华北克拉通火成碳酸岩时空分布和锶钕同位素特征及其地质意义

已有的研究得出：华北克拉通火成碳酸岩在时间上主要形成于古元代末—中元古代初、早中生代和晚中生代3个时段，反映了华北克拉通演化历史上3个拉张阶段。在空间上，前两个拉张阶段形成的火成碳酸岩分布在华北克拉通的北缘和南缘；后一个拉张阶段形成的火成碳酸岩分布在克拉通的中部和东部。华北克拉通火成碳酸岩的钕同位素主要特征是εNd(t )全为负值，而且随时间由老到新负值趋于降低。在εNd(t )-I Sr图解上，其投影点都落到了第3和第4象限，除莱芜—淄博地区火成碳酸岩外，其它都在富集地幔演化线附近，而且从老到新富集程度越来越高。推测华北克拉通火成碳酸岩形成的深部动力学机制可能是：在热地幔柱活动或周边板块向华北板块俯冲之后华北克拉通构造环境转变为伸展拉张环境，因压力减小而引起地幔部分熔融形成的碱性基性—超基性岩浆或碱性中性岩浆沿区域深断裂上侵，又经分异作用冷凝结晶形成杂岩体，火成碳酸岩是这些碱性岩浆演化晚阶段的主要产物。部分火成碳酸岩可能是从上地幔低程度部分熔融形成的碳酸岩浆直接上侵冷凝结晶而成。

Igneous Carbonatites in North China Craton: The temporal and Spatial Distribution, Sr and Nd Isotopic Charateristics and Their Geological Significance

According to the research of some predecessors and the present authors, the ages of the igneous carbonatites in North China Craton are focused in three stages: the latest Paleoproterozoic-early Mesoproterozoicera, the early Mesozoic era and the late Mesozoic era, which means that these three stages are obviously extensional periods in evolution history of North China Craton. The igneous carbonatites generated in the first and the second periods are distributed in northern and southern margins of the North China Craton, and the third one is recognized in the eastern and central parts of this craton. The isotopic geochemistry of these carbonatites is characteristic of negative εNd(t ) , decreasing with formation ages. On the εNd(t )-I Sr correlation diagram, most of the samples fall within the third and the forth quadrants, near the enriched mantle evolution trendline except for those located in Laiwu-Zibo area, where the enrichment degrees become higher and higher with their ages. This paper also suggests that the deep geodynamic mechanics of these carbonatites probably be related to the hot mantle plume activities or the subduction of the blocks around the North China Block (NCB), resulting in extensional setting of NCB. With the pressure reduction, the alkaline ultramafic-mafic or intermediate magmas were generated by partial melting of the mantle, and subsequent upwelling along the regional deep faults and formation of the complexes by fractional crystallization. The igneous carbonatites are mainly the late products of above-mentioned magmatic evolution. In addition, some carbonatites are argued most likely to be the direct crystallization products of the carbonatite magma derived from the low degree partial melting of the upper mantle.