J. Cent. South Univ. Technol. (2008) 15: 237-243

DOI: 10.1007/s11771-008-0045-4

Characteristics of soure region and tectonic implications of Early Indosinian Weiya gabbroic rock, eastern Tianshan mountains

ZHANG Zun-zhong(张遵忠)^{1, 2, 3}, GU Lian-xing(顾连兴)^{1, 2}, WU Chang-zhi(吴昌志)², SHAO Yi(邵 毅)^{1, 2},

XU Jian-rong(许建荣)^{1, 2}, ZHAI Jian-ping(翟建平)⁴, ZHENG Yuan-chuan(郑远川)²,

TANG Jun-hua(唐俊华)², WANG Chuan-sheng(汪传胜)², SAN Jin-zhu(三金柱)⁵

(1. East China Mineral Exploration and Development Bureau for Non-ferrous Metals, Nanjing 210007, China;

2. State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University,

Nanjing 210093, China;

3. State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China;

4. School of Environment, Nanjing University, Nanjing 210093, China;

5. No. 704 Brigade of Xinjiang Geoexploration Bureau for Non-ferrous Metals, Hami 839000, China)

Abstract: Based on the detailed petrographic study, the characteristics of source region of the Weiya gabbro and tectonic implications were studied. The results show that the gabbroic rock consists mainly of gabbro, with less amount of ultra-mafic rocks. The ultra-mafic rocks show cumulate texture and are gradually transitional contact with gabbro, indicating that they are cumulate products of parental magma. The ultra-mafic rocks consist mainly of spinel periodite and spinel clinopyroxenite. The former is mainly composed of olivine (65%-70%), spinel (10%-15%), hornblende (5%-10%) and phlogopite (5%-10%); the latter consists mainly of clinopyroxene (70%-80%), spinel (15%-20%) and phlogopite (0-10%), with minor amounts of carbonate (0-2%). No olivine or orthopyroxene is found. The gabbro is composed mainly of clinopyroxene (35%-40%), plagioclase (An 55-65, 40%-45%), hornblende (5%-15%), with variable amounts of carbonate (0-5%). Petrographic observations show that the source region of the Weiya gabbroic rock is water-rich due to intensive intra-continental A-type subduction occurring in this region during late Permian to early Triassic.

Key words: petrography; gabbro; indosinian; Weiya; eastern Tianshan

1 Introduction

It is generally accepted that mantle-derived magmas are formed either at convergent plate boundaries or within plate interiors^[1]. The petrogenesis of mantle-derived magmatic rocks can commonly be traced by their geochemical or isotopic data. However, the validity of geochemical data (e.g. trace elements and isotopes) interpretations become questionable without a clear understanding of the fundamentals of magma genesis and the first-hand field and petrographic observations. For example, gabbro samples taken from a modern oceanic lower crust are deficient in Ti-Fe oxides and show negative Nb-Ta-Ti anomalies on a spidergram^[2]. If this is interpreted as genetically associated with an arc setting, it is apparently inconsistent with geological evidence^[3]. It is worth emphasizing that modern analytical instruments make it possible to obtain high quality geochemical data, but any meaningful interpretations of these data cannot be divorced from basic petrological understanding and field observations.

Based on research of zircon geochemistry, ZHANG et al^[4] have concluded that two-generation zircons are found in the Weiya gabbroic rock. The first generation is crystallized from magma chamber, wheras the second generation is formed at shallower level, i.e. crystallization site. However, just as mentioned above, the source and tectonic setting of the Weiya gabbroic rock cannot be determined only by zircon geochemistry. Hence, in this work the characteristics of source region of the Weiya gabbroic rock and its tectonic implications were investigated based on detailed petrographic study.

2 Geological settings

The studied area (Fig.1) is situated in the eastern

Fig.1 Regional geology of eastern section of middle Tianshan mountains

segment of the middle Tianshan mountains, which is bounded by the Shaquanzi fault to the north and the Hongliuhe fault to the south and is referred to as middle Tianshan crystalline axis^[5] or middle Tianshan crystalline belt^[6]. The Precambrian volcanic-sedimentary rocks are divided into the middle Proterozoic Xingxingxia and Kavabulake groups and the late Proterozoic Tianhu group, which were deformed and metamorphosed around 1 000 Ma and 700 Ma^[7], respectively. These rocks are composed of migmatized schists, gneisses, marbles and amphibolites. This belt is geologically comparable to a zone at the northern margin of the Tarim Precambrian continent far to the south^[8] and is thus interpreted to have been pulled apart from the Tarim craton by the early Palaeozoic back-arc extension induced by the southward subduction of the Tianshan ocean along the Shaquanzi fault (Fig.1). The subduction direction of the Tianshan ocean was reverted towards the north during the late Palaeozoic time (Fig.1), producing Qoltag island arc (north Tianshan island arc) belt to the north of the Shaquanzi fault^[9-10]. The Qoltag island arc and the middle Tianshan passive continental margin collided along the Shaquanzi fault as the subducted Tianshan ocean was completely consumed at late Carboniferous epoch (around 300 Ma) ^{[7, 11]}, resulting in a dextral ductile shearing along the Shaquanzi fault during early Permian^[12].

3 Geology of Weiya complex

Weiya complex (Fig.2) is situated near the Weiya station of Nanxing railway and is 130 km to the

Fig.2 Geological map of Weiya complex: 1—Gabbro; 2—Quartz syenite; 3—Quartz diorite; 4—Biotite monzogranite;  5—Quartz diorite porphyrite (dykes); 6—Fine-grained granite; 7—Middle Proterozoic gneiss; 8—Quaternary sediments

southeast of the Hami, Xinjiang Uygur Autonomous Region, China. The complex, with nearly elliptical shape and an outcrop area of nearly 200 km², intrudes the middle-late Proterozoic schists and gneisses, and late Proterozoic (Chengjiangian) gneissic granites and is free of tectonic deformation.

The complex is composed of at least six intrusive phases in a sequence of gabbroic rock, quartz syenite, quartz diorite, biotite monzogranite, quartz diorite porphyrite and fine-grained granite. Such a sequence is defined based on the following field observations. The gabbroic rock is seen clearly cut by the quartz syenite. The sharp boundary between the quartz syenite and the biotite monzogranite dips outward beneath the quartz syenite with a pinkish hydrothermal alteration zone in the quartz syenite and a fine-grained chilled margin of the biotite monzogranite at two sides of the contact zone. The quartz syenite is cut by biotite monzogranite branches in some localities. It can also be found that patches of quartz diorite are scattered in and commonly penetrated by the biotite monzogranite. Contacts between these two rock types can either be sharp or transitional. K-feldspar porphyroblasts (1-2 cm) are present in the quartz diorite near the contacts. The biotite monzogranite, is commonly cut by and occurs as xenoliths inside the fine-grained granite. All the above rocks except the fine-grained granite are cut by the quartz diorite porphyrite dykes (Fig.2).

ZHANG et al^[13] reported zircon SHRIMP U-Pb dating of different lithological phases in the Weiya complex. The ages (Fig.2) of gabbro, quartz syenite, quartz diorite porphyrite dykes and fine-grained granite are   (236±6), (246±6), (233±8) and (237±8) Ma, respectively. These data are basically consistent with field observations within analytical errors. Consequently, it can be concluded that the Weiya complex has ages of 233-246 Ma corresponding to the early-middle Indosinian cycle.

4 Petrography of Weiya gabbroic rock

The gabbroic rock includes five small intrusions with an outcrop area of about 1 km². It consists mainly of gabbro, with less amount of ultra-mafic rocks. The ultra-mafic rocks show cumulate texture and are in transitional contact with gabbro, indicating that they are cumulates of parental magma. For the reason of their same origin, the five small intrusions were attributed to gabbroic rock in this work^[14].

4.1 Ultra-mafic rocks

Ultra-mafic rocks consist mainly of spinel periodite (Fig.3(a)) and spinel clinopyroxenite (Fig.3(b)) which are medium-fine grained massive rocks with black or greenish-grey color.

4.1.1 Spinel periodite

The rock is mainly composed of olivine (65%-70%), spinel (10%-15%), pargasite (5%-10%) and phlogopite (5%-10%).

Olivine occurs as panidiomorphic-granular crystal (Fig.3(a)), most grains are 1-3 mm in size with large grains up to 5 mm and small grains low to 0.5-1.0 mm. Typically, the olivines are replaced by serpentine along cracks in a mesh pattern. Small relict olivine islands commonly remain in the net centers, forming an array of sub-grains that have the same extinction direction, indicating that they are parts of a larger single crystal.

Spinel is greenish-dark idiomorphic crystal (Fig.3(a)), with most grains of 0.1-0.2 mm in size and minor ones up to 0.3 mm. The spinel grains are both scattered among olivine grains and included within olivine, indicating that the former crystallized earlier than the latter.

Pargasite is yellowish-brown (Fig.3(a)), with most grains of 0.2-0.3 mm in size. It is commonly scattered among olivine grains. The absence of the reaction-rim texture that pargasite replaces olivine suggests that the pargasite is primary mineral crystallized from magma and latter than spinel and olivine.

Phlogopite is sheet-like (Fig.3(a)) and is scattered among olivine, spinel and hornblende grains. The contact boundaries are straight and replacement texture or reaction-rim texture is absent, showing that phlogopite is primary mineral which is latest crystallized from melt than other minerals.

4.1.2 Spinel clinopyroxenite

This rock is mainly composed of clinopyroxene (70%-80%), spinel (15%-20%) and phlogopite (0-10%), with minor amounts of carbonate (0-2%). No olivine or orthopyroxene has been found.

Spinel is greenish-dark in color (Fig.3(b)), with most grains less than 0.05-0.10 mm and minor grains larger than 0.1 mm, the largest up to 0.4 mm. Most of the spinel grains are included within the clinopyroxene grains. Also, they can be scattered among clinopyroxene where abundant phlogopite grains occur, indicating that the spinel is crystallized earlier than the clinopyroxene.

Clinopyroxene can be divided into two generations. The earlier is crystallized from fluid-poor magma and occurs as anhedral grains, most of which show granular or bay texture and are 0.1-0.5 mm in diameter. The late-generation is formed from fluid-rich melt where abundant phlogopites are crystallized and shows perfect crystals. Particularly, the crystal-surfaces of clinopyroxenes are straight towards growth direction of phlogopite grains. Generally, the grain size of the late-generation is less than that of the early-generation.

Phlogopite is sheet-like (Fig.3(b)) and is commonly scattered among olivine and spinel grains. The contact boundaries are straight, no replacement texture or reaction-rim texture is developed, showing that phlogopite is primary mineral crystallized from magma.

Carbonate is colorless and transparent (Fig.3(b)) and occurs as anhedral grains scattered among clinopyroxene grains, with 0.1-0.3 mm in size. Contact boundaries between carbonate and clinopyroxene are straight, no replacement texture or reaction-rim texture of clinopy- roxene or other minerals can be found, suggesting that carbonate is crystallized latter than other minerals from the magma. The occurrence of carbonate in the rock indicates that the primary magma has some proportion of CO₂.

Fig.3 Microphotographs for textures of Weiya gabbroic rock (all under crossed nicols): (a) Microphotograph for texture of spinel periodite; (b) Microphotograph for texture of spinel clinopyroxenite; (c) Microphotograph for texture of Clinopyroxene in gabbro;  (d) Microphotograph for texture of Clinopyroxene in gabbro; (e) Microphotograph for texture of Hornblende in gabbro;          (f) Microphotograph for texture of Hornblende in gabbro; Hbl—Hornblende; Carb—Carbonate; Cpx—Clinopyroxene; Ol—Olivine; Par—Pargasite; Phl—Phlogopite; Pl—Plagioclase; Spl—Spinel; Ti-Fe—Ti-Fe oxides

4.2 Gabbro

Gabbro is a medium- to fine-grained massive rock with greenish-dark color. It is composed mainly of clinopyroxene (35%-40%), plagioclase (An55-65, 40%- 45%), hornblende (5%-15%), with minor amounts of carbonate (0-5%). No olivine or orthopyroxene has been found. The accessory minerals are dominated by apatite and Ti-Fe oxides besides zircon^[4].

Clinopyroxene can be generally divided into two generations. The early-generation, occurring as prism crystals (Fig.3(c)) and being similar to plagioclase in size (3-5 mm) is variably replaced by hornblende. However, the late-generation shows weak alteration, occurring as short-prism crystals (Fig.3(d)), with grains 1-2 mm in size less than those of the early-generation. Occasionally, reaction-rim texture of clinopyroxene enclosed by hornblende can be found.

Plagioclase can be also divided into two generations. The early-generation, with higher An contents, is dominated by polysynthetic twins with the width of lamellar in 0.05-0.10 mm. Plagioclase of this generation, 3-5 mm in size, is ubiquitously suffered from variable sericitization. In contrast, the late-generation, with lower An contents, is dominated by polysynthetic twins with width of lamellar less than 0.05-0.10 mm. Plagioclase of this generation, 1-2 mm in size, is mainly scattered among grains of clinopyroxene and shows clear crystal surfaces (Fig.3(d)).

Hornblende is greenish-dark in color and three types of hornblende can be distinguished in thin sections: 1) the first type of hornblende occurs as prism crystals with 0.05-0.20 mm in size (Fig.3(c)), and has clear cleavage, this type of hornblende commonly encloses the early-generation clinopyroxene and reaction-rim texture can be found; 2) the second type of hornblende occurs where the early-generation clinopyroxene is replaced by fluids along cracks, such type of hornblende shows characteristics of island- or bay-like texture (Fig.3(e)); and 3) the third type of hornblende, with poor crystals, is formed along the contact boundaries between the late-generation clinopyroxene and plagioclase (Fig.3(f)). The reaction-rim texture, with rim width 0.01-0.02 mm, can be identified. These three types of hornblende are likely to be the replacement products of clinopyroxene by fluids.

Carbonate is colorless and transparent. It occurs as anhedral grains, with 1-2 mm in size, scattering among clinopyroxene and plagioclase grains.

5 Discussion

5.1 Source characteristic of water-rich

NIU^[3] pointed out that anhydrous basaltic magma takes a different evolution path compared with hydrous basaltic magma. Anhydrous basaltic magma, such as MORB(middle ocean ridge basalt) one, follows an evolution path of dunite—troctolite—gabbro in terms of cumulate mineral assemblages. In contrast, hydrous basaltic melt such as IAB(island arc basalt) magma with abundant water takes a different path in that the cumulate equivalent to troctolite in anhydrous system becomes wehrlite in hydrous system. That is, differentiation of hydrous basaltic melts takes a path of dunite—wehrlite—gabbro in terms of mineral assemblages. The above cumulate assemblage is more effective than elemental geochemistry in tectonic settings discrimination.

As far as the Weiya gabbro is concerned, it is derived from basaltic parental magma that has experienced increasing fractional differentiation. Spinel (Al-rich) and olivine (Mg-rich) crystallized from parental magma lead to residual melt depleted in Al and Mg and enriched in Ca. As a result, the clinopyroxene is increasingly separated as a cumulate from the residual melt. Following gradual differentiation of clinopyroxene, the plagioclase will occur when the more evolved melt is increasingly poorer in Ca and Mg and richer in Al. The earlier fractionation of clinopyroxene relative to plagioclase from parental magma implies that parental magma is hydrous (water-rich) instead of anhydrous (water-poor). This is because the fractionation of clinopyroxene will be earlier than that of plagioclase if the parental magma is hydrous^[15]. On the contrary, the fractionation of clinopyroxene will be later than that of plagioclase if the parental magma is anhydrous. Although the crystallization of clinopyroxene in anhydrous basaltic parental magma may be earlier than that of plagioclase at elevated pressure, this requires pressures in excess of 800 MPa^[16]. Considering the fact that the parental magma of the Weiya gabbro emplaced at a depth where the wall-rocks are suffered from greenschist facies to amphibolite facies metamorphism, which corresponds to depths of 10-20 km^[17], it is suggested that clinopyroxene is unlikely to crystallize earlier than plagioclase from anhydrous magma under these pressure conditions. This can be understood experimentally by simple binary phase diagram (Fig.4) of diopside(Di)—anorthite(An)^[18] which indicates that a basalt with about 60% normative plagioclase (An) and about 40% normative diopside(Di) will crystallize An before Di begins to crystallize at the “dry” eutectic. However, the same basalt with abundant water (i.e. high water pressures) will crystallize Di before An joins at the “wet” eutectic. Di will crystallized from hydrous magma before An begins to crystallize. The water-rich characteristic of parental magma of Weiya gabbro can be verified by both occurrence of primary pargasite and phlogopite in spinel periodite and primary phlogopite and carbonate in spinel clinopyroxenite. Therefore, the source region of the Weiya gabbro is water-rich in terms of its hydrous parental magma.

Fig.4 Di-An binary phase diagram of Yoder^[18]

5.2 Tectonic implications

ZHANG et al^[19] suggested the source region of the Weiya gabbro was metasomatized by fluids released from young subducted continental crust, with limited continental contamination during magma ascent and emplacement. It is indicated that Weiya gabbro is formed in early Triassic when the middle Tianshan areas are situated in intraplate settings^[13,20]. Voluminous Indosinian granites have been reported from middle Tianshan and its adjacent areas such as Beishan and eastern Kunlun, indicating that tectonothermal events are significantly active in middle Tianshan and its adjacent areas in Indosinian period^[21-23]. Hence, it is supposed that dynamics and heat origin of Indosinian tectonothermal events in middle Tianshan and its adjacent areas are attributed to relation to northward subduction of Paleo-Tethyan ocean from late Permian to Triassic^[24]. It is the northward subduction of Paleo- Tethyan ocean from late Permian to Triassic that triggers intensively southward subduction of young terrane north to the middle Tianshan mountains. Such an inference is consistent with conclusion of ZHANG et al^[25], who, based on research of quartz syenite within the Weiya complex, have proposed that the thickened crust in Tianshan areas in Indosinian is likely related to northward subduction of Paleo-Tethyan ocean from late Permian to Triassic, which, in its turn, induced intensive intra-continental subduction (A-type subduction) of the Tianshan areas, and also consistent with conclusions of XIAO et al^[26-27], who, based on tectonic research of eastern Tianshan, have proposed that intensive Triassic (Indosinian) compression of this area is induced by northward subduction of the Paleo-Tethyan ocean plate.

From the foregoing, it can be concluded that the source region of the Weiya gabbro is characteristic of water-rich and might have been metasomatized by fluids released from young subducted continental crust (A-type subduction). Similar to B-type subduction, A-type subduction may play a fundamental role in causing source enrichment of mantle-derived magmas^[28]. From this point of view, further research on Indosinian magmatism will be of particular importance to evaluate geological evolution of the Tianshan orogenic belt and its adjacent areas in Triassic.

6 Conclusions

1) The source region of the Weiya gabbroic rock shows characteristics of water-rich.

2) Water-rich characteristics of the source region for the Weiya gabbroic rock implies that the source region is metasomatized by fluids released from young subducted continental crust (A-type subduction).

3) Intra-continental subduction (A-type subduction) may occur during late Late Permian or early Early Triassic in eastern Tianshan mountains.

Acknowledgements

We are grateful to Prof. ZHOU Jin-cheng and Dr. CHEN Li-hui, Department of the Earth Sciences, Nanjing University, for their constructive discussion, which is greatly contributed to improvement of this manuscript.

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(Edited by CHEN Wei-ping)

Foundation item: Projects(40672040, 40472042 and 40603008) supported by the National Natural Science Foundation of China; Project(2001CB409802) supported by the Major State Basic Research Development Program of China; Project(2005038237) supported by the Postdoctoral Science Foundation of China; Project supported by the Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University

Received date: 2007-04-29; Accepted date: 2007-06-11

Corresponding author: GU Lian-xing, Professor; Tel: +86-25-83686792; E-mail: lxgu@nju.edu.cn