西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究——来自斜长石斑晶的制约  

Deep magmatic process of new volcano ridge in Segment 27, Southwest Indian Ridge: Constraints from plagioclase phenocrysts

作　　者：王聪浩 刘佳[3] 陶春辉 李伟 WANG Conghao;LIU Jia;TAO Chunhui;LI Wei(Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China;Key Laboratory of Submarine Geosciences,Second Institute of Oceanography,Ministry of Natural Resources,Hangzhou 310012,China;School of Earth Sciences,Zhejiang University,Hangzhou 310027,China)

机构地区：[1]成都理工大学沉积地质研究院,成都610059 [2]自然资源部第二海洋研究所海底科学重点实验室,杭州310012 [3]浙江大学地球科学学院,杭州310027

出　　处：《海洋地质与第四纪地质》2022年第6期11-20,共10页Marine Geology & Quaternary Geology

基　　金：国家重点研发课题“超慢速扩张洋脊局部岩浆供给的深部过程及其成矿效应研究”(2018YFC0309902)。

摘　　要：前人对超慢速扩张西南印度洋脊(SWIR)玄武岩的研究多基于全岩粉末样品,而对能够记录更多岩浆过程的矿物斑晶开展的工作则较为匮乏。本文对西南印度洋脊断桥热液区所在的27段洋脊富含斜长石斑晶的玄武岩进行了全岩和单矿物的地球化学研究。玄武岩样品(34Ⅳ-TVG07和30Ⅲ-TVG14)SiO2含量为49.16%和49.50%, MgO含量分别为6.76%和6.52%。全岩微量元素总体上和N-MORB(normal mid-ocean ridge basalts)类似。电子探针测试结果显示,斜长石的An值范围变化较大(76.2~87.9),且绝大部分的斜长石斑晶An值都在80以上,比SWIR 64°E的Mount Jourdanne火山斜长石超斑玄武岩中的斜长石An值高得多(<70),暗示斜长石的成因不同于Mount Jourdanne,不是由下洋壳辉长岩中的斜长石被后期岩浆直接机械捕获携带上升而形成。运用Petrolg3软件计算模拟也显示斜长石无法由其寄主岩浆直接结晶产生。结合实验岩石学结果以及西南印度洋中脊地幔中存在古老地幔楔熔融残余的多方面地球化学证据推测,断桥区玄武岩中的高An值斜长石斑晶最有可能由软流圈地幔中的古老、亏损的岛弧地幔楔残余熔融形成的岩浆结晶形成。Most previous geochemical studies on basalts from the Southwest Indian Ridge(SWIR) were based on the analysis of bulk rocks, and those on phenocrysts are rare. We conducted bulk rock and mineral analyses of two rock samples of plagioclase-rich basalts from Segment 27,SWIR, where the Duanqiao hydrothermal field is located. The SiO_(2) and MgO contents of the two samples(34Ⅳ-TVG07 and 30Ⅲ-TVG14) are 49.16% and 6.76%, and 49.50 and 6.52%, respectively. Their trace elemental patterns are similar to typical N-MORB(normal mid-ocean ridge basalts). The EPMA analysis show that the An(% of anorthite) of the plagioclase phenocrysts vary in the range of 76.2 to 87.9, and most are above 80, which is significantly greater than those of plagioclase in the Mount Jordanne basalts, indicating that the An-rich plagioclase phenocrysts at 50.4°E are not derived from the lower oceanic crust of the Mount Jordanne. In addition, the Petrolog3 modeling shows that they could not crystallize directly from the mother magma. By combining the experimental constrains and previous evidence for ancient mantle wedge-like component entrained beneath this ridge, we believe that the An-rich plagioclase in Segment 27 basalts were most likely crystallized from magma due to partial melting of an ancient depleted sub-arc mantle.

关 键 词：玄武岩 斜长石斑晶 岛弧地幔 西南印度洋中脊 

分 类 号：P744[天文地球—海洋科学]