Influence of tectonic overpressure on P–T paths of HP–UHP rocks in continental collision zones: thermomechanical modelling

The principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P–T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P–T paths of HP–UHP rocks is located in the bottom corner of the wedge‐like confined channel with the characteristic magnitude of pressure deviation on the order of ～0.3 GPa and 10–20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP‐rocks subjected to significant (≥0.3 GPa) overpressure at intermediate subduction depth (50–70 km, P = 1.5–2.5 GPa) then underpressured at depth ≥100 km (P ≥ 3 GPa); (ii) HP‐rocks subjected to ≥0.3 GPa overpressure at peak P–T conditions reached at 50–70 km depth in the bottom corner of the wedge‐like confined subduction channel (P = 1.5–2.5 GPa); (iii) lower‐pressure rocks formed at shallower depths (≤40 km depth, P ≤ 1 GPa), which are not subjected to significant overpressure and/or underpressure.     

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area,northern Iran

The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.     

New data for water losses from mature Sitka spruce plantations in temperate upland catchments / Nouvelles données pour les pertes d’eau de plantations adultes de sapins de Sitka en bassins versants tempérés d’altitude

Abstract

Abstract Accurate estimates of water losses from mature Sitka spruce (Picea sitchensis) plantations in the UK uplands are required to assess the sustainability of water supply in the event of land-use change. Many investigations have demonstrated that afforestation increases water losses from temperate upland catchments, to up to 40% of annual site rainfall. In a 0.86 km² upland water supply catchment in southwest Scotland, interception loss in a Sitka spruce-dominated 37-year old plantation, was 52% of annual precipitation (2912 mm), considerably higher than reported in previous studies of similar catchments. From direct measurements of rainfall, cloudwater, discharge and soil evaporation, the catchment water balance was 96–117% complete, within the limits of measurement error. The most probable explanation for the higher forest interception loss reported here is the inclusion of cloudwater measurements.     

A window into the Early to mid‐Cretaceous infrastructure of the Yukon‐Tanana terrane recorded in multi‐stage garnet of west‐central Yukon,Canada

Amphibolite facies metasedimentary schists within the Yukon‐Tanana terrane in the northern Canadian Cordillera reveal a two‐stage, polymetamorphic garnet growth history. In situ U‐Th‐Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn‐kinematic, inclusion‐rich stage‐1 garnet core; and (ii) an inclusion‐poor, stage‐2 garnet rim that crystallized with syn‐ to post‐kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage‐1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular‐rich, pyrope‐poor inner portion of the stage‐2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage‐1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage‐2 metamorphism and the waning development of a composite transposition foliation (S_T) are dated at c. 118 Ma from monazite aligned parallel to S_T, and inclusions in syn‐ to post‐S_T staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y‐rich monazite that occurs within resorbed areas of both stage‐1 and stage‐2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near‐isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon‐Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high‐grade tectono‐metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east‐central Alaska and in the southeastern Canadian Cordillera.     

Metamorphic evolution of blueschists of the Altınekin Complex,Konya area,south central Turkey

The Altınekin Complex in south central Turkey forms part of the south‐easterly extension of the Tavşanlı Zone, a Cretaceous subduction complex formed during the closure of the Neo‐Tethys ocean. The protoliths of metamorphic rocks within the Altınekin Complex include peridotites, chromitites, basalts, ferruginous cherts and flysch‐facies impure carbonate sediments. Structurally, the complex consists of a stack of thrust slices, with massive ophiolite tectonically overlying a Cretaceous sediment‐hosted ophiolitic mélange, in turn overlying a sequence of Mesozoic sediments. Rocks within the two lower structural units have undergone blueschist–facies metamorphism. Petrographic, mineral–chemical and thermobarometric studies were undertaken on selected samples of metasedimentary and metabasic rock in order to establish the time relations of deformation and metamorphism and to constrain metamorphic conditions. Microstructures record two phases of plastic deformation, one predating the metamorphic peak, and one postdating it. Estimated peak metamorphic pressures mostly fall in the range 9–11 kbar, corresponding to burial depths of 31–38 km, equivalent to the base of a continental crust of normal thickness. Best‐fit peak metamorphic temperatures range from 375 to 450°C. Metamorphic fluids had high H₂O:CO₂ ratios. Peak metamorphic temperature/depth ratios (T/d values) were low (c. 10–14°C/km), consistent with metamorphism in a subduction zone. Lawsonite‐bearing rocks in the southern part of the ophiolitic mélange record lower peak temperatures and T/d values than epidote blueschists elsewhere in the unit, hinting that the latter may consist of two or more thrust slices with different metamorphic histories. Differences in peak metamorphic conditions also exist between the ophiolitic mélange and the underlying metasediments. Rocks of the Altınekin Complex were subducted to much shallower depths, and experienced higher geothermal gradients, than those of the NW Tavşanlı Zone, possibly indicating dramatic lateral variation in subduction style. Retrograde P–T paths in the Altınekin Complex were strongly decompressive, resulting in localized overprinting of epidote blueschists by greenschist–facies assemblages, and of lawsonite blueschists by pumpellyite–facies assemblages. The observation that the second deformation was associated with decompression is consistent with, but not proof of, exhumation by a process that involved deformation of the hanging‐wall wedge, such as gravitational spreading, corner flow or buoyancy‐driven shallowing of the subduction zone. Copyright © 2005 John Wiley & Sons, Ltd.     

海南省生活指数预报系统简介

以SGI工作站为基础,在工作站上进行紫外线指数、舒适度指数、穿衣指数的预报并建立预报系统,预报结果在微机上以图形和表格方式显示.系统充分利用现有的网络资源和T106数值预报产品,制作海南省生活指数预报,界面友好,操作简便.     

大地测量有关国际组织和科学规划的最新动态

叙述了大地测量有关国际组织和科学规划的动态及其发展趋势。由此我们可以了解有关国际组织正在做的工作、要达到的目标和有关科学规划的最新进展及已获得成果。这对我国的大地测量工作者来说 ,无疑是很有价值的。     

逐日滚动分县指导预报技术简介

简要介绍了利用 1995～ 1997年T10 6资料 ,建立新疆区域逐日滚动分县要素指导预报系统的方法和技术     

Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O system

Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity–composition relation for minerals, a P–T projection and a series of P–T pseudosections for specific samples of eclogite have been calculated in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P–T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P–T conditions, respectively, of 540–550 °C at c. 16 kbar, c. 560 °C at 15–17 kbar and c. 580 °C at 15–19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610–630 °C and 17–18 kbar. Together with H₂O‐content contours in the related P–T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P–T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature.     

Evidence of earth catastrophe by anomalous shocked quartz at the K/T boundary

Anomalous shocked quartz with high density (less than 1 % of density-deviation) is considered to be a relict of ultra high-pressure at meteoritic impact. The shocked quartz grains can be found only in terrestrial and artificial impact craters, meteorites and the Cretaceous-Tertiary (K/T) boundary samples. Volcanic activity is considered to be started or accelerated by enormous impact event.