An assessment of the magmatic conditions of Dinh Quan – Deo Ca – Ankroet granitoids: evaluating from the igneous rocks at SW Vietnam - SE Cambodia

Abstract

The Dinh Quan – Deo Ca – Ankroet granitoids in SW Vietnam - SE Cambodia occur in small bodies of Late Mesozoic igneous rocks, including Bay Nui - Ba Den (BN - BD) diorite (107 – 96 Ma), Bach Ho (BH) granitoid (110 Ma), Bay Nui - Phnom Den (BN - PD) granitoid (94 – 91 Ma); Con Dao (CD) suite (83 Ma), Pha Aok (PA) (78 Ma), and Nui Sam - Angkor Borei (NS - AB) granitoid suite (75 Ma). These granitoids correspond to two tectonic stages: (I) Paleo - Pacific subduction (110 – 90 Ma) and (II) transitional crust extension phase (83 – 75 Ma) which is prior to the sea - opening in Cenozoic. This study utilizes biotite composition as a geobarometer to estimate the magmatic conditions (P, T) of these granitoids. The Al - in - biotite method estimates magmatic conditions for I - type granitic rocks at P = 0.097 – 0.117 GPa, T = 749 – 761°C (BN - BD); P = 0.031 – 0.096 GPa, T = 628 – 759°C (BN - PD); and P = 0.262 – 0.272 GPa, T = 642°C (BH). A - type granites are characterized by P = 0.081 GPa, T = 885°C (CD), P = 0.311 – 0.408 GPa, T = 605 – 626°C (PA), and P = 0.292 – 0.316 GPa, T = 603 – 608°C (NS - AB). The classification of NS - AB and PA granites, previously identified as A - type, shows characteristics aligning with peraluminous, highly contaminated, and reduced I - type granites. Rhyolite MELTs modeling supports these findings, indicating I - type granitoids crystallized at P = 0.3 GPa and T = 1005 – 730°C under reduced oxygen conditions (ΔFMQ -1), except for BH, which shows a more oxidized state (ΔFMQ +1). A - type/SCR I - type granites formed at P = 0.35 – 0.4 GPa and T = 1105 – 758°C under reduced conditions (ΔFMQ -3 to -1). The Paleo - Pacific subduction - related granites formed at shallower depths, whereas the extensional related granitoids crystallized at greater depths with distinct redox conditions. These findings provide crucial insights into magmatic evolution and regional tectonics.