Ordovician to Silurian magmatism on the Utsira High, North Sea: implications for correlations between the onshore and offshore Caledonides

The age and tectonic significance of two basement granitoids cored on the Utsira High, Viking Graben, North Sea, are constrained by zircon LA-ICPMS data. Syenite in well 25/10-2R is dated to 482±2 Ma, with εHf(482) values from −4.4 to −5.8, and granite in well 16/1-12 yields an age of 436±3 Ma with εHf(436) values from +3.7 to −0.5. The evolved Hf-signature of the syenite demonstrates 482 Ma reworking of Palaeoproterozoic–Archaean crust, interpreted to reflect Taconian–Grampian orogenesis. New and recently published data from the offshore Caledonides allow a first-order correlation of the Utsira High basement with the surrounding on- and offshore Caledonides. The Utsira High occupies a similar position with respect to Dalradian rocks as the Scottish Midland Valley terrane. Geophysical data indicate a volcanic arc beneath the East Shetland Basin, forming a northwards continuation of the Utsira High basement. We propose that the offshore volcanic arc segments represent the northeastern continuation of the Scottish Grampian orogen across the Mesozoic North Sea rift. A north-striking terrane boundary fault is proposed to separate the volcanic arc from the East Shetland Platform and merge northwards with the offshore extension of the Great Glen Fault–Møre Trøndelag Fault Complex.针对北海维京地堑乌茨拉高地（Utsira High）两口钻井取心的基底花岗岩类岩石，本研究通过锆石LA-ICP-MS（激光剥蚀电感耦合等离子体质谱）数据限定了其形成时代与构造意义。25/10-2R井中的正长岩形成时代为482±2 Ma，其εHf(482)值介于-4.4至-5.8之间；16/1-12井中的花岗岩形成时代为436±3 Ma，εHf(436)值介于+3.7至-0.5之间。该正长岩演化的铪同位素特征表明，其形成于482 Ma的古元古代-太古代地壳再造作用，这一过程被认为对应塔康-格兰扁造山作用（Taconian–Grampian orogenesis）。来自近海加里东造山带（offshore Caledonides）的新数据与已发表数据，可实现乌茨拉高地基底与周边陆上及近海加里东造山带的一级区域对比。乌茨拉高地相对于达拉第岩层（Dalradian rocks）的展布位置，与苏格兰中部地体（Scottish Midland Valley terrane）一致。地球物理数据显示，设得兰海盆东侧（East Shetland Basin）下方存在火山弧，该火山弧为乌茨拉高地基底向北的延伸部分。本研究提出，近海火山弧段为苏格兰格兰扁造山带（Scottish Grampian orogen）跨中生代北海裂谷（Mesozoic North Sea rift）向东北方向的延伸部分。本次研究还提出一条走向近南北的地体边界断裂，可将该火山弧与设得兰平台东侧（East Shetland Platform）分隔开，并向北与大格伦断裂-莫尔-特伦德拉格断裂复合体（Great Glen Fault–Møre Trøndelag Fault Complex）的近海延伸段衔接。