辽河坳陷青龙台辉绿岩的Ar—Ar年代学和地球化学特征——对华北东部岩石圈减薄作用完成时间的限制

辽河坳陷青龙台辉绿岩全岩Ar—Ar同位素定年所获得的坪年龄为（35．5±0．7）Ma，与正反等时线年龄在误差范围内均相吻合，表明该辉绿岩侵位于新生代古近纪的沙河街期。地球化学分析结果显示，青龙台辉绿岩贫硅，富碱、钛，轻重稀土元素分异明显，略具Eu正异常，富集大离子亲石元素（LILE，如Sr、Ba等）和高场强元素（HFSE，如Nb、Ta等），显示与洋岛玄武岩（OIB）类似的地球化学特征。区域资料显示，华北克拉通东部晚白垩世一古近纪均以拉斑质玄武岩的喷发为主．为尖晶石二辉橄榄岩相高程度部分熔融的产物，新近纪以来则以碱性玄武岩的喷发为主，为石榴子石二辉橄榄岩相低程度部分熔融的产物。青龙台辉绿岩具较高的Lad／YbCr、Ce／Y值和较明显的Sr、Ba、Eu异常，应是尖晶石-石榴子石过渡相的二辉橄榄岩经较低程度部分熔融的产物。因此，青龙台辉绿岩的形成时间可代表华北东部岩石圈由减薄至增厚的转折时间，

Ar-Ar dating and geochemical characteristics of Qing- longtai diabase from the Liaohe depression, North China： finishing time restriction on thinning process of the eastern North China＇s lithosphere.

Ar-Ar daring for whole rock from the Qinglongtai diabase returns a plateau age of 35.5±1.7 Ma, which is in good accordance with their isochronal age within the uncertainty. It shows that the diabase has intruded in the Paleogene Shahejie period. Geochemically, these diabases have low SiO2, high Na2O＋K2O and TiO2 contents, with intensive differentiation between light REE and height REE, slight Eu positive anomaly, and generally enrich in large iron lithophile elements （LILE, such as Sr and Ba） and high field strength elements （HFSE, such Nb and Ta）, and can be comparable to those typical ocean island basahs （OIB）. Previous researches on basalts and lithospheric evolution of the eastern North China Craton （NCC） have indicated that basahs during late Cretaceous and Paleogene were dominated by tholeiites and derived by high degrees of partial melting of a spinel lherzolite at low pressure, while the Neogene basalts were dominated by alkaline basalt derived from low degrees of partial melting of a garnet lherzohte at high pressure. High LaCN/YbCN and Ce/Y ratios with positive anomalies of Sr, Ba and Eu of the Qinglongtai diabase imply that they could be derived from low partial melting of transitional zone between spinel lherzolite and garnet lherzotite. Consequendy, the intruding age of the Qinglongtai diabase from the Liaohe depression could be seem to the exact time for lithosphere thickness transposition of the eastern NCC from thinning to thickening.