重新认识宿松群

本文在对原宿松群进行深入研究和解剖基础上，将其解体为浦河片麻岩套与宿松岩群。重新厘定了宿松岩群自下而上，由甘田拗岩组（新建）、柳坪岩组、大新屋岩组、梓树坞岩组（新建）所组成的构造地层序列。分析了宿松岩群在南部与原大别群非不整合接触关系，在北部为过渡关系。从孢粉组合，地层序列等方面进一步论证了宿松岩群属震旦系。为重新认识大别山变质造山带的形成演化及高压带形成折返机制提供了新的地质依据。     

Permo–Triassic Barrovian-type metamorphism in the ultrahigh-temperature Kontum Massif, central Vietnam: Constraints on continental collision tectonics in Southeast Asia

The Kontum Massif in central Vietnam is composed of various metamorphic complexes including a high-temperature southern part (Kannak and Ngoc Linh complexes) and a low- to medium-temperature northern part (Kham Duc complex). The Kham Duc complex exhibits Barrovian-type medium-pressure metamorphism evidenced by kyanite- and/or staurolite-bearing metapelites. The garnet–gedrite–kyanite gneiss, which is the focus of the present study, preserves several mineral parageneses formed during a prograde and retrograde metamorphic history: staurolite + quartz in gedrite, garnet + gedrite + kyanite in the matrix, and spinel + cordierite symplectite between gedrite and sillimanite. The calculated semiquantitative petrogenetic grid reveals peak pressure conditions of 620–650°C at 1.1–1.2 GPa and peak temperature conditions of 730–750°C at 0.7–0.8 GPa. The monazite U–Th–Pb electron microprobe ages of the garnet–gedrite–kyanite gneiss and associated gneisses yield 246 ± 3 Ma for the Kham Duc complex, which is similar to the age of the high- to ultrahigh-temperature metamorphism in the adjacent Kannak and Ngoc Linh complexes of the southern Kontum Massif. The present results indicate that both the Barrovian-type and ultrahigh-temperature metamorphism occurred simultaneously in the Kontum Massif during an event strongly related to Permo–Triassic microcontinental collision tectonics in Asia.     

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx–Pl) but were metamorphosed on cooling into the high-pressure (Grt–Cpx–Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust.     

河北平山英云闪长质片麻岩锆石SHRIMP年代学

平山板桥沟地区的英云闪长质片麻岩常量元素具有高硅、高铝、高钠和低钾的特点,稀土元素显示强烈分异,具有较高的(La／Yb)。值(34),同时具有明显的Nb,Ta,Ti和P负异常,与太古宙TTG的特征完全一致。阴极发光和背散射研究表明,英云闪长质片麻岩中锆石具有密集震荡环带和较高的Th／U值,表现典型岩浆锆石的特点,但其外侧都有一高亮度(CL)的狭窄变质增生边。SHRIMP测试结果表明TTG的形成年龄为2536．5±9．8Ma,变质年龄为1802±43Ma。推测2500Ga西部块体向东部陆块俯冲形成了TTG,并导致五台陆块与太行山陆块拼合,使华北克拉通化,1．8Ga则为陆内的另一次构造事件。     

STUDY ON THE ISOTOPIC CHRONOLOGY AND THE TECTONIC SIGNIFICANCE OF DUGUER GRANITIC GNEISS IN CENTRAL QIANGTANG, TIBET

STUDY ON THE ISOTOPIC CHRONOLOGY AND THE TECTONIC SIGNIFICANCE OF DUGUER GRANITIC GNEISS IN CENTRAL QIANGTANG, TIBET     

The protoliths to the sillimanite gneisses from the Larsemann Hills and geological implication in their formation

The source rock from which the sillimanite gneisses derive mainly was the biotite plagioclase gneiss in the Larsemann Hills. It is the deformation-metamorphism process under special pressure and temperature condition, not the original rock compositions, that controls the presence of sillimanite. To a great degree, the sillimanite gneiss was the mixture of the detaining materials of the migrating felsic melt from the bt-plagioclase gneiss that underwent partial melting and the relics when the melt was removed. In sillimanitization the original rock had been changed substantially in chemical composition. The related metamorphism process severely deviated from the isochemical series, the process was of, therefore, an open system. In addition, the Al2O3 contents of the original rock was an important, but not critical factor for the formation of sillimanite, i.e., the sillimanite-bearing rock need not be of aluminum rich in composition, and vise contrarily, the aluminum rock may not produce sillimanite. The authors of the present paper postulate that the source rock from which the aluminum rich rock derives need not be of aluminum rich, but sillimanitization is generally the Al2O3 increasing process. The aluminum rich sediments such as clay or shale need not correspond directly to sillimanite-rich gneisses. No argillaceous rock present equals to sillimanite-rich gneiss in chemical composition. The protoliths to the sillimanite gneisses from the Larsemann Hills, east Antarctica, and their adjacent area may be pelite, shale greywacke, sub-greywacke, quartz sandstone and quartz-tourmalinite. If correct, the conclusion will be of significant implication for the determination of the sillimanite gneiss formation process and the reconstruction of the protolith setting.     

湖南省道县虎子岩片麻岩包体的岩石学特征和年代学研究

通过对道县虎子岩碱性玄武岩中片麻岩包体的岩石化学、Ｎｄ同位素模式年龄ＴＤＭ和颗粒锆石Ｕ－Ｐｂ法测年结果，确定了本区片麻岩包体的变质反应、变质相和原岩类型，恢复了本区前寒武纪地壳的形成和演化历史     

Geology,geochemistry and geochronology of the Archaean Peninsular Gneiss around Gorur,Hassan District,Karnataka, India

The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF₁). These have been over printed by near-coaxial open folding (DhF₁₂) and non-coaxial upright folding on almost N-S trend (DhF₂). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF₁ deformation. The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (Ce_N/Yb_N < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7). A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sr _i of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr _i = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.     

吉林省南部太古宙花岗质岩石及其成因探讨

吉林省南部早太古宙高级区和晚太古宙花岗岩—绿岩带中的英云闪长岩、奥长花岗岩、花岗闪长岩等广泛发育,各岩体分布明显受太古宙边缘穹隆及中央卵形隆起控制。在岩石化学特征上明显反映出英云闪长岩—奥长花岗岩和钙碱性两个演化趋势。经稀土模拟计算,英云闪长岩和奥长花岗岩可能由石英榴辉岩经10—40部分熔融形成。     

五台山早元古代碰撞造山带初步认识

五台山地区分布着复杂的变质杂岩。过去人们把这些变质岩作为地层分成群组段。龙泉关剪切构造岩的发现突破了这种传统观念。野外工作中发现所谓五台群主要包含了三个蛇绿混杂带;阜平群和恒山群的主要成分是灰片麻岩,它们构成两个太古代陆块的基底。五台地区的花岗岩类由代表弧环境的双花岗岩带的I型和S型花岗岩组成,它们与变质的钙碱性火山岩代表了古代的岩浆弧。滹沱群的豆村和东冶亚群以及过去划入阜平群和五台群的某些变沉积岩是阜平陆块被动大陆边缘的沉积。滹沱群的郭家寨亚群则是前陆盆地的磨拉石沉积。因此可以认为,这是一个由恒山仰冲陆块、北台-车厂弧和阜平俯冲陆块构成的碰撞造山带,碰撞时间大致是距今2050Ma。闭合的弧前大洋和弧后盆地形成了三条蛇绿混杂带。