## 6 Discussion
### Characteristics and Petrogenesis of Basalts  
Plate tectonic cycles drive the transition between oceanic and continental crust, which makes the study of the demise of ancient oceans a key to revealing Earth's dynamics [42; 43]. The oceanic plate stratigraphy (OPS) system records the complete sequence of the oceanic crust from its generation at mid-ocean ridges to its subduction and destruction at trenches (igneous basement + sedimentary-volcanic cover) [44; 45; 46]. Oceanic plate tectonics (OPG) identifies remnants of ancient oceans within continents through multidisciplinary integration (including igneous petrology, geochemistry, and structural deformation) [47; 48]. Since records of ancient oceans are often erased by subduction, igneous rock associations become the core carriers for reconstructing oceanic-continental transitions: from mid-ocean ridge basalts (MORBs), oceanic islands-seamount chains, to subduction-related arc magmatism and ophiolite/accretionary complexes [37; 49; 50; 51]. Their geochemical signatures can precisely define tectonic settings and provide direct evidence to constrain the subduction processes of the ancient Pacific Ocean. Middle Jurassic basalts exhibit alkaline and sub-alkaline, with calc-alkaline-tholeiitic trends diagnostic of subduction-related arcs. While Y-Sr/Y plots confirm arc signatures (Figure 7), Zr-Zr/Y and TiO\({}_{2}\)-MnO-P\({}_{2}\)O\({}_{5}\) diagrams paradoxically plot within WPB and OIA fields (Figure 8a,b), and Nb/Yb-Th/Yb systematics further demonstrate E-MORB-OIB transitional sources (Figure 8c,d), implying their formation in a young oceanic crust subduction setting. Early Cretaceous pillow basalts display distinct high-Mg# (mean 70) compositions compared to Jurassic counterparts. Despite arc-like Y-Sr/Y signals (Figure 7), their WPB-OIT overlaps in Zr-Zr/Y and TiO\({}_{2}\)-MnO-P\({}_{2}\)O\({}_{5}\) plots (Figure 8a,b), coupled with E-MORB--OIB hybridization in the Nb/Yb-Th/Yb space (Figure 8c,d), which argues for within-plate magmatism with OIB affinity. This likely records back-arc rifting following ridge-trench collision, marking the terminal stage of mid-ocean ridge subduction. The pillows of these pillow-like basalts range from 20 to 50 cm in diameter, also indicating formation in a shallow marine environment. Definition of the Raoe Subduction-Accretion Complex and Constraints on the Ocean-Continent Transition Process  
Ophiolites, defined as tectonically emplaced fragments of the ancient oceanic lithosphere, are characterized by a complete pseudostratigraphy including mantle peridotites, ultramafic-matic cumulates, sheeted dikes, pillow basalts, and pelagic sediments [57]. Figure 8: Tectonic discrimination diagrams for basalts from the Raoe Subduction–Accretion Complex of the Wandashan Block. (**a**) Zr vs. Zr/Y diagram (after [54]); (**b**) TiO\({}_{2}\)-MnO \(\times\) 10-P\({}_{2}\)O\({}_{5}\)\(\times\) 10 (after [55]); (**c**) Nb/Yb vs. Th/Yb (after [54]); (**d**) Nb/Yb vs. TiO\({}_{2}\)/Yb (after [56]). In contrast, subduction-accretion complexes comprise tectonically juxtaposed blocks (e.g., igneous rocks, cherts, limestones) within a matrix of trench-slope turbidites, recording progressive tectonic stacking during oceanic plate subduction [58, 59, 60, 61]. In contrast, the RSAC exhibits diagnostic features of a subduction-accretion system: (1) discrete tectonic blocks including ultramafic-gabbro suites (Xiangyangchuan-Dadingzishan, 220 Ma cumulate gabbros) and mid-ocean ridge basalt (MORB) fragments (Baliqiao, 170 Ma pillow basalts), occurring as fault-bounded slices; (2) cherts-shale-limestone assemblages with distinct matrix relationships--siliceous blocks (central Raohe-Yuejinshan) separated by a turbiditic matrix--while limestone blocks (Shichang-Hongqiling) are embedded within a weakly sheared matrix. These blocks, spanning Late Triassic to Cretaceous ages, are hosted in trench-slope turbidites of the Yongfuqiao Formation (silty slate/sandstone; [23].