Geochemistry and zircon U Pb geochronology of Late Mesozoic igneous rocks from SW Vietnam – SE Cambodia: Implications for episodic magmatism in the context of the Paleo-Pacific subduction
Anh Thi Quynh Nong, Christoph Hauzenberger, Daniela Gallhofer, Sang Quang Dinh
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).