The Ca Vịnh Complex [1] including orthogneiss and migmatite of banded or folded banded structure occurs mainly in the Fan Si Pan Zone of West Bắc Bộ. Its rocks are exposed in massifs of different sizes, among them the Ca Vịnh Massif is the greatest (over 200 km² in exposure) and considered as the typical and most studied massif of the complex [1].

The Ca Vịnh Complex is one of the crystalline basement complex of the Vietnamese territory. The determination of its original age has very important significance in the geological study of the territory, especially in the study on the tectonic evolution of the regional continental crust, the prospecting of mineral resources and the forecast of geological hazards. Formerly, French geologists described the rocks of the Ca Vịnh Complex as "Fan Si Pan Orthogneiss" of Huronian age; later geologists of the General Department of Mines and Geology [1] dated this complex as Neoproterozoic on the basis of the isotopic age of 2070-2300 Ma obtained by the K-Ar method from hornblende of gabbro-amphibolite (Bảo Hà Complex) by Nguyễn Văn Đễ, Kopytsa and Phan Trường Thị. Recently, we have analyzed the U-Pb age for zircon separated from an orthogneiss sample of the Ca Vịnh Complex by the SHRIMP method. Results of the first SHRIMP U-Pb analyses for zircon have been showing that some zircon grains have an ancient core of 2900 Ma which is superimposed by younger zircon generations of 2360 and 1960 Ma [7]. These are the most ancient minerals discovered in SE Asia. We have interpreted the 2900 Ma age value as age of primitive rocks (age of magmatic crystallization) of the Ca Vịnh Complex. The young zircon generations (2360 and 1960 Ma) covering the ancient core have been regarded as metamorphosed zircon developed during the impact of metamorphic phases on the complex, happened later in Neoproterozoic in the conditions of local fusion. However, due to zircon was analyzed from one sample (sample HK.22; see [7]), and the meeting frequency of ancient core in the analyzed sample is low (4 in 18 analyzed grains), one still can interpret the 2900 Ma zircon core as inheriting core in the magmatic complex of 2360 Ma age. So, for affirming the authenticity of the existence of a complex of ancient age (Late Archean age of the Ca Vịnh Complex) it is necessary to carry out further studies, supplementary analyses, especially on other samples of the complex.

This paper presents new results of U-Pb zircon isotopic age determination for two orthogneiss samples from the Ca Vịnh Complex by the SHRIMP (Sensitive High mass Resolution Ion MicroProbe) method. Together with published first results, these new results provide with reliable evidences for affirming the original age of the Ca Vịnh Complex, that still is an opened problem.

Zircon used in this study is beneficiated from 2 orthogneiss samples bearing the number RR03 (coordinates: 21^o37’17" N; 104^o46’58" E) and RR09 (coordinates: 21^o33’45" N; 104^o45’32" E) collected from the Ca Vịnh Complex. Recent studies on the geochemical and Nd isotopic characteristics of the two samples RR03 and RR09 have been giving the results of Nd model age of about 3400-3100 Ma, and the first zircon TIMS analysis has been giving the U-Pb age of 2800-2500 Ma [3]. Zircon from RR03 and RR09 was beneficiated at the National Taiwan University. Under the binocular microscopes, almost all zircon crystals are of short prism form of different sizes from 0.2 to 0.3 mm. The process of sample preparation and SHRIMP zircon analysis has been carried out in Japan (Tokyo and Hiroshima Universities). The detailed technique of sample preparation, SHRIMP U-Pb zircon analysis and calculation of experiments presented in this paper are completely similar to those presented in other recent papers [5, 6].

After the beneficiation zircon crystals subjected to analysis are sticked on an epoxy disk together with standard SL13 (of 572 Ma in age) and QGNG (1850 Ma in age) zircon grains. The probes are polished by diamond powder of 0.25 m m in size, then grinded to the middle part of the grain for exposing the central section of the analyzed zircon grain. After polishing the sample, the EPMA (Electron Probe MicroAnalyser) technique has been used for analyzing the zoning characteristics and interior structure of each zircon grain. This procedure allows to select the homogenous areas without any defect in each crystal for the SHRIMP U-Pb analysis. This analysis is realized for each different zoning "area" of each zircon crystal (surface of the analyzed point is of 20 m m in diameter).

The results of SHRIMP U-Th-Pb zircon analysis of the two samples RR03 and RR09 are presented in the Table 1. The results of calculation of isotopic age are presented in the Tera Wasserburg Diagram (Fig. 1).

The Figure 1 shows that almost all analyzed points have the age nearly coinciding with one another (distributed near the compatible curve) and are concentrated in the middle part between 2800 and 2900 Ma. The zircon sample RR03 (Fig. 1a) has 15 analyzed points giving the mean age value (weighted mean) of 2835 ± 6 Ma (error in the 2 sigmas level). The zircon sample RR09 (Fig. 1b) has 12 analyzed points giving the

Figure 1. Tera-Wasserburg Diagram for zircon of the Ca Vịnh Complex
(the curve determines the U-Pb age). a) Sample RR03; b) Sample RR09.
The mean age value (Ma) is the weight mean.

mean age value of 2843 ± 8 Ma. Three analyzed points of the sample RR03 and two points of the sample RR09 give the younger age value, which is the incompatible age (located far from the compatible curve); they have the overcut value of nearly 2400 and 2000 Ma (Fig. 1). These points of incompatible age reflect the influence of later thermo-tectonic phases (post-crystallization phases) impacting on the studied complex.

The basis of age determination for intrusive complexes consists of their geological relations with surrounding rocks, and the isotopic age of minerals and rocks existing in the complex. Geological observations carried out during the geological mapping work have been showing that granitoids of Xóm Giấu Complex penetrate and microclinize the rocks of the Ca Vịnh Complex [1], showing that the Ca Vịnh Complex is older than the Xóm Giấu one. Formerly, the radioactive age of the Ca Vịnh Complex has been usually cited as 2070-2300 Ma determined by the K-Ar method for hornblende from gabbro-amphibolite of the Bảo Hà Complex realized by Nguyễn Văn Đễ, Kopytsa and Phan Trường Thị, and of the Xóm Giấu Complex as 718-1369 Ma determined by the U-Pb monomineral cyrtolite method [1]. Recently, we have carried out thematic studies for clarifying the dating problem of the studied complex, that have been giving the age of syntectonic hornblende from orthogneiss of the Ca Vịnh Complex collected in the Hưng Khánh waterfall (Ca Vịnh Massif) determined by the K-Ar method as 1700 Ma [4], and by the Ar-Ar method on single crystalline grain as 2000 Ma [5]. These results of K-Ar and Ar-Ar age, in general, conform with the K-Ar age for hornblende of the Bảo Hà Complex (lying in the studied area and near the rocks of the Ca Vịnh Complex) by Nguyễn Văn Đễ, Kopytsa and Phan Trường Thị, affirming the presence of the highly metamorphic phase impacting on this complex during Paleoproterozoic, at the same time forecasting the possibility of older age of primitive formation of the complex. Recently, trying to determine the primitive age of this metamorphic complex, we have determined the radioactive age of zircon beneficiated from a rock sample of the Ca Vịnh Complex (with the number HK.22), and of zircon from a rock sample of the Xóm Giấu Complex (with the number HK.14), among which zircon of the Ca Vịnh Complex includes 3 generations giving the SHRIMP U-Pb age of 2900 Ma and 1960 Ma. The Xóm Giấu Complex is composed only of zircon crystals giving the age concentrated at 2260 Ma [4]. As presented above, we consider the 2900 Ma ancient core as reflecting the magmatic crystallization age of the Ca Vịnh Complex (primitive age of the rocks of the Ca Vịnh Complex). After the crystallization the rocks of the complex were involved into highly metamorphic processes (to the conditions of local fusion) which have been recognized by the development of the two younger zircon generations of 2360 and 1960 Ma. However, due to the superimposition of the ancient core (2900 Ma) by younger zircon generations (2360 and 1960 Ma), and the not great meeting frequency of ancient core in the sample (4 among 18 grains), there still is the second possibility explaining the presence of these ancient cores: the 2900 Ma ancient cores are inheriting ones. The inheriting cores are remnants of the Earth’s crust when the burnt magmatic massif of the Ca Vịnh Complex of 2360 Ma penetrated and carried them. According to this interpretation, the primitive age of the Ca Vịnh Complex lies in the 2360 Ma interval, and the 2900 Ma is the age of the Earth’s crust in the place where the burnt magmatic massif penetrated and was remained.

If we consider the 2900 Ma ancient core in the sample HK.22 as the inheriting core, its meeting frequency is very small, and the inheriting cores should have the age varying in a large interval. The new SHRIMP U-Pb zircon results presented in this paper let see that the two analyzed RR03 and RR09 samples having the U-Pb age as 2840 Ma, in general, conform with the result of 2900 Ma of the HK.22 zircon core. Synthesizing the analytic results of the three samples, the distribution of the age groups is presented in the Fig. 2. In this figure we can see three age groups through the analytic results: 2840-2900 Ma, 2400 Ma and about 2000 Ma. These three age groups correspond to three zircon generations observed in rock samples of the Ca Vịnh Complex [7], among them the ancient generation is most widespread. These results allow to determine the primitive age of the Ca Vịnh Complex as Late Archean, lying in the interval of about 2840-2900 Ma, excluding the possibility of above inheriting core. However, it is not clear that the magmatic activities of the Ca Vịnh Complex extended from 2900 Ma to about 2840 Ma, or it is the result of two or more phases?

The two younger age groups (2400 and 2000 Ma) exist both in the two RR03 and RR09 probes, although their meeting frequency is smaller than in the sample HK.22. These three samples were collected in different places in the Ca Vịnh Massif, and their metamorphic and deformational levels are not similar one with another. The two analyzed samples presented in this paper are less metamorphosed and deformed in comparing with the sample HK.22. The younger zircon generations have been interpreted as formed during the post-crystallization metamorphism, therefore the meeting frequency of young zircon generations in the two samples RR03 and RR09 being less than in the sample HK.22 is reasonable.

The results presented in this paper, together with those recently published [3, 7], let see that the ancient (2840-2900 Ma) zircon generation widespread in rock samples from the Ca Vịnh Complex affirms the authenticity of its Late Archean primitive age. From the published data, one can established the tectonothermal evolution of the Ca Vịnh Complex, as presented in the Figure 3.

Figure 3. Tectonothermal history of the Ca Vịnh Complex. Variance of one event
of cool down from the magma fluid is also presented for comparing
with the variance of thermotectonic multiple phases.

The figure 3 shows that magma of the Ca Vịnh Complex began to be crystallized since 2900-2840 Ma ago. This magmatic crystallization process is recognized by the widespread presence of zircon crystals of rather idiomorphic zoning structure having the U-Pb age from 2900 to 2849 Ma (samples RR03, RR09 and HK.22). Later, magmatic rocks of the complex were involved into the post-crystallization high metamorphism process happened in Paleoproterozoic, that is recognized by the presence of two zircon generations with the age of 2360 Ma and about 2000 Ma, some of them superimposed on the ancient core of 2900 Ma in age which was crystallized from the existed magma. These two post-crystallization metamorphic phases are still recognized by the development of syntectonic hornblende having the K-Ar and Ar-Ar ages of about 2000 Ma. Post-Paleoproterozoic metamorphic-deformational activities impacting on the complex has been recognized by the under-cut value of about 700 Ma on the compatible diagram of U-Pb zircon of analyzed probes (HK.22 and SH.14). A muscovite sample beneficiated from gneiss (BK.1106, collected near the sample SH.14) gave the K-Ar age as 770 Ma, conforming with this metamorphic phase. The later thermotectonic phases can be met in relation with the Indosinian Cycle (230-250 Ma) and the Yenshanian Cycle (Late Jurassic - Cretaceous). However, post-Paleoproterozoic metamorphism (after 2000 Ma) impacting on the complex took place in thermal conditions not surpassing 500^o C, that is the closed temperature for the K-Ar isotopic system.

The following conclusions can be drawn from above remarks:

1) The primitive age of the Ca Vịnh Complex is Late Archean, of about 2840-2900 Ma. The Ca Vịnh Complex is the first magmatic complex having reliable, authentic evidences on the oldest U-Pb isotopic age in the SE Asia.

2) There were at least two tectonothermal phases in highly metamorphic conditions impacting on the rocks of the Ca Vịnh Complex, that took place during Paleoproterozoic and were recognized by the presence and superimposition of two younger zircon generations in the rocks of the complex, having the age of about 2400 and 2000 Ma.

This work is completed in the framework of the Program of Basic Research (2002-2003) of the Commission of Natural Sciences (Ministry of Sciences, Technology and Environment).

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