Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

INTRODUCTION

The Gangdise tectonic-magmatic zone is located between the Indus-Yarlung tsangpo suture(IYS) zone to the south and the Bangong-Nujiang suture (BNS)zone to the north. It is a west-to-east striking, narrow terrain and a huge magmatic zone of about 2 500 km long and 100 -300 km wide, covering an area of about 400 000 km². The area of igne-ous rock exposures in the Gangdise zone is about 83% of the total area of magmatic rocks in Tibet. It is the most important continental magmatic arc on the Tibetan plateau and has attracted the attention of geologists for many years. The China National Land Geological Survey has confirmed thatGangdise is also a giant non-ferrous metal and noble metal mineralization zone resulting from magmatic responses related to the neo-Tethyan subduction and India-Asia collision processes(Zheng et al., 2004a, b; Wang et al., 2002).

Gangdise was traditionally believed to be a Yanshanian orogen and the volcanic and granitic rocks were formed in Cretaceous and Tertiary, which is related to the process of the closure of the neoTethys, subduction and arc-continent collision along the IYS(Li, 2002; Pearce and Mei, 1988; Huang and Chen, 1987; Jin and Zhou, 1978; Chang and Zheng, 1973). Geological field work has confirmed the existence of Late Triassic granitic intrusions, volcanic rocks, and tectonic unconformity between the upper Triassic and Permian. Some geologists suggested that there could be an Indosinian arc along Gangdise(Pan et al., 2004; Ren and Xiao, 2004; Li et al., 2003; Qu et al., 2003), but this requires more volcanic petrological and tectonic evidence.

Geochronological studies have been carried out on volcanic rocks in the Yeba Formation around Dagzê County, Tibet, which belongs to the" southern Gangdise Mesozoic and Cenozoic magmatic zone" (Pan et al., 1997). SHRIMP and ⁴⁰Ar/³⁹Ar measurements for zircon and feldspar have revealed that the Yeba volcanic rocks were formed at about (181.7± 5.2)Ma in the Early Jurassic and experienced metamorphism at about(131.9± 5.5)Ma and 73.2 Ma in the Cretaceous(Geng et al., 2006). In this paper, we intend to display their geochemical characteristics and confirm that it is a bimodal volcanic sequence formed in a mature island arc. The tectonic characteristics of Gangdise in the late Indosinian and early Yashannian(T₃-J₁)can be better constrained from this study, which will further confirm the existence of an Indosinian Gangdise magmatic arc.

REGIONAL GEOLOGY

The Yeba Formation was originally named the Yeba Group in 1974 by the Tibetan Team surveying the Valley of Yeba to define the volcanic rocks distributed between Lhasa and Dagzê County(Li et al., 1990; Tibetan Team of Collective Geological Survey, 1979). However, regional geological surveying in recent years suggests that the Yeba Formation is perhaps distributed in a much bigger area than originally expected, between Sangri and Gongbo'gyamda(Qinghai Team of Collective Geological Survey, 1994) (Fig. 1). Based on detailed field investigation and geochronologic studies, we have realized that the expanded Yeba Formation around Dagzê, Gongbo'gyamda and Sangri can be divided into two portions (Fig. 1, Tibetan Team of Regional Geological Survey, 2000; Qinghai Team of Collective Geological Survey, 1994), with different rock types, ages, and eruptive centers.

Figure  1.  Sketched geological map of studied region.1. Nyainqentanglha Group; 2. Lower Paleozoic; 3. Yongzhu Formation, composed of feldspar quartz sandstone, siltstone and mudstone; 4 -7. Upper Paleozoic, mainly composed of former Pangduo Group, Songduo Group, Luobadui Formation, etc.; 8. Lower to Middle Triassic, mainly composed of the Chaqupu Formation; 9. turbidite in Xiukang and Langjiexue groups; 10. sandstone and mudstone in Ridang Formation; 11. Upper Jurassic to Lower Cretaceous, mainly composed of former Duodigou Group(J3); 12. clastic rocks in the Jinzhushan Formation; 13. Early Tertiary volcanic rocks of the Linzizong Group; 14. studied volcanic rocks of the Yeba Formation near Dagzê; 15. eastern portion of the Yeba Formation to the south of Gongbo'gyamda and Maizhokunggar, outlined by geological mapping at scale of 1 ∶ 200 000, which is mainly silicic volcanic rocks and sedimentary rocks; 16. Gangdise granites; 17. locations of geological sections and sampling; 18. geological boundaries.

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We refer to the Yeba Formation exposed around Dagzê as the western portion. It is composed of slightly metamorphic basalt, schistic dacite, and silicic tuff. The lens-shaped volcanic terrain on the map extends for about 80 km, gradually declining on both sides. It is about 30 km wide in the widest place near the city of Dagzê with a thickness of about 4 000 m. Exposures of volcanic breccia and conglomerate in the Valley of Yeba, together with the thickest lava suggest that this valley was near the volcanic eruptive vent. The bottom of the Yeba volcanic sequence was intruded by Late Mesozoic granite. The volcanic terrain locally thrust on the sediments of Menzhong Formation(K_1-2m). The top of the volcanic terrain was faulted with sharp contact with limestone of the Duodigou Formation(J₃K₁). No fossiliferous sedimentary interbeds have been reported in this portion of the Yeba volcanic sequence. Therefore its age has been a major controversial problem for many years until our geochronologic measurements confirmed its age to be Early Jurassic(Geng et al., 2006; Pearce and Mei, 1988; Yin J X et al., 1988; Wang et al., 1983).

The eastern portion of the Yeba Formation is exposed in the region of Maizhokunggar, Gongbo'gyamda and Sangri, within a narrow region about 120 km long and 4 -10 km wide(Fig. 1). The eruptive center in this portion was around Jiama. The thickness of the volcanic rocks gradually decline from here. This is a volcanic-sedimentary sequence composed of dacite, rhyolite, felsic tuff and volcanic breccia, interbedded with sandstone, slate and limestone. It is covered unconformably by the Quesangwenquan Formation(T₃) and Duodigou Formation in different places and was intruded at the bottom by Late Mesozoic granites. The abundant bivalve fossils in its sedimentary interbeds suggest an age of the Bajocian stage in the Middle Jurassic(Mao et al., 2002; Gou, 1994; Yin J R et al., 1988).

The eastern portion of the Yeba Formation, around Jiama and Qulong to the south of Maizhokunggar, has been primarily studied geochemically and stratigraphically. The area represents the geographic environments of a sea beach and shallow continental platform on Gangdise volcanic island arc in the Middle Jurassic. The parent magma was derived from the partial melting of the lower crust (Mao et al., 2002). The bimodal volcanic sequence around Dagzê however, still lacks geochemical and petrogenetic research. This paper intends to discuss the tectonic settings of the Yeba bimodal volcanic rocks around the city of Dagzê based on its major and trace elemental and isotopic geochemistry.

PETROLOGIC CHARACTERISTICS

Field investigations have revealed that the Yeba volcanic rock sequence around Dagzê can be divided into three parts. These rocks have been folded, faulted and schisticly deformed in the tectonic background of Himalayan movement. The boundaries between each part are faults or have been erased due to tectonic crustal movements at later stages.

The first portion(J₁y¹)is exposed to the south of Baiding and Dagzê on the mountain slope. This west-east-striking lithology is about 3 000 m thick and was intruded by Mesozoic granite to the south and covered by Quaternary sediments in the broad riverbed of the Lhasa River. It was regarded as the middle portion of the Yeba Formation by previous geological mapping, at a scale of 1∶ 200 000. The typical section of this portion around the village of Baiding is composed of greenish massive meta-basalt with 3 -5 interbeds of reddish basaltic ignimbrite and volcanic breccia-bearing basalt. It has developed gentle folds and brittle fractures.

The middle portion(J₁y²)occurs on the mountain slope to the south of the Dagzê bridge. It was referred to as the lower part of the Yeba Formation by the previous geological mapping. Major rock types of this portion are grey and greenish meta-dacite. In the geological section to the south of Dagzê, this portion is composed of steeply northeastern-tilted volcanic rocks with a dipping angle greater than 60°. The strike and dip of minor cleavages within single layers suggest that the volcanic layers in this section were overturned.

The third or upper portion(J₁y³)is exposed to the east of Dagzê on both sides of the Lhasa River between the villages of Bagaxue and Segang, in a larger region than those of the lower and middle portions. It is mainly composed of slightly metamorphosed basalt, dacite and silicic tuff, which can be divided into two eruptive rhythms from basic to acid volcanic rocks. Secondary structures such as small or medium-sized folds, eastern-striking thrusts and uneven schistous deformation occur in this portion. Strong and weak schistous deformations occur repeatedly about every 70 -100 m in homogeneous thick volcanic lava. Strong schistous deformations usually occur near fault zones. These reflect the rule of stress concentration. Cleavages always parallel the volcanic layers. The orientation of asymmetrical secondary small folds here indicates southward thrust. These structural features reflect the strong compression during the period of Himalayan movement.

According to field investigation and thin section studies, the major volcanic rock types around Dagzê can be described as meta-basalt, dacite, metamorphic silicic tuff and fragment-bearing lava.

Meta-basalt occurs in the lower and upper portions of the volcanic sequence around Dagzê. This grey and greenish massive rock has porphyritic and cryptocrystalline structures with vesicular and amygdaloidal structure. It has blastoporphyritic and locally typical vitroporphyritic textures under a polarizing microscope. Phenocrysts are altered euhedral and semi-euhedral plagioclases and phenocryst assemblies locally. Anorthite molecule contents for unaltered plagioclase range from 30% -60% and belong to basic plagioclase. Pyroxene phenocrysts were seldom found, and were altered and replaced by epidote and chlorite with pyroxene pseudomorph. Phenocryst contents are about 5% -20% with sizes ranging between 1 and 5 mm. The matrix as the main part of basalt is composed of an intertexture of paralleled and semi-paralleled plagioclase microlites with the interspace filled by secondary minerals such as cryptocrystalline, vitric fragments, ferro-minerals, chlorite, epidote and actinolite, etc..

There are 5-8 layers of reddish basaltic ignimbrite interbedded in the basalt lava. These interbeds are 20-40 cm thick for a single layer and show textures of ignimbrite with fluidal and amygdaloidal structures(Fig. 2). Their major components were altered and replaced by chlorite, epidote and zoisite. These rocks were obviously formed in a continental environment.

Figure  2.  Basaltic ignimbrite showing vitric(dark) and magma(grey)fragments.Sample No. BD09, located on the hill about 2 km west of Baiding Village. Both dark basaltic vitric fragments(-) and lightcolored magma debris show features of plastic flow with secondary minerals filling the tuberose pore.

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The petrofacies observation suggests that most of the basaltic lava were results of submarine eruptions but the sea floor was uplifted occasionally to form continental volcanic flows.

Dacite is the major rock type in the middle and upper portions. It is grey and green colored massive lava rock with an aphanitic texture overprinted by the schistosity of a later stage. It shows blastoporphyritic, amygdaloidal, felsitic and fluidal structures under the microscope. Phenocrysts are mainly euhedral plagioclase and subordinate K-feldspar and quartz with a granularity of 1 -3 mm and contents of 10 % 15%. The matrix is a complex of sericite, micro-felsic minerals, epidote and biotite. Altered rocks are mainly composed of chlorite, sericite, epidote, carbonate and subordinate apatite and zircon. Pseudomorphous plagioclase occurs in parallel under the microscope. Carbonate and quartz fill the amygdaloidal pores in the rock.

Metamorphic silicic tuff is a light grey rock occurring in the upper portion to the east of Dagzê, with lepidoblastic and granoblastic structures and schistic texture. It occurs as interbeds in dacite layers. Major components are medium to fine-grained quartz, paralleled biotite and plagioclase with rare pseudomorphs of crystal and vitric fragments. Plasticly deformed biotite flakes appear in parallel and have augen-shaped assemblies locally under the microscope.

Fragment-bearing lava rocks sporadically occur as interbeds in lava flows. Granular fragments are formerly solidified basalt or dacite with a grain size of 1 -15 cm and 10% -45%(vol)of the bulk rock. Fragments show typical aphanitic or vitric textures with pores or amygdaloids as much as about 30% of a single fragment, which suggests that the fragments are the crashed exterior of lava flows. Cementing material among fragments is later solidified lava with a similar composition to the fragments. Around Baiding and the Valley of Yeba, the layers of fragment- bearing lava were partially replaced by white carbonate, which suggests strong metasomatism at a later stage.

Petrofacies studies in the field and under the microscope suggest that the volcanic rocks in the Yeba Formation were formed by strong and rapid flooding with occasional eruptions. Much of the volcanic activity was submarine, however, rocks with vitrophyric and fluidal textures and reddish basaltic ignimbrite, and fragment-bearing lava suggest a continental surface occasionally.

METHODOLOGY

We measured geological sections at the village of Baiding for the lower portion(J¹y¹) and in the valley to the south of Dagzê bridge for the middle portion (J₁y²), together equaling approximately 1.5 km in length. We also carried out geological investigations around the villages of Bagaxue and Segang, the Valley of Yeba, and between Dagzê bridge and Linzhou, for the upper and middle portions. Based on detailed observations, documentation, measurements, sampling and previous information, we carried out petrofacies studies on thin sections of the volcanic rocks around Dagzê and selected unaltered rocks for geochemical analysis. Major elements measurements were carried out in Chengdu Institute of Geology and Mineral Resources by atomic absorption spectroscopy with analytical precision better than 1%. Trace and rare earth element analyses were carried out by instrumental neutron activation analysis at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing. The instruments used for trace element analysis were IRIS, POEM and ICP AESICP MS; those for REE were IRIS and POEM, with analytical precision better than 1 %. See Zhang et al.(2001)and Zhang and Zhang(1995) for detailed methodology and process. Analytical data are listed in Table 1. Some geochemical plots were drawn using Geokit software(Lu, 2004).

Table  1.  Major (%) and trace ele ment (10-6) data for the Yeba volcanic rocks

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MAJOR ELEMENT PETROCHEMISTRY

Petrochemical classification shows characteristics of a bimodal suite(Table 1 and Fig. 3). The SiO₂ contents of these rocks fall into two ranges of 41%-50.4% and 64% -69%, belonging to basalt and dacite. There is an obvious" Daley gap" of medium volcanic rocks with SiO₂ content between 50.5% and 64%. The silicon-alkali plot shows features of calc-alkali series(Fig. 3a). We used stable element Zr/TiO₂-Nb/Y classification diagram as Si, K, Na are mobile elements and could be altered during metamorphism. This classification diagram also suggests ranges of basalt and dacite as in a bimodal suite(Fig. 3b).

Figure  3.  Classification diagrams for the Yeba volcanic rocks.(a) TAS silicon-alkali diagram(Rollison, 1992). Pc. picritic basalt; U1. tephrite, basanite; U2. phonolite, tephrite; U3. ventrallite; U4. phonolite; S1. trachybasalt; S2. basaltic trachyphonolite; S3. trachy andesite; T. trachyte; R. rhyolite; B. basalt; 01. basaltic andesite; 02. andesite; 03. dacite.(b)HFSE Nb/Y-Zr/TiO₂ diagram(Winchester and Floyd, 1977). (1)sub-alkaline basalt; (2) andesite/basalt; (3) andesite; (4)rhyodacite/dacite; (5)rhyolite; (6)comendite/pantellerite; (7)trachyte; (8)trachy andesite; (9)basanite/nephelinite; (10)alkali-basalt.

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A notable geochemical feature of basalt is its very low TiO₂ content of 0.66% -1.01%, with an average of 0.81%, much lower than that of continental tholeiite(2%). The average Fe2O3content for basalt is 5.18 %, higher than that of FeO at 4.41%. This suggests that the basalt experienced an oxidation process. Average TFeO(total iron oxide) content(9.07%)is lower than those of continental tholeiite(12.68%), MORB(10.16%) and alkali dorgalite(12.17%). The MgO content of basalt of 6.96% is similar to that of alkali dorgalite(7%, see Li(1992)for typical rock types). The primary basalt is characterized by MgO> 8%(Wang et al., 2001; McKenzie and Bickle, 1988) and Mg^#= 68 -75 (Wilson, 1989). These values for basalts in the Yeba Formation are 6.96% and 33 -65, lower than those of primary basalt.

The SiO₂ content for dacite is between 64.1% and 68.9%, averaging 66.28%. Its Na₂O+ K₂O is 5.57% -7.56%, averaging 6.63%, with Na₂O/K₂O> 1.1, which accords with the sodium-rich calcalkali series. The aluminiferous index ACNK for dacite ranges between 1.0 and 1.1, averaging 1.02, which accords with paraluminous acid volcanic rocks.

TRACE ELEMENT AND ISOTOPIC GEOCHEMISTRY

The ∑REE content for the Yeba basalt around Dagzê is 60.3-135 μg/g, averaging 104.4 μg/g. The dacite has relatively low REE contents, ∑ REE= 126.4 -167.9 μg/g, averaging 145 μg/g. On the chondrite-normalized REE patterns(Fig. 4), both basalt and dacite have similar LREE-enriched patterns with obvious LREE/HREE fractionation. The basalt samples have(Ce/Yb)_N= 2.8 -6.2, averaging 3.8;(La/Yb)_N= 3.3 -7.8, averaging 4.7. The dacite samples have(Ce/Yb)_N= 3.0 -6.0, averaging 4.6;(La/Yb)_N= 3.6 -7.3, averaging 6.0. The basalt samples haveδEu values of 0.96 -1.22, averaging 1.06 and without an Eu anomaly. The dacite has an obvious negative Eu anomaly, withδEu= 0.8 -1.0, averaging 0.88. No obvious plagioclase fractional crystallization could be confirmed for either rock type. Both rocks have similar MORB-normalized trace element patterns(spider diagram, Fig. 4), with enriched LILE and depleted HFS. The basalt was depleted in Ti, Ta and Zr with slightly depleted Nb and Ta(Nb^* = 0.54 -1.17, averaging 0.84). The basalt samples have stable La/Sm ratios. There are 8 out of 10 samples with La/Sm ratios between 3.2 and 3.75, averaging 3.64. The dacite samples have similar features with depleted P, Ti contents and other HFS, which might relate to the fractional crystallization of apatite. There is also slight Nb and Ta negative anomalies for dacite, with Nb^*= 0.74 1.06 averaging 0.86. Both rocks have relatively stable HFS but unstable K, Rb, Ba, and other LILE contents on the spider diagram, which could be affected by metamorphism at later stages(Fig. 4).

Figure  4.  Chondrite-normalized REE patterns and MORB-normalized trace element patterns(spider diagrams)for the Yeba volcanic rocks.(a) and(b)are REE plots for basalts and dacites, (c) and(d)are spider diagrams for the same samples.

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The slightly enriched LREE and minimal HREE fractionation for the basalt samples reflect the geochemical characteristics of its source rocks that could be spinal peridotite(Guo et al., 2001), as garnet peridotite would generate basalts with strong fractionated REE and HREE. The REE and HREE fractionation of basalts in the studied region is not high, with Ce/Yb ratios between 3 and 4 and averaging 3.8 for 9 out of 10 samples. Only one sample has a Ce/Yb ratio higher than 6.

Primary magma, which is derived from the depleted upper mantle and experienced partial melting to form MORB, must be characterized by highly depleted LILE and enriched HFSE contents. The volcanic rocks in the Yeba Formation however, have a much different geochemistry, and no obvious crustal contamination has been revealed. This may suggest that the source region experienced metasomatism by fluids with crustal geochemical properties before partial melting. The dehydration process of previously subducted oceanic and continental crusts in a deep subduction zone could form LILE and LREE-enriched fluids and may unevenly metasomatize the obducting mantle wedge to form regions selectively enriched in LILE and relatively depleted in HFSE and Ti(Guo et al., 2001).

Major and trace element and isotope geochemistry revealed their volcanic arc affinity for both basalt and dacite in the Yeba Formation. Their similar trace element and REE patterns may reflect their relationship of petrogenesis. The depleted mantle wedge under the subduction zone, which was affected by metasomatism of fluids with crustal geochemistry, should be the source rocks of the volcanic rocks in the Yeba Formation.

PETROGENESIS

Bimodal suites of volcanic rocks could be formed in different tectonic settings, not only continental rifts but oceanic islands, oceanic arcs, temporal extension periods within mature island arcs and marginal arcs(Gu et al., 2000; Wang et al., 2000; Qian and Wang, 1999; Shoichi and Asahiko, 1998; Christian and Paquette, 1997).

Tectonic Settings

The basalt and dacite in the Yeba Formation belong to the calc-alkali series. The basalt is depleted in Ti, K, P, Nb and Ta while the dacite is also depleted in Ti and P. Both have enriched LILE and LREE contents. These features are in accord with those of island arc volcanic rocks. Typical calc-alkali island arc dacite has obviously negative HFSE anomalies and enriched LREE with low REE fractionation ((La/Yb)_N < 10, Yb> 2.5× 10^-6, Y≥ 25× 10^-6) and a Mg^# value of about 0.36(Qian, 2001). The dacite in the Yeba Formation has similar geochemical features to those of island arc dacite. The Ti/V ratio can be used to distinguish basalts formed in different tectonic settings(Rollison, 1993). The Ti/V ratios for MORB, arc tholeiite, and arc calc-alkali basalt are 20 -50, 10 -20, and 15 -40 respectively. This ratio for basalt in the Yeba Formation is 16 -30 averaging 21.9, similar to that of arc calc-alkali basalt.

HFSE is often used to discriminate tectonic set tings for basalts. On Zr-Nb-Y and Ta/Hf-Th/Hf discrimination diagrams(Fig. 5), most samples of the Yeba basalt fall in regions of marginal arc and volcanic arc basalts, a few samples fall in the regions of continental rift and inland extension zone. These suggest a tectonic setting of continental marginal arc.

Figure  5.  Tectonomagmatic discrimination diagrams for basalts in the Yeba volcanic rocks.(a)Zr-Y-Nb diagram(from Rollison, 1993). A1. withinplate alkali basalt; A2. withinplate alkali basalt+ withinplate tholeiite; B. E-MORB; C. withinplate tholeiite + volcanic arc basalt; D. volcanic arc basalt + NMORB.(b) Th/Hf-Ta/Hf diagram(from Wang et al., 2001). Ⅰ. spreading marginal N-MORB; Ⅱ. converging margin(Ⅱ 1. oceanic arc basalt; Ⅱ 2. continental marginal arc basalt); Ⅲ. oceanic island basalt and T-MORB, E-MORB; Ⅳ. continental withinplate(Ⅳ 1. withinplate rift and marginal rift tholeiite; Ⅳ 2. withinplate rift alkali basalt; Ⅳ 3. extensional zone and primary rift basalt); Ⅴ. mantle plume basalt.

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Petrogenesis of Dacite

The petrogenesis of silicic rocks is the key for the petrogenetic study of bimodal volcanic rocks (Wang et al., 2000). There are two types of petrogenesis for silicic rocks: (1) Hot basic magma, derived from upper mantle partial melting, injects into cold crustal rocks and induces melting of crust rocks to produce silicic magma. This kind of silicic rock is usually exposed in a larger region than basalt. Both are different in trace element contents and Sr, Nd and Pb isotopic compositions(Davies and Macdonald, 1987; Doe et al., 1982).(2) The fractional crystallization process of basaltic magma may produce silicic magma. But it is most possible that the fractional crystallization process results in a petrochemically continuous sequence of basalt, basaltic andesite and andesite. This kind of continuous sequence is often formed by mantle melting under subduction zone (Machado et al., 2005; Jörg et al., 2002; Stefanie and Ernst, 1999). Volcanic rocks of this genetic type should have similar trace element and isotopic characteristics, and the silicic rocks must be much less than basalts.

Rayleigh's law, C^l /C⁰= F^(D-1), is used to model trace element behavior. Based on Rayleigh's law, volcanic series with common source rocks show logarithm lineations for paired stable trace elements(see Geng and Mao(1991)for detailed discussion). The Th-Hf diagram for the Yeba volcanic rocks does not show this kind of lineation(Fig. 6), nor do the other stable trace elements. It suggests that the dacite in the Yeba Formation could not be formed by the fractional crystallization of basaltic magma.

Figure  6.  lnTh versus lnHf plot of the Yeba volcanic rocks.

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Tectonic extension within an arc zone or back arc basins may result in the partial melting of the upper mantle and form different volcanic rock types as happened in the East China Sea and the Sea of Japan (Kozo et al., 2004). The partial melting of upper mantle peridotite at degrees less than 5% results in dacite magma(Li and Li, 2004), and may form basaltic magma at 5% -20% of partial melting(Jörg et al., 2002). Coexisting mafic and felsic rocks could be generated from the same source rocks(Bonin, 2004).

During the tectonic transition process from compression to extension at the late period of island arc evolution, unstable tectonic decompression could probably cause different degrees of upper mantle partial melting to form basic to acid magmas(Andrew et al., 2005). Periodic extension and rifting often happened in mature island arcs. Silicic magma could be formed at the early period of inner arc extension. Otherwise mafic dykes injection and eruptions could happen at the stage of arc rifting(Busby, 2004).

The diagrams of hyper-lithophile elements(Ta, Th, La, Ce) versus lithophile elements(Zr, Hf, Sm)can be used to distinguish fractional crystallization and partial melting(Li, 1992; Allegre and Minster, 1978). On these diagrams, the volcanic rocks in the Yeba Formation show inclined patterns, different from the fractional crystallization patterns of horizontal lines(Fig. 7a-c). These suggest that both dacite and basalt were formed by partial melting. Rollison(1993)presented a quantitative model to estimate partial melting degrees from the Ce and Sm contents of volcanic rocks. On this diagram, dacite was from 1% -2% partial melting of peridotite, while the basalt was from 4% -10% partial melting (Fig. 7d).

Figure  7.  Partial melting discrimination and its degree diagrams.(a)-(c)black dots. basalts; black squares. dacites. Both show inclined lines; (d)black dots. basalts; cross. dacites. Percentage numbers on the line indicate degrees of partial melting.

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Gangdise Tectonic Evolution in Indosinian to Yanshanian Epoch

The Yeba Formation is situated between the WE strike Late Paleozoic"faulted uplift on arc back" to the north and the IYS to the south(Xiao et al., 1988), which belongs to" the Mesozoic to Cenozoic southern Gangdise magmatic arc" (Pan et al., 1997). The Mesozoic to Cenozoic magmatic arc here has been believed to be related to the northward subduction of the Yarlung tsangpo neo-Tethys(Li, 2002; Pearce and Mei, 1988; Jin and Zhou, 1978). Although Late Triassic granite and volcanic rocks have been discovered sporadically along this zone of Gangdise(Pan et al., 2004; Ren and Xiao, 2004; Li et al., 2003; Qu et al., 2003), Early Jurassic magmatism has not been reported yet.

Geochemical studies on the Early Jurassic bimodal volcanic rocks in the Yeba Formation suggest that a mature magmatic arc actually existed in Gangdise between the late Indosinian and early Yanshanian epochs. It also confirmed the existence of an Indosinian magmatic arc that has been discussed in recent years.

Although Late Triassic to Early Jurassic ophiolite and flysch have been confirmed in the IYS(Geng et al., 2004; Pan et al., 2004; Ren and Xiao, 2004; Gao and Song, 1995; Honegger et al., 1982), geologists believe that the Yarlung tsangpo neo-Tethys did not begin subducting until the Late Jurassic(Pearce and Mei, 1988; Honegger et al., 1982). Therefore, the Indosinian to early Yanshanian magmatism and orogeny in Gangdise could be related to the southward subduction of the Bangong-Nujiang Ocean or to the Yongzhu-Polong interarc ocean.

Geological mapping in recent years confirmed the existence of interarc ophiolitic mélanges in several places within Gangdise: Yongzhu-Polong(T₃-J₃), Guiya-Juewong(J), Shiquanhe(K₁), Shenzha(J₃K₁), and Asuo(J_2-3). They may represent small oceanic basins or rifts within the Mesozoic Gangdise arc(Wang et al., 2003; Yang et al., 2003; Zheng et al., 2003). These ophiolitic mélanges join the BNS around Shiquanhe and Baingoin. Among these sutures and interarc rifts were micro terrains, magmatic arc and back arc basins. This suggests a tectonic and paleogeographic model of archipelago(Pan et al., 1997; Hsu et al., 1995). Geologists have realized that the Mesozoic Gangdise arc is more complicated than previously expected. More geological work should be done to better constrain its magmatism and tectonic settings.

As part of the Mesozoic Gangdise magmatism, the eastern part of the Yeba Formation occurring around Sangri and Gongbo'gyamda is a succession of silicic volcanic rocks and sediments of the Middle Jurassic(Mao et al., 2002; Yin J R et al., 1998; Gou, 1994). The sedimentary structures and rock types suggest that these sediment-volcanic rocks were formed in a shallow marine or continental shelf environment(Mao et al., 2002), differing from the Early Jurassic volcanic sequence around Dagzê(western part of the Yeba Formation). This reflects that the paleogeography for the whole Yeba Formation evolved from a continent-ocean transitional zone to a shallow marine environment. Meanwhile, magmatic activity evolved from a bimodal volcanic suite to silicic rocks, which may suggest tectonic evolution from a continental magmatic arc to an extensional margin. The magmatic source rocks were transferred from the upper mantle to the lower crust.

CONCLUSIONS

The Early Jurassic bimodal volcanic suite in the Yeba Formation occurs around Dagzê with an eruptive center around the Valley of Yeba. The Middle Jurassic Yeba Formation exposed around Gongbo'gyamda and Sangri is a volcanic-sedimentary sequence with an eruptive center around Jiama, to the south of Maizhokunggar.

The Early Jurassic bimodal volcanic suite in the Yeba Formation is characterized by island arc affinities geochemically, with enriched LREE, LILE and depleted HREE, HFS. Basaltic rocks have isotopic features of ε_Nd(t)= 0.96 -10.03 and(⁸⁷Sr/⁸⁶Sr)_i= 0.704 3 -0.706 4. The ε_Nd(t) and(⁸⁷Sr/⁸⁶Sr)_i for dacites are -1.42 to 1.08 and 0.703 8 -0.704 9 respectively. Geochemical studies suggest that both basalt and dacite originated from the partial melting of the upper mantle wedge at different melting degrees. Their source rock was affected by selective metasomatism by fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages.

The bimodal volcanic suite in the Yeba Formation was formed in a temporal extension period in the late stages of the Indosinian Gangdise island arc. But the Mesozoic Gangdise volcanic rocks and their relationship with the BNS, the IYS, and interarc riftbasin systems still need better constraint.