SUMMARY

Petrology of granitoids in the White Tiger, Dragon oil field basement and adjacent areas of the Cửu Long Basin

Vũ Như Hùng, Trịnh Long

Based on materials of quantitative study on the geology-structure side and material composition of granitoids lying in the basement of White Tiger and Dragon oil fields and adjacent areas of the Cửu Long Basin, these granitoids are divided into 4 genetic types as follows:

1. Type of post-collision continental margin arc magmas of T₃-J₁ age, including an association of differentiated from intermediate to felsic rocks closely related to activities of subduction zone, having deep origin from mantle strongly contaminated by the crust, with: IR_Sr = 0.70448; IR_Nd = 0.512207; ε_Nd = -2.86, belonging to the I- S-granite type. Their isotopic age is 178±5 - 222±5 Ma..

2. Type of active continental margin arc magmas of K₁ age, including an association of differentiated from mafic, intermediate to felsic rocks closely related to subduction zone settings. Behaviours of trace elements reflect that they took source from magmas of mantle partly mixed and contaminated by the crust. They have low IR_Sr (0.70645 - 0.70695); IR_Nd = 0.512309 - 0.512361; ε_Nd = (-3.43) - (-1.92) with ¹⁴⁷Sm/¹⁴⁴Nd = 0.098 - 0.1238, showing that their magmas took source from enriched mantle with the somewhat contaminated and mixed tendency. Their isotopic age is 111±5 - 143,80 Ma..

3. Type of spreading on the continental margin arc magmas of K₂ age, including an association of aluminium-high granite of the potassium-high and silica-rich calc-alkaline series. According to the composition of trace elements, they characterize granites of deep source, but formed in the correlation with the crust, bearing some characters of A-granite. They have IR_Sr = 0.70712; IR_Nd = 0.512343; e_Nd = -3.54, showing that their magmas took source from enriched mantle or from the crust having the contaminated and mixed tendency between magmas of mantle source and those of crust source in the setting of spreading on the continental margin arc of the subduction zone. Their isotopic age is 78±5 - 115±5 Ma..

4. Type of anorogenic magmas of Paleogene age, including 2 bimodal associations: felsic (with IR_Sr = 0.70639; IR_Nd = 0.512532; e_Nd = -0.81), taking source from metasomatic mantle, rich in trace elements of EMII type, and mafic (with IR_Sr = 0.70614; IR_Nd = 0.512606; e_Nd = -0.23), taking source from mantle having the composition corresponding to homogenous chondrite source domain.

Người biên tập: GS.TS. Trần Văn Trị.