首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 125 毫秒
1.
Whole-rock geochemical analyses using major and trace elements in combination with the Sm–Nd and Pb–Pb isotope systems, together with SHRIMP age dating on metasedimentary rocks from the Sierras de Chepes, the Sierras de Córdoba, the Sierra Norte and the San Luis Formation in the Sierra de San Luis, have been carried out to unravel the provenance and the geodynamic history of the Eastern Sierras Pampeanas, Central Argentina. The geochemical and the Sm–Nd data point to a slightly stronger mafic and less-fractionated material in the provenance area of the Sierras de Córdoba when compared to the other units. The TDM model ages from the Sierras de Chepes (~1.82 Ga) and the Sierra Norte (~1.79 Ga) are significantly older than the data from the Sierras de Córdoba (1.67 Ga). The Pb data are homogeneous for the different units. Only the 208Pb/204Pb ratios of some samples from the Sierras de Córdoba are higher. A late Pampean detrital zircon peak around 520 Ma from the Sierras de Chepes is in accordance with the new data from the San Luis Formation. This is similar to the literature data from the Famatina Belt located to the northwest of the Sierras de Chepes and also fits the detrital zircon peaks in the Mesón group. These maximum depositional ages were also reported from some locations in the Puncoviscana Formation but are absent in the Sierras de Córdoba. An improved model for the development of the Eastern Sierras Pampeanas in the area between the Sierras de Córdoba and the Puncoviscana Formation is provided. This gives new insights into the late Pampean development of the Sierra de San Luis and the complex development of the Eastern Sierras Pampeanas. This new model explains the younger detrital ages in the Puncoviscana Formation compared with the older ages of the Sierras de Córdoba. Another model of the Sierra de San Luis explains the younger depositional ages of the Pringles Metamorphic Complex and the San Luis Formation when compared to the Nogolí Metamorphic Complex and the Conlara Metamorphic Complex. Additionally, the rather fast change of the high-grade metamorphic conditions in the Pringles Metamorphic Complex and the low-grade metamorphic conditions in the San Luis Formation is explained by extension, the ascent of (ultra) mafic material and later folding and erosion.  相似文献   

2.
《Ore Geology Reviews》2003,22(1-2):41-59
In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Córdoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Marañón Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane.The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.  相似文献   

3.
Miocene sedimentary successions of the Ñirihuau and Collón Cura formations east of the El Maitén Belt constitute a partial record of the Andean exhumation, defining a synorogenic infill of the Ñirihuau Basin in the foothills of the North Patagonian fold and thrust belt. Gravimetric and seismic data allow recognizing the internal arrangement and geometry of these depocenters that host both units, separating a synextensional section previous to the Andean development at these latitudes, from a series of syncontractional units above. A series of progressive unconformities in the upper terms shows the synorogenic character of these units corresponding to the different pulses of deformation that occurred during the middle Miocene. New U–Pb ages constrain these pulses to the ∼13.5–12.9 Ma interval and allow reconstructing the tectonic history of this region based on the detrital zircon source populations. The U–Pb maximum ages of sedimentation give to the Ñirihuau Formation in particular a younger age than previously assumed. Additionally, synsedimentary deformation in strata of the upper exposures of the Collón Cura Formation associated with contractional structures and U–Pb ages allow identifying a younger paleoseismogenic pulse in ∼11.3 Ma. Thus, based on these data and a compilation of previous datasets, a tectonic evolution is proposed characterized by a contractional episode that migrated eastwardly since ∼19 to 15 Ma producing the Gastre broken foreland and then retracted to the eastern North Patagonian Precordillera, where out-of-sequence thrusts cannibalized the wedge top zone in the El Maitén belt at ∼13.5–11.3 Ma.  相似文献   

4.
The Sierras Pampeanas in central and north-western Argentina constitute a distinct morphotectonic feature between 27°S and 33°S. The last stage of uplift and deformation in this area are interpreted to be closely related to the Andean flat-slab subduction of the Nazca plate beneath the South American plate. K–Ar fault gouge dating and low-temperature thermochronology along two transects within the Sierra de Comechingones reveal a minimum age for the onset of brittle deformation about 340 Ma, very low exhumation rates since Late Paleozoic time, as well as a total exhumation of about 2.3 km since the Late Cretaceous. New Ar–Ar ages (7.54–1.91 Ma) of volcanic rocks from the San Luis volcanic belt support the eastward propagation of the flat-slab magmatic front, confirming the onset of flat-slab related deformation in this region at 11.2 Ma. Although low-temperature thermochronology does not clearly constrain the signal of the Andean uplift, it is understood that the current structural relief related to the Comechingones range has been achieved after the exhumation of both fault walls (circa 80–70 Ma).  相似文献   

5.
A‐type orthogneisses of mid Neoproterozoic age (774 ± 6 Ma, U‐Pb SHRIMP zircon age), are reported for the first time from the Grenvillian basement of the Western Sierras Pampeanas in Argentina. These anorogenic meta‐igneous rocks represent the latest event of Rodinia break‐up so far recognized in Grenvillian basement exposures across Andean South America. Moreover, they compare well with A‐type granitoids and volcanic rocks along the Appalachian margin of Laurentia (Blue Ridge), thus adding to former evidence that the Western Sierras Pampeanas Grenvillian basement was left on the conjugate rifted margin of eastern Laurentia during Rodinia break‐up and the consequent opening of the Iapetus ocean.  相似文献   

6.
The Vinchina Formation is one of the thickest Cenozoic units related to the Andean orogeny in Argentina totaling more than 5100 m in thickness. Different ages, from Eocene to latest Miocene, have been postulated for this red-bed succession based on fission track, magnetostratigraphy and whole rock isotopic analyses. Two new high precision U-Pb zircon ages are reported herein for this unit. A maximum U-Pb age of 15.6 ± 0.4 Ma was obtained from detritic zircons collected from a thick tuffaceous interval of the Lower Member of the Vinchina Formation at La Cueva (Precordillera), while a depositional U-Pb age of 9.24 ± 0.034 Ma was derived from volcanic zircons collected from a thin tuff bed in the Upper Member at Quebrada de Los Pozuelos (Northwestern Sierras Pampeanas).At La Cueva, the Vinchina Formation unconformably overlies eolian sandstones of the Vallecito Formation and was divided into four units representing 1) deposits of high-sinuosity ephemeral rivers associated with 2) a playa-lake passing upwards to 3) low-sinuosity sandy ephemeral rivers and finally, 4) a gravelly-sandy braided plain. The tuffaceous level corresponding to unit 1 is located 280 m above the base of the formation.At Quebrada de Los Pozuelos, the Vinchina Formation unconformably overlies the Vallecito Formation and is covered by a deeply incised surface at the base of the Toro Negro Formation. We divided the Vinchina Formation into four units. Unit 1 represents sedimentation in shallow fluvial channels with sandy to muddy floodplains. Units 2 and 3 record sedimentation in braided, meandering and anastomosing rivers. Finally unit 4 represents deposition in braided and wandering fluvial systems. The sampled tuff is located within unit 4 at ∼3470 m above the base of the formation.The new ages indicate that the bulk of the Vinchina Formation is Miocene in age but they do not preclude a longer time span for the sedimentation of the whole unit. Ages of the sampled volcanic zircons match an important episode of volcanism recorded in the Cerro Las Tórtolas Formation, located ∼90 km to the west in the Andean Cordillera, but also the upper tuff could be related to the late Miocene Puna volcanism. Comparison of the new ages with previous chronological data suggests coetaneous sedimentation along different depocenters of the Bermejo basin (e.g., Vinchina and Talampaya depocenters in Western Sierras Pampeanas and La Troya depocenter and Huaco-Mogna sections in Precordillera) and strenghten the need for correlation among them. In addition the age of 15.6 ± 0.4 Ma constrains the end of the severe arid conditions recorded in the Sierras Pampeanas and Precordillera region.  相似文献   

7.
The subduction of the Nazca plate under the South American plate around 31°S is characterized by flat slab geometry. The (Chilean) Pampean flat slab of Argentina associated with the subduction of the Juan Fernandez ridge lies in a region of a series of foreland uplifts corresponding to the thin-skinned Precordillera and basement cored Sierras Pampeanas ranges. The SIEMBRA project deployed 40 broadband stations in 2008–2009 in both the Precordillera and the Sierras Pampeanas with the aim to foster the understanding of the entire central Andean flat slab region. One of the SIEMBRA station (DOCA) located on the western flank of Sierra de la Invernada in the Central Precordillera appears particularly appropriate to study the crustal structure and eventually detect discontinuities related to terranes establishment. We thus performed a receiver function analysis using teleseismic data recorded at the DOCA station during the SIEMBRA project and from October 2011 to June 2012 using a broadband UNSJ (National University of San Juan) seismic station with the purpose to obtain crustal images with details of the intracrustal structure consistent with a mechanism that could explains both the observed earthquake depths and the uplift pattern in the Central Precordillera. Our results show that the Moho beneath the Precordillera lies at a depth of about 66 km. The Moho signal appears diminished and behaves irregularly as a function of azimuthal orientations. Although this observation could be the result of an irregular geometry it also correlates with the hypothesis of partial eclogitisation in the lower crust. Two mid-crustal discontinuities have also been revealed. The shallower one could correspond to a décollement level between the Precordilleran strata and the Cuyania basement at 21 km depth. The deeper one which the presence has been matched with a sharp decrease of the crustal seismic activity drove us to the hypothesis of a major change in crustal composition at 36 km. Finally the flat portion of the subducted slab has been imaged lying at about 100 km depth.  相似文献   

8.
The Sierra de Pie de Palo located between 67°30′–68°30′ W and 31°00′–32°00′ S in the Argentine Western Sierras Pampeanas in Argentina is a distinct basement range, which lacks thermochronological data deciphering its exhumation and uplift history below 200 °C. Integrated cooling histories constrained by apatite fission-track data as well as (U–Th)/He measurements of zircon and apatite reveal that the structural evolution of this mountain range commenced during the Late Paleozoic and was mainly controlled by tectonically triggered erosion. Following further erosional controlled exhumation in a more or less extensional regime during the Mesozoic, the modern topography was generated by denudation in the Paleogene during the early stage of the Andean deformation, whereupon deformation propagated towards the west since the Late Mesozoic to Paleogene. This evolution is characterised by a total of 3.7–4.2 km vertical rock uplift and by 1.7–2.2 km exhumation with a rate of 0.03–0.04 mm/a within the Sierra de Pie de Palo since ca. 60 Ma. Onset of uplift of peak level is also referred to that time resulting in a less Pliocene amount of uplift than previously assumed.  相似文献   

9.
This paper discusses new structural, kinematic and geochronological data from polymetallic (Pb-Zn-Cu-Ag ± Au) vein-type deposits hosted in the metamorphic basement of the southern Sierras de Córdoba. A Carboniferous age was established for the hydrothermal event between ∼329 and 315 Ma (Late Mississippian-Early Pennsylvanian) by the K/Ar fine-fraction dating method of sericitic alteration related to metallic ore deposition in the Las Guindas and Oro districts. The obtained ages postdate the spatially associated Devonian magmatism and overlap the A-type Early Carboniferous magmatism defined for the Eastern Sierras Pampeanas. The presence of non-exhumed granitic bodies at shallow depths, possibly related to mineralization, is supported by available geophysical and field evidence.The strain fabric and 3-D kinematic analyses constitute first kinematic data for the Carboniferous basement of the Southern Sierras Pampeanas demonstrating that mineralization was controlled by NNW- and ENE-trending brittle-ductile transtensional shear zones that overprint the earlier high-strain deformation fabrics of the basement. Transtensional deformation has accommodated large amounts of strike-slip movements and subordinated extensional components. The calculated kinematic axes indicate a coherent kinematic pattern of the mineralized systems in the two studied districts, with a maximum extension direction oriented NNE- to NE and maximum shortening direction oriented WNW- to NW. This deformation regime, active during mineralization, point to a non-compressive setting at the Late Mississippian-Early Pennsylvanian boundary. In line with other regional evidence, we propose a distinctive Carboniferous deformational phase in the Eastern Sierras Pampeanas, dominated by transtension. This period would have occurred after the transition with the Devonian compressional/transpressional orogenic regime.  相似文献   

10.
Detrital modes of sandstones and conglomerates of the Toro Negro Formation (Late Miocene-early Pliocene) were used to analyze the evolution of the broken-foreland stage of the Vinchina Basin (28°30′–29°00′ S and 68°30′–68°20′ W) of NW Argentina. This basin located in the Western Sierras Pampeanas is bounded to the west by the Precordillera and to the east by the Famatina System. Three sandstone petrofacies: plutonic-metamorphic, volcanic and mixed petrofacies and three conglomerate lithic associations: basement, sedimentary and volcanic lithic associations were recognized, allowing to establish three source areas: Western Sierras Pampeanas (Toro Negro and Umango Ranges), Cordillera Frontal and Precordillera.During the Late Miocene, the Toro Negro Range (to the north) together with the Cordillera Frontal and Precordillera (to the west) were the main sources for depositional sequences I and II (lower member of the Toro Negro Formation). On the contrary, during the latest Miocene-early Pliocene, Depositional Sequence III (upper member) exhibited a progressive increase in the supply from the eastern Precordillera (to the west) with additional material from the Umango Range to the south. Besides, evidence of synchronic volcanism is recorded in the upper part of Depositional Sequence II and the lower part of Depositional Sequence III.The coexistence of the three source areas and the changing distribution patterns due to re-accommodation of sediment dispersal routes demonstrate that the evolution of this type of basin is much more complex than previously envisaged. Therefore, an integrated analysis using different tools (sedimentary facies, paleocurrent measurements, sandstone petrography and conglomerate composition) is needed for a clearer understanding of broken-foreland basins.  相似文献   

11.
The provenance of Neoproterozoic to Early Paleozoic sedimentary rocks in the Sierras Pampeanas has been established using U–Pb SHRIMP age determination of detrital zircons in twelve metasedimentary samples, with supplementary Hf and O isotope analyses of selected samples. The detrital zircon age patterns show that the western and eastern sectors of the Sierras Pampeanas are derived from different sources, and were juxtaposed during the Early Cambrian ‘Pampean’ collision orogeny, thus defining initiation of the supercontinent stage of southwestern Gondwana. The Western Sierras Pampeanas (WSP), which extend northwards to the southern Puna (Antofalla) and the Arequipa Massif (Peru), constitute a single large continental basement of Paleoproterozoic age — the MARA block — that was reworked during the Grenvillian orogeny. The MARA block probably extends eastwards to include the Río Apa block (southern Brazil), but in this case without a Mesoproterozoic overprint. Detrital zircons from the WSP and Antofalla yield age peaks between 1330 and 1030 Ma, remarkably similar to the range of ages in the Grenville province of eastern Laurentia. The WSP Neoproterozoic sedimentary cover to this basement shows the same 1330–1030 component, but also includes important 1430–1380 Ma zircons whose juvenile Hf and O isotopic signatures strongly suggest derivation from the Grenville and the Southern Granite–Rhyolite provinces of eastern Laurentia. In contrast the Eastern Sierras Pampeanas metasedimentary rocks have a typically bimodal detrital zircon pattern with peaks at ca. 1000 and 600 Ma, which respectively indicate sources in the Natal–Namaqua belt and the East African orogen and/or the Dom Feliciano belt of SE Brazil and Uruguay. Sedimentary rocks in the Eastern Sierras Pampeanas and Patagonia deposited during the Late Early Cambrian–Early Ordovician interval, after the Pampean orogeny, have detrital patterns common to many sectors along the Terra Australis orogen, reflecting increasingly dominant input to the Paleozoic basins from the Neoproterozoic to Early Cambrian orogenic belts of the Gondwana margin.  相似文献   

12.
碰撞带前陆盆地的建立是大陆碰撞的直接标志和随后造山带构造变形的忠实记录。本文对欧亚板块与印度板块碰撞前后发育在拉萨地块上的冈底斯弧背前陆盆地,同碰撞产生的雅鲁藏布江周缘前陆盆地,以及碰撞后陆内变形产生的喜马拉雅前陆盆地的沉积地层演化以及碎屑锆石物源特征等进行了系统分析,结合前人及我们近些年的研究成果,认为冈底斯岛弧北侧发育一个典型的弧背前陆盆地系统而不是以前普遍接受的伸展盆地。除传统认为的喜马拉雅前陆盆地系统外,在碰撞造山带中还发育一个雅鲁藏布江前陆盆地系统,它是欧亚板块与印度板块碰撞以后,欧亚板块加载到印度被动大陆边缘产生的典型周缘前陆盆地。上述2个造山带前陆盆地系统的识别,大大提高了对新特提斯洋俯冲、碰撞过程的认识。造山带前陆盆地证据指示,新特提斯洋至少于140 Ma以前就已开始俯冲, 110 Ma俯冲速度开始提高,在65 Ma前后印度大陆与欧亚大陆发生碰撞,喜马拉雅山于40 Ma开始隆升,其剥蚀物质大量堆积在喜马拉雅前陆盆地中。  相似文献   

13.
The Tucumán foreland basin is bounded by: 1) basement cored ranges with elevations over 6000 m in the west; 2) inverted extensional grabens to the north; 3) basement thrust blocks in the south and 4) basement cored small ranges in the east. This foreland basin is located between two geological provinces: the Sierras Pampeanas and the Santa Bárbara system.Cretaceous Salta rifting extended southwards covering the entire eastern part of the province of Tucumán in NW Argentina. Syn-rift and post-rift deposits can be recognized in accordance with their architectural geometries. Foreland basin sediments progressively covered the rift deposits as the Andean orogen propagated towards the east.Despite some early studies, the Tucumán basin is poorly documented. For the present study, 44 sections of 2D seismic surveys amounting to more than 730 km were used to describe the structure and the depositional evolution of the basin. The present structure is the result of a long sequence of events that includes a compressional deformation during the Paleozoic, a rifting stage during the Cretaceous and a foreland stage during the late Cenozoic. Although tectonic inversion, which has played a role during the foreland stage since the Miocene, can be observed in many sectors of the basin, it is more prominent along the margins. Reactivation of old basement discontinuities and inversion of Cretaceous normal faults produced the compartmentalization of the foreland, giving rise to the present shape of the Tucumán basin. This evolution is recorded in the Neogene deposits.  相似文献   

14.
Fluorite mineralization at the La Nueva and Bubu mines yields Sm-Nd ages of 131 ± 22 and 117 ± 26 Ma, respectively. Thus, the mineralization most probably is related to a late Gondwanian (Lower Cretaceous) extensional and magmatic event that affected the Sierras Pampeanas basement during the opening of the Atlantic Ocean. Hydrothermal fluids involved in the formation of the fluorite probably were of meteoric origin, their isotopic composition (Sr and Nd) resulting largely from the incongruent dissolution of feldspars in the host porphyritic granites.  相似文献   

15.
The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian.

The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.  相似文献   


16.
The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 ± 10 and 1250 ± 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 ± 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 4° towards 130°, corresponding to a nearly vertical southwest–northeast (50–230°) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.  相似文献   

17.
The Toro Negro Formation is a foreland sequence in western La Rioja province, Argentina, which records the late-stage tectonic evolution of the Vinchina Basin. Together with the underlying Vinchina Formation, these two units represent one of the thickest and longest continually exposed foreland sections in northwest Argentina. The Vinchina basin is uniquely situated between the Toro Negro and Umango blocks of the Western Sierra Pampeanas to the north and south, the Precordillera to the west, and the Sierra de Famatina to the east. New U-Pb dating of volcanic tephra provides improved age constraints on the pace of sedimentation, and U-Pb ages of detrital zircons serve to strengthen existing provenance interpretations. We show that deposition of the Toro Negro Formation spans roughly 6.9 to 2.3 Ma: Late Miocene to Early Pleistocene. A high-relief, erosional unconformity with the underlying Vinchina Formation developed sometime between 9.3 and 6.9 Ma, although stratigraphic considerations suggest it spanned only the later part of this time interval (perhaps 7.5–6.9 Ma). Above this unconformity, undecompacted sedimentation rates are remarkably high at ∼1.2 mm/yr, slowing to ∼0.3 mm/yr after ∼6 Ma. An unconformity in the upper part of the section is constrained to occur sometime between 5.0 and 3.0 Ma, probably beginning not long after 5.0 Ma. The timing of both unconformities broadly Matches the timing of inferred tectonic events in the Sierra Famatina ∼50 km to the east, the Fiambalá basin to the north, and the Bermejo basin to the south, suggesting they May record regional tectonism at these times. Provenance interpretations of detrital zircon spectra are consistent with previous interpretations based on sediment petrography. They show that provenance did not change significantly during the course of Toro Negro deposition, precluding major tectonically-induced drainage reorganization events. Sediments were derived primarily from the north (Toro Negro Block) and west (Precordillera). The data are consistent with a subtle increase in sediment supply from the Precordillera beginning around 6.5 Ma.  相似文献   

18.
The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580?C510?Ma) Pampean, the late Cambrian?COrdovician (500?C440?Ma) Famatinian and the Devonian-Carboniferous (400?C350?Ma) Achalian orogenies. Geochronological and Sm?CNd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm?CNd results for the metamorphic basement suggest that the T DM age interval of 1.8?C1.7?Ga, which is associated with the less radiogenic values of ??Nd(540) (?6 to ?8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger T DM age and a more positive ??Nd(540) value. Pampean pre-540?Ma granitoids form two clusters, one with T DM ages between 2.0 and 1.75?Ga and another between 1.6 and 1.5?Ga. Pampean post-540?Ma granitoids exhibit more homogenous T DM ages ranging from 2.0 to 1.75?Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. T DM values for the Ordovician Famatinian granitoids define a main interval of 1.8?C1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger T DM ages ranging from 1.3 to 1.0?Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic ??Nd(540) values that range between 0.5 and ?4 and T DM ages younger than 1.3?Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.  相似文献   

19.
As an important part of South China Old Land, the Jiangnan Orogenic Belt plays a significant role in explaining the assembly and the evolution of the Upper Yangtze Block and Cathaysia, as well as the structure and growth mechanism of continental lithosphere in South China.The Lengjiaxi and the Banxi groups are the base strata of the west section of the Jiangnan Orogenic Belt.Thus, the research of geochronology and tectonic evolution of the Lengjiaxi and the Banxi groups is significant.The maximum sedimentary age of the Lengjiaxi Group is ca.862 Ma, and the minimum is ca.822 Ma.The Zhangjiawan Formation, which is situated in the upper part of the Banxi Group is ca.802 Ma.The Lengjiaxi Group and equivalent strata should thus belong to the Neoproterozoic in age.The Jiangnan Orogenic Belt consisting of the Lengjiaxi and the Banxi groups as important constituents is not a Greenville Orogen Belt(1.3 Ga–1.0 Ga).The Jiangnan Orogenic Belt is a recyclic orogenic belt, and the prototype basin is a foreland basin with materials derived from the southwest and the sediments belong to the active continental sedimentation.By combining large amounts of dating data of the Lengjiaxi and the Banxi groups as well as equivalent strata, the evolutionary model of the western section of the Jiangnan Orogenic Belt is established as follows: Before 862 Ma, the South China Ocean was subducted beneath the Upper Yangtze Block, while a continental island arc was formed on the side near the Upper Yangtze Block.The South China Ocean was not closed in this period.From 862 Ma to 822 Ma, the Upper Yangtze Block was collided with Cathaysia; and sediments began to be deposited in the foreland basin between the two blocks.The Lengjiaxi Group and equivalent strata were thus formed and the materials might be derived from the recyclic orogenic belt.From 822 Ma to 802 Ma, Cathaysia continued pushing to the Upper Yangtze Block, experienced the Jinning-Sibao Movement(Wuling Movement); as result, the folded basement of the Jiangnan Orogenic Belt was formed.After 802 Ma, Cathaysia and the Upper Yangtze Block were separated from each other, the Nanhua rift basin was formed and began to receive the sediments of the Banxi Group and equivalent strata.These large amounts of dating data and research results also indicate that before the collision of the Upper Yangtze Block with Cathaysia, materials of the continental crust became less and less from the southwest to the east in the Jiangnan Orogeneic Belt; only island arc and neomagmatic arc were developed in the eastern section.Ocean-continent subduction or continent-continent subduction took place in the western and southern sections, while intra-oceanic subduction occurred in the eastern section.Comprehensive analyses on U-Pb ages and Hf model ages of zircons, the main provenance of the Lengjiaxi Group is Cathaysia.  相似文献   

20.
A new paleomagnetic study on well-dated (~ 155 Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A95 = 5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~ 155 Ma. Our calculation shows a ~ 1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~ 155 Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~ 155 Ma.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号