Geochemistry and zircon U[sbnd]Pb geochronology of Late Mesozoic igneous rocks from SW Vietnam – SE Cambodia: Implications for episodic magmatism in the context of the Paleo-Pacific subduction

Abstract

Late Mesozoic igneous rocks in Vietnam are predominantly found in the Dalat zone (southern Vietnam), but smaller volumes also occur within southernmost Indochina (SW Vietnam-SE Cambodia). Within the latter area, six different rock suites can be distinguished based on field relations, geochemical characteristics, and zircon UPb geochronology: (1) Bay Nui-Ba Den diorite suite; (2) Bay Nui-Phnom Den granitoid suite, (3) Pha Aok granite suite, (4) Nui Sam-Angkor Borei suite, (5) Bach Ho granitoid suite, and (6) Con Dao gabbro/granite suite. These igneous rock suites indicate several pulses of magmatism during the late stages of a long-lived convergent margin. The Bay Nui-Ba Den diorite suite (ca. 107–96 Ma) is composed of monzogabbro, monzodiorite, diorite, and syenite, characterized by a metaluminous (I-type) nature, and shows several characteristics of alkali-rich primitive arc melts. The Bach Ho granitoid suite (ca. 110 Ma) consists of granodiorite and granite and displays a calc-alkaline (moderate in potassium) signature. The Bay Nui-Phnom Den granitoid suite (ca. 94–91 Ma) is the most widespread igneous group, consists mainly of granodiorite and granite of I-type affinity. These three rock suites show typical characteristics of arc-related magmatism similar to the I-type granitoids previously described in the Dalat area. The younger magmatic suites (<90 Ma) include the fined-grained granites of the Con Dao suite (ca. 83 Ma), coarse-grained two-mica granites of the Pha Aok suite (ca. 78 Ma), and fine-grained fluorite-bearing biotite granites of the Nui Sam-Angkor Borei granitoid suite (ca. 75 Ma). They display characteristics of A-type affinity with strongly negative Eu, Sr, Ba, Ti, and P anomalies. Geochemical signatures of these young igneous rocks indicate an extensional tectonic setting, possibly related to basaltic injection/asthenospheric upwelling. This additional heat input is required for partial melting of a dry residual crustal source. Geochemical and geochronological results demonstrate that the Late Mesozoic magmatism in southern Indochina can be related to two significant tectonic stages in Indochina and adjacent areas: (1) the Paleo-Pacific subduction (110–90 Ma) and (2) a transitional phase leading to the opening of the South China Sea (83–75 Ma).