Journal of Petrology Advance Access originally published online on September 3, 2007
Journal of Petrology 2007 48(10):1973-1997; doi:10.1093/petrology/egm046
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Sources and Petrogenesis of Late Triassic Dolerite Dikes in the Liaodong Peninsula: Implications for Post-collisional Lithosphere Thinning of the Eastern North China Craton
1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy Of Sciences, PO Box 9825, Beijing 100029, China
2Department of Applied Geology, Curtin University of Technology, PO Box U1987, Perth, WA 6845, Australia
RECEIVED NOVEMBER 15, 2006; ACCEPTED JULY 18, 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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A combination of major and trace element, whole-rock Sr, Nd and Hf isotope, and zircon U–Pb isotopic data are reported for a suite of dolerite dikes from the Liaodong Peninsula in the northeastern North China Craton. The study aimed to investigate the source, petrogenesis and tectonic setting of the dikes. Sensitive high-resolution ion microprobe U–Pb zircon analyses yield a Late Triassic emplacement age of
213 Ma for these dikes, post-dating the collision between the North China and Yangtze cratons and consequent ultrahigh-pressure metamorphism. Three geochemical groups of dikes have been identified in the Liaodong Peninsula based on their geochemical and Sr–Nd–Hf isotope characteristics. Group 1 dikes are tholeiitic, with high TiO2 and total Fe2O3 and low MgO contents, absent to weak negative Nb and Ta anomalies, variable (87Sr/86Sr)i (0·7060–0·7153), 
Nd(t) (– 0·8 to –6·5) and Hf(t) (–2·7 to –7·8) values, and negative 
Hf(t) (–1·1 to –7·8). They are inferred to be derived from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with some upper crustal assimilation and fractional crystallization. Group 2 dikes have geochemical features of high-Mg andesites with (87Sr/86Sr)i values of 0·7063–0·7072, and negative Nd(t) (–3·0 to –9·5) and Hf(t) (–3·2 to –10·1) values, and may have originated as melts of foundered lower crust, with subsequent interaction with mantle peridotite. Group 3 dikes are shoshonitic in composition with relatively low (87Sr/86Sr)i values (0·7061–0·7063), and negative Nd(t) (–13·2 to –13·4) and Hf(t) (–11·0 to –11·5) values, and were derived by partial melting of an ancient, re-enriched, refractory lithospheric mantle in the garnet stability field. The geochemical and geochronological data presented here indicate that Late Triassic magmatism occurred in an extensional setting, most probably related to post-orogenic lithospheric delamination. KEY WORDS: mafic dike; asthenospheric mantle; lithospheric mantle; delamination; North China Craton
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The tectonic evolution of orogenic belts is typically marked by changes in the composition of the associated magmatism (Harris et al., 1986
). Post-collisional magmatism is one of the common features of many orogens around the world, and may indicate that the orogen is in the process of collapsing (Dewey, 1988). Petrogenetic studies of post-collisional magmatism not only provide constraints on the geodynamic processes responsible for the cessation of collision and onset of extensional collapse (Bonin et al., 1998), but also reveal changes in magma source regions associated with such processes (Turner et al., 1992, 1996; Kay & Kay, 1993; Bonin et al., 1998). Thus, the geochemical characteristics of post-collisional magmatism allow the evaluation of its mantle source and provide constraints on the cessation of collision and onset of extensional collapse.
The Dabie–Sulu orogen in central China was formed by collision between the North China Craton (NCC) and Yangtze Craton (YC), and is known to contain the largest distribution of ultrahigh-pressure (UHP) metamorphic rocks in the world (Fig. 1a; Cong, 1996; Hacker et al., 1998; Liu et al., 2004; Wan et al., 2005), which were formed by deep subduction (over 200 km, Ye et al., 2000) in the Early–Middle Triassic and exhumation to mid-crustal levels by the Late Triassic (Liu et al., 2004; Wan et al., 2005). Post-collisional magmatism is weakly developed in the Dabie–Sulu UHP orogenic belt, and only a single intrusion, the Jiazishan complex with an age of 215–200 Ma, has been identified in the Sulu part of the belt (Chen et al., 2003; Guo et al., 2005; Yang et al., 2005). However, Late Triassic magmatism consisting of mafic dikes, diorites, syenites and monzogranites with mafic enclaves has been identified in the Liaodong Peninsula of the northeastern NCC (Yang et al., 2004a; Wu et al., 2005a; Yang et al., 2007a), located to the north of the Sulu UHP orogen (Fig. 1a and b).
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Mafic dike swarms are a common expression of mantle-derived magma generation associated with extensional tectonic regimes in post-collisional or intraplate extensional settings (e.g. Rock, 1991
). They can thus provide important information for understanding not only the mantle source of the magmas, but also the tectonic evolution of the orogenic belt and adjacent regions (Gorring et al., 2001, 2003). In this paper, we report zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotopic compositions of mafic dikes from the southern Liaodong Peninsula to: (1) document the geochemical characteristics of these rocks; (2) investigate their mantle sources and petrogenesis; (3) interpret the evolution of lithospheric mantle beneath the eastern NCC; (4) evaluate the tectonic implications for the eastern NCC during the Late Triassic. |
GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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East China is composed of the Central Asian Orogenic Belt (CAOB) in the north, the NCC in the centre and the Yangtze Craton in the south (Fig. 1a; Wang & Mo, 1996
). The Dabie–Sulu orogenic belt in east–central China (Fig. 1a) marks the collision zone between the North China and Yangtze cratons (Cong, 1996; Wang & Mo, 1996). Deep subduction of the Yangtze continental crust to >120 km depth is evident by the occurrence of micro-diamond in eclogites (Xu et al., 1992); exhumation of even deeper rocks from >200 km depth was deduced from mineral exsolution relations (Ye et al., 2000). Detailed zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb data and inferred P–T paths indicate that the protoliths of the UHP metamorphic rocks were subducted to mantle depths in the Early–Middle Triassic (244–226 Ma) and exhumed to mid-crustal levels by the Late Triassic (221–218 Ma) (Liu et al., 2004; Wan et al., 2005).
The Liaodong Peninsula is located in the northeastern segment of the NCC, to the north of the Sulu belt (Fig. 1a). It consists of Early Archean to Paleoproterozoic basement rocks overlain by unmetamorphosed Mesoproterozoic to Cenozoic cover. The Late Archean basement rocks consist of 2·5 Ga diorite–tonalite–granodiorite suites (Lu et al., 2004) that were deformed during the Paleoproterozoic (Yin & Nie, 1996) and Early Cretaceous (120–110 Ma) (Yang et al., 2004b, 2007b). Paleoproterozoic rocks (the Liaohe Group) unconformably overlie the Late Archean rocks, and were deposited and then metamorphosed during a 1·9–1·85 Ga orogenic event (Lu et al., 2004; Luo et al., 2004). Subsequently, the Liaodong Peninsula was covered by thick sequences of Meso- to Neoproterozoic and Paleozoic sediments. Before the Mesozoic, igneous activity was weak, indicating that the Liaodong Peninsula was tectonically stable.
Mesozoic intrusive rocks are widely distributed in the Liaodong Peninsula and cover an area of 20 000 km2 (Fig. 1b); magmatism mostly occurred in the Late Jurassic (173–150 Ma) and Early Cretaceous (135–110 Ma) (Yang et al., 2004c, 2006a, 2007c; Wu et al., 2005b, 2005c). However, minor Triassic magmatism (230–210 Ma) has been identified (Wu et al., 2005a; Yang et al., 2007a), consisting of mafic dikes, nepheline syenites, syenites, diorites and monzogranites with mafic enclaves. Many mafic dikes are exposed in the Dalian area, in the southern Liaodong Peninsula (Fig. 1c).
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PETROLOGY OF THE MAFIC DIKES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The mafic dikes intrude Neoproterozoic sandstones and limestones and Middle Triassic syenites; some of these dikes were also emplaced during the Late Mesozoic (Wang et al., 2000
). The dikes are tens of meters to 300 m in width and several hundred meters to several kilometers in length. They have a doleritic composition, with a medium- to fine-grained ophitic texture. The main minerals are plagioclase (40–50%), clinopyroxene (45–50%), K-feldspar (5%), and minor quartz and biotite. Accessory minerals include magnetite, ilmenite, zircon, apatite and titanite. The detailed mineralogy of all samples is listed in Table 1.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Zircon U–Pb dating
Two dike samples (03JH011 and 03JH025) from the Liaodong Peninsula were chosen for zircon U–Pb dating. Samples of fresh rock (about 25 kg) were crushed and the heavy mineral fraction, including zircon, was extracted using standard techniques including heavy liquid separation. Zircon was extracted from each crushed-rock concentrate by hand-picking under a binocular microscope. The zircon grains were placed on a mount, sectioned and polished. The mounts were carbon coated for cathodoluminescence (CL) imaging and gold coated for the ion microprobe analyses. Measurements of U, Th, and Pb (Table 1) were conducted using the SHRIMP II ion microprobe at the Beijing SHRIMP Laboratory. U–Th–Pb ratios and absolute abundances were determined relative to the SL13 standard zircon (206Pb/238U = 0·092821 corresponding to 572 Ma; 238 ppm 238U; Claoué-Long et al., 1995
). Analyses of the TEM zircon standard were interspersed with those of unknown grains, to monitor analytical conditions throughout the session. The machine operating conditions were similar to those described by Nelson (1997). Measured compositions were corrected for common Pb assuming the composition of Broken Hill lead, and the data were recalculated on the basis of the measured non-radiogenic 204Pb content. The SQUID 1.0 and ISOPLOT 3.0 software of Ludwig (2003) were used for data processing. Uncertainties on individual analyses in data tables are reported at the 1
level. The 206Pb/238U ages are considered the most reliable for Phanerozoic zircons such as these, as the zircons are concordant and the low count rates on 207Pb result in large statistical uncertainties, making the 207Pb/206Pb and 207Pb/235U ratios a less sensitive measure of age for younger zircons (Compston et al., 1992). Weighted mean ages for pooled 206Pb/238U analyses are quoted at the 95% confidence level, except where noted. The zircon UPb data are presented in Table 2 and shown in Fig. 2.
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Major and trace elements
After petrographic examination, 17 of 19 samples from the southern part of the Liaodong Peninsula (Table 2 and Fig. 1c) are fresh and were selected, crushed and powdered in an agate mill for geochemical analysis. Elemental analyses were conducted in the Department of Geology, Northwest University in Xi’an, China. Major elements were determined by X-ray fluorescence (XRF), with analytical uncertainties ranging from 1 to 3%. Trace element concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7500a system. For trace element determination, about 50 mg of crushed whole-rock powder was dissolved for about 7 days at
100°C using HF–HNO3 (10:1) mixtures in screw-top Teflon beakers, followed by evaporation to dryness. The material was dissolved in 7N HNO3 and taken to incipient dryness again, and then was re-dissolved in 2% HNO3 to a sample/solution weight ratio of 1:1000. The analytical errors vary from 5 to 10% depending on the concentration of any given element. An internal standard was used for monitoring drift during analysis; further details have been given by Gao et al. (2002).
Nd, Sr and Hf isotopes
Rock powders for Nd–Sr isotopic analysis were dissolved in Teflon bombs after being spiked with 84Sr, 87Rb, 150Nd, and 147Sm enriched isotope tracers prior to HF + HNO3 (in a ratio of 2:1) dissolution. Rubidium, Sr, Sm, and Nd were separated using conventional ion exchange procedures as described by Yang et al. (2004d) and measured using a Finnigan MAT 262 multi-collector mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Procedural blanks were <100 pg for Sm and Nd and <500 pg for Rb and Sr. 143Nd/144Nd values were corrected for mass fractionation by normalization to 146Nd/144Nd = 0·7219, and 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0·1194. Typical within-run precision (2) for Sr and Nd was estimated to be ± 0·000015. The analytical errors for Rb, Sr, Sm and Nd contents are better than 2%, distinct from those determined by the ICP-MS technique, which resulted in differences in Rb, Sr, Sm and Nd concentrations between the isotope dilution (ID) and ICP-MS methods. The measured values for the La Jolla and BCR-1 Nd standards and NBS-607 Sr standard were 143Nd/144Nd = 0·511853 ± 7 (2n, n = 3) and 0·512604 ± 7 (2n, n = 3) and 87Sr/86Sr = 1·20042 ± 2 (2n, n = 12) during the period of data acquisition.
Hf isotopic analyses were conducted in the multicollector (MC)-ICP-MS Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences (Beijing), using a ThermoElectron Neptune MC-ICP-MS system. Sample powder (100 mg) was dissolved in sealed Teflon bombs. Separation of Hf was performed using a routine two-column ion exchange technique which includes: (1) group separation of high field strength elements (HFSE) through a cation exchange column (1 cm x 8 cm, packed with Bio-Rad AG50X8, 200–400 mesh resin); (2) purification of Hf through a second exchange column packed with Ln resin. 176Hf/177Hf measurements were normalized to 179Hf/177Hf = 0·7325. During the period of data acquisition, standard BHVO-1 was also processed for Hf isotopes, and gave a ratio of 0·283105 ± 6 (2m) for 176Hf/177Hf, agreeing with the recommended value (0·283100 ± 3, Weis et al., 2005).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Geochronology
Zircons from mafic dike sample 03JH025 show oscillatory zoning, typical of igneous crystallization, and have light margins in CL images (Fig. 2a). Eleven analyses yield ages between 222 ± 4 and 204 ± 2 Ma (1
) and form a coherent group with a weighted mean 206Pb/238U age of 213 ± 5 Ma (2, MSWD = 1·2) (Fig. 2a). However, zircons from sample 03JH011, previously reported in Yang et al. (2004a), are relatively homogeneous and grey in CL, although one grain has oscillatory zoning and a large apatite inclusion in the core that appears white in the CL image (Fig. 2b). This grain yields a 206Pb/238U age of 211 ± 2 Ma, whereas the other zircon grains give 207Pb/206Pb ages of 1·1–0·8 Ga (Fig. 2b) and are interpreted either to result from wall-rock contamination of the mafic magma during emplacement or to be inherited from the source rock. The Late Triassic age (213 Ma) is consistent with the previous result [213·0 ± 1·5 Ma, zircon thermal ionization mass spectrometry (TIMS) U–Pb age] of Wang et al. (2000) and the field observation that some dikes intrude deformed Triassic syenites with an age of 220 ± 2 Ma (zircon LA-ICP-MS U–Pb age, Wu et al., 2005a). Thus, the age of 213 Ma represents the emplacement age of the dolerites in the Liaodong Peninsula.
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Geochemistry
Full major and trace element analyses of the Liaodong dikes are presented in Table 3. Rubidium, Sr, Sm and Nd concentrations, 143Nd/144Nd, 87Sr/86Sr and 176Hf/177Hf ratios, and Nd and Hf model ages for these mafic dikes are listed in Table 4 and shown in Fig. 7. The (87Sr/86Sr)i ratios,
Nd(t) and Hf(t) values have been calculated at 213 Ma on the basis of the U–Pb zircon dating. Hf(t) is defined as Hf(t) = 1·33Nd(t) + 3·19 such that a sample with positive Hf(t) lies above and a sample with negative Hf(t) lies below the mantle array of Vervoort et al. (1999).
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The samples exhibit significant variations in major and trace element and Sr–Nd–Hf isotopic compositions (Tables 3 and 4). The dikes are subalkaline to alkaline and range in composition from basalt to basaltic andesite and basaltic trachyandesite (Fig. 3a). On the basis of their distinct petrography (Table 1), geochemistry (Table 3) and Sr–Nd–Hf isotopic compositions (Table 4), three groups of dikes can be identified.
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Group 1
Group 1 dikes, mainly composed of clinopyroxene, plagioclase and Fe–Ti oxides, are basic to intermediate in composition (SiO2 47·2–55·1 wt%) and belong to the tholeiitic series (Fig. 3a and b), with high abundances of total Fe2O3 (13·4–17·6 wt%), TiO2 (2·4–4·2 wt%) and CaO (5·7–10·0 wt%) and low concentrations of Al2O3 (11·8–13·7 wt%), K2O (0·5–2·0 wt%), Cr (1·1–101 ppm) and Ni (1·5–73·6 ppm) (Table 3; Figs 3c and 4). They have low MgO (2·01–6·07 wt%) and mg-numbers [where mg-number = 100 x Mg/(Mg + Fe2+) = 20·1–47·5] (Fig. 3d). Group 1 dikes are light rare earth element (LREE) enriched with (La/Yb)CN (subscript ‘CN’ refers to values normalized to chondrite) ranging from 3·0 to 6·6 and LREE contents up to 118 times chondrite (Fig. 5a). All samples have relatively flat middle REE (MREE) to heavy REE (HREE) chondrite-normalized REE patterns with (Dy/Yb)CN values of 1·2–1·4. In primitive mantle (PM)-normalized trace element patterns (Fig. 6a), they are enriched in large ion lithophile elements (LILE), including Rb, Th and U, and LREE, but with variable negative Ba and Sr anomalies. They are also enriched in HFSE and lack negative Nb, Ta, Zr and Hf anomalies.
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Group 1 dikes display considerable variations in (87Sr/86Sr)i (0·7060–0·7153),
Nd(t) values (–0·8 to –6·5), Hf(t) values (–2·5 to –7·8), and Hf(t) values of –1·1 to –6·4 (Fig. 7 and Table 4). In the Nd(t) and Hf(t) vs (87Sr/86Sr)i diagrams (Fig. 7a and b), Group 1 dikes define a linear trend, indicating a two-component mixing process. They have negative Hf(t) values (–6·4 to –1·1) [where Hf(t) = 1·33Nd(t) + 3·19 – Hf(t)] and plot close to or below the mantle array [Hf(t) = 1·33Nd(t) + 3·19] of Vervoort et al. (1999) in the Hf(t) and Hf(t) vs Nd(t) diagrams (Fig. 7c and d).
Group 2
Group 2 dikes also plot in the subalkaline field (Fig. 3a) and are characterized by high MgO (7·38–8·94 wt%), Cr (397–795 ppm) and Ni (110–238 ppm), low TiO2 (0·66–0·81 wt%) and total Fe2O3 (7·6–7·9 wt%) concentrations at intermediate silica contents (SiO2 53·1–55·6 wt%; Table 3, Figs 3 and 4), similar to the geochemical characteristics of high-Mg andesites (Tatsumi, 1982, 2006; Tatsumi & Maruyama, 1989; Shinjo, 1999; Yogodzinski et al., 2001; Wang et al., 2002; Zhang et al., 2003). They are medium-K to high-K calc-alkaline (Fig. 3c). Group 2 intermediate dikes are enriched in LREE and are relatively depleted in HREE with (La/Yb)CN values of 8·2–12·0 and (Dy/Yb)CN values of 1·2–1·4 with no negative Eu anomalies in the chondrite-normalized REE patterns (Fig. 5b). In the PM-normalized trace element diagram (Fig. 6b), Group 2 dikes are enriched in LILE (including Rb, Ba, Th and U) and LREE with positive Ba and Sr anomalies, and are depleted in HFSE with strong negative Nb, Ta and Ti anomalies. Group 2 dikes have relatively low (87Sr/86Sr)i ratios (0·7063–0·7072) compared with Group 1 and variable Nd(t) values of –3·0 to –9·5, Hf(t) values of –3·2 to –10·1 and Hf(t) values of –3·9 to + 3·5 (Fig. 7 and Table 4).
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Group 3
Two dikes sampled from the Liaodong Peninsula are identified as Group 3. They are alkaline (Fig. 3a) with K2O/Na2O ratios of 1·2–1·5 and total alkali (K2O + Na2O) contents of 7·1–7·6 wt%; they are thus shoshonitic (Fig. 3c). They have mafic to intermediate compositions (SiO2 49·6–51·9 wt%, MgO 5·8–6·5 wt%), and Cr and Ni contents between those of Groups 1 and 2 (Table 3 and Fig. 3b). They are highly enriched in LREE with (La/Yb)CN values of 42·7–44·6 and (Dy/Yb)CN values of 1·9–2·0, distinct from Groups 1 and 2, and are without negative Eu anomalies (Fig. 5b). In the PM-normalized trace element diagram (Fig. 6b), Group 3 rocks are enriched in LILE with positive K, Ba and Sr anomalies, and are depleted in HFSE with pronounced negative Nb, Ta, Zr, Hf and Ti anomalies. Nb–Ta fractionation is evident in the PM-normalized trace element patterns, with Nb/Ta ratios of 26·7–29·3 (Fig. 6b). The mafic dikes of Group 3 have relatively low (87Sr/86Sr)i ratios (0·7061–0·7062) and strongly negative
Nd(t) (–13·2 to –13·4) and Hf(t) values (–11·0 to –11·5). They have positive Hf(t) values of + 2·9 to + 3·6 (Fig. 7 and Table 4), distinct from Group 1 dikes.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The major and trace element characteristics and Sr–Nd–Hf isotope compositions of the primitive and evolved magmatic rocks in the Liaodong Peninsula can provide constraints on the nature of the mantle source, the processes of partial melting, crustal-level evolution and geodynamic setting. Moreover, there may be a relationship between the petrogenesis of the post-collisional mafic dikes and the evolution of the Sulu orogenic belt.
Magma sources and petrogenesis
Group 1
The Group 1 rocks have variable SiO2 (47·2–55·1 wt%), MgO (2·0–6·4 wt%), Cr (1·1–101 ppm), Ni (1·5–74 ppm), (87Sr/86Sr)i (0·7060–0·7153), Nd(t) (–0·8 to –6·5) and Hf(t) (–2·5 to –7·8) values. All these features indicate that none of these dike samples represent primary magma compositions. In the Nd(t) and Hf(t) vs (87Sr/86Sr)i diagrams (Fig. 7a and b), the Group 1 dikes define a mixing line between two components; that is, a component (mafic magma) with low (87Sr/86Sr)i (< 0·7060) and high Nd(t) (> 0) and Hf(t) (> –1·3) values, and another component (crustal material) with high (87Sr/86Sr)i (> 0·7153) and negative Nd(t) (< –6·5) and Hf(t) (< –7·8) values. Furthermore, the positive correlations between CaO, Al2O3 (and Ca/Al ratios, not shown) and MgO (Fig. 4) are consistent with extensive clinopyroxene and feldspar fractionation. Plagioclase fractionation is also indicated by the negative Sr and Ba anomalies in the PM-normalized trace element patterns (Fig. 6b). The increase in total Fe2O3 and TiO2 concentrations with decreasing MgO abundances (Fig. 4) may be the result of accumulation of Fe–Ti oxides. All these characteristics preclude a simple, common evolution by closed-system fractionation processes; instead, they suggest the operation of combined processes of crustal assimilation and fractional crystallization (AFC) in the petrogenesis of the magma.
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The energy-constrained assimilation and fractional crystallization (EC-AFC) model (Bohrson & Spera, 2001
; Spera & Bohrson, 2001) provides a rigorous approach to the simulation of AFC processes. First, we selected reasonable ranges of the initial parameters required in the EC-AFC model based on the isotopic and geochemical characteristics of the magmas in the Liaodong Peninsula. The likely crustal materials involved in the petrogenesis of Group 1 dikes have high (87Sr/86Sr)i ratios, similar to those of upper crustal materials and crustal-derived granites in the Liaodong Peninsula and NCC as a whole (Jahn et al., 1999; Yang et al., 2006a). Therefore, an upper crustal composition was selected with a Sr content of 350 ppm, Nd concentration of 26 ppm, (87Sr/86Sr)i of 0·7250 and Nd(t) value of –10 [(143Nd/144Nd)i = 0·51185]. As the parental magma of Group 1 dikes had (87Sr/86Sr)i <0·706 and Nd(t) >0, we selected values of 0·7038 for Sr and Nd(t) = +3·0 for the Nd isotopic compositions of the parental magma. The Sr and Nd contents of Group 1 dikes range from 221 to 554 ppm and 20 to 34 ppm, respectively (Table 3). In the (87Sr/86Sr)i vs Sr diagram (Fig. 8b), the Group 1 dikes define a rough line back to a (87Sr/86Sr)i of 0·7038 and Sr contents of 160–180 ppm in the parental magma. However, fractionation of clinopyroxene and plagioclase will decreases the Sr content of magmas, whereas crustal assimilation will increase the Nd concentration. Thus, we selected Sr and Nd abundances for the parental magma of 200 ppm and 10 ppm, respectively. The physical characteristics of the magma body–country rock system are similar to those suggested by Bohrson & Spera (2001) and are listed in Table 5. Using the selected values of these initial parameters, we performed calculations using the EC-AFC (http://magma.geol.ucsb.edu) calculation programs. In addition, AFC calculations (DePaolo, 1981) were also performed. The output from our EC-AFC and AFC simulations provides model curves to fit the correlation trends between (87Sr/86Sr)i and Sr, and Nd(t) and (87Sr/86Sr)i (Fig. 8). These parameters provide important constraints on the characteristics of the primitive magma, the nature of the crustal contaminant and AFC processes involved in the petrogenesis of the Group 1 dikes.
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Although none of these dike samples represent primary melts, the samples with high MgO contents (
6·0 wt%) are tholeiitic and have low SiO2 abundances (47·1–49·2 wt%), relatively low (87Sr/86Sr)i (down to 0·7060) and relatively high Nd(t) (up to –0·8) and Hf(t) (up to –2·5) values, indicating that the parental magmas to Group 1 dikes have (87Sr/86Sr)i <0·7060, Nd(t) >0 and Hf(t) > –2·5. The high-MgO samples (MgO 6·0 wt%) have relatively high Nb, Ta, and (Nb/Th)N up to 1·1 (Fig. 9a) with no negative Nb and Ta anomalies in the PM-normalized trace element patterns (Fig. 6a), indicating that the parental magma to Group 1 mafic dikes was enriched in HFSE and LREE relative to LILE. The high-MgO samples also have high total Fe2O3, TiO2, Al2O3 and CaO contents (Table 3), between the major element compositions of glasses produced by partial melting experiments on natural anhydrous fertile spinel peridotite at conditions ranging from 0·2 to 3·0 GPa and 1100 to 1375°C at melt fractionation of <30% (HK-66; Hirose & Kushiro, 1993) and on a fertile peridotite at conditions ranging from 2 to 30 kbar (Falloon et al., 1988) (Fig. 9b). Therefore, the parental magmas of Group 1 dikes should have low SiO2 abundances and high total Fe2O3, TiO2, Al2O3 and CaO contents, with no negative Nb, Ta and Ti anomalies, similar to experimental melts derived from partial melting of fertile peridotites (Falloon et al., 1988; Hirose & Kushiro, 1993).
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The variable major and trace element abundances and Sr and Nd isotopic compositions of the Group 1 dikes strongly suggest the involvement of a crustal component in their petrogenesis. The positive correlation between (87Sr/86Sr)i and (La/Yb)CN ratios and negative correlation between
Nd(t) values and (La/Yb)CN ratios (Fig. 9c and d) indicate that the variable major and trace element characteristics were not produced by simple differentiation of a mafic magma, but resulted from assimilation of a crustal component with low MgO, Nd(t) and Hf(t) values and high LREE abundance and (87Sr/86Sr)i compared with the products of fractional crystallization. The Group 1 dikes have variable Ce/Pb ratios (3·8–15·7) intermediate between those of oceanic basalts (25, Sun & McDonough, 1989) and the upper continental crust (3·7, Rudnick & Gao, 2003). In addition, Ce/Pb ratios decrease and Zr/Sm ratios (not shown) increase with increasing (87Sr/86Sr)i, suggesting that upper continental crustal material was involved in their petrogenesis. This is also evidenced by the elevated Rb/Sr, Th, U and La contents with decreasing MgO concentration (Table 2) and the presence of inherited zircons with ages of 1·1–0·8 Ga in the Group 1 dikes (Fig. 2b) (Yang et al., 2004a). In addition, the crustal component ought to have relatively high (87Sr/86Sr)i (> 0·715) and moderately negative Nd(t) and Hf(t) values, distinct from those of Late Triassic granites with (87Sr/86Sr)i < 0·710, Nd(t) values of –14 to –16 and Hf(t) values of –13 (Fig. 7) that were derived from partial melting of lower crustal materials (Yang et al., 2007a). All these factors suggest an involvement of upper crustal materials in the origin of Group 1 dikes.
In general, low La/Yb ratios reflect a melting regime dominated by relatively large melt fractions and/or spinel as the predominant residual phase, whereas high La/Yb ratios are indicative of smaller melt fractions and/or garnet control. Sample 03JH007, having relatively lower MgO and higher SiO2 and trace element concentrations than other samples of Group 1, may have formed by relatively low degrees of partial melting. In the LaCN vs MgO (not shown), (87Sr/86Sr)i and Nd(t) vs (La/Yb)CN diagrams (Fig. 9c and d), the trends defined by the Group 1 dikes show that the high-MgO samples have low La/Yb ratios and (87Sr/86Sr)i ratios and high Nd(t) values, indicating that the primary magma parental to Group 1 dikes would have had low La/Yb and (87Sr/86Sr)i ratios and high Nd(t) values. Based on geochemical modeling, a 1–15% partial melt of spinel peridotite ought to have relatively low La/Yb and Dy/Yb. In the Dy/Yb vs La/Yb diagram, (Fig. 10) the Group 1 dikes define a trend, showing that the high-MgO samples have low La/Yb and Dy/Yb. Therefore, the relatively low (La/Yb)CN (3·0–6·6) and (Dy/Yb)CN (1·2–1·4) (Table 2), in combination with relatively flat MREE to HREE patterns (Fig. 5a) for the high-MgO samples of the Group 1 dikes, suggest that they may have formed by relatively high degree melting of a mantle source in the spinel stability field.
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The geochemical and isotopic data indicate that the Group 1 dikes were the result of upper crustal assimilation and crystal fractionation of mafic magmas derived from high percentage partial melting of a fertile mantle in the spinel stability field. The Group 1 dikes have negative
Hf signatures, indicating that melts in equilibrium with garnet-bearing residua exhibit greater Lu/Hf fractionation than Sm/Nd, such that with time the crystallization products of these melts will evolve, over time periods in excess of 1 Gyr, to isotopic compositions below the Nd–Hf isotope array with negative Hf (Fig. 7c and d). As summarized by Davies et al. (2006), two mantle source components have been proposed to explain the origin of negative Hf signatures: sub-cratonic lithospheric mantle and recycled subducted oceanic crust (Salters & White, 1998; Griffin et al., 2000; Janney et al., 2002; Nowell et al., 2004). However, mineral assemblages, and geochemical and isotopic compositions of mantle xenoliths in Paleozoic kimberlites show that the lithospheric mantle beneath both the North China and Yangtze cratons was relatively ancient, refractory mantle (Menzies et al., 1993; Griffin et al., 1998; Xu et al., 2001; Zhang et al., 2001, 2004). The lithospheric mantle has negative Nb and Ta anomalies in PM-normalized trace element diagrams (Wu et al., 2006a) and has enriched Sr and Nd isotopic compositions with 87Sr/86Sr(213 Ma) of 0·7045–0·7094 and Nd(213 Ma) values of –4·3 to –8·0 (Fig. 7a; Zhang et al., 2001; Wu et al., 2006a). These characteristics are distinct from those of the parental magmas of the Group 1 dikes, precluding an ancient lithospheric mantle source for the Group 1 dikes. Thus, we propose that the parental magmas of Group 1 dikes were derived from partial melting of a relatively fertile, sub-lithospheric (asthenospheric) mantle source beneath the continental lithospheric mantle in the Liaodong Peninsula.
Group 2
The Group 2 dikes have variable Sr, Nd and Hf isotopic compositions (Fig. 7). In the Nd(t) and Hf(t) vs (87Sr/86Sr)i diagrams (Fig. 7a and b), the trends of Group 2 dikes suggest mixing between a component with high Nd(t) and Hf(t) values and low (87Sr/86Sr)i ratios, and one with low Nd(t) and Hf(t) values and low (87Sr/86Sr)i ratios. They have intermediate silica contents (SiO2 53·1–55·6 wt%), but with high MgO (7·4–8·9 wt%, mg-number = 65·9–69·4), Cr (397–795 ppm) and Ni (110–238 ppm) abundances (Table 2; Fig. 3d), similar to the geochemical characteristics of high-Mg andesites (Tatsumi & Maruyama, 1989; Shinjo, 1999; Yogodzinski et al., 2001; Wang et al., 2002; Zhang et al., 2003).
Three processes can generate a primitive high-Mg andesite: (1) partial melting of a hydrous mantle source (e.g. Kushiro, 1974; Tatsumi, 1982, 2006; Hirose, 1997); (2) interaction of melts formed by slab melting with a mantle wedge component (e.g. Yogodzinski et al., 1994; Kelemen, 1995; Stern & Kilian, 1996; Rapp et al., 1999); (3) melting of foundered lower crust, followed by interaction with mantle peridotite (Kay & Kay, 1993; Xu et al., 2002). However, partial melts from a hydrous mantle source at high temperature (>900°C) should be in isotopic equilibrium with their source, and thus would have homogeneous isotopic compositions, which is not consistent with the observations in this study (Fig. 7). Crustal assimilation by mantle-derived melts would reduce their MgO, Cr and Ni concentrations (Fig. 3d), which is not consistent with the high MgO, Cr and Ni contents of the Group 2 dikes. Thus, the high-Mg andesitic magmas parental to the Group 2 dikes could be the products of interaction between mantle peridotite and ascending intermediate to felsic melts originating from recycled crustal materials (Fig. 3d) (Kay & Kay, 1993; Yogodzinski et al., 1994; Kelemen, 1995; Stern & Kilian, 1996; Xu et al., 2002). Interaction of intermediate to felsic melts with mantle peridotite should enrich the melts in MgO, Cr and Ni (Fig. 3d), which is consistent with the observations. The mantle peridotite would have relatively high Nd(t) and Hf(t) values and low (87Sr/86Sr)i ratios, similar to the source of the Group 1 dikes. The low Nb and Ta contents of the Group 2 dikes, coupled with high Th/La ratios and low Nb/Th and (87Sr/86Sr)i ratios (0·7063–0·7073) and Nd(t) and Hf(t) values, at intermediate silica contents, similar to those of lower crustal-derived xenoliths (Zheng et al., 2004), indicate that the melts could have originated from a lower crustal source with low (87Sr/86Sr)i, Nd(t) (–3·0 to –9·5) and Hf(t) (–3·2 to –10·1) values.
According to the experimental work of Rapp et al. (1999), the interaction of melts with mantle peridotite causes decreases in the SiO2 and Al2O3 contents of the hybridized melts, with simultaneous increases in the contents of MgO, FeO, Na2O, K2O, TiO2 and trace element abundances, but significantly, most element ratios (e.g. Sr/Y, La/Yb, Sr/Nd, Nb/La, K/La, etc.) remain nearly constant. Therefore, these ratios should reflect those of the parental magmas to the Group 2 dikes. The parental magmas of Group 2 dikes have relatively high SiO2 and Sr contents and low Y (15·8–17·3 ppm) and Yb (1·3–1·5 ppm) abundances, with high Sr/Y ratios similar to those of adakites (Defant & Drummond, 1990), indicating that these melts were derived by partial melting of recycled oceanic or continental crustal materials at high pressures, with garnet in the residue (Rapp et al., 1999; Xu et al., 2002). The (87Sr/86Sr)i (> 0·7060) and negative Nd(t) values (< –9·5) of Group 2 dikes are distinct from those of partial melts of subducted oceanic slabs (Nd(t) > 0; Defant & Drummond, 1990) and those of deeply subducted Yangtze continental crustal materials with high Nd(t) values (> –3; eclogites; Jahn et al., 2003), precluding subducted oceanic slab or subducted Yangtze crust being the source of the Group 2 dikes. However, the Sr, Nd and Hf isotopic compositions of the intermediate to felsic parental magmas of the Group 2 dikes are similar to those of lower crustal-derived Late Triassic granodiorites and monzogranites in the Liaodong Peninsula (Fig. 7; Yang et al., 2007a) and lower crustal-derived mafic granulite xenoliths (Zheng et al., 2004), indicating that the crustal component in the origin of Group 2 dikes may be lower crust of the NCC. Thus, the Group 2 dikes with high-Mg andesitic compositions may have originated as melts of recycled NCC lower crust with subsequent interaction with mantle peridotite, as proposed by Kay & Kay (1993) and Xu et al. (2002).
Group 3
The geochemical and Sr–Nd–Hf isotopic compositions of the Group 3 dikes are distinct from those of Groups 1 and 2 (Figs 3–7
), precluding a common source and petrogenesis. The Group 3 dikes have low SiO2, MgO, Cr and Ni contents, distinct from those of Group 2 (Table 2 and Fig. 3). They have relatively low (87Sr/86Sr)i and strongly negative Nd(t) and Hf(t) values, similar to those of Triassic nepheline syenites in the Liaodong Peninsula (Fig. 7a) and the worldwide lamproites and Group II kimberlites (Nowell et al., 2004; Davies et al., 2006), suggesting that they could be derived from a common source; for example, subcontinental lithospheric mantle (Nowell et al., 2004; Davies et al., 2006). In addition, the geochemistry of the Triassic nepheline syenites also suggests they were derived by partial melting of an enriched, refractory lithospheric mantle source (mica- or amphibole–pyroxene-bearing harzburgite) beneath the NCC (Wu et al., 2006b). The Group 3 dikes have low TiO2 and total Fe2O3 concentrations, similar to those of experimental melts derived from partial melting of a relatively refractory mantle (harzburgite) source (KLB-1; Hirose & Kushiro, 1993; Hirose, 1997; refractory peridotite; Falloon et al., 1988) (Fig. 9b). As partial melts of refractory peridotite should have low CaO and Al2O3 concentrations, the high Al2O3 concentrations of the Group 3 dikes may be the result of melting of pyroxene veins in harzburgite (Foley, 1992). Their shoshonitic characteristics, with high K contents, indicate a K-bearing phase, such as amphibole or phlogopite in their source. Melts in equilibrium with phlogopite are expected to have higher Rb/Sr (> 0·1) and lower Ba/Rb (< 20) ratios than those from amphibole-bearing sources (Furman & Graham, 1999). In Fig. 10b, the Group 3 dikes display high Ba/Rb (> 30) and low Rb/Sr (< 0·1), suggesting an amphibole-bearing source. Thus, the Group 3 dikes could have been derived from an amphibole-bearing, refractory lithospheric mantle source that was metasomatized by recycled crustal materials prior to magma generation.
The Group 3 dikes are enriched in LILE and LREE and depleted in Nb and Ta, with negative Nb, Ta and Ti anomalies in PM-normalized trace element patterns (Fig. 6b), suggesting that metasomatism of the source was triggered by subduction-related fluids or melts. Niobium is depleted relative to the LREE, and negative Nb, Ta, Zr and Hf anomalies in the PM-normalized trace element diagram (Fig. 6b), together with low Nd(t) values and relatively high LILE/HREE and LILE/HFSE, are commonly interpreted to be the result of addition of subuduction-related fluids and melts to a refractory mantle source (Kepezhinskas et al., 1997).
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The Group 3 dikes are relatively depleted in HREE compared with LREE. They have extremely high (La/Yb)CN and (Gd/Yb)CN ratios (Table 3), combined with low HREE abundances (Fig. 5b). Based on geochemical modeling, small-degree partial melts from an amphibole–garnet peridotite ought to have relatively high La/Yb and low Dy/Yb (Fig. 10a). The Group 3 dikes show decoupled Nd [Nd(t) –13] and Hf [Hf(t) –11·5] isotopic compositions with Hf >0 (Table 4), indicating an ancient source in the garnet stability field. Despite the effects of parent/daughter ratio fractionation during the partial melting event that produced the Group 3 dikes, calculated depleted mantle model ages for Nd and Hf are consistently ancient in the range of 1·86–1·58 Ga. The Sr–Nd–Hf isotopic characteristics, therefore, suggest that the Group 3 dikes were derived from a mantle source characterized by long-term LREE enrichment and weak to moderate Rb/Sr enrichment. As the relative order of partition coefficients for Lu–Hf–Sm–Nd in pyrope (and majoritic) garnet is DSm > DNd, DLu > DHf and IHf > DNd, extraction of melt within the garnet stability field will lead to residual peridotite compositions that evolve rapidly with time to radiogenic Hf and Nd isotopic compositions above the mantle Nd–Hf isotope array, and thus, to positive 
Hf values. Therefore, these characteristics, including relatively high (La/Yb)CN (42·7–44·6) and (Dy/Yb)CN (1·9–2· 0) ratios (Table 2), relatively steep MREE to HREE patterns (Fig. 5b) and decoupled Nd and Hf isotopic compositions, are garnet signatures (Kay & Gast, 1973; Salters & Hart, 1989; Hirschmann & Stolper, 1996), indicating that the Group 3 dikes may have formed by relatively low-percentage melting of an amphibole-bearing mantle source in the garnet stability field.
In summary, geochemical and Sr–Nd–Hf isotopic compositions of the Late Triassic mafic dikes in the Liaodong Peninsula suggest that at least four components were involved in their petrogenesis (i.e. asthenospheric mantle, lithospheric mantle, upper crust and foundered lower crust). The Group 1 dikes were most probably produced by partial melting of a depleted, asthenospheric mantle source, with subsequent crustal assimilation and fractional crystallization during magma ascent and emplacement. The Group 2 dikes may have originated as melts of foundered NCC lower crust with subsequent interaction of the melts with mantle peridotite. In contrast, the Group 3 dikes were possibly generated by partial melting of enriched lithospheric mantle with garnet in the residue.
Tectonic implications
Mafic dike swarms are a common expression of magma generation associated with extension in a post-collisional continental tectonic setting (e.g. Rock, 1991). Together with the presence of Late Triassic alkaline to calc-alkaline intrusions (Chen et al., 2003; Guo et al., 2005; Yang et al., 2005, 2007a), the mafic dikes of the Liaodong Peninsula suggest that the Sulu orogenic belt underwent extension during the Late Triassic. As discussed in previous sections, the parental magma of the Group 1 dikes was derived by partial melting of asthenospheric mantle and experienced upper crustal assimilation and fractional crystallization. In contrast, the Group 2 dikes originated from interaction of foundered NCC lower crustal-derived melts with mantle peridotite, whereas the Group 3 dikes were derived from partial melting of enriched, refractory lithospheric mantle that experienced some fractional crystallization. Residual garnet is not required to generate the trace element characteristics and Nd–Hf isotopic correlations of the Group 1 dikes, and thus the pressure of melt generation is constrained in the region of the spinel facies. However, the trace element characteristics of the Group 2 and 3 dikes, with decoupled Nd and Hf isotope compositions, require garnet in the residue, indicating that the depth of melt generation was in the garnet stability field.
Two alternative geodynamic models have been proposed to explain Late Triassic magmatism within the Dabie–Sulu orogenic belt. (1) In the slab break-off model, the leading portions of high-density oceanic lithosphere founder and result in buoyancy-driven uplift and exhumation of the UHP metamorphic rocks within low-density slices, accompanied by normal faulting of the hanging-wall plate, with crustal extension and magmatism (Ernst & Liou, 1995; Chen et al., 2003). (2) In the lithospheric delamination model, continent–continent collision leads to thickening of the lithosphere, including the lower continental crust. Depending on the extent of the pressure increase, lower crustal mafic paragneiss may be transformed to amphibolite (P < 1 GPa) or to granulite–eclogite–garnet clinopyroxenite assemblages at higher pressures (2–3GPa; e.g. Wolf & Wyllie, 1994; Rapp & Watson, 1995). The density increase leads to gravitational instability of an over-thickened lithospheric keel. In particular, dense garnet-rich lower crustal restite allows for detachment of the lithospheric mantle from the upper lithosphere and leads to its sinking into the asthenosphere (Bird, 1979; England & Houseman, 1989; Houseman & England, 1993; Lustrino, 2005). In this case, the change in potential energy in the remaining lithosphere would lead to a sudden uplift of the Sulu orogen and surrounding areas (e.g. Bird, 1979; England & Houseman, 1989; Houseman & England, 1993) with crustal extension and Late Triassic magmatism (as suggested by Kay & Kay, 1993; Lustrino, 2005; Yang et al., 2005, 2006b).
The slab break-off model can explain the exhumation of UHP rocks from mantle depths to crustal levels (Davies & von Blanckenburg, 1995; Ernst & Liou, 1995) and a four-component source for the Late Triassic dolerite dikes (Gorring & Kay, 2001; Gorring et al., 2003). In the slab break-off model, resultant upwelling of the asthenosphere would heat the overlying lithosphere and lead to melting of the metasomatized and hydrated portions of the lithospheric mantle, which would in turn result in alkaline to ultrapotassic magmas by small degrees of melting or calc-alkaline magmas by larger degrees of melting (Davies & von Blanckenburg, 1995). However, it cannot explain the fact that the depth of partial melting of the asthenospheric mantle was at shallow levels in the spinel stability field, whereas the depth of partial melting of lithospheric mantle was in the region of garnet stability. In addition, magma generation requires break-off to occur at shallow depths, thus causing significant thermal perturbation capable of melting metasomatized or hydrated peridotite that has a solidus temperature of 1100°C at 80 km depth (Davies & von Blanckenburg, 1995). In the Sulu orogenic belt, where continental subduction extended to > 200 km depth (Ye et al., 2000), slab break-off would need to be at such great depth that it would have led to negligible thermal perturbation in the overriding lithosphere and hence no magmatism (Davies & von Blanckenburg, 1995). Furthermore, detailed zircon and monazite SHRIMP U–Pb dating and inferred P–T paths of the metamorphic rocks show that UHP metamorphism occurred between 244 and 226 Ma, followed by retrograde metamorphism at 221–218 Ma, during exhumation to at least mid-crustal levels (Liu et al., 2004; Wan et al., 2005). All these data indicate that the protoliths of the UHP metamorphic rocks were subducted to mantle depths in the Early–Middle Triassic (before 226 Ma) and rapidly exhumed to mid-crustal levels by 218 Ma.
The slab break-off model is commonly used to explain the initial stage of rapid exhumation of the UHP metamorphic rocks (at about 244–226 Ma), as proposed by Davies & von Blanckenburg (1995) and Ernst & Liou (1995). However, the emplacement ages of the Late Triassic magmatic rocks [215–200 Ma for syenites in the Sulu orogenic belt (Guo et al., 2005; Yang et al., 2005) and 213–212 Ma for dolerite dikes and monzogranites in the Liaodong Peninsula (Yang et al., 2007a; this study)] indicate that magmatism significantly post-dates the 240–226 Ma collision between the North China and Yangtze cratons and ultrahigh-pressure metamorphism (e.g. Chavagnac & Jahn, 1996; Hacker et al., 1998; Liu et al., 2004; Wan et al., 2005).
An alternative hypothesis is that lithospheric delamination, accompanied by crustal extension, was induced by instability of the thickened lithosphere (Houseman et al., 1981). The change in potential energy in the remaining lithosphere would then lead to a sudden uplift of the Sulu orogen and surrounding areas (e.g. Bird, 1979; England & Houseman, 1989; Houseman & England, 1993). This is supported by the zircon U–Pb and single mineral Sm–Nd and Rb–Sr isotope geochronology of ultrahigh-pressure metamorphic rocks and adjacent country rocks, which distinguish two stages of rapid cooling corresponding to two stages of fast uplift: an initial period of rapid uplift and cooling between 226 and 218 Ma that could be caused by continental subduction or slab break-off, and a later period of rapid cooling that could be caused by partial delamination or convective removal of lithopheric mantle and related extension during the Late Triassic to Middle Jurassic (218–173 Ma) (Li et al., 2000; Liu et al., 2004; Wan et al., 2005).
Furthermore, the delamination model can also explain the geochemical and isotopic variations in the Late Triassic dolerite dikes in the Liaodong Peninsula. The geochemical and geochronological data presented here favor four source components that can be explained as a fertile asthenosphere, a re-enriched, refractory lithospheric mantle, a detached lower crust and an upper crustal component. Our preferred model for the petrogenesis of the Late Triassic doleritic magmas and their relationship to the regional tectonic evolution is shown in Fig. 11. Subduction of the Yangtze Craton beneath the North China Craton forced down lower crustal rocks into the mantle (Fig. 11a) at about 245 Ma (Cong, 1996; Hacker et al., 1998; Liu et al., 2004; Wan et al., 2005). Oceanic slab break-off caused the UHP–HP rocks to be rapidly exhumed between 244 and 226 Ma and taken to middle levels at about 221–218 Ma (Fig. 11b) (Davies & von Blanckenburg, 1995; Ernst & Liou, 1995; Liu et al., 2004; Wan et al., 2005). Continent–continent collision had thickened the lithosphere of the Sulu orogenic belt and surrounding areas, and this induced mafic lower crust transformation to high-density eclogite or garnet clinopyroxenite. The density increase led to gravitational instability of an over-thickened lithospheric keel and induced detachment of the lithospheric mantle from the upper lithosphere and subsequent sinking into the asthenosphere (Bird, 1979; England & Houseman, 1989; Houseman & England, 1993; Lustrino, 2005). The change in potential energy in the remaining lithosphere would then lead to a sudden uplift of the Sulu orogen and surrounding areas, accompanied by crustal extension (e.g. Bird, 1979; England & Houseman, 1989; Houseman & England, 1993; Kay & Kay, 1993; Lustrino, 2005). The volume formerly occupied by the lithospheric keel was replaced by new, fertile asthenosphere, which underwent decompressional melting during upwelling. Asthenosphere-derived melts assimilated crustal materials during their ascent and formed the Group 1 dikes in the Liaodong Peninsula at 213 Ma. The partial melts of delaminated lower crust and enriched, refractory lithospheric mantle interacted with asthenospheric peridotite and produced the coeval Group 2 dikes with high-Mg andesitic compositions and the Group 3 dikes with shoshonitic signatures (Fig. 11c).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROLOGY OF THE MAFIC...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Three suites of dolerite dikes in the Liaodong Peninsula of the northeastern North China Craton have distinct geochemical and Sr–Nd–Hf isotopic compositions, indicating they were derived from discrete sources by various petrogenetic processes. The Group 1 dikes, with low SiO2, intermediate MgO concentrations, high total Fe2O3 and TiO2 contents, and weak to negligible negative Nb and Ta anomalies, formed by crustal assimilation and fractional crystallization of mafic magmas derived from partial melting of a fertile asthenospheric mantle source in the spinel stability field. The Group 2 dikes, with geochemical signatures of high-Mg andesites, were the result of interaction between melts derived from partial melting of delaminated lower crust and asthenospheric peridotite. The Group 3 shoshonitic dikes originated from partial melting of an ancient, refractory lithospheric mantle source at high pressures with garnet in the residue. The zircon U–Pb ages of the dolerite dikes in the Liaodong Peninsula indicate that they intruded at
213 Ma, post-dating the collision of the North China and Yangtze cratons and the UHP metamorphism. Late Triassic magmatism is probably related to lithospheric delamination following continent–continent collision.
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ACKNOWLEDGEMENTS |
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We thank Qian Mao and Yu-Guang Ma for help with zircon CL imaging; Yan-Bin Zhang, Jing Lü and Pei-Yan Guan for their help in the field; and Ping Jian, Dun-Yi Liu and Yu-Hai Zhang for their help during SHRIMP U–Pb analyses. We thank Marjorie Wilson (Editor), Michele Lustrino, Tanya Furman and Yaoling Niu for their constructive reviews and comments. This study was supported by the National Science Foundation of China (Grants 40672055, 40525007 and 40325006) and is The Institute for Geoscience Research (TIGeR) publication number 49.
*Corresponding author. Telephone: +86-10-6200-7900 (O). Fax: +86- 10-6201-0846. E-mail: jinhui{at}mail.igcas.ac.cn
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