摘要
在兴蒙造山带北部的贺根山蛇绿岩东侧新识别出扎很花前弧蛇绿岩。野外填图结果显示,扎很花蛇绿岩呈NE向展布,出露宽约3km,长约10km,蛇绿岩各单元出露较齐全,岩性主要为蛇纹石化方辉橄榄岩、层状辉长岩、暗色均质辉长岩、浅色均质辉长岩、块状前弧玄武岩、枕状前弧玄武岩(fore-arc basalt,FAB)和泥质硅质岩,其岩性组合和岩石序列可与代表洋内初始俯冲的迪彦庙前弧蛇绿岩相对比。扎很花蛇绿岩岩石序列中玄武岩锆石U-Pb年龄约326Ma,与东部迪彦庙蛇绿岩的辉长岩同期。扎很花前弧玄武岩主要发育无斑结构,基质呈球颗结构、间粒结构,枕状构造。岩石SiO_(2)含量为45.67%~58.70%,较低的TiO_(2)(0.82%~0.98%),富Na_(2)O(0.01%~5.09%)而贫K_(2)O(0.04%~1.31%),Na_(2)O/K_(2)O为0.01~93.25;岩石具有亏损型稀土元素配分模式,类似N-MORB,但高场强元素(HFSE)Nb、Ta和Ti等比N-MORB稍低,相容元素Cr、Ni和大离子亲石元素(LILE)K、Rb、Ba、U比N-MORB稍高,与弧拉斑玄武岩相似。地球化学特征指示扎很花玄武岩兼有洋中脊(MORB)与岛弧双重特性,而更像MORB,与马里亚纳前弧玄武岩(FAB)和迪彦庙前弧玄武岩相一致。扎很花前弧蛇绿岩和贺根山玄武岩的微量元素和Nd同位素揭示,它们都起源于亏损的MORB型地幔,源区未受俯冲组分影响。但是随着俯冲作用的进行,扎很花玄武岩的源区由于熔体的抽提作用源区逐渐更加亏损,同时也逐渐有来自俯冲板片的熔体或流体的加入,并由此也导致了源区的氧逸度的不断升高,揭示扎很花蛇绿岩具有前弧环境岩浆源区演化的特征,类似于IBM前弧和迪彦庙前弧蛇绿岩的演化。这一发现为明确贺根山蛇绿岩带早石炭世存在前弧玄武岩提供了岩石学和地球化学佐证,表明贺根山洋早石炭世发生了洋内初始俯冲岩浆作用,记录了早石炭世古亚洲洋东段洋内初始俯冲事件。
A newly identified Zhahenhua fore-arc ophiolite has been recognized on the eastern side of the Hegenshan ophiolite in the northern Xing-Meng orogenic belt.Field mapping reveals that the Zhahenhua ophiolite strikes NE-trending,with an outcrop width of~3km and a length of~10km.Various units of the ophiolite are relatively well-exposed,and the lithology mainly includes serpentinized harzburgite,layered gabbro,dark homogeneous gabbro,light homogeneous gabbro,massive fore-arc basalt(FAB),pillow fore-arc basalt,and pelitic siliceous rocks.Its lithological association and lithostratigraphic sequence are comparable to those of the Diyanmiao fore-arc ophiolite,which represents intra-oceanic initial subduction.The basalt in the Zhahenhua ophiolite has a zircon U-Pb age of~326Ma,contemporary with the gabbro in the eastern Diyanmiao ophiolite.The Zhahenhua fore-arc basalts predominantly exhibit aphanitic texture,with a matrix showing spherulitic and intergranular textures,and pillow structures.The rocks have SiO_(2)contents of 45.67%~58.70%,low TiO_(2)(0.82%~0.98%),enriched Na_(2)O(0.01%~5.09%)and depleted K_(2)O(0.04%~1.31%),with Na_(2)O/K_(2)O ratios ranging from 0.01 to 93.25.Geochemically,they display depleted rare earth element(REE)distribution patterns similar to N-MORB,but with slightly lower high field strength elements(HFSE:Nb,Ta,Ti)and higher compatible elements(Cr,Ni)and large ion lithophile elements(LILE:K,Rb,Ba,U)compared to N-MORB,resembling arc tholeiites.The geochemical features indicate that the Zhahenhua basalts exhibit dual characteristics of mid-ocean ridge basalt(MORB)and island arc basalt,but are more MORB-like,consistent with the Mariana fore-arc basalt(FAB)and the Diyanmiao fore-arc basalt.Trace element and Nd isotope data from the Zhahenhua fore-arc ophiolite and the Hegenshan basalts indicate that both originated from a depleted MORB-type mantle(DMM)source,which was not affected by subduction components initially.However,as subduction progressed,the source of the Zhahenhua basalts became progressively more depleted due to melt extraction,while also experiencing input from subducted slab-derived melts or fluids.This process led to a gradual increase in oxygen fugacity of the source,revealing evolutionary characteristics of a fore-arc magmatic source,analogous to the IBM fore-arc and Diyanmiao fore-arc ophiolites.This discovery provides petrological and geochemical evidence for the presence of fore-arc basalts in the Hegenshan ophiolite belt during the Early Carboniferous,indicating that intra-oceanic initial subduction magmatism occurred in the Hegenshan Ocean during the Early Carboniferous,and records an Early Carboniferous intra-oceanic initial subduction event in the eastern segment of the Paleo-Asian Ocean.
作者
李英杰
董培培
李红阳
郭菁华
张夏炜
王帅
LI YingJie;DONG PeiPei;LI HongYang;GUO JingHua;ZHANG XiaWei;WANG Shuai(Hebei Key Laboratory of Strategic Critical Mineral Resources,Heibei GEO University,Shijiazhuang 050031,China;Innovation Base for Mineralization Theory and Prospecting Technology of Ophiolite Belt,Hebei GEO University,Shijiazhuang 050031,China)
出处
《岩石学报》
北大核心
2025年第11期3802-3817,共16页
Acta Petrologica Sinica
基金
科技部科技基础资源调查专项(2022FY101704)
科技部深地国家科技重大专项(2025ZD1006106)
河北地质大学科技创新团队(岩浆作用及其地球动力学过程)联合资助.
关键词
前弧玄武岩
初始俯冲
早石炭世
贺根山蛇绿岩
古亚洲洋
内蒙古
Fore-arc basalt
Initial subduction
Early Carboniferous
Hegenshan ophiolite
Paleo-Asian Ocean
Inner Mongolia
