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Abstract The tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized. 

Abstract Mantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge. 

Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at >800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.