Archaean-Palaeoproterozoic cherts are rare and contain valuable information to understand the chemical evolution of oceans in the past, given that during most of this period an abiological deposition process has been interpreted for their formation, in which silica precipitated directly from seawater. In the Palaeoproterozoic basement of the Tandilia Belt (Argentina), Río de la Plata Craton (RPC), cherts have been mentioned as part of the El Cortijo Formation, which includes low-grade metamorphic rocks, such as metacherts, metawackes and metabasites. In order to provide conclusive evidence about the origin of this unit, detailed geochemical analysis and petrographic and field studies of the metacherts were performed. They allowed two groups of metacherts to be identified: gray-whitish (GWM) and green-bluish metacherts (GBM). The GWM present nodular and lens morphologies and are mainly composed of micro- and mesoquartz crystals with evidence of recrystallization processes. The GBM show a layered structure that consists of micro- and mesoquartz crystals recrystallized in fine bands, interspersed with bands composed of epidote, zoisite, chlorite and sericite. The multielement diagram (normalized to post-Archaean Australian shale) shows positive anomalies of La, Eu and Y and a slight enrichment of the heavy rare earth elements (HREE) with respect to the light rare earth elements (LREE), similar to seawater signatures, for the GWM. Eu anomalies between 1.2 and 1.4, a flat distribution of LREE and depleted contents of HREE are shown for the GBM. These characteristics indicate that the GWM derived from C-cherts (chemical cherts) sensu-stricto, while the GBM protolith is compatible with silicification of volcanic rocks, or S-cherts (silicified cherts), also considering the high lithophile element contents. Furthermore, the Ge/Si atomic ratios, from 1.48 x 10⁻⁶ to 4.79 x 10 ⁻⁶, and Y/Ho ratios lower than 43.36 (for both chert types) indicate an influence of hydrothermal silica source. The study of the El Cortijo Formation cherts has revealed conclusive evidence about the origin of these marine rocks allowing a reinterpretation of the whole formation as an Oceanic Plate Stratigraphy complex in the Tandilia Belt basement, southern margin of the RPC.

太古宙-古元古代燧石非常罕见，其中包含了解过去海洋化学演化的宝贵信息，因为在这一时期的大部分时间里，它们的形成都被解释为生物沉积过程，其中二氧化硅直接从海水中沉淀出来。在坦迪利亚带（阿根廷）、拉普拉塔克拉通 (RPC) 的古元古代基底中，燧石被称为 El Cortijo 地层的一部分，其中包括低品位变质岩，如变质岩、变质岩和变质岩。为了提供关于该单元起源的确凿证据，进行了详细的地球化学分析、岩相学和野外研究。他们允许识别两组 metachert：灰白色 (GWM) 和绿蓝色 metachert (GBM)。GWM 呈现结节状和透镜状形态，主要由具有再结晶过程证据的微晶和中晶石英组成。GBM 显示层状结构，由细带重结晶的微晶和中晶石英晶体组成，散布着由绿帘石、黝帘石、绿泥石和绢云母组成的带。多元素图（归一化为后太古代澳大利亚页岩）显示 La、Eu 和 Y 的正异常以及重稀土元素 (HREE) 相对于轻稀土元素 (LREE) 的轻微富集，类似于海水特征，对于长城汽车。GBM 的 Eu 异常在 1.2 和 1.4 之间，LREE 分布平坦，HREE 含量减少。这些特征表明 GWM 源自 C-cherts（化学燧石） sensu-stricto，而GBM原岩与火山岩的硅化相容，或S-cherts（硅化燧石），也考虑到高亲石元素含量。此外，Ge/Si 原子比，从 1.48 x 10^-6到 4.79 x 10 ^-6和 Y/Ho 比低于 43.36（对于两种燧石类型）表明水热二氧化硅源的影响。对 El Cortijo 组燧石的研究揭示了关于这些海相岩石起源的确凿证据，允许将整个地层重新解释为 RPC 南缘 Tandilia 带基底的大洋板块地层复合体。