An oceanic plate sinks beneath a continental plate; it gradually moves deeper to the hot dense mantle core. Under the increased heat and pressure conditions, it begins to melt itself and produce a magma fluid. But the most significant mechanism in the process is the release of water into the mantle by metamorphosing of hydrated serpentinites of the oceanic crust. The amount of H2O then reduces the melting point of the surrounding mantle. A partial melt of the mantle occurs and generates magma fluid 1, 2. That magma fluid produced within the mantle is lighter compare to the surrounding environment moves up into the continental crust where it resides in the magma chambers. Inside the magma chamber hot magma cooler and solidifies and forms granitoid pluton within the crust, also some amount of SiO2 rich magma differentiated within the magma chamber could escape to the surface and forms an acidic volcanic chain.

Granitoid rocks have been well studied for a long time; their classification is varied and depends on the scope of researchs. In general, a classification of granitoid rocks based on tectonic setting can be seen as the studies of Pitcher (1983, 1993), Barbarin (1990) Figure 2.

In a basic and simple model of convergent tectonic between India and Eurasia plates, different types of granitoid forming through each stage of the evolution Figure 3.

(A) Initial subduction of India oceanic plate beneath Eurasia plate; VAG I type granitoid produced from the partial melt of mafic material of the oceanic crust itself and from the surrounding mantle; also S type granitoid forming as mixing of mantle magma with continental crust material;

(B) Further subduction of India into the core of Eurasia, India continental crust closer to India, magma produced from mantle cease stop;

(C) The final stage of collision, heat caused by decaying radioactive materials within the crust melt felsic continental materials and produce a COLG and WPG type granitoid.

Separated from the northeastern part of the Gondwana supercontinent in the southern hemisphere began Devonian time, through different phases of the opening of the Tethyan sea, small continental blocks gradually moved northward to the Eurasia continent and finally assembled together to the south and southeast Asia by the Cretaceous time during long and complex geological activities. The Indochina block (e.g., Vietnam) has at least three main tectonic events that have been recognized in relation to complicated tectonic evolutions with other blocks in the region.

Beginning of Devonian with the opening of the Paleo-Tethyan sea, numerous small geological blocks such as Indochina, South China, North China, Tarim,…drifted away from Gondwanaland supercontinent and move northward. Silurian fish fossils have been distributed within the South China block, also discovered in Vietnam (Indochina block) 3. It is suggested that these two blocks stay close to each other during Silurian time. No connected marine fossils have been reported since early Devonian time, it is suggested toward Devonian time, these two blocks connected and welded together. The amalgamation could happen toward the end of Devonian or Early Permian to form the Cathaysialand 4Figure 4. However, a small sea branch believed to be presented between these blocks.

This Devonian tectonic event is referred to Caledonian Orogeny 5, although this name is inappropriate to be applied to Asia, it has affected different parts of China and particularly in South China. In Vietnam, tectonic events reported by the Early Paleozoic are not well recognized. However, Devonian granitoids have been reported in the central area of Vietnam (Kontum Massif, Dai Loc Massif, Song Chay Massif …) (Figure 5). It is highly possible that the tectonic event is largely distributed throughout Vietnam and likely is suitable to use as Caledonian orogeny.

The Indochina block finally welded to the South China block by the middle Triassic and was named as Indosinian orogeny. Nature of the evolution between these two blocks is still in debate, it may be purely subduction of South China block under Indochina block from north to south6Figure 6, or it may be in the opposite direction by Indochina block from south to north under South China block 7Figure 7.

Another tectonic model also being suggested as the reactivation of an old suture 8Figure 8. The Sibumasu block which separated from Gonwanaland by the opening of the Meso-Tethyan ocean at late Permian moved northward and docked to the southeast side of the Indochina block. This tectonic activity has reactivated the old suture which situated here between the two blocks and completely amalgamated them by middle Triassic time

Moved northward after separated from Gondwanaland by early Permian, the Sibumasu came and docked in the southwest side of Indochina, also Tarim and North China, South China … blocks. Different stages of tectonic evolution were reported in a long time span from Early Permian to Early Jurassic, specified in Sibumasu and Indochina blocks 9Figure 9, associated with different granitoid emplacements in Thailand, Malaysian peninsula, Cambodia and southwest Vietnam Figure 10.

Cretaceous time, a major tectonic event occurs on the east side, the Paleo_Pacific oceanic crust subducted beneath the Eurasia continent 10. Numerous granitoid pluton largely distributed along the eastern coast of Asia from the north in China as Yanshannian granitoid toward the south in Vietnam as Truong Son granitoid Figure 11 .

Recent studies in the southern part, in Vietnam, there is not only a simple subduction of Paleo-Pacific oceanic plate beneath the Eurasia plate, there is another collision of Indochina block with a continental fragment on the Paleo-Pacific oceanic plate – the Luconia Dangerous Ground 11 by Cretaceous Figure 12.

Rock samples collected by Hieu P. T. and Hao V..in their researches 12, 13 zone A, Figure 1. Geochemical data analysis were performed by University of Science and Technology of China, Hefei and Geological Processes and Mineral Resources, China University of Geoscience Table 1.

Geotectonic diagram by Canabis et al., 1989 on Y, La, and Nb and Harris et al., 1986 on Rb, Hf and Ta showed as Figure 13.

Most of the samples fall into active margin between two blocks; some can be seen go further into the collision between two blocks.

Around Devonian time, the S Vietnam block believed to subduct beneath the N Vietnam, which at that time belongs to the South China Block 14. The tectonic activity continued to bring those two plates close to each other, and finally, weld together formed the Cathaysialand at the end Carbonifereous 15. The almagamation of these two blocks not so intense some syn-collision granites could be found, but post COL granites could not be found. The boundary between these two blocks is suggested as Tam Ky - Phuoc Son suture Figure 1.

Indosinian is an important tectonic event in Indochina block; within Vietnam, many studies are conducted by Vietnamese and foreign geologists. Result of this tectonic Indochina block welded to South China block, and the suture is confirmed by the Song Ma ophiolitic belt of Triassic time. Rock samples were collected by Pham T. H., zone B, Figure 1, and results of geochemical analysis were performed by the Institute of Geology and Geophysics, Chinese Academy and re-used in this study Table 216.

In geotectonic diagrams, almost all of the samples fall into the field of the active continental margin, and some samples could be found in the collision zone Figure 14.

The convergence between Indochina and South China blocks produces both VAG and COLG granitoid rocks. It may not clearly demonstrate the full scope of tectonic evolution in detail, but it may prove that the tectonic activities composed of the first stage of subduction continued with the next stage of the collision of these two blocks. It is suggested that South China was subducted beneath the Indochina block based on most of Ben Giang and Hai Van granitoids have stayed in the southern part of Song Ma suture, the location where two blocks amalgamated together Figure 1.

At the same time in the west side of the Indochina block, the Sibumasu block, after broken up from Gondwanaland, came and subducted beneath the Indochina block. Granitoid emplacements for this tectonic evolution were found in a long time span from the early Permian toward the end of Cretaceous time from northern Thailand toward the south to the Malaysia peninsula. In Vietnam, a conductive study by the HCM University of Science with the cooperative of Academia Sinica, Taiwan, by Ching Yin Lan (2009) to collect samples of the Nui Cam batholith in the SW corner Vietnam. Rock analysis was performed at the Academia Sinica, EPMA laboratory. Research study has been presented in the Goldschmith Geology Conferences, Canada 2009.

In the geotectonic plots, these samples are falling mostly in the field of VAG, Syn-COLG, and WPG. The Sibumasu block came and subducted beneath the Indochina block, the tectonic activities and granitoid emplacement could be found at the first stage of subduction with VAG and later on with Syn COLG and WPG in the next phase of collision COLG between two plates and finally induced a type of within plate granitoid WPG in the last phase, Figure 15.

By the Cretaceous time, the Paleo-Pacific oceanic plate subducted beneath the Asia continental plate, this type of convergence produced a broad and long granitoid belt from north China toward the south to Vietnam as Truong Son belt. Rock samples collected in Dalat zone by Bich Thuy N. T. and analysis performed by Memorial University of St. John’s Newfoundland, U K. 17, Table 3. However, the nature of this tectonic event is not simple with just a merely subduction of the Paleo-Pacific oceanic crust beneath the Eurasia continental crust. It is more complex as 10 suggestion that there is a continental fragment that possibly on the Paleo-Pacific plate, following the course of subduction, this block came to collide with Indochina block Figure 16.