Introduction

Separated from the Gondwana Supercontinent in the southern hemisphere, during the opening of the Meso-Tethys sea by Early Permian time, the Sibumasu block migrated toward the north and finally subducted beneath the Indochina block around Triassictime (Figure 1)1, 2.

The suture between Indochina and Sibumasu blocks, where these two blocks amalgamated by the Middle Triassic are defined by many unique geological features; among them, the island volcanic arc is one of the most favorite features.

Within Thailand, at least three volcanic belts have been reported (Figure 2):

1. The Chiang Rai–Chiang Mai Volcanic Belt in the west;

2. The Chiang Khong–Lampang–Tak Volcanic Belt;

3. The Loei–Phetchabun Volcanic Belt in the East.

Each of these volcanic belts represents a specific tectonic environment: the Chiang Rai–Chiang Mai Volcanic Belt represents a type of oceanic island arc3; the Chiang Khong–Lampang–Tak volcanic Belt shows a typical continental margin volcanic arc4; while the Loei–Petchabun Volcanic Belt, distinctively has an island volcanic arc imprint5.

Traces of the Triassic suture between Indochina and Sibumasu has been studied in detail from northern Thailand toward the south to Malaysia; however, there is still a missing gap along with the Vietnam territory. Recently, ultramafic pyroxenite rock – the 95 hill - in Binh Long province, southern Vietnam, has been studied in detail6. This type of ultramafic rock represented a closed backarc basin which was reported in many places within Thailand territory, but the first time in Vietnam.

Chau Thoi and Nui Gio are small hills located within Dong Nai and Binh Long province area in southern Vietnam. These volcanic rocks consist of dacite, andesite, basalto-andesite, and basalt. Chau Thoi rock has been referred to as extrusive volcanic rocks classified as Deo Bao Loc formation, ages reported from the late Triassic to early Cretaceous7.

Method

A joined field trip has been held in 1999 between the University of Science (VNUHCM) and the Academia Sinica, Taiwan, represented by Dr. Lan Ching Yin. Volcanic rocks collected at Chau Thoi hill (Bien Hoa province) and Nui Gio hill (Binh Long province) have been studied and analyzed at the IES (Institute of Earth Science), Academia Sinica with XRF, and XRF+ICP-AES spectrometers. Nine selected samples consist of 03 CT (Chau Thoi) andesites, dacites; 04 NGa (Nui Gio) andesites and 02 NGb (Nui Gio) basalto-andesites with whole-rock geochemistry, and trace elements results are listed in Table 1.

Result

In Chau Thoi hill and the surrounding area, dacites, andesite , and tuffaceous rocks are collected in quarries and borehole cores.

Dacite has a porphyritic texture which is characterized by phenocrysts. Phenocrystals occupy a high amount in volume rock (about 70%), including most of the quartz (15%), plagioclase (30%), K-feldspar (20%), and altered color minerals (probably biotite, 5%). Plagioclase crystals with size change from 0.1mm to 2.0mm, commonly 1.4mm, subhedral to euhedral in shape, almost of crystals display polysynthetic twinning. Many crystals are frequently replaced by sericite/clay minerals and locally epidote. Color mineral are probably biotite which is completely altered to chlorite and epidote minerals. Groundmass occupies approximately 30% of the rock volume, consisting of microcrystals of plagioclase and quartz (Figure 3).

Andesite of Nui Gio is green color, due to strongly altered to epidote (~65% - 70%), with a subordinate amount of quartz/silica minerals (~29%), minor calcite (~1%). Quartz is present in fractures and scattered distribution in the rock, while silica minerals with calcite concentrate in patches width of 0.5-2.0 mm. The rock contains some fractures filled with quartz crystals (Figure 4).

Basalto-andesite has a porphyritic texture which is characterized by phenocrysts. The amount of phenocryst is occupying about 10% - 15% of the rock volume, consist of pyroxene and hornblende. Groundmass component occupies about 85 - 90% of the rock volume, comprising a high amount of plagioclase microcrystals (30 - 35%) and volcanic glass (25-30%), with a considerable amount of secondary minerals as epidote (10%), calcite (2%), chlorite (8%)and opaque minerals (5%). Euhedral lath-shaped plagioclase microcrystals aligned in the direction of flow, which is partly altered to clay minerals; however, crystal boundaries are usually well defined and the texture of some crystals is still partly preserved. A small amount of euhedral opaque minerals are very tiny and distribute in the rock (Figure 5).

In the initial subduction of an oceanic crust beneath another oceanic crust, produced magma has the characteristic of tholeiitic material of the original mantle – island arc tholeiitic series of the early stage (Figure 6)8. Later and deeper subduction away from the trench, produced magma has the characteristic of calc-alkaline materials of the later mature arc stage. In such island arc calc-alkaline series rock, the melting magma is formed by mixing more felsic material of the subducted crust and the original mafic mantle material (Figure 7).

On the AFM diagram, samples fall mainly into the field of the cal-alkaline series, but not in the tholeiitic series, which is the characteristic of the mature island volcanic arc (Figure 8). In more detail, they are defined as dacite, andesite, and basalt type in the Jensen (1976) petrochemical classification(Figure 9).

Trace elements have been used to evaluate the orogenic environment, from Pearce (2008) diagram (Figure 10), these rocks are found in the field of volcanic arc array base on Nb/Yb – Th/Yb trace elements.

In the other geotectonic environment plot by Cabanis and Nicolle 1989, based on the tertiary plot of Y/15-La/10-Nb/8, most samples belong to island arc rock suites, and two samples are in the transitional area toward the tholeiitic rock suites (Figure 11).

All analyzed rocks have chondrite-normalized patterns enriched in LREE relatively to depleted flat HREE, which is typical for island arc rock suites (Figure 12).

MORB-normalized trace element of CT-NG samples shows a prominent Nb trough, characteristic of subduction zone magmas (Figure 13). This pattern distribution could be seen as the same as the MORB normalized trace element of standard world data for island arc volcanic rocks (Figure 14).

Discussion

Three different volcanic belts reported in Thailand:

1. the Chiang Rai–Chiang Mai Volcanic Belt in the west;

2. the Chiang Khong–Lampang–Tak Volcanic Belt;

3. the Loei–Petchabun Volcanic Belt in the East.

The Chiang Rai-Chiang Mai volcanic rocks have been analyzed and are found to be mid-ocean ridge and ocean island basalts.

The Chiang Khong-Lampang volcanic rocks have a characteristic of a back-arc basin or continental rifting environment.

The Loei-Petchabun volcanic belt, especially the Western Loei volcanic rocks, are andesitic and dacitic in composition and are interpreted to have formed by east-dipping subduction of Sibumasu under Indochina9.

The major oxides and rare elements of the Chau Thoi and Nui Gio volcanic rocks show the distinctive characteristic of calc-alkaline rocks in the island arc tectonic environment. It could be the best fit for the Loei volcanic belt within Thailand.

More specifically, the nature geochemical of dacite, andesite, basalto-andesite, is believed to be formed later and deeper toward the Indochina block, a kind of mature island volcanic arc system.

A tectonic model of the geotectonic development of the Indochina and Sibumasu blocks and the Loei volcanic belt could be described as the bellow schematic diagram (Figure 15)9.

The PaleoTethys oceanic crust began to subduct beneath the Indochina block in Permian time. This type of tectonic evolution produced a calc-alkaline island volcanic arc on the front of the Indochina block. The following Sibumasu block finally comes, collides, and amalgamates with the Indochina block.

The Loei-Petchabun volcanic belt, a type of island arc volcanic belt, is the oldest and farthest eastward on the Indochina block, among the other volcanic belts reported in Thailand territory.

At the current time, Chau Thoi (Nui Gio) rocks and other similar extrusive volcanic rocks in central Vietnam have been classified as Deo Bao Loc formation; and the extrusive rocks belong to the Truong Son granitoid belt. They are the magmatic products resulting from the convergence tectonic activity between the PaleoPacific oceanic plate and the Indochina/China block during late Triassic to early Cretaceous time 7, the Yanshanian orogeny. Not geochemical analysis and trace element study for these extrusive rocks. The current classification of these rocks is merely based on petrographical studies.

Conclusion

Major and trace elements of Nui Gio and Chau Thoi volcanic rocks show the typical calc-alkaline rock series composed of dacite, andesite, basalto-andesite, basalt. These rock suites demonstrate a type of mature volcanic chain product from an island arc tectonic environment. They are formed when the Sibumasu oceanic crust portion (PaleoTethys oceanic crust) is subducted beneath the Indochina block around Permian time. This tectonic process closed the Paleo-Tethys sea, amalgamated Indochina and Sibumasu blocks together by middle Triassic time.

This study is the first discovery in the region and completes the geography of the suture forming by the amalgamation of the Indochina and Sibumasu in the Triassic time. In comparison, these have been intensively studied and researched in Thailand territory, but not in Vietnam until this current time.

On the other hand, the Chau Thoi (Nui Gio) volcanic rocks have been classified as Deo Bao Loc formation, which belongs to the Truong Son granitoid belt, age from late Jurassic to early Cretaceous, however, in this study, they are more likely classified and correlated to Loei-Petchanbun volcanic belt of Permian age. Therefore, more studies, especially geochronological studies, are needed to confirm and support this research.

Authors' Contributions

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Competing Interest

The author declares that this paper has no competing interests.