THE GENESIS AND FORMING CONDITIONS
OF GRANITOIDS IN THE ĐÀ LẠT ZONE

NGUYỄN THỊ BÍCH THỦY

Scientific Institute of Geology and Mineral Resources,
Thanh Xuân, Hà Nội.

Abstract: Numerous bodies of granitoids and volcanic rocks exposed in the Đà Lạt zone are subduction-related products. The granitoids belong to an Andean-type arc in the west of Pacific region extending from SE China through South Việt Nam to SW Borneo. Based on the studies on petrography, mineralogy as well as geochemistry, they are subdivided into three complexes: 1) The Định Quán, 2) Đèo Cả, and 3) Cà Ná ones. Based on the major and trace element concentrations, these rocks belong to the high-K calc-alkaline series. Most of them display typical features of I-type granites (e.g., ASIs <1.1, normative corundum <1% and d¹⁸O <10‰). The Định Quán Complex consists of hornblende-biotite-bearing granodiorites, diorites, and minor granites. Whereas the Cà Ná Complex contains biotite-bearing granites, but poor in hornblende. The Đèo Cả Complex is made up of pink porphyritic hornblende-biotite-bearing granodiorites, monzogranites and minor diorites.

Based on the chemical concentration of hornblende separated from granitoid samples, the pressure and depth of granitoid formation are calculated as follows: 2.7 – 2.2 kbar for the Định Quán granitoid corresponding to a depth of about 9.7 - 7.9 km, 2.5 kbar for Cà Ná granite with an equivalent depth of 9 km; and 0.8 to 2.2 kbar with an equivalent depth of 2.9 to 7.9 km for the Đèo Cả granites. The emplacement ages of granitoids from all three complexes are constrained by U-Pb zircon ages. Emplacement times of these three magmatic complexes and an intrusion sequence can be established as: the Định Quán (112-100 Ma), Cà Ná (96-93 Ma), and Đèo Cả (92-88 Ma) complexes.

I. INTRODUCTION

The Đà Lạt zone of 300x150 km in size is located in the South Việt Nam (Fig. 1). It is made up by Precambrian basement rocks, Jurassic sediments, and Late Mezosoic igneous and Cenozoic basaltic rocks [16, 29, 33]. In the Đà Lạt zone, basalts are Cenozoic tholeiitic and sub-alkaline in composition that are associated with extension tectonics, following the collision of the Eurasian and the Indian plates at around 65 ± 10 Ma [3, 16].

The numerous granitoids and corresponding felsic volcanic rocks in the Đà Lạt zone have been interpreted as resulted from the NW subduction of the Pacific plate under the Southeast Asian continental margin [1, 31]. They belong to an Andean-type arc, which initially was formed in south-east China from the middle Jurassic to Early Cretaceous [7], extending through South Việt Nam during the Middle Cretaceous to SW Borneo during the Late Cretaceous and Early Paleocene [10].

Based on previous studies on petrography, mineralogy and chemical composition, these granitoids were subdivided into three complexes. Generally, from east to west, these are the Định Quán, Đèo Cả [19] and Cà Ná Complexes [15].

In general, the Đà Lạt zone granitoids have been studied by many geologists [15, 19, 20, 25]. However, the viewpoints on their origin, time of emplacement as well as forming conditions are different. Therefore, this paper focuses on the origin and forming conditions of these granitoids, using numerous quantitative analytical results, especially the chemical analytic results of hornblendes separated from granitoids of the three above mentioned complexes.

Figure 1. Simplified geological map of the Đà Lạt zone showing the distribution of granitoids of the Định Quán, Cà Ná and Đèo Cả Complexes [25]. The upper inset shows that from Middle Jurassic to Middle Cretaceous times the SE Asian margin was an Andean-type arc [31]. NW-direction subduction beneath the continent is evidenced by widespread rhyolitic volcanism and granitic intrusions along SE China [e.g. 12] and SE Việt Nam. The lower inset shows Việt Nam and location of study area.

II. ANALYTICAL METHODS

1. Determination of whole-rock major and trace element concentrations

Rocks are crushed in a jaw crusher and powdered in an agate mill to a grain size of < 0.063 mm. Major and trace element (Rb, Ba, Nb, Sr, Zr, Y, Cr, Ni, and Zn) analyses are performed on fused glass discs, which are made from rock powder mixed with Li₂B₂O₇ (1.5/7.5) and fused at high temperature. Loss on ignition (LOI) was calculated after igniting one gram of the sample powder to 1000^oC for one hour. Analytical uncertainties range from 0.3 to 0.5% and from 3 to 5% for major and trace elements, respectively, depending on their concentration level. The analytical results are given in Tab.1.

2. U-Pb isotope analysis

The collected samples for analyses are CN-13 and DC-28. Sample CN-13 is taken from the Cà Ná massif (Cà Ná railway station). The sample is from leucocratic and coarse-grained granite. Sample DC-28 is selected from Núi Dinh massif belonging to the Đèo Cả Complex. Rock samples of 5-7 kg are splitted by employing a jaw crusher and still roller mill. The broken material is sorted by a Wilfley table for initial density separation. The heavy minerals are collected and dried in an oven. Further mineral concentration is carried out with a Frantz isodynamic magnetic separator. Zircon in non-magnetic part is separated by using heavy fluid and hand picked under a binocular microscope. Finally, zircons were washed in hot HCL (6N) và HNO₃ (7N).

3. Chemical composition of hornblende

To analyse the concentration of major elements and chlore in hornblende, the samples were prepared as thin sections but without resin cover. After that, the polished samples were washed in a ultrasound basin, dried and covered by a thin carbon layer before analysis.

The concentration of elements in hornblende is analyzed by electromicroprobe with a beam of 2-5 µ, acceleration of 15-20 kv and intensity of 100-150 nA. The analytical results are shown in Tab.2.

III. ANALYTICAL RESULTS

1. Major and trace element geochemistry

Representative chemical analyses of samples are listed in Tab.1. The bulk-rock concentrations of the Đà Lạt zone granitoids are characterized by relative high SiO₂, low MgO, and low abundance of high-field strength elements (Nb, Ti, Zr). For example, Nb is generally lower than the average value of I-type (14 ppm) and felsic I-type (21 ppm) granites in the Lachlan Fold Belt of southern Australia [7,14].

On the QAP [8] and Anorthite-Albite-Orthoclase diagrams [13] (Fig.2a-b), the Định Quán rocks have from tonalitic to granodioritic composition. In contrast, most of Cà Ná rocks are granite and the Đèo Cả rocks are of granodioritic, monzogranitic, and granitic composition. Figs. 3a-b show that all studied samples from the Đà Lạt zone belong to calc-alkaline series in an AFM plot (Fig.3a).

In Fig.3b, nearly all granitoid samples plot in the high-K field. Total alkaline content ranges from 6.5 to 8.5% with the content of potassium higher than that of sodium in almost analyzed samples.

The A/CNK vs. A/NK diagram defines the rocks as metaluminous to slightly peraluminous, and of I-type character (Fig. 4a). The mineralogy supports the weakly peraluminous character of the granitoid samples as they lack muscovite and other primary alumino-silicates, like garnet, cordierite, or sillimanite. Some of the Cà Ná samples (with ASI ≥ 1.1) display weakly peraluminous character, as they contain secondary muscovite.

Table 1. Major and trace element concentration of granitoids in the Đà Lạt zone

DQ = Định Quán, DC = Đèo Cả, CN = Cà Ná, ASI = aluminium saturation index (molecule Al₂O₃ / (CaO+K₂O+Na₂O),

Co = corundum, total Fe is expressed as Fe₂O₃.

Figure. Ternary diagrams illustrating the composition of granitoids in the Đà Lạt zone.

a) the modal diagram in terms of quartz (Q), K-feldspar (A), and plagioclase (P). Nomenclature taken from [13]. b) the molecular nomative Anorthite-Albite-Orthoclase diagram O’Connor [21]. à Định Quán, ○ Cà Ná, + Đèo Cả

Figure. Diagram demonstrating geochemistry of Đà Lạt granitoids.

a)Diagram Na₂O+K₂O-Fe₂O₃-MgO (AFM) with a line separating
for tholeiitic and calc-alkaline rocks. b) Diagram K₂O-SiO₂ [13] with lines separating K-high, K-medium and K-low rocks

2. Pressure and temperature estimation for Đà Lạt granitoids

Hornblende geobarometry

Since pressure correlates linearly with total aluminium (Al^T) the content of hornblende is buffered by a mineral assemblage of quartz + alkali feldspar + plagioclase + hornblende + biotite + iron titanium + titanite together with melt [11, 21], therefore, hornblende geobaromerty used to estimate the pressure and depth of emplacement of the granitoid. Since some of the granitoids from the Đà Lạt zone containing the critical mineral assemblage are required for application of the Al-in-hornblende barometer. Electron-microprobe analyses of hornblende from six massifs are listed in Tab.2.

Table 2. Chemical analytical results of hornblende by EMPA

Định Quán Complex

Sample	DQ-1	DQ-8	DQ-8	DQ-8	DQ-8	DQ-8	DQ-8
Crystal	1	1	1	2	3	4	4
Note	rim	core	rim	rim	core	rim	Core-rim
SiO2	48.1	50.3	45.2	46.1	46.1	46.5	47.8
TiO2	0.4	0.1	0.7	0.7	0.6	0.6	0.5
Al2O3	5.8	4.1	7.2	7.1	6.7	6.6	5.7
FeO	20.0	14.9	16.8	16.6	16.6	16.4	16.0
MnO	0.5	0.5	0.4	0.5	0.5	0.5	0.4
MgO	10.1	14.2	12.1	11.8	11.8	11.7	12.3
CaO	10.6	11.9	11.2	11.3	11.4	11.4	11.6
Na2O	1.1	0.7	1.5	1.3	1.3	1.3	0.9
K2O	0.6	0.2	0.8	0.8	0.8	0.8	0.6
Cl	0.2	0.0	0.3	0.3	0.2	0.2	0.1
Total	97.3	96.9	96.2	96.5	95.6	96.0	95.8

Cà Ná Complex

Sample	CN-7	CN-7	CN-7	CN-7	CN-7	CN-7	CN-7	CN-7
Crystal	1	1	2	2	3	4	4	4
Note	rim	rim	core	rim	core	core	rim	rim
SiO2	46.8	46.2	48.9	47.5	46.1	48.3	46.9	46.8
TiO2	0.7	0.7	0.3	0.3	0.6	0.4	0.3	0.4
Al2O3	6.2	6.2	4.1	5.1	6.5	4.1	5.6	5.6
FeO	21.0	21.1	20.7	20.8	22.1	20.2	21.2	21.4
MnO	0.5	0.4	0.5	0.5	0.6	0.5	0.5	0.5
MgO	9.1	9.3	10.1	9.6	8.6	10.2	9.4	9.2
CaO	10.2	10.3	10.8	10.8	10.5	10.9	10.6	10.5
Na2O	1.4	1.5	0.8	0.9	1.3	0.8	1.1	1.2
K2O	0.7	0.7	0.4	0.5	0.7	0.4	0.6	0.6
Cl	0.4	0.4	0.3	0.3	0.4	0.3	0.4	0.4
Total	97.0	96.8	96.7	96.4	97.2	96.1	96.5	96.6

Đèo Cả Complex

Sample	DC-3	DC-3	DC-18	DC-18	DC-26	DC-26	DC-26	DC-26
Crystal	1	1	7	7	1	1	1	2
Note	core	rim	rim	rim	rim	core	rim	core
SiO2	45.7	46.1	51.5	50.6	44.8	44.5	44.8	44.8
TiO2	0.9	0.8	0.2	0.2	0.7	0.7	0.7	0.7
Al2O3	7.9	7.2	3.4	3.9	4.8	4.7	4.8	4.5
FeO	18.6	18.8	15.6	15.6	31.3	32.3	31.3	31.3
MnO	0.5	0.4	0.4	0.4	0.7	0.8	0.7	0.8
MgO	10.1	9.8	13.7	13.4	2.4	2.4	2.4	2.6
CaO	10.2	10.8	11.6	11.6	9.5	9.0	9.5	9.5
Na2O	1.4	1.6	0.8	0.9	2.0	2.1	2.0	2.0
K2O	0.9	0.9	0.3	0.4	0.9	0.9	0.9	0.9
Cl	0.2	0.3	0.1	0.1	0.3	0.3	0.3	0.2
Total	96.4	96.6	97.4	97.0	97.4	97.6	97.4	97.3

3. U-Pb isotopic composition

The U-Pb zircon isotopic ratios of Cà Ná and Đèo Cả Complexes are given in Tab.3 and Fig.5a-b. Pb isotopic composition of K-feldspar from the Đà Lạt granitoids (Tab.4) are used for correction of Pb measured ratios.

Table 3. Analytical results of U-Pb zircon from Cà Ná and Đèo Cả granitoids

Sample	^207Pb/^235U	Error (%)	^206Pb/^238U	Error (%)	Correlation
CN-13
CN-13/1	0.081313	2.38	0.012384	2.27	0.96
CN-13/2	0.093966	1.02	0.014228	0.82	0.80
CN-13/3	0.087129	1.19	0.013189	1.16	0.98
CN-13/4	0.094324	1.72	0.014188	1.33	0.78
DC-28
DC-28/1	0.109727	1.04	0.104727	0.96	0.99
DC-28/2	0.095240	0.86	0.012282	0.85	0.97
DC-28/3	0.106595	0.70	0.013679	0.64	0.99
DC-28/4	0.099703	1.39	0.012790	1.35	0.98
DC-28/5	0.096653	0.66	0.012502	0.47	0.74

Table 4. Pb isotopic composition of K-feldspar from the Đà Lạt granitoids

K-feldspar	^206Pb/^204Pb	^207Pb/^204Pb	^208Pb/^204Pb
DQ-8	18.449	15.536	38.310
CN-10	18.471	15.542	38.356
CN-16	18.495	15.537	38.354
DC-4	18.569	15.550	38.448
DC-27	18.487	15.532	38.343

Errors of the analysed ratios are <0.1%

Granite sample CN-13: In the Fig. 5a, four zircon fractions are selected for U-Pb analysis. Two of them are ploted on the concordia curve having an average ²⁰⁶Pb/²³⁸U age of 92.8 Ma. Two other points ploted below the concordia curve and having age of 78 and 83 Ma may indicate Pb loss. We conclude that the average ²⁰⁶Pb/²³⁸U age of ~93 Ma represents the emplacement time of this granite.

Granite sample DC-28: Five zircon fractions from sample DC-28 are selected for U-Pb analysis and the results are plotted on the Fig.5b. Five data points scatter around two groups of ages of 90.4-92.1 Ma and 101-103 Ma. On the Fig. 5b, three data points are plotted closely to the concordia curve and clustered around 90.4-92 Ma giving a mean ²⁰⁶Pb/²³⁸U age of 91.6 Ma. Two other zircon fractions yield concordant, but older, U-Pb ages of ~108 and ~102 Ma, possibly inherited from earlier granitoids with ages similar to the Định Quán granitoid samples [18]. The age of ~91.6 Ma is considered to be the best estimation for the emplacement time of this granite.

Đèo Cả Complex

Cà Ná Complex

Figure 5a-b. Zircon ages of the Cà Ná and Đèo Cả Complexes: a) U-Pb concordia plot for zircon analyses of sample CN-13 indicating crystallization at 92.8+/-1.8 Ma; b) U-Pb concordia plot for zircon analyses of sample DC-28. One data point plots

IV. DISCUSSION

1. Classification of granite

Chappell and White (1974) showed that there are two contrast types of granites in the Lachlan Fold Belt of Southeast Australia. These were referred to as I- and S-types to indicate their different origins. The S (sedimentary)-type is derived from partial melting of sedimentary source rocks and I (igneous)-type - from melting of source rocks of igneous composition that have not experienced surface weathering processes. Many studies on granites have shown that S- and I-type granites can be distinguished on the basis of δ¹⁸O values [6, 7, 22]. From those studies, I-type granite values of δ¹⁸O <10‰, where as S-type granite have δ¹⁸O values >10‰. Classification criteria for I-type and S-type granites are summarised in Tab.5.

According to the classification of all above mentioned authors, the Đà Lạt granitoids can be classified as I-type granites. Most analyzed samples have normative corundum ranging from 0.3 to 0.8% and aluminium saturation index <1.1, Sr_i and δ¹⁸O values that range from 0.7049 to 0.7069 and 7.5 to 8.9‰, respectively. In addition, the presence of hornblende, titanite and lack of primary muscovite and other alumino-silicate minerals, such as sillimanite, cordierite, and garnet, strongly support I-type origin.

The S-type granites usually have lower values of HFS elements. This led Harris [in 9] to use the diagram Rb/Zr -SiO₂ ratio as an effective and powerful tool to distinguish S-type granite from I-type one. According to this classification criterion, the Đà Lạt granitoids fall in the field of I-type (Fig. 4b). On the Fig. 4a, almost all analytic samples fall in the I-type field, some of the Cà Ná granites, which are hydrothermal altered and contain secondary muscovite, plot on the boundary between I- and S-type granites.

Compared to typical Lachlan S-type granites [27] the Cà Ná and Đèo Cả rocks have higher Na₂O and K₂O contents and lower A/CNK ratios. In fact, granitoids of both suites are compositionally transitional between Lachlan I-and S-type granites (Fig. 4c). This implies that the Cà Ná and Đèo Cả granitoids might be derived from partial melting of acidic to intermediate igneous rocks or immature sediments. All plots in Fig. 7a-c indicate an origin of the Cà Ná and Đèo Cả magmas by dehydration melting of greywacke-type source rocks. Some of the Cà Ná granites fall in the range of felsic pelite (Fig. 7b), as these samples contain secondary muscovite. Muscovitization of feldspar and chloritization of biotite in these samples are evidences for post-magmatic hydrothermal alteration.

Table 5. Summary of classification criteria for I-type and S-type granites
(White and Chappell 1974, 1983, 1987)

I-type	S-type
(i) Broad spectrum of compositions from mafic to felsic rocks	Narrow range of felsic to SiO2-rich rocks
(ii) Metaluminous mineralogy; hornblende common and more abundant than biotite in mafic samples; accessory titanite common	Peraluminous mineralogy: biotite, muscovite predominante; no hornblende; some cordierite and/or aluminosilicates
(iii) Hornblende-rich igneous-appearing xenoliths	Pelitic or quartzose metasedimentary xenoliths
(iv) Na2O>3.2wt% in felsic rocks and 2.2wt% in mafic rocks	Na2O <3.2wt%
(v) Molecular Al2O3/(CaO+Na2O+K2O)<1.1	Molecular Al2O3/(CaO+Na2O3+K2O)>1.1
(vi) C.I.P.W normative corundum <1%	C.I.P.W normative corundum >1%
(vii) Regular inter-element variations within massifs; linear or near linear	More irregular variation diagrams
(viii) Initial strontium (Sri)<0.708	Initial strontium (Sri)>0.708
(ix) High and variable eNd values	Low and constant eNd values
(x) Contacts strongly discordant	Sometimes surrounded by high grade metamorphic rocks
(xi) d^18O<10‰ SMOW	d^18O>10‰ SMOW

2. Forming conditions of granitoids in the Đà Lạt zone

a. Pressure and depth of emplacement of granitoids in the Đà Lạt zone

Since pressure correlates linearly with total aluminium (Al^T) content of hornblende buffered by nine phases assemblage [28] therefore, hornblende geobaromerty has been used to estimate the pressure and depth of emplacement of the granitoids. Since some of the granitoids from the Đà Lạt zone contain the critical mineral assemblage quartz + alkali feldspar + plagioclase + hornblende + biotite + iron titanite that together with melt is required for application of the Al-in-hornblende barometer. The Al^T cation calculations are based on 23 oxygens and a cation number of 15 and the pressure are calculated using the calibration of Schmidt [22]; P (± 0.6 kbar) = ± 3.01 - 4.76 Al^T and the pressure estimates and corresponding depths from all three complexes are presented in Tab.6. The results are still limited, however, the first calculations indicate low-and medium pressure regimes for the Đà Lạt granitoids. Hornblende of the Đèo Cả complex has lower Al^(T) content compared to those from the two other complexes, reflecting the shallower intrusion depth. Two samples from the Định Quán Complex yield crystallization pressures between 2.2 and 2.7 kbar corresponding to a depth of about 7.9 to 9.7 km. Solidification of the Đèo Cả magmas took place at relatively lower pressures, ranging from 0.8 to 2.2 kbar with a depth of ca. 2.9 to 7.9 km. Pressure investigation for the Cà Ná magma is restricted, because most of collected samples do not contain the required mineral assemblage. However, a sample CN-7 being present limit the application of the Al-in-hornblende barometer and therefore amphibole rims in this sample were analyzed and yielded a solidus pressure of 2.5 kbar, corresponding to a depth of about 9.0 km

Table 6. Pressure estimations for the intrusion of granitoid magmas
from the Đà Lạt zone obtained from Al-in-hornblende barometry
using the calibration of Schmidt [30]

Sample	Rock type	Al^(T)(cations)	Pressure (kbar)	Depth (km)^a
DQ-8	Granodiorite	1.21	2.7	9.7
DQ-1	Granodiorite	1.09	2.2	7.9
CN-7	Granite	1.16	2.5	9.0
DC-3	Granodiorite	1.09	2.2	7.9
DC-18	Granite	0.80	0.8	2.9
DC-26	Granodiorite	1.01	1.8	6.5

^a- Values were calculated assuming an average density of 2.8 g/cm³

b. Tectonic setting

Granitoids of all three complexes are K-high, calc-alkaline rocks enriched in LILEs such as Cs, K, Rb, U and Th and depleted in Nb and Ti. Magmas with these chemical features are generally believed to generate in subduction-related environments [9, 27, 28]

On the Rb-(Y+Nb) diagram (Fig. 6a) [24] almost all analyzed samples plot on
VAG field. Some of the Đèo Cả and Cà Ná samples plot on the boundary between syn-COLG and WPG fields resulting from highly fractionated or altered samples and contained secondary muscovite. On Ta vs. Yb (Fig. 6b) all analyzed samples fall into the VAG field in conjunction with their geochemistry and mineralogy and demonstrate that the Đà Lạt granitoids were generated in subduction-related environments.

c. Genesis of granitoids in the Đà Lạt zone

Compositional diversity among crustal magmas may arise partly from different source compositions, but also from variations of melting conditions, such as H₂O content, pressure, temperature, and oxygen fugacity [9, 23, 32, 35]. Compositional differences of magmas produced by partial melting under variable melting conditions of different crustal source rocks, such as amphibolite, gneisses, metagraywackes and meta-pelites, may be visualized in terms of major oxide ratios. Partial melts originated from mafic source rocks, for example, have lower Al₂O₃/(FeOtotal+MgO+TiO₂) and (Na₂O+ K₂O)/(FeOtotal+MgO+TiO₂), but higher CaO/Al₂O₃ than those derived from meta-pelite rocks. As can be seen in Figs. 4, 7a-d, the Định Quán rocks have lower values of Al₂O₃/(FeOtotal+MgO+TiO₂) and (Na₂O+ K₂O)/(FeOtotal+MgO+TiO₂), higher values of CaO/Al₂O₃ and narrow range of CaO/(FeOtotal+MgO+TiO₂) ratios in comparison to Cà Ná and Đèo Cả rocks. These features in combination with relatively high values of Mg# (68 to 58, except of one granite sample having Mg# = 38) preclude a derivation from felsic pelite and metagreywacke rocks of the Định Quán granitoids. Instead, the partial melting of mafic lower crustal source rocks probably is generated from the Định Quán magmas.

Figure 6a-b. Chemical composition of the Đà Lạt granitoids in tectonic discrimination diagrams with the fields of volcanic-arc granitoids (VAG); syn-collisional granitoids (syn-COLG); within-plate granitoids (WPG); ocean-ridge granitoids (ORG). a) and b) Rb vs. (Y+Nb) and Ta vs. Yb discrimination diagrams of Pearce et al. (1984).

In the Figs. 7a-d, almost all analyzed samples of Cà Ná and Đèo Cả granitoids fall in the metagreywack field. Some of Cà Ná samples fall in the range of felsic pelite (Fig. 7b), as these samples contain secondary muscovite. Muscovitization of feldspar and chloritization of biotite in these samples are evidences for post-magmatic hydrothermal alteration. The Cà Ná granites have a narrow range in initial Sr-isotopic ratios (0.7060 to 0.7064) and nearly constant ε_Nd values (–2.5 to –2.7) indicate the derivation from relative homogeneous melt. In contrast to Cà Ná granites, the Đèo Cả samples show relatively large variations in isotopic compositions (ε_{Nd (T)} = ~ 1 to ~ −3; Sr_i = 0.7055-0.7069), suggesting their derivation from heterogeneous sources. ε_Nd values vary from +1 to –3, reflecting a significant input of mantle-derived components.

It is worth to note that rocks of the Cà Ná and Đèo Cả Complexes do not fit into alphabetical classification of Chappell and White [6]. Granitoids originated by melting of sedimentary rocks should have S-type characters. This study however shows that the Cà Ná and Đèo Cả granitoids were generated from greywacke-type sources but they have I-type characters. A similar recognition was reported by Barker et al. [2], that the generation of Alaskan granodiorites from melting of flyschoid sediments mainly consists of mudstones and graywackes. Nevertheless, these granodiorites typically show I-type features. Maybe the geochemical characteristics of the Cà Ná and Đèo Cả granitoids reflect largely the igneous parentage of these graywackes.

Figure 7a-d. Plots showing compositional fields of experimental melts derived from partial melting of felsic pelites, greywackes, and amphibolites [23] and composition of studied samples.

à Định Quán, ○ Cà Ná, + Đèo Cả

V. CONCLUSIONS

The Đà Lạt granitoids, having I-type characteristics, belong to the K-high calc-alkaline series, and are subduction-related products of the Pacific plate subducting beneath the Eurasian continental crust during Cretacous.

The Định Quán granitoids were generated from dehydration melting of mafic lower crustal source rocks, while the Cà Ná magmas -- from relatively homogeneous greywacke-type source. The Đèo Cả granitoids most probably were originated from the partial melting of heterogeneous greywacke-type sources with an additional contribution of mantle components.

Hornblende geobarometry indicates that the Đà Lạt granitoids were formed in the medium pressure regimes. The formation pressures and depths of the Định Quán, Cà Ná and Đèo Cả are 2.2 - 2.7 kbar and 7.9 - 9.7 km; 2.5 bar and 9 km; 0.8 - 2.2 kbar and 2.9 - 7.9 km, respectively.

The results of this study in combination with the results of Nguyen et al [18], U-Pb zircon dating constrain the emplacement of granitoids at ~112-100 Ma for the Định Quán, ~96-93 Ma for Cà Ná, and ~92-88 Ma for Đèo Cả Complexes.

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