SYNKINEMATIC ULTRAMAFIC-MAFIC MAGMATISM IN THE RED RIVER SHEAR ZONE

A. E. IZOKH¹, TRẦN TRỌNG HOÀ², G. V. POLYAKOV¹,
NGÔ THỊ PHƯỢNG², TRẦN TUẤN ANH², A. V. TRAVIN³

¹Institute of Geology, SB RAS, Novosibirsk, Russia;
²Institute of Geological Sciences, NCNST, Hoàng Quốc Việt Road, Hà Nội, Việt Nam;
³Analytical Center of UIGGM, SB RAS, Novosibirsk, Russia

Abstract: A multistage syn-shearing differentiated lherzolite-websterite-gabbro association is distinguished as a part of the Red River Shear zone (RRSZ). Isotope studies provided some tentative forming stages of the ultramafic-mafic association of the RRSZ. The eraly stage (49-42 Ma) is recorded by garnet-bearing amphibole gabbro and peridotite with Al-high clinopyroxene at the M#u A cross-section, which were intruded under deep conditions before the exhumation of the matter from the shear zone. Younger ages (35 Ma) has been obtained for the amphibole gabbro at the marginal part of the shear zone (Mậu A cross-section) and for the websterite from the Việt Trì area, which coincides with the time of formation of Phan Si Pan lamproite. The youngest age (25 Ma) has been obtained for amphibole gabbro with Al-low clinopyroxene, which probably was generated at lower depth after exhumation. The presence of high-temperature mantle-derived melts, which intruded at different stages of the RRSZ formation, provides a new view on its tectonic history.

 

INTRODUCTION

The Red River or Ailao Shan - Red River shear zone (RRSZ) attracts the attention of geologists because the formation of this large-amplitude shear zone (more than 600 km) is associated with the Indostan-Eurasia collision [1]. Earlier, the metamorphic rocks of this zone were considered as an Archean metamorphic structure [2]. Recent geochronological studies have shown, however, that they were formed in the Late Oligocene-Early Miocene [3-5]. Igneous rocks of the RRSZ have received less attention. But the post-shear granite massifs of this zone are shown to be formed 22-24 Ma years ago [6]. There are some bodies of lherzolites, olivine pyroxenites, and pyroxenites (Bảo ái Complex) in the RRSZ, whose igneous genesis is proven, but no data are available about their age, the formation to which they belong, and their geodynamic setting [7,8]. Amphibolites, gabbro-amphibolites, and garnet amphibolites are also described as counterparts of the Proterozoic Cẩm An Complex. Some researchers describe these formations as amphibolites of the metamorphic formations of the Dãy Núi Con Voi or RRSZ zone [5].

GEOLOGIC SETTING

In 2002, in the framework of the project "Within-plate magmatism of North Việt Nam and its metallogeny" we have studied on amphibolites, amphibole gabbros, pyroxenites, peridotites, and different granitoids of the Dãy Núi Con Voi zone in some cross-sections across the strike of the RRSZ zone (Fig. 1).

In the cross-sections Km 9 southeast of Bảo Yên along the road, thin (5-10 cm) interbeds of garnet amphibolites with metamorphic foliation occur within garnet-sillimanite-biotite gneisses with bedding-plane partings of migmatites. Combined with gneisses, these rocks form complicated folds, which are distinguishable as metamorphic formations (paramphibolites). There are also massive garnet-bearing metagabbros (H1591) with fine impregnation of pyrrhotite and chalcopyrite and with poikilitic grains of biotite. In composition, these rocks correspond to subalkalic gabbros (monzogabbros) and are characterized by high contents of TiO₂ and K₂O (Table 5). They are intruded by veins of cordierite granite-pegmatite. In the central part of the zone, fresh clean-ups along the road expose dike bodies of amphibole gabbros (H1701/1) and hornblendites (H1701/2), which are experienced viscoplastic deformations and lensing (Fig. 2). The amphibole gabbros in these bodies are characterized by uniform composition and massive structure. Unlike the amphibolites, they show neither metamorphic differentiation nor folded deformations. Thin sections display igneous gabbro texture, thus implying their magmatic genesis. The gabbroids contain pyrrhotite-chalcopyrite impregnation. The host metamorphic rocks are also sulfidized and graphitized in places. Veins of leucocratic granites and granite pegmatites break through the gabbroids. Lherzolites and pyroxenites are absent in this site.

Table 1. Olivine composition from peridotite from the Red River shear zone

Note: H1715 - coarse-grained lherzolite with orthopyroxene poikilitic texture; H1718 - biotite-bearing lherzolite; H1720/1, H1721/2 - lherzolite. Analysis was made on "Camebaxmicro" in Analitical Center UIGGM SB RAS, Nobosibirsk by E. Nigmatylina.

The second cross-section of the RRSZ extends from the town of Mậu A northward (Fig. 1). At this cross-section small boudins and separate large bodies (up to 100 m) of amphibole gabbros, garnet-bearing amphibole gabbros, amphibole websterites, lherzolites, and schriesheimites are observed within gneisses. In the southern part of this cross-section in the RRSZ, gneisses host a large body made up from amphibole gabbros and websterites. Contacts of gabbroids with gneisses and blastomylonites are distinct, but cooling zones are absent (Fig. 3). The central part of the body is made up from websterites grading upward into gabbroids (Fig. 4). Some outcrops of large-grained lherzolites with megacrysts of orthopyroxene (H1715) also occur there. Bodies of lherzolites, schriesheimites, and websterites are mapped in the central part of the shear zone. Garnet amphibole gabbros and diorites are also widespread there. Some bodies are differentiated from lherzolites to olivine websterites, and others, from olivine websterites to websterites and amphibole melanogabbros. In the same cross-section, a dike of small-grained amphibole gabbro-dolerite (H1712) is found within gneisses. The dike is 3-4 m thick. Unlike the earlier described bodies, no lensing occurs there. These observations imply that picrobasalt magmas were intruded many times and this process continued even after the main shear movements had been completed.

Table 2. Orthopyroxene composition of amphibole gabbro, pyroxenite and peridotite from the Red River shear zone

Note: H1718 - biotite-bearing lherzolite; H1721/2, H1720/1, - lherzolite, H1713, H1738 - websterite; H1737 - amphibole gabbro.

Table 3. Clinopyroxene composition from amphibole gabbro, pyroxenite and peridotite from the Red River shear zone

Note: H1718 - biotite-bearing lherzolite; H1719/1 - lherzolite,

H1719/2 - garnet-bearing diorite; H1726, H1701/1 - amphibole gabbro.

In the Việt Trì area, amphibolites that underwent metamorphic differentiation are exposed in a marl quarry. Between them, massive outcrops of amphibole gabbros, hornblendites, as well as amphibole and olivine websterites occur, whose composition is similar to the composition of rocks of the previous cross-sections (Table 5).

Table 4. Amphibole composition of amphibole gabbro, pyroxenite and peridotite from the Red River shear zone

Note: H1591- amphibolite (metamonzogabbro); H1701/1; H1704, H1706, H1711/1, H1713, H1716, H1717, H1726, H1737 ? amphibole gabbro; H1712 ? amphibole gabbrodolerite; H1713, H1736, H1738 ? websterite; H1715 - coarse-grained lherzolite with orthopyroxene poikilitic texture; H1722 ? amphibole peridotite; H1718 ? biotite-bearing lherzolite; H1719/1, H1720/1, H1721/2 - lherzolite, H1719/2 ? garnet-bearing diorite; H1734, H1735 - amphibolite

MINERALOGY AND PETROGRAPHY OF THE ROCKS

The lherzolites and websterites from the Yên Bái area have been mineralogically and petrographically characterized in earlier works [8]. The ultrabasic rocks at the Mậu A cross-section are similar in petrography and petrochemistry. Compositionally, olivine in the lherzolites corresponds to chrysolite (f = 17-27). Olivines have low content of CaO, which is typical for plutonic rocks (Table 1). The Mậu A lherzolites and pyroxenites are characterized by high content of alumina in clinopyroxenes (up to 7-8%) (Table 3), which indicates a great depth of their generation unlike the Yên Bái peridotites. Clinopyroxenes from the lherzolites of this region contain no more than 3% alumina [8]. High-Al clinopyroxenes are typical for garnet amphibole gabbros and diorites, which are associated with lherzolites and websterites from the Mậu A area, which is also indicative for higher pressure of their formation. Primary-magmatic amphibole in peridotites and pyroxenites corresponds to pale-coloured Mg-hornblende with high alumina of 8-11% (Table 4). For amphibole from the lherzolites, the content of alumina and sodium shows a positive correlation, which is not true for pyroxenites.

Fig.2. Synkinematic dykes of amphibole gabbro and plagioclase-bearing hornblendite in gneisses and blastomylonitesy of the Red River shear zone (cross - section near Bảo Yên (Fig.1).

 

 

 Fig. 3. The contact between amphibole gabbro (H 1716), garnet- sillimanite biotite gneisses (H 1716/1) and blastomylonites in the cross - section near Mậu A (Fig.2)

The amphibole gabbros have the following petrographic composition: basic plagioclase, clinopyroxene - magnesian augite-augite (f =10-35) and brown amphibole. Orthopyroxene (bronzite) is not ubiquitous. Compositionally, amphibole corresponds to Mg-hornblende and, less frequently, to pargasite. Edenite was found only in the dikes of amphibole gabbrodolerite (Table 4). Ore minerals are represented by Ti-magnetite and ilmenite. Impregnation of pyrrhotite and chalcopyrite is rather widespread. No olivine is found in gabbroids. The rocks have a gabbro texture and a massive structure with rare by oriented arrangement of amphiboles. Thin sections show no signs of plastic deformations which are typical for the host gneisses. In gabbroids, amphibole often displays a higher degree of idiomorphism than plagioclase, which implies a higher content of water in the melt. No cocrystallization of olivine with plagioclase, early crystallization of orthopyroxene relative to clinopyroxene in websterites, and the presence of garnet in some amphibole gabbros are indicative for deep-seated crystallization (more than 6 kbar).

According to alumina content in clinopyroxene, the amphibole gabbros are subdivided into two groups (Table 3). The first cross-section, the Bảo Yên area, is characterized only by amphibole gabbros with low content of alumina in pyroxene (2.5-3%), and the second, Mậu A cross-section, by amphibole gabbros and garnet amphibole gabbros with high content of alumina in clinopyroxene (up to 8%). This suggests different facies of depth involved in the generation of these rocks. It is possibly connected with different stages of their formation. Gabbroids, pyroxenites, and peridotites with Al-rich pyroxenes were formed at great depth under 6-8 kbar, as inferred from the host gneisses and garnet amphibolites [5]. The latter correspond to the garnet amphibole gabbros we have studied. Their emplacement occurred before gneiss exhumation. Unlike them, the gabbroids with low-Al pyroxenes were formed after exhumation at a shallower depth. As will be shown below, these stages of evolution conform with the Ar-Ar ages.

PETROCHEMISTRY AND GEOCHEMISTRY OF ROCKS

Petrochemical composition of the RRSZ ultrabasic-basic association is given in Table 5. On the variation diagrams MgO-Al₂O₃ and MgO-CaO, lherzolites, pyroxenites, and amphibole gabbros form a single trend, which permits us to refer them to a single differentiated association. This conforms with geological observations of some bodies with magmatic differentiation. The lherzolites from the Mậu A cross-section differ a little from the Yên Bái lherzolites [8]. MgO content varies from 35 to 25% (Fig. 5). A significant negative correlation of Mg with Al and Ca is observed in lherzolites, that coincides with the general trend of differentiation in this association. No correlation between Si and Mg suggests that the differentiation is due to olivine fractionation. Websterites and olivine pyroxenites form a discrete group, whose position on variation diagrams matches olivine fractionation (Fig. 5). They have higher content of Al₂O₃, CaO, and higher Fe/(Fe+Mg). The amphibole gabbros form a compact group, which also follows the general trend of fractionation. Garnet-bearing and garnet-free gabbroids do not differ in chemical composition. They are moderately enriched in Al₂O₃ (up to 17%) and Ti (up to 1.3%) and are depleted in phosphorus. In content of alkali they correspond to basalts of normal series (Table 5). In this aspect they drastically differ from metamorphosed monzogabbronorite, which is characterized by high concentration of potassium (up to 2%). High content of Ti and P and REE pattern (Fig. 6) in this rock allows them to be classified as gabbroids of subalkalic series. These observations raise the question of the presence of subalkalic synmetamorphic magmatism in the RRSZ.

Table 5. Whole rock composition (in mass %) of lherzolite-websterite-gabbro association from Red River shear zone

Note: analysis was made by RFA method in Analytical Center UIGGM SB RAS, Nobosibirsk, by A.D. Kireev. Iron ? in ferrous oxide

Table 6. Content of trace elements in rocks (%) of lherzolite-websterite-gabbro association from the Red River shear zone

Note: analysis was made by ICP-MS method in Irkutsk, using the following standards: IP-1, BHVO-1, W-2, JB-2, RGM-1.

  

 

 Figure 4. The large intrusive body with differentiation from websterite (H1713) to amphibole gabbro (H1714) (H1713) in the cross - section near Mậu A (Fig.2).

Interesting data have been obtained by analyzing rare and rare-earth elements of ultrabasic-basic association of the RRSZ (Table 6). Despite close chemical composition of amphibole gabbros in different areas, they appeared to be of same difference in distribution of REE (Fig. 6). The gabbroids from the Bảo Yên area display a pattern of weakly fractionated REE distribution at the level of 10 chondrite units and a weak positive Eu anomaly. The ratios La/Sm and Ce/Yb are close to unity (0.94-1.11 and 1.11-2.5, respectively). The primitive mantle-normalized spidergram shows Zr and Hf minima and Rb, Th, U, and Sr maxima. In geochemistry, metamorphosed monzogabbronorite differs drastically from these gabbroids. It is characterized by higher contents of REE and their strongly differentiated distribution (La/Sm = 2.6; Ce/Yb = 6.55). Eu anomaly is absent. The spidergram displays minima for Zr, Hf, and Y, and maxima for Rb, Nb, Ba, U, and Th.

 

 

Figure 5. Whole rock composition of the synkinematic lherzolite-websterite-gabbro association from Red River shear zone on the MgO-Al₂O₃, MgO- CaO diagrams.
1 ? amphibolite; 2 ? amphibolitic gabbro; 3 ? websterite; 4 ? lherzolite; 5 lherzolite from Yên Bái area [7].

 

Figure 6. REE distribution patterns in the rock of the synkinematic lherzolite- websterite-gabbro association Red River shear zone. Samples number see in plate 6. Trace elements are chondrite-normalized as recommended by Boyton (1984).

 

Amphibole gabbroids, pyroxenites, and peridotites from the Mậu A area have similar REE patterns at concentration levels of more than 10 units (Fig. 6). Differences in levels of their concentration are in agreement with the model of fractionation of a picrite or picrodolerite melt. The lowest concentrations are typical of lherzolites and some olivine websterites. Nevertheless, REE concentrations in lherzolites exceed the typical level of mantle peridotite. A weak Eu minimum is observed in them. Monotypic REE spectra are established for amphibole garnet-free and garnet-bearing gabbros, which indicate that they belong to the same series. The presence or absence of garnet seems to be related to the specific features of composition (Fe/(Fe+Mg)) or conditions of generation (high pressure). In general, REE are more differentiated as compared with the first area. The ratios La/Sm and Ce/Yb are close for peridotites (2.4 and 3.0), pyroxenites (2.15 and 2.4), and gabbroids (2.2 and 2.3). As for the Bảo Yên area, spidergrams of the Mậu A cross-section show Zr, Y, and Hf minima and, generally, U, Th, Sr, Rb, and Ba maxima, though the behaviour of the latter elements is not unambiguous, which probably is linked with metamorphism. In geochemical signature, the dike of amphibole gabbro-dolerites is similar to the differentiated series (Fig. 6). This suggests that the parent melt was composed of them. The coarse-grained lherzolite (H1715) differs from the whole series of rocks in a drastic Eu minimum, though in other elements it is close to other lherzolites.

RESULT OF Ar-Ar DATING OF AMPHIBOLE GABBRO

Earlier attempts of Ar-Ar dating of lherzolites from the Yên Bái area were not successful [8]. A possible reason is that these rocks are poor in potassium and the excessive argon borrowed from the host rocks had an effect on their isotopic composition. To carry out isotope analysis, we chose six specimens of the freshest amphibole gabbros and schriesheimite from different areas. Specimen H1701/2 is a plagioclase hornblendite from an isolated lens of the Bảo Yên cross-section (Fig. 2). It is made up of brown Mg-hornblende with subordinate plagioclase. Specimens H1706, H1711, H1716, H1722 were taken from the Mậu A cross-section. Specimen H1706/1 is garnet-free amphibole gabbro from a large boudin among gneisses; H1711 is garnet-bearing amphibole gabbro from an outcrop of basement near a spring; H1716 is amphibole gabbro from the contact with blastomylonites from a differentiated body; H1722 is fresh schriesheimite among lherzolites, and H1736 is amphibole olivine websterite from a marl quarry in the Việt Trì cross-section.

Results of bulk analysis are shown in Fig. 7. The oldest ages are obtained from amphibole garnet gabbro (H1711), amphibole gabbro (H1716), and schriesheimite (H1722) from the Mậu A cross-section: 42 ± 2.4; 51 ± 2; and 49 ± 2.6 Ma, respectively. The amphibole gabbro from the southern part of the Mậu A cross-section (H1706) is younger - 35.5 ± 1 Ma. The olivine websterite from the quarry is of the same age - 35 ± 2.3 Ma. The youngest age is obtained from the Bảo Yên hornblendite - 25 ± 1.6 Ma. As inferred from the spectra (Fig. 7), the amphibole gabbros, crystallized under low pressures, have no excessive argon, which suggests that they formed synchronously with the displaced metamorphic formations of the RRSZ [5]. The other specimens have an excess of argon, which naturally hampers the estimation on the time of origin of the whole complex. Nevertheless, the obtained data provide a lower time limit of formation of the considered intrusions, which prevents them from to be referred to as fragments of the ancient layered ultramafic-mafic massifs of the Cam An Complex or as fragments of an ophiolite association.

DISCUSSION

Geological, mineralo-petrographic, and petrochemical data show that lherzolites, websterites, and amphibole gabbros form a single lherzolite-websterite-gabbro differentiated association. These rocks form small dike-shaped bodies disturbed by strike-slip faults and subjected to lensing. Intact dikes of amphibole gabbro-dolerites are found in places. According to petrographic and mineralogical signatures, two depth facies of generation of these rocks are recognized. Garnet-bearing amphibole gabbroids as well as websterites and lherzolites with high content of alumina in pyroxenes were crystallized at a pressure of no less than 6 kbar. However, the amphibole gabbros from the Bảo Yên and Yên Bái cross-sections are characterized by a shallower depth of their generation. Geological data, fine-grained fabric, and dolerite texture suggest that the dike of amphibole dolerite was crystallized after the main stage of exhumation. Having much in common, the parenty melts from different cross-sections differ in geochemistry. Within some clusters, ultrabasic rocks and gabbroids have complementary geochemical features, which suggests that they were differentiated from the same magma.

Preliminary isotope studies provided some tentative stages of formation of the ultrabasic-basic association of the RRSZ. The early stage (49-42 Ma) gave rise to the garnet-bearing amphibole gabbros and peridotites of the Mậu A cross-section, which were intruded under depth condition before the exhumation of the matter from the shear zone. Younger ages (35 Ma) have been obtained from the amphibole gabbros in the marginal part of the shear zone of the Mậu A cross-section and for the websterite from the Việt Trì cross-section, which coincides with the time of formation of lamproites in the Phan Si Pan province [9]. The youngest age (25 Ma) is obtained from amphibole gabbroids, which were generated at lower depths, most likely, after exhumation. It is probable for this reason that they virtually have no excessive argon. Of course, the isotope age of the formation of this association require a more precise determination, but the available geological and mineralo-petrographic data confirm that there were several stages of intrusion of the single ultrabasic-basic association.

Thus, a multistage synshear differentiated lherzolite-websterite-gabbro association is to be distinguished as part of the RRSZ. The presence of high-temperature mantle-derived melts, which intruded at different stages of formation of the RRSZ, provides a new vision of its tectonic history. Moreover, the ore potential of this structure needs revision in the new context.

This work is supported by the Ministry for Science and Technology of Việt Nam in the framework of the project "Intra-plate magmatism of North Việt Nam and its metallogeny", and by grant 03-05-65088 from the Russian Foundation for Basic Research and "Leading Scientific Schools" (grant NSH-1573.2003.5).

REFERENCES

1. Tapponier P., Lacassin R., Leloup P.H., Scharer U., Zhong D., Wu H., Liu X., Ji S., Zhang L., and Zhong J., 1990. The Ailao-Shan - Red River metamorphic belt: Tertiary left-lateral shear between Sundaland and South China. Nature, 343: 431-437.

2. Dovzhikov A.E., Bui Phu My, Vasilevskaya E.D., et al., 1965. Geology of North Việt Nam. Hà Nội, 667 p. (in Russian).

3. Harrison T.M., Wenji C., Leloup P.H., Ryerson F.J., and Tapponier P., 1992. An Early Miocene transition in deformation regime within the Red River fault zone, Yunnan, and its significance for Indo-Asian tectonics. J. Geophys. Res, 97: 7159-7182.

4. Leloup P.H., Lacassin R., Tapponnier P., Scharer U., Zhong D., Liu X., Zhang L., Shaocheng, J., and Phan T.T., 1995. The Ailao Shan - RRSZ (Yunnan, China), Tertiary transform boundary of Indochina. Tectonophysics, 251: 3-84.

5. Trần Ngọc Nam, Toriumi M., Itaya T., 1998. P-T-t paths and post-metamorphic exhumation of the Dãy Núi Con Voi shear zone in Việt Nam. Tectonophysics, 290: 299-318.

6. Trần Trọng Hoà, Phan Lưu Anh, Ngô Thị Phượng, Nguyễn Văn Thế, Trần Tất Thắng, 2000. The Cenozoic granites in Red River zone. J. of Sci. of the Earth, 22/4: 306-318

7. Trần Trọng Hoà, Ngô Thị Phượng, Nguyễn Văn Thế, 1999. Subdivision and correlation of mafic-ultramafic intrusive formations in the Lục Yên Châu sheet series. Geol. & Min. Res. of Việt Nam, III: Hà Nội (in Vietnamese).

8. Trần Trọng Hoà, Trần Tuấn Anh, Ngô Thị Phượng, et al, 2000. Ultramafic formation in the Red River Zone. J. Earth Sci., 22/3. Hà Nội (in Vietnamese).

9. Polyakov G.V., Nguyen Trong Yem, Balykin P.A., et al, 1997. New data on cocite, ultrapotassic basic rock in North Việt Nam. Geologia I Geofizika, 38/1: 154-167. Moskva (in Russian).