摘要:
地球化学分析结果显示，宽坪群变基性火山岩和斜长角闪岩SiO2含量均小于53%，TiO2的含量分别是0.87%~2.03%和0.92%~2.12%，MgO含量较低（Mg#=29~49），原岩类型为亚碱性TH系列玄武质岩石。宽坪群变基性火山岩和斜长角闪岩的轻稀土、重稀土元素无明显分异，LREE呈现亏损—略富集，稀土元素配分模式呈平坦型，与E-MORB的配分模式相似。相对于N-MORB，宽坪群变基性火山岩和斜长角闪岩的大离子亲石元素（LILE, 如Sr、K、Rb和Ba）轻度富集，高场强元素（HFSE，如Ta、Nb、Zr、Hf、Ti等）既不富集也不亏损, 显示与E-MORB相类似的地球化学特征。宽坪群变基性火山岩和斜长角闪岩的初始锶比值比较分散，但其初始钕比值(143Nd/144Nd)(t)比较集中和均一，分别为0.511962~0.512192和0.512028~0.512157，??Nd(t)值均为正值，分别是+5.7~+10.2和+7.0~+9.5，表明2类岩石来自轻稀土和大离子亲石元素略呈亏损的源区。SHRIMP锆石U-Pb年代分析揭示，宽坪群变基性火山岩形成于晚新元古代Ediacaran期（611Ma±13Ma），较老的残余晶核锆石可能是岩浆活动过程中捕获古老地壳成分的记录。黑云母40Ar/39Ar热年代学分析表明，宽坪群原岩变质时代为石炭纪Serpukhovian期（319.1Ma±3.6Ma）。认为宽坪群中的变铁镁岩块/片形成于晚新元古代被动陆缘裂谷洋盆，是华北陆块南缘大陆裂解作用的产物。
Abstract:
Geochemical analysis shows that: metabasic volcanic rocks (greenschist) and amphibolite in the Kuanping Group both have lower SiO2 (<53%) and MgO (Mg#=29-49) and their TiO2 contents are 0.87-2.03% and 0.92-2.12% respectively, indicating that their protoliths are basaltic rocks of the subalkaline .tholeiitic series. The chondrite-normalized REE patterns of greenschist and amphibolite in the Kuanping Group show no pronounced differentiation between LREE and HREE ((La/YbN)=0.82-1.42), with LREE showing depletion or slight enrichment. The REE patterns are flat, similar to the N-MORB-normalized patterns. The two types of metamafic rocks are slightly enriched in large-ion lithophile elements (LILE) (e.g., K, Rb, and Ba) and neither depleted nor enriched in high-field strength elements (HFSE) (e.g., Nb, Ta, Zr and Ti), indicative of a strong geochemical affinity to E-MORB. The two types of mafic rocks have relatively dispersed initial Sr ratios, but their initial Nd ratios (143Nd/144Nd)(t)) are relatively concentrated and homogeneous, being 0.511962 to 0.512192 and 0.512028 to 0.512157 respectively ? Nd(t) values of the two types of mafic rocks are both positive, +5.7 - +10.2 and +7.0 - +9.5 respectively, indicating that they were sourced from source regions slightly depleted in LREE and LILE. SHRIMP U-Pb zircon age analysis reveals that greenschist in the Kuanping Group formed in the late Neoproterozoic Ediacaran, at ~611±13 Ma. The older relict core zircons are most likely to be the record of the older crust component captured during magmatism. 40Ar/39Ar thermochronological analysis of biotite reveals that the protoliths of the Kuanping Group were metamorphosed in the Carboniferous Serpukhovian, at ~319.1±3.6 Ma. Conclusively, all those lines of evidence show that the two types of metamafic blocks in the Kuanping Group formed in a late Neoproterozoic passive continental-margin rift ocean basin and were derived from the rifting of the south passive continental margin of the North China block.
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