Seve terranes of the Kebnekaise Mts., Swedish Caledonides, and their amalgamation, accretion and affinity

A major allochthon of the Scandinavian Caledonides, the Seve belt has traditionally been considered to be derived from the rifted margin and continent-ocean transition (COT) of Baltica. However, geochronological results obtained from its inferred northern equivalent, the Kalak Nappe Complex (KNC), have been taken to indicate an exotic affinity of this complex and of also Seve terranes, an interpretation adopted in recent palaeogeographic models. In the Kebnekaise Mts., the COT is represented by the Kebnekaise terrane composed of amphibolitized dykes of gabbro and dolerite of depleted magma source and rare felsic and ultramafic rocks. Coronitic dolerite and gabbro with abundant rutile suggest high pressures before or during amalgamation with the underlying Mårma terrane, composed of quartzofeldspathic gneisses intruded by mafic and granitic rocks, the latter including a previously dated c. 845 Ma-old granite. The granite mingled with mildly alkaline dolerites chemically similar to transitional basalts of continental rifts. Following emplacement at shallow (andalusite stability) crustal levels, the igneous complex and host rocks underwent extensive deformation, metamorphism within the sillimanite-kyanite stability field and local migmatization. The Kebnekaise and Mårma terranes amalgamated in early Ordovician as indicated by the U-Pb age of 487±7 Ma obtained from titanite fabrics of deformed granite in thrust vicinity, and by ⁴⁰Ar-³⁹Ar results. The pressure increase in both terranes suggests that amalgamation occurred during initial subduction and imbrication. Correlation of the Mårma terrane with the KNC is discussed. Results obtained in this study give no reason to ascribe an exotic affinity to the Seve terranes of the Kebnekaise Mts.

作为斯堪的纳维亚加里东造山带（Scandinavian Caledonides）的一套大型外来岩席，塞韦岩带（Seve belt）传统上被认为源自波罗的古大陆（Baltica）的裂谷边缘与大陆-洋壳过渡带（COT）。然而，从其被推测的北部对应体——卡拉克推覆体杂岩（Kalak Nappe Complex, KNC）中获得的地质年代学数据，被用以表明该杂岩与塞韦地体（Seve terranes）均具有外来亲缘属性，这一解释已被近年的古地理模型所采纳。在凯布讷山（Kebnekaise Mts.）地区，大陆-洋壳过渡带由凯布讷地体（Kebnekaise terrane）代表，该岩系由源自亏损岩浆源区的辉长岩与辉绿岩闪岩化岩墙，以及少量长英质、超镁铁质岩石构成。发育大量金红石的冠状结构辉绿岩与辉长岩，指示其在与下伏马尔马地体（Mårma terrane）拼合之前或拼合过程中曾经历高压环境；马尔马地体由长英质片麻岩构成，该类片麻岩被镁铁质与花岗质岩石侵入，其中花岗质岩石包含一颗此前已完成定年的约845 Ma的花岗岩。该花岗岩与弱碱性辉绿岩发生混合，后者的地球化学特征与大陆裂谷的过渡型玄武岩相似。这套火成杂岩及其围岩在红柱石稳定域（andalusite stability）对应的浅部地壳就位后，经历了强烈变形、矽线石-蓝晶石稳定域（sillimanite-kyanite stability field）内的变质作用以及局部混合岩化作用。凯布讷地体与马尔马地体于早奥陶世完成拼合，这一结论得到了逆冲带附近变形花岗岩中榍石（titanite）结构获得的487±7 Ma铀-铅年龄（U-Pb age），以及40Ar-39Ar法（40Ar-39Ar）定年结果的支持。两地体均出现的压力升高现象表明，拼合作用发生于初始俯冲与叠瓦构造形成阶段。本文还讨论了马尔马地体与卡拉克推覆体杂岩的对比关系。本研究获得的结果不支持将凯布讷山地区的塞韦地体归为外来亲缘属性。