Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatífera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P–T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720–850 °C and 10–14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite–melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet.The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction–accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge.     

P–T–t conditions of high-grade metamorphic rocks of the Garzon Massif, Andean basement, SE Colombia

The metamorphic evolution of the Garzón Massif, Colombia, is established on the basis of the textural, goethermobarometric, and geochronological relationships of the metamorphic minerals. The geothermobarometric data define a clockwise, nearly isothermal decompression path (ITD) for rocks from Las Margaritas migmatites, constrained by four P–T areas: 780–826 °C and 6.3–8.0 kbar, 760–820 °C and 8.0–8.8 kbar, 680–755 °C and 6.6–9.0 kbar, and 630 °C and 4 kbar. For the a garnet-bearing charnockitic gneiss from the Vergel granulites, the path is counterclockwise, constrained by geothermobarometric data of 5.3–6.2 kbar and 700–780 °C and 6.2–7.2 kbar and 685–740 °C. The clockwise ITD path represents a loop followed by the orogen during the transitional granulite–amphibolite metamorphic conditions, probably associated with a subduction process followed by a collisional tectonic event. This subduction framework produced continental crust thickening between 1148 and 1034 Ma and later collision with another continental block approximately 1000 Ma ago. The orogenic exhumation occurred with moderate uplift rate. The counterclockwise trajectory and two metamorphic events suggest a vertical displacement between the Vergel granulites and Las Margaritas migmatites units, because there is no isotopic difference that indicates the existence of different terranes. The data confirm that the metamorphic evolution for this domain was more dynamic than previously believed and includes: (1) metamorphic processes with the generation of new crust with a possible mixture of old material and (2) metamorphic recycling of continental crust. These geological processes characterize a complex Mesoproterozoic orogenic event that shares certain features with the Grenvillian basement rocks participating in the formation of Rodinia.     

Geochemistry of granitoids and their minerals from Rebordelo–Agrochão area, northern Portugal

Deformed Hercynian peraluminous granitoids ranging from tonalite to granite crop out in the Rebordelo–Agrochão area, northern Portugal and some of them contain tonalitic and granodioritic enclaves. Variation diagrams of major and trace elements of the rocks, biotites and sphenes show fractionation trends. The most- and the least-deformed samples of granite and their biotites also define fractionation trends. There is decrease in all rare earth element (REE) contents and increase in the Eu anomaly in REE patterns from the most- to the least-deformed samples of granite. All the granitoids define a whole-rock Rb–Sr errorchron. A whole-rock Rb–Sr isochron for the least-deformed samples of granite yields an age of 357±9 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7087±0.0007. Geochemical modelling suggests that the tonalitic magma evolved by AFC (fractional crystallization of magnesiohornblende, plagioclase, quartz, biotite and ilmenite, and assimilation of metasediments) to originate tonalitic and granodioritic enclaves, granodiorite and granite. δ¹⁸O values support this mechanism. The tonalite is hybrid and derived by interaction of a mantle-derived magma and crustal materials.     

Kinematic link between episodic trapdoor collapse of the Negra Muerta Caldera and motion on the Olacapato-El Toro Fault Zone, southern central Andes

A combined geochronological and structural analysis of the Miocene Negra Muerta Caldera was designed to better understand caldera formation associated with prominent faults on the central Andean plateau. Rb–Sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano-tectonic episodes. The first episode commenced with explosive eruption of the 9.0±0.1 Ma andesitic Acay Ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity. Dominant volcanic and tectonic activity occurred during the second episode, which is bracketed by eruption of the 7.6±0.1 Ma rhyolitic Toba 1 Ignimbrite and effusive discharge of the 7.3±0.1 Ma rhyodacitic to andesitic lava flows. Structural relationships between rocks of the Negra Muerta Volcanic Complex and collapse-induced normal faults, notably NE-striking normal faults, agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode. Floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor. This induced horizontal extension of the caldera floor and was accomplished by fragmentation of, and intrusion of dikes into, the floor. Collapse-induced and post-collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left-lateral slip on the nearby Olacapato-El Toro Fault Zone. This furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central Andes. The structural analysis of the Negra Muerta Caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene–Pliocene epizonal gold deposit

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured ⁸⁷Sr/⁸⁶Sr between 0.70440 and 0.70460, ε_Nd between + 2.2 and + 3.2 and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The ⁸⁷Sr/⁸⁶Sr and ε_Nd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. ⁸⁷Sr/⁸⁶Sr and ε_Nd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, ε_Nd values lie between + 1.25 and + 3.28 and the ⁸⁷Sr/⁸⁶Sr of calcite and pyrite fall between 0.70444 and 0.70930. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.     

Palaeomagnetic,geochronological and geological constraints on the tectonic evolution of the Mejillones Peninsula,northern Chile

Palaeomagnetic and geochronological data from an Early Palaeozoic high grade metamorphic complex (Jorgina Formation) and Jurassic layered basic intrusion (Moreno Complex) are reported from the Mejillones Peninsula of northern Chile (23–23°30'S). ⁴⁰Ar–³⁹Ar dates from the Lower Palaeozoic Jorgina Formation and the Moreno Complex are between 170 and 158 Ma, coincident with a phase of emplacement of the north Chilean coastal batholith. This suggests that intrusion and magnetization of the Moreno Complex and the metamorphism and remagnetization of the Jorgina Formation were related to batholith emplacement. Extracted stable components of magnetization from all units (17 sites) define site-mean directions with a scattered distribution. The scatter in site-mean directions is interpreted as being due to minor, localized, non-uniform, block-fault related (normal or strike-slip, or both) rotation after 158 Ma. The palaeomagnetic and geochronological data indicate that no significant large-scale latitudinal translation of crustal blocks has taken place in this part of northern Chile since the Late Jurassic. In addition, they indicate that the uniform clockwise rotation after the mid-Cretaceous which affected the adjacent Cordillera de la Costa either did not extend into the Mejillones Peninsula or took the form of localized block-fault rotations. The restriction of palaeomagnetically defined styles of rotation to discrete areas within the north Chilean forearc indicates that forearc wide block-fault rotation models are not applicable to the Pacific margin of northern Chile.     

Interaction between the Nazca and South American plates and formation of the Altiplano–Puna plateau: Review of a flexural analysis along the Andean margin (15°–34°S)

The results of a two-dimensional flexural analysis applied to the Andean margin, which is based on the correlation between topography and Bouguer anomaly, are here reviewed in order to characterize rigidity variations across and along the forearc–arc transition of the Central Andes and to understand the role of the forearc in the formation of the Altiplano Plateau. The forearc has maximum rigidities between 15° and 23°S. Forearc rigidity decreases gradually southward and sharply toward the plateau. The main orogen (elevations higher than 3000 m) is very weak along the entire Central Andes. A semi-quantitative interpretation of these trends, based on the relationship between flexural rigidity and the thermo-mechanically- and compositionally-controlled strength of the lithosphere, allows the following conclusions to be made: (1) across-strike rigidity variations are dominated by the thermal structure derived from the subduction process; (2) the forearc constitutes a strong, cold and rigid geotectonic element; (3) southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab; (4) very low rigidities along the main orogen are caused by the existence of a thick, quartz-rich crust with a low strain rate-to-heat flow ratio; (5) the strength of the plateau lithosphere is localized in an upper-crustal layer whose base at 15 km could be correlated with a P-to-S seismic wave converter (TRAC1 of Yuan et al., 2000 [Yuan, X., Sobolev, S., Kind, R., Oncken, O. et al. 2000. Subduction and collision processes in the Central Andes constrained by converted seismic phases. Nature, V 408, 21/28 Diciembre, p. 958–961]); (6) the forearc–plateau rigidity boundary corresponds to a zone of changing thermal conditions, eastward-increasing crustal thickness and felsic component in the crust, and low strain-rate deformation, which correlates with a west-verging structural system at the surface. These conclusions suggest that the rigid forearc acts as a pseudo-indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland. This pseudo-indenter is geometrically represented by a crustal-scale triangular zone rooted at TRAC1. This model allows the integration of existing contradictory ideas on the dynamics of forearc–plateau interaction that are related to the relative importance of upper-crustal compressive structures and lower crustal accumulation below the forearc.     

索伦-林西缝合带北缘构造属性：来自内蒙古扎鲁特旗北部寿山沟组变沉积岩及其安山岩透镜体的制约

本文选择扎鲁特旗北部达巴艾勒地区出露的一套下二叠统寿山沟组变沉积岩及其安山岩透镜体作为研究对象,在野外地质调查的基础上,通过岩相学、年代学、岩石地球化学等方面的系统研究,深入探讨索伦-林西缝合带北缘晚古生代的构造属性。下二叠统寿山沟组变质沉积岩LA-ICP-MS锆石U-Pb测年结果显示其最小峰值年龄为282.3±4.2Ma和285.9±5.6Ma,限定了沉积下限为早二叠世,另外还含有少量400~500Ma以及个别700~800Ma、1700Ma的单颗粒碎屑锆石年龄,表明其物源区主要来自于北侧的锡林浩特岩浆弧。碎屑锆石年龄图谱反映其沉积背景为汇聚背景下的弧前盆地。地球化学特征表明寿山沟组变质沉积岩物源主要来自中酸性岩浆岩源区,形成于大陆岛弧环境。该套沉积岩中的晚石炭世安山岩透镜体锆石U-Pb测年结果为304.9±4.7Ma,地球化学特征显示其具有高镁安山岩特征,为俯冲洋壳释放含水流体交代地幔楔发生部分熔融的产物,形成于大陆边缘弧构造背景。综合分析,索伦-林西缝合带北缘在晚石炭世-早二叠世处于洋内俯冲向洋陆俯冲过渡的转换环境。

     

Analysis of the North Anatolian Shear Zone in Central Pontides (northern Turkey): Insight for geometries and kinematics of deformation structures in a transpressional zone

The western part of the North Anatolian Shear Zone at the southern boundary of the Central Pontides in Turkey, was investigated in the Kurşunlu-Araç area by means of a geological-structural field study. In this area the North Anatolian Shear Zone results in a transpressional deformation zone that extends between two master faults striking parallel to the main shear direction. The main systems of structures identified in the deformation zone appear to be oriented parallel to the directions predicted by Riedel theoretical model. Nevertheless, the strain partitioning is more complicated than predicted by theory. The structural analysis suggests a polyphase deformation characterized by a steady component of transcurrence associated with alternance of compression and extension. Along each of theoretical directions the combination of double verging structures can be observed, with folds and thrust surfaces root into high-angle shear zones, according to flower-type geometries. The discrepancies of directions, kinematics and geometries from theoretical models are due to transpressive and/or transtensive nature of the deformation. According to the observed outcropping structures, we propose a conceptual model for the North Anatolian Shear Zone, interpreting it as a crustal-scale positive flower structure.     

Geochronology and geochemistry of the Parashi granitoid,NE Colombia: Tectonic implication of short-lived Early Eocene plutonism along the SE Caribbean margin

The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al₂O₃, Na₂O and Ba–Sr contents. Initial ⁸⁷Sr/⁸⁶Sr isotopic values range between 0.7050 and 0.7054, with ¹⁴³Nd/¹⁴⁴Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.     

Transpression,extrusion, partitioning,and lateral escape in the middle crust: Significance of structures,fabrics, and kinematics in the Bronson Hill zone,southern New England,U.S.A.

Fabrics in the mid-crustal Bronson Hill zone of the southern New England Appalachian orogen record a range of apparent finite strains and conflicting kinematics, but structural relationships indicate coeval development. At the smallest scale of this study, shortening was accommodated in granitic orthogneiss, while transcurrent deformation was partitioned into relatively thin zones of metastratified rocks along the margins. The Monson orthogneiss can be broadly characterized by subvertical to steeply dipping S > L tectonites, subvertical to subhorizontal stretching lineations, closed to isoclinal folds, and dextral/reverse kinematics. The east-bounding Conant Brook shear zone and Greenwich syncline are characterized by steeply dipping mylonitic foliations, a range of lineations, and dextral/reverse kinematic indicators. The west-bounding Mt. Dumplin high strain zone is comprised of steeply dipping mylonites, subhorizontal lineations, and sinistral/normal kinematics. These structures reflect coeval partitioned dextral transpression, vertical extrusion, and north-directed lateral escape of the orthogneiss that was facilitated by bounding conjugate shear zones. Comparison of structural subdomains with transpressional modeling indicates vertical pseudo-monoclinic to inclined triclinic coaxial to simple shear influenced transpression. Compatibility between laterally adjacent subdomains was maintained by meso-/microscale partitioning. Absolute and relative timing constraints show that transpression was sustained from 330 Ma to 300 Ma.     

The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina

The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (100 km²) composed of andesite to basaltic andesite rocks that crop out 75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures.The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions.Petrographic and chemical features suggest that the andesitic units (SiO₂ > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally.Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.     

Age and origin of gem corundum and zircon megacrysts from the Mercaderes–Rio Mayo area, South-west Colombia, South America

Potential sources for alluvial gem corundum and zircon from the Rio Mayo area, near Mercaderes, Colombia are reviewed, based on U–Pb dating of syngenetic and protogenetic mineral inclusions in corundum samples and on a zircon megacryst. Corundum recovered from the region (approx. 99% sapphire, 1% ruby) commonly shows growth banding, includes colour change stones and exhibits overlaps in colour ranges and inclusion characteristics. This suggests a contiguous genetic suite. The U–Pb dating used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Because of the young ages and low-U contents of the dated zircons, the acquired data required a special statistical treatment. The results from zircon, fluorapatite and allanite-(Ce) inclusions provide a corundum crystallization age of 8 to 11 Ma, in relation to northern Andean Miocene uplift and magmatism. The zircon megacryst gave a younger crystallization age of c. 0.6 Ma, unrelated to the corundum genesis. Geochemical parameters (trace element and O isotope ranges) for corundum samples suggest a metamorphic/metasomatic origin. The age data rules out corundum genesis during the Late Cretaceous ophiolitic generation, but leave open possible later metasomatic interactions with this substrate. The Cr/Ga and Ga/Mg ratios and O isotope range for the corundum fall within the known limits for metasomatic, desilicated felsic/ultramafic ‘plumasitic’ associations, suggesting a possible parental source. Allanite, extremely rare as an inclusion in corundum elsewhere, may prove a characteristic inclusion for Rio Mayo corundum.     

苏北锦屏地区构造变形及片麻岩舌状体成因研究

通过对锦屏山地区详细填图,确认了苏鲁造山带南缘超高压变质带与高压变质带的界线性质为一韧性剪切带,锦屏地区不存在区域性褶皱,而是由于朐山花岗质片麻岩和云台岩群变质火山岩构成强干岩层,锦屏岩群变质沉积岩构成软弱岩层,形成夹心饼式(layer cake assemble）组合。在印支期右行剪切作用下,形成不均一剪切,使朐山花岗质片麻岩形成长约10km,宽约5km的无根剪切舌状体,物质运动方向指向南。     

Late Paleozoic–Early Triassic magmatism on the western margin of Gondwana: Collahuasi area, Northern Chile

The basement in the ‘Altiplano’ high plateau of the Andes of northern Chile mostly consists of late Paleozoic to Early Triassic felsic igneous rocks (Collahuasi Group) that were emplaced and extruded along the western margin of the Gondwana supercontinent. This igneous suite crops out in the Collahuasi area and forms the backbone of most of the high Andes from latitude 20° to 22°S. Rocks of the Collahuasi Group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main Andes of Chile, the Frontal Cordillera of Argentina (Choiyoi Group or Choiyoi Granite-Rhyolite Province), and the Eastern Cordillera of Peru.Thirteen new SHRIMP U–Pb zircon ages from the Collahuasi area document a bimodal timing for magmatism, with a dominant peak at about 300 Ma and a less significant one at 244 Ma. Copper–Mo porphyry mineralization is related to the younger igneous event.Initial Hf isotopic ratios for the ~ 300 Ma zircons range from about − 2 to + 6 indicating that the magmas incorporated components with a significant crustal residence time. The 244 Ma magmas were derived from a less enriched source, with the initial Hf values ranging from + 2 to + 6, suggestive of a mixture with a more depleted component. Limited whole rock ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr isotopic ratios further support the likelihood that the Collahuasi Group magmatism incorporated significant older crustal components, or at least a mixture of crustal sources with more and less evolved isotopic signatures.     

Tectonic and hydrodynamic control of landslides in the northern area of the Central Rif, Morocco

The northern edge of the Central Rif (Morocco) is subject to numerous landslides where mechanisms do not correspond to the classical models used by geomechanics specialists. It is necessary to adopt a multidisciplinary approach that combines geomorphology, geology, hydrogeology, and geotechnics in order to understand how such slope failures are generated, especially in a region with a heterogeneous structure characterised by significant lithological differences, severe fracturing, and thrust sheets where tectonic contacts play a major role in groundwater circulation. This report shows that these failures are essentially controlled by the tectonic contact separating the Tisirene and Chouamat thrust sheets and by subsurface hydrodynamic conditions. A model of spatial and temporal variations in the factor of safety is proposed.     

冀北北岔沟门地区中生代侵入岩地质年代学和地球化学特征研究

北岔沟门地区广泛分布中生代侵入岩体.本文选取了该地区有代表性的8个岩体进行地质年代学和地球化学研究,旨在建立其年代格架并探讨岩石的成因演化.主要岩石类型为闪长玢岩、花岗闪长岩、石英二长岩、二长花岗岩、钾长花岗岩等.颗粒级锆石的U-Pb同位素测年数据表明所研究的岩体主要有三期早三叠世(245～250Ma)、晚侏罗世(140～147Ma)和早白垩世(125～137Ma).地球化学数据表明岩石普遍具有较高的钾含量、准铝质,主要属高钾钙碱性系列,部分为橄榄玄粗岩系列.岩石不同程度地显示轻稀土元素富集、高场强元素(HFSE)和大离子亲石性元素(LILE)解偶等特点.进一步的岩石地球化学研究表明区内侵入岩可划分为高BaSr和低BaSr两种类型,其中高BaSr型具有高Al2O3(≥15%)、Sr(≥400×10-6) 和低Y(≤18×10-6)、Yb(≤1.9×10-6),高的Na2O/K2O(>1), Sr/Y(>20)、La/Yb(≥10)比值;较高的Mg#(38.47～57.78)、具正的或弱的铕负异常等地球化学特征,这些特征与埃达克岩具有某些相似性;而低BaSr型岩石SiO2、K2O、Y、Yb含量相对较高、Al2O3、Sr、Ba含量相对较低、具明显的铕负异常等地球化学特征.尽管两类岩石在时空上密切共生,但可能具有不同的源区和成因机制.综合研究表明本区中生代侵入岩总体形成在加厚地壳的构造背景下,与底侵作用或壳幔相互作用有关.     

Pollen‐ and diatom based environmental history since the Last Glacial Maximum from the Andean core Fúquene‐7, Colombia

The late Pleistocene–Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene‐7. Time control is provided by 11 accelerator mass spectrometry (AMS) ¹⁴C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake‐levels were low and the area was surrounded by paramo and subparamo vegetation. Late‐glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake‐level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late‐glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake‐level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. Copyright © 2003 John Wiley & Sons, Ltd.