Rutile U-Pb age for the ultra-high pressure eclogite from the dabie Mountains,central China:Evideence for rapid cooling

The U-Pb isotope compositions of rutile, omphacite and garnet in the eclogite from the Jinheqiao area in the Southern Dabie ultrahigh-pressure metamorphic zone were analyzed. The consistent high precision U-Pb age (218 ±(1.2) Ma of rutile in eclogite from the Dabie Mountains was obtained by two ways of isochron and common Pb correction based on the composition of omphacite. This proves that the omphacite in eclogite has a U/Pb ratio (μ = 2.8) low enough to be used for common Pb correction in the analyses of rutile. Under the rapid cooling condition (40℃/Ma) the closure temperature for U-Pb diffusion in rutile is about 470℃. Thus, this U-Pb age of rutile proves that 218 Ma should be the cooling age of eclogite at 470℃ instead of the peak metamorphic age.     

东准噶尔红柳峡辉长岩锆石U-Pb年龄及地质意义

准噶尔盆地基底性质及形成时期问题对研究新疆地壳演化、油气资源的勘探与评价均有重要意义,但长期以来一直存在很大争议。准噶尔盆地东部红柳峡地区是探讨准噶尔盆地是否存在前寒武纪基底的代表性地区之一,因此对该区进行同位素定年分析,以期获得古老锆石年龄的同位素证据。选取原定为中元古界札曼苏岩群中的大量发育的变质辉长岩进行了LA-ICP-MS锆石U-Pb定年研究。发现U-Pb表面年龄值分布的范围很宽,从385±7 Ma至2 322±45 Ma.岩浆锆石206Pb/238U加权平均年龄为（396±5）Ma（MSWD=1.15,n=7）,代表辉长岩的结晶年龄为早泥盆世晚期。此外辉长岩中尚有4粒年龄为（842～2 322）Ma捕获锆石,代表了岩浆上升溢流路经区围岩中的锆石形成时代。札曼苏岩群被划分为中元古代是不准确的,综合该区的变质砂岩和玄武岩锆石同位素定年数据,认为札曼苏岩群形成时期的下限不早于早泥盆世。结合区域地质综合分析认为,准噶尔盆地东部可能存在前寒武纪的古老结晶基底。     

北秦岭罗汉寺岩群锆石年龄及地质意义

目的准确测定北秦岭罗汉寺岩群辉长岩脉的形成时代,探讨其地质意义。方法采用激光剥蚀等离子质谱法,对辉长岩中锆石进行U-Pb同位素定年。结果辉长岩形成年龄为475 Ma±4 Ma。结论罗汉寺辉长岩脉的形成时代与北秦岭两条高压—超高压变质带和富水杂岩成岩时代接近或一致,它的形成很可能与北秦岭早古生代岛弧体系碰撞有关。     

新疆东准噶尔铜华岭中酸性侵入体锆石U-Pb年代学研究

伊吾县铜华岭中酸性侵入体位于新疆北部东准噶尔阿尔曼太断裂和克拉麦里断裂之间的野马泉岛弧区, 岩性主要为闪长玢岩、英云闪长岩和二长花岗岩等。通过锆石LA-ICP-MS U-Pb年代学方法, 对这3种岩性进行精确定年, 其²⁰⁶Pb/₂₃₈U的加权平均年龄分别为427.4±3.2,422.8±3.2和418.5±2.6Ma,说明铜华岭岩体是早古生代岩浆活动的产物, 为东准噶尔早古生代造山带演化提供了花岗岩证据。     

陇县地区新街片麻岩套锆石年龄及其地质意义

目的寻找秦-祁结合部位早古生代岛弧型岩浆活动的证据,为秦岭和祁连造山带的对比研究提供依据。方法依据岩石主、微量地球化学特征判别新街片麻岩套的构造属性,并通过LA-ICP-MS对其中的锆石进行U-Pb测年。结果新街片麻岩套富集大离子亲石元素（LILE）和轻稀土元素（LREE）,亏损高场强元素（HSFE）和重稀土元素（HREE）,指示其形成与俯冲作用有关,在构造判别图解中落入岛弧火山岩范围;U-Pb锆石测年结果显示,花岗质片麻岩的形成年龄为447.7±5.4Ma。结论该年龄与北秦岭和北祁连造山带内部的岛弧火山岩活动的时限一致。     

SHRIMP zircon U-Pb dating for two episodes of migmatization in the Dabie orogen

Zircon CL imaging and SHRIMP U-Pb dating were carried out for migmatite in the Dabie orogen. Zircons from the Manshuihe migmatite show clear core-rim structures. The cores display sector or weak zoning and low Th/U ratios of 0.01 to 0.17, indicating their precipitation from metamorphic fluid. They yield a weighted mean age of 137±5 Ma. By contrast, the rims exhibit planar or nebulous zoning with relatively high Th/U ratios of 0.35 to 0.69, suggesting their growth from metamorphic melt. They give a weighted mean age of 124±2 Ma. Zircons from the Fenghuangguan migmatite also display core-rim structures. The cores are weakly oscillatory zoned or unzoned with high Th/U ratios of 0.21 to 3.03, representing inherited zircons of magmatic origin that experienced different degrees of solid-state recrystallization. SHRIMP U-Pb analyses obtain that its protolith was emplaced at 768±12 Ma, consistent with middle Neoproterozoic ages for protoliths of most UHP metaigneous rocks in the Dabie-Sulu orogenic belt. By contrast, the rims do not show significant zoning and have very low Th/U ratios of 0.01 to 0.09, typical of zircon crystallized from metamorphic fluid. They yield a weighted 206Pb/238U age of 137±4 Ma. Taking the two case dates together, it appears that there are two episodes of zircon growth and thus migmati-tization at 137±2 Ma and 124±2 Ma, respectively, due to metamorphic dehydration and partial melting. The appearance of metamorphic dehydration corresponds to the beginning of tectonic extension thus to the tectonic switch from crustal compression to extension in the Dabie orogen. On the other hand, the partial melting is responsible for the extensional climax, resulting in formation of coeval migmatite, granitoid and granulite. They share the common protolith, the collision-thickened continental crust of mid-Neoproterozoic ages.     

川西须家河组火山岩夹层时代及其地质意义

四川盆地西部的上三叠统须家河组作为反演前陆盆地构造演化的物质基础，因缺少精确的同位素地层年龄阻碍了对盆山演化的进一步理解。作者在四川盆地西部须家河组中发现了一套火山（碎屑）岩夹层，通过 LA-ICP-MS U-Pb 同位素定量分析，获得1件凝灰岩样品中岩浆锆石的结晶年龄，其加权平均值为（214．7±1．6）Ma，属于晚三叠世诺利期。该年龄结果对四川盆地西部须家河组岩石地层单元的划分与对比可提供重要参考，同时为进一步约束龙门山隆升与川西前陆盆地的演化、古特提斯洋关闭等重大事件时限提供了新证据。基于四川盆地及周缘的区域火山活动对比结果，表明诺利期中期（210～222 Ma B．P．）是中国西部地区一次强烈的区域火山喷发阶段，其范围从羌塘地体延伸到东昆仑地体，所对应的构造事件并非传统的印支运动作用。     

U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207Pb/206Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891 ±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zir- cons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.     

Single zircon LAM－ICPMS U－Pb dating of Guidong complex （SE China） and its petrogenetic significance

Because of the strong effects of the Caledonian and Indosinian orogenies and Yanshanian magmatism since early Paleozoic time, different types of granites and ore deposits occur widely in the Nanling region of SE China[1]. The Guidong complex is situated at the border between the South Jiangxi—North Guangdong post-Caledonian orogenic belt and the Hercynian-Indosinian belt, which is constrained by an E-W regional deep fault (Guidong-Da- dongshan deep fault) and therefore is elongated nea…     

中国大别山双子叶植物化石木材（Ⅱ）：橄榄科,山茱萸科

湖北省新州县大别山下阳逻镇双子叶植物硅化木系列报道之二，在前文里我们描述了樟科一种化石木。本文增加二科进行描述，它们是橄榄科植物化石新种小节橄榄木和山茱萸科植物化石新种湖北山茱萸木。这些木化石的解剖学特征都显示了大别山地区在第三纪时气候更倾温暖湿润。     

中国大别山双子叶植物化石木材（Ⅰ）—樟科

描述了湖北省新州县大别山下（北纬３０°３１′；东径１１４°３５′）阳逻镇新近采到的一块木化石，其结构近似于现存樟科樟属植物。木化石的解剖学特征显示了大别山地区在第三纪时气候倾向更温暖湿润。