赣中早第三纪镁铁质岩石的地质地球化学及其地质意义

Geological and geochemical characteristics of early Tertiary basaltic rocks in center Jiangxi province, Southeast China and their geological implication

赣中吉安-秦和白垩纪红盆中发育有大量的镁铁质岩石,呈岩墙、岩瘤等多种产状,野外穿插关系和全岩K-Ar法年龄(为49-65Ma,相当于古新世)均表明它们形成于早第三纪,微量元素以明显富集大离子亲石元素、中度富集高场强元素和轻稀土元素为特征,无明显的Nb和Ta负异常,同位素具有相对较低87Sr/86Sr(0.7041-0.7064)和较高143Nd/144Nd(εNd(T)=+0.8-+6.2)的特征。赣中古新世镁铁质岩石的地球化学特征表明它们与岛弧玄武岩明显不同,类似于洋岛玄武岩(OIB)。结合岩石的地质地球化学特征和区域地质发展史,认为赣中古新世镁铁质岩石的源区是EMⅡ型富集地幔和亏损地幔(DMM)混合,暗示古新世时太平洋板块对中国东南部的影响很弱,中国东南部自古新世初期进入大陆边缘的板内岩石圈伸展环境,岩石圈伸展与软流圈上涌有关。

Geological and geochemical characteristics of Paleocene basaltic rocks in center Jiangxi province, Southeast China is systematically researched in this article, aiming to discuss genesis of those and their geodynamics implication. Plenty of basaltic rocks occurred dikes and stock as Jian-Taihe later Cretaceous basin. On the basic of field characteristics and K-Ar dating (49 similar to 65 Ma, Paleocene) for them, the basaltic rocks were formed during early Tertiary. The trace element and rare earth element chemistry of the basaltic rocks are characterized by enrichment of large ion element, moderate enrichment high field strength element and light rare earth element, and all samples do not display significantly Nb and Ta negative anomalies and are similar to within-plate basalts. The initial Sr-87/Sr-16 compositions of the basaltic rocks are low, ranging from 0.7041 similar to 0.7064, and initial Nd-143/Nd-144 of them are high with epsilon(Nd) (T) = +0.8 similar to +6.2. Combining geological and geochemical characteristics with regional geological history, the Paleocene basaltic rock studied are related to asthenospheric mantle uplift and lithospheric extension, and were generated by partial melting and mixing of different proportions of depleted asthenospheric mantle and EM II-type lithospheric mantle and had undergone different degree of fractional crystallization en rote to the surface. All features indicate that southeast China was dominated by lithosphere uplift and extension rather than related to pacific plate subduction during the Paleoeene time. The occurrence of Paleocene basaltic rock are geochemical distinct from the Cretaceous magmas, similar to Cenozoic basalts, indicting tectonics setting of southeast China change from continental are to intra-plate regime, and this significant change result from lithosphere extension and asthenosphere upwelling.