The genetic link between kamafugite magmatism and alkaline–carbonatite complexes in the Late Cretaceous Alto Paranaíba Igneous Province, Central Brazil

The Late Cretaceous Mata da Corda Formation, located in the eastern part of the Alto Paranaíba Igneous Province (APIP), Central Brazil, is one of the few places on Earth where kamafugite melts reached the surface generating large volumes of lava, pyroclastic rocks and shallow intrusions over an area of 4,500 km². The western part of the APIP, however, is dominated by hundreds of diatreme-like kamafugites and shallow kimberlite intrusions and by the occurrence of multi-stage alkaline-carbonatite complexes. These complexes feature silica-undersaturated K-rich alkaline rocks, such as aillikite, that closely resemble the mineralogy and geochemistry of kamafugite, albeit lacking feldspathoids. The spatial and temporal distribution of kamafugite and aillikite within the APIP suggests a connection between them. In addition, on a regional scale, airborne magnetic data show three highly magnetic dipole-like structures to the south of the Mata da Corda Formation of an undisclosed nature, which bear geophysical similar responses to the neighbouring alkaline-carbonatite complexes. Links between kamafugite and aillikite are evaluated by the following chemical and isotopic evidence: (1) kamafugite and aillikite compositions plot in the kamafugite field of Foley’s ultrapotassic rock classification; (2) similar CI chondrite-normalized REE distribution, with aillikite enriched up to 2 times in REE compared to kamafugite; (3) both lithologies share almost the same rock-forming minerals; and (4) similar ¹⁴³Nd/¹⁴⁴Nd_(i) and ⁸⁷Sr/⁸⁶Sr_(i) ratios for all the APIP alkaline-carbonatite rocks, indicating a common source from an enriched lithospheric mantle. Therefore, silica-undersaturated rocks from alkaline-carbonatite complexes display an evolved ultrapotassic affinity indicative of a genetic link.

位于巴西中部阿尔托帕拉奈巴火成岩省（Alto Paranaíba Igneous Province, APIP）东部的晚白垩世马塔达科达组（Mata da Corda Formation），是全球少数几处钾霞橄黄长岩（kamafugite）熔浆抵达地表，并在4500平方千米范围内形成大规模熔岩、火山碎屑岩及浅成侵入体的区域之一。然而，阿尔托帕拉奈巴火成岩省西部则以数百个类火山筒状钾霞橄黄长岩体、浅成金伯利岩侵入体，以及多阶段碱性-碳酸岩杂岩体的产出为典型特征。这类杂岩体发育硅不饱和富钾碱性岩，例如阿利基岩（aillikite），其矿物学特征与地球化学组成与钾霞橄黄长岩极为相似，但缺乏似长石。阿尔托帕拉奈巴火成岩省内钾霞橄黄长岩与阿利基岩的时空分布暗示二者之间存在成因联系。此外，在区域尺度上，航空磁测数据显示，马塔达科达组南部存在三个性质未明的强磁类偶极子构造，其地球物理响应特征与邻近的碱性-碳酸岩杂岩体高度相似。我们通过以下地球化学与同位素证据对二者间的成因联系进行了评估：（1）钾霞橄黄长岩与阿利基岩的化学成分均落入Foley超钾质岩石分类方案中的钾霞橄黄长岩范畴；（2）二者均呈现相似的CI球粒陨石标准化稀土元素（REE）分布模式，其中阿利基岩的稀土元素丰度较钾霞橄黄长岩富集达2倍；（3）两种岩性的造岩矿物组成几乎完全一致；（4）阿尔托帕拉奈巴火成岩省所有碱性-碳酸岩岩石均具有相近的初始¹⁴³Nd/¹⁴⁴Nd与初始⁸⁷Sr/⁸⁶Sr比值，指示其源区均为富集型岩石圈地幔。因此，碱性-碳酸岩杂岩体中的硅不饱和岩石展现出演化后的超钾质亲缘性，表明二者存在明确的成因关联。