologically identical to the rocks of western Dronning Maud Land (DML), which were previously studied and interpreted as a manifestation of the Karoo-Maud plume in Antarctica. The spatial distribution of the dikes suggests eastward spreading of the plume material, up to the Schirmacher Oasis for at least 10 Ma. The geochemical characteristics of magmas from the Schirmacher Oasis reflect the influence of crustal contamination, which accompanied both the ascent and spreading of the plume. The magmas of the initial stage of plume activity (western DML) appeared to be the most contaminated in crustal components.It was found that the geochemical characteristics of Mesozoic magmas from the Schirmacher Oasis are identical to those of enriched tholeiites from the Afanasy Nikitin Rise and the central Kerguelen Plateau (Hole 749), which indicates that their enrichment was related to the ancient material of the Gondwana continent. This was caused by the opening of the Indian Ocean under the influence of the Karoo-Maud plume. This process was peculiar in that it occurred in the presence of nonspreading blocks of varying thickness, for instance, Elan Bank in the central Kerguelen Plateau, and was accompanied by the formation of intraplate volcanic rises, which are documented in the seafloor relief of basins around Antarctica. The geochemical characteristics of igneous rocks from the resulting rises (Afanasy Nikitin, Kerguelen, Naturaliste, and Ninetyeast Ridge) indicate the influence of processes related to crustal assimilation. The magmatism that occurred 40 Ma after the main phase of the Karoo-Maud volcanism at the margins of the adjacent continents of Australia (Bunbury basalts) and India (Rajmahal trapps) could be generated by the Karoo-Maud plume flowing along the developing spreading zone. The plume moved subsequently and was localized at the Kerguelen Plateau, where it occurs at present as an active hotspot.     

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is D_s = 1.7°, I_s = 43.1°, k_s = 403, α₉₅ = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A₉₅ = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.     

Multiphase carbonate-salt immiscibility in carbonatite melts: data on melt inclusions from the Krestovskiy massif minerals (Polar Siberia)

Minerals of olivine–melilite and olivine–monticellite rocks from the Krestovskiy massif contain primary silicate-salt, carbonate-salt, and salt melt inclusions. Silicate-salt inclusions are present in perovskite I and melilite. Thermometric experiments conducted on these inclusions at 1,230–1,250°C showed silicate–carbonate liquid immiscibility. Globules of composite carbonate-salt melt rich in alkalies, P, S, and Cl separated in silicate melt. Carbonate salt globules in some inclusions from perovskite II at 1,190–1,200°C separated into immiscible liquid phases of simpler composition. Carbonate-salt and salt inclusions occur in monticellite, melilite, and garnet and homogenize at close temperatures (980–780°C). They contain alkalies, Ca, P, SO₃, Cl, and CO₂. According to the ratio of these components and predominance of one of them, melt inclusions are divided into 6 types: I—hyperalkaline (CaO/(Na₂O+K₂O)≤1) carbonate melts; II—moderately alkaline (CaO/(Na₂O+K₂O)>1) carbonate melts; III—sulfate-alkaline melts; IV—phosphate-alkaline melts; V—alkali-chloridic melts, and VI—calc-carbonate melts. Joint occurrence of all the above types and their syngenetic character were established. Some inclusions demonstrated carbonate-salt immiscibility phenomena at 840–800°C. A conclusion in made that the origin of carbonate melts during the formation of intrusion rocks is related to silicate–carbonate immiscibility in parental alkali-ultrabasic magma. The separated carbonate melt had a complex alkaline composition. Under unstable conditions the melt began to decompose into simpler immiscible fractions. Different types of carbonate-salt and salt inclusions seem to reflect the composition of these spatially isolated immiscible fractions. Liquid carbonate-salt immiscibility took place in a wide temperature range from 1,200–1,190°C to 800°C. The occurrence of this kind of processes under macroconditions might, most likely, cause the appearance of different types of immiscible carbonate-salt melts and lead to the formation of different types of carbonatites: alkali-phosphatic, alkali-sulfatic, alkali-chloridic, and, most widespread, calcitic ones.     

Re-Os同位素在晚新元古代至早寒武世测年及古环境演绎中的应用进展

晚新元古代至早寒武世是地球演化历史上极其重要的时期,该时期地层年龄框架的厘定以及古海水、古大气、古气候等方面的研究一直是国际地学界的关注热点。Re-Os同位素测年法是近十几年来兴起的地层测年法,其对于晚新元古代至早寒武世重要时间节点上沉积的富有机质黑色页岩具有测年及示踪古环境的双重优势。本文以晚新元古代至早寒武世地层为例,阐述了Re-Os同位素测年法在年代地层学研究中的应用,通过与大量的、精确的锆石U-Pb年龄对比,论证了Re-Os同位素测定沉积地层年龄的可靠性。对于震旦系陡山沱组地层建阶及留茶坡组地层的年代学研究,黑色页岩Re-Os同位素测年法可发挥重要的作用。结合前人所做的Sr同位素变化曲线以及收集的晚新元古代—早寒武世Os同位素初始值的测试数据,阐述了Os同位素在该时段古环境演绎及Ni-Mo-V多金属层来源追溯上发挥的作用。黑色页岩难以进行~(87)Sr/~(86)Sr的分析,而Os同位素初始值能较好地弥补这一缺陷,二者的相互结合将为晚新元古代至早寒武世古环境演化的研究作出贡献。本文指出,随着Re-Os同位素分析技术的发展以及更多实验流程的建立,该项技术在未来年代地层学,尤其对一些古老地层的年代厘定具有广阔的应用前景。     

Architecture and composition of ocean floor subducted beneath northern Gondwana during Neoproterozoic to Cambrian: A palinspastic reconstruction based on Ocean Plate Stratigraphy (OPS)

The Blovice accretionary complex, Bohemian Massif, hosts well-preserved basaltic blocks derived from an oceanic plate subducted beneath the northern active margin of Gondwana during late Neoproterozoic to early Cambrian. The major and trace element and Hf–Nd isotope systematics revealed two different suites, tholeiitic and alkaline, whose composition reflects different sources of melts within a back-arc basin setting. The former suite has composition similar to mid-ocean ridge basalts (MORB), yet with striking enrichment in large-ion lithophile elements (LILE) and Pb paralleled by depletion in Nb, in agreement with its derivation from depleted mantle fluxed by subduction-related fluids. In contrast, the latter suite has composition similar to ocean island basalts (OIB) with variable contribution of ancient, recycled crustal material. We argue that both suites represent volcanic members of Ocean Plate Stratigraphy (OPS) and indicate that the oceanic realm consumed by the Cadomian subduction was a complex mosaic of intra-oceanic subduction zones, volcanic island arcs, and back-arc basins with mantle plume impinging the spreading centre. Hence, the basalt geochemistry implies that two distinct domains of oceanic lithosphere may have existed off the Gondwana’s continental edge: an outboard domain, made up of old and less buoyant oceanic lithosphere (remnants of the Mirovoi Ocean surrounding former Rodinia?) that was steeply subducted and generated the back-arcs, and young, hot, and more buoyant oceanic lithosphere generated in the back-arcs and later involved in accretionary complexes as dismembered OPS. Perhaps the best recent analogy of this setting is the Izu Bonin–Mariana arc–Philippine Sea in the western Pacific.     

晚震旦世至早寒武世扬子地台北缘碳同位素研究

震旦-寒武交变期是地史上一个重大转折期,亦是一个具有特殊意义的过渡时期.运用碳同位素地球化学方法,探讨扬子地台北缘四川南江地区晚震旦世-早寒武世交变期,碳同位素地球化学异常与环境变化和生物演化的相互联系.南江剖面沉积岩有机碳同位素组成在-35.8‰~-30.1‰间变化;碳酸盐碳同位素组成从-3.5‰~+0.5‰.在灯影组顶部、牛蹄塘组下部和上部,变化的碳、硫同位素组成和不同的黄铁矿与有机碳含量反映了有机碳埋藏量和环境的变化.下寒武统富有机碳和黄铁矿的黑色页岩沉积,暗示了早寒武世早期缺氧环境的存在.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.     

An isotopic biogeochemical study of Neoproterozoic and Early Cambrian sediments from the Centralian Superbasin, Australia

Organic matter from Neoproterozoic and Early Cambrian sediments of the Amadeus and Officer basins of the Centralian Superbasin, Australia, has been studied for biomarker distributions and the carbon isotopic compositions of kerogen and individual hydrocarbons. These sediments represent both shallow and deep water marine facies in the older sections and marine and saline lacustrine carbonate deposits in the Cambrian. Hydrocarbon biomarker patterns were found to be quite consistent with the known sedimentary environments and provide valuable insights into the biogeochemical changes which accompanied the transition from a microbially-dominated ocean to the early stages of metazoan radiation. In particular, carbon isotopic data for n-alkyl and isoprenoid lipids presented here, and in earlier studies, showed a reversal in carbon isotopic ordering between the Proterozoic and Phanerozoic. By comparison with the delta 13C of kerogen, n-alkyl lipids from deep-water Proterozoic sediments were enriched in 13C and appear to be derived mainly from heterotrophs whilst open marine Phanerozoic counterparts are 13C depleted and evidently derived mainly from autotrophs. Data from the samples studied here are consistent with a model invoking a change in the redox structure of the ocean, possibly aided by the innovation of faecal pellets.     

Transmantle (intratelluric) fluid flows: a new model for plumes and plume magmatism

A fluid model for the formation of mantle plumes is proposed. During the emission of gas from the Earth’s core, it accumulates as lenses at the core-mantle boundary. Reaching a critical size, the lenses burst out into the mantle and migrate to the surface. A relatively stationary transmantle fluid flow from the core-mantle boundary arises, which heats the mantle and the layer interacting with it. The flow stops in the base of the hard lithosphere and spreads laterally, causing its melting accompanied by the formation of magma chambers, which, reaching critical sizes, massively intrude and flow out.     

Dating of Neoproterozoic and Cambrian orogenies in Tasmania*

A coherent set of timing constraints is produced for Tasmania's Proterozoic and Cambrian geology when only mineral ages are considered and whole‐rock ages excluded. The oldest recognised event is the formation of sedimentary deposits which contain detrital zircons that indirectly indicate a depositional age younger than 1180 Ma. Partial melts of these sedimentary rocks were incorporated in Neoproterozoic, Devonian and probably Cambrian felsic magmas. Neoproterozoic granite on King Island has an age of 760 ± 12 Ma and is part of a high‐level intrusive episode that accompanied the Wickham Orogeny, an event with regionally varied strain that is represented in northwestern Tasmania by a low‐angle unconformity, by altered granitoid with a magmatic age of 777 ± 7 Ma, and by the thick turbidite pile of the Burnie and Oonah Formations with its syndepositional intrusions of Cooee Dolerite. The late Neoproterozoic was relatively quiet tectonically but by early in the Middle Cambrian a crustal collision which marked the early phase of the Tyennan Orogeny brought about high‐level emplacement of ultramafic‐bearing allochthons and deep‐seated metamorphism of quartzose sedimentary and basaltic rocks. The ultramafic allochthons carried tonalite that had crystallised only shortly before at 510 ± 6 Ma, while the deep‐seated metamorphism produced eclogite at 502 ± 8 Ma. By middle Middle Cambrian times the metamorphic rocks had been uplifted and they experienced repeated uplift during the period of Mt Read volcanism and onward to the close of the Tyennan Orogeny in the Early Ordovician, an overall period of some 20 million years from the early Middle Cambrian. Regionally varied strain was again a feature during the Tyennan Orogeny, with the Smithton area in northwestern Tasmania and King Island occupying relatively undeformed cratonic positions.     

Early Paleozoic magmatism in the Bumbat-Hairhan area of the Lake Zone in western Mongolia (geological,petrochemical, and geochronological data)

We consider the structure, composition, and age of Early Paleozoic intrusive associations of the Bumbat-Hairhan area located in the central part of the island-arc system of the Lake Zone, western Mongolia. There are large polychronous plutons (Bumbat-Hairhan and Gundguzin) and small intrusions in this area. We have established that they are composed of granitoid and gabbroid associations of different lithology, ages, and geodynamic nature. The results of U-Pb dating show that granitoid and basic magmatism took place within 551–465 Ma. Two main stages of intrusive magmatism are recognized, which reflect two stages of the geodynamic evolution of the region: island-arc (551–524 Ma) and accretion–collision (511–465 Ma). At all stages, predominant rocks in the region were diorite–tonalite–plagiogranite and plagiogranite associations of calc-alkaline series..     

Ediacaran mafic magmatism recorded in Cambrian eclogites of the Ross orogen,Antarctica: Implications for the Neoproterozoic rifting episodes along the Pacific-Gondwana margin

We report the first finding of Ediacaran mafic magmatism in northern Victoria Land of the Ross orogen, Antarctica, based on ca. 600–590 Ma magmatic zircon cores in Cambrian eclogites. The mafic magmatism could be either linked to ca. 615–590 Ma incipient convergent margin magmatism in central segment of the Ross orogen, or ca. 600–580 Ma continental rift-related volcanism widespread in eastern Australia. The latter is preferred based on the trace-element compositions distinctive from those of arc-related basalts and the depleted mantle-like Hf isotopic compositions of zircon. Our results suggest dual rifting episodes during both the Ediacaran and Cryogenian (ca. 670–650 Ma) in the East Antarctic margin, correlative with those in eastern Australia. A spatial distribution of coeval rifting and subduction along the Ediacaran margin of East Antarctica is readily accounted for by rift inheritance; the upper- and lower-plate geometry resulting from detachment and transform faulting.     

Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic)

Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe₂O_3tot TiO₂, and P₂O₅ concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ?_Nd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ?_Nd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.     

原特提斯洋俯冲起始——来自北祁连早寒武世弧前岩浆作用的新证据

形成于弧前背景的岩浆岩可能记录了俯冲起始的重要证据。本文以北祁连走廊南山蛇绿混杂岩带新识别出的富铌辉长岩和斜长花岗岩为研究对象,在野外观察的基础上,结合岩石学、地球化学、同位素地球化学和年代学的综合研究,对原特提斯洋初始俯冲的时间进行约束,并探讨其形成过程及构造意义。锆石U-Pb定年结果显示富铌辉长岩样品锆石加权平均²⁰⁶Pb/²³⁸U年龄为512±4 Ma;两个斜长花岗岩样品的锆石加权平均²⁰⁶Pb/²³⁸U年龄分别为522±3 Ma和519±1 Ma。全岩地球化学结果显示富铌辉长岩的Nb含量为7.49×10^-6～10.80×10^-6,TiO₂含量为1.50%～2.08%,Nb/U值为11.9～13.4,(Nb/La)_N>0.5,明显高于岛弧玄武岩,其εNd(t)值介于+4.38～+5.78之间。斜长花岗岩样品K₂O含量(0.31%～1.66%)和K₂O/Na<...     

LA-ICPMS analyses of silicate melt inclusions in co-precipitated minerals: Quantification, data analysis and mineral/melt partitioning

We present a new approach to determine the composition of silicate melt inclusions (SMI) using LA-ICPMS. In this study, we take advantage of the occurrence of SMI in co-precipitated mineral phases to quantify their composition without depending on additional sources of information. Quantitative SMI analyses are obtained by assuming that the ratio of selected elements in SMI trapped in different phases are identical. In addition Fe/Mg exchange equilibrium between olivine and melt was successfully used to quantify LA-ICPMS analyses of SMI in olivine. Results show that compositions of SMI from the different host minerals are identical within their uncertainty. Thus (1) the quantification approach is valid; (2) analyses are not affected by the composition of the host phase; (3) the derived melt compositions are representative of the original melt, excluding significant syn- or postentrapment modification such as boundary layer effects or diffusive reequilibration with the host mineral. With this data we established a large dataset of mineral/melt partition coefficients for the investigated mineral phases in hydrous calc-alkaline basaltic-andesitic melts. The clinopyroxene/melt and plagioclase/melt partition coefficients are consistent with the lattice strain model of Blundy and Wood [Blundy, J., Wood B., 1994. Prediction of crystal-melt partition-coefficients from elastic-moduli. Nature372, 452-454].     

Late Neoproterozoic (Ediacaran)-Early Paleozoic (Cambrian) acritarchs from the Marwar Supergroup,Bikaner-Nagaur Basin,Rajasthan

Characteristic latest Neoproterozoic and Early Paleozoic acritarchs and associated organic-walled microfossils are recorded from the sediments of Marwar Supergroup encountered in BGW-A well (Bikaner-Nagaur Basin) from 1123–481 m depth. Six distinct acritarch assemblages, broadly comparable with globally known Ediacaran (Vendian) and Cambrian assemblages are recognised. The recovered microfossils provide precise age for different units of the Marwar Supergroup whose ages, till now, were poorly understood due to absence or paucity of invertebrate and other mega and microfossils.     

熔体包裹体挥发分应用的研究进展

挥发分(例如H2O、CO2、F、Cl和S)是地幔的重要组成部分,虽然它们在地幔中的含量很低,但是在地幔熔融和熔体演化、地幔不均一、地幔流变学、地幔地震特性和电导率等研究方面具有重要作用。对矿物熔体包裹体和玻璃挥发分的研究已经成为当前的研究热点。其中,熔体包裹体研究凭借其独特的优势成为研究地幔和岩浆挥发分组成的重要手段。熔体包裹体直接捕获了矿物形成时岩浆中的成分,且由于寄主矿物的存在使得熔体包裹体能够保持独立演化而不受外界环境影响,因此能够较为完整地保存岩浆中的挥发分信息。同时,研究熔体包裹体中的挥发分是恢复岩浆喷发前挥发分含量最直接的途径。如果通过现代分析方法(如扫描电镜、电子探针和离子探针等)对熔体包裹体进行详细的岩相学观察以及对后期可能影响熔体包裹体原始挥发组分的作用(地壳混染、岩浆去气、扩散和水化作用)进行评估,并结合实验研究熔体包裹体被捕获后发生的变化而对数据进行矫正,那熔体包裹体对研究岩浆体系中的挥发分将大有可为。基于此,本文系统介绍了熔体包裹体挥发分研究的现状及主要研究内容,主要包括熔体包裹体挥发分的测试方法、挥发分在岩浆中的溶解度、判断挥发分数据可靠性和挥发分研究的经典应用等4个方面。