赣南白面石盆地双峰式火山岩的Rb—Sr和Sm—Nd同位素特征

江西南部寻邬县的白面石盆地,分布一套菖蒲组基性玄武岩与酸性流纹岩互层的火山岩组合。其Ｒｂ－Ｓｒ等时线年龄：玄武岩为１７３Ｍａ,流纹岩为１６５Ｍａ,说明这是一套中侏罗世双峰式火山岩。通过进一步的同位素研究,玄武岩具有高εｓｒ及近于零的εｅｒ值,显示其明显受到地壳物质的混染;流纹央则以其高εｅｒ及低εｅｒ值,表现为地壳成因。     

U-Pb zircon,Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the western Alps

Three meta-acidic rocks from the western Italian Alps, a magnesiochloritoid-bearing metapelite from the Monte Rosa massif, a coesite-pyrope-quartzite from the Dora Maira massif and the Monte Mucrone granite in the Sesia Zone, have been studied by U-Pb zircon, Rb-Sr on whole-rock, apatite and phengite and Sm-Nd wholerock methods. The mineral parageneses of the investigated rocks indicate high- to very-high-pressure and medium-to-high-temperature metamorphism. This combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of U-Pb zircon systems in three intensely metamorphosed areas of the Pennine domain. The U-Pb zircon data have yielded a magmatic age for the Monte Mucrone granite at 286±2 Ma. This result confirms the occurence of late-Hercynian magmatism in the Sesia Zone, as in other Austro-Alpine units and in other areas of the European crystalline basement. In the Monte Rosa massif, a geologically meaningless lower intercept age of 192±2 Ma has been interpreted as an artefact due to a complex evolution of the U-Pb zircon system. The magmatic shape of the zircons implies a magmatic or volcano-sedimentary protolith for this rock, originally considered as a metasediment. The very-high-pressure metamorphism in the Dora Maira quartzite has produced an opening of the U-Pb zircon system at 121+12–29 Ma. The Rb-Sr data support the occurence of high-grade metamorphism during Cretaceous times. Phengites model ages are slightly younger than the U-Pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation. The Nd model ages from the whole-rock samples, as well as the U-Pb upper intercept ages from zircons of all three investigated rocks, indicate the presence of Proterozoic crustal components inherited from the precursors of these meta-acidic rocks. The studied zircon populations and their U-Pb systems apparently showed open-system behaviour only when affected by extreme metamorphic conditions (700–750° C, > 28 kbar), whereas eclogite-facies conditions of 500–550° C and 14–16 kbar were not enough to disturb significantly the U-Pb zircon evolution. It is also probable that the sedimentary or magmatic origin of the protoliths of these meta-acidic rocks, which involved different characteristics such as grain-size and fluid phase concentration and composition, could be another important factor controlling the U-Pb zircon system behaviour during metamorphic events.     

Rb-Sr age of lunar igneous rocks 62295 and 14310

We report an age of crystallization for spinel-troctolite (VHA basalt) 62295 of 4.00 ± 0.06 × 10⁹ yr (I = 0.69956 ± 6) and an age of crystallization for KREEP-rich basalt 14310 of [3.94 ± 0.03 × 10⁹yr (I = 0.70041 ± 5). The ages probably date the cooling of shock melts.     

Isotopic age of the Permian-Triassic basaltic magmatism in the Polar Cis-Urals: Rb-Sr and Sm-Nd data

Two basaltic lava flows are confined to the Permian-Triassic boundary in succession of continental deposits of the Polar Cis-Urals that is confirmed by Rb-Sr (250 ± 15 Ma) and Sm-Nd (249 ± 17 Ma) dates obtained for the lower flow in the Paemboi-Khalmer-Yu area of the Korotaikha depression. The analyzed basalts are correlative in age with Siberian plateau basalts (250 Ma) and the Permian-Triassic boundary dated at 251 Ma in the Meishan section of marine deposits in South China.     

Garnet-pyroxene and lawsonite-bearing rocks of the Maksyutov Complex (Southern Urals)

The new petrological and geochrological data are used to constrain the nature of garnet-clinopyroxene and lawsonite-bearing rocks, which contain a rare variety of Fe-Ca-rich garnets. These rocks associated with antigorite serpentinites have no equivalents in the other units of the Maksyutov eclogite-glaucophane schist complex and, thus, can be regarded as a separate “mafic-ultramafic” unit. Based on their mineral and chemical composition, the garnet-clinopyroxene and lawsonite-bearing rocks can be interpreted as HP associations formed within a deep continental rifting setting. They experienced a series of metasomatic alterations during decompression exhumation and were accreted to the Maksyutov Complex as a result of the arc-continent collision. The U–Pb zircon data indicate that a Late Riphean–Lower Paleozoic age (824 and 440–470 Ma) was a crystallization age of garnet-clinopyroxene rocks and Ar–Ar white mica age (341 ± 2 Ma) represents the timing of the final accretion of all structural unit to the Maksyutov Complex.     

Perspectives of phosphorite mineralization in Lower Paleozoic rocks of the Zilair synclinorium,Southern Urals

Data on the phosphorite mineralization, lithology, and variability of Lower Paleozoic rocks in the Zilair synclinorium and Yuryuzan basin are given. Comparison of Lower Paleozoic rocks with type sections of phosphate-bearing rocks in the world suggested their compliance with the phosphorite-enclosing rocks of the fine-grained phosphorite association. Analysis of the facies variability of rocks indicate the possibility of discovery of large phosphorite deposits in the Yuryuzan and Syuren subzones of the Western structural-facies zone. Phosphorite deposits are unknown in the synclinorium because of their instability during hypergenesis and burial under allochthonous loose sediments of the Cenozoic.     

内蒙古西北部宝音图群Sm—Nd和Rb—Sr地质年代学研究

内蒙古西北部的宝音图群由角闪岩相的变质地层组成。该群中斜长角闪片岩的Sm-Nd全岩等时线年龄为2485±128Ma,可代表宝音图群的形成年龄。等时线的ε_(Nd)(t)值为2.73±2.81,说明岩浆来源于亏损的地幔源区。相同样品的Rb-Sr全岩等时线年龄为623±36Ma,显然反映后期Rb-Sr体系重新均一化的强烈影响,因此该年龄被解释为变质年龄。上述结果表明宝音图群形成于古元古代早期,应属华北板块北缘已知的最古老岩石地层单位,并保留了新元古代末期构造—变质事件的记录。这些认识有助于查明华北板块北缘元古宙构造演化的历史。     

Zircon geochronology of the Mashak volcanic rocks and the problem of the age of the lower-middle Riphean boundary (Southern Urals)

In the type sections of the Riphean within the Bashkirian mega-anticlinorium (Southern Urals), the Mashak Formation represents a basal unit of the Middle Riphean erathem. The formation comprises throughout its area of distribution the alternation of volcanic, volcano-sedimentary, and sedimentary sequences and is divided into the lower, middle, and upper subformations. The volcanic rocks containing zircons (four samples, rhyodacite and rhyolite collected at Mashak, Berezyak, and Bolshoi Shatak ranges) are largely confined to the lower subformation. Analyses were performed using a SHRIMP II methodology, with special attention to the mineralogical characteristics of zircons, including their habit, morphology, preservation, and inclusions. All zircons show similarities in their mineral chemistry and geochemistry, which are indicative of the geochemical affinity of the volcanic rocks. At the same time, all zircon grains are characterized by specific typological parameters, which may equally reflect the parameters involved in the development of such volcanic rocks under different conditions. The integrated U-Pb age of zircons (SHRIMP II, VSEGEI, St. Petersbrug) from the four samples is 1383 ± 3 Ma. On the basis of the age of the Berdyaush gabbro-granitoid intrusion (up to 1410 Ma), the most likely age of this boundary is 1400 Ma, which is equated to the Calymmian and Ectasian of the International Stratigraphic Scale.     

Zirconology of ultrabasic rocks of the Karabash massif (Southern Urals)

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530–560 Ma) and Early–Late Ordovician (440–480 Ma).     

Chronological and genetic relationships between intrusive rocks of the Berdyaush pluton,South Urals,in light of new U-Pb and Sm-Nd isotopic data

Geological observations in combination with previously published and new isotopic data allowed us to reconstruct the history of geological events that eventually gave rise to the formation of the Berdyaush pluton situated on the western slope of the South Urals: (1) emplacement of gabbro into Lower Riphean sedimentary rocks (1410–1390 Ma); an enriched mantle source of gabbro arising in the Archean or Paleoproterozoic; (2) formation of granitic melt in the lower crust; Archean TTG association subsequently enriched in K and correlative elements as a result of interaction with enriched mantle-derived magmas and related fluids was a magma source; mixing of mantle and crustal magmas in the course of their synchronous ascent with formation of hybrid intrusive rocks; injections of mafic and hybrid melts into incompletely solidified granite; fragmentation of such injections with the formation of melanocratic nodules; emplacement of basic dikes into the cooled granite—all these events took place 1410–1360 Ma ago; (3) discrete episodes of partial melting of enriched mantle source with waning intensity; formation of minor volumes of melt, which solidified under auto- and paraautochthonous conditions as local domains highly enriched in incompatible elements (1360–1270 Ma); (4) partial melting of those domains resulting in the formation of minor nepheline syenite intrusions (915–800 Ma), containing relict zircon grains dated at >1270 Ma; (5) injections of mantle-derived alkaline melt contaminated with crustal granitic material as microsyenite and syenite porphyry dikes (700–500 Ma ?). Thus, the Berdyaush pluton is a projection of a local domain of mantle and crustal magma generation, which periodically resumed its activity over almost a billion years.