南祁连化隆岩群LA-ICP-MS锆石U-Pb年龄及其地质意义

侵入于化隆岩群中的基性-超基性岩中因产有拉水峡、裕龙沟小型铜镍矿床和一大批铜镍矿点,而备受关注.其化隆岩群形成时代的确定直接影响着祁连山地区铜镍矿找矿方向和找矿工作布署.近年来,随着同位素年代测试技术的不断发展,高精度数据出现,对化隆岩群形成时代的认识逐步明朗化.通过高精度的LA-ICP-MS锆石微区原位U-Pb同位素测年,获得青海省化隆县拉水峡铜镍矿区含矿基性-超基性岩体围岩化隆岩群黑云斜长片麻岩的形成时代为(910±7)Ma,代表着化隆岩群形成时代的下限,结合前人研究成果,进一步确定化隆岩群的形成时代为新元古代.并认为该区有与金川铜镍矿相似的成矿地质背景,含矿基性-超基性岩均为Rodinia超大陆裂解的产物,具有良好的找矿前景.     

Stratigraphic relationships of Cryogenian strata disconformably overlying the Bitter Springs Formation, northeastern Amadeus Basin, Central Australia

Detailed mapping and C and O stable isotopic data from sedimentary carbonate in units both above and below the paleo-erosion surface on the Bitter Springs Formation (BSF) in the northeastern Amadeus Basin, Australia, have clarified the stratigraphy of the area. Isotopic data indicate that the top of the Loves Creek Member of the Bitter Springs Formation is preserved near Corroboree Rock, and is overlain by fenestrate-carbonate-clast breccia, and dolomitic quartz sandstone and chert-pebble conglomerate of the Pioneer Sandstone. The isotopic data, as well as lithologic data, indicate the presence of a 1–2 m-thick cap carbonate preserved between Corroboree Rock and areas 10 km to the northeast. In many places the cap carbonate layer is mostly a syn-sedimentary dolomite-clast breccia, consistent with deposition and disturbance in shallow water. C and O isotopic data also indicate that thin-bedded sandstone and dolomite above the Bitter Springs Formation at Ellery Creek, and a newly discovered massive chert-bearing dolomite at Ross River could both belong to the glaciogenic Olympic Formation. Detailed mapping also provides a more detailed context for the famous black chert microfossil locality in the Bitter Springs Formation at Ross River.     

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.     

Petrogenesis and tectonic implications of Neoproterozoic, highly fractionated A-type granites from Mianning, South China

The origin of Neoproterozoic intrusions (ca. 860–750 Ma) along the western part of the Yangtze Craton has been the subject of debate in recent years, with two competing models proposed. The plume model argues for an extensional setting and emphasizes the role of a superplume in the Rodinia breakup, whereas the arc model argues for the presence of a subduction zone in the Yangtze Craton. As a contribution to this animated dispute, geochronologic and geochemical analyses have been carried out on the Mianning granite, which is the largest pluton (700 km²) in the northern Kangdian rift of the western Yangtze Craton. It is shown that the Mianning granites were emplaced at ca. 780 Ma and display highly fractionated feature (i.e., SiO₂ > 75 wt%; Eu/Eu* = 0.03–0.50; enrichment of K, Rb, Th, U, Zr, Hf, Y and REEs; depletion of Nb, Ta, Ba, Sr, P, Eu and Ti). They are metaluminous to strongly peraluminous (A/CNK = 0.93–1.55) and contain abundant perthite and minor alkali riebeckite and sphene, sharing the petrological and geochemical characters of A₂-type granites. Positive _Nd (t) (2.97–5.24) and zircon _Hf (t) (9.2–12.1) values are consistent with a derivation by partial melting of a relatively young crust formed about 1000–900 Ma. Given the general absence of A-type granites in arc settings, the Mianning A-type granites are suggestive of an anorogenic, crustal extensional environment for the western Yangtze Craton during the Neoproterozoic. The data presented in this study are therefore consistent with an intracontinental rift model, but are not sufficient to identify plume involvement in the Neoproterozoic magmatism.     

桂北平英花岗岩锆石U-Pb年代学、Hf同位素、地球化学特征及其地质意义

通过对桂北平英花岗质岩体详细的锆石U-Pb年代学、Hf同位素组成及岩石地球化学特征的研究,论证了岩体的形成时代、成因类型、源区性质及其与宝坛锡矿的成矿关系。该岩体中心相-粗粒黑云母花岗岩的锆石LA-MC-ICP-MS U-Pb定年表明,其~(206)Pb/~(238)U加权平均年龄为834.2±5.1 Ma,属新元古代构造岩浆活动的产物。平英花岗岩具有高硅、富碱、强过铝质的特征,岩石富集Cs、Rb、U、Ta而亏损Ba、Sr、Ti等元素。球粒陨石标准化稀土配分曲线呈右倾形和强烈的Eu负异常(Eu/Eu*=0.05~0.31)。花岗岩中锆石的εHf(t)值介于-12.6~-1.6之间,峰值在-4.8~-3.0之间;二阶段模式年龄T_(DM)~C(Hf)在1.83~2.51 Ga之间,峰值在1.9~2.0 Ga之间。这些特征表明平英岩体形成于该区古元古代富硼基底的部分熔融作用,并经历了高度的分异演化过程。桂北九万大山—元宝山地区的新元古代黑云母花岗岩具有良好的锡成矿潜力,是华南多时代花岗岩演化及锡多金属成矿系列的重要组成部分。     

Neoproterozoic ana texts of the yunkai metamorphic Complex; Evidence from U-Pb geochronology of zircon in migniatite北大核心CSCD

In order to determine the migmatization time of the Preeambrian metamorphic basement(the Yunkai metamorphic complex) of the Yunkai terrain, a study of LA-MC-1CP-MS U-Pb dating of zircon in matrixes and felsic veins of mig-matites from the Tiantangshan Group. Results show that the inherited magmatic cores of zircon grains from the matrix of migmatite have oscillatory zoning and yield a U-Pb age of 1086 ±24 Ma, representing the pmtolith age, whereas the metamprohic rims of zircon grains from the felsic vein of migmatite yield an age of 852. 6±8. 8 Ma, representing the timing of the anatex is (migmatization). These results imply that there is a Mesoproterozoic metamorphic basement with a Neoproterozoic anatexis event in the Yunkai area. The timing of this anatexis event is consistent with that of the Sibao Movement. Therefore, we conclude that the timing of the anataxis could be an important geological record for the amalgamation of the Yangtze and Cathaysian blocks.     

南秦岭城口火山岩锆石LA-ICP-MS U-Pb定年和地球化学研究

南秦岭大巴山城口断裂带出露一套玄武安山岩、安山岩组合,火山岩锆石LA-ICP-MS U-Pb定年测试结果为716±4Ma,表明其为新元古代岩浆产物;岩石地球化学研究表明火山岩富集轻稀土元素,原始地幔标准化微量元素蛛网图显示以富集大离子亲石元素Cs、Ba、Th、U及高场强元素分异为特征,Nb、Ta强烈亏损以及低的Ti(Ti O_20.85%)含量,与典型的岛弧火山岩相似;微量元素La/Nb、Th/Yb及Hf/Ta比值特征也显示岛弧岩浆属性,相对高的Zr/Y、Ta/Yb和低的Zr/Nb比值区别于大洋岛弧火山岩,具有明显的大陆亲缘性,表明城口火山岩形成于陆缘岛弧环境。综合已有的地质、地球化学及同位素年代学研究表明新元古代晚期扬子板块北缘及南秦岭地区为一活动陆缘岩浆杂岩弧,暗示中国华南板块很可能位于Rodinia超大陆的边缘部位。     

周庵铜镍-铂族矿床锆石U-Pb年代学、地球化学及Sr-Nd同位素特征:对周庵基性-超基性岩体及矿床成因的探讨

周庵铜镍铂族矿床位于南秦岭构造带北缘,商丹断裂带南侧。岩体隐伏于新生代沉积物和中元古代大雀山组地层之下,主要由二辉橄榄岩组成,见有少量纯橄榄岩、角闪岩、辉长岩,岩体大部分经受了蛇纹石化、次闪石化、绿泥石化等蚀变作用,矿体主要赋存于周庵岩体与大雀山组地层的内接触带。对辉长岩的LA-ICP-MS锆石U-Pb测年表明周庵岩体形成于621±1.5Ma,形成时代为新元古代晚期。Sr-Nd同位素测试得到的Sm-Nd等时线年龄为622±59Ma,与锆石U-Pb年龄一致。(~(87)Sr/~(86)Sr)_t=0.705726~0.706655,(~(147)Nd/~(144)Nd)_t=0.511730~0.511863,ε_(Nd)(t)=-2.05~0.55,显示弱富集的特征。超基性岩主量元素含量显示岩体m/f值为4.47~4.99,为铁质超基性岩,Cu、Ni含量与烧失量之间没有相关关系。矿石和不含矿岩体具相同的稀土元素配分模式,显示轻稀土富集,具弱的正Eu异常。微量元素特征显示Rb、Pb强烈富集,Nb、Ta相对亏损,说明岩浆受到了地壳物质的混染。综合分析认为周庵岩体形成与新元古代晚期,岩浆来源于软流圈地幔,代表Rodinia超大陆裂解事件在南秦岭的地质记录。岩浆侵位时受到了地壳物质的强烈混染,利用Nd同位素估算得混染程度为18.57%,这可能是促发岩浆体系达到硫饱和而发生金属硫化物熔离的主要因素。     

华南早新元古代莲沱组地层磁倾角偏低研究及其古地理意义

华南莲沱组最新的年龄结果表明,其时代结束于715Ma,因此,准确确定莲沱组的古纬度对"雪球地球"的研究具有重要意义。通过对莲沱组红层进行等温剩磁各向异性研究,获得其磁倾角校正因子为0.8719,校正后的磁倾角为70.4°,对比热退磁实验测得的莲沱组磁倾角为67.8°,则其磁倾角偏低量为2.6°。通过校正前后的磁倾角分别计算古纬度,获得磁倾角偏低所引起的古纬度变化为3.9°±6°。通过对比华南与澳大利亚-东南极板块的720Ma古地理位置,发现这一时期冰碛岩从中纬度到赤道广泛分布,验证了当时的"雪球地球"环境。     

江南造山带东段800～780Ma陆内裂谷岩浆活动——来自浙皖赣邻区约790Ma铝质A型花岗岩的证据

对江南造山带东段浙皖赣邻区的灵山花岗斑岩进行了同位素年代学和岩石地球化学研究。LA-ICP-MS锆石U-Pb年龄测定结果表明,灵山花岗斑岩形成于791.8±2.6Ma。在岩石地球化学组成上,灵山花岗斑岩具有高Si,高TFe O/(TFe O+Mg O)值,低Mg、Ca、Mn和P,显示出过铝质的特征(A/CNK=1.04~1.18);微量元素富集Rb、Ga、Th、Zr、Y,贫Sr、P、Ti、Ba;稀土元素总量较高,轻、重稀土元素分异较明显,并表现出强烈的负Eu异常,这些特点与典型的铝质A型花岗岩一致。地球化学特征和前人研究成果表明,灵山花岗斑岩形成于陆内裂谷环境,最有可能来源于早期初生地壳的部分熔融。结合前人研究成果认为,800~780Ma的岩浆活动是华南新元古代一期重要的构造岩浆事件,该期岩浆活动不仅在华南新元古代盆地的形成过程中扮演了重要角色,而且在该时期构造环境的探讨、板溪期沉积旋回时限的标定等方面都发挥了重要作用。