Tectonomagmatic evolution of the Early Ordovician suprasubduction-zone ophiolites of the Trondheim Region, Mid-Norwegian Caledonides

The Trondheim Region ophiolites in the Mid-Norwegian Caledonides represent variably tectonized ophiolite fragments. We present high-precision thermal-ionization mass spectrometry and secondary-ion mass spectrometry (SIMS) U–Pb zircon dates, whole-rock geochemical and Sm–Nd data and Lu–Hf zircon analyses that permit the timing and nature of various stages in the evolution of the ophiolite to be elucidated. Plagiogranite intrusions dated at 487 and 480 Ma have relatively juvenile Nd and Hf isotopic compositions (εNd(t)=6.3, εHf(t)=8.2–12.4). Geochemical data indicate a subduction-zone influence, suggesting formation in an oceanic back-arc setting. At 481 Ma, a granitoid body with a relatively strong unradiogenic Nd and Hf isotopic composition (εNd(t)=−2.6 to −4.0, εHf(t)=3.8–6.4) and subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect the formation or migration of an oceanic arc above a subduction zone influenced by continentally derived sediments. At c. 475–465 Ma, a greenstone-dominated conglomerate and volcaniclastic sequence was deposited on the eroded ophiolite, indicating obduction between about 480 and 475 Ma. At c. 468–467 Ma, the deformed ophiolite and its sedimentary cover was intruded by trondhjemite dykes and shoshonitic volcanic rocks with intermediate Nd and Hf isotopic compositions (εNd(t)=3.0–3.9, εHf(t)=4.4–10.2). We interpret this magmatism to reflect subduction-polarity reversal and establishment of a magmatic arc at the continental margin shortly after obduction.

挪威中部加里东造山带内的特伦德拉格地区蛇绿岩（Trondheim Region ophiolites）为经历不同程度构造变形的蛇绿岩碎块。本文报道了高精度热电离质谱（thermal-ionization mass spectrometry）与二次离子质谱（SIMS）锆石U-Pb定年数据、全岩地球化学数据、钐-钕（Sm–Nd）同位素数据及锆石Lu-Hf同位素分析结果，借此阐明该蛇绿岩演化各阶段的时间尺度与本质特征。年龄为487 Ma与480 Ma的斜长花岗岩侵入体具有相对亏损的Nd与Hf同位素组成（εNd(t)=6.3，εHf(t)=8.2–12.4）。地球化学数据显示其受俯冲带作用影响，表明该岩体形成于洋弧后盆地环境。在481 Ma时，一具具有显著非放射成因Nd与Hf同位素组成（εNd(t)=−2.6至−4.0，εHf(t)=3.8–6.4）且具俯冲带地球化学特征的花岗岩类岩体侵入该蛇绿岩。本文将该阶段解读为受大陆源沉积物影响的俯冲带上方洋弧的形成或迁移过程。约475~465 Ma期间，一套以绿岩为主的砾岩与火山碎屑岩序列沉积于遭受侵蚀的蛇绿岩之上，表明蛇绿岩仰冲作用发生于约480 Ma至475 Ma之间。约468~467 Ma时，变形后的蛇绿岩及其沉积盖层被奥长花岗岩（trondhjemite）岩脉与钾玄岩质火山岩侵入，二者具有中等的Nd与Hf同位素组成（εNd(t)=3.0–3.9，εHf(t)=4.4–10.2）。本文将该期岩浆作用解读为仰冲作用后不久发生的俯冲极性反转，并在大陆边缘建立了岩浆弧。