Petrographical, petrochemical characteristics of Ta Kou massif granitoids, Ham Thuan Nam, Binh Thuan

Nguyen Thi Thu Thuy and Nguyen Kim Hoang

Abstract

Ta Kou is an isometric shape with an area of ​​about 15 km2. Petrographical composition is mainly biotite-hornblende granodiorite; minors are altered light-colored fine-grained biotite granite. Mineral composition is mainly plagioclase 4550, quartz 25, potassium feldspar 1520, biotite 510, hornblende (57); secondary is pyroxen; Accessory minerals include zircon, apatite, orthit, muscovite and rare ore. In some places, near fault or high arch of massif, rocks have been altered by other magma, especially near faults due to post-magma activity including fine-grained biotite granite which caused strongly by alkalization such as increasing the content of potassium felspar (3550%) and quartz 30; reducing plagioclase content (3025%); amphibole -colored minerals is rare, and pyroxene is absent. Chemical composition of medium acid granodiorite SiO2 61.0862.14 (61.85); total alkalinity (K2O+Na2O) 5,996,04 (6.00); ratio of alkaline K2O/Na2O: 0.74-0.77 (0.75 <1). Characterized trace elements content of granodiorite: Rb and Ba are low but Sr is quite high; ratios Rb/Sr: 0.24; Ba/Sr 1.40, Ba/Rb: 5.85; K/Rb: 245.39; La/Yb: 10.33; Ce/Yb: 22.11; normal Eu anomalies. When influencing the post-magmatic activity, some granite is more acidific, the chemical composition of altered granite SiO2 72.2774.07 (73.17); total alkalinity (K2O+Na2O): 7.487.96 (7.72); the ratio of alkaline  K2O/Na2O: 1.601.69 (1.64>1). Characterized trace elements content of altered granite: Ba and Sr are low but high Rb; ratios of Rb/Sr: 1.43; Ba/Sr: 3.06, Ba/Rb: 2.79; K/Rb: 218.05; La/Yb: 8.60 and Ce/Yb: 15.74; strong Eu anomalies.Ta Kou granitoids belong to the medium to high aluminum series, medium to high potassium alkaline series, negative Eu anomalies are from normal to strong, type of I- granite. Granitoid characterized subduction-related formation and altered, which may be due to the effects of later phase magmatic activity. Compared with granitoid formations in South Vietnam, Ta Kou massif granitoids belong to phase 2 of Định Quán complex.

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