数值拟合法在修正K-Ar全岩年龄值方面的应用

K Ar法是地质工作中常用的测年方法 ,根据测试对象的不同 ,可分为全岩以及单矿物K Ar法。通常情况下 ,前者的结果要比后者的结果偏低。本文在对华南及华北地区部分K Ar同位素年龄值进行详细分析的基础上 ,以黑云母年龄为基准值 ,采用数值拟合法 ,对该样品的全岩年龄值的修正问题进行了初步探讨     

Dunite, Glimmerite and Spinellite in Achankovil Shear Zone, South India: Implications for Highly Potassic CO2-rich Melt Influx Along an Intra-continental Shear Zone

In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39–50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al₂O₃ (7.8–8.98 wt.%), and low Ni (10–14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K₂O (%%5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = 0.96) and extremely low NiO (<0.14 wt.%). Spinels in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = 59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO₂ fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO₂-rich melts injected into lower crustal levels. K-Ar ages of 470.5±9.3 and 464.5±9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO₂-rich fluid/melt influx along shear zones in this part of East Gondwana continent.     

Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.     

K-Ar和Ar-Ar方法的精细测年及其检验

活动构造研究、近现代火山研究、古地磁研究等要求K-Ar和Ar-Ar年代学方法不断向越来越小的年龄尺度和越来越精细的方向发展。文中以腾冲等火山地区的K-Ar和Ar-Ar年代学研究结果及实验方法研究结果为根据,对K-Ar和Ar-Ar方法的精细测年及其检验进行阐述。说明K-Ar和Ar-Ar年代测定结果是单个样品的单次测量结果,其影响因素包括样品因素和实验因素。为了提高测年结果的准确度和精确度,必须有严格而合理的野外采样和实验过程,并且在具体的地质样品年代测量之外,还应经常利用一系列可能的方法对实验结果进行检验     

The Matsue Formation: Evidence for gross mantle heterogeneity beneath Southwest Japan at 11 Ma

Abstract Middle Miocene basalts and basaltic andesites of the Matsue Formation outcrop within a 5 km radius of Matsue city in eastern Shimane Prefecture. Despite their limited outcrop and age (11.0 ± 1.5 Ma), they show a wide range in ⁸⁷Sr-⁸⁶Sr(0.70370–0.70593), ¹⁴³Nd-¹⁴⁴Nd(0.512904–0.512471) and large ion lithophile element (LILE) contents, but a relatively narrow range for some high field strength elements (HFSE) such as Nb and Ti. These basalts and andesites can be divided into three groups based on petrography, major element, trace element and isotope chemistry. Although one group has undergone some fractional crystallization, isotope chemistry precludes linkage of the groups by a closed-system process. Crustal contamination can explain isotope chemistry, but is not consistent with trace element variations. The most satisfactory model is eruption of two compositionally distinct magmas, with limited magma mixing and fractional crystallization. Published experimental work shows that one end-member resulted from shallow melting of upwelling mantle at ∼25 km. The simultaneous eruption of the other end member magma in the same area points towards a heterogeneous mantle. The isotopic composition of Matsue Formation basalts and andesites covers the entire range of Late Miocene mafic volcanic rocks of southwest Japan. Such gross heterogeneity developed on a local scale has implications for models that deal with regional chemical variations of mafic volcanic rocks in southwest Honshu.     

雷琼地区第四纪地磁极性年表——火山岩钾-氩年龄及古地磁学证据

雷琼地区火山岩的钾-氩同位素年龄测定和古地磁极性研究表明,新生代在该地区的岩浆活动一直很强烈。火山岩的生成年代不晚于中新世晚期,至更新世岩浆活动尤为强烈。 研究也表明中新世晚期以来这里的地球磁场极性变化规律与近期发表的地磁极性年表大体相符,并肯定了数个尚有争议的短期极性事件的存在。年代为0.58±0.13Ma的反极性火山岩的资料肯定了安比拉（Emperor）事件的存在。研究还证实了布容/松山极性时界线之下和哈拉米洛极性亚时之上有一短期的极性事件,其年代约为0.78±0.03Ma.在奥尔都维和哈拉米洛极性亚时之间存在有另一正极性事件,这可能相当于吉尔萨（Gilsa）事件。     

以火山岩层序剖面K-Ar测年确定辽西四合屯古生物化石地层的年代

从系统剖面连续测年的角度，利用K-Ar年代学方法并结合前人工作，确定了辽西四合屯剖面的六个火山活动期和含古生物化石的湖相沉积地层的顶底时间界限。六期火山活动分别为133．6～133．1Ma、129．3．129．2Ma、．126．1Ma、125．1．124．2Ma、-122Ma和～113Ma。其中第四期活动最为强烈。侵入含古生物化石的湖相沉积层的火山岩的年龄为124．4Ma，其上覆火山岩年龄为124．2Ma，下伏火山岩的年龄为124．6～125．1Ma，因此，该湖相沉积地层的持续时间应为大约1Ma。     

山西义兴寨岩体和车厂-北台岩体的变质程度及变质作用K-Ar定年

义兴寨岩体和车厂-北台岩体位于华北克拉通中部造山带中段,均被变质辉绿岩墙穿切,分别出露于山西省恒山南麓和五台山北麓。义兴寨岩体出露的岩石类型呈现渐变过渡的特征,主要为花岗闪长片麻岩、石英闪长片麻岩和闪长片麻岩;变质高峰期温压条件达到710~760℃/0.65~0.75 GPa,记录的降温降压条件为560~690℃/0.46GPa;变质高峰期为高角闪岩相,属于中压变质相系;变质角闪石K-Ar定年表明义兴寨岩体变质时代为1.84~1.82Ga。车厂-北台岩体主要由二云母花岗片麻岩构成,变质高峰期温压条件达到550~690℃/0.92~1.14 GPa;变质高峰期为中角闪岩相,属于中压变质相系顶部;变质黑云母K-Ar定年表明车厂-北台岩体变质时代为1.80 Ga。这两个岩体变质时代与华北克拉通中部造山带普遍记录的1.96~1.80 Ga之间的变质事件一致,说明它们同样卷入了华北中部造山带早元古代末期的造山过程。     

兰家沟钼矿床同位素年代学研究

辽宁西部是我国钼矿资源的主要矿产地。作者根据30个K—Ar同位素年龄和一条Rb—Sr等时线年龄讨论了兰家沟钼矿区两种主要类型花岗岩和钼矿化的时代。研究表明,分布广泛的似斑状中粗粒花岗岩K—Ar年龄为219—188Ma,与钼矿化有密切关系的似斑状细粒花岗岩Rb—Sr等时线年龄为154Ma,K—Ar等时线年龄为152Ma。根据与钼矿化关系最密切的钾化细粒花岗岩、云英岩化花岗岩和石英斑岩的年龄,认为钼矿化富集时代为148—60Ma,作者还运用辉钼矿的Re、Os的化学分析数据,计算了Re—Os年龄,其年龄值为180Ma,与中粗粒似斑状花岗岩年龄相近。     

Chemical Features of White Micas from the Piemonte Calc-schists, Western Alps and Implications for K–Ar Ages of Metamorphism

Anomalously large chemical ranges in muscovite-paragonite and muscovite-celadonite systems are observed in white micas from the Piemonte calcschists in the Chisone valley area, internal western Alps. The petrographical and chemical observations on white mica strongly suggest that most mica crystals with high Na/K ratios in the chlorite zone are of detrital origin, and were derived from the pre-Alpine high-temperature metamorphic sequence such the Caledonian and/or Variscan. Submicroscopic muscovite (Ms) - paragonite (Pg) composite aggregates occur in the chlorite zone and their EPMA analyses give an apparent chemical composition range from Ms_0.6Pg_0.4 to Ms_0.2Pg_0.8. In the rutile zone, the paragonite content of the white micas is less than 20%, suggesting that the white micas have been homogenized during the Alpine metamorphism even if detrital white micas existed.Metamorphic mica is also very heterogeneous. The total range in Si content becomes wider with increasing of metamorphic grade: 3.22–3.39 pfu for the chlorite zone, 3.07–3.45 pfu for the chloritoid zone and 3.06–3.59 pfu for the rutile zone. This clearly indicates that the micas have experienced significant retrogressive chemical reactions during cooling and exhumations of the host schists.The detrital white mica in the chlorite zone has not reset well in its K-Ar system during the Alpine subduction-related metamorphism. The wide range of the white mica K-Ar ages from 115 to 41 Ma must be due to a mixture of various amounts of detrital white mica in the separates. This feature is also observed in the chloritoid zone though the age variation is not so large as that in the chlorite zone. In contrast, the mica in the rutile zone, which was higher than 450°C, has been reset completely during Alpine HP metamorphism.