Devonian alkaline magmatism in South Qinling,China: evidence from the Taohekou Formation,Northern Daba Mountain

The Langao area in South Qinling consists of a poorly dated sequence of volcano-sedimentary units (Taohekou Formation). This study focuses on volcanic edifice of Taohekou Formation. Detailed field mapping, section facies analysis, geochemistry, and Ar–Ar geochronology is presented here to develop a more accurate understanding of the tectonic evolution of the Qinling Orogen. Detailed field mapping and section facies analysis reveals the presence of a seamount volcanic edifice in western Xiaozhen area. Ar–Ar dating of alkaline basalts in the sequence indicates an Early Devonian magmatic age. Whole rock geochemistry of these alkaline basalts indicates that both potassic and sodic basalts were erupted respectively in the eastern and western areas. Both types of samples are typically characterized by high concentrations of TiO₂, Nb, Ta, and Zr. Ocean-island basalt (OIB)-like rare earth element distribution patterns, trace element spider diagrams, and similar Sr and Nd isotopic compositions (i.e. relatively low Sr and moderate Nd isotopic ratios) likely indicate that the samples were comagmatic but have experienced varying degrees of fractional crystallization. In addition, the samples appear to have all originated from the asthenospheric mantle and undergone mantle metasomatism, ultimately resulting in an OIB-type provenance. Most importantly, in combination with its regional tectonic evolution, the Taohekou Formation is constrained to a seamount environment. This formation does not represent a plume but, rather, an extensional fracture zone related to subduction. Thus, this geologic interpretation generates pivotal evidence for accurately investigating the properties and temporal constraints of the Mianlue oceanic basin.     

Chemical and mineralogical heterogeneity in the basal zone of the Partridge River Intrusion: implications for the origin of Cu–Ni sulfide mineralization in the Duluth Complex,midcontinent rift system

Copper–nickel sulfide mineralization in the Partridge River Intrusion of the 1.1 Ga Duluth Complex is restricted primarily to a 100 m thick zone near the base of the intrusion, which is heterogeneous at meter scales in terms of both sulfide contents and rock types, which include dunite, melatroctolite, troctolite, leucotroctolite, gabbro, olivine gabbro, gabbronorite, and rare norite. Olivine-rich troctolites and melatroctolites appear to have required mineral accumulation on a substrate, whereas augite troctolite and gabbros are thought to have formed via in situ crystallization of magmas ranging in composition from high-Al olivine tholeiite to high-Ti tholeiite. δ¹⁸O values of orthopyroxene-poor rocks in the Partridge River Intrusion range from 5.2 to 6.7‰. δ¹⁸O values of 6.7‰ are consistent with less than 20% contamination by high-¹⁸O metasedimentary country rock, either via devolatilization or local partial melting. Rocks with greater than ∼15% orthopyroxene, gabbronorites, and norites, are characterized by δ¹⁸O values in excess of 6.9‰, and required the assimilation of larger amounts of siliceous country rocks. Sulfur isotopic values in leucotroctolitic rocks that contain less than ∼400 ppm S and that overlie the basal zone range between −1.5 and 2‰, values that are consistent with those of mantle-derived sulfur. In contrast, δ³⁴S values in the basal zone range from −1.4 to 10.5‰, where the ³⁴S-enriched samples require an input of sulfur from metasedimentary country rocks. δ³⁴S values of the rocks in the basal zone correlate with variations in olivine Fo content but not with S abundance. The wide range in δ³⁴S values of rocks in the basal zone strongly suggests that magmas interacted with layers in the sedimentary country rocks that were themselves characterized by variable sulfide contents and δ³⁴S values. The S isotopic data suggest that the heterogeneity observed in the basal zone results from the emplacement of relatively thin sheets of compositionally distinct magma. All rock types present in the basal zone can be produced as a result of variable degrees of fractionation of a parental high-Al olivine tholeiite, followed by varying degrees of contamination of derivative liquids by country rocks. The S-contamination process was essential for the development of Cu–Ni mineralization, and was restricted to the earliest stages in the development of the Duluth Complex at a time when volatile species such as S and H₂O, and low-T partial melts of country rocks, were available to magmas. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada

Nepheline natrolite syenites of Center 2 of the Coldwell alkalinecomplex exhibit rare wispy mafic-rich modal layering, extensivexenolith-rich zones and a wide variety of textural types, thelatter resulting from the imposition of high temperature shearingand recrystallization on consolidated syenite. The texturesdeveloped range from allotriomorphic granular to porphyroclasticto mosaic granulob-lastic. The nepheline syenites are pyroxene-poor.Pyroxenes occur most commonly as corroded diopside to diopsidichedenbergite cores surrounded by amphibole and less commonlyas acmitic hedenbergite overgrowths upon cores of iron-richamphiboles. Amphiboles are the dominant mafic phase and rangefrom magnesian hastingsitic hornblende to hastingsite to hastingsitichornblende to ferroedentic hornblende. Nephelines contain excesssilica and have not equilibrated to compositions characteristicof low temperatures. Feldspars lack microcline twinning andperthites and have undergone extensive ion exchange at highsub-solidus temperatutes with sodium-rich fluids. Formationof late stage primary and replacement natrolite, muscovite andthomsonite is characteristic. The nepheline syenites are consideredto be a part of a cycle of continental rift magmatism and tohave been emplaced by cauldron subsidence as a hot hydrous magma.The rocks did not undergo long term subsolidus re-equilibrationas the high temperature mineral assemblage has been preservedby uplift during post-intrusive regional block faulting. Thenepheline syenites were probably dervied by extensive fractionalcrystallization of alkali basaltic magmas.     

Feldspar microtextures and multistage thermal history of syenites from the Coldwell Complex,Ontario

Optical and TEM (transmission electron microscopy) observations of perthites from augite syenites in the Coldwell Complex (Ontario) reveal a complex set of microtextures that outline a multistage thermal history. Regular microtextures (linear or braid texture, straincontrolled, coherent intergrowths) show a progressive evolution from the margin of the intrusion inwards with lamellar spacings in the range 40–100 nm. The textures evolve in a manner similar to those for the Klokken intrusion and reflect differences in cooling rates and bulk composition. Superimposed upon the regular microtexture are 10 m scale compositional fluctuations which we call ripples. The boundary relationships and bulk composition of ripples, which are themselves Ab-rich and Or-rich linear coherent cryptoperthites, suggest that they formed by coarsening during a phase of high-temperature (530°C) fluid-feldspar interaction. This was followed by a return to coherent exsolution in which fluid was not involved. Coarse, irregular, patch microperthite cross-cuts all other microtextures. These final deuteric intergrowths are believed to result from a further low-temperature (< 380° C) fluid-feldspar interaction and are associated with subgrain formation and the presence of micropores. The outermost syenite sample, against a gabbro ring structure, has distinctive, modified microtextures, indicating that the gabbro is, at least in part, a later intrusion. Our findings show that TEM work on alkali feldspar microtextures can identify discrete thermal events in the cooling history of igneous plutons and illustrates the potential of such microtextures for establishing the relative ages of intrusive rocks.     

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997     

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa_83–93), ferroaugite (Di₅₀Hd₄₅Ac₅–Di₅Hd₉₀Ac₅)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa₉₃), and acmitic-hedenbergite (Di₅Hd₅₀Ac₅–Ac₅₀Hd₅₀).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10^–15 to 10^–23–10^–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.     

Platinum-group minerals in fluid inclusions from the Marathon deposit,Coldwell Complex,Canada

Summary Platinum-group minerals (PGM) project into fluid inclusions that occur in chalcopyrite and cubanite from the Marathon deposit, Two Duck Lake gabbro, Coldwell Complex, Ontario. Semi-quantitative analyses of the micron-sized PGM were made by SEM-EDS; they reveal Ag-bearing intermetallic compounds of Pd₃Sn-Pd₃Pb-Pd₃Te (i.e., atokitezvyagintsevite-keithconnite) and telargpalite (Pd₂AgTe) on broken, irregular surfaces of the Cu-Fe-S minerals. Halite daughter minerals, and quenched brine occur in and around some opened fluid inclusions. These data confirm the hypothesis based on petrography and mineral compositions that saline fluids remobilized PGE, Cu, and other elements and precipitated them well after crystallization of sulfide and silicate magmas in the Marathon deposit.

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Platin-Gruppenminerale in Flüssigkeitseinschlüssen aus der Marathon Lagerstdtte, Coldwall Komplex, Kanada

Zusammenfassung Platin-Gruppenminerale (PGM) ragen in Flüzssigkeiteinschliisse, die im Kupferkies und Cubanit der Marathon Lagerstätte, Two Duke Lake Gabbro, Coldwell Komplex, Ontario, vorkommen, hinein. Semiqantitative Analysen der mikrongroßen PGM wurden mit einem SEM-EDS durchgeführt. Sie ergaben Ag-führende, intermetallische Verbindungen von Pd₃Sn-Pd₃Pb-Pd₃Te (Atokit-Zvyagintsevit-Keithconnit) und Telargpalit (Pd₂AgTe) auf zerbrochenen, irregulären Oberflächen der Cu-Fe-SMinerale. Steinsalz-Tochterkristalle und abgeschreckte Salzlösungen treten innerhalb und in der unmittelbaren Umgebung geöffneter Flüssigkeitseinschlüsse auf. Diese Daten bestätigen die Hypothese, basierend auf der Petrographie und der Mineralzusammensetzung, daß saline Fluida PGE, Cu und andere Elemente remobilisieren können, und daß d iese nach der Kristallisation der Sulfide und des Silikatmagmas in der Marathon Lagerstätte zur Ablagerung gekommen sind.

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With 3 Figures     

The Neoproterozoic continental rift magmatism of the eastern Jiangnan orogen: new evidence from the mafic sheets in northern Zhejiang Province,South China

Massive mafic sheets were recently recognized intruding the Neoproterozoic strata in Fuyang area, eastern Jiangnan orogen. Geochronological, geochemical, and isotopic studies were carried out to understand their mantle source, crust–mantle interaction, and tectonic setting. LA-ICP-MS U-Pb zircon data indicate that the sheets were generated at 808 ± 7 Ma. The mafic sheets consist of two groups: high Ti and low Ti. They are enriched in light rare earth elements (LREE; 3.3–5.3 ppm) and show negligible Eu anomalies (δEu = 0.77–1.12). They also have strong large-ion lithophile element (LILE; Sr, K, Rb, Ba) enrichment, moderately strong high-field-strength element (HFSE) enrichment (except for Nb-Ta depletion), and positive εNd(t) (5.1–9.1). Geochemical and isotopic data indicate that the mafic sheets were generated from a depleted asthenospheric mantle source. The high-Ti mafic sheets have higher HFSE contents and less Nb-Ta depletion than the low Ti series, indicating a lower degree of partial melting and crustal contamination. The mafic sheets grew in a within-plate setting, concurrent with the ~820–750 Ma rifting events in the eastern Jiangnan orogen. They are likely related to the breakup of the Rodinia supercontinent.     

Paleozoic tholeiitic magmatism of the Kola province: Spatial distribution,age, and relation to alkaline magmatism

This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380–390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume–lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.     

Devonian alkaline magmatism in the northern North China Craton: Geochemistry,SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton(NCC).The intrusion is mainly composed of quartz-monzonite.Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca.381.5 Ma.The rock is metaluminous with high(Na_2O + K_2O) values ranging from 8.46 to 9.66 wt.%.The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies.The Wulanhada rocks exhibit high initial values of(~(87)Sr/~(86)Sr)_t = 0.70762-0.70809,low ε_(Nd)(t) =-12.76 to-12.15 values and negative values of ε_(Hf)(t) =-23.49 to-17.02 with small variations in(~(176)Hf/~(177)Hf),(0.281873-0.282049).These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC.The Wulanhada rocks,together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions,constitute a post-collisional magmatic belt along the northern NCC.     

Parental melts of melilitolite and origin of alkaline carbonatite: evidence from crystallised melt inclusions, Gardiner complex

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na₂O + K₂O/Al₂O₃ > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai. Received: 18 January 1996 / Accepted: 2 September 1996     

Complex rift geometries resulting from inheritance of pre-existing structures: Insights and regional implications from the Barmer Basin rift

Structural studies of the Barmer Basin in Rajasthan, northwest India, demonstrate the important effect that pre-existing faults can have on the geometries of evolving fault systems at both the outcrop and basin-scale. Outcrop exposures on opposing rift margins reveal two distinct, non-coaxial extensional events. On the eastern rift margin northwest–southeast extension was accommodated on southwest- and west-striking faults that form a complex, zig-zag fault network. On the western rift margin northeast–southwest extension was accommodated on northwest-striking faults that form classical extensional geometries.Combining these outcrop studies with subsurface interpretations demonstrates that northwest–southeast extension preceded northeast–southwest extension. Structures active during the early, previously unrecognised extensional event were variably incorporated into the evolving fault systems during the second. In the study area, an inherited rift-oblique fault transferred extension from the rift margin to a mid-rift fault, rather than linking rift margin fault systems directly. The resultant rift margin accommodation structure has important implications for early sediment routing and depocentre evolution, as well as wider reaching implications for the evolution of the rift basin and West Indian Rift System. The discovery of early rifting in the Barmer Basin supports that extension along the West Indian Rift System was long-lived, multi-event, and likely resulted from far-field plate reorganisations.     

Cryptic sediment-hosted critical element mineralization from eastern Yunnan Province,southwestern China: Mineralogy,geochemistry, relationship to Emeishan alkaline magmatism and possible origin

A previous study briefly described the occurrence of a new type of Nb(Ta)-Zr(Hf)-REY-Ga (REY: rare earth elements and yttrium) polymetallic mineralization in eastern Yunnan, southwest China. In this paper, the mineralogical and geochemical features have been further advanced through a study of two regionally extensive and relatively flat-lying mineralized layers from No. XW drill core. The layers are clay-altered volcanic ash and tuffaceous clay, and are dominated by clay minerals (mixed layer illite/smectite, kaolinite, berthierine, and chamosite); with lesser amounts of quartz and variable amounts of anatase, siderite and calcite; along with trace pyrite, barite, zircon, ilmenite, galena, chalcopyrite, and REE-bearing minerals. The mineralized samples have higher Al₂O₃/TiO₂ values (13.7–41.4) and abundant rare metal elements (Nb, Ta, Zr, Hf, REE, Ga, Th, and U) whereas less mineralized samples are rich in V, Cr, Co, and Ni and have lower Al₂O₃/TiO₂ values (2.32–7.67). The mineralized samples also have strong negative δEu in chondrite-normalized REE patterns. Two processes are most likely responsible for the geochemical and mineralogical anomalies of the mineralized samples: airborne volcanic ash and multi-stage injection of low-temperature hydrothermal fluids. Based on paragenetic analysis, this polymetallic mineralization is derived from the interaction between alkaline volcanic ashes and subsequent percolation of low-temperature fluids. The intense and extensive alkaline volcanism of the early Late Permian inferred from this study possibly originated from the coeval Emeishan large igneous province (ELIP). This unique Nb(Ta)-Zr(Hf)-REE-Ga mineralization style has significant economic and geological potential for the study of mineralization of the lowest Xuanwei Formation.     

Late paleozoic-Early Mesozoic within-plate magmatism in North Asia: traps, rifts, giant batholiths, and the geodynamics of their origin

A number of large areas of igneous provinces produced in North Asia in the Late Paleozoic and Early Mesozoic include Siberian and Tarim traps and giant rift systems. Among them, the Central Asian Rift System (CARS) has the most complicated structure, evolved during the longest time, and is a large (3000 × 600 km) latitudinally oriented belt of rift zones extending from Transbaikalia and Mongolia to Middle Asia and including the Tarim traps in western China. CARS was produced in the Late Carboniferous, and its further evolution was associated with the lateral migration of rifting zones; it ended in the Early Jurassic and lasted for approximately 110 Ma. CARS was produced on an active continental margin of the Siberian continent and is noted for largest batholiths, which were emplaced simultaneously with rifting. The batholiths are surrounded by rift zones and compose, together with them, concentrically zoned magmatic areas, with crustal (granitoid) magmatism focused within their central portions, whereas mantle (rift-related) magmatism is predominant in troughs and grabens in peripheral zones. The batholiths show geological and isotopic geochemical evidence that their granitoids were produced by the anatexis of the host rocks at active involvement of mantle magmas. Zonal magmatic areas of the type are viewed as analogues of large igneous provinces formed in the environments characteristic of active continental margins. Large within-plate magmatic provinces in North Asia are thought to have been generated in relation to the overlap of at least two mantle plumes by the Siberian continent during its movement above the hot mantle field. In the continental lithosphere, mantle plumes initiated within-plate magmatic activity and facilitated rifting and the generation of traps and alkaline basite and alkali-salic magmatic associations. Because of the stressed states during collision of various type in the continental margin, the mantle melts did not ascend higher than the lowest crustal levels. The thermal effect of these melts on the crustal rocks induced anatexis and eventually predetermined the generation of the batholiths.     

华北克拉通中元古代裂解事件:以渣尔泰-白云鄂博-化德裂谷带岩浆与沉积作用研究为例

华北克拉通存在三个主要的中元古代裂谷带,从南到北分别为熊耳裂谷带、燕辽裂谷带以及渣尔泰-白云鄂博-化德裂谷带。其中熊耳群中火山岩的峰期年龄为1780~1750Ma,其上还有形成于被动大陆边缘的五佛山群、汝阳群以及官道口群。中北部的燕辽裂谷带包括长城系、蓟县系和青白口系,其中长城系团山子组和大红峪组火山岩的年龄分别为~1640Ma和1626~1622Ma,蓟县系高于庄组、雾迷山组和铁岭组凝灰岩的年龄分别为1560Ma、1485Ma和1437Ma,而下马岭组凝灰岩年龄为1366~1380Ma。北缘渣尔泰-白云鄂博-化德裂谷带中渣尔泰群书记沟组玄武岩年龄为1743Ma,阿古鲁沟组酸性火山岩年龄为~810Ma,白云鄂博群尖山组中基性火山岩年龄为1728Ma,化德群比鲁特组火山碎屑岩年龄为1515Ma。中元古代岩浆事件除了裂谷带中的火山作用外,还包括三期基性岩墙群(~1780Ma太行-吕梁岩墙群、~1730Ma密云岩墙群和~1620Ma泰山岩墙群)以及1.76Ga到1.65Ga非造山岩浆组合(斜长岩-环斑花岗岩体-A型花岗岩)。中元古代中期,华北克拉通北缘发育了基性岩席(墙)、A型花岗岩以及碳酸岩脉,双峰式岩浆作用说明华北北缘在中元古代中期经历了裂谷作用,与哥伦比亚超大陆的最终裂解有关,并且与白云鄂博巨型REE-Nb-Fe矿床的形成具有成因上的联系。华北克拉通北部两个裂谷带中的地层具有可以对比的层序以及时代,而中元古代中期辉绿岩墙、A型花岗岩以及碳酸岩脉可以与其它克拉通同时期的非造山岩浆作用对比,证明华北克拉通经历了哥伦比亚超大陆的最终裂解。古地磁数据已经证明在哥伦比亚超大陆时期Siberia、Laurentia、Baltica、Amazion以及华北克拉通是连接在一起的,而北缘中元古代中期大陆裂谷相关岩浆岩的发现也说明它是与另一个古大陆相连的。华北克拉通南缘熊耳火山岩的构造背景到底是大陆裂谷还是大陆边缘弧则关系着其是与另一个克拉通相连还是面向大海,这需要我们进一步深入研究。     

Evolution of late Triassic rift basin evaporites (Passaic Formation): Newark Basin, Eastern North America

The Passaic Formation of the late Triassic Newark Supergroup is 2700 m thick and was deposited in series of wide, deep to shallow lacustrine environments in the Newark rift basin (eastern North America). The Passaic Formation can be divided into lower, middle, and upper sections based on depositional structures, composition and the distribution and morphology of its evaporites. Evaporites formed as a result of syndiagenetic cementation and/or displacive processes. Evaporitive minerals now include gypsum and anhydrite, although other mineral species, such as glauberite, may have originally existed. Most of the evaporites of the Passaic Formation occur within massive red mudstone and siltstone lithologies in the form of diffuse cements, void-fillings, euhedral crystals, crystal clusters and nodules. These evaporites grew displacively within the fine siliciclastic matrix as a result of changes in the hydrochemical regimes of the rift basin. A well-developed upward increase in the amount of evaporite material is present in the Passaic Formation. This resulted from: (1) long-term, progressive increase in aridity, and (2) significant increase in evaporation surface area of the basin during its tectonic evolution. A nonmarine source for the evaporites is evident from the isotopic data. Sulphate δ³⁴S ranges from 11%. to 3.3%. CDT, while δ¹⁸O ranges from + 15.1%. to + 20.9%. SMOW, indicating derivation from early diagenetic oxidation of organic sulphur and pyrite within the organic-rich, lacustrine deposits. The ⁸⁷Sr/⁸⁶Sr ratios in sulphate are radiogenic (average 0.71211), showing the interaction of basin waters with detrital components and that the Newark Basin was isolated from the world ocean. Most of the original evaporites show evidence of diagenetic change to polycrystalline and polymineralic pseudomorphs now filled with recrystallized coarse-grained anhydrite (1–3 mm size) and low-temperature albite. Homogenization temperatures of fluid inclusions within the coarse-grained anhydrite indicate crystallization temperatures for anhydrite in the range of 150° to 280°C. Such elevated temperatures resulted from circulation of hot water in the basin. Later exhumation of these rocks caused partial to total replacement of anhydrite by gypsum in the upper part of the section. The resulting increase in volume due to hydration of anhydrite at shallow depths also emplaced non-evaporative satin-spar veins (fibrous gypsum) along bedding planes and in fractures. While the local geology of the Newark rift basin controlled the distribution of facies, the sedimentological development of the Passaic Formation evaporites resulted from the world-wide climatic aridity that prevailed during the late Triassic. because the Newark Basin sequence was only covered with about 3 km of sedimentary overburden that correspond to about 100°C and hence suggests that evaporites have experienced alteration by hot fluids. 5 As the Triassic marks the greatest evaporite formation world-wide and profound sense of parched continentality throughout the world existed before the final break-up of the Pangea, the Passaic Formation evaporites are an example of the influence of these palaeoclimatic conditions at the eastern margin of North America.     

Petrological and age relationship between emplacement of magmatic breccia, alkaline magmatism, and static metamorphism in the North Pyrenean Zone

A magmatic breccia showing peculiar structural and geochemical features has been found in the North Pyrenean Zone (Lherz area). The features (concentric zoning, “fluidal” structure, reaction rim at the matrix-clast interface, high Co, Sc and REE contents) indicate a genetic relationship between the breccia and fluids of deep origin which have infiltrated the Mesozoic series. These fluids, probably related to the alkaline magmatism commonly observed in the Pyrenees, could be locally responsible for the static Pyrenean metamorphism.³⁹Ar/⁴⁰Ar age determinations have been made in order to compare (a) the age of the neoblastesis in the magmatic breccia matrix (95 ± 2 Ma), (b) the age of the unoriented metamorphic minerals in the surrounding marbles (98.5 ± 2.2 Ma) and (c) the age of the alkaline magmatism in the Corbières and Lherz areas (95.4 ± 2.3 Ma and 101.2 ± 2.5 Ma, respectively).     

Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism from Kutch basin, northwest India

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill.IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° (α₉₅=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (dp/dm=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.