Magmatic arc metamorphism: petrology and temperature history of metabasic rocks in the Coastal Cordillera of northern Chile

We investigated the metamorphic cooling history of underplated magmatic rocks at midcrustal depth. Granulites and amphibolites occur within the Jurassic magmatic belt of the Coast Range south of Antofagasta in northern Chile between 23°25' and 24°20' S. The protoliths of the metamorphic rocks are basic intrusions of Early Mesozoic age. They are part of the magmatically formed crust, and the essentially dry magmas were emplaced in an extensional regime. The granulites (clinopyroxene–orthopyroxene–plagioclase) show all stages of fabric development from magmatic to granoblastic fabrics. Pyroxene compositions were reset at temperatures around 800°  C independent of the stage of textural equilibration. The granulites were partially amphibolitized at upper amphibolite facies temperatures of 600–700°  C. Following cooling, a possible reheating to greenschist facies temperatures around 500°  C is indicated by prograde zoning in magnetite–ilmenite pairs. Mineral assemblages are not suitable for barometry, but a conservative estimation of the garnet-in reaction at given whole-rock compositions suggests maximum pressures in the granulite facies of around 5 kbar, and similar pressures are indicated by phengite barometry for the greenschist facies. The P–T  path of granulite–amphibolite metamorphism is one of slow cooling from magmatic temperatures with heterogeneous deformation. The thinning of the pre-Andean (Precambrian–Triassic) crust was apparently compensated by the magmatic underplating and this special tectonomagmatic setting caused the prolonged residence of the accreted rocks at midcrustal levels.     

赣北修水观音阁砾岩特征及其构造意义

有关赣北地区双桥山群内角度不整合和地壳运动存在与否问题,一直争论不休。论文以较丰富的详细野外调查资料为基础,并结合前人研究成果和室内各类样品测试结果,围绕这个赣北地区重要的基础地质问题进行了讨论。作者认为“修水运动”不能成立,并提出了砾岩的沉积模式。     

Geochronology, geochemistry and tectonic implications of Xiongshan diabasic dike swarm, northern Fujian

Sm/Nd isotopic age determination showed that Xiongshan dike swarm was at 585.7 Ma ± 30 Ma. The trace element geochemistry and Sr/Nd/Pb isotope gemhemistry studies indicate that the dike swarm was products of back-arc basin spreading ridge and the magma originated from the depleted mantle region which was metasomatized by LILE-rich liquids/melts derived from subduction slab. Project supported by the National Natural Science Foundation of China.     

Spatial-temporal Frame,Evolution and Mineralization of the Northern Qilian Metallogenic Province

Four metallogenic epochs occurred in different tectonic environments during theevolution of the Northern Qilian metallogenic province through the geological time. The Mid-dle Proterozoic metallogenic epoch witnessed the tectonic environment of crustal breakupcaused by mantle diapirism, in which ultramafic-mafic rocks were intruded along beep faultbelts and the superlarge Jinchuan magmatic Cu-Ni sulphide deposit was formed. In theMiddle-Late Proterozoic metallogenic epoch the crust was further broken to form anintracontinental rift, in which the Chenjiamiao style massive Cu-Fe sulphide deposits hosted bybasic volcanic tuff were formed in the lower volcano-sedimentary sequence, while the largesedex type Jingtieshan style Fe-Cu deposits were formed within the upper abyssal carbon-richargillaceous sedimentary sequence. The Early Palaeozoic saw the aulacogen environment, with-in which the Baiyinchang style superlarge massive base and precious metal sulphide depositshosted by quartz keratophyric tuff were formed in the Middle-Late Cambrian rifted island arcand the massive Cu-Zn sulphide deposits and magmatic chromite deposits associated with theophiolite suite were formed in the Early-Middle Ordovician, and the Honggou style massiveCu-Fe sulphide deposits hosted by spilite were formed in the Late Ordovician back-arc basinenvironment. In the Late Palaeozoic-Meso-Cenozoic, the metallogenic province went into anintracontinental orogenic stage characterized by compressive tectonic environment, in whichthere occurred carbonate-quartz vein type and tectono-alteration gold deposits associated withductile-shear structures.     

巴基斯坦北Potwar形变区地震的震源机制研究

巴基斯坦北Potwar形变区是西北喜马拉雅褶皱逆冲带前陆区的一部分，绘制了该区的地震活动性图. 与相邻地区比较，该区地震并不活跃，没有显示出与地表地质构造相关的清晰地震活动图象. 做出了4次地震的震源机制解. 结果表明，有3次地震是左旋走滑断层活动，另一次地震是逆断层活动. 地震震源机制解的P轴方向为NW-SE和NE-SW. 现今的构造形变很可能也包括基底的形变.      

新疆北部中二叠统烃源岩地球化学特征与沉积环境

首次对新疆北部准噶尔、吐哈及伊犁盆地中二叠统碳酸盐岩类、泥岩类烃源岩的微量元素、稀土元素和有机地球化学丰度进行了系统分析 ,其主要特征是微量元素含量与同类岩石的克拉克值相比 ,有的较高 ,有的偏低 ;轻稀土富集程度较高 ,重稀土相对较低 ,Ce/ Ce*具有较明显的正异常 ,反映出烃源岩沉积环境总体为还原环境。有机地球化学分析证实 ,烃源岩中微量元素含量对沉积水体中生物的分布有一定的影响 ,部分微量元素丰度与有机质含量呈明显的正相关关系。还原程度强的沉积相带是原始有机质沉积的良好场所 ,沉积有机质能及时沉积、保存 ,烃源岩大多有机质丰度高 ;反之 ,则明显变差     

Sedimentary environment and development controls of the hydrocarbon source beds:Middle and Upper Proterozoic in northern North China

Problem on development control of marine source bed hold in Chinese petroleum industry progression. The Hongshuizhuang Formation,Tieling Formation and Xiamaling Formation in the Middle and Upper Proterozoic are important hydrocarbon source beds in northern North China, and investigation of their sedimentary environments and the controls has great significance for petroleum exploration in North China. Based on sedimentology (sequence stratigraphy), palaeoecology, sedimentary geochemistry, and sedimentary palaeogeography, their development pattern is discussed. All these studies indicate that the development controls of the hydrocarbon source beds include a favorite palaeogeographic location, exceeding propagation of biomes in low and middle latitudes, anoxic environments, enrichment of phosphorus element and the adsorption of clay minerals during the preservation of organic matter in the marine carbonates.     

Petrogenesis of orthopyroxene- and amphibole-bearing andesites, Mustique, Grenadine Islands, Lesser Antilles Arc: Isotope, trace element and physical constraints

Abstract On the island of Mustique, fresh and propylitized olivine–plagioclase–clinopyroxene basalt, plagioclase–clinopyroxene–orthopyroxene and plagioclase–clinopyroxene–amphibole andesite lavas and minor intrusions are interbedded with Oligocene pyroclastic and epiclastic rocks. Chemical data show that two isotopically identical, but chemically different, suites of lava are present: (i) the OPXS (⁸⁷Sr/⁸⁶Sr 0.70403–0.70454; ¹⁴³Nd/¹⁴⁴Nd 0.512952–0.512986; δ¹⁸O_cpx 5.49 and 5.61), comprising basalts and orthopyroxene‐bearing andesites; and (ii) the AMPHS (⁸⁷Sr/⁸⁶Sr 0.70401–0.70457; ¹⁴³Nd/¹⁴⁴Nd 0.512981–0.513037; δ¹⁸O_cpx 5.54), made up of basalts and amphibole‐bearing andesites. The OPXS has higher contents of TiO₂, P₂O₅, light rare earth elements, Sm, Pb, Th, U, Zr, Y and Nb, and higher La/Yb ratios than the AMPHS. The isotopic data suggest that both suites formed from melts derived from the same subduction‐modified depleted mantle source as the volcanic rocks of nearby St Vincent and Bequia, and the northern islands of the Lesser Antilles Arc. The immobile trace element contents, and La/Yb ratios, of the OPXS are indicative of ～10% partial melting of the source, whereas those of the AMPHS are indicative of ～25% partial melting. The within‐suite chemical variation of the OPXS is consistent with ～45% fractional crystallization of its intratelluric mineral assemblages, and that of the AMPHS is consistent with the removal of ～65% of its intratelluric assemblages. Experimental evidence suggests that both suites of basalt crystallized at pressures <8 kbar from melts containing 1–2 wt% water. After extensive fractional crystallization, the andesites crystallized at pressures between approximately 5 and 2 kbar. The OPXS magmas appear to have lost more of their water content than the AMPHS magmas. Thus, the OPXS andesites formed from melts with an estimated water content of 2–3 wt%, whereas the AMPHS andesites formed from melts containing at least 4.5 wt% water.     

Physicochemistry and Mineralogy of Storm Dust and Dust Sediment in Northern China

Dust sediments collected from 1995 to 1998 in Beijing, Dunhuang, Inner Mongolia, Kashi, the Kunlun Mountains, Lanzhou, Ningxia, the Taklimakan Desert, and Xi‘an, China, were characterized in terms of their physical, chemical, and mineralogical properties. Most aerosols and dust analysed ranged in texture from silty clay to clay loam. Their median particle diameters (Mds) generally ranged between 5 to 63μm,coinciding with those of loess from central China and the finest sand from northwestern China. The dust sediments were characterized by a predominance of SiO2 and Al2O3, followed by K2O. Their SiO2/Al2O3and K2O/SiO2 molar ratios ranged from 5.17 to 8.43 and from 0.009 to 0.0368, respectively. The mass concentration spectrum during a dust storm showed a single peak, rather than the triple peak generally observed under clear sky conditions. The dominant minerals were chlorite, illite, calcite, and dolomite.These physical, chemical, and mineralogical properties were consistent with those of aeolian soils and loess in western and central China. The results suggest that aerosols and fine-gained fractions of dust sediments collected in northern China are mainly composed of soil material transported from the arid and semiarid regions of China and Mongolia by prevailing winds. The rate of deposition and properties of dust falling on eastern China were strongly influenced by meteorological conditions, season, latitude, longitude, and altitude of the sampling sites.     

柴达木盆地北缘"沙柳河岩群"的重新启用

柴北缘变质基底岩系中的表壳岩包含了岩石组合、沉积建造、变质程度和地质时代不同的两套岩石组合。其中，分布在柴达木地块的沙柳河、鱼卡河等地的以大理岩、石英岩和含石榴石英片岩为主含榴辉岩透镜体的表壳岩系，时代限定在1．0～1．3Ga，与分布在欧龙布鲁克微陆块的全吉山、德令哈等地形成时代大约2.3～2.4Ga的达肯大坂岩群有着明显的不同，后者以斜长角闪岩、石榴石英片岩和黑云变粒岩为主，并具有较强的钾质混合岩化。由于前者岩石组合清楚，沉积建造特征明显，构造意义独特，且有一定的区域分布性和可比性，所以具备建立新的岩群的条件。根据定名优先的原则，决定重新启用沙柳河岩群。