北京王府井东方广场古人类遗迹热释光断代

东方广场位于北京王府井地区。施工开挖中发现,在埋深大约10.5m和12m处分别有2层河流砂层,含化石、石器、灰烬等物。经热释光断代方法测定,上层约为距今1.5至1.9万年,下层约为距今2.2至2.6万年。     

第四纪沉积物常用测年方法及其适用性研究

由于沉积环境与成因的不同,第四纪地层的岩性、成分、结构和厚度在短尺度空间范围内可能发生明显的变化,地表松散沉积物在自然营力的作用下容易受到剥蚀、搬运和再沉积;在气候变化、生物活动、特别是水的作用下,其化学成分和结构也会发生变化,因此,第四纪地层不仅在空间上复杂多变,而且其组成、岩性等也随时间变化。本文讨论了第四纪松散沉积物的测年问题,对各种测年方法适用的测定对象与测年范围作了分析和对比,力求获得最接近于地层沉积年龄或地质事件发生年龄的数据。     

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.     

Zircon SIMS ages and chemical compositions from Northern Dabie Terrain: Its implication for pyroxenite genesis

We present the results of a detailed micro-scale investigation of zircons from pyroxenites, Daoshicong Northern Dabie using a combination of SIMS and ICPMS. The SIMS measurements gave ages of 134—159 Ma. Its average of (144.5± 6.2) Ma is interpreted as the best estimate of the pyroxenite intrusion. The crystallization of zircons continued for quite a long time and underwent slow cooling. The pyroxenites are products of post-collision magmatism. The REE pattern is HREE-enriched, and its HREE concentrations fall between the magmatic and metamorphic range of gneissic zircons from the Dabie area, which indicate the involvement of crust material in its mantle source.     

Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively. Both whole-rock and rock-forming minerals show large δ18O variations up to 5.5%. with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the δ18O values of zircon are tightly clustered between 3.12%. and 4.19%. and thus lower than the normal-mantle δ18O values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially. The remarkably low zircon δ18O values are genetically due to sea-water exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydro thermal alteration in the second stage. It is inferred that the 18O-depleted A-type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures. In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.     

Nd isotopic disequilibrium between minerals and Rb-Sr age of the secondary phengite in eclogite from the Yangkou area, Qingdao, eastern China

There are two generations of white micas in retrograded coesite-bearing eclogite from the Yangkou area near Qingdao, eastern China. The secondary phengite developed along the folliations in eclogite is the majority of the white micas. Nd and Sr isotopic disequilibriums between garnet and retrograded omphacite as well as secondary phengite have been observed. Consequently, the Rb-Sr ages ((193 ± 4) Ma―(195 ± 4) Ma) given by the tie lines of the secondary phengite + garnet or whole rock may predate the formation time of the phengite. The Rb-Sr age of (183 ± 4) Ma given by the secondary phengite + retrograde omphacite is much closer to the formation time of the phengite indicating the retrograde age of eclogite instead of a cooling age of eclogite at 500℃.     

祁连山东段冷龙岭南麓白水河冰碛物ESR测年研究

白水河是祁连山东段冷龙岭南麓大通河的一条支流,在其谷口保存有形态较完整的冰川终碛,终碛剖面中有将该终碛垄分成两次冰川作用的一推积垄,此次用ESR测年技术对采自该剖面中的冰碛物进行了测年研究,推积垄前部上样品测得的ESR年龄为73．0ka,55．8ka;推积垄后部靠近推积垄的一个样品,测得ESR年龄为36．7ka,所测得的ESR年龄与终碛垄沉积的新老关系相一致,经过ESR测年结果的误差与其可靠性讨论并结合终碛垄沉积关系和推积垄前后的^14C年龄得出;推积垄前部的冰碛物为末次冰期早期沉积的,与深海氧同位素的4阶段相对应;推积垄后部的冰碛物为末次冰期晚期所沉积的,可与深海氧同位素的2阶段相对应,此结论也进一步验证以往学者所得出的结论,说明冰碛物可以用ESR技术进行测年。     

20世纪末激光显微探针定年把K-Ar年代学推向了新的里程碑

已有50年历史的K-Ar定年法,由于过剩Ar和Ar丢失的普遍发现,使其最广泛的应用面临着严重挑战。在20世纪的最后几年,以激光显微探针⁴⁰Ar/³⁹Ar定年方法的完善和精度的提高为标志,把K-Ar年代学研究推向了一个新高度。微区微量高精度高分辨定年,把定年时限扩展到人类历史范畴,精细的分析技术拓宽了年代学的应用范围,使之解决的地质问题更广泛。     

无稀释剂法K-Ar定年的原理及地质应用

年轻火山物质K-Ar定年中Ar的精确测量一直是地质年代学研究的重要课题。在常规K-Ar定年法中,样品外来氩的⁴⁰Ar/³⁶Ar比值被假定为295.5。但研究表明,这种假定在某些情况下,尤其是在年轻样品的测量中受到了限制。一种无稀释剂的测Ar技术已在近年来得到了发展和应用。该方法的优势在于测氩系统不受³⁸Ar稀释剂的混染,利用标准大气的校准可测得样品中的⁴⁰Ar、³⁸Ar和³⁶Ar。考虑火山岩中大气氩分馏造成的影响,利用测得的³⁸Ar/³⁶Ar比值还可以对所测样品中的外来⁴⁰Ar/³⁶Ar比值进一步加以校正,给出精确的放射性成因⁴⁰Ar^*含量计算和K-Ar定年。目前,无稀释剂的K-Ar定年技术在地质上已有广泛应用,并能取得晚第三纪到第四纪可靠的年龄,对地质事件做出合理地解释和重建岩浆活动历史。对无稀释剂法K-Ar定年原理及地质应用的介绍,将为K-Ar年代学应用于研究晚第三纪以来的年轻火山岩提供一种有力的手段和解决途径。     

电熔镁锆合成料中SiO_2的赋存形态及分布特征

研究了以轻烧氧化镁和锆英石为原料 ,以电熔法合成的 Mg O-Zr O2 复相材料中 Si O2 的赋存形态。结果表明 :系统中占 (3 0～ 40 ) %的 Si O2 成分与 Mg O反应形成镁橄榄石相 (M2 S) ,余下的 Si O2 成分将进入玻璃相 ,且玻璃相的组成与钙镁橄榄石的理论组成大致相当。