GABBRO-SYENITE
ASSOCIATIONS OF
TRẦN TRỌNG HÒA1, TRẦN TUẤN ANH1,
NGÔ THỊ PHƯỢNG1,
A.E. IZOKH2, G.V. POLYAKOV2, P.A. BALYKIN2,
CHING-YING LAN3,
HOÀNG HỮU THÀNH1, BÙI ẤN NIÊN1, PHẠM THỊ DUNG1.
1Institute of Geological Sciences, NCNST, Hoàng Quốc
Việt Road, Hà Nội, Việt Nam;
2Institute of Geology SB RAS, Novosibirsk, Russia;
3Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
Abstract: Detailed
research on material substances of gabbro and syenite at Lô-Gâm and Phú Ngữ
structures of the
Gabbro of the first type
belongs to titanium-low, alkali-low, aluminium-high type, relatively rich in
LREE and depleted in Nb, Ta, that characterizes calc-alkaline mafic intrusions.
Syenite of this type has wide variation of trace elements and REE, but dapleted
in Nb, Ta and Zr. Initial ratios of 87Sr/86Sr = 0.71014-
0.71055 for gabbro and 0.70819 - 0.71023 for syenite; 143Nd/144Nd
(T [250 Ma]) = 0.51198 - 0.51213, (e
N[T]) = -3.58 -8.51) for syenite respectively.
Monzogabbro, monzonite and
syenite of Tích Cốc association have significant difference in mineral
assemblage (hypersthene, high in Ti, low in aluminium biotite, rich in
apatite), geochemistry (high in Ti, high in alkali, rich in K, Clay shale, Pb,
REE, and depleted in Nb, Ta, Zr, Hf). Initial ratios of 87Sr/86Sr
= 0.71296 - 0.71502 for gabbro and 0.71193 for syenite.
Gabbro pyroxenite,
melagabbro, melasyenite and nepheline syenite of Bằng Phú cassociation are
different in Ti, Al, Fe, Mg, Na, K content. Gabbro and gabbro-pyroxenite are
rich in K, Rb, Sr, Th, Pb, REE, depleted in Nb, Ta and
Ba. Nb, Ta and Ba concentration is lower significantly in comparison to
alkaline ultramafic rocks (ijolite, urtite, melteigite). Initial 87Sr/86Sr
ratio of gabbro-pyroxenite is relatively hugh
(0.7100). Ne-Bi syenite is rich in Rb, Nb, Ta, Zr, Hf, Th, U, but depleted in
Ba, Sr and very low in REE, along with initial 87Sr/86Sr
= 0.71913.
The homogenization of
geochemical and isotopic significances of mafic-intermediate rocks implies that
they have close genetic relation. This basis allows to have
a conclusion that investigated gabbro-syenite associations are products of
differentiated mantle-derived magma, which is contamined strogly by crustal
material.
INTRODUCTION
Complex intrusions of gabbro, gabbrodiorite,
diorito-syenite, monzonite and syenite are widely spread over East Bắc Bộ structure.
As they were formed in small bodies, they were grouped into different complexes
of intrusive rocks, which have widespread exposure. For example, gabbroid rocks
were grouped into Núi Chúa Complex of Mesozoic age (Late Triassic), syenitoid
varieties were either joined to Middle Paleozoic Phia Ma Complex or Paleogene
Chợ Đồn Complex [1, 2, 8]. The formation of syenites of Phia Ma type has been
assigned to alkaline magmatism that end up the Middle Paleozoic stage, while
Chợ Đồn Paleogene syenites have been considered as late products of Late
Mesozoic Early Cenozoic magmatism [2]. Thus, gabbroid rocks are known as
orogenic product, while syenites are considered as of intraplate magmatism. In
recent years, a number of detailed research on gabbro-syenite from Lục Yên (Yên
Bái) and Tích Cốc (Tuyên Quang) of Lô-Gâm structure [4, 7] have been showing
that mafic rocks do not belong to layered intrusion of Núi Chúa type. These
mafic varieties are accompanied with syenite ones to form the gabbro-syenite
association, which has geochemical significances of calc-alkaline
subduction-related magmas. According to these researches, two different types
of gabbro-syenite associations can be classified: Lục Yên, and Tích Cốc types.
In Phú Ngữ structure, similar association was described [2]. During the
realization of the project named as "Intraplate magmatism of Việt
SAMPLES AND ANALYZED TECHNIQUES
Representative gabbro and syenite samples (and other intermediate varieties as well) from massifs: Tích Cốc (Hàm Yên District) of Lô-Gâm Structure, Bằng Phúc and Khuổi Loỏng (Chợ Đồn District) of Phú Ngữ Structure, are collected by Project: "Intraplate magmatism and related mineralization of Việt Nam". In addition, samples from Lục Yên (Tân Lĩnh and Yên Minh massifs) are provided by Project: "Geological mapping of Lục Yên Châu Map sheet". The sample localities are shown in Fig.1.
Mineral composition of the rocks were analyzed in
polished thin sections by an electron-microprobe CAMECA Kevex, major elements
of whole-rock by XRF, trace and rare earth elements by ICP-MS at the United
Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian
Academy of Sciences and the Department of GeoSciences, National Taiwan
University. Nd and Sr isotopic data were analyzed by a VG354 mass spectrometer
for Sr and a MAT 262 for Sm and Nd at the
GEOLOGICAL BACKGROUND
The East Bắc Bộ region is a folding system that
belongs to the southern margin of
The Lô-Gâm structure is located at the southeast
of Sông Chảy Dome, adjacent to the Phú Ngữ Zone at the east and
Table 1
(cont.). Representative composition of
gabbro-syenite association of
|
Location |
Chợ Đồn |
||||||||
|
Sample |
H-1204 |
H-1218/1 |
H 1538 |
H-1215 |
H-1211 |
H-1221 |
H-1540 |
H-1536 |
H-1216 |
|
Massif |
Khuổi Loỏng |
Bằng Phúc |
|||||||
|
Rock type |
Gabbro |
Syenite |
|||||||
|
SiO2 |
47.78 |
42.08 |
43.23 |
44.95 |
53.03 |
55.95 |
56.83 |
57.43 |
60.47 |
|
TiO2 |
0.63 |
0.76 |
0.902 |
0.98 |
0.40 |
0.14 |
0.080 |
0.712 |
0.17 |
|
Al2O3 |
16.98 |
18.78 |
16.68 |
12.33 |
19.40 |
26.57 |
24.31 |
15.86 |
22.63 |
|
Fe2O3 |
6.13 |
7.14 |
9.19 |
10.13 |
3.58 |
1.40 |
1.08 |
7.43 |
1.63 |
|
FeO |
|
|
|
|
|
|
|
|
|
|
MnO |
0.08 |
0.12 |
0.164 |
0.16 |
0.06 |
0.04 |
0.030 |
0.138 |
0.04 |
|
MgO |
7.10 |
2.84 |
4.87 |
4.42 |
1.31 |
0.00 |
0.23 |
2.33 |
0.00 |
|
CaO |
19.35 |
21.41 |
17.04 |
19.92 |
9.83 |
1.24 |
1.02 |
7.47 |
1.27 |
|
Na2O |
0.96 |
1.68 |
1.77 |
1.63 |
2.81 |
7.05 |
7.43 |
2.76 |
5.36 |
|
K2O |
0.22 |
0.96 |
1.92 |
2.04 |
6.00 |
7.29 |
8.08 |
4.75 |
7.56 |
|
P2O5 |
0.03 |
0.76 |
0.616 |
0.64 |
0.48 |
0.01 |
0.030 |
0.229 |
0.02 |
|
LOI |
|
|
3.91 |
|
|
|
0.99 |
0.26 |
|
|
TOTAL |
99.28 |
96.51 |
100.29 |
97.18 |
96.92 |
99.68 |
100.11 |
99.37 |
99.15 |
|
Sc |
26.03 |
9.56 |
|
21.72 |
5.98 |
0.72 |
|
|
1.86 |
|
V |
151.3 |
68.14 |
59 |
109.90 |
38.65 |
|
3.4 |
39 |
5.79 |
|
Cr |
47.84 |
12.78 |
29 |
19.60 |
-0.33 |
|
35 |
38 |
-0.07 |
|
Co |
32.47 |
21.75 |
20 |
36.05 |
9.38 |
0.50 |
34 |
11 |
0.77 |
|
Ni |
55.6 |
17.29 |
23 |
24.39 |
4.43 |
|
325 |
29 |
|
|
Cu |
43.6 |
23.17 |
115 |
56.08 |
55.31 |
9.34 |
8661 |
2.07 |
7.57 |
|
Zn |
40.29 |
45.6 |
|
84.8 |
62.8 |
21.9 |
|
|
54.3 |
|
Ga |
14.17 |
14.09 |
|
16.04 |
14.89 |
27.46 |
|
|
23.45 |
|
Cs |
2.6 |
2.28 |
|
3.437 |
4.698 |
20.43 |
|
|
16.71 |
|
Rb |
12.03 |
38.0 |
51 |
85.5 |
242.8 |
451.1 |
266 |
126 |
436.7 |
|
Sr |
292.1 |
604.5 |
441 |
334.5 |
562.4 |
90.4 |
41 |
263 |
126.3 |
|
Y |
14.48 |
23.74 |
23 |
29.32 |
23.89 |
18.81 |
15 |
47 |
30.30 |
|
Zr |
53.59 |
78.14 |
183 |
143.90 |
67.90 |
216.00 |
239 |
444 |
338.70 |
|
Nb |
1.546 |
2.893 |
8 |
6.635 |
6.41 |
30.77 |
|
|
35.22 |
|
Ba |
77.03 |
169 |
167 |
109 |
416 |
82 |
20 |
850 |
156 |
|
La |
5.399 |
28.38 |
20 |
35.08 |
30.74 |
9.10 |
3.7 |
45 |
9.20 |
|
Ce |
12.08 |
54.8 |
40 |
66.1 |
57.0 |
18.2 |
8.5 |
8.1 |
20.3 |
|
Pr |
1.575 |
6.85 |
5.3 |
8.13 |
6.73 |
2.18 |
1.2 |
10 |
2.73 |
|
Nd |
7.784 |
25.62 |
19 |
30.09 |
23.63 |
7.44 |
4.3 |
34 |
10.29 |
|
Sm |
2.016 |
4.94 |
3.5 |
5.93 |
4.37 |
1.58 |
1 |
6 |
2.46 |
|
Eu |
0.604 |
1.136 |
0.87 |
1.146 |
1.064 |
0.397 |
0.31 |
1.12 |
0.599 |
|
Gd |
2.019 |
4.18 |
3.2 |
5.03 |
3.82 |
1.51 |
1.04 |
5.4 |
2.38 |
|
Tb |
0.377 |
0.698 |
0.56 |
0.852 |
0.646 |
0.315 |
0.25 |
0.99 |
0.533 |
|
Dy |
2.423 |
3.91 |
3.3 |
4.83 |
3.74 |
2.12 |
1.7 |
5.9 |
3.64 |
|
Ho |
0.491 |
0.759 |
0.62 |
0.964 |
0.753 |
0.466 |
0.37 |
1.18 |
0.838 |
|
Er |
1.357 |
2.14 |
1.8 |
2.77 |
2.13 |
1.52 |
1.2 |
3.5 |
2.63 |
|
Tm |
0.206 |
0.314 |
|
0.420 |
0.315 |
0.261 |
|
|
0.445 |
|
Yb |
1.352 |
2.09 |
1.7 |
2.82 |
1.95 |
1.75 |
1.2 |
3.2 |
2.76 |
|
Lu |
0.204 |
0.325 |
0.25 |
0.45 |
0.273 |
0.229 |
0.12 |
0.43 |
0.351 |
|
Hf |
1.794 |
1.85 |
4 |
3.52 |
1.58 |
4.52 |
4.5 |
9.7 |
5.78 |
|
Ta |
0.111 |
0.277 |
0.42 |
0.362 |
0.298 |
2.260 |
2.2 |
1.7 |
2.869 |
|
Pb |
8.892 |
6.46 |
|
12.45 |
20.00 |
88.05 |
|
|
89.99 |
|
Th |
2.48 |
7.48 |
12 |
15.81 |
12.49 |
27.71 |
38 |
65 |
37.97 |
|
U |
0.984 |
2.143 |
1.5 |
4.527 |
3.270 |
9.709 |
8 |
8.4 |
16.770 |
Table 1 (cont).
Representative composition of gabbro-syenite association
of
|
Location |
Lục Yên |
Tích Cốc |
||||||||
|
Sample |
LY3145 |
LY2515 |
LY4136 |
LY425/A |
LY4014 |
LY367/2 |
HG-108 |
LY-H1 |
HG-112 |
HG-111 |
|
Massif |
TL |
YM |
TL |
LY |
LY |
YM |
Tích Cốc |
|||
|
Rock type |
Gabbro |
Syenite |
Monzogabbro |
Syenite |
||||||
|
SiO2 |
43.57 |
49.18 |
54.23 |
62.92 |
66.31 |
63.73 |
50.42 |
50.59 |
51.27 |
54.63 |
|
TiO2 |
1 |
0.49 |
0.67 |
0.28 |
0.02 |
0.2 |
2.51 |
3.34 |
1.66 |
1.36 |
|
Al2O3 |
17.94 |
16.97 |
15.57 |
17.03 |
17.17 |
17.56 |
15.82 |
15.09 |
15.36 |
19.64 |
|
Fe2O3 |
5.62 |
0.74 |
1.29 |
1.26 |
0.36 |
1.5 |
13.11 |
14.57 |
11.87 |
9.09 |
|
FeO |
3.98 |
5.82 |
4.14 |
1.77 |
0.19 |
1.93 |
|
|
|
|
|
MnO |
0.14 |
0.12 |
0.09 |
0.06 |
0.01 |
0.05 |
0.18 |
0.19 |
0.18 |
0.12 |
|
MgO |
5.74 |
8.41 |
6.53 |
1.9 |
0.4 |
0.28 |
3.73 |
4.17 |
4.31 |
0.59 |
|
CaO |
18.67 |
12.82 |
12.52 |
1.77 |
1.13 |
1.94 |
6.68 |
6.86 |
7.06 |
4.13 |
|
Na2O |
0.5 |
2.08 |
1.5 |
3.97 |
1.94 |
4.26 |
2.76 |
2.62 |
2.63 |
3.10 |
|
K2O |
0.22 |
0.63 |
1.38 |
7.12 |
11.36 |
6.72 |
3.66 |
2.91 |
4.16 |
5.43 |
|
P2O5 |
0.19 |
0.05 |
0.15 |
0.12 |
0.03 |
0.06 |
0.61 |
0.53 |
0.70 |
0.33 |
|
LOI |
2.08 |
2.61 |
1.6 |
0.93 |
0.37 |
1.19 |
0.65 |
0.54 |
0.89 |
|
|
TOTAL |
99.65 |
99.92 |
99.67 |
99.13 |
99.29 |
99.42 |
100.13 |
101.41 |
100.09 |
98.42 |
|
Sc |
29 |
31 |
31 |
7 |
3 |
3 |
74.75 |
30.89 |
26.1 |
11.08 |
|
V |
171 |
193 |
167 |
28 |
6 |
6 |
252.1 |
386.1 |
235.5 |
|
|
Cr |
20 |
216 |
60 |
91 |
8 |
4 |
12.95 |
6.204 |
19.48 |
|
|
Co |
37 |
34 |
24 |
6 |
6 |
2 |
21.5 |
24.97 |
21.29 |
10.84 |
|
Ni |
70 |
62 |
66 |
49 |
6 |
5 |
4.926 |
4.938 |
12.66 |
|
|
Cu |
76 |
43 |
44 |
2 |
19 |
13 |
14.99 |
32.54 |
17.36 |
41.03 |
|
Zn |
62 |
56 |
45 |
77 |
8 |
49 |
133.5 |
175.2 |
127.3 |
121.8 |
|
Ga |
25.5 |
13.8 |
16 |
16.3 |
9.3 |
20.3 |
21.68 |
22.85 |
20.94 |
23.23 |
|
Cs |
|
|
|
|
|
|
4.018 |
3.223 |
3.893 |
1.26 |
|
Rb |
9 |
22.8 |
45.1 |
241 |
272 |
253 |
115.4 |
106.3 |
133.7 |
137.5 |
|
Sr |
231 |
268 |
389 |
127 |
309 |
180 |
333.9 |
306.1 |
308.7 |
721.4 |
|
Y |
27.4 |
14.6 |
20.7 |
14.5 |
2.5 |
49 |
28.3 |
28.1 |
32.86 |
22.28 |
|
Zr |
79 |
45 |
38 |
323 |
19 |
569 |
116.3 |
115.5 |
113.2 |
65.13 |
|
Nb |
14.1 |
4.1 |
9 |
8.7 |
1.6 |
24.9 |
13.26 |
24.54 |
9.976 |
8.53 |
|
Ba |
46 |
110 |
380 |
538 |
2395 |
667 |
1396. |
1005. |
1332. |
1919 |
|
La |
22.86 |
6.4 |
17.84 |
32.76 |
10.08 |
493.56 |
31.41 |
27.66 |
33.94 |
24.85 |
|
Ce |
52.93 |
14.53 |
38.97 |
56.39 |
16.67 |
679.33 |
63.33 |
60.28 |
71.54 |
52.16 |
|
Pr |
6.85 |
1.89 |
5.3 |
6.59 |
1.63 |
71.7 |
7.394 |
7.785 |
8.779 |
7.395 |
|
Nd |
28.56 |
9.35 |
22.21 |
23.62 |
7 |
197.39 |
31.04 |
35.98 |
37.5 |
31.2 |
|
Sm |
6.19 |
2.3 |
4.6 |
4.11 |
1.11 |
25.62 |
6.493 |
9.223 |
8.046 |
7.183 |
|
Eu |
1.45 |
0.68 |
1.03 |
0.66 |
0.81 |
1.61 |
2.599 |
2.271 |
2.41 |
1.637 |
|
Gd |
5.73 |
2.52 |
4.19 |
3.68 |
1.22 |
21.88 |
5.603 |
7.345 |
6.722 |
5.811 |
|
Tb |
0.88 |
0.47 |
0.71 |
0.49 |
0.19 |
2.25 |
0.892 |
1.134 |
1.071 |
0.868 |
|
Dy |
5.05 |
2.85 |
4.04 |
2.43 |
0.82 |
9.88 |
5.093 |
5.766 |
5.942 |
4.386 |
|
Ho |
1.05 |
0.61 |
0.81 |
0.48 |
0.15 |
1.91 |
1.014 |
1.013 |
1.149 |
0.738 |
|
Er |
2.75 |
1.65 |
2.14 |
1.3 |
0.35 |
5.64 |
2.766 |
2.589 |
3.087 |
1.978 |
|
Tm |
0.38 |
0.27 |
0.32 |
0.22 |
0.08 |
0.7 |
0.432 |
0.392 |
0.48 |
0.27 |
|
Yb |
2.42 |
1.59 |
1.88 |
1.33 |
0.32 |
4.51 |
2.646 |
2.419 |
2.888 |
1.618 |
|
Lu |
0.35 |
0.25 |
0.29 |
0.25 |
0.07 |
0.66 |
0.402 |
0.357 |
0.433 |
0.243 |
|
Hf |
2.86 |
1.84 |
2.26 |
6.75 |
1.51 |
12.31 |
2.931 |
2.868 |
2.924 |
1.583 |
|
Ta |
0.86 |
0.28 |
0.67 |
0.64 |
0.24 |
2.22 |
1.166 |
2.718 |
0.835 |
0.718 |
|
Pb |
14 |
10 |
15 |
20 |
32 |
40 |
14.41 |
12.49 |
17.38 |
17.43 |
|
Th |
1.8 |
1.8 |
4 |
49 |
4.4 |
65 |
4.934 |
1.986 |
4.901 |
2.546 |
|
U |
0.5 |
0.7 |
1.1 |
6 |
3.8 |
9.2 |
0.996 |
0.958 |
1.275 |
0.717 |
YM- Yên Minh; TL- Tân Lĩnh; LY-
Lục Yên
The Phú Ngữ structure is an arc-like structure, which adjoins the Sông Hiến Mesozoic volcanic-sedimentary basin through the Yên Minh - Phú Lương Fault at the east, and Lô-Gâm Structure at the west through the Trần Phú - Sơn Dương sub-meridional fault [9]. This structure consists of Early-Middle Paleozoic metasediments (Ordovician-Silurian and Devonian): shale, siliceous shale, sericite schist, sandstone. At Chợ Đồn, sillimanite-garnet schists are met, which probably belong to older metasediment formations. Magmatic formations are divided into 3 types: 1/ layered mafic-ultramafic intrusives of Núi Chúa type of Late Triassic [1] or T3-J [5]; 2/ aluminous granite of Phia Bioc type (Late Triassic); 3/ subalkaline and alkaline intermediate intrusives (nepheline-bearing syenite) of Paleogene age (?). Gabbro and syenite intrusives at Chợ Đồn form 2 massifs: Khuổi Loỏng gabbro massif and Bằng Phúc gabbro- syenite massif.
The Khuổi Loỏng massif is located 2.5 km northeast of Bằng Lũng Townlet. It is an isometric massif, which crops out on an area of 1 km2, injecting through two- mica schist, sillimanite-cordierite bearing schist. Especially, the whole massif is chapped and brecciated, in which syenites are cement, and gabbro is rock fragments. The Bằng Phúc massif is located farther northeast (10-12 km), cropping out on about 2-3 km2. Two rock associations can be identified: 1/ gabbro, medium-grained monzonite with Pl+Cpx+Amf+Fsp+Bi assemblage (that is similar to Khuổi Loỏng gabbro), and pyroxene syenites; 2/ nepheline-biotite syenite (miaskite), minority of gabbro and/or gabbro-pyroxenite with Cpx+K-Fsp+Amf+Calcite+Ne+SpheneGrt. The garnet-bearing type is assigned to alkaline metasomatism between gabbro and syenite [1].
About 27 km north there is the biggest syenite and nepheline syenite massif of the East Bắc Bộ region named as Phia Ma. In this massif, alkaline gabbro and pyroxenite similar to those at Bằng Phúc are also found [Nguyễn Trung Chí et al., 2003. Petrology and metallogeny of alkaline formations in North Việt Nam, in Archives]. In this case, the classification into two complexes Chợ Đồn and Phia Ma is not comprehensive. The age of Bằng Phúc nepheline syenite is 220-230 Ma [Nguyễn Trung Chí et al., 2003].
PETROGRAPHY AND MINERALOGY
Gabbroids of Tân Lĩnh, Yên Minh (Lục Yên), Khuổi Loỏng and southeast Bằng Phúc massifs are normal gabbro (Pl+Cpx), amphibole gabbro (Pl+residual Cpx+Amf) and/or gabbrodiorite. Sometimes, pyroxenite and gabbropyroxenite (Cpx+Pl+Amf) are found, but they are different only in the content of colour minerals.
Monzogabbro and monzodiorite are popular in the Tích Cốc massif. Their mineral assemblages are relatively stable: Opx+CpxAmf+Bi+Fsp+Pl, with typical monzonitic structure. Accessory minerals are ilmenite and apatite.
As described above, the Bằng Phúc massif comprises 2 associations: gabbro- syenite (not containing colour minerals and foid) and alkaline gabbro-syenite (containing foid). Gabbro of the first association has common mineral assemblage: Pl+Cpx+Amf+BiFsp, which is similar to Lục Yên gabbro and sometimes similar to Tích Cốc monzogabbro. Associated syenite often has Fsp+Cpx+Bi assemblage. Mineral composition of gabbro and gabbropyroxenite of the second association is relatively complicated, consisting of Cpx+Amf+Pl+FspNeBi+CaGr. Mineral composition of nepheline-biotite syenite is Fsp+Ne+BiPl. Detailed mineral composition is described in Ngô Thị Phượng et al [in this jourrnal issue].
GEOCHEMISTRY AND ISOTOPE CHARACTERISTICS
Lục Yên gabbrosyenite association: the Lục Yên gabbroid belongs to the type of low titanium (TiO2 = 0.49-1.0 wt.%), low magnesium (MgO = 5.74-8.41 wt.%), relatively high aluminium (Al2O3 = 16.97-17-94 wt.%), and low alkali (Na2O+K2O = 0.72-2.71 wt.%) (Fig.2). It is similar also to tholeiitic series in titanium and alkalinity, but similar also to calc-alkaline series by aluminium content. Concentration of Cu, Ni, Co, Cr, V is uniformly low, while Rb, Sr, Th, U concentration is relatively high, along with low concentration of Nb, Ta (Table 1), representing significantly the primitive mantle - normalized diagram (Fig. 4). Gabbroic rocks are relatively rich in REE, especially in LREE. Their distribution pattern is similar to calc-alkaline mafic series (Fig. 3). Their Nb/Ta, Nb/La, Ce/Yb and Th/U ratios are similar to those of mantle origin. Lục Yên syenite (SiO2 = 54.23-66.31 wt.%; Na2O = 1.5-4.26 wt.%; K2O = 6.72-11.36 wt%) is characterized by the enrichment in Rb, K, Th, U, and REE along with the depletion in Nb, Ta, Zr concentration, that is close to calc-alkaline intermediate-acid association [7]. However, the exceptional enrichment of REE, Zr, Ba in sample LY-367/2 is possibly related to the presence of REE-bearing mineral phases. Widely variation of Nb/Ta, Nb/La, Ce/Yb n m Th/U ratios imply that the syenite was derived from a strongly crustal contaminated magma. Strontium isotopic ratios are high (87Sr/86Sr 250Ma = 0.71014 - 0.71055 in gabbro and 0.70819 - 0.71023 in syenite), and low eNd(T250Ma) (-3.58 á -6.45 and -5.19 á -8.51, respectively) also implying that gabbro and syenite were derived from isotopically similarly crustal contaminated sources.
Tích Cốc monzogabbro and monzodiorite have distinct chemical composition. Monzogabbro (SiO2 = 50.42-51.27 wt.%) is high in titanium (TiO2 = 1.66-3.34 wt.%), relatively high in aluminium (Al2O3 = 15.00-15.82 wt%), high in P2O5, low in magnesium (MgO = 3.73-4.31 wt.%), high in iron and alkalinity (Na2O+K2O = 5.53-6.79 wt.%), especially high in potassium (K2O = 2.91- 4.16 wt.%). In the correlation diagrams of TiO2, CaO, MgO, Fe2O3, K2O, Na2O, Al2O3 versus SiO2 (Fig. 2), Tích Cốc monzogabbro often gathers in a distinct group differing from Lục Yên and Chợ Đồn gabbros. It is strongly poor in Cu, Ni, Co, Cr, relatively rich in V, Rb, Sr, Zr, Ba, Y, and REE, poor in Nb, Ta (Table 1). Ratios of Nb/La (0.29-0.88), Nb/Ta (9.02-12.0) are relatively lower than those of Lục Yên gabbro, but significantly higher in Ce/Yb (23.9-32.2) ratio. In comparison to chondrite, their REE concentration exceeds 20 times (for HREE) and 160 times (for LREE). In the normalized-REE and multi-element diagrams (Fig.3,4), significant depletion of Nb, Ta, Zr, Hf, La, Ce is recorded. Compared to other gabbros, the Tích Cốc gabbro represents no Eu negative anomaly. The Tích Cốc monzodiorite (sample HG-111) has higher alkalinity (Na2O = 3.1%, K2O = 5.43%) than other monzogabbro, but their trace element and REE distributions are quite similar. Nb, Ta, Zr and Hf depletion in monzodiorite is stronger (Fig .4). Isotopic composition in gabbro and syenite is relatively homogenous (87Sr/86Sr = 0.71296 - 0.71502, and 0.71193, respectively), implying that they share the same isotopical sources that were strongly contaminated by crustal materials.
Gabbro and syenite of Khuổi Loỏng and southeastern Bằng Phúc massif belong to low in titanium, alkali, high in aluminium, and calcium type. They are poor in Cu, Ni, Cr, Co, and REE, relatively rich in Sr (Table 1). Compared to Lục Yên and Tích Cốc gabbro, their Nb/Ta (14), Nb/La (0.3), Ce/Yb (8.9), Th/U (2.5) ratios are relatively lower, but variation trend of trace element and REE is similar (Fig. 5). Initial 87Sr/86Sr ratio of Khuổi Loỏng gabbro (0.70937) is similar to those of Lục Yên and Tích Cốc gabbro. Pyroxene-bearing syenite (H-1536) of Bằng Phúc is relatively low in Al2O3 (15.86 wt.%), high Fe2O3 (7.43 wt.%), medium alkalinity (Na2O+K2O = 7.51 wt.%, K2O/Na2O > 1), rich in Rb (126 ppm), Sr (263 ppm), Zr (444 ppm), Ba (850 ppm), Th (65 ppm) v# REE (Table 1). Its trace element and REE patterns (Fig. 3) are similar to Lục Yên and Tích Cốc syenites, except abnormal Ce depletion. This feature requires further detailed investigation.
Bằng Phúc alkaline rocks are
classified into 3 groups, on the basis of correlation (Na2O+K2O)
SiO2: gabbro-pyroxenite (SiO2 = 42.08 - 44.95 wt.%; Na2O+K2O
= 2.64 - 3.69 wt.%), mela-gabbro-syenite (SiO2 = 53.03 wt.%; Na2O+K2O=8.81wt.%)
and nepheline biotite syenite (SiO2 = 55.95 60.47 wt.%; Na2O+K2O
= 12.92-15.41wt.%), K2O/Na2O > 1. Each group has
distinct content of Ti, Al, Fe, Mg, Ca. Na, and K (Table 1, Fig. 2). In
the correlation diagrams, no transitional trend between 3 groups is represented
(Fig. 2). They are scattered apart into 3 fields. Gabbro and
gabbro-pyroxenite are characterized by the enrichment of K, Rb, Sr, Th, Pb,
REE, and depletion of Nb, Ta, Ba (Table 1, Fig. 3,4).
Compared to typical alkaline ultramafic rocks (ijolite, urtite, melteigite from
Ilimausssaq,
Gabbro-syenite (H-1211/1) has similar trace element distribution pattern, but they have higher concentration, along with higher value of Ce/Yb (30) ratio. The rock is relatively rich in Rb (243 ppm), Sr (562 ppm), Th (12 ppm), and poor in Nb (6 ppm), Ta (0.3 ppm). Its Nb/Ta (21.4), Nb/La (0.2), Th/U (3.8) ratios and REE pattern shows the similarity to above mentioned gabbro and pyroxenite.
Ne-Bi syenite (H-1216, H-1221, and H-1540) has
different geochemical significance in comparison to pyroxene-bearing syenite:
relatively rich in Rb, Nb, Ta, Zr, Hf, Th, U, poor in Ba, Sr and extremely low
in REE (Table 1). Compared to nepheline syenite of Ilimaussaq,
DISCUSSION AND CONCLUSIONS
Research on material composition of gabbro and syenite in the Lô-Gâm and Phú Ngữ structures shows that they are specific gabbro-syenite associations. Three distinct gabbro-syenite associations have been classified: Lục Yên, Tích Cốc and Chợ Đồn. The first and the second type are normal and of subalkaline mafic association, and the last type is of alkaline mafic one. Although they have high sodium mineral phase such as nepheline, they belong to potassic-sodic alkalinity. By correlation of alkalinity and aluminium content, they are of miaskitic series. According to our research, alkaline colour minerals consist only of pargasite.
Mafic rocks of the first type are low in titanium, low in alkalinity (sodic series), high in aluminium, especially rich in LREE along with the depletion of Nb, Ta.., implying that they belong to the calc-alkaline series. Intermediate members of this series are potassic subalkaline formations, characterized by highly variation of trace and rare earth elements, which are typical for subductionrelated formations [4, 7]. The genetic relationship between gabbro and syenite of this association is proved by the compatibility of trace, rare earth elements and isotopic significance. They are all products of a differentiated magma, which was contaminated by crustal materials.
Monzogabbro and monzodiorite and syenite of the Tích Cốc association has significant difference of mineral assemblage (presence of hypersthene, titano-biotite, and abundance of apatite), geochemistry (low aluminium, high titanium, alkalinity and K, Cs, Pb, REE) but still represented the subduction - related magma (depletion of Nb, Ta, Zr, and Hf). The homogeneity of geochemical and isotopic significances of mafic and intermediate rocks shows that they have close genetic relationship. Similar to the first type, they show crustal contaminated manifestation, represented by high Ba, Ce/Yb and 87Sr/86Sr ratios in both gabbro and syenite rocks.
The presence of foid-bearing alkaline gabbro and alkaline syenite in the Bằng Phúc massif changes the identification of the Chợ Đồn Complex. Although having distinct features in mineral assemblage, and geochemistry as well, Chợ Đồn rocks also show similar variation of trace element and Sr isotopic data as well. After Nguyễn Trung Chí et al., 2003, e Nd(T) isotopic data shows the value of -2.9á -9.7, similar to our Nd data from Lục Yên gabbro and syenite. Thus, the studied gabbro-syenite associations were isotopically originated from a similar crustal contaminated magma. The compositional differences of gabbro-syenite associations are probably due to host rocks, where they were crystallized, by assimilation, contamination and metasomatism in segregated magma chamber, especially carbonate assimilation. This feature is expressed significantly in the Bằng Phúc massif, by the replacement of pargasite, surrounding titanium-high hedenbergite, and/or REE-low carbonate, and the presence of andradite-grossular garnet [Ngô Thị Phượng et al, in this jourual issue]. In addition, the depletion of Zr, Nb, Sr, Ta and REE excludes that they are formed at greater depth (upper mantle) as seen in alkaline rocks of the Siberian platform (Maimete and Tugu), Kola Peninsula (Lovozero and Khibina), Urals (Vishnevogorsk) (Fig. 5). It presents that the alkaline magma is affected by carbonate assimilation.
In short, the gabbro-syenite associations of Lô-Gâm and Phú Ngữ structures are differentiated from magma with significant contribution of crustal materials. This condition (depletion of Ta, Nb, and Zr) is quite similar to subduction-related magmatism. These magmatic associations are probably of post-collision, which are related to extension in the continental margin, contrary to intraplate magmatic associations that rich in LILE, HFSE and REE. The age of Lục Yên gabbro and Chợ Đồn nepheline syenite are 244 Ma and 233 Ma, respectively, which represents Indosinian event. They were formed possibly in the relaxation between Indosinian and Yanshanian orogeny in the southeast margin of South China Block. However, it needs to have detailed research on trace elements and isotopic geochemistry of whole rock and minerals as well.
Acknowledgement
The authors would like to thank Prof. Phan Trường
Thị, Ph. D. Nguyễn Trung Chí for their help in the fieldwork at Bằng Phúc; Eng.
Nguyễn Văn Thế for giving samples of Yên Minh and Lục Yên massifs; Profs.. Chung Sun-Lin and Lo Ching-Hua,
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