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Northeastern territory of Vietnam is covered by contrasting tectonostratigraphic assemblages representing various ages, origins and formation environments. Parts of the area are overlain by thick volcano-sedimentary and intrusive assemblages, which were previously interpreted as to have derived from intraplate subsidence and rifting processes during latest Paleozoic to Early Mesozoic. These rocks either overlay on or intruded into the variably deformed Early Paleozoic shallow marine terrigenous to carbonate and Middle to Late Paleozoic, moderate to deep water, locally chert-bearing associations. All of the above assemblages are in turn variably overlain by continental molass deposits and minor volcanic and intrusive rocks of Early Cenozoic in age. Recent studies, for the first time, have identified a distinctive mafic magmatic assemblages that exposed scatteredly as discontinuous, northwest-southeast-trending blocks within northeastern part of Vietnam along the Sino-Vietnam international border. These assemblages comprise well preserved pillow basalt successions, which are tectonically intercalated with gabbro-diabasic rocks, reef-style limestone and deep-marine chert-bearing deposits. The contacts between the pillow successions with surrounding sedimentary rocks are tectonized, in which the pillow sequences occur as exotic blocks surrounded by large brittle-ductile thrusts within a large zone of several km thick tectonic melanges. Preliminary Rb-Sr isotopic dating has obtained a formation age of the pillow basalt at c.a. 334 Ma, much older than the Late Permian to Early Triassic age range proposed previously. The presence of pillow basalt in the study area is a direct evidence of the submarine volcanism and the exposed pillow sequences are possible remnants of an ancient oceanic floor or island arc systems. The pillow successions in the study area are similar, in term of field occurrence, structure and relationship with surrounding rocks, to those pillow basalts occurring within southwestern part of Guanxi Province in China that borders the study area, where they have been interpreted to be remnants of oceanic crust underneath Paleotethys. It is possible that the dismemberment and obduction of the fragments of Paleotethys's floor in the form of tectonic melanges within an accretionary complex, which latter evolved to an orogenic belt as consequence of the convergence of continental blocks such as Indochina and Yangtze or South China were the cause of the occurrence of abundant pillow basalts in parts of northeast Vietnam and environs. In this case, the area along northeastern border of Vietnam may have been part, or near the site, of a paleosuture zone that was formed by the destruction of Paleotethys to create the Southeast Asia segment of the present-day Euroasia.
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