Pacific-type transform and convergent margins: igneous rocks, geochemical contrasts and discriminant diagrams

Transform margins represent lithospheric plate boundaries with horizontal sliding of the oceanic plate, which in time and space replaced the subduction-related convergent margins. This happened due to the following: ridge-crest–trench intersection or ridge death along a continental margin (recent California and Baja California, Queen Charlotte–Northern Cordilleran, west of the Antarctic Peninsula, and probably Late Miocene–Pleistocene southernmost South America); change in the direction of oceanic plate movement (western Aleutian–Komandorsk and southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted from the spreading ridge collision (subduction) with a continental margin or slab tear formation after subduction cessation. Igneous magmatic series formed above the slab window or slab tear are similar in composition and show diversity of tholeiitic (sub-alkaline), alkaline, or even calc-alkaline and peraluminous rocks. The comprehensive geochemical dataset for igneous rocks (more than 2400 analyses) from the recent model geodynamic settings allowed us to build discriminant diagrams for the petrogenic oxides TiO₂ × 10–Fe₂O₃^Tot–MgO and trace elements Nb*5–Ba/La–Yb*10, which show distinctive rock features present on both convergent and Pacific-type transform margins. The author’s diagrams are capable of distinguishing volcanic and plutonic rocks formed above the subduction zones at an island arc and continental margin (related to convergent margins), from those formed in the strike-slip tectonic setting of transform margins along continents or island arcs.

转换边缘（Transform margins）指大洋板块发生水平滑动的岩石圈板块边界，这类边界在时空维度上替代了与俯冲作用相关的会聚边缘。其形成原因可归纳为以下三类：一是脊峰-海沟交汇或沿大陆边缘的脊消亡，典型实例包括现今的加利福尼亚及下加利福尼亚地区、夏洛特皇后群岛-北科迪勒拉区域、南极半岛西侧，以及可能的晚中新世-更新世南美最南端区域；二是大洋板块运动方向发生改变，涉及西阿留申-科曼多尔尔斯克区域与安第斯山脉最南端区域；三是岛弧与大陆发生碰撞，如新几内亚岛。俯冲后岩浆作用与两类构造过程相关：一是扩张脊与大陆边缘发生俯冲（碰撞）而形成的板片窗，二是俯冲作用停止后形成的板片撕裂。形成于板片窗或板片撕裂之上的火成岩岩浆系列在成分上具有整体相似性，同时兼具拉斑玄武质（亚碱性）、碱性乃至钙碱性和过铝质岩石的多样性。基于当前地质动力学背景下的火成岩（共2400余件分析数据）综合地球化学数据集，我们构建了针对成岩氧化物TiO₂×10–全Fe₂O₃–MgO以及微量元素Nb*5–Ba/La–Yb*10的判别图解，这些图解可体现会聚边缘与太平洋型转换边缘所特有的岩石学特征。本文提出的判别图解能够区分两类岩石：一类是形成于岛弧及大陆边缘俯冲带之上、与会聚边缘相关的火山岩与侵入岩，另一类是形成于沿大陆或岛弧分布的转换边缘走滑构造环境中的岩石。