黔中隆起金沙地区中新生代隆升剥蚀的裂变径迹分析

对贵州金沙地区取样进行了裂变径迹热史模拟,结合地质分析再现了黔中隆起的沉降和隆升剥蚀作用过程。该地区经历了180～70Ma时期的沉降埋藏增温和70Ma之后的剥蚀冷却降温两大阶段。沉降阶段的增温速率为0.909℃/Ma,对应的T3—K的沉积厚度约为4550m。冷却降温曲线分为三段:70～15Ma,样品温度从120℃降低到60℃,冷却速率为1.09℃/Ma;15～5Ma为一个"平台"期,样品温度维持在60℃左右;5Ma至今,样品温度从60℃快速冷却至20℃,冷却速率达8℃/Ma,这与喜马拉雅期云贵高原的整体隆升和晚期快速崛起相对应。计算表明,晚白垩世以来的隆升剥蚀量达2800m左右。     

Fission track analysis and thermotectonic history of the main borehole of the Chinese Continental Scientific Drilling project

The Chinese Continental Scientific Drilling (CCSD) project, part of the International Continental Drilling Program (ICDP), has completed drilling a 5158 m hole in the eastern part of the Dabie-Sulu ultrahigh-pressure metamorphic belt. This study reports on an apatite fission track analysis of core samples from 0 to 4000 m depth in the CCSD main hole (CCSD-MH). We determined the fission track ages of 38 apatite samples from different depths. The ages range between 98.6 ± 17.0 and 3.2 ± 1.3 Ma, showing a general decreasing trend with depth, from 87.1 ± 11.2 Ma at the surface to 3.2 ± 1.3 Ma at 3899 m depth. As a first approximation, an average uplift rate of ~ 35 m/Ma is calculated for the period 90-30 Ma. The trend in ages within the borehole shows some fluctuations, and indicates movements along major faults. It is inferred that the highest-level major normal fault occurs at a depth of ~ 350 m, recording a vertical displacement of ~ 400 m. Movement along another prominent normal fault at a depth of ~ 2150 m occurred subsequent to ~ 25 Ma. Three major reverse faults occur at about 2450, 3050 and 3250 m depth. Testing geological constrains against the fission track data set indicated an agreement with a reheating of the area during the late Cretaceous and Eocene, followed by cooling to ~ 80 °C during the Eocene and a low cooling until the samples reached their present-day position in the Donghai area.     

苏鲁超高压变质岩的构造热历史：中国大陆科学钻探工程(CCSD)主孔（MH）和先导孔（PP2）的磷灰石裂变径迹约束

本文包括中国大陆科学钻（CCSD）主孔（MH）0~5000m和先导孔（PP2）0~1000m的磷灰石裂变径迹分折结果,先导孔PP2的裂变径迹表观年龄变化范围为79.5±5.1~50.4±6.2Ma,主孔的裂变径迹表观年龄变化范围为98.6±17.0~2.9±2.0Ma,主孔在4200m以下,磷灰石样品中实际上已不存在自发裂变径迹,表明裂变径迹时钟已经“置零”。实验资料表明,裂变径迹表观年龄值随样品深度的增加而逐渐减少,直到一定深度,即达到磷灰石的裂变径迹封闭温度（~120℃）以后,年龄值为零。根据主孔0~2000m和先导孔0~1000m的裂变径迹年龄剖面,作为一级近似,计算出超高压变质岩体在90~30Ma期间,平均隆升速度为~35m/Ma。对主孔测定了9个样品的约束径迹（Confined track）长度,样品约束径迹平均长度的变化范围约为13.1~7.4μm,总的变化趋势是：约束径迹平均长度随样品深度的增加而逐渐减少。样品的约束径迹长度分布都具有双峰型特征。根据裂变径迹年龄和约束径迹长度的资料,应用计算机模拟得到了样品的时间-温度（t-T）轨迹。结果表明,岩体从早白垩世（~120Ma）快速冷却以后,在晚白垩世和始新世又经历了两次加热作用,始新世末岩体所达到的温度大约是80℃,随后岩体则一直上升和缓慢冷却到现今所处的位置。在最后~30Ma岩体的平均隆升速度为~53m/Ma。     

SHRIMP in situ isotopic analyses of REE,Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (?-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.     

孔雀河斜坡与库鲁克塔格隆起构造事件的裂变径迹证据

文章以塔里木盆地东北缘库鲁克塔格隆起与孔雀河斜坡盆山系统为主要研究对象,在该地区露头和钻井样品开展碎屑磷灰石、锆石裂变径迹研究,对库鲁克塔格构造演化中关键构造事件提供热年代学约束.锫石样品在加里东晚期-早海西期达到最大古地温,之后经历了长期的抬升降温过程,锆石最小峰值年龄记录了371～392 Ma 和328～305.7...     

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone,New Zealand

 Large volume (100–1000 km³), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of ⁴⁰Ar/³⁹Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession. Received: 20 November 1995 / Accepted: 8 May 1996     

Thermal history of the Krishna–Godavari basin,India: Constraints from apatite fission track thermochronology and organic maturity data

The Krishna–Godavari (KG) basin, a passive margin Late Carboniferous to Holocene basin along the rifted east coast of India, includes the deltaic and inter-deltaic regions of the Krishna and Godavari rivers onshore and extends into the offshore. It is one of India’s premier hydrocarbon-bearing basins. In an attempt to better understand the thermal history of the basin, apatite fission track (AFT) data has been obtained from six exploration wells (five onshore and one offshore). AFT thermal history models as well as other thermal indicators e.g. vitrinite reflectance (VR), Rock–Eval T_max data reveal that the host rocks are currently at their maximum post-depositional temperatures and that any possible heating related to small-scale tectonism or rifting episodes in the basin bears little significance on the maturation of the sediments. In the case of one borehole (M-1) however, the organic maturity data reveals a period of Oligocene cooling across an unconformity when ∼1000 m of section was eroded due to falling sea-level. This information offers the potential for improved basin modeling of the KG basin.     

长江中下游成矿带抛刀岭金矿裂变径迹研究及大地构造意义

抛刀岭金矿位于长江中下游成矿带中的安庆-贵池矿集区,是长江中下游多金属成矿带中首次发现的斑岩型独立金矿床。通过磷灰石裂变径迹研究分析区内的构造演化特征,裂变径迹长度处于11.9±1.8μm和13.2±1.6μm之间,表明样品在后期受构造热事件的影响,样品长时间处于退火带温度,长度直方图总体呈现单峰特征,受隆升冷却作用控制明显。通过对同组年龄(x2检验值>5%)和混合年龄(x2检验值<5%)进行分析,所测年龄位于123Ma和19Ma之间,可以将年龄分为5组:123.0Ma,107Ma,86Ma,52Ma,19Ma。裂变径迹热历史可以分为4个时期:缓慢冷却期,从150～90Ma;快速冷却期,从90Ma至65～58Ma;缓慢冷却期,从65～58Ma至18～12Ma;快速降温期,从18～12Ma至今。自中侏罗世以来,江南隆起带受控于太平洋板块构造,太平洋板块在不同时期有着不同的活动,其运移方向和活动强度直接控制着中国东部的构造演化。研究地区所测裂变径迹年龄划分时代与太平洋形成后不同产状洋岛链出现的时代吻合,热历史模拟结果和太平洋板块不同期次活动相关。通过热历史模拟和构造分析,可推测热历史不同阶段的形成是不同时期太平洋板块运动的结果,样品混合年龄亦证明了构造活动的多期次性。可以推断贵池地区自白垩纪以来的构造运动与太平洋构造带活动密切相关,其多期构造演化是对太平洋板块活动的响应。     

On statistical estimation in fission track dating

Statistical methodology is described for obtaining point and interval estimates of age in fission track dating experiments, particularly for (a) the external detector method, which is reviewed briefly, and (b) the population method, which is considered in more detail. Some mathematical background to models, based on the underlying physical process, is given. This is used to suggest appropriate statistical methods, to assess them and to help understand sources of variation in the data. For the external detector method, a model is required for numbers of tracks given the amount of uranium; the usual Poisson model is derived as a consequence of independent thinning of a bivariate point process. For the population method, uranium heterogeneity does not necessarily invalidate the estimate but does represent a source of variation that must be allowed for in its error; thus an interval estimate of age might be relatively uninformative.     

Thermochronology of allochthonous terranes in Ecuador: Unravelling the accretionary and post-accretionary history of the Northern Andes

The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present ⁴⁰Ar/³⁹Ar and fission-track data from the Cordillera Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of 540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at 13 and 9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since 6 Ma.