Journal of Petrology Advance Access originally published online on March 11, 2005
Journal of Petrology 2005 46(6):1121-1154; doi:10.1093/petrology/egi012
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Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China
STATE KEY LABORATORY FOR MINERAL DEPOSITS RESEARCH, DEPARTMENT OF EARTH SCIENCES, NANJING UNIVERSITY, NANJING 210093, P.R. CHINA
RECEIVED AUGUST 15, 2003; ACCEPTED DECEMBER 10, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135 Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850°C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65·4% and 76·8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200°C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial
Nd values (–4·2) and high initial 87Sr/86Sr ratios (0·7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves. KEY WORDS: volcanic complex; quenched enclaves; petrology; geochemistry; back-arc extension setting; Xiangshan; SE China
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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During the late Mesozoic, extensive magmatism took place in SE China, forming a belt of volcanic–intrusive complexes
600 km wide, parallel to the present coastline (Fig. 1). Despite intensive scientific research, the geodynamic setting of these complexes remains controversial, and several models have been suggested. These include (1) subduction of the Palaeo-Pacific plate (e.g. Charvet et al., 1994
; Martin et al., 1994; Zhou & Wu, 1994; Lapierre et al., 1997; Zhou & Li, 2000); (2) continental rifting and basin formation caused by collision between Indochina and South China during the early Mesozoic (e.g. Gilder et al., 1991; Li, 1999); (3) a Mesozoic sinistral shear system (e.g. Xu, 1964, 1993; Xu et al., 1987); (4) Alpine-type collision between the Yangtze and Cathaysia Blocks (Hsü et al., 1988).
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The Xiangshan volcanic complex, comprising a resurgent caldera (collapse caldera and resurgent dome association), is located in the northern part of the magmatic province close to the southern margin of the Yangtze Block. Since the late 1950s, a number of giant uranium deposits have been found in this area; consequently, the uranium deposits and their host volcanic rocks have been the focus of intensive study, although the petrogenesis of the volcanic complex remains controversial. Fang et al. (1982)
, Liu (1985), Chen (1990), Wang et al. (1991) and Liu et al. (1992) noted that the magmatic rocks forming the Xiangshan volcanic complex are peraluminous and may have been derived from partial melting of crustal rocks. However, Xia et al. (1992) argued that these characteristics are the result of mixing of crustal-derived melts with large ion lithophile element (LILE)-enriched mantle-derived magma. More recently, Gilder et al. (1996) suggested that the Xiangshan subvolcanic rocks are probably A-type granites. In this study we present a detailed investigation of the mineralogy, petrology, trace and rare earth element and Sr–Nd–O isotopic geochemistry of the Xiangshan volcanic complex and quenched intermediate composition enclaves entrained within the sub-volcanic rocks. We attempt to better constrain both the petrogenetic processes and their significance in understanding the geodynamic setting of late Mesozoic tectonics and magmatism in SE China. These new data allow us to explore the following questions. (1) What is the nature of the source of the magmas which formed the Xiangshan volcanic complex? (2) Under which P–T conditions did the Xiangshan magmas evolve? (3) What is the relationship between the sub-volcanic rocks and their entrained quenched enclaves? (4) In what kind of tectonic setting was the volcanic complex emplaced? (5) What are the regional geodynamic implications of this late Mesozoic activity?
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GEOLOGICAL SETTING |
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Xiangshan is located in Jiangxi Province, SE China, close to the tectonic suture zone between the Yangtze and Cathaysia Blocks (Fig. 1). In this part of SE China a series of late Mesozoic volcanic–intrusive complexes were emplaced during three major stages from the Early to Late Yanshanian: 180–160 Ma; 160–135 Ma and 135–90 Ma (Zhou & Li, 2000
). As indicated in Fig. 1, the associated granitoids appear to young towards the coast.
A series of Cretaceous–Tertiary red-bed basins such as the Gan-Hang Basin [the ‘Gan-Hang Rift’ of Gilder et al. (1996)] and Shiwandashan Basin formed concurrently with the latest phase of the regional magmatism (Fig. 1). These NE-trending basins are infilled with red clastic sedimentary rocks along with marl, gypsum, and evaporites, which are locally interlayered with volcanic rocks. The basins are mainly located to the NW of the volcanic–intrusive complex belt, and are thought to have been deposited in a back-arc extensional environment (Zhou & Li, 2000). Bimodal mafic–felsic magmatism occurred in basins such as the Gan-Hang Basin in the Early Cretaceous. A series of granite–porphyry dyke swarms intruded into the Early Cretaceous red beds. One of these granite–porphyries has a U–Pb zircon age of 105 ± 1 Ma (Yu et al., 2001). The basic magmatism forms compact massive basalt lava flow sequences interbedded with Early Cretaceous red mudstones. Individual lava flows are around 10 m thick, with the thickest around 30 m. These basaltic rocks have K–Ar ages of 104–99 Ma (Yu et al., 2001) and are enriched in alkalis, light rare earth elements (LREE) and LILE, belong to the shoshonitic magma series and were inferred by Liao et al. (1999) to be derived by partial melting of enriched lithospheric mantle. Gilder et al. (1996) proposed the name ‘Shi-Hang zone’ for this NE-trending zone of extensional basins, which are arguably parts of a single continental rift–shear-zone system.
The Xiangshan resurgent caldera is approximately 26 km east–west and 16 km north–south (Fig. 2). The basement to the caldera consists mainly of Early–Middle Proterozoic and Sinian (700 Ma) metamorphic rocks. The northwestern part of the caldera is overlain by Cretaceous red beds of the Gan-Hang Basin (Fig. 2).
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Xiangshan volcanic activity took place mainly in the Late Jurassic and included two subcycles, each of which has an early and late stage. The major rock types and facies are summarized in Table 1 and available geochronological data are given in Table 2. Rhyolite of the first subcycle has a U–Pb zircon age of 158 ± 0·2 Ma (Yu, 2001
) and a sanidine 40Ar/39Ar age of 149 ± 1 Ma (Chen, 1990). After rhyolite effusion, the volcano collapsed, creating the inward-dipping volcanic beds of the first subcycle and ring fractures. At about 140 ± 7 Ma (U–Pb zircon age, Chen et al., 1999), a resurgent dome composed of weakly welded rhyolitic tuff, crystal tuff and porphyritic lava formed. At about 135 ± 7 Ma (U–Pb zircon age, Chen et al., 1999), subvolcanic rocks such as monzogranite–porphyry and syenogranite–porphyry were intruded along the ring fractures, forming the last stage of the volcanic activity. These high-level intrusions contain quenched enclaves (Fan et al., 2001).
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The Middle Proterozoic basement to the complex consists mainly of schists, granulites and amphibolites. Hu (1998)
suggested that the protoliths of the schists and granulites were of sedimentary origin, whereas the precursor to the amphibolites was basaltic. The amphibolites have yielded a Sm–Nd isochron age of 1113 ± 49 Ma (MSWD = 0·22) (Hu, 1998). Phyllites, slates and meta-sandstones comprise the Sinian (700 Ma) metamorphic rocks.
Early Proterozoic metamorphic rocks, exposed in the northwestern part of the Fujian province to the east of the Xiangshan area, also consist mainly of granulites, schists and amphibolites. The amphibolites have yielded a SHRIMP U–Pb zircon age of 1766 ± 19 Ma (Li et al., 1998).
Contemporaneous (154–146 Ma) mafic magmatism within the Shi-Hang zone also occurs further to the SW of Xiangshan in the Daoxian–Guiyang area (Fig. 1), including the Daoxian basaltic lava and Zhicun (Guiyang) mafic dykes (Wang et al., 2003). These basaltic rocks (SiO2 45·0–52·7 wt %) have high MgO contents (6·8–17·9 wt %) and compatible trace element concentrations (Ni, 123–647 ppm), and have high K2O contents (3·4–4·3 wt %) with K2O/Na2O ratios of 1·7–4·2. The rocks have initial Nd values of –0·7 to –3·8 and 87Sr/86Sr ratios of 0·7053–0·7070 (Wang et al., 2003). Wang et al. (2003) have suggested that these high-Mg potassic rocks (referred to below as the ‘Daoxian high-Mg potassic rocks’) originated from an enriched mantle source, and did not experience significant crustal contamination during magma ascent.
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SAMPLES AND ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Forty samples from surface exposures, underground workings and open pits have been collected from the Xiangshan volcanic complex, including four quenched enclaves. All the samples were collected away from areas of uranium mineralization, and are thus largely unaltered. For the large enclaves (
60 cm axis), samples from enclave centres and quenched margins were selected for analysis; for the small enclaves (20 cm axis), which do not contain quenched margins, samples close to the boundary between the enclave and the host rock were selected for analysis. The sample locations are shown in Fig. 2, and lithology and mineralogy of the collected magmatic rock samples are summarized in Table 3.
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Electron microprobe analysis has been carried out on biotite, amphibole, pyroxene, plagioclase, and K-feldspar in samples from the volcanic complex and the quenched enclaves, using a JEOL JXA-8800 Superprobe at the State Key Laboratory for Mineral Deposits Research in Nanjing University. Operating conditions were 15 kV at 10 nA beam current. For amphibole, pyroxene and biotite, the standards used were hornblende (for Si, Ti, Al, Fe, Ca, Mg, Na and K) and fayalite (for Mn). For feldspar, the standards used were hornblende (for Si, Ti, Al, Fe, Ca and Mg), albite (for Na), orthoclase (for K) and fayalite (for Mn).
Whole-rock chemistry was determined at the State Key Laboratory for Mineral Deposits Research in Nanjing University by conventional wet chemical methods for major elements and by inductively coupled plasma mass spectrometry (ICP-MS) using a Finnigan Element II system for REE and trace elements. Oxygen isotopic compositions were measured by conventional BrF5 extraction in the Open Laboratory for Isotope Geology of the Chinese Academy of Geological Sciences (Beijing). Nd and Sr isotopic compositions were analysed at the Institute of Geology, Academica Sinica, following the methods of Huang & DePaolo (1989). 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0·7219 and 87Sr/86Sr ratios to 86Sr/88Sr = 0·1194. During the period of analysis, measurements for the La Jolla Nd and NBS-987 Sr standards yielded a 143Nd/144Nd ratio of 0·511848 ± 18 (2
, n = 27) and a 87Sr/86Sr ratio of 0·710266 ± 35 (2, n = 30), respectively. Total analytical blanks were 5 x 10–11 g for Sm and Nd and (2–5) x 10–10 g for Rb and Sr. The isotopic compositions of present-day CHUR used for calculating the initial Nd are 143Nd/144Nd = 0·512636 and 147Sm/144Nd = 0·1967 (Jacobsen & Wasserburg, 1980).
Analytical data are reported in Tables 4–8, and in Electronic Appendix Tables 1 and 2 (which may be downloaded from the Journal of Petrology web site at http://www.petrology.oupjournals.org).
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CLASSIFICATION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The volcanic rocks are all rhyolitic, according to the total alkali–silica diagram (TAS) of Le Bas et al. (1986)
(Fig. 3a). The subvolcanic rocks are classified as syenogranite and monzogranite, according to the QAP diagram (Streckeisen, 1976; Fig. 3b).
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According to the IUGS classification scheme (Le Maitre, 1989
; Le Bas, 2000), the quenched enclaves show boninitic affinities (SiO2 >52 wt %, MgO >8 wt % and TiO2 <0·5 wt %) with high MgO (9·6–10·2 wt %), SiO2 (53·8–54·7 %) and low TiO2 (0·47–0·50 wt %) contents in enclave centres. The quenched enclaves are also enriched in alkalis and have high K2O contents. In the TAS diagram (Fig. 3a), the quenched enclaves plot in the fields of shoshonite and latite, corresponding to the (quartz) monzonite compositions of the QAP diagram (Fig. 3b).
Petrography
First subcycle
Rhyolitic crystal tuff, rhyolitic welded tuff (early stage) and rhyolite (late stage) are the main volcanic rock types in the first subcycle. The crystal tuff is a light grey to purplish-red rock with a fragmental texture, consisting of crystal fragments (0·1–2 mm, 50%,), glass fragments (0·05–0·1 mm, 10%,), fine-grained volcanic ash (<0·05 mm, 35%) and minor lithic fragments such as schist (5%). The crystal phases include alkali feldspar (20%), quartz (20%), plagioclase (5%), biotite (5%) and minor accessory minerals such as garnet, apatite, zircon and allanite. Alkali feldspar generally contains inclusions of plagioclase and biotite. Plagioclase is albite and usually sericitized. Biotite is locally altered to chlorite. Garnet is almandine, and contains apatite inclusions.
The welded tuff is purplish-red, with a well-developed welded texture consisting mainly of crystal fragments (25%), plastically deformed vitric fragments (20%) and fine-grained volcanic ash (55%). The crystal fragments include quartz, plagioclase (albite) and minor alkali feldspar and biotite.
The rhyolite lava is purplish-red with a porphyritic texture. Phenocrysts (40%) consist of plagioclase, alkali feldspar, biotite, amphibole and quartz. Plagioclase generally contains inclusions of biotite and apatite. Amphibole and biotite are locally altered to chlorite. The groundmass consists of feldspar and quartz with a felsitic or spherulitic texture.
Second subcycle
Weakly welded rhyolitic tuff, porphyritic lava (early stage) and subvolcanic rocks including monzogranite–porphyry and syenogranite–porphyry (late stage) are the main volcanic rock types in the second subcycle. The weakly welded tuff is purplish-red and consists mainly of crystal fragments (30%), plastically deformed vitric fragments (20%) and fine-grained volcanic ash (50%). The crystal fragments include quartz, alkali feldspar and plagioclase (albite).
The porphyritic lava is a light grey rock. Phenocrysts, with a cataclastic texture, include quartz, alkali feldspar, plagioclase and biotite. Plagioclase is usually normally zoned, and has been variably sericitized. Biotite is locally altered to chlorite. The groundmass consists mainly of feldspar, quartz and biotite with a microcrystalline or fine-grained granitic texture.
The subvolcanic rocks (monzogranite–porphyry and syenogranite–porphyry) are light grey with a porphyritic texture. Phenocrysts include plagioclase, alkali feldspar, quartz, biotite and minor amphibole and pyroxene. Plagioclase is usually normally zoned and sericitized. Biotite and amphibole are locally altered to chlorite, and pyroxene is generally altered to amphibole and chlorite. The groundmass consists mainly of alkali feldspar, quartz, plagioclase and biotite with a fine-grained granitic texture.
Quenched enclaves
Quenched enclaves, hosted by the subvolcanic rocks, are ovoid bodies with the long axis ranging from several centimetres to 60 cm. Large enclaves contain back-veins (Fig. 4a) and quenched margins (Fig. 4b). The enclaves are greyish black in hand specimen and contain alkali feldspar (36–44%), plagioclase (11–22%), quartz (0–14%), pyroxene (20%), biotite (20%) and amphibole (5%) with a microcrystalline texture. Accessory minerals are mainly magnetite and apatite. Within the enclaves, or at the boundary between the enclaves and the host rock, K-feldspar xenocrysts are rounded and show sieve-like resorption textures, filled by the enclave material (Fig. 4c and d). They are compositionally identical to K-feldspar in the host rocks (Fan et al., 2001).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Electron microprobe analyses of representative minerals in the samples from the volcanic complex and the quenched enclaves are presented in Tables 4–6. The compositional variations of feldspar, biotite, amphibole and pyroxene are illustrated in Figs 5 and 6.
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Feldspars
Plagioclase in the first subcycle volcanic rocks is either albite (early stage) or andesine (late stage). Normal zoning is typical, with labradorite in the core of phenocrysts (Table 4, Fig. 5a). In the second subcycle volcanic rocks, plagioclase varies from albite (crystal fragments in the weakly welded rhyolitic tuff), through oligoclase and andesine (phenocrysts, microphenocrysts and microlites in the porphyritic lava), to andesine and labradorite (phenocrysts and microphenocrysts in the subvolcanic rocks). Normal zoning is typical from andesine in the core to oligoclase at the rim (Table 4, Fig. 5a). In the quenched enclaves, plagioclase shows typical normal zoning from labradorite in the core to andesine at the rim (Table 4, Fig. 5a). Alkali feldspars in the volcanic rocks and quenched enclaves are all sanidine with the composition ranging from Or58 to Or98 (Table 4, Fig. 5a).
Biotite
Biotite occurs mainly in the rhyolite of the first subcycle and in the porphyritic lava and subvolcanic rocks of the second subcycle, and also in the quenched enclaves. Biotite in the rhyolite is magnesio-biotite, whereas biotite in the porphyritic lava is ferribiotite and annite (siderophyllite). Biotite in the subvolcanic rocks is ferribiotite (Fig. 5b). The biotites in the Xiangshan volcanic and subvolcanic rocks are relatively Fe rich, which is typical of A-type silica-rich magmas (Collins et al., 1982; Jiang & Yang, 2001; Jiang et al., 2002). Biotites in the quenched enclaves are mainly phlogopites (Fig. 5b) with higher MgO contents and Mg/(Mg + FeT) ratios than those of biotites in the volcanic and subvolcanic rocks (Table 5). The enclave biotites also have high Fe3+/Fe2+ ratios (>1) (Table 5), which is typical of shoshonitic rocks (Jiang et al., 2002). Biotites in the volcanic and subvolcanic rocks are characterized by very high F contents, varying from 0·8 to 2·9 wt % (Xia et al., 1992).
Amphibole
Amphibole occurs mainly in the subvolcanic rocks and quenched enclaves. All the amphiboles are calcic according to the classification scheme of Leake (1978). Amphiboles in the quenched enclaves are edenite and magnesio-hornblende and have higher MgO contents and Mg/(Mg + FeT) ratios than amphibole in the host subvolcanic rocks, which is magnesian hastingsitic hornblende (Table 5). Amphiboles in the subvolcanic rocks also have high F contents, varying from 0·2 to 1·0 wt % (Xia et al., 1992).
The pressure of calcic amphibole crystallization can be estimated using Al-in-hornblende geobarometer (Johnson & Rutherford, 1989):
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) and are thus suitable for estimations of crystallization pressure. The results are 4·4–4·5 kbar [average 4·4 kbar (15 km, if 1 kbar 
3·3 km)] for the phenocryst hornblende in the subvolcanic rocks, and 1·9–2·5 kbar [average 2·2 kbar (7 km)] for the amphibole in the quenched enclaves.
Experimental studies have shown that AlIV and (Na + K)A in amphibole increase with increasing crystallization temperature (e.g. Fig. 6, Helz, 1973). Amphiboles in the quenched enclaves plot at the low-temperature end of the experimental data array in Fig. 6, about 710°C, whereas the amphibole phenocrysts in the subvolcanic rocks give higher temperatures of >825°C.
Holland & Blundy (1994) developed an improved amphibole–plagioclase thermometer based on their earlier one (Blundy & Holland, 1990). They suggested that application of the thermometer for assemblages with quartz be restricted to amphiboles that have NaA > 0·02, AlVI < 1·8 and Si in the range 6·0–7·7, and to plagioclase with Xan < 0·90. Compared with these conditions, the amphibole–plagioclase assemblages discussed here are suitable for the application of this thermometer. The amphibole compositions have been recalculated following the procedure of Holland & Blundy (1994). As a result, the amphibole–plagioclase assemblages in the subvolcanic rocks give higher temperatures of 828–925°C (average 871°C), whereas those in the quenched enclaves give lower temperatures of 678–763°C (average 720°C), which is in good agreement with the estimations by the method of Helz (1973) mentioned above.
Pyroxene
Pyroxene occurs mainly in the subvolcanic rocks and quenched enclaves. Pyroxene in the subvolcanic rocks is ferrosilite, whereas that in the quenched enclaves shows a range of compositions from diopside to subcalcic augite (Fig. 5c). Lindsley (1983) proposed a graphical thermometer based on experimental data for Ca–Mg–Fe pyroxenes. It is suggested that application be limited to pyroxenes in which Wo + En + Fs exceeds 90%. For the pyroxenes discussed here, Wo + En + Fs does exceed 95%, and they are, therefore, suitable for the application of this geothermometer. The compositions of the studied pyroxenes, using the correction procedure of Lindsley & Andersen (1983) (Fig. 5c), indicate a temperature range of 900–950°C for orthopyroxene in the subvolcanic rocks and a variable temperature range of 700–1300°C for clinopyroxene in the enclaves, for pressures between 5 and 10 kbar.
Experimental studies have shown that Al contents in Ca-rich clinopyroxene increase with increasing crystallization pressure (Thompson, 1974). High-temperature (1300–1200°C) augites, which have relatively low Wo (24·5–26·4%), have relatively higher Al2O3 contents (2·4–3·2 wt %) than medium-temperature (1050–850°C) augites (0·5–1·3 wt %), which have medium Wo (37·1–45·7%) (Table 6). Lower-temperature (700°C) diopsides, which have high Wo (46·7%), have the lowest Al2O3 contents (0·2 wt %) (Table 6). This could imply that high-temperature (1300–1200°C) augites represent phenocrysts that crystallized from magmas in the lower crust prior to injection into the felsic magma chamber, where they formed the enclaves. Medium-temperature (1050–850°C) augites formed during rapid cooling of the mafic magma after its injection into the felsic magma chamber (4·4 kbar, based on the Al-in-hornblende geobarometer), whereas the diopsides, together with the amphiboles, equilibrated below 700°C and at low pressure, and are most likely to have formed after the emplacement of the host subvolcanic rocks, in good agreement with the low pressure (2·2 kbar) estimated using the Al-in-hornblende geobarometer. This is strongly supported by the crystallization temperature for the orthopyroxene (900°C from Fig. 5c at 5 kbar pressure) and amphibole [880–940°C from Fig. 6, and 870°C by the method of Holland & Blundy (1994)] in the host magma chamber.
Garnet
Garnet occurs in the rhyolitic crystal tuff as crystal fragments, and in the porphyritic lava as an accessory mineral. Previous studies indicate that garnets in the crystal tuff, which are almandine in composition, have low CaO contents (0·3–0·6 wt %) with a grossular component of 0·9–1·8 wt %. They show simple Mn zoning from Mn-poor cores (2·0–2·6 wt % MnO) to Mn-rich rims (4·6–8·2 wt % MnO), with no change in CaO content (Xia et al., 1992). Garnets in the porphyritic lava, which are also almandine, have high CaO (11·1–13·9 wt %) and low MnO contents (0·2–1·0 wt %) (Liu et al., 1992).
Almandine garnet is relatively rare in volcanic rocks worldwide (Harangi et al., 2001). This is probably due to the restricted conditions under which such garnets can form (hydrous mantle source, high-pressure crystallization from hydrous Al-rich magmas) and to the particular geodynamic setting (tensional stress field), which enhances the rapid ascent of garnet-bearing melts. Green & Ringwood (1968) demonstrated that almandine-rich garnet could be a liquidus or near-liquidus mineral in silicic magmas at high pressure (9–18 kbar). Garnet with 2–6 mol % grossular and 2–10 mol % spessartine could crystallize from a silicic magma at pressures of 5–7 kbar (Green, 1976, 1977). Further experimental studies (e.g. Hensen & Green, 1973; Clemens & Wall, 1981; Green, 1982, 1992; Conrad et al., 1988) have suggested that Ca-rich (CaO >4%) and Mn-poor (MnO <4%) almandine could crystallize at relatively high pressure (7 kbar) and at temperatures of >900°C. Simple Mn zoning with no change in CaO content may represent growth of garnet at constant pressure. The compositions of garnets discussed here suggest high pressures (5–7 kbar for the first subcycle and 7 kbar for the second subcycle) and high temperatures (>900°C) for garnet crystallization.
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element chemistry
Volcanic complex
The rocks comprising the volcanic complex are peraluminous with values of the alumina saturation index ASI [= molar Al2O3/(CaO + Na2O + K2O)] > 1·00 except for some subvolcanic rocks that are metaluminous (ASI < 1·00) (Table 7). The studied samples have high alkali contents. In a plot of Alkalinity Ratio vs SiO2 all samples plot in the alkaline field except for a few that overlap the boundary with the calc-alkaline field (Fig. 7). The samples also have high K2O contents, with all data plotting in the high-K calc-alkaline and shoshonite fields in a K2O vs SiO2 classification diagram (Fig. 8a). On binary major element vs SiO2 variation diagrams (Fig. 8a), samples from both the first subcycle and second subcycle show linear trends, although there is some scatter. There is a general decrease in TiO2, Al2O3, total Fe (FeO + Fe2O3), MgO, CaO and P2O5 with increasing SiO2, whereas K2O and Na2O have relatively constant abundances over a wide range of SiO2.
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Variation diagrams of SiO2 vs trace elements (Fig. 8b) show a general decrease of Sc, Co, Ba, Sr and Zr with increasing SiO2, relatively constant Nb and Y contents over a wide range of SiO2 and considerable scatter for Rb. In general, all the samples from the volcanic complex (with the exception of the mafic enclaves) bear the distinctive trace element geochemical signature of A-type granitoids such as: (1) enrichment in REE, high field strength elements (HFSE) and Ga; (2) depletion of Ba, Sr and transition metals; (3) high Ga/Al ratios (Whalen et al., 1987
, 1996). The volcanic complex samples have high Ga/Al ratios and plot in the A-type field (Fig. 9).
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Primitive mantle normalized trace element patterns (Fig. 10a) show significant negative Ba, Sr, Nb, Ti and P anomalies. The negative Nb, Ti and P anomalies might suggest a destructive margin tectonic setting or derivation from a crustal source or crustal contamination coupled with advanced degrees of differentiation. The rocks of both the first subcycle and second subcycle show negative Eu anomalies; in each cycle from the early stage to late stage, the size of the Eu anomaly decreases and LREE/HREE ratios increase (Fig. 11a and b).
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Quenched enclaves
Samples from enclave centres, quenched margins and rims (between centre and quenched margin) were selected for analysis. The enclave centre samples show the least chemical modification and could, therefore, be considered as representative compositions of the quenched enclave magma. These have SiO2 contents of 53·8–54·7 wt %, with high MgO (>8 wt %) and low TiO2 (
0·5 wt %) contents (Table 7). Their high Mg/(Mg + Fe) ratios (0·76–0·78) and compatible trace element contents (e.g. 213–223 ppm Ni, 136–273 ppm V) are also similar to those of boninites that have Mg/(Mg + Fe) = 0·55–0·83, 70–450 ppm Ni and 132–187 ppm V (Hickey & Frey, 1982). These chemical characteristics are consistent with equilibration of the primitive magma with mantle peridotite (Hickey & Frey, 1982; Bloomer & Hawkins, 1987). The quenched enclaves also show shoshonitic affinities. They are enriched in alkalis (Fig. 3) and have high K2O contents (Fig. 8a) with K2O/Na2O ratios from 2·0 to 4·3. Their total REE contents range from 153 to 157 ppm, with strong LREE enrichment (Fig. 11c). Compared with mid-ocean ridge basalt (MORB), the quenched enclaves are enriched in LILE and depleted in Ti, Y and Yb, similar to the Daoxian high-Mg potassic rocks (Fig. 10b).
Isotope geochemistry
Both the first and second subcycle volcanic rocks show a narrow range of Nd (T) values (–7·3 to –8·3), whereas their initial 87Sr/86Sr ratios vary from 0·7063 to 0·7108. The samples from the volcanic complex have 
18O values of 8·9–11·0
that are between those of the Middle Proterozoic orthometamorphic and parametamorphic rocks (Table 8), and show a positive correlation between initial 87Sr/86Sr and 18O values. The Nd and Sr isotopic compositions of the Xiangshan volcanic complex plot between those of the Early–Middle Proterozoic orthometamorphic and parametamorphic rocks at the age of the volcanic activity (Table 8, Fig. 12).
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The quenched enclave centre has a relatively high
Nd (T) value (–4·2) and a low (87Sr/86Sr)i value (0·7081). A traverse from the enclave centre through the enclave rim and quenched margin, to the host rock (monzogranite–porphyry) shows a gradual decrease in the Nd (T) values and a gradual increase in the (87Sr/86Sr)i and 18O values, suggesting isotopic exchange between the quenched enclave and host rock. Therefore the actual Nd (T) value of the primitive magma may be higher than that of the quenched enclave centre (–4·2), and the actual (87Sr/86Sr)i and 18O values of the primitive magma may be lower than those of the quenched enclave centre (0·7081 and 9·5, respectively). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Origin of the magmatic rocks within the volcanic complex
Pressure and temperature (P–T) constraints
Pressure estimations made on the basis of mineral chemistry show similar values for the first subcycle volcanic rocks (5–7 kbar) and second subcycle volcanic rocks (4·4–7 kbar), suggesting magma chamber depths between 15 and 23 km (for 1 kbar = 3·3 km). Magmatic temperatures, estimated from pyroxene and amphibole mineral chemistry, vary from 850 to 940°C, in broad agreement with the homogenization temperatures (838–1130°C) of magmatic melt inclusions determined by Xia et al. (1992)
(Table 9). Table 9 also shows that the magmatic temperatures of the late-stage volcanic rocks are slightly higher than those of the early stage volcanic rocks of each subcycle; these are similar to those inferred for the formation of A-type granitoid magmas (>850–900°C: Clemens et al., 1986; >950–1000°C: Creaser & White, 1991). Attainment of such high temperatures to induce partial melting of the crust probably requires an injection of mantle-derived melt into the lower crust (e.g. Whalen et al., 1987).
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Partial melting vs fractionation
Petrogenetic models for A-type granitoid magmatism have involved either partial melting of specific crustal protoliths (e.g. Collins et al., 1982
; Whalen et al., 1987; Creaser et al., 1991), or extensive fractional crystallization of mantle-derived basaltic magmas (e.g. Loiselle & Wones, 1979; Turner et al., 1992). At Xiangshan, both the first and second subcycle volcanic rocks show a steep trend in a La/Sm vs La diagram (Fig. 13), suggesting that the effects of partial melting and source composition were more important than fractional crystallization in controlling the compositional variation within the volcanic complex. Figure 13 also implies that both Middle Proterozoic orthometamorphic and parametamorphic rocks could be possible crustal sources for the magmas of the Xiangshan volcanic complex. This is also supported by the Nd and Sr isotopic data.
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As described above, from the early to late stages of each subcycle, SiO2 contents decrease, whereas the contents of refractory components such as MgO, Sc and Co all increase (Fig. 8). Together with the lack of hornblende and pyroxene in the early stage volcanic rocks (felsic), this is considered to favour a progressive partial melting model. The magmas that produced the rhyolites, porphyritic lavas and subvolcanic rocks probably experienced subsequent fractionation after partial melting (Fig. 13). Both the compositional gap and discontinuous compositional trend between the quenched enclaves and their host volcanic rocks (Fig. 8) suggest minimal scale mixing between mantle-derived magma (represented by the enclaves) and crustal melts (represented by the volcanic–subvolcanic rocks) in the petrogenesis of the magmas of Xiangshan volcanic complex.
Nature of the magma source regions
Both the first and second subcycle volcanic and subvolcanic rocks exhibit similar compositional trends in major and trace element variation diagrams (Fig. 8), and have similar Nd–Sr isotopic compositions, suggesting that they were probably derived from similar source regions. The narrow range in Nd (T) values (–7·3 to –8·3), is higher than that of the Early–Middle Proterozoic parametamorphic rocks (–9·7 to –13·1), but lower than that of the orthometamorphic rocks (2·2 to –0·3) at the age of volcanic activity (Table 8, Fig. 12). The (87Sr/86Sr)i of the magmatic rocks is lower than that of the Early–Middle Proterozoic parametamorphic rocks, but similar to that of orthometamorphic rocks at the age of volcanic activity (Table 8, Fig. 12). The Nd (T) values of the volcanic complex are also within the range (–6·2 to –12·0) of the Sinian (700 Ma) metamorphic basement rocks at the age of volcanic activity (Table 8). Because the Sinian strata were uplifted to shallow crustal levels before volcanic activity in the district (Fig. 2), we, therefore, rule out these rocks as a source for the volcanic rocks. The calculated TDM values of the volcanic rocks range from 1254 to 1906 Ma (average 1637 Ma). This could imply that the volcanic rocks were probably derived from partial melting of Middle Proterozoic metamorphic rocks including both orthometamorphic and parametamorphic rocks at depth. However, the Middle Proterozoic metamorphic rocks have higher 87Sr/86Sr ratios (at the age of volcanic activity) than the volcanic complex; we, thus, cannot rule out an isotopic contribution from underplated mafic magma to the magmas of the Xiangshan volcanic complex, although the major and trace element geochemistry does not provide any evidence for large-scale mixing between mantle-derived magma and crustal melts.
High and constant values of both K2O and Na2O (Fig. 8a) may require the presence of a K-rich phase such as K-feldspar or biotite and a Na-rich phase such as plagioclase in the source region. The Sr- and Ba-depleted nature of the magmatic rocks (Fig. 10a) could also indicate that plagioclase and K-feldspar were present in the source region. In addition, with increasing degrees of partial melting from the early to late stages of each subcycle, the Rb/Ba and Rb/Sr ratios of the volcanic rocks decrease significantly (Fig. 8c), which also suggests that K-feldspar and plagioclase are stable mineral phases in the source region. Furthermore, the lower Rb/Ba and K/Ba, and higher K/Rb ratios of the late-stage volcanic rocks, with respect to the early stage volcanic rocks, suggest that K enrichment may be due to the melting of K-feldspar rather than biotite (Landenberger & Collins, 1996). This implies that the source-rock is K-feldspar rich, rather than biotite rich. In contrast, the constant and lower Ca/Sr ratios of both the early stage and late-stage volcanic rocks of each subcycle indicate that both hornblende and plagioclase were present in the source region before partial melting (Landenberger & Collins, 1996). In the binary system Ab–An, the initial melt of a given composition plagioclase is enriched in the Ab component relative to the bulk composition, and the melt becomes progressively more enriched in the An component with increasing temperature. Therefore, the progressive melting of plagioclase in the source region could result in an increase in the CaO content of the melt from the early to late stages of each subcycle (Figs 5a and 8a), whereas the magnitude of the negative Eu anomaly in the whole-rock REE pattern decreases (Figs. 11a and b).
The more felsic volcanic rocks (the early stage of each subcycle) are depleted in LREE and slightly enriched in heavy REE (HREE) relative to the less felsic volcanic rocks (the late stage of each subcycle), suggesting relatively higher concentrations of pyroxene in the source region during the initial stages of melting (Mark, 1999). The Y contents of all the volcanic rocks (Fig. 10a) and the flat HREE patterns (Fig. 11a and b) indicate that garnet was absent from the source region during partial melting.
In summary, we propose an alternative model for the source rocks of the Xiangshan A-type magmas in which Middle Proterozoic metamorphic rocks in the lower crust were dehydrated, but not geochemically depleted, under granulite-facies conditions during an earlier thermal event. The probable reactions are
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; Landenberger & Collins, 1996) for parametamorphic rocks (schist and granulite) and
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; Landenberger & Collins, 1996) for orthometamorphic rocks (amphibolite). Once the temperature buffering effect of these reactions was removed, temperatures would continue to rise to >900°C and melting of such granulitized source rocks would then proceed by the congruent fluid-absent melting reaction
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; Whitney, 1988), it is possible, provided underplating by mantle-derived magmas continues after dehydration, as Landenberger & Collins (1996) suggested for the Chaelundi (eastern Australia) A-type granite. They suggested that the Chaelundi granite could have been derived from a dehydrated charnockitic lower crust in a subduction-related environment. Our model is similar to that of Kilpatrick & Ellis (1992) for the origin of ‘C-type’ magmas (igneous charnockites), in which a fertile, but ‘anhydrous’ granulite is melted at high temperatures.
Origin of quenched enclave magma
Original composition of the enclave magma
Most petrologists now accept that quenched mafic enclaves form through injection of mafic melt into a magma chamber containing cooler, partially crystalline felsic magma (Waight et al., 2001, and references therein), and this is the interpretation we favour for the Xiangshan quenched enclaves. In this model, the enclaves represent small volumes of mafic melt that, once injected into the felsic magma, cool rapidly, partially crystallize and become more viscous to form discrete magma blobs within the chamber. Because thermal diffusion is 103–105 times faster than chemical diffusion, initially hotter, more mafic magma batches reach thermal equilibrium with felsic host magma long before chemical equilibrium is attained (Waight et al., 2001, and references therein). The mafic blob then cools at the same rate as the host magma, allowing for a complex process of chemical diffusion between the contrasting magmas (Johnston & Wyllie, 1988; Allen, 1991; Holden et al., 1991; Lesher, 1994; Elburg, 1996; Waight et al., 2001).
The degree of diffusive equilibration would be variable, because of different proportions of mafic to felsic magma, enclave size and residence time in the host magma (Allen, 1991). Thorough equilibration may result in similar mineral assemblages, mineral compositions of amphibole and biotite, whole-rock Rb contents, and initial 87Sr/86Sr ratios between enclaves and host rocks (Allen, 1991). At Xiangshan, although chemical diffusion may be minimized in the fast cooling volcanic rocks, it does exist between the quenched enclaves and the host subvolcanic rocks. This is observed as a gradual increase in SiO2, Al2O3, Zr contents and (87Sr/86Sr)i values, and a gradual decrease in MgO, CaO, total Fe, transition element contents and Nd (T) values, from enclave centre to enclave rim (quenched margin), and to the host rock (Tables 7 and 8; Figs 8 and 12). This signature also indicates that chemical diffusion did not result in complete equilibration, and that the quenched enclave centres show the least modification in chemical and isotopic composition and, thus, could be approximately taken to represent the pristine composition of the enclave magma. This conclusion is also due to the following.
- The enclave centres and their host rocks show very different mineral assemblages and mineral compositions as indicated above.
- The elemental and isotopic compositions of enclave centres are also distinct from those of the host subvolcanic rocks, but similar to those of the coeval Daoxian high-Mg potassic rocks (Figs 10b, 11c and 12). Furthermore, high Mg/(Mg + Fe) ratios (0·76–0·78) and compatible element contents (e.g. 213–223 ppm Ni) of the enclave centres also show the chemical characteristics of ‘primitive’ mantle-derived magmas [Mg/(Mg + Fe) >0·7, 100–450 ppm Ni] (Bloomer & Hawkins, 1987).
- Diffusion coefficients for K2O and 87Sr/86Sr are similar (Waight et al., 2001, and references therein), and, as suggested by Waight et al. (2001), a correlation should exist between K2O and 87Sr/86Sr or Sr if the isotopic compositions of the enclaves reflect diffusive equilibration. At Xiangshan no clear correlation exists between K2O and 87Sr/86Sr or Sr for quenched enclaves and host rocks, suggesting no equilibration for K2O and 87Sr/86Sr. Furthermore, the high K2O contents in the enclave centres are similar to those of the Daoxian high-Mg potassic rocks, as are their Sr–Nd isotopic compositions. The high K2O contents may therefore probably reflect the original signature of the quenched enclave magma. In fact, similar K2O contents between the quenched enclaves and their host rocks (limited chemical gradient) may minimize the chemical diffusion (Snyder & Tait, 1998).
- It is not clear that the similar contents of Rb, Sr, Ba, Y and Nb between quenched enclaves and their host subvolcanic rocks (Fig. 8b) are the result of diffusive equilibration or reflect the original composition of the quenched enclave magma. Holden et al. (1991) have indicated that the HFSE and REE are less mobile in diffusive processes. This, together with the similar contents of HFSE and REE and similar HFSE/HFSE, HFSE/REE ratios between the quenched enclave centres and Daoxian high-Mg potassic rocks, suggests that the compositions of the enclaves probably reflect the original signature of the parent magma. Previous studies (e.g. Lesher, 1994; Waight et al., 2001) have indicated that Sr and Nd isotopic compositions undergo diffusional exchange roughly 10 times faster than the elements Sr and Nd, respectively. As indicated above, the Sr and Nd isotopic compositions of the quenched enclave centres are out of equilibrium with their host rocks. We, therefore, conclude that these enclave compositions represent that of a more primitive magma.
Origin of the enclave magma
A key question for the origin of the Xiangshan quenched enclave magma is to explain its boninitic and shoshonitic affinities. The minimum conditions necessary to produce primary boninitic magma include anomalously high temperatures (>1100°C) and the introduction of subduction-zone fluids to a shallow (<50 km) harzburgitic mantle (Meijer, 1980; Hickey & Frey, 1982; Bloomer & Hawkins, 1987; Crawford et al., 1989; Sobolev & Danyushevsky, 1994; Kim & Jacobi, 2002). Such a mantle source had been previously depleted and is believed to be highly refractory. Two possible mechanisms have been suggested for generation of this depleted mantle: production of MORB (Hickey & Frey, 1982), or production of arc tholeiites (Meijer, 1980); that is, at a mid-ocean ridge or in a subduction zone.
It has been proposed that the majority of shoshonitic rocks are derived from the partial melting of continental lithospheric mantle modified by previous introduction of subducted slab-derived fluids or melts (Peccerillo, 1990; Foley & Peccerillo, 1992; Rogers, 1992; Turner et al., 1996; Jiang et al., 2002). Interaction between such hydrous fluids and mantle can lead to the formation of potassic magmas as suggested by the experimental data of Wyllie & Sekine (1982). These experiments demonstrate that reaction between such a fluid (or melt) and mantle peridotite can produce a hybrid phlogopite pyroxenite; partial melting of such ‘hybridized’ mantle would produce potassic melts.
Integrating the petrogenetic models, we suggest that the parental magma to the Xiangshan high-Mg potassic quenched enclaves originated from a phlogopite-bearing harzburgitic source within the lithospheric mantle, depleted and hybridized during earlier extraction of arc tholeiites and metasomatized by subducted sediment-derived fluid or melt associated with Palaeo-Pacific plate subduction. Such a hydrous mantle source at shallow depth (<50 km) could have melted in response to thermal perturbations associated with back-arc extension in the northwestern volcanic–intrusive complex belt of SE China (see below for more discussion). The centres of the Xiangshan quenched enclaves have low TiO2 contents and plot in the field defined by experimental melts of depleted peridotite (Falloon et al., 1988) (Fig. 14), similar to the Daoxian high-Mg potassic rocks and also to the Italian lamproites that have been proposed as originating from a harzburgitic mantle source contaminated by subducted sediments (see Peccerillo, 1999). At Tsaolingshan (Taiwan) high-Mg potassic rocks have also been proposed as deriving from a phlogopite-bearing harzburgitic source in the lithospheric mantle (Chung et al., 2001). The Xiangshan quenched enclaves are enriched in LILE (e.g. K, Rb, Cs, Th, U). Such enrichment has also been observed in phlogopite-bearing harzburgite xenoliths from Batan, the Luzon arc (Maury et al., 1992). Both the Xiangshan quenched enclaves and Batan xenoliths have similar K/Rb and Rb/Cs ratios, within the ranges of oceanic sediments. Negative Ce anomalies in the Xiangshan quenched enclaves (Fig. 11c) also suggest that the mantle source may have been affected by oceanic sediments (e.g. Shimizu et al., 1992). In addition, the Sr and Nd isotopic compositions [Nd (T) = –4·2, (87Sr/86Sr)i = 0·7081] are consistent with an enriched mantle source.
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The high K2O contents (>3%) in enclave centres require a potassic phase in the source region. Melts in equilibrium with phlogopite are expected to have significantly higher Rb/Sr (>0·1) and lower Ba/Rb (<20) ratios than those (Rb/Sr <0·06, Ba/Rb >20, respectively) formed from amphibole-bearing mantle sources (Furman & Graham, 1999
). The Xiangshan enclave centres show higher Rb/Sr (0·61–0·85) and lower Ba/Rb (1·74–2·41) ratios, strongly suggesting that the enclave magma formed through melting of a phlogopite-bearing mantle source. The flat HREE patterns (Fig. 11c) in the Xiangshan quenched enclaves are consistent with derivation from spinel-facies mantle, contrasting with a garnet-facies source that shows a steep HREE pattern (Xu et al., 2001). It follows that partial melting took place at a shallow depth in the mantle. There is little change in La/Yb ratios and Dy/Yb ratios remain almost constant during melting in the spinel stability field, whereas melting in the garnet stability field produces large changes in Dy/Yb and La/Yb ratios (Fig. 15). Figure 15 shows that partial melting of a hypothetical LREE-enriched mantle source in the spinel stability field can generate the La/Yb–Dy/Yb systematics of the Xiangshan enclave magma. The La/Yb–Dy/Yb systematics of the Daoxian high-Mg potassic rocks suggest that they may probably be derived from mixing of partial melt from spinel-facies mantle with a small portion of partial melt from garnet-facies mantle (about 1% of melting).
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Tectonic environment
The consensus view is that boninites are erupted in suprasubduction-zone settings, typically in the forearc region of oceanic arcs (Hickey & Frey, 1982
; Hawkins et al., 1984), or in the back-arc basin setting (Crawford et al., 1981). Flower & Levine (1987) concluded that the appearance of boninite might reflect arc splitting as a possible precursor to back-arc basin formation. In addition to oceanic arcs, rocks of boninitic affinity occur in some continental arcs (e.g. Piercey et al., 2001). Piercey et al. (2001) suggested that the boninitic rocks from SE Yukon, Canada, were part of an ancient continental margin arc–back-arc magmatic system. Recently, Smithies (2002) described Archaean boninite-like rocks (2950 Ma) from the Mallina Basin, of the Pilbara Craton, NW Australia. They showed that these rocks probably formed more than 40 Myr after regional magmatism and 60 Myr after the last regional event linked to subduction, suggesting an intracontinental extensional setting for their origin. This extension was linked either to local lithospheric detachment delamination or to a mantle plume (Smithies & Champion, 2000). The mantle source for these boninite-like rocks was metasomatically enriched by an earlier subduction event at, or before, 3015 Ma, and the refractory source may have also been enriched during this event. Boninites are therefore powerful geodynamic tracers, and their recognition places important constraints on the tectonic settings of ancient orogens (e.g. Smithies, 2002).
Shoshonitic rocks typically occur in destructive plate margin settings as the arc matures and are generally younger than associated tholeiitic and calc-alkaline rocks (Morrison, 1980). It has been proposed that such potassic rocks also occur in extensional environments such as post-collisional and continental rift tectonic settings (Muller et al., 1992; Turner et al., 1996; Eklund et al., 1998; Liégeois et al., 1998; Rogers et al., 1998; Jiang et al., 2002). Muller et al. (1992) suggested a series of discrimination diagrams for potassic rocks formed in different tectonic settings. The Xiangshan quenched enclave data plot in the continental arc field in these diagrams (not shown). Such a setting is characterized by relatively flat subduction angles and broad Benioff zones (Muller et al., 1992).
It has been recognized that A-type granitoid magmas can be generally formed in a variety of extensional regimes, from continental back-arc extension to post-collision extension and to within-plate tectonic settings (e.g. Eby, 1992; Whalen et al., 1996; Förster et al., 1997). At Xiangshan, the volcanic complex has an A-type affinity, suggesting an extensional tectonic setting, as discussed above.
To sum up, an oceanic arc or continental margin arc, or intracontinental extension linked to an earlier phase of subduction could be suggested to explain the petrogenesis and boninitic affinity of the Xiangshan quenched enclave magma. However, its shoshonitic affinity implies only a continental arc tectonic setting. Together with the extension-related setting proposed for the A-type granitoids of the volcanic complex, we suggest a continental back-arc extensional setting as the succession of a continental arc.
An integrated model for origin of the Xiangshan volcanic complex
Based on the petrological and geochemical data presented in this paper, an integrated model for the origin of the Xiangshan volcanic complex is proposed, as shown in Figs 16 and 17. Zhou & Li (2000) have suggested a model associated with Palaeo-Pacific plate subduction and underplating of mafic magmas for the origin of Late Mesozoic igneous rocks in SE China. From 180 to 160 Ma, along the eastern flank of the Central Range in Taiwan, the Palaeo-Pacific plate was subducted underneath the Chinese continent at a very low angle (Fig. 16a). Slab dehydration at depths of around 110–200 km (Tatsumi & Eggins, 1995) triggered partial melting in the mantle wedge to generate basaltic magmas. An enormous heat source provided by the underplated basaltic magmas gave rise to partial melting of lower-crustal rocks to generate the 180–160 Ma plutons in SE China, generating localized areas of depleted residue. A large proportion of the source region (above or around the melt zone) dehydrated and later became the source rocks for A-type magmas such as those of the Xiangshan volcanic complex. In the lithospheric mantle, metasomatism by slab-related fluids produced a phlogopite-bearing harzburgite source region for the high-Mg potassic magmas.
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Between 160 and 135 Ma, it is suggested that the dip angle of the subducted slab increased, resulting in oceanward migration of the active magmatic zone (Fig. 16b). At the same time, back-arc extension and influx of asthenophere as a consequence of slab roll-back resulted in partial melting of the phlogopite-bearing spinel harzburgitic lithospheric mantle, generating the high-Mg potassic magmas. Injection of these anomalously high-temperature (>1200°C) melts into the dehydrated and granulitized lower-crustal source region induced it to partially melt. Such crustal melts assembled at depths of
15–23 km to form a magma chamber, which subsequently was erupted to form the Xiangshan volcanic complex (Fig. 17). Meanwhile, the high-Mg potassic magma was injected into the subvolcanic magma chambers, resulting in the quenched enclaves hosted by the subvolcanic rocks. The same tectonic setting is inferred for the Daoxian high-Mg potassic rocks. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Late Jurassic (158–135 Ma) peraluminous volcanic complex at Xiangshan has A-type affinities. The dominant felsic magmas are probably derived from partial melting of lower-crustal rocks that consist of Middle Proterozoic metamorphic rocks. These metamorphic rocks include both orthometamorphic and parametamorphic rocks that had been dehydrated but not melt-depleted during an earlier (180–160 Ma) thermal event. The quenched mafic enclaves within the subvolcanic rocks have both boninitic and shoshonitic affinities, suggesting derivation from a phlogopite-bearing spinel harzburgitic lithospheric mantle source.
Detailed petrological and geochemical data for the Xiangshan volcanic complex and its quenched mafic enclaves suggest that the magmas were probably generated in a back-arc extensional regime. This suggests that generation of the extensive late Mesozoic belt of volcanic–intrusive complexes in SE China was related to subduction of the Palaeo-Pacific Plate. Between 180 and 160 Ma SE China was a continental arc environment, forming the 180–160 Ma plutons of the late Mesozoic volcanic–intrusive complex belt. Since 160 Ma the northwestern belt has been a back-arc extension setting, resulting in an influx of asthenophere and partial melting of the phlogopite-bearing harzburgitic lithospheric mantle to form high-Mg potassic magmas. Pulsatory injection of such anomalously high-temperature (>1200°C) magmas into the dehydrated and granulitized crustal source region induced them to partially melt and generate the high-temperature (>850°C) A-type magmas of the Xiangshan volcanic complex.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL METHODSCLASSIFICATIONMINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We are grateful to X. Huang from Institute of Geology, Academica Sinica, for his assistance with measurements of Nd and Sr isotope compositions, to D.-F. Wan from the Isotope Geochemistry Laboratory of the Chinese Academy of Geological Sciences (Beijing) for her assistance with stable isotope analyses, and to R.-C. Wang from the State Key Laboratory for Mineral Deposits Research of Nanjing University for his help with electron microprobe analyses and beneficial discussion on mineral chemistry. We thank Tod Waight and Paul Morris for their constructive reviews, and John Gamble for his helpful editorial handling of the manuscript and good suggestions for improvement. The Executive Editor, M. Wilson, is thanked for helpful comments in her handling of this manuscript. This work was financially supported by the National Natural Science Foundation (40221301) and the National Key Basic Research Projects (G1999043211 and 2002CB412603), and by the Nanjing University Talent Development Foundation.
* Corresponding author. E-mail: yhj186{at}hotmail.com
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