河北平泉光头山碱性花岗岩体的形成时代

光头山碱性花岗岩体位于河北省与内蒙古自治区交界处的平泉县北部,地理坐标为东经118°27′北纬41°17′。岩体出露面积约20km²,四周为太古代变质岩(图1)。岩石为灰白色中细粒花岗结构,含椭圆状、次圆状的细粒暗色包体,还有晚期的含碱性矿物的伟晶囊状体。主要造岩矿物有碱性长石、石英、钠铁闪石、霓辉石,副矿物主要有钠铁非石(Aenigmatite)、羟铁云母、硅铈铁矿、褐帘石、星叶石等。     

山东沂水杂岩中变基性岩类的Sm—Nd同位素年龄及其地质意义

在山东太古宙沂水杂岩的变基性岩类中首次获得的Sm-Nd全岩等时线同位素年龄为2997Ma,证实了山东中部的郯庐断裂带中有中太古代麻粒岩相表壳岩存在。沂水杂岩的岩石组合主要有二辉斜长麻粒岩、二辉斜长片麻岩、二辉铁英岩(少量)、二辉斜长角闪岩和紫苏花岗岩类等,它们组成了山东最老的汞丹山地块,其时代大致相当于胶东群,但层位可能更为偏下。沂水杂岩为高角闪岩相至麻粒岩相。变基性岩的原岩相当低钾拉斑玄武岩,稀土配分具有较平坦型和轻稀土富集型两种模式。岩石的ε_(Nd)(t)值为+3.8±0.3,物源来自亏损(上)地幔。它与华北陆台北缘麻粒岩相中部分岩石的岩性十分相似,表明其源区的化学特征相似,但主体的稀土模式有所不同,说明太古宙不同地区相似岩石地球化学的不均匀性。     

Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachment

Abstract   Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble-sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri-platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non-metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The ⁴⁰Ar–³⁹Ar ages of muscovite from the Yellow Band show two-phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian-type Eo-Himalayan metamorphic event and the younger age records a decompressional high-temperature Neo-Himalayan metamorphic event. A muscovite whole-rock ⁸⁷Rb–⁸⁶Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre-Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission-track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period (15.5–14.4 Ma).     

豫西东秦岭造山带核部杂岩中大理岩Pb—Pb等时线年龄及其地质意义

本文报道了豫西东秦岭核部杂岩中大理岩919±53Ma的Ph－Pb等时线年龄测定结果,结合本地区变质作用及其年龄研究结果,指出大理岩919±53Ma的Pb-Pb等时线年龄是碳酸盐岩区域变质作用形成大理岩后近于等压冷却的年龄,为秦岭地区加厚的地壳均衡隆升,到达地壳一定深度后近于等压的冷却提供了同位亲年龄证据。     

Age and Tectonic Significance of the Mafic Dyke Swarm in the Kuruktag Region, Xinjiang

The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar / 36Ar-40K/ 36Ar isochron age of 282 Ma with the initial 40Ar/ 36Ar = 507. The characteristics of the major, trace and rare earth elements of the dyke rocks indicate that they belong to the calc-alkaline series and were emplaced in extensional structures at an intermediate depth of the crust. This shows that there was a large-scale extension in the Permian Period, which might be associated with the post-collisional extension of the ancient Tianshan Mountains.     

石源龙地区铀矿床的同位素地质特征

本文详细研究石源龙地区铀矿床的同位素地质特征，并由此得出：ａ．成矿时间大约为９２Ｍａ～１１５Ｍａ；ｂ．成矿溶液来自当地中生代的大气降水；ｃ．成矿溶液中锶和稀土元素的来源与南园组的流纹质熔结凝灰岩有关；ｄ．成矿过程中伴随着ＣＯ２的逸失发生了同位素交换动力学效应，而且ＣＯ２大量逸失是导致沥青铀矿沉淀的主要原因。     

A counterclockwise PTt path of high-pressure/low-temperature rocks from the Coastal Cordillera accretionary complex of south-central Chile: constraints for the earliest stage of subduction mass flow

Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of 15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to 300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist.

The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic.     

张宣隆起核部的转枝莲辉石闪长岩

位于冀西北张家口-宣化地区的转枝莲辉石闪长岩侵入水泉沟正长岩杂岩体中。该岩石由紫苏辉石、次透辉石、黑云母、中长石、正长石以及磷灰石、钛铁矿等副矿物组成。全岩分析结果SiO253％～59％。本文根据上述两点将其命名为辉石闪长岩。通过岩石成因、稀土元素分布模式和岩石的Sr-Nd同位素组成研究，认为岩体是上地幔和下地壳物质部分熔融的产物。辉石闪长岩的Rh-Sr等时线年龄为143Ma。     

40Ar/39Ar and Rb-Sr isochron dating of the gold deposits on northern margin of the Jiaolai Basin, Shandong, China

The Pengjiakuang, Dazhuangzi and Fayunkuang gold deposits, located on the northern margin of the Mesozoic Jiaolai Basin, east of Shandong Province, are controlled by a low-angle normal fault. Gold ores are typically brecciated, veinlet and disseminated. The Ar-Ar and Rb-Sr isochron dating methods were adopted to date ores and lamprophyre dike. The results indicate that the age of the Pengjiakuang gold deposit is 117.33–118.42 Ma, that of the Dazhuangzi gold deposit is 117.39 Ma, and that of the Fayunkuang gold deposit is (128.49±7.2) Ma. The consistency in metallogenic age between the gold deposits on the margin of the Jiaolai Basin and the gold deposits (115–126 Ma) of the northern uplift area suggests that both were formed in the same metallogenic period. That is to say, the large-scale metallogeny of the Jiaodong region took place in late-Yanshannian ((120±10) Ma).     

副矿物的电子探针化学定年方法原理及应用

化学定年方法是利用电子探针分析副矿物中的U、Th、Pb含量而进行定年的一种新方法。基本原理是，假设可以忽赂矿物韧始铅，封闭体系中矿物的总铅含量代表U—Th衰变形成的放射性成因铅含量，即：Pb总＝^238U(e^λ238r-1) ^235U(e^λ235r—1) ^232Th(e^λ232r—1)，该方法可利用的副矿物包括：锆石、独居石、褐帘石、碘纪矿、铀石、钍石、晶质铀矿等。本文以华南西华山岩体漂塘花岗岩中的锆石为例，对该方法进行了尝试性的研究。分析结果计算表明，两颗锆石的平均年龄分别为143．9Ma和143Ma，与前人通过同位素方法获得的年龄基本相符。结果还显示，锆石颗粒从中心到边缘的年龄呈现从老到新的趋势。