应用数理统计与蛛网图相结合的方法研究成矿物质来源——以赣南6722铀矿床为例

文章通过赣南6722铀矿床实例,运用Q型模糊聚类分析与蛛网作图相结合的对比方法,对6722铀矿床的成矿物质来源进行了研究,判明了含矿隐爆角砾岩、蚀变花岗岩与黑云母花岗岩具有密切的成因关系。笔者认为,采用多元数理统计分析与蛛网图相结合的研究方法可获得较多精细的、为蛛网图所隐匿的、但具有重要地质意义的地球化学信息,从而得到更为严谨、客观、并具有科学再现性的结论。

多体系微量元素地球化学对比中存在的问题及解决途径

在微量、稀土元素地球化学对比研究中,直观地根据蛛网图上曲线相似程度而得出的结论带有很大的主观随意性,可信度低。通过2个应用实例(云南北衙金矿床;江西相山变质岩),运用Q型逐步聚类分析及模糊聚类分析与蛛网作图相结合的对比研究方法,分别对云南北衙金矿的成矿物质来源和江西相山变质岩的地层单元归属问题进行了探讨,发现该方法可获得较多精细的、为蛛网图所隐匿的、但具有重要地质意义的地球化学信息,从而得出更为严谨、客观、并具有科学再现性的结论。

Appraisal of Crustal Contamination in Southern Bastar Mafic Dykes in Chattisgarh, India

The present work is an attempt to assess the effect of crustal contamination through the Granitoids host rock, within the southern Bastar mafic dykes of Chhattisgarh, India, in the light of geochemical characteristics. Petrographically, these dykes are classified as Amphibolite, Dolerite/Meta-Dolerite and Diorite. Geochemically, all dyke samples have been classified as “high iron sub-alkaline Tholeiites”. On account of overlapping magnesium and iron concentration in Amphibolite and Dolerite dykes, distinctly higher High-Field Strength Element (HFSE), higher Rare-Earth Element (REE) concentrations in the Dolerite dykes than in the Amphibolite dykes, it is inferred that both dyke swarms are fed from two different Tholeiitic magmas. Conclusively, these dykes are recognized as belonging to two different swarms, BD1 and BD2 respectively. This is also corroborated by differences in the LREE patterns i.e. BD2 dykes have relatively enriched LREE pattern than that of BD1 dykes. It is evident from higher LaN/LuN ratio in the BD2 swarm, than in BD1 dyke swarm. These dykes intrude in the coarse-grained leucocratic Bastar Granitoids/Granite Gneisses, which are the host for these dykes. The comparative study of the Primordial mantle-normalized multi-element spider grams, and Chondrite-normalized rare-earth element patterns for the average of BD1 and BD2 dykes and the average of Bastar Granitoids, clearly reflect that the great degree of variation in LIL elements, observed in the Bastar dykes, is either due to metamorphism or due to secondary alteration, and not due to crustal contamination. This is further supported by much higher average “Nb”/“La” ratios in Bastar Mafic Dykes, than in the Bastar Granitoids. Crustal assimilation plays almost no role in the petrogenesis of the Bastar mafic dykes. The Bastar mafic dykes owe their incompatible element characteristics, certainly mantle derived.

塔里木盆地东南缘安南坝地区镁铁质麻粒岩的成因--来自地球化学及Sr-Nd-Pb同位素的制约

甘肃阿克塞县安南坝地区镁铁质麻粒岩呈脉状、透镜状赋存于新太古代米兰岩群和TTG片麻岩中。岩石主要由斜长石(Pl)+斜方辉石(Opx)+单斜辉石(Cpx)+角闪石(Amp)+磁铁矿(Mt)等组成。安南坝镁铁质麻粒岩中Ti、P、Nb、Ta、Th、Hf、Sr及REE等元素与Zr相关性较好,表明其在变质作用过程中保持基本稳定。地球化学数据显示其原岩属于拉斑玄武质岩系列,Si O_2、Ti O_2、Al_2O_3、P_2O_5含量相对较低,Ca O、Mg O含量相对较高。Mg~#值为41.52~43.09,低于原生玄武质岩石的Mg~#值,Fe_2O_3~T、Mg O、Ca O与Si O_2含量呈负相关性,指示原岩岩浆演化过程中可能发生了辉石、角闪石等镁铁质矿物的分异结晶作用。镁铁质麻粒岩∑REE较低,稀土元素配分模式为轻稀土元素弱富集、重稀土元素相对平坦的右倾型,Eu异常不明显(Eu/Eu~*=0.91~1.01)。岩石富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素,具有显生宙典型岛弧玄武质岩石的地球化学特征。Sr、Nd、Pb同位素组成显示镁铁质麻粒岩原岩源自富集地幔,并受到一定程度的地壳物质混染。构造环境分析表明安南坝镁铁质麻粒岩原岩形成于与俯冲有关的岛弧环境。在俯冲作用机制下,俯冲板片流体交代使地幔楔发生富集,形成富集地幔,随着(弧后)伸展作用的加强,进一步诱发富集地幔的部分熔融形成镁铁质岩浆,最终岩浆就位形成辉长岩或辉绿岩脉,后期在麻粒岩相变质作用条件下变质为镁铁质麻粒岩。