华南新元古代玄武质岩石成因与构造意义：从造山运动到陆内裂谷

华南广泛发育新元古代岩浆岩,深入了解这些新元古代岩浆岩的成因对研究区域大地构造及其在Rodinia超大陆的聚合一裂解演化中的作用有重要意义.目前学术界对这些岩浆岩的成因及其大地构造意义还存在很大的分歧.我们在系统分析华南新元古代玄武质岩石地球化学特征的基础上,通过对这些玄武质岩石的原始熔体成分和熔体温度,探讨其地幔源区的组成和热结构,为其形成的构造背景提供新的约束;结合近期发表的各种地质观察资料和玄武质岩石的研究结果,提出了华南晚中元古代.新元古代中期从造山运动到陆内裂谷的地球动力学演化的模型.     

米仓山坪河新元古代二长花岗岩成因及其地质意义

扬子板块西北缘新元古代岩浆作用成因的研究对于研究Rodinia超大陆在该区的构造演化具有重要意义。本文对扬子板块西北缘米仓山地区二长花岗岩进行了LA-ICP-MS锆石U-Pb定年和同位素、全岩主量和微量元素分析。结果表明二长花岗岩锆石U-Pb年龄为742.1±5.9 Ma,属于新元古代花岗岩。岩石具有高Si O_2(76.84%~80.08%),高碱(Na_2O+K_2O=7.64%~8.99%),相对富钾(K_2O/Na_2O=0.91~1.36),低P_2O_5含量等特征,铝饱和指数(A/CNK)介于0.77~0.89之间。岩石具有稀土元素含量较高、相对富集轻稀土元素的特征,具有明显负Eu异常,Eu*=0.05~0.13,Rb、U、Th、K、Pb等元素相对富集,Ba、Nb、Sr、P、Zr、Ti和Eu等元素明显亏损。岩石n(~(87)Sr)/n(~(86)Sr)=0.747067~0.795283,n(~(143)Nd)/n(~(144)Nd)=0.512472~0.512661,ε_(Nd)(t)=+3.6~+5.2,二阶段模式年龄T_(2DM)值介于0.96~1.07Ga之间。综合地球化学、同位素特征及米仓山区域构造资料,我们认为坪河二长花岗岩为高钾钙碱性高分异I型花岗岩,该岩浆起源于新生玄武质下地壳岩石的局部熔融,在上升阶段可能经历了地壳的混染作用,形成于伸展的构造环境中,是新元古代晚期Rodinia超大陆裂解作用的岩浆响应。     

Petrology and Geochemistry of the Banded Iron Formation of the Kuluketage Block,Xinjiang, NW China: Implication for BIF Depositional Setting

The Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon ²⁰⁶Pb/²³⁸U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO₂, Zr(Nb) and Al₂O₃ and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al₂O₃ contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.     

阿尔金南缘帕夏拉依档沟斜长角闪岩年代学、地球化学及其构造意义

阿尔金造山带南缘帕夏拉依档沟零星发育一些斜长角闪岩,主要呈透镜状或豆荚状产于阿尔金岩群大理岩中。文章对这些斜长角闪岩开展了岩相学、年代学以及地球化学方面的研究,获得斜长角闪岩LA-ICP-MS锆石U-Pb年龄为(857.4±7.1)Ma和(432.5±5)Ma,前者代表其形成年龄,后者代表其变质年龄。地球化学特征显示斜长角闪岩轻稀土微弱富集,轻重稀土分异不显著,并有部分消减带玄武岩地球化学特征,源区为古俯冲带再循环的富集地幔源区,并受到流体或熔体的改造。结合判别图解和区域构造演化,分析得出斜长角闪岩原岩形成于初始裂谷环境,是大陆拉张向裂谷转换机制下形成的过渡岩石类型,较新年龄(432.5±5)Ma为早志留世陆壳伸展和软流圈底辟作用引起的变质年龄。本次研究为新元古代中晚期阿尔金南缘基性岩浆活动提供了新的年代学和地球化学方面的证据,这期基性岩浆活动可能为Rodinia超大陆初始裂解事件在阿尔金南缘的响应。     

关于板块运动启动时间的争论

"板块运动何时启动?"是当前重大和争议较大的科学问题。文章从超大陆研究、板块运动的地质学标志和热动力学数据模拟等3方面的研究进展介绍国际地学界对这一问题的不同认识。在分析各种不同观点的基础上,作者阐述了对该问题的倾向性认识,赞同地球板块构造不是在一个特定时期出现的独立全球"事件",即不是一个等时的重大热-构造事件的认识。但从新太古代开始,地球上已出现具有板块水平运动特征的俯冲作用。由于地幔温度的差异,早期的板块运动缺少深俯冲形成的高压和超高压变质记录,而从新元古代开始,出现与现代板块一致的动力学机制。因此,赞同大致以新元古代作为"原板块样式"和"现代样式"板块运动的转换时间。     

Late Palaeoproterozoic Hekou Group in Sichuan,Southwest China: geochronological framework and tectonic implications

The Palaeoproterozoic Hekou Group, an outcrop along the SW-margin of the Yangtze Block, consists of volcanic and sedimentary rocks that experienced greenschist facies metamorphism and was intruded by gabbroic and granitic plutons. The sedimentary rocks consist of coarse to fine-grained siliciclastic and carbonate rocks including quartzite, mica schists, polymictic meta-conglomerates and marble, whereas volcanic rocks consist of sodic lava and pyroclastic rocks including albitites, interbeded metatuffs, and metabasalts. Metatuffs from five layers have zircon U–Pb age of 1710 ± 18 Ma (MSWD = 1.6), 1637 ± 7 Ma (MSWD = 0.65), 1601 ± 15 Ma (MSWD = 0.94), 1661 ± 7 Ma (MSWD = 1.4), and 1718 ± 11 Ma (MSWD = 0.3) and these ages show that the Hekou Group deposited at ~1.7–1.6 Ga. The high content of light rare earth element (LREE), the low content of highrare earth element (HREE) and negative Ti anomalies, relatively high content of incompatible fluid-insoluble elements (Nb, Ta, and Th), and the high varied εNd(t) values (?6.0 to +4.6) of the metavolcanic rocks show that these rocks are formed in back-arc basin. Our study also implies that the Yangtze Block also underwent subduction-related, continental margin accretion on its SW-margin during the growth of the Nuna supercontinent at ~1.7–1.6 Ga.     

Transparent gemstones and the most recent supercontinent cycle

Certain hard and normally opaque minerals, both rare and common, occasionally crystallize as transparent or partly transparent ‘gem’ crystals. Such stones, designated ‘crystalline coloured gemstones’ (CCGs), form under highly constrained just-so ‘Goldilocks’ conditions that did not exist prior to the last supercontinent cycle. One set of gem-forming conditions arises during continent-to-continent collisions. Another comes into existence during the breakup of continents and produces distinctively different gems. Over 50 different CCG-forming metamorphic and pegmatitic minerals of the first type crystallize in certain orogenic areas near the edges of continents. In East Africa, the Himalayas, Pamirs and Hindu Kush, groups of CCG deposits formed near the leading edge of the upper plate during the collisions, and are associated with large scale circular arcs. Gems of the second type, which are for the most part blue–green–yellow magmatic sapphires, occur in conjunction with continental volcanism in eastern Australia, far eastern Russia, eastern China, Laos, Vietnam, Thailand, Rwanda, Cameroon, Nigeria and elsewhere. Orogeny causes deep fractures to be regenerated upward into younger rocks. CCG deposits of the first type are systematically associated with ancient fractures originating from below, vestiges of the Earth’s early history. Fractures may be older than the rocks in which they are observed.     

南秦岭桐柏地区程湾花岗岩LA-MC-ICP-MS锆石U-Pb年龄及地球化学特征

程湾花岗岩位于南秦岭桐柏地区,岩性主要为片麻状(黑云母)正长花岗岩,岩石地球化学成分显示其高硅、富碱、贫镁、低磷和低钛的特征,A/CNK=0.99~1.40,平均为1.17,为高钾钙碱性系列过铝质岩石,稀土元素配分曲线总体表现为右倾斜的"海鸥式",具有负Eu异常(δEu=0.11~0.81,平均0.46)。岩石富集高场强元素Zr、Th、Hf和Rb、K,强烈亏损Ba、Sr、P、Ti,10000Ga/Al值在2.84~3.63之间,计算锆石饱和温度为817~879℃,属于A2型花岗岩。正长花岗岩LA-MC-ICP-MS锆石UPb测年结果表明,其~(206)Pb/~(238)U年龄加权平均值为793.0±4.1Ma,表明岩体形成于新元古代青白口纪晚期,而非前人认为的早古生代,其形成与Rodinia超大陆的裂解有关,且形成于裂解的初期阶段。桐柏地区当时仍为扬子大陆北缘的南秦岭,在新元古代中—晚期(825~611Ma)整体上处于陆内裂解的构造体制。     

吉林省张广才岭塔东岩群年代学:Rodinia超大陆裂解的响应

塔东岩群是一套高绿片岩相-角闪岩相变质岩系,被认为是"松嫩-张广才岭地块"前寒武纪变质基底的一部分,但其形成时代一直没有定论.采用锆石U-Pb(LA-MC-ICP-MS)测试方法,对塔东岩群朱敦店岩组黑云二长片麻岩进行年代学研究;105次测试中104个有效测点年龄介于239～2690 Ma之间.年龄频谱图显示众多年龄组:2690～2679 Ma(n=2)、2509～2304 Ma(n=8)、2287～2173 Ma(n=5)、1924～1836 Ma(n=4)、1428～1352 Ma(n=2)、1171～1017 Ma(n=3)、997～929 Ma(n=12)、922～822 Ma(n=40)、812～793 Ma(n=14)、745～738 Ma(n=2)、279～239 Ma(n=8);其中早前寒武纪锆石占～18％、格林威尔造山期锆石占～53％、Rodinia超大陆裂解期锆石占～14％.锆石年龄谐和图反映前寒武纪锆石都有不同程度的Pb丢失,以1428～239 Ma、2690～1836 Ma两个年龄区段分别进行数据处理,获得前者不一致线下交点为476±84 Ma,后者不一致线下交点1711±210 Ma,可能反映两次重要构造热事件.年龄为晚古生代者,尽管锆石Th/U比值普遍大于0.1,显示为岩浆锆石特点,但其复杂的CL图像特征,包括云雾状、流动状、楔状、冷杉叶状环带,并具有规则的外形,表明为后期热事件改造的结果.结合前人研究成果及区域地质建造特征,认为塔东岩群主体形成于新元古代晚期,可能是对Rodinia超大陆裂解的响应;经历了～516 Ma、～476 Ma、～426 Ma构造改造事件,可能为早古生代末的构造杂岩,于～260 Ma又遭受后期强烈热事件的影响;大量早前寒武纪碎屑锆石的存在支持"松嫩-张广才岭地块"古老基底存在的认识.     

世界伟晶岩型锂矿床地质研究进展

稀有金属伟晶岩主要分为LCT (Li-Cs-Ta)型和NYF (Nb-Y-F)型,其中LCT型伟晶岩是全球重要锂矿来源.本文分析了全球伟晶岩型锂矿床的地质勘查和研究成果,简要介绍了各大陆代表性伟晶岩型锂矿床,发现锂矿空间分布不均匀,成矿时间具有多期性和阶段性,成矿事件主要发生在汇聚造山作用的晚期,伴随超大陆汇聚事件.LCT型伟晶岩富含挥发分,与后碰撞S型花岗岩密切相关,多产于中高级变质岩区,就位深度较大,多沿断裂构造贯入.