江西相山铀矿田地貌剥蚀特征及其控矿意义——磷灰石裂变径迹证据

本文以数字高程(DEM)地貌特征分析为基础,采用磷灰石裂变径迹测试和温度-时间反演模拟研究,分析江西相山铀矿田铀成矿后剥蚀程度的差异性,结合已知矿床的成矿特征,探讨地貌剥蚀程度与矿体保存之间的关系,为区域找矿提供指导.通过DEM合成图像和水系分布特征,表明相山铀矿田是一个遭受中等侵蚀的地貌区,相山主峰南北和东西侧地貌侵蚀差异特征明显.统计分析表明,已经发现的铀矿床、点的分布与次级火山机构关系密切,相山南部的次火山机构剥蚀较深,西部次火山机构剥蚀相对弱,而北部和西北部则处于中等剥蚀程度.磷灰石裂变径迹测试结果表明,相山铀矿田的南部和东部开始剥蚀的时间早于西部,但晚于相山主峰的剥蚀.利用磷灰石的裂变径迹长度和温度参数,进一步开展了温度-时间的反演模拟研究,结果显示相山西部快速隆升发生于40~60 Ma之间,相山南部和东部的快速隆升发生于60~75 Ma之间,相山主峰的快速隆升发生于75~100 Ma之间,表明相山主峰、相山东部及南部较西部经历了较长时间的剥蚀.结合现今区域地质体出露特征及铀矿化蚀变类型的空间展布规律、成矿深度的估算等,推测相山铀矿田东部和南部剥蚀程度较深,早期可能形成的中低温铀矿体被剥蚀殆尽;北部剥蚀程度中等,地表出露了形成深度稍深的碱交代蚀变矿床;而西部剥蚀程度较低,地表发育浅部低温成矿的酸交代蚀变铀矿床.据此推断,相山铀矿田的西部深部具有很好的找矿潜力.     

Apatite fission track geochronology of the Southern Hunan province across the Shi-Hang Belt: insights into the Cenozoic dynamic topography of South China

The Shi-Hang Belt is a Mesozoic tectonic zone and has always been regarded as the boundary between the Yangtze and Cathaysia blocks. It occupies a key tectonic location and attracts considerable attention due to its dynamic formation mechanism. However, its Cenozoic dynamic process is poorly constrained. The Cenozoic activation of the Shi-Hang Belt, as well as its cooling and exhumation, aids in dating the onset time of the formation of the mountain ranges and reveals the deformation process of the South China Block. To uncover the history of its Cenozoic cooling and denudation, apatite fission-track (AFT) thermochronology was applied to batholiths and strata spread across the Shi-Hang Belt in the Hunan Province. Twenty-three samples are dated with ages ranging from 23.6 ± 1.5 to 45.8 ± 3.0 Ma. Except for two older ages (42.1 ± 2.6 and 45.8 ± 3.0 Ma), the other ages range from 23 to 36 Ma with less variation on both sides of the Chenzhou–Linwu fault. The thermochronological modelling of 15 measured samples demonstrates that rocks rapidly passed through the AFT partial annealing zone to the near surface at different onset times from 36 to 23 Ma. The regional AFT cooling pattern is unrelated to the internal structures of the Shi-Hang Belt characterized by a Mesozoic fold-thrust feature. We attribute the Cenozoic exhumation of the Shi-Hang Belt to the dynamic topography of the South China Block, which is related to mantle downwellings and upwellings due to several episodes of quick subduction of the Pacific Plate underneath Eurasia during the Late Cretaceous–early Cenozoic and the Oligocene–early Miocene. The far-field effect of the India–Tibet collision may have contributed to the exhumation of the Shi-Hang Belt.     

白垩纪以来米仓山-汉南穹窿剥蚀过程及其构造意义：磷灰石裂变径迹的证据

报道了米仓山-汉南穹窿一带磷灰石裂变径迹分析结果,以制约该区白垩纪以来的剥蚀-演化历史.露头样品磷灰石裂变径迹年龄分布显示从汉南穹窿南部的核部地区向南至四川盆地北部裂变径迹的年龄逐渐变新,这与米仓山地区逆冲断裂以背驮式扩展的构造样式从汉南穹窿向南经米仓山褶皱-逆冲带发育到四川盆地北缘的构造模式相吻合.热模拟的结果显示米仓山-汉南穹窿经历了两期快速的剥蚀,其分别发生在白垩纪（约90 Ma之前）和15 Ma以来.研究区白垩纪的快速剥蚀反映了秦岭-大别造山带白垩纪的区域性剥蚀事件,这可能是对临区诸多构造事件（如西伯利亚-蒙古-中朝板块的碰撞,拉萨-羌塘-思茅-印支块体的碰撞,太平洋板块向欧亚板块的俯冲及其相关的岩浆活动）远场效应的响应;约15 Ma以来的快速剥蚀是对青藏高原隆升向东北方向传递的响应.     

中亚萨亚克大型铜矿田矽卡岩型铜成矿作用的年代学制约

萨亚克铜矿田是中亚成矿域巴尔喀什成矿带唯一的以矽卡岩型铜矿化为主的大型铜矿床,产在哈萨克斯坦的萨亚克复向斜内。铜矿田包括了几个在空间上相对独立的矽卡岩型铜矿床、斑岩型铜钼网状脉矿床和一系列石英脉型矿脉,构成了斑岩型和矽卡岩型两个端元形成的成矿系列,但以矽卡岩型为主。铜矿田花岗岩类岩石的锆石SHRIMPU-Pb定年,给出了两期与矽卡岩型铜成矿作用有关的深成岩浆作用的时代,早期的闪长岩结晶年龄为335±2Ma,晚期的花岗闪长岩结晶年龄为308±10Ma,反映了两期矽卡岩型铜成矿作用的年龄。结合成矿元素分析,认为早期335±2Ma为主要成矿期年龄,而308±10Ma为次要成矿期年龄。区域成矿时代的分析表明,主矽卡岩型成矿期先于区域的斑岩型铜成矿作用而发生,次要的矽卡岩型成矿期略晚于区域斑岩铜成矿期。花岗岩类岩石中的角闪石、黑云母、钾长石40Ar/39Ar测年结果进一步限定了矽卡岩型铜成矿作用的时代,分别给出冷却年龄为286.6±6.7Ma、306.6±2.9Ma和257±11Ma。4个花岗岩类岩石中的磷灰石裂变径迹测年和热历史模拟给出年龄范围为85.4±4.5Ma至66.9±4.1Ma,说明了矽卡岩型萨亚克铜矿田的剥露作用主要发生在晚白垩世晚期。本文花岗岩类岩石的U-Pb、40Ar/39Ar和裂变径迹热年代学研究,揭示了萨亚克铜矿田从深成的岩浆侵入活动、成矿作用、区域冷却到剥露作用的全过程。     

郯庐断裂带对鲁西隆升过程的影响:磷灰石裂变径迹证据

郯庐断裂带(TLFZ)是一条贯穿华北的NNE向巨型断裂带。新生代以来,在郯庐断裂带的两侧及其内部发生了显著的伸展构造变形,形成了泰安-莱芜-蒙阴NW向断陷盆地群,并使鲁西块体发生了急剧的陆内伸展隆升。本文在前人研究的基础上,分别在鲁西沂山、徂徕山和蒙山三处进行了大量的样品采集,总计完成了25个样品的测试,获得了一系列新的磷灰石裂变径迹(AFT)年代学结果。结合前人已发表的裂变径迹结果,对鲁西地区新生代与伸展变形有关的剥露-隆升作用的时空分布特征、隆升剥露模式及隆升幅度进行分析,并揭示郯庐断裂带在鲁西新生代热隆升过程中的影响。主要认识有:1)新生代以来,鲁西主要经历了始新世-早渐新世和新近纪以来两期快速剥露-隆升阶段。2)始新世-早渐新世主要表现为幕式差异性快速剥露-隆升,鲁西南受NW向断层控制形成向北、向东的掀斜抬升作用,鲁西北受NE向断裂控制,形成向北、向西的掀斜抬升作用。新近纪以来,进入相对低速区域性剥露-隆升阶段。3)AFT模拟显示,与始新世-早渐新世的幕式快速剥露-隆升相比,中新世以来,鲁西剥露-隆升速率相对减小,但剥蚀量剥露-抬升量较大。故鲁西整体抬升于中新世以来。4)结合前人研究成果,新生代以来,鲁西宏观上受郯庐断裂带伸展活动影响,越靠近郯庐断裂带剥蚀量越大,局部受NW或NE向断裂控制。     

相山铀矿田铀多金属成矿时代与成矿热历史

相山铀矿田的铀多金属矿化主要可划分为碱性铀矿化、酸性铀矿化、铅锌银铜矿化和金矿化四种类型。通过沥青铀矿和矿化岩石U-Pb等时线、黄铁矿Rb-Sr等时线、绢云母~(40)Ar-~(39)Ar同位素年龄测定,结合铀多金属成矿特征研究,厘定了相山铀矿田铀多金属成矿时代,确定铀多金属矿化的成矿时序为:碱性铀矿化、铅锌银铜矿化、金矿化、酸性铀矿化。锆石裂变径迹研究表明,相山矿田铀多金属矿化样品的锆石裂变径迹峰值年龄与U-Pb、Rb-Sr和~(40)Ar-~(39)Ar同位素年龄一致性良好,裂变径迹年龄(峰值年龄)可以限定热液铀多金属成矿热事件时代。碱性铀成矿热事件的锆石裂变径迹峰值年龄为119. 8～125. 6Ma;金成矿热事件和铅锌银铜多金属成矿热事件的锆石裂变径迹峰值年龄为106. 1～113. 8Ma;酸性铀成矿热事件的锆石裂变径迹峰值年龄为86. 7～100. 0Ma;新发现一期锆石裂变径迹峰值年龄为66. 4～78. 6Ma的热事件,该期热事件可能为相山矿田最晚一期酸性铀成矿热事件。相山矿田66. 4～78. 6Ma的铀成矿热事件,与华南花岗岩型热液铀矿床的区域成矿热事件时代耦合,该发现对华南火山岩型铀矿成矿时代的重新认识,对火山岩型、花岗岩型铀矿床成矿统一性认识具有重要意义。     

西藏羌塘盆地白垩纪中期构造事件的磷灰石裂变径迹证据

拉萨与羌塘地块于白垩纪中期的碰撞造山对羌塘原型盆地的热体制和构造演化有着重要影响.运用磷灰石裂变径迹方法,对羌塘盆地隆鄂尼夏里组和托纳木雪山组砂岩分析表明,裂变径迹年龄集中在120～ 80Ma之间,表明在白垩纪中期,羌塘盆地普遍发生了一次构造抬升事件,该期构造事件的年龄与盆地内早白垩世的岩浆热事件、主要构造变形作用发生在晚白垩世以及雪山组和阿布山组角度不整合的时代(125～75Ma)较一致,是拉萨与羌塘地块碰撞造山事件的记录.热历史模拟表明,白垩纪中期构造事件对羌塘盆地南部和北部的热演化历史有着差异影响,羌塘盆地南部降温速率相对不大,抬升剥蚀厚度约1500m,而北部古地温迅速降温到近地表温度,抬升剥蚀厚度近4000m.这种差异抬升剥蚀可能与班公湖-怒江洋壳向南俯冲使得因拉萨地块构造负载而导致羌塘地块的挠曲有关.     

Integration of zircon color and zircon fission-track zonation patterns in orogenic belts: application to the Southern Alps, New Zealand

An exhumed crustal section of the Mesozoic Torlesse terrane underlies the Southern Alps collision zone in New Zealand. Since the Late Miocene, oblique horizontal shortening has formed the northeastern–southwestern trending orogen and exhumed the crustal section within it. On the eastern side, rocks are zeolite- to prehnite–pumpellyite-grade greywacke; on the western side rocks, they have the same protolith, but are greenschist to amphibolite facies of the Alpine Schist. Zircon crystals from sediments in east-flowing rivers (hinterland) have pre-orogenic fission-track ages (>80 Ma) and are dominated by pink, radiation-damaged grains (up to 60%). These zircons are derived from the upper 10 km crustal section (unreset FT color zone) that includes the Late Cenozoic zircon partial annealing zone; both fission tracks and color remain intact and unaffected by orogenesis. Many zircon crystals from sediments in west-flowing rivers (foreland) have synorogenic FT ages, and about 80% are colorless due to thermal annealing. They have been derived from rocks that originally lay in the reset FT color zone and the underlying reset FT colorless zone. The reset FT color zone occurs between 250 and 400 °C. In this zone, zircon crystals have color but reset FT ages that reflect the timing of orogenesis.     

Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos Ridge: constraints from thermochronology

The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as ⁴⁰Ar–³⁹Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases ⁴⁰Ar–³⁹Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.     

Fission track dating obsidians in Central and Northern Anatolia

Twenty obsidian samples from Central and North Anatolia were dated by the fission track method. Split samples analyzed in two different laboratories give consistent results. Data presented refer to three volcanic fields and are summarized as follows: Orta-Sakaeli (Northern Anatolia) obsidians with ages from 21–23 Ma; Göllü Dagi dome complex near Çiftlik (Central anatolia) has given ages between 1.33–0.98 Ma; Acigöl caldera (Central Anatolia) shows three age groups: (1) Eruptions of precaldera (Bogazköy) obsidians with ages 0.18–0.15 Ma; (2) Formation of the large intracaldera dome Kocadag-Taskesiktepe at about 0.08 Ma (uncorrected apparent age of 0.075 Ma); and (3) Acigöl peripheral obsidian domes with plateau ages of 0.020 and 0.019 Ma, and an additional uncorrected age of 0.018 Ma form a distinct age group. This fission track parameters discriminate between the various obsidian sources of prehistoric obsidian artifacts.