中国大陆科学钻探工程主孔3000～3500 m花岗质片麻岩地球化学性质及其成因机制

中国大陆科学钻探工程主孔(CCSD-MH)3000~3500m深度范围内的岩心揭示的主要岩石类型以正、副片麻岩为主,夹少量的榴辉岩、片岩及斜长角闪岩等。正片麻岩可进一步划分为钾长花岗质片麻岩和二长花岗质片麻岩。地球化学研究表明,花岗质片麻岩的SiO2含量普遍高,为73.56%~79.01%,Al2O3含量为10.49%~13.75%;TiO2、Fe2O3、FeO、MnO和MgO的含量明显偏低,其中Fe2O3为0.26%~2.56%,FeO为0.09%~1.13%,MgO为0.01%~0.63%;岩石明显富含Na2O、K2O,强烈贫钙,Na2O K2O的含量为6.20%~9.15%,K2O的含量为1.26%~5.67%,CaO的含量为0.08%~1.59%。花岗质片麻岩在洋脊玄武岩标准化蛛网图上都表现出强烈富集大离子亲石元素(K、Rb、Th)的特点,高场强元素Ti、Ta和Nb表现为明显的负异常,而Hf和Zr则为明显正异常。3000~3500m花岗质片麻岩的原岩具A型花岗岩的特征,形成于大陆裂谷环境,记录了Rodinia超大陆裂解时苏鲁地区的花岗岩浆事件,表明新元古代晚期扬子板块北缘的构造属性为大陆裂谷环境。     

Origin and Geological Significance of TTG Gneisses from the Maevatanana Greenstone Belt in North–Central Madagascar, and A Comparison with India

The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite(TTG) gneisses,and is important for its concentrations of various metal deposits(e.g.,chromium,nickle,iron,gold).In this paper we report on the petrography,and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area,as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses.The gneisses consist mainly of granitoid gneiss and biotite(±hornblende) plagiogneiss,and analysis of thin sections provides evidence of crushing,recrystallization,and metasomatism related to dynamic metamorphism.Samples have large variations in their major and trace element contents,with SiO_2 = 55.87-68.06 wt%,Al_2O_3 = 13.9-17.8 wt%,and Na_2O/K_2O =0.97-2.13.Geochemically,the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend,while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage.Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at2.5-2.4 Ga.Most ε_(Hf(t)) values of zircons from the biotite(± hornblende) plagiogneisses are positive,while most ε_(Hf(t)) values from the granitoid gneisses are negative,suggesting a degree of crustal contamination.Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca.3.4-2.6 Ga,representing a period of paleocontinent formation in the Mesoarchean.Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃.Based on these data,the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt(mostly lower amphibolite facies) in the Maevatanana area,and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent.The lithological similarity of Precambrian basement,the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India,which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics,and also important constraints on the correlation of the two continental fragments.     

An extensional and transtensional origin of elongated magmatic domes and localised transfer faults in the northern Menderes metamorphic core complex,western Turkey

The northern Menderes metamorphic core complex has complex exhumation history and is one of the key localities to investigate the spatial and temporal relationships of extensional and compressional structures. Detachment faults and syn-extensional plutons are linked to a series of antiforms and synforms and the denudation of the northern Menderes Massif occurred in three stages. The first stage is related to the development of detachment faults under the consistent NE–SW-directed extension. The second stage is represented by a series of elongated magmatic domes that were oriented parallel, oblique and perpendicular to the regional extension direction. Emplacement of these asymmetrical magmatic domes appears to have been controlled by heterogeneous extension and post-dates the extensional Simav detachment fault. On the third stage, progressive heterogeneous extension that led to updoming of plutons has been finally accommodated by a localised and short-lived transfer zone, which was described as the Gerni shear zone for the first time in this study. The transfer zone is formed by a NE-striking, dextral ductile/brittle shear zone that accommodated the propagation of folds, conjugated strike-slip faults and normal- and oblique-slip faults. Mylonites associated with the transfer zone are related to the localisation of strain along the thermally weakened strike-slip fault systems by short-lived intrusions rather than to the development of regional-scale detachment faults. These structures are consistent with a transtensional simple shear model, which properly explains the evolution of extensional and compressional structures exposed in the northern Menderes core complex. Structural setting of the E?rigöz region is somewhat similar to that of the NE-trending gneiss domes in the northern Menderes Massif and updoming of magma during late stages of detachment faulting appears to have played an important role in the exhumation of lower and upper plate rocks.     

赞皇变基性岩中锆石的U-Pb定年及其地质意义

赞皇变质杂岩区位于阜平杂岩南部,地处华北克拉通中部造山带的中段,和中部带北段杂岩一样,是洞悉华北克拉通前寒武纪基底构造演化历史的一个重要窗口.研究区变基性岩可分为斜长角闪岩和角闪斜长片麻岩两种,二者均以似层状方式产于黑云斜长片麻岩或长英质片麻岩中,斜长角闪岩亦可呈透镜状,二者后期与围岩一起共同经历了高角闪岩相变质作用....     

东喜马拉雅构造结南迦巴瓦岩群花岗质片麻岩的初步研究

野外地质填图和研究发现,东喜马拉雅构造结高喜马拉雅结晶岩系中有古老的花岗岩侵入,并在鲁霞地区圈定了9个花岗质侵入体。古老的花岗质岩石主要侵位于南迦巴瓦岩群直白岩组中,与南迦巴瓦岩群一起经历了麻粒岩相变质作用而形成花岗质片麻岩套。岩石类型有花岗闪长质片麻岩、黑云母花岗质片麻岩、闪长质片麻岩等。岩石化学研究表明这些花岗片麻岩套具“S”型特征,可能有深部幔源物质的加入。花岗岩形成深度在2～5km之间．侵位时代为552～525Ma,为新元古代晚期,属泛非期陆内演化阶段的产物。高喜马拉雅地区在元古宙末期形成了结晶基底。