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对北京西山髫髻山组和东岭台组火山岩锆石U-PbSHRIMP和LA-ICP-MS定年及地球化学的研究结果表明,髫髻山组上部火山岩时代为137.1±4.5Ma(2σ),东岭台组中部火山岩时代为130~134Ma.前者略老于后者,但两者时代相差<5Ma,证明两组火山岩的喷发是在很短的时间内完成的.北京西山地区髫髻山组火山岩的年龄与承德盆地髫髻山组火山岩的年龄明显不同,表明髫髻山组火山岩年龄可能存在明显区域变化.北京西山髫髻山组中存在典型的埃达克岩(SiO2含量约为56%,Na2O=3.99~6.17,Na2O/K2O=2.2~3.1,Sr=(680~1074)×10?6,Y=(13.2~16.3)×10?6,Yb=(1.13~1.52)×10?6,Sr/Y=43~66),高镁埃达克岩和高镁安山岩(Mg#=54~55).髫髻山组埃达克岩及与其共生的其它中酸性火山岩均具有典型的Nb-Ta负异常和Pb正异常等大陆地壳特征,表明它们是陆壳岩石部分熔融的产物.上述结果表明,北京西山地区髫髻山组埃达克岩代表了中生代曾存在于华北克拉通的加厚榴辉岩下地壳与岩石圈地幔一同拆沉至软流圈中,榴辉岩随后发生部分熔融,熔体上升过程中与地幔橄榄岩发生相互作用的产物.髫髻山组火山岩的年龄制约了拆沉作用至少延续至137Ma.东岭台组火山岩为Mg#正常的流纹岩和英安岩,表明它们没有与地幔发生反应,它们可能代表了拆沉作用导致的软流圈地幔物质上涌造成的深部地壳物质重熔的产物.
The zircon U-Pb SHRIMP and LA-ICP-MS dating and geochemistry of volcanic rocks in the Xiushan Qiushan and Donglingtai formations show that the Upper Volcanic Age of the Qijiashan Formation is 137.1 ± 4.5Ma (2σ) The volcanic age of the middle part of the group is 130 ~ 134Ma, while the former is slightly older than the latter, but the difference between the two ages is less than 5Ma, which proves that the volcanic eruptions of the two groups were completed in a very short period of time. The age of volcanic rocks in the Qiaomishan Formation of the Chengde Basin is obviously different, which indicates that there may be obvious regional variations in volcanic age in the Qiaomishan Formation.Modern adakites (SiO 2 content about 56%, Na 2 O = 3.99 ~ 6.17, The results show that Na2O / K2O = 2.2 ~ 3.1, Sr = (680-1074) × 10-6, Y = 13.2-16.3 × 10-6, Yb = 1.13-1.52 × 10-6, Sr / Y = 66), high-magnesium adakite and high-magnesium andesite (Mg # = 54-55). The adakite and other intermediate-acid volcanic rocks in the Qiongjiashan Formation have typical Nb-Ta negative anomalies and Pb positive anomalies These continental crustal features indicate that they are part of the melting products of continental crustal rocks.The above results show that the Adakite of the Qiaomishan Formation in the Xishan area of Beijing represents the Mesozoic In the crust of the North China Craton, the crust beneath the eclogite and the lithospheric mantle subsided into the asthenosphere, and the eclogite then partially melted and interacted with the mantle peridotite during the melt ascending. The age of the volcanic rocks restricted the detachment effect to at least 137 Ma. The Donglingtai volcanic rocks are rhyolite of Mg # normal, indicating that they did not react with the mantle and may represent asthenosphere due to delamination The product of remelting of deep crustal material caused by upwelling of mantle material.