内蒙狼山新元古代酸性火山岩的发现及其地质意义
摘要
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 <sup>206</sup>Pb/<sup>238</sup>U 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 <sup>206</sup>Pb/<sup>238</sup>U 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 <sup>206</sup>Pb/<sup>238</sup>U 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