New U-Pb age constraints on the upper Banxi Group and synchrony of the Sturtian glaciation in South China

The Nanhua basin in South China hosts well-preserved middle–late Neoproterozoic sedimentary and volcanic rocks that are critical for studying the basin evolution, the breakup of the supercontinent Rodinia, the nature and dynamics of the “snowball” Earth and diversification of metazoans. Establishing a stratigraphic framework is crucial for better understanding the interactions between tectonic, paleoclimatic and biotic events recorded in the Nanhua basin, but existing stratigraphic correlations remain debated, particularly for pre-Ediacaran strata. Here we report new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) U-Pb zircon ages from the middle and topmost Wuqiangxi Formation (the upper stratigraphic unit of the Banxi Group) in Siduping, Hunan Province, South China. Two samples show similar age distribution, with two major peaks at ca. 820 Ma and 780 Ma and one minor peak at ca. 910 Ma, suggesting that the Wuqiangxi sandstone was mainly sourced from Neoproterozoic rocks. Two major age peaks correspond to two phases of magmatic events associated with the rifting of the Nanhua basin, and the minor peak at ca. 910 Ma may correspond to the Shuangxiwu volcanic arc magmatism, which represents pre-collision/amalgamation subduction on the southeastern margin of the Yangtze Block. The youngest zircon group from the topmost Wuqiangxi Formation has a weighted mean age of 714.6 ± 5.2 Ma, which is likely close to the depositional age of the uppermost Banxi Group. This age, along with the ages reported from other sections, constrains that the Banxi Group was deposited between ca. 820 Ma and ca. 715 Ma. The age of 714.6 ± 5.2 Ma from the top of the Wuqiangxi Formation is indistinguishable with the SIMS U-Pb age of 715.9 ± 2.8 Ma from the upper Gongdong Formation in the Sibao village section of northern Guangxi, South China. It is also, within uncertainties, overlapped with two TIMS U-Pb ages from pre-Sturtian strata in Oman and Canada. These ages indicate that the Jiangkou (Sturtian) glaciation in South China started at ca. 715 Ma instead of ca. 780 Ma and support a globally synchronous initiation of the Sturtian glaciation at ca. 715 Ma.     

胶东金矿成矿时代及区域地壳演化——基性脉岩的SHRIMP锆石U-Pb年龄及其地质意义

关于胶东金矿的成矿时代前人已进行了较多研究,但其成矿时限尚没有准确确定。对胶东焦家金矿床成矿前和成矿后的2个基性脉岩进行了锆石SHRIMP年龄测定和锆石氧同位素分析。锆石阴极发光图像显示,样品中有少量岩浆新生锆石,其晶形、环带清晰。在2个成矿前脉岩样品中测得岩浆锆石206Pb/238U年龄最小值为124.9±1.8Ma和124.2±1.1Ma,1个成矿后脉岩样品中测得最小年龄值为112.2±0.7Ma。故成矿前脉岩的侵位年龄小于等于124.9Ma和小于等于124.2Ma,成矿后脉岩的侵位年龄小于等于112.2Ma,金矿的成矿年龄被限制在124~112Ma之间。此外,较多的锆石年龄数据记录了早侏罗世、三叠纪、古生代、新元古代、中元古代和新太古代的岩浆事件和变质事件,指示胶北隆起深部既有华北克拉通新太古代、中元古代和新元古代的地质体或地质事件,也有苏鲁造山带古生代和三叠纪的地质体或地质事件,苏鲁超高压变质带曾俯冲到胶北隆起之下。锆石氧同位素组成变化范围较大,δ18O值为3.86‰~11.37‰,指示了不同时代锆石氧同位素组成的不均一性和物质来源的差异性。     

安徽铜陵矿集区中生代侵入岩体年代学研究及其成矿指示意义

基于前人在铜陵矿集区获得的大量高精度同位素定年数据,本文补充测定了研究程度相对较低的棋子坑、瑶山、荷花塘等11个小型侵入体的LA-ICP-MS锆石U-Pb年龄,年龄分布在147～131 Ma之间。通过对区内成岩-成矿时代进行系统分析,提出了该区为一燕山期陆内"瞬时大规模成矿作用"形成的大型矿集区。区内燕山期成矿岩体主要包括花岗闪长岩类、石英(二长)闪长岩类和辉石(二长)闪长岩类等3类中酸性侵入岩,其成岩时代主要集中在152～130Ma之间,其中,花岗闪长岩集中在148～137 Ma、石英(二长)闪长岩在152～130 Ma、辉石(二长)闪长岩在150～134 Ma,区内主要矿床中获得的辉钼矿Re-Os成矿年龄分布在141～137Ma,成岩与成矿时代相差约10Ma以内,且成矿年龄具有自东向西逐渐变新的特征,区内相邻矿田成矿时代相差2Ma左右。铜陵矿集区在141～137 Ma的短时限内三次幕式大规模成矿作用为全球陆内岩浆成矿作用中所罕见。     

西藏谢通门县雄村铜金矿主要地质体的40Ar/39Ar年龄及地质意义

雄村铜金矿Ⅱ号矿体在2007-2008年取得了重大的找矿突破,详细的地质编录成果表明,矿体同样受含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩控制.含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩的锆石U-Pb年龄已经确定(164～177 Ma),因此,雄村铜金矿不同地质体的云母类、长石类矿物Ar-Ar同位素年龄的测定显得尤为关键.文章通过对穿切I号矿体的黑云母花岗闪长岩、云煌岩脉的黑云母Ar-Ar同位素测年,结合其他研究者的成果,得到一组十分重要的年龄数据.穿插矿体的黑云母花岗闪长岩中的黑云母(样号6187-335)坪年龄(46.96±0.42)Ma,穿插矿体的无矿化的云煌岩(5053-324.4)(Cu含量0.0551%,Au含量0.034 g/t,Ag0.6 g/t)中的黑云母给出了一个较好的似坪年龄,加权平均年龄为(49.59±0.58)Ma.结合其他研究者测定的中侏罗世侵位的角闪石英闪长玢岩(不含矿)[锆石U-Pb年龄为(177.1±2.0)Ma]中黑云母的At-At同位素年龄为(48.57±0.31)Ma;含矿凝灰岩围岩[锆石U-Pb年龄为(176±5)Ma,MS3VD=0.63}的蚀变绢云母Ar-Ar年龄为(47.07±0.30)Ma;似伟晶岩中长石的At-At年龄为(47.62±0.7)Ma,认为不同形成时代、不同产出空间、不同矿化程度的地质体的云母类、长石类矿物的Ar-Ar同位素年龄的一致性,反映了后期岩浆热事件对中侏罗世早期形成的地质体和矿体的黑云母氩同位素体系产生了较强的扰动或置换.谢通门大岩基黑云母花岗闪长岩的侵位致使各地质体发生显著的退变质,形成典型的角岩化带,这种退变质的时限在46-48 Ma之间的始新世lutetian期,进而认为各地质体中云母类矿物的~(40)Ar/~(39)Ar同位素年龄不能作为成矿年龄.     

棉花坑铀矿床花岗质脉岩地球化学特征及其与铀成矿的关系

粤北棉花坑铀矿床矿体产于一组陡倾的、近南北向展布的构造蚀变带中,在矿体附近多见有细粒花岗质岩脉穿插,通过对该类脉岩的研究表明,其锆石 U Pb 年龄为138．6±1．3 Ma （MSWD＝0.81）,属于晚燕山期岩浆活动的产物.脉岩的硅含量高,镁、铁、钙和钛含量低, ACNK 值均＞1.1, CaO/Na2 O值＜0.3,刚玉分子数C＞1.8％,属于典型的 S型花岗岩类;脉岩中富集大离子元素K、Rb、Th而亏损Ba、Sr、Ti和P,稀土总量低,配分模式明显呈海鸥型,且表现出一定程度的“四分组效应”, Eu亏损明显.细粒花岗质脉岩的侵入作用使围岩中的铀活化、预富集,因此铀矿化与其有成因联系.     

钦杭成矿带西段罗容杂岩体岩石学、地球化学及年代学

罗容杂岩体主要由二长闪长岩、二长岩、二长正长岩和正长岩组成。岩石以富碱、富含Ba、K、Rb、Sr、Ga等大离子亲石元素(LILE)和LREE、Nb、Ta、Zr、Hf、Th、U、Y等高场强元素(HFSE)为主要特征,属钾玄岩系列。对罗容杂岩体进行了高精度的LA-ICP-MS锆石U-Pb定年,获得角闪辉石二长岩等时线年龄为163.40±0.40 Ma(MSWD=0.12)。研究认为罗容杂岩体形成于中侏罗世,属于A1亚型花岗岩,形成于燕山早期华南后造山阶段大陆地壳拉张减薄的构造环境。     

桂东北越城岭—苗儿山地区印支期成矿作用：油麻岭和界牌矿区成矿花岗岩锆石U-Pb年龄和Hf同位素制约

提要：油麻岭和界牌矿床是越城岭—苗儿山地区两个钨多金属矿床,规模均达到中型,主要矿体均产在中细粒(斑状)二长花岗岩接触带附近矽卡岩和破碎带中。本研究利用LA-MC-ICP-MS微区原位测试技术,对油麻岭矿化花岗岩和界牌内接触带花岗岩中的锆石进行了U-Th-Pb和Hf同位素分析。结果显示,油麻岭矿化花岗岩锆石U-Pb年龄为（220±1）Ma（MSWD=0.035）,界牌内接触带花岗岩锆石U-Pb年龄为（215±1）Ma（MSWD=0.055）,间接指示越城岭—苗儿山地区存在印支期成矿作用。油麻岭花岗岩样品εHf(t)值在-2.47~-6.67,界牌花岗岩样品εHf(t)值在-3.52~-6.79,表明这两个矿区花岗岩都是源于地壳物质的重熔。与南岭地区其他成钨花岗岩比较,两者源区均具有高εHf(t)值特征,可能是较多εHf(t)值＞0的新元古代新生岛弧地壳组分加入引起的。本文获得及前人发表的印支期同位素年龄表明,桂北地区经历了较强的印支期成岩成矿事件,有较好的找矿前景,今后应注意区内印支期花岗岩出露区的勘查评价工作。     

The Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China — Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton; the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.     

佳木斯地块及邻区早古生代——晚中生代构造演化

为厘清嘉荫—牡丹江缝合带的形成时代,确定佳木斯地块早古生代—晚中生代构造演化过程,笔者对佳木斯地块及邻区出露的一些特征岩石的相关资料进行了收集与系统分析。分析结果显示,黑龙江杂岩外来岩块原岩具有类似蛇绿岩的特征,锆石U--Pb年龄结果显示其形成时代为257～210 Ma,云母及蓝闪石Ar--Ar年龄显示基质与外来岩块共同发生高压变质与退变质事件的时间为193～145 Ma,结合出露于佳木斯地块与松嫩地块的早古生代-早侏罗世火成岩地球化学特征及锆石U-Pb年龄,提出佳木斯地块与松嫩地块间经历了早古生代拼合—二叠纪末期三叠纪裂解—侏罗纪-白垩纪初期再次拼合的构造演化过程。古太平洋板块向西的俯冲作用,驱动了其最终的拼合,形成了以黑龙江杂岩为标志的嘉荫—牡丹江缝合带。     

原地生成宇宙成因核素测年技术思考(三):暴露、侵蚀、埋藏、再暴露和地表升降过程的双核素投影

双核素稳态侵蚀岛投影是原地生成宇宙成因核素测年技术中判断样品是否具有复杂暴露历史和估计侵蚀速率的常用方法。暴露、侵蚀、埋藏和再暴露的投影图可依据传统核素浓度计算等式利用计算机编程绘出。利用新推出的地表持续抬升与下降过程中样品的核素浓度与生成速率(及加速率)、侵蚀速率及暴露时间的计算等式,对其26Al/10Be浓度比值相对于10Be浓度/10Be生成速率比值投影分别进行了计算机模拟,发现下降样品的投影可高于或右偏于稳态侵蚀岛,而抬升样品的投影仍然在稳态侵蚀岛内。利用持续地表下降,可以解释已往文献数据中一次性暴露样品投影高于或右偏于稳态侵蚀岛的"不合理"现象,而持续抬升对投影结果的影响类似于侵蚀,因此如果忽略抬升,计算的暴露年龄结果将偏小。