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1.
Halkieria is an Early Cambrian fossil genus with proposed phylogenetic affinities ranging from stem group lophotrochozoan to stem-group aculiferan. Skeletal similarities are often cited as evidence in support of a mollusk interpretation for Halkieria and related Cambrian fossils. The skeletal microstructures of the polyplacophoran mollusk （chiton） Ischnochiton hakodadensis, which bears perinotum elements resembling sclerites of halkieriids and other coeloscleritophoran small shelly fossils （SSFs）, were studied to evaluate the interpretations. There are both similarities and differences between chiton skeletal structures and halkieriid sclerites： 1） the head valve of L hakodadensis and the dorsal shell of Halkieria evangelista share a similar pattern of fine concentric growth lines, but halkieriid dorsal shells lack other polyplacophoran valve characteristics, including V-shaped indentation, radiate ribs; 2） the arrangement of I. hakodadensis perinotum elements in three zones is somewhat similar to the arrangement pattern of halkieriid sclerites; 3） the inner perinotum scales of L hakodadensis （-200 μm in diameter） have a base and a blade, remarkably similar in morphology to halkieriid palmate sclerites （-400 μm in diameter）. However, I. hakodadensis perinotum scales are nearly solid whereas halkieriid sclerites are hollow. These observations add a microstructural dimension to the skeletal comparison between mollusks and halkeriids, and may prove to be useful in resolving the phylogenetic affinity of Halkieria and the Halwaxiidae.  相似文献   

2.
In recent years, an increasing number of plant fossil leaves and petrified woods have been discovered from the Middle Jurassic Wanbao Formation in Moguqi Town of Inner Mongolia, NE China. Here, we describe a new species of Coniopteris moguqiensis sp. nov. preserved as a fragment with fertile and sterile pinnules. The sterile ultimate pinnules are elongate ovate with sphenopteriod type venation, and fertile pinnules are usually isolated, bipinnate at least with the sorus apical, elliptical, 1 mm in diameter; sporangia are almost globular, 100–150 μm in diameter, and the annulus is vertical. In situ spores are rounded-triangular in polar view, 25–30 μm in diameter with sides straight and slightly convex; trilete, laesurae are thin and slightly straight; the exine surface is usually psilate under the light microscope but finely reticuloid sculptured on the proximal view under a scanning electronic microscope. The fern genus Coniopteris usually suggests a warm and humid environment, which is consistent with the palaeoclimatic conditions of petrified wood and megafossil plants. The new discovery further supplements the floral composition of the Wanbao Formation, providing new material for understanding the evolutionary trend and classification of Coniopteris.  相似文献   

3.
New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the ～3.5 Ga North Pole area of Western Australia. Their carbon isotopic compositions were measured in situ by secondary-ion mass spectrometry. The carbonaceous filaments are ～1μm wide, 10 to 100 μm long, and are permineralized in a fine-grained (～1 μm) silica matrix. They are morphologically divided into three types (i.e., spiral, threadlike, and branched filaments). Their sizes and morphologies resemble modern and previously reported fossil bacteria. These similarities and their complex three-dimensional geometry suggest that they may represent morphologically preserved fossil bacteria. δ13C values of the carbonaceous filaments range from ?42 to ?32‰, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Calvin cycle. This is consistent with the hypothesis that autotrophs already existed on the Archean Earth.  相似文献   

4.
Cambrian Fossil Embryos from Western Hunan,South China   总被引：1，自引：1，他引：0  
The exquisitely preserved fossil embryos of Markuelia recovered from the limestones of the Middle Cambrian Haoqiao Formation and Upper Cambrian Bitiao formation in western Hunan, South China are described and illustrated in detail for the first time.A new species Markuelia elegans sp.nov.is established based mainly on embryos from the Upper Cambrian.A few of animal's resting eggs,which are comparable with those of the Ediacaran Doushantuo Formation,have been also found in the Upper Cambrian of western Hu...  相似文献   

5.
Coniopteris was a ubiquitous plant of the Jurassic and Cretaceous periods and played an important role in the flora of the time. However, its anatomical structure is relatively poorly known. The specimens of Coniopteris hymenophylloides(Brongniart) Seward described here were collected from the Yaojie Formation in Gansu Province, northwestern China. The sterile fronds are characterized as being at least bipinnate with alternate arranged linear pinnae covered by thin cuticles. Fertile fronds are linear-lanceolate, with single sorus at the margin of each fertile pinnule. In situ spores are typically trilete, triangle to subcircular in polar view, cap-shaped in equatorial view, and 37 μm in average diameter. The trilete marking is straight and narrow, generally extends to 4/5 of the spore radius. The spore surface is smooth, and parts of the exine are granulated. The epidermal cells of cuticles are irregular in shape, approximately 40-60μm long and 10-20 μm wide. The elliptical stomatal complexes are paracytic,approximately 30 μm long and 19 μm wide,and irregularly distributed. On the basis of its epidermal structures and comparisons with extant ferns, we consider that Coniopteris displays combined features of the related extant genera.  相似文献   

6.
Fossil mammal sites of late Miocene age (ca 9 Ma) occur in hourglass‐shaped, non‐interconnected cavities up to 15 m deep, hosted in mudstone (mostly sepiolite), chert and carbonate bedrock in Cerro de los Batallones. This paper provides a model for the sedimentary infilling of the cavities, which functioned as traps for vertebrate faunas and contain one of the richest and best preserved Neogene mammal assemblages of the Iberian Peninsula. Generation of the mammal‐bearing cavities started with the solution of underlying evaporites, which resulted in fissures that were subsequently enlarged by subsurface piping, a process rarely preserved in the ancient sedimentary record. The system of subterranean cavities evolved into a pseudokarst landscape, resulting in doline‐like shafts reaching the ancient land surface. The sedimentary infilling of the cavities comprises both clastic and carbonate lithofacies that were investigated by outcrop observation, standard and scanning electron microscope petrography, mineralogical analysis, and stable isotope geochemistry. Gravel and breccia talus deposits, clast and mud‐supported gravel, pebbly sandstone and mudstone are common detrital infill deposits mostly derived by overflow erosion of bedrock. The deposits containing the mammal bones are marls, and occur both in subsurface cavities and doline‐like depressions. In the underground cavities, marlstone was mainly of clastic origin and accumulated in ponds scattered over the floor of the cavity. In contrast, marlstone deposits in the surface dolines formed mostly as a result of biochemical carbonate deposition in small shallow lakes subjected to fluctuation of the water level. The δ18O and δ13C carbonate values indicate different origins for the two kinds of marls. During the final phases of pipe infill the doline marlstone sealed the mammal sites, usually off‐lapping the adjacent bedrock.  相似文献   

7.
靖安油田五里湾一区长6储层孔隙结构特征   总被引：8，自引：0，他引：8  
靖安油田五里湾一区是一个大型的低渗透油气开发区，长6储层具有高孔低渗的特征，储层以原生剩余粒间孔隙为主，长石及早期碳酸盐胶结物的溶解是形成次生孔隙的主要原因，中孔（20um-50um)和大孔（50um-100um)是油气的主要储集空间，而细喉（1um-3um)和中喉（3um-7um)则是长6基本的渗流通道，根据均值与细喉以上孔隙所占百分比的关系曲线，可将长6储层孔隙结构分为4种类型，其中1类，II类具有较好的孔渗性，Ⅲ类孔隙结构较差，Ⅳ类为非储层孔隙结构，铸体薄片孔喉图像分析表明，长6储层具有复杂的孔喉分布特征，不同微相孔喉分布及孔隙结构特征是造成油井产能差异的重要因素。  相似文献   

8.
本文结合生产实践,对煤田钻探绳索取心技术的钻孔结构、钻头选择、粉煤取心、护孔堵漏、钻具配套等问题,提出了一些认识。包括孔径确定、钻具级配、孔身构造、技术套管;钻头选型、胎体磨料、水口形态;粉煤钻头、碎岩取心器、自锁半合管;冲洗液选型、随钻堵漏、停钻堵漏工艺;以及应用加长岩心管、钻杆接手、防斜接手、可退式打捞矛等。   相似文献   

9.
Werner, K., Tarasov, P. E., Andreev, A. A., Müller, S., Kienast, F., Zech, M., Zech, W. & Diekmann, B. 2009: A 12.5‐kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia. Boreas, 10.1111/j.1502‐3885.2009.00116.x. ISSN 0300‐9483. A 415 cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon‐dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic‐rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ～12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well‐preserved and radiocarbon‐dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe‐like communities around the site, possibly in response to a relatively warm and dry climate ～11.4–11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ～11.0–8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ～8.2–6.9 and ～6.7–0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (～170 km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.  相似文献   

10.
The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix-supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM-CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM-CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a-axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ18O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ18O composition of +20·0‰. Estimates of the δ18O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ18O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.  相似文献   

11.
Szewczyk, J. & Nawrocki, J. 2011: Deep‐seated relict permafrost in northeastern Poland. Boreas, Vol. 40, pp. 385–388. 10.1111/j.1502‐3885.2011.00218.x. ISSN 0300‐9483. A research borehole drilled in northeastern Poland revealed a permafrost layer at least 93 m thick within the Cretaceous sedimentary succession below a depth of 357 m. Its entire thickness has not been determined, owing to the borehole depth limit (450 m). The relict of permafrost, unexpected in this part of Europe, is in the ice–water transition phase at a temperature close to 0 °C. We estimate that the permafrost has been preserved in an area of several square kilometres located above the central part of the Suwa?ki anorthosite intrusion, which is covered by ～800 m of sedimentary rocks. We show that the remnants of the Last Glaciation, which ended at c. 13.4 ka BP, can, even at present, affect the thermal regime of the outer zone of the Earth's crust. These findings are of significant importance for the reconstruction of past climate conditions.  相似文献   

12.
A new macrofossil Lagerstatte was discovered from the uppermost Ediacaran Jiucheng Member at Jinning and Jiangchuan of the eastern Yunnan, yielding numerous diverse well-preserved thallophyte macrofossils. These include the previously-known representatives of vendobionts, Vendotaenia and Tyrasotaenia, and the biggish multicellular benthos such as Chuaria-like and Tawuia- like fossiis, as well as Longfengshaniaceaens with diverse holdfast structures. There are still some other problematic macrofossils with peculiar configurations as well as uncertain relatives. The distinct dominance of the giant, unbranching thallophytes occasionally with holdfast structures distinguishes this assemblage from the other Ediacaran macrofossil Lagerstattes in the Doushantuo Formation at Miaohe, Wenghui and Lantian, and the contemporary assemblage in the Shibantan Member of Dengying Formation, Yangtze Gorges area. This paper outlines the characteristics of some of the multicellular macrofossils from the Jiucheng Member at Jiangchuan. They include some macrofossils with different types of holdfast structure, larger Chuaria-like and Tawuia-like morphology and questionable affinities as well. The discovery of greater diverse macrofossil assemblages from the Jiucheng Member of eastern Yunnan has further indicated that an important diversification and evolutionary radiation of metaphytes took place in the latest Ediacaran time. This radiation of largescale, benthic metaphyte along with phytoplankton was likely important contributors to the early Cambrian explosion of metazoans.  相似文献   

13.
There is a cross-cutting relationship between the E-W trending structures and the NE-trending structures in the northern Longmen-Micang Mountains region,which reflects possible regional tectonic transi...  相似文献   

14.
A Pliocene strandline system in the Hanson Plain Sands, between the volcanic plains of the Western District and the northern flanks of the Otway Range in southwest Victoria, provides an excellent datum against which to measure Late Neogene fault‐related uplift. Individual strandlines that can be traced from elevations of ～120 m near Cobden to ～245 m on the Ferguson Hill structure are displaced across northeast‐trending faults and monoclines associated with the Simpson and Ferguson Hill structures. A break in slope in the Otway Range front at elevations of 230–280 m, beneath which drainage incision parallels the trends of the strandlines, probably correlates with the Pliocene coastline on the ancestral Otway Range. By analogy with correlative systems in the Murray Basin, deposition occurred during sea‐stands between 0 and 65 m above present‐day sea‐level, implying uplift of between 175 and 240 m since the Early Pliocene. Enhanced incision parallel to the strandlines, in combination with tilting of fault blocks about northeast axes at a high angle to the strandlines, has facilitated the development of a remarkable rectilinear drainage net. Local inversion of the drainage where it focused basalt flows constrains the age of incision and faulting to greater than ca 1 Ma and, most probably, less than ca 2 Ma.  相似文献   

15.
Physical evidence for the drainage of glacial lakes remains relatively rare in depositional records, giving rise to much debate on the location of outlets and discharge pathways, as well as on the climate impact of the attendant meltwater forcing. Lake Ojibway developed following the withdrawal of the Laurentide Ice Sheet in northern Ontario and Quebec, Canada. The late‐stage evolution of this large ice‐dammed lake was influenced by the complex dynamics of the retreating ice margin, which highly complicates the identification of the termination of Lake Ojibway in glaciolacustrine sediment records. Here, we document the composition of sections of rhythmites that contain in their upper part an anomalously thick and whitish bed (10–15 cm) that is in turn overlain by ～1 m of faintly bedded rhythmites. Grain‐size analyses showed that the thick whitish bed consists primarily of fine to coarse silt (2–63 μm), contrasting with the lower and upper rhythmites that are largely dominated by clay (<2 μm). The detrital carbonate content of the thick silt bed is characterized by consistently high values (2.5 to 2.8%), whereas the bounding rhythmites show lower and highly variable values. Oxygen isotope measurements further show a marked change going from typical glacial meltwater values (～ ?29.6 to ?27.7‰; VSMOW) for the lower rhythmites and the silt bed to modern‐like meteoric values (?18.4 to ?14.6‰) for the uppermost rhythmites. These data suggest that this marker bed may be associated with a major drawdown event that possibly corresponds to the final drainage of Lake Ojibway. AMS radiocarbon dating of ostracods extracted from the drainage bed also documents an important hardwater effect within the Ojibway basin.  相似文献   

16.
The Hongtoushan Volcanogenic Massive Sulphide Deposit (VMSD) occurs in the Hunbei granite-greenstone terrane, Liaoning Province, NE China. Rocks in the mining area have been metamorphosed around 3.0–2.8 Ga to upper amphibolite facies at temperatures between 600°C and 650°C. Cordierite-anthophyllite gneiss (CAG) in the Hongtoushan mining area, which occurs hundreds of meters below the ore horizon, corresponds to the metamorphosed semi-conformable alteration zone of the VMSD hydrothermal system, whereas the one immediately below the main ore layer represents the metamorphosed pipe-like alteration zone. Whole-rock oxygen isotope signatures were well preserved in both types of CAGs, although the mineral components have been entirely changed during regional metamorphism. Therefore, whole-rock oxygen isotopes can be used to estimate the formation temperature of both types of alteration zone. Calculations show that the semi-conformable and pipe-like alteration zones for the Hongtoushan submarine hydrothermal system were formed at 290–360°C and 285–320°C, respectively, whereas estimates for the former were slightly higher than that of the latter, indicating that the semi-conformable alteration zone represents the deep part of the Hongtoushan seafloor hydrothermal system, while the pipe-like alteration zone represents the discharge conduits for metal-rich fluids, which is closer to the seafloor.  相似文献   

17.
The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ～530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ～530 °C and 4.6 kbar to ～630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ～4‐kbar decompression during Early Devonian exhumation of the Greenvale Province.  相似文献   

18.
This article is to review results from scientific drilling and fault-zone trapped waves(FZTWs) at the south Longman-Shan fault(LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan, China. Immediately after the mainshock, two Wenchuan Fault Scientific Drilling(WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault(YBF), the middle fault strand of the south LSF zone. Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m, respectively. The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge, breccia, cataclasite and fractures. Close to WFSD-1 site, the nearly-vertical slip of ~4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake. A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks. Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths. The zone is several hundred meters wide along the principal slip, within which seismic velocities are reduced by ~30–55% from wall-rock velocities and with the maximum velocity reduction in the ~200-m-wide rupture core zone at shallow depth. The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes. We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake. We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ~10% due to severe damage of fault rocks during the M8 mainshock. Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault(AGF), two strands of the south LSF at shallow depth. A combination of seismic, petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics, and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.  相似文献   

19.
车载钻机是矿山事故地面钻孔救援的核心装备,在国内外历次成功救援中发挥了重要作用,给进系统是车载钻机的主要执行机构,使用过程中暴露出长工作行程制约整机通过性、定位轮系维护困难、双油缸运动同步性不强、动力头偏载引起导轨磨损、液压表显示起拔力延时等问题。研制的ZMK5550TZJF50/120型救援车载钻机采用基于油缸—钢丝绳倍速传动机构的伸缩桅杆式结构,实现了长工作行程和短运输尺寸;设计快换式轮系、钢丝绳防扭转张紧系统,解决一二级给进机身的辅助定位系统安全保护和维护困难的难题;采用给进系统油路同步补偿和先导控制技术,解决给进装置双油缸同步的难题;研发抽屉式调平体和测力销轴结构,实现动力头钻进过程中自平衡式调节及起拔力监测。经过静态、模态分析、应变测试和现场工业性应用,给进系统最大起拔力达1 205 kN,能够有效处理大直径救援井施工过程中卡钻、埋钻等孔内事故,配合动力头、钻杆加卸系统,具有快速起下钻具功能,可大幅度提高倒杆效率,单根起下钻效率小于3 min/根。研究结果表明:救援车载钻机伸缩桅杆式给进装置运输尺寸短,钻机机动性强,道路通过性好;现场试验中处理了多次孔内卡钻、埋钻事故隐患,稳定和可靠性较高、可维护性好、起拔力监测准确、倒杆效率高,可以满足600 m深度救援井施工需求。给进装置轻量化,以及执行系统、传动系统、动力系统之间的响应匹配优化研究是今后的重点方向。   相似文献   

20.
Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.  相似文献   

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