西南印度洋中脊63.9°E斜长石超斑状玄武岩对超慢速扩张洋脊岩浆过程的指示     

程石  周怀阳 《岩石学报》2019,35(11):3565-3577

人们对超慢速扩张洋中脊深部岩浆过程的了解至今仍十分模糊。我们对西南印度洋洋中脊(Southwest Indian Ridge,SWIR) 63. 9°E处采集到的斜长石超斑状玄武岩(Plagioclase Ultra-Phyric Basalt,PUB)进行了岩石学和地球化学研究。样品具有以下几个特征:斜长石斑晶的体积分数高达～25%,而橄榄石斑晶的体积分数约1%;尽管该样品中玻璃的成分与同一洋脊段玄武岩的成分基本一致,但高Fo橄榄石斑晶与玻璃基质的成分不平衡;不同类型的斜长石晶体之间存在成分差异,单个斜长石大斑晶中的An值也呈现出与正常的结晶分异过程不符的环带;斜长石斑晶中发育溶蚀、筛状等不平衡结构。因此,我们认为,斜长石超斑状玄武岩经历了多期次熔体的作用,是由通过密度分选聚集在岩浆房顶部的斜长石斑晶被之后的火山喷发带出海底形成。尽管斜长石超斑状玄武岩与同一洋脊段的非斑状玄武岩之间并不存在母熔体成分上的差别,但超斑状玄武岩的出现进一步反映了超慢速扩张洋壳岩浆活动的多样性。  相似文献   

Geochemistry and fluid inclusions of scheelite-mineralized granodiorite porphyries from southern Anhui Province,China     

《Ore Geology Reviews》2017

We report petrological, cathodoluminescence (CL), major and trace element analyses and fluid inclusion studies on scheelite (W)-mineralized granodiorite porphyries from Dongyuan and Zhuxiling, southern Anhui Province (China). In Dongyuan, the larger part of the granodiorite porphyry body is mineralized with W concentrations up to 1140 g/t (total WO₃ reserves >140 000 tonnes), whereas in Zhuxiling mineralization is spatially more limited. All mineralized rocks are strongly altered, containing abundant calcite and no fresh plagioclase. W-mineralized rocks show higher K, Pb and lower Na, Sr, Ti contents compared to the non-mineralized ones. Co-variations between mobile elements (K, Na, Pb, Sr, Rb, etc.) and W, combined with petrological observations, demonstrate that fluid alteration must have controlled the mineralization. In quartz from both deposits 4 types of fluid inclusions have been recognized, i.e., aqueous-carbonic (WC-type), aqueous (W-type, subdivided into Wm-type containing minor detected CO₂ and Wn containing no CO₂), carbonic (pure CO₂, C-type) and late secondary aqueous inclusions (LW-type). WC- and LW-type inclusions represent the original magmatic fluids and meteoric waters, respectively. The other inclusions represent evolved magmatic fluids which are closely related to alteration and mineralization. WC-, Wm- and Wn-type inclusions show higher salinity and lower homogenization temperatures, indicating fluid immiscibility with CO₂ effervescence. Additionally, during plagioclase alteration and precipitation of K-feldspar and sericite the K/Na ratio is lowered and Ca released. The precipitation of scheelite must have been promoted by increasing Ca contents in the fluid and also by increasing pH due to CO₂ loss. The present study demonstrates that for granitic rocks without calcareous wall rock, plagioclase breakdown must have been the key factor for scheelite mineralization. The presence of CO₂-rich fluid indicates that both deposits formed in the same intracontinental extension setting as those in the Nanling region. Thus, the Yanshanian granites from southern Anhui Province may have a great potential for W mineralization, especially the blind ones in deep levels.  相似文献