GEOTHERMODYNAMIC FORMING CONDITIONS OF THE LATE JURASSIC - EARLY CRETACEOUS KRϡNG PHA GRANITOID TYPE IN THE SOUTHEAST MARGIN OF KON TUM GEOBLOCK

VŨ VĂN VẤN, PHAN LƯU ANH, TRẦN TUẤN ANH,
TRẦN HỒNG LAM, PHẠM THỊ DUNG

Institute of Geological Sciences, Hoàng Quốc Việt, Cầu Giấy, Hà Nội

Abstract: The new quantitative chemical, mineralogical, rare earth and rare element analysis data as well as the results of many years of research on the material composition, age, origin and forming conditions of the J₃-K₁ Krông Pha granitoid type located at the SE margin of the Kon Tum Geoblock have allowed to make some conclusions on the characteristics and geothermodynamic conditions of their formation as follows:

- The Krông Pha type granitoids are the products of continuous, prolonged differentiation of a magma fluids, creating a rock assemblage with the petrographic composition varying in the increasing direction of SiO₂ as follows: gabbrodiorite - quartz diorite - granodiorite - biotite-hornblende granite - granite. They have the same characteristics showing that they all belong to the normal petrochemical sequence and to the potassium-high calc-alkaline series.

- The Định Quán - Đèo Cả granitoids are of deep (mantle) origin from the magma fluid where their crystallization started at the temperature of 1000 - 1030^oC and ended at the temperature of 600^oC - 680^oC in the condition of vapour pressure of 1.4 - 4.3 kbar, at the depth of about 10 - 12 km.

- The Krông Pha type granitoids have typical features characterizing a type of magma of the active continental margin region (Andes type) generated during the period of plate collision (orogeny) lasting from Late Jurassic to Early Cretaceous in the process of forming the geologic structure in South Central Vi#t Nam .

The characteristics of Late Jurassic – Early Cretaceuos granitoids at the southeast margin of Kon Tum Geoblock has been studied by many Vietnamese and foreign geologists during the last half century. It is notable that on the side of foreign geologists, they are object of study frequently touched upon in the works by Saurin (1933, 1944, 1962, 1964), Fromaget (1961), Faure (1964), Lassere, Fontaine (1934, 1978), Lacroix (1970), Hoffet (1935). Among Vietnamese geologists, first of all mention should be made for the authors who have published a large amount of materials on geology, composition, age of granitoids in the area and related minerals since the 1975 year. These are the results obtained during the survey and compilation of the 1:500,000 geological map of South Việt Nam carried out by Geological party N⁰ 500 led by Nguyễn Xuân Bao. Then, there are the results of the compilation of the series of 1:200,000 scale geological maps (Bến Khế - Đồng Nai area) by the Geological Party No.20B led by Nguyễn Đức Thắng during 1987 - 1989. Besides, there are many deeply thematic research projects related to intrusive and extrusive magmatic formations in the region which have been published and kept in archives. Among them there are: Vũ Vấn Vắn, Nguyễn Việt ý (1985-1990); Nguyễn Kinh Quốc (1990); Nguyễn Việt Ý, Trần Trọng Hoà (1990); Huỳnh Trung, Nguyễn Xuân Bao (1991); Nguyễn T­ờng Tri et al. (1999); Bùi Minh Tâm et al. (2002); Vũ Nh­ Hùng (2002); Vũ Văn Vấn, Trần Hồng Lam (2000 - 2002).

However, it should be noted that among the above works the issues on geothermodynamic forming conditions of the magmatic intrusions in the studied area has not been paid due attention. Therefore, in order to clarify them, the existing materials [6.13] and new quantitative data obtained by SEM, ICP, INAA, XRF, etc. published below will be one of the basis for meeting to some extent the needs of data serving the discussion and understanding the geothermodynamic forming conditions of granitoids in the SE margin of Kon Tum Geoblock - the granitoid of Krông Pha type [7]

I. BASIC CHARACTERISTICS OF THE ROCKS IN THE KRϡNG PHA TYPE GRANITOID ASSEMBLAGE

The rocks of the studied Krông Pha type granitoid assemblage occur in the following intrusive formations: Định Quán, Krông Pha, Đèo Bảo Lộc, Đèo Cả, SW Nha Trang, Phan Rang.

These granitoids have been described in many geologic literatures as belonging to ##nh Quán Complex and Đèo Cả Complex [2,3,4], distributed in the area of Đà Lạt zone, in the SE margin of Kon Tum Geoblock, with a petrographic composition widely varying from gabbrodiorite to granite. Later, in some deeply thematic research works, they have been reclassified as Krông Pha type granitoid assemblage [7] or Định Quán - Đèo Cả Granitoid [3].

These granitoids are most concentrated in Khánh Hòa, Ninh Thuận, Bình Thuận and Đồng Nai areas. They form intrusive bodies of large size and complicated boundary. The most typical are the Định Quán, Krông Pha, Đèo Cả intrusive massifs. Based on the composition, structure, texture and geological relationship the studied granitoid formations can be classified into 3 intrusive phases and one vein rock phase as follows:

Phase 1: gabbrodiorite, diorite and quartz diorite; phase 2: granodiorite, biotite-hornblende granite of coarse to medium grain, porphyritic texture; phase 3: biotite-hornblende granite, granophyric granite, biotite granite, of fine grain and porphyritic texture; vein rock phase: porphyritic granodiorite, granite-aplite, granite-pegmatite.

According to their degree of distribution, the rocks of phase 2 occupy about 80 % of the exposed area of the studied granitoid massifs in the area, followed by those of phase 3, phase 1 and vein rock phase.

On the relationship with the country rocks, in many sites of field survey, the penetration of the Krông Pha type granitoids into terrigenous sedimentary rocks of the Lower-Middle Jurassic Bản Đôn Group (J_1-2 bd) has been observed. In the internal contact zone the granitoids are altered with the development of sericitization, feldspathization, quartzification with the usual increase of chromatic minerals. In the external contact zone the sedimentary rocks are subjected to thermal alteration into hornfels with elongate zones of unstable width.

Besides, in the granitoids are met relatively abundant pocket-like and vein-like pegmatite bodies and xenoliths. They are multiform, diverse in size, and of very relatively complicated composition. The pegmatite veins and pockets are composed of feldspar, quartz, biotite, some hornblende; in some places they contain molybdenite and iron sulphide. The xenoliths are mostly of sedimentary rocks, although their boundaries are still remained, most of minerals in them were altered and recrystallized Some xenoliths are originally magmatic rocks; they are less altered, with the composition corresponding to diorite, granodiorite and felsite.

The results of analysis and geological researches on the petrography of granitoids [6, 7, 9-13] point out that the intrusive massifs are formed from many rock varieties with continuously changing petrographic composition, forming the rock series: gabbrodiorite - quartz diorite - granodiorite - biotite-hornblende granite - granite.

The new results of chemical analysis of biotite and amphibole by SEM are shown in Tables 2, 3. These are two closely associated minerals in all rock varieties in the Krông Pha type assemblage. The chemical analysis of biotite shows that it has the following specific characteristics: high in magnesium, high in titanium, low in aluminium and relatively low in iron content. The MgO content in biotite varies between 9.04 - 12.4%, the iron content varies between 40.6 - 60, while the aluminium content (L) lies mainly within the limit of the biotites having medium or low aluminium content (L = 16.0 - 21.3). The chemical composition of amphibole indicates that it is rich in calcium (10.74 - 11.26%). According to its graphic representation, it belongs to the hornblende field (Dodge et al. 1968). The MgO content in amphibole varies between 9.7 - 11.66%, Al₂O₃ = 5.67 - 8.32% and here there is a difference that the hornblende is relatively rich in iron: FeO = 15.5 - 20%. In general the specific characteristics on titanium, iron, aluminium contents as well as the excessive aluminium content in biotite in the studied granitoids are nearly conformable with the average characteristics of biotite in biotite-amphibole granitoid assemblages (Usakova, 1980; Panek, 1982; Distanova, 1990).

The chemical composition of granitoids are shown in the diagram of Fig. 1 and their average representative values are shown in Table 1. The general analysis of these data shows that in the classification diagram they have been recognised to have their composition similar to that in the results of petrographic study, that in the Krông Pha type granitoid assemblage are present mainly the rocks from granodiorite to granite and slightly alkaline granite. In composition and content, the variable chemical elements found in them indicate that in the direction of increasing the quartz content in the studied rock assemblages there is a variation in chemical composition reflecting that they are the products of a continuous and prolonged differentiation from a magmatic source: the SiO₂ content evenly increases from ~ 60% to 75%, the total alkali (Na₂O + K₂O) from ~ 6% to 10%, and on the contrary, the Al₂O₃, å FeO and CaO contents steadily decrease: Al₂O₃ from 16 to 12%, å FeO from 6 to 1% and CaO from 4 to <1%.

In general, the Krông Pha type granitoids have the typical characteristics of a normal petrochemical sequence belonging to the calc-alkaline series [13].

Geochemical characteristics of some rare elements: in the direction of increasing SiO₂ content it is noticed that the Rb increases (» 100 - 400 g/T) and the K/Rb ratio decreases (300 - 100 g/T), while Ba, Sr decrease (300 - 70 g/T) and Rb, Li increase. In particular, the Rb, Li contents in diorite are relatively low (Li: 23 g/T, Rb: 90 g/T) and high in granite (Li: 403 g/T, Rb: 300 g/T). Respectively their Rb/Sr ratio increases from 0.5 to >2, in exceptional cases up to 8.0 and Ba/Rb ratio decreases from 3.7 to 0.3 [13].

Table 1. Average chemical composition of representative rocks of the Krông Pha type granitoid assemblage

Name of rock	Number of samples	Massif	SiO2	TiO2	Al2O3	Fe2O3	FeO	MnO	MgO	CaO	Na2O	K2O	LOI	P2O5	Total
Granodiorite	11	Krông Pha	65.71	0.48	15.46	1.20	3.86	0.07	1.70	3.67	3.11	3.49	1.21	0.12	99.74
Bi-Hb granite	4	Krông Pha	67.63	0.42	14.96	1.14	2.56	0.08	1.41	3.33	2.96	3.69	1.27	0.15	99.59
Qtz diorite	4	Krông Pha	65.17	0.53	15.68	1.00	3.10	0.08	1.67	4.04	2.99	3.44	1.23	0.12	99.04
Bi-Hb granite	7	Định Quán	69.32	0.37	14.30	0.67	2.85	0.06	1.20	2.81	2.95	3.80	0.92	0.12	99.37
Bi granite	2	Định Quán	75.28	0.08	13.11	1.04	1.13	0.02	0.50	0.85	2.25	5.50	0.72	0.30	100.78
Granite	2	Định Quán	75.99	0.05	12.34	0.30	1.50	0.02	0.30	0.71	2.82	5.83	1.00	0.03	100.87
Granodiorite	4	Định Quán	64.32	0.57	15.72	2.95	4.63	0.09	2.06	4.08	3.19	3.41	0.99	0.15	99.83
Diorite	7	Đèo Cả	62.45	0.64	15.98	3.09	2.80	0.16	1.95	4.30	3.21	3.07	0.84	0.21	98.58
Granodiorite	8	Đèo Cả	67.43	0.57	15.08	2.63	2.29	0.08	1.35	3.11	3.47	4.03	0.41	0.15	99.74
Bi granite	7	Đèo Cả	70.93	0.35	14.26	1.38	1.14	0.16	0.88	2.40	3.43	4.44	0.52	0.11	99.86
Granite	4	Đèo Cả	74.33	0.16	13.00	1.03	1.09	0.08	0.53	1.07	3.23	4.81	0.47	0.05	99.68
Bi-Hb granite	7	Nha Trang City	68,28	0, 50	15,19	1,48	1,98	0,17	0,82	3,36	3,39	4,59	0,20	0,03	99,97
Granite	9	Phan Rang	74,04	0,35	12,22	0 ,75	0,13	0,70	0,19	0,93	4,42	5,03	0,01	0,12	98.89

 

 

By integrating the results of isotope dating [Nguyễn Xuân Bao et al, 2000, in Archives] it is to note that the studied granitoids are of common age from 140 to 82 Ma (K/Ar), from 108 to 109 Ma (Rb-Sr) and 80 - 108 Ma (Ar-Ar). Based on the reliability of the isotope analysis method and comparing it with the ages of magmatic rocks of the same type in the SE Asia region as well as in South China, it is appropriate to attribute the Krông Pha type granitoid assemblage to the Late Jurassic - Early Cretaceous age.

II. GEOTHERMODYNAMIC FORMING CONDITIONS OF THE KRϡNG PHA TYPE GRANITOIDS

For the petrologic purpose, it is not sufficient only to know the parameters deciding the generation of the magma, but it requires also to have information on the crystallization conditions of the magma formed under certain geologic settings. On this basis, we can solve some problems related to the genesis, depth, temperature and pressure of formation, degree of denudation, etc. of specific intrusive massifs. In order to solve these problems, in the seventies of the XX century some geologists have carried out theoretical and experimental researches and have set forth the method for calculating the temperature and pressure of generation and crystallization of magma based on the chemico-mineral and chemical composition of rocks. Among these, from the former Soviet Union school there are L. L. Perchuk (1973, 1976) and V. A. Rudnik G. M. Belaev (1978). In recent years, based on great achievements obtained from both theoretical and experimental domains and with more and more improved means of quantitative analysis, the study of geothermodynamic conditions on the formation of magmatic rocks have been more inclined to isotopic analysis, rare/trace and rare earth elements (M. Wilson, 1989).

Aiming to understand the geothermodynamic forming conditions of the Krông Pha type granitoids, the authors have used integratedly the above calculation methods. The basis of these methods may be summazised as follows:

L. L. Perchuk has used the chemomineralogic composition on the basis of the phase correlation principle of mineral couples in the calculation to serve the determination of the magmatic crystallization temperature. The geothermodynamic basis of this principle is that a part of the geothermodynamic conditions of the phases change. As we know, in the exchange reactions the distribution of cations and anions follow Sobolev and Ramberg rules, i. e. the reaction will move to the formation of compounds of strong acids and strong bases and those of weak acids and weak bases. On this basis, by many results of theoretical and experimental researches, L. L. Perchuk has come to a general phase correlation principle as follows: if there are two isomorphic cations different in negative charge, in given conditions the more dependenct of their migration among paragenetic silicates on the temperature happens when there is the more different in acidity of the radicals and in crystallographic system of those minerals.

In order to evaluate the distribution of cations in the paragenetic minerals, one uses the coefficient Kd which reflects the dependence on temperature and pressure conditions. Based on this dependence and through experiments, various geothermometers (Am - Bi, Py - Am, Pi - Bi, Bi - Gr, etc.) and geobarometers (Cor - Gr - Sil - Qz, Am - Pl, etc.) have been developed depending on the paragenetic mineral couples.

Inheriting the above achievements, Rudnik and Belaev on the basis of the chemical composition of rocks have set forth a method for calculation of PH_2O (water vapour pressure) and Tmin (crystallization temperature) based on the interaction between the PH_2O/PS ratio (geostatic pressure) and Tmin calculated by L. L. Perchuk for the solidus of the physico-chemical system corresponding to the melting of eutectic granites.

The forming temperature-pressure parameters of Krông Pha type granitoids are extracted from [6] and Fig. 2; the results newly analyzed and calculated for this paper are shown in Tables 2, 3, 4, 5.

As regards the geothermodynamic conditions of the granitoids of Định Quán and Krông Pha massifs, on the basis of calculation from the new chemomineralogic analysis of Bi-Am geothermometer and comparison with the curves of L. L. Perchuk (1976) their crystallization temperature has been determined as varying within 600^oC - 750^oC (Table 4, Fig. 2). By calculations based on the chemical composition of the rocks, this range of temperature has been proved to be appropriate for crystallization and solidification temperature of the Định Quán and Krông Pha intrusions. Besides, it is also allowed to recognise some characteristics of the geothermodynamic conditions in the formation and crystallization of these intrusions as follows:

The Định Quán - Krông Pha granitoids were formed from a same magma source. The crystallization was started to form the rocks of Định Quán - Krông Pha granitoids at the geostatic pressure of about 6 kbar (corresponding to the depth of 18 km), the vapour pressure of about 4.3 kbar and the temperature of 1030^oC and was terminated at the temperature of 650^oC.

Table 2. Mineral composition and crystallochemical coefficient of amphibole in the rocks of Krông Pha type granitoid assemblage

 

Table 3. Mineral composition and crystallochemical coefficient of biotite in the rocks
of Krông Pha type granitoid assemblage

+ The results in Tables 2, 3 are of the research project DTDL-2003/0 ; analyzed in the Novosibirsk Geological - Geophysical Instituet, Russian Federation, by the SEM method. For the symbols and rock names see Table 2.

The vapour pressure variation curve throughout the crystallization process of Định Quán - Krông Pha magma shows that the vapour pressure varied from 0.3 to 4.3 kbar. This is conforms with the results of petrographic researches: the more and more frequent apparition and eventual domination of minerals containing the OH group (Am, Bi), in the mean time there is a decrease and absence of pyroxenes in the rocks from the lower part (gabbro-diorite) to the upper part (biotite-hornblende granite). This proves that the role of PH_2O becomes greater in the last stage of the forming process of Định Quán Granitoid.

The Đèo Cả granitoid and the rock massifs with similar composition: The results of calculation in Table 4, Fig 1. allow to make the following main remarks: the crystallization of the magma began at the temperature approximating 1000^oC, vapour pressure: 0.3 kbar and terminated at 680^oC (approximating the results of calculation for the Bi-Am geothermometer), the vapour pressure about 1.4 kbar (corresponding to the depth of 10-12 km).

Detailed analysis of the thermodynamic parameters shows that the granitoid of Đèo Cả, SW Nha Trang massifs and the Phan Rang granitoid were formed in various conditions: the forming pressure and temperature of granitoids from Đèo Cả to Phan Rang tends to decrease with the transition from each other, reflecting partially the stage of magmatic evolution corresponding to the gradual migration of the magma to a higher position in the earth crust with its crystallization at the depth of 10-12 km.

 

 Table 4. Crystallization temperature of Krông Pha type granitoids
(on the basis of Bi-Am geothermometer)

(Analytical data of the research project DTDL-2003/07, by Neutron activation method at the Novosibirsk of Geory and Mineralogy, Russian Federation)

An integrated study of a relationship between the Krông Pha type granitoids through the variation of the thermodynamic parameters P and T throughout the crystallization process of Định Quán - Đèo Cả magma has been showing that the studied granitoids have undergone the differentiated crystallization from a same source, and it is possible that the first products are the basic-intermediate rocks (Định Quán and Krông Pha massifs) and the final products are intermediate alkali-high and potassium-high basic-acidic rocks (Đèo Cả, SW Nha Trang and Phan Rang massifs).

 Table 5. Results of rare and trace elements from the Krông Pha type granitoids (ppm)

(Analytical data of the research project DTDL-2003/07, by Neutron activation method at the Novosibirsk Institute of Geology and Mineralogy, Russian Federation)

The above judgements about geothermodynamic forming conditions of the Định Quán - Đèo Cả granitoids have been confirmed once more through quantitative analyzes of isotopes and rare-earth elements. In particular, by analyzing the variation in content of some rare elements (Table 5), [13] such as Li, Rb, Ba, Sr it is to remark that the Định Quán and Đèo Cả granitoids have some common characteristics: they both reflect the differentiation in magmatic crystallization, and are similar in origin of formation. From early crystallizing rocks (diorite) to late ones (granite), they reflect a trend of increase in content of some elements such as Li, Rb, Zr, and vice-versa for Ba, Sr with increased K/Rb, Ba/Sr ratios and decreased Rb/Sr ratio.

By analyzing the isotope and rare earth data it is to remark that the intrusive rocks have low original isotope ratios ⁸⁷Sr/⁸⁶Sr = 0.703 - 0.706 [8], nearly equal those of original basalt [14]. The distribution curves of rare earth elements [13] are inclined lines, with strong decrease from light rare earth elements to the heavy ones, with a clear negative anomaly of Eu (Fig. 3), corresponding to the rocks of deep origin – from the mantle, and with evidences of containing crust materials (Rb/Sr: 0.5 - 3) formed during the Yenshan orogeny (J - K).

The tectonic setting of the formation of the Krông Pha type granitoids have been studied thoroughly in various works [2,8,12,13] and it has been agreed that they are the magmatic intrusions of an active continental margin region, existing from Late Jurassic to Early Cretaceous. They are calc-alkaline, potassium-high rocks with the tendency of increasing potassium towards the west of the territory, reflecting that the studied magmatic intrusions belong to the posterior zone of this active continental margin.

CONCLUSIONS

The above results of researches on the material composition, origin and geothermodynamic forming conditions of Late Jurassic – Early Cretaceous granitoids of Krông Pha type allow to make the following main conclusions:

- The Krông Pha type granitoids are the products of continuous prolonged differentiation of a magma fluid, creating a rock assemblage with the petrographic composition varying in the increasing direction of SiO₂ as follows: gabbrodiorite - quartz diorite - granodiorite - biotite-hornblende granite - granite. They have the same characteristics showing that they all belong to a normal petrochemical sequence and to the potassium-high calc-alkaline series.

- The Định Quán - Đèo Cả granitoids are of deep (mantle) origin from the magma body where their crystallization started at the temperature of 1000 - 1030^oC and ended at the temperature of 600^oC - 680^oC in the condition of vapour pressure of 1.4 - 4.3 kbar, at the depth of about 10 - 12 km.

- The Krông Pha type granitoids have the typical characteristics of products of the magmatic activities in an active continental margin (Andes type) related to the plate collision-orogeny process lasting from Late Jurassic to Early Cretaceous during the formation of the Đà Lạt - Nha Trang folded structure.

This paper is compiled on the basis of data from the research project DL-2003/7 and the Fundamental Research Project No 71.31.04.

Abbreviations in the paper: Bi - biotite; Am - amphibole; K.fsp - potassium feldspar; Pl - plagioclase; Py - pyroxene; Gr - garnet; Qz - quartz; Cor - cordierite; Sil - sillimanite; Hb - hornblende.

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