内蒙古迪彦庙蛇绿岩带达哈特前弧玄武岩的发现及其地质意义

内蒙古SSZ型迪彦庙蛇绿岩位于兴蒙造山带北部的二连浩特-贺根山蛇绿岩带。笔者最近在迪彦庙蛇绿岩带西侧达哈特一带新发现前弧枕状玄武岩。该玄武岩岩性为低钾拉斑玄武岩,发育球颗结构和枕状构造。通过LA-ICP-MS U-Pb测年,获得玄武岩中锆石的206Pb/238U加权平均年龄为333.4±8.5Ma。岩石SiO_2 42.97%~50.9%,较低的TiO_2 0.59%~0.94%,富Na_2O(1.58%~4.26%)而贫K_2O(0.04%~0.44%),Na_2O/K_2O为9.59~47.3;岩石具有亏损型稀土配分模式,类似N-MORB,但高场强元素(HFSE)Nb、Ta和Ti等比N-MORB稍低,相容元素Cr、Ni和大离子亲石元素(LILE)K、Rb、Ba、U比N-MORB稍高,与弧拉斑玄武岩相似。地球化学特征指示达哈特枕状玄武岩兼有洋中脊与岛弧双重特性,而更像洋中脊,与马里亚纳前弧玄武岩(FAB)相一致。这一发现为明确二连-贺根山蛇绿岩带早石炭世存在前弧玄武岩提供了岩石学、年代学和地球化学佐证,表明二连-贺根山洋早石炭世发生了洋内初始俯冲和洋陆转化岩浆作用。

Discovery and significance of the Dahate fore-arc basalts from the Diyanmiao ophiolite in Inner Mongolia

The Diyanmiao SSZ-type ophiolite is in the eastern part of the Erenhot-Hegenshan ophiolite belt that is belong to the Xing''anling-Mongolian Orogenic Belt. The Dahate fore-arc basalts were newly discovered in the Diyanmiao ophiolite. The Dahate basalts are low-K arc tholeiite with variolitic texture and pillow structure. U-Pb analyses of zircon gains from basalts by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)yielded ²⁰⁶Pb/²³⁸U weighted mean age of 333.4±8.5Ma. Geochemically, the Dahate pillow basalts are characterized by moderate concentrations of SiO₂ (42.97%~50.9%), low TiO₂(0.59%~0.94%)and rich in Na₂O(1.58%~4.26%)and poor in K₂O(0.04%~0.44%)and high ratio of Na₂O/K₂O (9.59~47.33), and show light REE(LREE)-depleted chondrite-normalized REE patterns like N-MORB. However,the Dahate pillow basalts, compared with N-MORB, show slightly lower content of HFSE such as Nb, Ta and Ti, and slightly higher content of incompatible elements Cr, Ni and LILE such as K,Rb,Ba and U, similar to those of island arc tholeiite(IAT). These geochemical characteristics show that the Dahate basalts have both the characteristics of N-MORB and island arc tholeiite, and are more like N-MORB, which is consistent with the fore-arc basalts(FABs)of Mariana forearc. The discovery is petrological, chronological and geochemical evidence for the existence of Early Carboniferous fore-arc basalts in the Erenhot-Hegenshan ophiolite belt, and suggest that the Erenhot-Hegenshan ancient ocean began to subduct initially and occurred magmatism of the ocean-land transformation in Early Carboniferous.