Multi-layer perceptron-based tectonic discrimination of basaltic rocks and an application on the Paleoproterozoic Xiong'er volcanic province in the North China Craton

Abstract

The geochemistry of basaltic rocks is widely used to investigate the tectonic setting of magmatism. The limitation of traditional two-dimensional tectonic discrimination diagrams is mainly risen from the fact that they can only simultaneously use the information of two (x-y plots) or three (ternary diagrams) elements (or element ratios) for discrimination. This obstacle can be overcome with the assistance of machine learning method, which shows great performances in classification of multidimensional datasets. In this study, we present a neural network-based model that uses whole rock major and trace elements to discriminate basaltic rocks (SiO2 45–55 wt%) from a wide range of tectonic settings, including continental arc basalt (CAB), island arc basalt (IAB), intra-oceanic arc basalt (IOAB), mid-ocean ridge basalt (MORB), oceanic plateau basalt (OPB), oceanic island basalt (OIB), continental flood basalt (CFB) and continental rift basalt (CRB). Using a modified method of cross validation, it is estimated that the model can discriminate the tectonic setting with an average accuracy of ~86%, and ~98% in discriminating the major tectonic regimes (arc, spreading center, or within-plate magmatism). This discrimination model was programed as a stand-alone Microsoft Excel spreadsheet that can be directly used by pasting the whole rock data into it. The discriminator was then applied to investigate the geodynamic background of the Paleoproterozoic (~1.75 Ga) Xiong'er volcanism in the southern margin of the North China Craton (NCC). It has long been debated whether this magmatism took place in a continental arc or within-plate rift environment. The discrimination result shows that both the Xiong'er Group volcanic rocks and coeval intrusive rocks have CFB affinities, indicating that they were products of mantle plume activity and defines a large igneous province (LIP) in a within-plate setting. This, along with previous studies, constrains the breakup of NCC, a part of the Columbia supercontinent, not later than ~1.79 Ga, and supports the idea that the fragmentation of the Columbia was triggered by mantle plume impingement.