Elsevier

Lithos

Volume 117, Issues 1–4, June 2010, Pages 247-268
Lithos

Zircon U–Pb ages, REE concentrations and Hf isotope compositions of granitic leucosome and pegmatite from the north Sulu UHP terrane in China: Constraints on the timing and nature of partial melting

https://doi.org/10.1016/j.lithos.2010.03.002Get rights and content

Abstract

Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz + Kfs + Pl + Ap inclusions, which record a Neoproterozoic protolith age of ∼ 790 Ma, (2) mantles with Coe + Phe + Ap inclusions that record Triassic UHP age at 227 ± 3 Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ∼ 210 ± 3 Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe + Phe + Ap inclusions record Triassic UHP age of 227 ± 3 Ma, and (2) outer magmatic areas with Qtz + Kfs + Ab + Ap inclusions that record partial melting time of 212 ± 2 Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz + Kfs + Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212 ± 2 Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201 ± 2 Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism.

Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/177Hf of 0.28187 ± 0.00003 (2 SD; standard deviation) corresponding to εHf(790) and Hf model ages (TDM2) of about − 16.3 and 2.41 Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (< 0.006), low Th/U (< 0.1) and considerably higher, 176Hf/177Hf (0.28233 ± 0.00002; 2 SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP (εHf(227) =  14.6 ± 0.8; 2 SD) and pre-metamorphic (εHf(227) =  27.7) domains indicates equilibration of the Lu–Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212 Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower 176Hf/177Hf (∼ 0.28216) and εHf(t) (− 17.6 ± 1.2; 2 SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression.

Introduction

Ultrahigh-pressure (UHP) metamorphic terranes form at depths in excess of 100 km, and studies of these regions are one of the few ways to obtain information on processes occurring in the root zones of major collisional belts (Wallis et al., 2005). Typically UHP terranes, such as the Sulu–Dabie and Kokchetav UHP terranes, are almost entirely comprised of felsic gneisses and the properties of these rocks mostly determine the large-scale geodynamic behavior of the terranes. In the Sulu–Dabie UHP terrane, however, the felsic gneisses rarely preserve direct evidence of UHP metamorphism (e.g., Ye et al., 2000), and thus most studies have focused on the blocks and lenses of eclogite that occur within the gneisses.

However, recent studies have shown that UHP minerals commonly occur as inclusions in zircon from the felsic gneisses of the Sulu terrane (e.g., Liu et al., 2004a, Liu et al., 2004b, Liu et al., 2007). This discovery shows that the voluminous continental materials, mostly felsic gneisses with minor mafic–ultramafic intrusive and extrusive rocks, were subducted to mantle depths in excess of 100 km where they were metamorphosed and then exhumed. All of these rocks experienced similar isothermal-decompression PTt evolutional histories (Liu et al., 2004b, Liu et al., 2004c, Liu et al., 2008). UHP metamorphism in the Sulu terrane reached temperatures in excess of 800 °C at pressures of 3.0–4.0 GPa or more (e.g., Zhang et al., 1995, Ernst and Liou, 1999, Liu et al., 2004b, Liu et al., 2004c). Under these conditions, partial melting of the felsic gneisses would have been likely, particularly in the presence of free water (e.g., Huang and Wyllie, 1981). The presence of such melts in the Sulu terrane are suggested by the emplacement of K–feldspar-rich dikes at the peak UHP metamorphism (Wallis et al., 2005). However, in situ U–Pb zircon analyses from K–feldspar-rich dikes record an age range from 237 to 200 Ma, which overlap with both UHP and late amphibolite-facies metamorphic ages of previous studies (e.g., Liu et al., 2006, Liu et al., 2008). Liou and Zhang, 1996, Zheng et al., 2003 demonstrated that free water or fluid flow is very limited under peak UHP metamorphic conditions, and thus it appears that there was fluid unavailability for partial melting of the Sulu felsic gneisses under the peak UHP conditions. Such melting could also take place during breakdown of hydrous minerals such as phengite during exhumation of UHP rocks (Ye et al., 2001). Partial melting of gneisses during exhumation of the Sulu terrane was also suggested by the development of an isothermal-decompression PT path for these rocks, which would favor decompression melting (Banno et al., 2000, Nakamura and Hirajima, 2000). However, there is no geochronological evidence to support these various hypotheses.

Partial melting is a common geological phenomenon of high-grade metamorphic orogens (e.g., Clemens, 1990, Gerdes et al., 2000, Gerdes, 2001, Brown, 2001, Villaseca et al., 2001, Wu et al., 2007). Direct dating of felsic vein formed by partial melting in an orogen is therefore crucial for understanding the relationships among partial melting, metamorphic evolution and orogenic processes. Zircon is a stable mineral and has very low rates of Pb diffusion (Cherniak and Watson, 2003), thus the U–Pb ages of zircon grains formed during partial melting should reflect the time of crystallization from the melt rather than cooling along a metamorphic PT path (Wu et al., 2007). The extremely stable nature of zircon and its high closure temperature for U–Pb diffusion means that its isotopic system is little disturbed by metamorphism and migmatization. In situ U–Pb dating of zoned zircon can thus give significant age information on the complex evolution history of its migmatitic host (Foster et al., 2001, Keay et al., 2001, Buick et al., 2008). The Hf isotopic compositions of zircon in complex migmatites can be used not only to trace source characteristics, but also to reveal processes involved in the generation of crustal melts and the crystallization of zircon during partial melting (Flowerdew et al., 2006, Wu et al., 2007).

Rocks that contain evidence for partial melting, such as granitic leucosomes and pegmatites, occur widely within biotite-bearing orthogneiss from Weihai, in the north Sulu UHP terrane. However, very few zircon U–Pb ages are available for these rocks (e.g., Wallis et al., 2005). Zircon in granitic leucosomes of migmatite from this area is very complex, commonly containing inherited cores from the UHP protolith, mantles related to different stages of metamorphism and newly crystallized rims produced during partial melting of the host rock. A detailed geochronological study has to be combined with a careful investigation of the mineral inclusions and the internal zonation of zircon by cathodoluminescence (CL). For this reason, the timing of partial melting in the Sulu terrane is still highly controversial (Banno et al., 2000, Nakamura and Hirajima, 2000, Ye et al., 2001, Wallis et al., 2005). In this paper, we present the results of an integrated study of zircon grains from granitic leucosome and pegmatite in Weihai, north Sulu UHP terrane, involving mineral inclusion identification, CL imaging, U–Pb sensitive high-resolution ion-microprobe (SHRIMP) dating, and in situ trace element and Lu–Hf analysis by laser ablation–inductively coupled plasma–mass spectrometry (LA–(MC)–ICP–MS). Our new results not only provide precise age information about partial melting, but also have implications for the nature of the protolith source, and the relationships between partial melting, metamorphic evolution and orogenic process in the Sulu UHP terrane.

Section snippets

Geological setting and studied samples

The Sulu terrane and its western extension (Dabie and Hong'an UHP terranes) in eastern China mark the collision zone between the Sino–Korean and the Yangtze craton. The Sulu HP–UHP metamorphic belt is bounded by the Yantai–Qingdao–Wulian Fault (YQWF) on the north, and the Jiashan–Xiangshui Fault (JXF) on the south (Fig. 1). The Sulu UHP metamorphic belt is mainly composed of amphibolite-facies orthogneiss with subordinate amounts of coesite-bearing eclogite (Hirajima et al., 1990, Hirajima et

U–Pb SHRIMP dating

Zircon was separated from five samples (WH1–WH5) using standard heavy-liquid and magnetic techniques, and then handpicked under a binocular microscope. The selected crystals, together with the zircon standard TEMORA 1 (Black et al., 2003) were embedded in 25 mm epoxy discs and ground to approximately half their thickness. Cathodoluminescence (CL) images of the zircon grains were obtained at Peking University, using a FEI PHILIPS XL30 SFEG SEM set at 15 kV and 120 μA with a scanning time of 2 min.

Biotite-bearing orthogneiss (WH1)

Zircons from orthogneiss WH1 form rounded anhedral, transparent and yellowish to colourless grains (Fig. 4a, c and e). Based on CL images the zircon from WH1 is divided into two groups. The first group is characterized by distinct, low-luminescent cores surrounded by high-luminescent mantles and low-luminescent rims (Fig. 4b), whereas the second group is characterized by high-luminescent cores and low-luminescent rims (Fig. 4d and f). The first group has distinctive mineral assemblages in the

Biotite-bearing orthogneiss (WH1)

Trace element data of different zircon domains from orthogneiss sample WH1 are listed in Table 5, and presented graphically in Fig. 9. All pre-metamorphic zircon cores have similar chondrite-normalized REE patterns strongly enriched in heavy (HREE) and depleted in light (LREE) (Fig. 9a, and b). In general, patterns for the cores differ somewhat in shape from those of the coesite-bearing mantles and retrograde rims. The cores are characterized by higher contents of (light) LREE and (middle) MREE

U–Pb SHRIMP zircon dating

The results of 102 U–Pb SHRIMP analyses on 68 zircon grains from biotite-bearing orthogneiss WH1, granitic orthogneiss WH2 and WH3, and pegmatite WH4 and WH5 are summarized in Table 2, Table 3, Table 4, and graphically presented on Tera–Wasserburg diagrams with 2σ errors (Fig. 11, Fig. 12).

Zircon Lu–Hf isotope composition

The results of the Lu–Hf isotope analyses on zircon from all five samples (WH1–WH5) are shown in Fig. 13, Fig. 14 and listed in Table 6.

Protolith origin of the biotite-bearing orthogneiss

Biotite-bearing orthogneiss is widespread in north Sulu UHP terrane. Mineral inclusion analysis, CL imaging, U–Pb SHRIMP ages, REE abundances and Lu–Hf isotope compositions reveal that the zircon grains separated from the orthogneiss have pre-metamorphic, magmatic cores, UHP metamorphic mantles and retrograde rims. A mineral inclusion assemblage in the cores composed of Qtz + Kfs + Pl + Ap implies that zircon grew during granite crystallization.

These zircon cores from sample WH1 recorded a

Concluding remarks

Detailed geochemical and geochronological SHRIMP and LA–(MC)–ICP–MS analyses of zircon separates from UHP orthogneiss, granitic leucosome and pegmatite in the north Sulu UHP terrane, together with available data yield the following conclusions:

  • (1)

    Three discrete U–Pb SHRIMP ages are recorded in zoned zircons from the UHP orthogneiss: A Neoproterozoic protolith age (∼ 790 Ma) for the pre-metamorphic magmatic cores, Triassic UHP age of 227 ± 3 Ma for the coesite-bearing mantle and subsequent HP

Acknowledgements

This work was supported financially by the National Natural Scientific Foundation of China (Grants No. 40725007 and 40399143), National 973 project of the Chinese Ministry of Science and Technology (Grant No. 2003CB716502) and the German Science Foundation (DFG; Grants No. GE1152/2-2 and WE2850/3-1). The manuscript benefited from extended discussions with Profs. Qinhan Shen and Lingsen Zeng, and constructive reviews by Dr. Ryan Ickert, Dr. Tony Kemp and Prof. Ian Buick are greatly appreciated.

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