The transition from rifting to sea-floor spreading within a magma-poor rifted margin: field and isotopic constraints

We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean–continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from gabbros and albitite yield concordant U–Pb ages of 161 ± 1 Ma; their initial ɛ_Hf (+ 14.4 to + 14.9) as well as bulk rock ɛ_Nd values of from gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle.     

The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)

The Zone of Samedan is part of a fossil, early Mesozoic rift system originally situated in the distal, Lower Austro-Alpine domain of the Adriatic passive continental margin. An early Mesozoic configuration of asymmetrical rift basins bounded by relative structural highs compartmentalized Late Cretaceous active margin tectonics; Jurassic half-grabens were folded into arcuate synclines, whereas relative structural highs engendered thin, imbricated thrust sheets. West-directed thrusting and folding initiated at the surface and continued to depths favoring mylonitization under lower greenschist-facies conditions. At this time Liguria-Piemontese ophiolites were accreted to Lower Austro-Alpine units directly underlying the Zone of Samedan. Late Cretaceous orogenic collapse of the Adriatic active margin involved the reactivation of west-directed thrusts as low-angle, top-to-the-east, normal faults. These faults accommodated extensional uplift of Liguria-Piemontese ophiolites and Lower Austro-Alpine units beneath and within the Zone of Samedan. During Paleogene collision, some Late Cretaceous faults in the Zone of Samedan were reactivated under lower anchizonal conditions as north-directed thrusts. The latter stages of this early Tertiary thickening were transitional to brittle, high-angle normal faulting associated with top-to-the-east extension and spreading above the warm, uplifting Lepontine dome.     

Evidence of hydrothermal fluid flow in a hyperextended rifted margin: the case study of the Err nappe (SE Switzerland)

This paper investigates hydrothermal fluid circulation in pre- and syn-tectonic sediments associated with detachments faults. The study area, located in the Err Nappe (SE-Switzerland), preserves a portion of the Adriatic distal margin. Two sites were studied in combining fieldwork, petrography, geochemistry and fluid inclusion analysis: the Piz Val Lunga and Fuorcla Cotschna areas. Both preserve relationships between a spectacularly exposed rift-related extensional detachment fault and its footwall and hangingwall that consist of extensional allochthons and syn- to post-tectonic sediments. These areas register a complex fluid flow history characterized by dolomitization, de-dolomitization, calcite cementation, dolomite and quartz veining and diffuse silicification. Meso- and micro-scale observations allow defining two steps in fluid evolution, which are related to Jurassic rift activity. A first carbonate-rich event occurred before the exhumation of the granitic basement, and this was followed by a second event marked by a change in the fluid towards a silica-dominated chemistry. Homogenization temperatures of fluid inclusions (average T_h = 120?130 °C), negative δ¹⁸O values and a radiogenic ⁸⁷Sr/⁸⁶Sr signatures of carbonate minerals support the hypothesis that both the pre-tectonic rocks constituting the allochthons and the syn-tectonic sediments overlying the detachment fault were crossed by a flux of over-pressured hydrothermal fluids originating from seawater that penetrated into the basement through fault and fracture systems. Field relationships show that this fluid circulation started latest in middle Early Jurassic time, when fault activity migrated from the proximal to the future distal margin. We propose that it evolved chemically as a result of the involvement of the granitic basement forming the footwall of the extensional detachment system. Hydrothermal activity continued until the Middle/Late Jurassic, when tectonic activity shifted outwards leading to the exhumation of mantle rocks. This paper provides an original contribution to better understand the complex evolution of hyperextended continental rift domains and to constrain their thermal regimes.     

Heat flow of the eastern Canadian rifted continental margin revisited

Continental shelves are regions where heat flow is not measured directly by conventional marine techniques, but estimated from oil exploration data with possible bias. In order to overcome this problem, we recently proposed a method based on neural networks (Geophys J Int 2006, 166 :115) that provides better constraints on thermal conductivity: it allowed us to derive 161 heat flow estimates on the eastern margin of Canada, where previous studies had concluded to the existence of heat flow higher than that in the adjacent continent and ocean. We conversely found rather homogeneous values (∼45–55 mW m⁻²) all along and across the margins, from Labrador to Nova Scotia. The difference seems essentially related to the porosity of sediments that was not taken into account in previous studies. Our results support a high asymmetry for the conjugate Iberian‐Canadian margins, and could indicate that mantle heat flow increases abruptly from the Canadian shield to the margins.     

Preservation of a Paleoproterozoic rifted margin in the Himalaya:Insight from the Ulleri-Phaplu-Melung orthogneiss

Orthogneiss within the Paleoproterozoic strata of Lesser Himalayan sequence across the Himalaya has been variably linked to development in a continental arc setting, Indian basement, or a continental rift.New whole rock and trace element geochemical data and U/Pb zircon geochronology indicate that the granitoid protoliths to these rocks were derived from upper crustal sources in the Paleoproterozoic and have within-plate, A-type affinities. This is consistent with their generation in a rifted margin and is compatible with paleogeographic reconstructions that indicate an open boundary for present-day northern India in the Paleoproterozoic.     

Thermotectonic evolution of a rifted continental margin: fission track evidence from the Kangerlussuaq area, SE Greenland

Fission track analyses of apatites from sediments, Precambrian gneisses and Caledonian to Tertiary intrusive rocks from the Kangerlussuaq region reveal its post-Caledonian thermotectonic history. Inland the history involves cooling to temperatures within the high temperature part of the apatite annealing interval and slow cooling (or reheating) continued in Cretaceous times through this interval. Apatites from coastal areas between Kangerlussuaq and Tasiilaq reveal only Tertiary cooling. In Tertiary times cooling accelerated after the main intrusive phase in the Tertiary. The evolution is taken as evidence for a general uplift/erosion since Caledonian times probably disturbed by basin formation and sedimentation and reheating due to magmatic activity. Thermal subsidence of the rift shoulders following the opening of the adjacent oceanic basin is not indicated. Annealing patterns inland of the plume centre in the Kangerlussuaq provide no evidence for the earlier movement of the plume from a westerly direction.     

New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps

Direct observation and extensive sampling in ancient margins exposed in the Alps, combined with drill-hole and geophysical data from the present-day Iberia margin, result in new concepts for the strain evolution and near-surface response to lithospheric rupturing at magma-poor rifted margins. This paper reviews data and tectonic concepts derived from these two margins and proposes that extension, leading to thinning and final rupturing of the continental lithosphere, is accommodated by three fault systems, each of them characterized by a specific temporal and spatial evolution during rifting of the margin, by its fault geometry, and its surface response. The data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting. The place of final break-up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres. During final break-up, the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes.Dedicated to Daniel Bernoulli who taught me to compare the geological record of oceans and orogens     

Inheritance of Jurassic rifted margin architecture into the Apennines Neogene mountain building: a case history from the Lucretili Mts. (Latium, Central Italy)

The western Lucretili Mts. in the central Apennines (Latium, Italy) have been recently re-mapped in great detail and are the subject of combined stratigraphic, sedimentological and structural investigations. In this paper, we present a new stratigraphic interpretation of the Jurassic paleogeography of western Lucretili Mts., where a rift-derived intrabasinal paleo-high of the Alpine Tethys has been identified for the first time by means of facies analysis and biostratigraphic dating. Recognised facies associations, combined with dated stratigraphic sections, allow to define the morphology of the structural paleo-high and to identify the associated gravity-driven deposits (olistoliths) accumulated in the surrounding basin. Furthermore, we investigated the modes of interaction between Jurassic extensional structures and the subsequent contractional patterns developed during the Tertiary mountain building. In detail, the role played during Apennines tectonics by the paleo-escarpments bounding the paleo-high and by the surrounding olistoliths has been analysed. The paleo-escarpments either acted as focussing features for ENE-directed frontal thrust ramp localisation and were offset with small shortening amounts or reactivated as NNE striking high angle transpressional faults or preserved the original geometries as a result of variable orientation of paleo-escarpments with respect to the Neogene compressive stress field (with ENE oriented sigma1). Newly formed ENE striking tear faults connect these either inherited or neo-formed discontinuities. This complex stratigraphic and structural pattern is substantially different from the previous interpretations of this portion of the central Apennines based on a hypothesised layer-cake stratigraphy deformed by neo-formed Neogene thrusts. This contribution strengthens the importance of integrating facies analyses and structural investigations to detect the influence of pre-orogenic structures on compressive structural patterns, in an area where commercial seismic lines are not available and cannot help in reconstructing the subsurface geometries.     

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO₂ and incompatible trace element contents, and a large range of Nd_i values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in Nd_i–(²⁰⁶Pb/²⁰⁴Pb)_i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO₂, La and Nb contents, and lower and more homogeneous Nd_i values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative Nd_i (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative Nd_i values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.     

天山古生代洋陆转化特点的几点思考

本文简要评述了天山地区前寒武纪洋陆格局、古生代洋盆的开启与闭合的时限、古生代洋陆格局、下石炭统底部碰撞造山磨拉石发现的地质意义、以及石炭—二叠纪大规模裂谷岩浆作用形成的地球动力学背景等问题,并且提出了一些新的思考     

The transition from the Younger Dryas to the Preboreal: a case study from the Kattegat, Scandinavia

A two-step climatic warming and oceanographic change during the Younger Dryas/Preboreal transition was registered by diatom, foraminiferal, mollusc, lithologic data and sediment accumulation rates in a high resolution sediment core from the Swedish west coast. An abrupt climatic warming in the surface water of the Kattegat occurred at c . 10 200 BP, resulting in a rapid increase in sea surface water temperatures. The attenuation of meltwater discharge into the Kattegat led to an increase in sea surface salinity. Consequently, the difference in salinity through the water column diminished. This change happened within less than 80 years. The warming of bottom water in the deeper parts of the region took place a few hundred years after the surface water warming. The climatic amelioration was recorded by increased meltwater discharge and a slight increase in abundance of relatively warm diatoms around 10 600 BP at the time of the recession of the Fennoscandian ice sheet. An increase in the number of arctic/subarctic benthic foraminifera shows that the bottom water temperature during this period was still relatively low.     

Gully development in eastern Romania: a case study from Falciu Hills

The gullied systems from the Falciu Hills within the Chioara catchment (2997 ha) consist of both main types of gullies, discontinuous and large continuous ones along valley bottoms, and lots of ephemeral gullies. Several methods have been used to measure and estimate gully characteristics. Then, the gully development stages, the effect of the natural conditions, and especially the impact of land management on gullying in the Falciu Hills over the last two centuries have been defined. In addition, the role of gully erosion in triggering landslides has also been studied. Two main periods have been distinguished (until 1960 and 1961–2012) for assessing major characteristics of land degradation. The results show that total gully area in the Chioara catchment is 66.4 ha excepting for the ephemeral gullies, and areas occupied by gullies from the five study sub-catchments (2334 ha) account for two-thirds. Total length of the main gully network in the entire catchment is 33.2 km from which the five sub-catchments account for 71 %. The mean gully density of 1.11 km km⁻² supports the evidence that here gullying is the major environmental threat. Half of the gully areal growth and three-quarters of the new landslide area occurred over the 1961–2012 period. Delayed deforestation peaking during 1830–1930 and land conversion to arable use resulted in severe soil erosion, high aggradation along the non-gullied valley bottoms, and severe gullying. The average gully head retreat rate over the last two centuries from four trunk continuous gullies is 14 m year⁻¹, and the sediment yield from gullying only accounted for 54–69 % of the sediment mass produced by water erosion. The evolution of gullies is linked to major land-use changes in the study area. Despite a decreasing tendency of gullying and catchment area over the last half century, gullying still remains problematically high in East Romania.

     

Generation and metamorphism of new crust in magmatic arcs: a case study from northern Chile

In a deep section of the Jurassic Andean arc, coastal Cordillera of northern Chile, south of Antofagasta, basic magmas from a mantle source accreted large masses of crust in an early stage of arc development. These gabbroic intrusions were isochemically metamorphosed at moderate pressures (≤ 5 kbar) and high temperatures (∼ 800°C) into uniform garnet-absent dino+ortho-pyroxene gneisses (granulites). Subsequently they were partly transformed into amphibolites ( T ≤ 700°C) during tectonically controlled access of fluid without major changes in whole rock chemistry. Locally migmatization occurs in the amphibolites. The metamorphic unit was diversified by enduring magmatic activity in the form of chemically distinct gabbroic and quartz dioritic intrusions. Formation and composition of continental crust might be a typical example for the deeper plutonic sections of this continental magmatic arc.     

Fluid flow characteristics of Bandanna Coal Formation: a case study from the Fairview Field,eastern Australia

Fluid flow characteristics of cleat systems in coalbed methane reservoirs are crucial in reservoir management and field development plans. This paper aims to evaluate the cleat system properties including cleat porosity, permeability, and aperture as well as the impact of permeability growth on production performance in the Bandanna Coal Formation of the Fairview Field, eastern Queensland. Owing to the presence of bad hole conditions and poor core recovery of the coal intervals, the petrophysical well logs and laboratory measurements cannot be used as a source of information for this purpose. Hence, a new approach is employed that utilises early water production data to measure water in place and absolute permeability of the coal. In addition, micro-computed tomography (CT) scan method is used to investigate the cleat system that is preserved in a core sample and results are compared with the ones obtained by analysis of production data. Cleat system evaluation by analysis of production data and micro-CT scan technique provides a comprehensive approach that brings confidence in measurements and helps to obtain cleat properties at the sufficient scale for reservoir engineering purposes. The necessary information including production data and core samples are collected from a dewatering well and the nearby observation well in the study area. Analysis of early water production data (single-phase flow) indicates that coal permeability is 189 mD and the average cleat porosity is approximately 5%. High cleat porosity describes the large volume of water produced over the life of the study well. The 3D model of the fossilised cleat system constructed by the micro-CT scan method reveals that coal is well-cleated and cleat spacing and mean cleat aperture are 4 and 0.136 mm, respectively. The average cleat porosity that is measured by the micro-CT scan method is 5.7%, which is fairly close to the cleat porosity measured by analysis of production data. Production data analysis indicates that effective permeability to gas starts to grow at the midlife of the well and it strongly controls the shape of the production profile. The results of this study help in future field development and infill drilling programs in the Fairview Field and provide important insights into cleat system of Bandanna Coal Formation.     

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300?million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The S?o Gabriel block in the west is characterized by c.760?C690?Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880?Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ??Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the S?o Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D₃) yield an U?CPb zircon age (LA-ICP-MS) of 701?±?10?Ma giving the approximate minimum age of the S?o Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789?±?7?Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234?±?28?Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100?C2,000?Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800?C700?Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and S?o Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880?Ma (Passinho event) and (ii) accretion of the 760?C700?Ma Cambaí/Vila Nova magmatic arc (S?o Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW?CNE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590?Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530?Ma and can be assigned to final stages of the amalgamation of West Gondwana.     

Multi-stage crustal accretion by magmatic flare-up and quiescence intervals in the western margin of the São Francisco Craton: U-Pb-Hf and geochemical constraints from the Almas Terrane

The global plate tectonic regime in early Paleoproterozoic times is highly debated. The interval 2.45–2.2 Ga is known for a minima in juvenile magmatism, but this is not a global phenomenon. New results of whole-rock geochemistry and U–Pb-Hf analysis in igneous and detrital zircons, allied with existing isotopic and geophysical data, allow to identify and constrain the duration of magmatic flare-up and quiescence events in the western São Francisco Paleoplate. Igneous samples yield ages indicating three accretionary magmatic events, an older with ages ca. 2476.4 ± 9 Ma to 2462 ± 13 Ma, an intermediate at 2390 ± 14 Ma, and a younger from 2235 ± 26 Ma to 2201 ± 5 Ma, all presenting magmatic arc geochemical signatures. Xenoliths of quartzite and volcanic tuff from the upper greenstone sequence (Morro do Carneiro Fm.) are hosted in the 2211 ± 9 Ma tonalite and the maximum depositional age of the Morro do Carneiro basin is dated 2234 ± 12 Ma, indicating a syn-orogenic setting for this basin. Detrital zircon UPb age distribution for quartzites of the greenstone sequence shows peaks at 2.65, 2.47, 2.39, 2.27 and 2.23 Ga. Altogether, the studied rocks record an accretionary orogeny with four distinct episodes: Episode S1: 2.52–2.46 Ga, ɛHf(t) values from +0.57 to +6.36; Episode S2: 2.43–2.37 Ga, ɛHf(t) values from +0.10 to +4.30; Episode R1: 2.32–2.26 Ga, ɛHf(t) values from +1.61 to −7.23 (from detrital zircons); Episode R2: 2.24–2.20 Ga, ɛHf(t) values from +0.39 to −2.73. These early Paleoproterozoic accretionary orogenies mark the onset of amalgamation of the São Francisco continental paleoplate that surrounds the craton, with accretions of an exotic micro-block and continental magmatic arcs, indicating evolution from dominant Siderian juvenile magmatism to Rhyacian crustal magmatism. These patterns show striking similarities to the orogenies in the Mineiro Belt and North China Craton.     

Pastoralists in transition — a case study from Lengijape,Maasai Steppe

The change of the Maasai from pure pastoralists to agropastoralists has brought about serious problems for them. This paper tries to analyse this process of change and to throw some light on the resultant problems. First, it is being shown how the climate of the Maasai Steppe is too dry for a successful cultivation of maize by them, which has become their main food. Successive crop failures and high labour costs have rendered maize-growing a losing-business for the Maasai. Moreover, the State policy of granting certificates of occupancy in the Maasai land to outsiders allowing them to make large-mechanized farms, has alarmed the Maasai so much that many of them rushed back to their home areas to rescue their land from being grapped by other ethnic groups. Thus, they began to abandon their traditional grazing-movements, which are highly adapted to the ecological conditions of the Maasai Steppe. At the same time, the herds of the Maasai were decimated, because of the spread of animal diseases following the deterioration of the State veterinary service system. Added to this are the losses incurred through distress sales of cattle for buying maize for food, at a time at which the maize prices are increasing at a much higher rate than the prices of livestock. This dilemma of the Maasai and their indigenous coping mechanisms are being illustrated by the case of the people of Mokia at Namalulu, with special consideration of the role of women in solving these problems.     

Possible correlation between a mantle plume and the evolution of Paleo-Tethys Jinshajiang Ocean: Evidence from a volcanic rifted margin in the Xiaru-Tuoding area, Yunnan, SW China

Several Paleozoic sutures in Southwestern China provide a record of the history of the Paleo-Tethys Ocean, whose birth and final closure are associated with the breakup and assembly of Gondwanaland. Recent studies indicate that there are widespread OIB-type mafic volcanic rocks within these suture zones and intervening terranes. This paper examines the geology and geochemistry of volcanic rocks in the Xiaruo-Tuoding area, a remnant passive margin succession of the Jinshajiang Paleo-Tethyan Ocean. The sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults. The available geochemistry of these volcanic rocks suggest that they are OIB-like basalts, characterised by SiO₂ = 42.78–50.46 wt.%, high TiO₂ contents (TiO₂ = 2.2–3.55 wt.%), moderate MgO = 4.15–6.49 wt.%, Mg# = 0.37–0.50, high Ti/Y ratios (mostly > 450), large ion lithosphere elements enrichment, high strength field elements and rare earth elements, with La/Nb = 1.04–1.39, Ce/Yb = 18.38–30, Sm/Yb = 2.16–3.52, (⁸⁷Sr/⁸⁶Sr)i = 0.705350–0.707867, and Nd(t) = − 1.43–1.90. These geochemical and isotopic signatures are generally similar to those of the Emeishan flood basalts, which together with stratigraphic constraints, demonstrate that these volcanics were formed in a volcanic rifted margin, probably associated with a mantle plume. A new model is proposed to interpret the evolution of the Jinshajiang Paleo-Tethyan Ocean and its possible relationship to the Emeishan mantle plume. In this model, we argue that the opening of the Jinshajiang Paleo-Tethyan Ocean in the Carboniferous was caused by a mantle plume. The mantle plume was active to the east along the western margin of the Yangtze Craton between 300 and 260 Ma, from which the voluminous Emeishan flood basalts were erupted at 260 Ma. The closure of the Jinshajiang Ocean occurred since the Middle Permian. Continuous westward subduction generated the Jiangda-Weixi magmatic arc to the west of the Jinshajiang suture. This subduction also partly destroyed and/or tectonically sliced the volcanic rifted margin. Some seaward dipping volcanic-sedimentary sequences on the east flank of the Jinshajiang Ocean were preserved, but are strongly deformed.     

扬子板块北缘新元古代构造属性的岩浆事件制约

黄陵地区新元古代侵入杂岩可为研究扬子板块北缘新元古代构造演化过程及其深部动力学机制提供关键信息。依据岩石组合及分布特征,可将黄陵杂岩划分为黄陵庙岩套、三斗坪岩套、大老岭岩套和晓峰岩套四个单元。本文以黄陵杂岩的围岩崆岭杂岩中花岗片麻岩、黄陵庙黑云母花岗岩和三斗坪闪长岩为研究对象,在系统的野外地质和岩石学研究基础上,开展了LA-ICP-MS锆石U-Pb年代学分析。结果表明,崆岭杂岩花岗片麻岩原岩年龄为1978±13 Ma,且记录了2.5 Ga的构造-热事件。黄陵庙黑云母花岗岩和三斗坪闪长岩分别形成于815±9 Ma和813±6 Ma,与黄陵庙岩套和三斗坪岩套的侵位时限基本一致。综合分析本次研究结果与前人资料,认为黄陵杂岩主要形成于863～794 Ma,为新元古代早期岩浆活动的产物。通过综述区域地质和地球化学研究资料,认为黄陵杂岩形成于新元古代早期活动大陆边缘的构造环境,提出扬子板块北缘在新元古代早期经历了长期的俯冲-增生造山过程。