Characterisation of Apatites as Potential Uranium Reference Materials for Fission‐track Dating by LA‐ICP‐MS

We report homogeneity tests on large natural apatite crystals to evaluate their potential as U reference materials for apatite fission‐track (AFT) thermochronology by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). The homogeneity tests include the measurements of major element concentrations by electron probe microanalysis (EPMA), whereas for U concentration, isotope dilution (ID) ICP‐MS and laser ablation (LA) ICP‐MS were employed. Two apatite crystals are potential reference materials for LA‐ICP‐MS analysis: a 1 cm³ fraction of a Durango crystal (7.5 μg g^?1 U) and a 1 cm³ Mud Tank crystal (6.9 μg g^?1 U). The relative standard deviation (1 RSD) of the U concentration determined by ID‐ICP‐MS of both apatite crystals was ≤ 1.5%, whereas 1 RSD for the LA‐ICP‐MS results was better than 4%, providing sufficient homogeneity for fission‐track dating. The results on the U homogeneity for two different apatite samples are an important step towards establishing in situ dating routines for AFT analysis by LA‐ICP‐MS.     

浙西北湖安地区花岗岩剥露程度及其对萤石矿找矿的启示——来自于磷灰石裂变径迹的证据

浙西北湖(州)—安(吉)地区近年来新发现一系列的大中型萤石矿,显示出该地区良好的萤石矿成矿潜力.随着勘查工作的持续推进,地表找矿难度的增加,对浙西北萤石矿深部找矿潜力的研究逐渐被大家所重视.本文选择浙西北湖安地区典型萤石矿区出露的花岗岩体,通过磷灰石裂变径迹技术,获得花岗岩体的裂变径迹年龄,分析花岗岩经历的热演化历史,通过温度-时间热史模拟反演,探讨湖安地区不同区域的剥蚀程度,推测研究区深部的找矿潜力.研究结果显示,样品的裂变径迹年龄集中在(29±2)～(53±3)Ma之间,且年龄有自西向东逐渐变新的趋势.温度-时间反演模拟显示,湖安地区自晚白垩世以来一直处于隆升-剥蚀的状态,局部地区在新生代经历过快速的抬升和伸展作用,平均剥蚀速率为0.062 mm/a,剥蚀量在2 km以上.利用温度-时间模式曲线,计算了不同区域的典型萤石矿区的剥蚀程度,结果显示研究区西侧平均剥蚀量为2.89 km,中间平均剥蚀量为3.69 km,研究区东侧平均剥蚀量为4.08 km.结合地表地形地貌特征,以及萤石矿成矿年龄与矿区花岗岩年龄之间的年龄间隔,认为研究区现今出露地表的花岗岩可能并非萤石矿的成矿地质体,现今萤石矿的成矿地质体可能还在深部.根据研究区西侧剥蚀量低于东侧,推断西侧萤石矿深部可能具有更好的成矿潜力.这一结论可以为湖安地区萤石矿下一步勘探工作提供依据.     

吉林桦甸市夹皮沟本区金矿剥露历史和矿床保存变化——来自磷灰石裂变径迹年代学证据

有关吉林省桦甸市夹皮沟金矿区剥露历史的研究,至今尚不多见,然而成矿后的变化与保存是矿床学研究的一个重要领域,并且矿区剥露历史对于区内矿体保存状况具有重要的指示意义。裂变径迹是研究区域隆升剥露的一种有效手段,能提供剥蚀速率和剥蚀量的定量数据,本文应用裂变径迹技术研究夹皮沟金矿区中生代成矿以来的剥露历史。使用磷灰石裂变径迹年龄数据进行热历史模拟,揭示其演变机理。实验中获得8个了磷灰石裂变径迹测试结果,裂变径迹年龄可分为3组:(128~111Ma、86~64Ma和48~29Ma)。夹皮沟金矿区的热演化历史可分为了3个阶段,分别为:第1阶段160Ma到85±10 Ma;第2阶段85±10 Ma到28±2 Ma;第3阶段28±2 Ma至今,相应的剥露深度为2.90km、0km和1.17km。根据剥露厚度及成矿深度推测,夹皮沟本区中下戏台、立山两矿区金矿床保存状况良好,下部仍有较大探矿潜力。     

江南隆起带幕阜山岩体新生代剥蚀冷却的低温热年代学证据

江南隆起位于扬子与华夏地块的碰撞汇聚带,是研究华南大地构造演化的关键地质单元.本文采用磷灰石裂变径迹及(U-Th-Sm)/He年龄分布特征定性分析与径迹长度分布数据定量模拟相结合,主要研究了幕阜山岩体新生代的隆升与剥蚀过程,并在此基础上结合区域构造背景, 对其构造-热演化之间的关系进行了探讨.自晚白垩世持续隆升以来,幕阜山岩体经历的平均剥蚀厚度约4800 m.在不同岩体间,隆升过程及幅度存在差异,空间上具有非均匀性.热史结果显示幕阜山岩体经历了3期剥蚀, 其中两期快速剥蚀分别发生在晚白垩世-古近纪(80~50 Ma)和10 Ma以来,而这之间为一期缓慢剥蚀过程.研究区古近纪的快速剥蚀反映了中-下扬子喜山期大规模伸展断陷作用造成的肩部块体快速剥蚀事件; 约10 Ma以来的快速剥蚀是对太平洋板块向西运动的响应.幕阜山岩体自燕山晚期以来的隆升剥蚀作用具有良好的盆地沉积响应, 三期隆升剥蚀事件与研究区构造演化的动力学背景相吻合.     

低温热年代学数据对龙门山推覆构造带南段新生代构造活动的约束

龙门山位于青藏高原东边缘,地形陡变,剥蚀作用强烈.近年来先后经历了2008年汶川MW7.9地震和2013年芦山MS7.0地震,多位学者对龙门山地区做了大量的低温热年代学研究.文中在研究程度相对薄弱的龙门山南段补充了4个锆石裂变径迹年龄和4个磷灰石裂变径迹年龄,结合前人的低温热年代学数据结果得出,宝兴杂岩从新生代早期开始快速冷却,降温幅度超过225℃,而龙门山中段的彭灌杂岩降温幅度为185 ～225℃.宝兴杂岩的4个裂变径迹年龄分布在2.7 ～5Ma,相对彭灌杂岩较年轻,表明晚新生代以来宝兴杂岩的冷却速率要高于彭灌杂岩.在地表温度为15℃和古地温梯度为30℃/km的假设下,宝兴杂岩距今3～5Ma以来的平均剥露速率为0.63 ～ 1.17mm/a.低温热年代学数据揭示出龙门山中段的差异剥蚀集中在北川-映秀断裂和江油-灌县断裂上,而南段的差异剥蚀分散在更宽范围内的双石-大川断裂(南、北2个分支)及其东侧的断层和褶皱.     

Denudation history and landscape evolution of the northern East-Brazilian continental margin from apatite fission-track thermochronology

We reconstruct the history of denudation and landscape evolution of the northern East- Brazilian continental margin using apatite fission-track thermochronology and thermal history modeling. This part of the Brazilian Atlantic margin is morphologically characterized by inland and coastal plateaus surrounding a wide low-lying inland region, the Sertaneja Depression. The apatite fission track ages and mean track lengths vary from 39 ± 4 to 350 ± 57 Ma and from 10.0 ± 0.3 to 14.2 ± 0.2 μm, respectively, implying a protracted history of spatially variable denudation since the Permian at relatively low rates (<50 m My⁻¹). The Sertaneja Depression and inland plateaus record Permian-Early Jurassic (300–180 Ma) denudation that precedes rifting of the margin by > 60 Myrs. In contrast, the coastal regions record up to 2.5 km of Late Jurassic-Early Cretaceous (150–120 Ma) denudation, coeval with rifting of the margin. The samples from elevated coastal regions, the Borborema Plateau and the Mantiqueira Range, record cooling from temperatures above 120 °C since the Late Cretaceous extending to the Cenozoic. We interpret this denudation as related to post-rift uplift of these parts of the margin, possibly resulting from compressional stresses transmitted from the Andes and/or magmatism at that time. Several samples from these areas also record accelerated Neogene (<30 Ma) cooling, which may record landscape response to a change from a tropical to a more erosive semi-arid climate during this time. The inferred denudation history is consistent with the offshore sedimentary record, but not with evolutionary scenarios inferred from the recognition of “planation surfaces” on the margin. The denudation history of the northeastern Brazilian margin implies a control of pre-, syn- and post-rift tectonic and climatic events on landscape evolution.     

Denudation history of the Snowy Mountains: Constraints from apatite fission track thermochronology

Apatite fission track thermochronology from Early Palaeozoic granitoids centred around the Kosciuszko massif of the Snowy Mountains, records a denudation history that was episodic and highly variable. The form of the apatite fission track age profile assembled from vertical sections and hydroelectric tunnels traversing the mountains, together with numerical forward modelling, provide strong evidence for two episodes of accelerated denudation, commencing in Late Permian—Early Triassic (ca 270–250 Ma) and mid‐Cretaceous (ca 110–100 Ma) times, and a possible third episode in the Cenozoic. Denudation commencing in the Late Permian—Early Triassic was widespread in the eastern and central Snowy Mountains area, continued through much of the Triassic, and amounted to at least ～2.0–2.4 km. This episode was probably the geomorphic response to the Hunter‐Bowen Orogeny. Post‐Triassic denudation to the present in these areas amounted to ～2.0–2.2 km. Unambiguous evidence for mid‐Cretaceous cooling and possible later cooling is confined to a north‐south‐trending sinuous belt, up to ～15 km wide by at least 35 km long, of major reactivated Palaeozoic faults on the western side of the mountains. This zone is the most deeply exposed area of the Kosciuszko block. Denudation accompanying these later events totalled up to ～1.8–2.0 km and ～2.0–2.25 km respectively. Mid‐Cretaceous denudation marks the onset of renewed tectonic activity in the southeastern highlands following a period of relative quiescence since the Late Triassic, and establishes a temporal link with the onset of extension related to the opening of the Tasman Sea. Much of the present day relief of the mountains resulted from surface uplift which disrupted the post‐mid‐Cretaceous apatite fission track profile by variable offsets on faults.     

西昆仑及邻区新生代差异隆升的热年代学证据

通过总结和分析有关热年代学的资料, 认为西昆仑及邻区的隆升存在明显的时空差异性.第一, 隆升历史表现出明显的阶段性: 晚渐新世到早中新世(25~16Ma) 的部分隆升阶段、中新世中后期的快速隆升阶段(14~8Ma) 和晚中新世以来的整体强烈隆升阶段(6Ma以来).第二, 隆升在空间上显示出明显的差异性: 塔什库尔干-公格尔山地区的隆升主要集中在9Ma以来, 红其拉甫-库地地区的隆升主要集中在25~16Ma, 康西瓦-普鲁地区的隆升则主要在9Ma以来(集中在9~2Ma) 和25~12Ma.东西方向上表现为东西两端靠近构造结(喜玛拉雅西构造结和西昆仑-阿尔金构造结合部位) 的地方较新(主要在9Ma以来), 中间较老(主要在9Ma以前); 南北方向上, 西昆北地体在20Ma左右发生快速隆升, 西昆南地体在9~5Ma发生快速隆升, 而甜水海地体在5~2Ma发生快速隆升, 由北向南总体上呈现出由老到新的空间差异性.      

Variable Phanerozoic thermal history in the Southern Canadian Shield: Evidence from an apatite fission track profile at the Underground Research Laboratory (URL), Manitoba

Analysis of a 1.15 km deep apatite fission track (AFT) thermochronology profile at the Underground Research Laboratory (URL), in the southwestern Canadian Shield suggests two Phanerozoic heating and cooling episodes indicating significant, previously unsuspected, Phanerozoic heat flow variations. Phanerozoic temperature and heat flow variations are temporally associated with burial and erosion of the Precambrian crystalline shield and its overlying Phanerozoic successions, which are now eroded completely. Maximum Phanerozoic temperatures occurred in the late Paleozoic when the geothermal gradient is estimated to have been ~ 40-50 °C/km (compared to a present day gradient of ~ 14 ± 2 °C/km) and the sedimentary cover was ~ 800-1100 m thick. Our thermal history models, confirm regional stratigraphic relationships that suggest that the Paleozoic succession was completely eroded prior to beginning of Mesozoic sedimentation. A second heating phase occurred during Late Cretaceous-Paleogene burial when the geothermal gradient is estimated to have been ~ 20-25 °C/km and the Mesozoic and Cenozoic succession was ~ 1200 to 1400 m thick. The Phanerozoic thermal history at the URL site shows a pattern similar to that inferred previously for the epicratonic Williston Basin, the centre of which lies several 100 km to the west. This implies a common regional thermal history for cratonic rocks underlying both the basin and the currently exposed shield. It is suggested that the morphotectonic differences between the Williston Basin and the exposed shield at the URL are due to a dissimilar thermomechanical response to a common, but more complicated than previously inferred, Phanerozoic geodynamic history. The two Phanerozoic periods of variations in geothermal gradient (heat flow) were coeval with epeirogenic movements related to the deposition and erosion of sediments. These paleogeodynamic variations are tentatively attributed to far-field effects of orogenic processes occurring at the plate margin (i.e. the Antler and the Cordilleran orogenies) and the associated accumulation of cratonic seaway sedimentary sequences (Kaskaskia and Zuni sequences).     

湘东北望湘岩体的热年代学与幕阜山隆升

应用热年代学方法,测定了湘东北望湘岩体的热历史,分析了九岭-幕阜山岭的隆升过程.望湘岩体约于140.0 Ma侵位,其4个不同高程的样品的锆石裂变径迹年龄范围为85.5～74.3 Ma,磷灰石裂变径迹年龄范围为55.6～45.2 Ma.结果表明,九岭-幕阜山岭自中新生代以来,经历了3期(132.0～120.0,81.1～55.6,47.0～30.0 Ma)较强烈的隆升和剥蚀夷平过程.