内蒙狼山新元古代酸性火山岩的发现及其地质意义

在内蒙狼山西南段原定为中元古代沉积建造(渣尔泰山群)中, 新发现呈层产出、具有变余聚斑状和斑状结构、斑晶主要由石英和钠长石组成、岩石化学成分富钠低钾的酸性火山岩, 其顶、底板均为细晶方解石大理岩(或结晶灰岩). 两件代表性样品的锆石呈短柱状-长柱状, 内部环状结构发育, 具有较高的Th/U比值, 表明是岩浆成因. A8-0样品20粒锆石的²⁰⁶Pb/²³⁸U加权平均年龄为(816.9±4.5) Ma. A14-5样品也测定20粒锆石, 15粒锆石的²⁰⁶Pb/²³⁸U加权平均年龄为(805±5.0) Ma, 其余5粒锆石(3.1, 6.1, 7.1, 20.1, 21.1)依次为(867.0±10), (829.1±8.0), (849.7±9.8), (839.2±9.7), (830.7±9.3) Ma. 这证实酸性火山岩属于新元古代产物, 其年龄区间正好在 Rodinia(罗迪尼亚)超大陆裂解的时限范围内. 且酸性火山岩轻稀土富集, 重稀土亏损, 铕负异常明显, 高场强元素(HFSE)富集, 大离子亲石元素等相对亏损, 稀土和微量元素组成与华南扬子块体西缘和浙江次坞地区裂谷环境的新元古代酸性火山岩总体相似(但华南酸性火山岩的高场强元素Th, Nb, Ta, Hf, Zr均明显相对亏损), 具有形成于张性裂谷盆地的特征. 这表明内蒙狼山西南段存在与新元古代Rodinia超大陆裂解作用相呼应的海相火山-沉积变质建造, 内蒙狼山地区可能有与Rodinia超大陆裂解作用相呼应的构造-岩浆-热事件发生. 这一发现, 结合在西部阿拉善邻区发现的晋宁期花岗岩年龄(814~872 Ma)和金川超基性岩体的年龄(827 Ma)及华北地台邻区新元古代岩浆(火山)活动的最新研究成果, 对进一步研究Rodinia超大陆的构造演化历史有重要意义.

Discovery of Neoproterozoic acid volcanic rock in the south-western section of Langshan, Inner Mongolia

Bedded acid volcanic rocks, which preserve blasto-glomeroporphyritic or porphyritic crystals consisting of quartz and albite with clear evidence of alterations and deformations, from the Mesoproterozoic Zha’ertaishan Group in the southwestern section of Langshan, Inner Mongolia were analyzed. Zircons from two acid volcanic rock samples, A8-0 and A14-5, were separated. The zircons exhibited an igneous morphology, oscillatory zonings and relatively high Th/U ratios, and they were mostly prismatic or short prisms either colorless or brown in color, suggesting a magmatic origin. SHRIMP U-Pb dating was carried out and the 20 measurement points obtained from sample A8-0 yielded a weighted average ²⁰⁶Pb/²³⁸U age of 805±5 Ma. Twenty measurements were also taken on the A14-5 sample, but five points (3.1, 6.1, 7.1, 20.1 and 21.1) were discarded because of their higher ²⁰⁶Pb/²³⁸U ages (867.0 ± 10, 829.1 ± 8.0, 849.7 ± 9.8, 839.2 ± 9.7 and 830.7 ± 9.3 Ma, respectively). The remaining 15 points yielded a weighted average ²⁰⁶Pb/²³⁸U age of 814 ± 4.6 Ma. These ages are interpreted as the timing of an acid volcanic eruption and thus represent a Neoproterozoic (805–814 Ma) submarine volcanic eruption in the southwestern section of Langshan, Inner Mongolia. Geochemical data show that the acid volcanic rock is enriched in light rare earth and high field strength elements, but depleted in heavy rare earth and lithophile elements that have well-developed Eu anomalies similar to those of the Neoproterozoic volcanic rocks. These rocks are comparatively depleted in high field strength elements Th, Nb, Ta, Hf, Zr, and from the western Yangtze block and Ciwu, Zhejiang, revealing that the acidic magma had erupted in. We suggest that this submarine volcanic eruption is probably related to the breakup of Rodinia in Langshan, Inner Mongolia. This discovery, together with the ages of 814–872 Ma reported for the neighboring terranes of Jinningian deformed granites in the Alax area and the age of 827 Ma for the ultramafic rock (ultramafite) in Jinchuan, as well as the recent report of magma activities during the Neoproterozoic in the North China Platform and adjacent areas, is of significance help in understanding the Rodinia evolution in the western section of Langshan, Inner Mongolia.