Yê Yên Sun Granitoid in West Bắc Bộ has been considered as a complex consisting only of one massif, but its composition is not simple with: granodiorite, biotite granite, granosyenite and alkaline feldspar granite [2], among them biotite granites role. With a big size as the Yê Yên Sun Massif (about 1000 km²) this diversity in composition is easy to understand. However, if they belong to a complex there must have a given common feature which, first of all, expresses in characteristics of material composition showing that they were formed from a same magmatic source in a same tectonic setting. Up to now, there still are different opinions on the nature of the Yê Yên Sun Granite, such as: a) products of calc-alkaline magmatism [2]; b) subalkaline formation of intraplate magmatism [1]. Specially, according to [2] the studied granite belongs to the abyssal plutonic facies of the Phu Sa Phìn Granite and is related in origin to the Tú Lệ subalkaline rhyodacite-rhyolite.

At the end of the 2001 year, the authors of this paper have realized a field trip along the Highway No.4d from Sa Pa (Lào Cai) to Bình Lư (Lai Châu) cutting transversely the exposure of the central part of the Yê Yên Sun Massif. The analysis results of the collected samples has been showing that granite in this area belongs to two types: 1) the one having the intermediate composition between S- and I-granite (mainly of I-granite character); 2) the one corresponds to the A-granite type. Naturally, a question has been posed before us: what petrological model can explain the magmatic origin of two different granite types which were assigned to a same complex and, even, drawn as a same intrusive massif on the map? In this paper, we will present the newest analysis results on the geochemistry of rare and trace elements of samples from our above said field trip, aiming to clarify the above problem.

The chemical composition of biotite was analyzed by Camebax-Micro microsonde of the Institute of Geology, Novosibirsk (Russia). The major rock-making oxides were analyzed by X-ray fluorescence method, trace and rare earth elements ICP-MS – by HP-Argilent instrument of the Dept of Geology, Taipei University (Taiwan).

The results of sample analysis and investigation on the Sa Pa - Bình Lư Section have been allowing to distinctly distinguish the exposing area of the two granite types (Fig.1). Accidentally, their contact has been found near the top of the Ô Quy Hồ Pass, which is also the boundary between the two Lai Châu and Lào Cai provinces. The relatively leucocratic, fine-grained granite is exposed on the west of the pass (in the Lai Châu side), while relatively more melanocratic, medium-grained biotite granite - on the east of the pass (in the Lào Cai side). Furtherly, they will be called as the west granite and the east granite for convinient description. Regrettably, the direct contact between these two granites has not been observed yet. However, in the east granite there have been met the veins of leucocratic aplitic granite or quartz veins containing molybdenite mineralization which may be the late products of the west granite. This can prove that the west granite was formed later than the east one.

The west granite is fairly homogeneous, consisting mainly of leucocratic, biotite-bearing granite (sometimes, containing hornblende), small- or fine-grained of the aplite type. In general, the rock is massive, relatively homogeneous or weakly compressed and having directive structure (Photos 1 & 2); only near or just in the faulted area it is compressed and strongly mylonitized.

Mineral composition: Q+Fsp+Pl+Bi (a little) ± Hbl+Sphene+Fluorite. Biotite consists usually of greenish (weakly chloritized) or light brown-green small flakes. The chemical composition of biotite (Table 1) is characterized by the titanium-low (TiO₂ = 0.83-1.10%), relatively aluminium-low (Al₂O₃ = 13.71-14.92%), high magnesium (MgO = 11.83-13.10%) properties, therefore the mineral bears the intermediate character between biotite of high aluminium granite and that of typical calc-alkaline granite [7].

Table 1. Chemical composition of biotite from granite in the Ô Quy Hồ Pass

Note: V-9274 - biotite granite from the west of the Ô Quy Hồ Pass; V-9263 - biotite granite from the east of this pass.

The east granite is mainly small- to medium-grained, banded by pressure, strongly foliated and, in comparing with the west granite, more melanocratic (biotite content usually reaches 5-10%). The foliation expresses itself distinctly with the directive arrangement of elongated minerals, such as: kalifeldspath and biotite (Photos 3 & 4). On this side the studied rock has a fairly similar banded and foliated type with that of alkaline granite of the Mường Hum Complex. Differing from the west granite, the east granite does not contain amphibole and is richer in kalifeldspath and sphene. Biotite is usually green-brown coloured, with the composition relatively high in titanium (TiO₂ = 2.41%), very low in aluminium (Al₂O₃ = 9.27%), relatively low in iron (FeO = 15.91%) and fairly high in magnesium (MgO = 13.1%). This biotite type is usually met in granite of high alkalinity [7].

The analysis results of chemical composition and content of rare and trace elements of the two studied granites are presented in the Table 2. This table shows that, basing only on some main characteristics: content of Al₂O₃, Fe₂O₃, CaO, Na₂O and K₂O, one can distinctly distinguish them. Based on the K₂O-SiO₂ correlation, the two west and east granites correspond to the high potassium calc-alkaline series, similar to the Phu Sa Phìn Granite and Tú Lệ Rhyolite (Fig. 2), but the west granite is characterized by its high aluminium content (Al₂O₃ = 14.89-16.56%), relatively low in iron (Fe₂O₃ = O.19-1.33%), high in calcium (CaO = 1.45-1.75%) and lower in potassium (K₂O = 4.03-4.72%) in comparing with the east granite (Table 1). According to the ANK-ACNK correlation (Fig. 3) the studied rocks belong to the group of saturated in aluminium granite, but still not to the true per-aluminium granite yet. The west leucocratic granite tends to be close to the richer in aluminium rocks.

On the characteristics of content of large ion lithophile elements (LILE), the west granite is relatively rich in Rb (135-163 ppm), very rich in Sr 216-1447 ppm) and Ba (481-3319 ppm); according to the ratios K/Rb = 178-289; Rb/Sr = 0.1-0.86; Rb/Ba = 0.04-0.45 they are closer to I-granite type than S-granite type. In comparison with the west granite, the east granite is relatively rich in Rb (201-219 ppm), very poor in Sr (26-47 ppm) and Ba (109-349 ppm); the ratios K/Rb = 229-232; Rb/Sr = 4.5-8.3; Rb/Ba = 0.62-1.2 fairly conform to those of the A-granite (according to Whalen J.B. et al [10], in the average composition of A-granite the ratios K/Rb = 229; Rb/Sr = 3.52; Rb/Ba = 0.48), showing that this granite could be formed from a magmatic melt of enriched mantle origin or influenced by mantle materials. The characteristics on lithophile elements of the east granite is completely close to the geochemical features of subalkaline granite of the Phu Sa Phìn type and subalkaline rhyodacite, rhyolite of the Tú Lệ type in West Bắc Bộ, and a little lower than alkaline granite of the Mường Hum type (Fig. 4) in the east of the Phan Si Pan Range, the adjoining area with the Red River tectonic shear zone.

About the characteristics on high field strenght elements (HFSE), the difference between the two granite types most distinctly expresses itself in the content of Nb, Zr, Y, Ta and Hf. The peculiarity of their content (Table 1) and their distribution on the PM - normalized (PM) diagram (Fig. 4) shows that: the west granite is distinctly poorer in Nb (7.1-17.9 ppm), Ta (0.45-1.59 ppm), Zr (92-178 ppm), Y (6-11.58 ppm), Hf (3-5.3 ppm) than the east granite (Nb = 94.12-167.7 ppm; Ta = 6.86-12.98 ppm; Zr = 728.7-1013 ppm; Y = 64.37-97.52 ppm; Hf = 20.3-28.1 ppm). The negative anomalies of Nb, Ta and Ti in the west granite is close to the rocks of high potassium calc-alkaline series which are formed in magmatic activities related to the subduction zone. On the contrary, the east granite, based on the distributive characteristics of Nb, Zr, Hf, Ta and Y, is completely close to the A-granite type formed in the intraplate setting [11], similar to the Jurassic-Cretaceous Tú Lệ Volcanites and Phu Sa Phìn Granite in the Tú Lệ Depression (Fig. 4).

The studied granites are different from one another on the content and distributive characteristic of rare earth elements (REE) (Table 2, Fig. 5). If the east granite is very rich in REE, especially the light rare earth elements (LREE) with the ratio (La/Yb)_N = 8.65-12.88, the west granite is relatively poorer in REE with the strongly changing ratio (La/Yb)_N: 5.38-41.46. The east granite is characterized by strong negative Eu anomaly (Eu/Eu* = 0.11-0.30), while the west granite has a positive Eu anomaly or weak negative Eu anomaly (Eu/Eu* = 0.62-1.30). The absence of negative Eu anomaly and the relative richness in HREE is a particular feature of the west granite, that needs further study for clarifying the problem.

Table 2. Major (% of weight) and trace elements (ppm) of the Ô Quy Hồ Granite

On the FeO*/MgO – (Zr+Nb+Ce+Y) and (Na₂O + K₂O) / CaO - (Zr+Nb+Ce+Y) discriminate diagram of Whalen J.B. et al [10] the componential points of the west granite coincide with the granite field of I-, S- and M-type, while the those of the east granite coincide with the A-granite (Fig.6). They are completely close to subalkaline effusives-intrusives rocks of the Tú Lệ Depression as well as to the Mường Hum Granite, that are typical representatives of intraplate magmatism.

From the above stated things, one can come to following judgement: there are two granite types different in composition in the Sa Pa - Bình Lư section cutting through the centre of the Yê Yên Sun Massif. The granite exposed in the west of the Ô Quy Hồ Pass bears the characteristics of high-K calc-alkaline series of I-granite type, while that exposed in the east has the composition completely corresponding to subalkaline or alkaline rocks of A-granite type. Even, in comparison with the average composition of A-granite (after Whalen [10]) the studied rocks are richer in Rb, Zr, Nb and Ce (according to Whalen the A-granite contains in average: Rb = 169; Zr = 528; Nb = 37; Ce = 137). If the evolution of granite goes from the east westwards, it will be difficult to explain the formation of the west granite by the process of differentiation or mixed assimilation of the primary magma, which was, in the case of the subalkaline east granite, melted from plutonic substance with the composition corresponding to the enriched mantle. Furthermore, in the case of differentiation and mixed assimilation the intermediate products are widespread, and the geochemical characteristics of the whole differentiation series will have the successive character. In the studied case, the rocks have a distinct contrast character. On the Nb/U-Nb correlation diagram the position of the east granite as well as of the Phu Sa Phìn and Mường Hum granites tends to be close to the enriched mantle source of the oceanic island basalt type – intraplate magmatic activity type, while the west granite has a composition close to that of the continental crust (Fig.7). Based on the Th/Ta-La/Yb correlation the studied rocks have been divided into 2 distinct groups with different evolutional tendencies: the west granite has the source close to the continental crust type, while the east granite, as well as the Phu Sa Phìn Granite, the Tú Lệ rhyodacite, rhyolite, and granite of the Mường Hum type, is completely close to the enriched mantle source (EM₂ and EM₁) (Fig.8).

Basing on the high potassium, rich in Rb, Zr, Nb,Ta and rare earth elements characters of intraplate magmatic activities, we could come to the judgement that the granite exposed in the east of the Ô Quy Hồ Pass belongs to the Phu Sa Phìn Complex. The study on the Nd isotope character of this A-granite has been showing that its ẻ Nd(0) value (-3.41 á -2.34) is fairly close to that of the Tú Lệ rhyodacite-rhyolite and the Phu Sa Phìn A-granite (-1.70 á -2.81) [4], and characterizes the products of plutonic origin. The TDM age of the east granite is similar to that of subalkaline rhyodacite, rhyolite and granite of the Tú Lệ Depression (0.67-1.07 Ga.).

Thus, if the subalkaline east granite is excluded from the Yê Yên Sun Complex the remaining content of this complex will consist only of granite (possibly, even granodiorite and granosyenite) and leucogranite bearing the geochemical characteristics of high-K calc-alkaline series. The mantle source from which this magma is formed consists, possibly, of lower crust origin. Judging the low content of Nb, Ta and Hf, the absence of negative Eu anomaly we can suppose that this source material has the composition close to that of calc-alkaline granite of the Điện Biên or Pò Sen types in West Bắc Bộ, that means the materials formed from subduction activities occurring in the previous geodynamic history. The study on isotopic characteristics of the west granite is being carried out with the hope to make clearer this problem.

The determination of the geodynamic settings, in which the studied magmatic rocks were formed, based on the Pearce’s (1984) discrimination diagram following the Rb-(Y+Nb) and Nb-Y correlations, has been showing that the west granite belongs to the volcanic arc or syn-collision magma type (Fig.9a, b), while the east granite conforms to the character of products from intraplate magmatic activities. In fact, analyzing the tectonic setting of SE Asia at the end of Mesozoic – beginning of Cenozoic we can see that the Late Cretaceous - Paleogene magmatic activities belong all to the intraplate regime. Therefore, it is impossible to interprete the formation of the west granite as products of subduction activity in the plate margin setting. In the intraplate extension, in the West Bắc Bộ territory only the Pu Sam Cáp ultrapotassic complex was formed [8].

According to the geological and isotopic datings, the west granite was formed at the end of Cretaceous - beginning of Paleogene, corresponding to 72-41 Ma. [6], that means this event coincides with the Eurasia-India collision, before the occurrence of dextral strike-slip displacement along the Ailaoshan - Red River Fault Zone (27-22 Ma.) [3]. Therefore, the appearance of granite and leucogranite with the characteristics of high-K calc-alkaline rocks related to the compressional mechanism as the west granite can be interpreted as the result of the Eurasia-India collision activity trasmitted from the west through the Điện Biên - Lai Châu Fault Zone. The Yê Yên Sun Granite (after the viewpoint of authors of this paper) is considered as the product of post-collision magmatic activities, and does not belong to the plutonic facies of the Phu Sa Phìn Granite. This event occurs, possibly, before extension leading to the formation of a series of mafic, intermediate and felsic-alkaline rocks (lamproite, lamprophyre, trachyte and syenite) in the Pu Sam Cáp Mt area situated in the boundary between the Sông Đŕ Intracontinental Rift and the Phan Si Pan Uplift.

Related to the strong Oligocene dextral strike-slip (27-22 Ma.) there was the formation of leucocratic granite and pegmatite of anatectic origin as well as biotite granite and leucocratic granite of palingenic origin along the Red River Fault Zone and adjacent areas [9]. These two above granite types in the Red River Zone differ from the biotite granite and leucocratic granite of the Yê Yên Sun Complex. Therefore, based on the characteristics of their composition and position in the geological development history of West Bắc Bộ in Late Cretaceous - Paleogene (72-41 Ma.) the judgement on the nature of Yê Yên Sun biotite and leucocratic granites as products of local collision movement on the general background of the Eurasia-India collision is completely reasonable.

Because of the lack of references, the authors cannnot correlate the Yê Yên Sun Granite with the similar rocks distributed in the Yunnan territory, China. According to Zhang and Xie (1995) Jurassic-Cretaceous granites in the west of Yunnan have mainly the composition corresponding to S-granite (according to [4]) while the A-granite is really widespread in Paleogene. The high potassium alkaline granite, rich in Nb, Ta, Zr and rare earth elements as that in the east of the Ô Quy Hồ Pass and Phu Sa Phìn Granite has not been found in the description of the Upper Cretaceous volcano-plutonic belt in Southeast China. According to Li X.H. [5], in this area the Late Cretaceous A-granite is usually related in space with granite of the calc-alkaline series (I-granite). The content of Nb (23.5-47 ppm), Ta (2.11-3.27 ppm), Zr (203-633 ppm) and REE of this granite is relatively rich, but very lower (eg. La = 30.5-74.9 ppm) in comparing with granite in east Ô Quy Hồ and Phu Sa Phìn in West Bắc Bộ. The magmatic source of subvolcanic and intrusive rocks of the Tú Lệ, Phu Sa Phìn and east Ô Quy Hồ types, possibly, consists of subalkaline basalt of intraplate type, that is in this case the Permian-Triassic basalt of the Emeishan type (China) or Viên Nam type in West Bắc Bộ (Việt Nam). The tectonic nature and magmatic activity origin of the Tú Lệ Zone will be touched upon in another paper.

The result of detailed analysis of geochemical characteristics of major and trace elements (including rare earth elements) of biotite granite and leucocratic granite of the Yê Yên Sun Complex distributed in the east and the west of the Ô Quy Hồ Pass has been allowing to distinguish two different in origin granite types: leucocratic granite with the high-K calc-alkaline character of I-granite type in the west, and subalkaline biotite granite of A-granite type in the east.

The subalkaline biotite granite in the east, based on the following characters: rich in potassium, in Rb, Zr, Nb, Ta and rare earth Th, U elements, poor in Sr, Ba, is completely close to the subalkaline granite of the Phu Sa Phìn Complex, and bears the characteristics of intraplate magmatic activity. It needs to be separated and, according to authors of the paper, considered as products of magmatic activity of the Phu Sa Phìn type.

The leucocratic granite in the west of the pass is characterized by its relatively high alkalinity of the potassium-sodium type; it is rich in Rb, Sr, Ba and relatively poor in Nb, Ta, Hf, Zr, bearing complex characters of magmatic activities of continental arc or after collision. Correlating to the Paleogene geodynamic settings of West Bắc Bộ, we can consider this granite as post-collision product related to compressional movement caused by the Eurasia-India collision. This new judgement on granite of the Yê Yên Sun Complex needs further detailed studies on isotopes in association with isotopic dating for clarifying the problem.

The nature determination of the Yê Yên Sun Granite consists of the premise that allows to evaluate the perspective of Cu-Mo, maybe also Au, mineralizations, related to magmatic activities of this type.

Acknowledgements. The authors would like to express their thanks to Dr. Bùi ấn Niên and Prof. Dr. Andrzej Zelazniewicz for their collaboration in field investigation, to M.S. Trần Việt Anh for his help in the data processing and figure presentation.

The paper is completed with the financial support of the Project 750.401 of the Basic Scientific Research Programme.

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