Age and geochemistry of the Early Mesozoic granitoid massifs of the southern Bureya terrane of the Russian Far East

A massif of porhyritic microcline biotite granites located in the southern part of the Bureya (Turan) terrane has an age of 185 ± 1 Ma. The granites are characterized by (K₂O + Na₂O) > 8%, a K₂O predominance over Na₂O, and a moderately differentiated REE distribution pattern ((La/Yb)_n = 14.7–28.5). The obtained age indicates that at least one stage of the Early Mesozoic granitoid magmatism in the Bureya terrane occurred in the Early Jurassic. The formation of early Mesozoic granitoids was presumably related to collision between North Asian and Sino-Korean cratons, and the intervening Amur superterrane, although a subduction origin also cannot be completely ruled out.     

Serpentinized Peridotite as Source of Aeromagnetic Anomalies

The source of long-wavelength aeromagnetic anomalies appears to originate from the eartlrs deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust. Results show that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.     

冀东麻粒岩相区的变质作用及其演化

根据对表壳岩、中酸性侵入岩和基性岩墙的变质变形作用及相互关系的深入研究,确定冀东麻粒岩相区在早前寒武纪时期主要经历了两期变质作用,每期变质作用均经历了一个长期的演化过程,峰期阶段的变质条件均达到了麻粒岩相。第一期变质作用(3000～2500Ma)的PTt演化具近等压冷却(IBC)特点的逆时针轨迹,第二期变质作用(2300～2000Ma)是近等压加热(IBH)特点的顺时针轨迹,反映两期变质作用具有不同的构造环境和演化过程。     

湘中地体—一个古澳大利亚克拉通的碎片

华南及澳大利亚的地质、地球物理资料对比研究表明:①湘黔桂拼合线,分开了岩石圈结构不同的两个地体,即扬子复合地体和湘中地体;②湘中地体,原系西澳大陆块的一部分,它们在1100～750Ma前从南纬58°漂移万里到达北半球,并与扬子复合地体拼合;③西澳主大陆与湘中地体的离散,发生在奥陶纪晚期或志留纪早期。     

Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km^-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.     

佳木斯地体晚侏罗世－白垩纪古地磁研究及其构造意义

对佳木斯地体盖层鸡西群和桦山群１９８个标本的古地磁研究，确定了该地体晚侏罗世和白垩纪的古磁极位置（６３．４Ｎ，７１．ＯＥ，Ａ９５＝６．１和７２．２Ｎ，７３．７Ｅ，Ａ９５＝３．６）和古地理纬度（５２．５Ｎ和５２．６Ｎ）。白垩纪以后，佳木斯地体相对于松辽地体又有大幅度的位移。     

桐柏─大别山西段的伸展构造

桐柏─大别造山带经历了两阶段的伸展作用。早阶段伸展形成了局限海，并且形成了桐柏─大别山变质核杂岩的雏形。二郎坪岩群、龟山岩组、南湾岩组沉积在局限海中，沉积层序表现为从北向南的迁移。宽坪岩群和二郎坪岩群以低压高温的拉张变质作用为特征。晚期伸展对桐柏─大别山变质核杂岩的最后成型起着重要作用，并产生了中生代的花岗岩侵入和混合岩化。     

Discussion on the Transform of Paleotethys into the Pacific

1.IntroductionTherehavebeenalargeamountofresearchesontheN--SextendingPaleotethysinthePaleozoicandonthemodernE--WextendingPacificOceanintheCenozoic.ButfewhavetouchedonhowthePaleotethysevolvedintothemodemPacific,andthereareonlyguessesaboutthesitllationofthePaleo--Pacificbetweenthetwostages.ThispapertriestodiscussthedynamicmechanismofthePaleo--PacificplateandthecharacteristicsoftheactivitiesofEastAsiacontinentalmarginintheMesozoicthroughtheterranesfromPaleotethysremainingintheEastAsiacont…     

辽宁红透山块状硫化物矿床高级变质下盘蚀变带研究

辽宁红透山块状硫化物矿床(MSD)位于华北地台东北部的浑北花岗岩-绿岩地体内,矿区岩石受到了高级角闪岩相变质.我们的研究表明,广泛分布于红透山层状矿体下盘数百米处的和直接产于矿体下盘的堇青-直闪片麻岩(COG),分别代表了经历过变质的MSD成矿系统的半整合状和筒状海底热液蚀变带.元素地球特征显示,层状COG并非同一种岩石,而是由五种不同岩性的岩石组成,筒状COG的原岩主要由流纹质岩石组成,而在走向上与COG过渡的角闪片麻岩和黑云片麻岩则代表了各种岩性蚀变岩变质后的未蚀变原岩.相对于未蚀变原岩而言,层状COG具富Fe、Mg,贫Na、K、Ca、Rb、Sr和Ba的特征;筒状COG则富Fe、Mg、Si,贫K、Rb、Sr和Ba.蚀变带内Fe、Mg的增加,K的强烈丢失表明,蚀变带在变质前应以绿泥石化为特征.     

Gangdese retroarc thrust belt and foreland basin deposits in the Damxung area, southern Tibet

Geologic mapping and U–Pb detrital zircon geochronologic studies of (meta)sedimentary rocks in the Damxung area (90 km north of Lhasa) of the southern Lhasa terrane in Tibet provide new insights into the history of deformation and clastic sedimentation prior to late Cenozoic extension. Cretaceous nonmarine clastic rocks 10 km southeast of Damxung are exposed as structural windows in the footwall of a thrust fault (the Damxung thrust) that carries Paleozoic strata in the hanging wall. To the north of Damxung in the southern part of the northern Nyainqentanglha Range (NNQTL), metaclastic rocks of previously inferred Paleozoic age are shown to range in depositional age from Late Cretaceous to Eocene. The metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the Damxung thrust prior to being tectonized during late Cenozoic transtension. Along the northern flank of the NNQTL, Lower Eocene syncontractional redbeds were deposited in a triangle zone structural setting. All detrital zircon samples of Cretaceous–Eocene strata in the Damxung area include Early Cretaceous grains that were likely sourced from the Gangdese arc to the south. We suggest that the that newly recognized Late Cretaceous to Early Eocene (meta)clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating Gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern Tibet prior to India-Asian collision.