Kui-Dong Zhao

Contrasting origins of late Mesozoic adakitic granitoids from the northwestern Jiaodong Peninsula, east China: implications for crustal thickening to delamination

Two suites of granitoids, the Late Jurassic (158 ± 3 Ma) Linglong suite and the Early Cretaceous (130–126 Ma) Guojialing suite, crop out in the northwestern Jiaodong Peninsula, eastern China. The Linglong suite is a monzogranite, comprising alkali feldspar, plagioclase, quartz and Fe-rich biotite. The Guojialing suite includes at least five plutonic bodies of both granodiorite and monzo-granite. The rocks are composed of plagioclase, alkali feldspar, quartz, Mg-rich amphibole and Mg-rich biotite. Both the Linglong and Guojialing suites have adakitic affinity. They are enriched in LREE with high La/Yb ratios and show positive Eu anomalies. The rocks are also enriched in LILE and depleted in HFSE with high Sr/Y ratios. The Linglong granite shows very uniform Sr–Nd isotopic compositions with initial 87 Sr/ 86 Sr ratios of 0.7119–0.7126 and e Nd (T) values of −21.3 to −21.6, which are similar to those of the local Neoarchaean basement. The Guojialing suite has variable initial 87 Sr/ 86 Sr ratios (0.7108–0.7120) and e Nd (T) values (−10.8 to −17.2), which are distinct both from those of the Neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area. Detailed petrological and geochemical data indicate that the Linglong suite was derived by partial melting of Neoarchaean metamorphic lower-crustal rocks at depth of > 50 km with a eclogite residue, whereas the Guojialing suite was formed by the reaction of delaminated eclogitic crust-derived melt with the upwelling asthenospheric mantle. The petrogenesis of these two contrasting adakitic granitoids suggests intensive lower-crustal delamination during Early Cretaceous times, following a crustal thickening process from the late stage of the Early Jurassic to early stage of the Late Jurassic with crustal thickness of 50 km, respectively.

Petrogenesis of Late Jurassic Qianlishan granites and mafic dykes, Southeast China: implications for a back-arc extension setting

A late Mesozoic belt of volcanic-intrusive complexes occurs in Southeast China. The Qianlishan granites are distributed in the northwest of the belt. The pluton is composed of porphyritic biotite granite (153 Ma) and equigranular biotite granite (151 Ma) and was intruded by granite-porphyry dykes (144 Ma) and mafic dykes such as lamprophyre and diabase (142 Ma). The granitic rocks, consisting mainly of K-feldspar, plagioclase, quartz and Fe-rich biotite, have SiO 2 contents of 72.9–76.9%, and are enriched in alkalis, rare earth elements (REE), high field strength elements (HFSE) and Ga with high Ga/Al ratios, but depleted in Ba, Sr and transition metals. Trace-element geochemistry and Sr–Nd isotope systematics further imply that the Qianlishan granitic magmas were most probably derived by partial melting of Palaeo- to Mesoproterozoic metamorphic lower-crustal rocks that had been granulitized during an earlier thermal event. These features suggest an A-type affinity. The Qianlishan lamprophyre and neighbouring coeval mafic dykes (SiO 2 = 47.9–53.8 wt%) have high MgO and compatible element contents. These rocks also have high K 2 O contents and are enriched in alkalis, light REE, large ion lithophile elements, and depleted in HFSE. They have low initial e Nd values and relatively high initial 87 Sr/ 86 Sr ratios. We suggest a subduction-modified refractory lithospheric mantle (phlogopite-bearing harzburgite or lherzolite) for these high-Mg potassic magmas. The Qianlishan diabases (SiO 2 = 48.4–48.7 wt%) are alkaline and have high TiO 2 and total Fe 2 O 3 contents, together with the positive initial e Nd value, suggesting derivation from fertile asthenopheric mantle (phlogopite-bearing lherzolite). A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Qianlishan granites and associated mafic dykes. Between 180 and 160 Ma, Southeast China was a continental arc, forming the 180–160 Ma plutons of the late Mesozoic volcanic-intrusive complex belt, and the lower-crust was granulitized. Since 160 Ma the northwestern belt has been in a back-arc extensional setting as a consequence of slab roll-back, resulting in the lithosphere thinning and an influx of asthenophere. The upwelling asthenosphere, on the one hand, induced the local lithospheric mantle to melt partially, forming high-Mg potassic magmas, and on the other hand it underwent decompression melting itself to form alkaline diabase magma. Pulsatory injection of such high-temperature magmas into the granulitized crustal source region induced them to partially melt and generate the A-type magmas of the Qianlishan granitic rocks.

Mineral chemistry of the Qitianling granitoid and the Furong tin oredeposit in Hunan Province, South China implication for the genesis of granite and related tin mineralization

The Qitianling granitic stock is a stanniferous granite located in Hunan Province, China. Recently, the Furong tin ore deposit was found in this stock, with a tin reserve of approximately 600,000 tons. The main rock-forming minerals including amphibole, biotite, plagioclase and ore minerals including cassiterite and rutile were analysed chemical compositions by electron microprobe. The biotite is Fe-rich annite, and has high Ti and Cl concentration. The biotite has high Fe 3+ /(Fe 2+ +Fe 3+ ) ratios and the oxygen fugacity calculated by biotite compositions is above Ni-NiO (NNO), and near the Fe 2 O 3- Fe 3 O 4 (MH). The amphiboles are ferropargasite and ferro-edenite hornblende. The pressure of the granite estimated by Al-in-hornblende barometer is 3.6 ± 0.9 kbar. An amphibole-plagioclase thermometry and a semiquantitative hornblende thermometer yield a forming temperature of 750∼820°C. The chlorite from the orebody has negligible K 2 O, Ti 2 O, F, and Cl, but shows similar Fe/(Fe+Mg) ratios with amphibole and biotite. Cassiterite is observed closely associated with rutile disseminated in chlorite alteration veins and envelopes. The ore-forming temperature is estimated to be 290∼405°C from chlorite geothermometry. The Qitianling granite contains Sn-rich hornblende and biotite, and shows a high oxygen fugacity for the magma, which is slightly different from the common S-type stanniferous granites worldwide. Fractional crystallization of the magma and tin deposition directly from exsolved magmatic-hydrothermal fluids may not be the major mechanism for the tin mineralization in this deposit. Instead, we suggest that post-magmatic hydrothermal alteration of the granite may have released tin and other metals ( e.g. , Ti) from the Sn-bearing biotite and hornblende in the granite. Then cassiterite and rutile precipitated together with chlorite when the physical and chemical condition of the Sn- and Ti-rich fluids changed.

SHRIMP U-Pb zircon dating for lamprophyre from Liaodong Peninsula： Constraints on the initial time of Mesozoic lithosphere thinning beneath eastern China

It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning started. The Liaodong Peninsula is located in the eastern segment of the North China Craton and is one of the important domains to explore the event of lithosphere thinning. SHRIMP U-Pb zircon dating and geochemical study were carried out for the lamprophyre dike swarm that intruded into the magnesite ore-beds in the Dashiqiao Formation of Paleoproterozoic Liaohe Group at the Huaziyu magnesite ore district, Liaodong Peninsula. The results indicate that these lamprophyre dikes were intruded in late Jurassic (155±4 Ma) and show some geochemical characteristics of potassic magmas. It is now accepted that the lithosphere thinning took place in the late Mesozoic, and the peak thinning stage occurred in early Cretaceous (130–120 Ma). Considering the potassic mafic magmatism marking the onset of the lithospheric thinning, we therefore suggest that the studied late Jurassic potassic lamprophyre dike swarm could imply that the late Jurassic is the time that lithosphere thinning started.

Zircon effect alone insufficient to generate seawater Nd‐Hf isotope relationships

[1] Many studies have suggested that continental weathering inputs have controlled the dissolved oceanic budget of hafnium (Hf). However, whether the offset of seawater Nd‐Hf isotope compositions from the terrestrial array can be fully generated by incongruent weathering of continental rocks (the zircon effect) is still not well constrained. In recent years, an increasing amount of combined U‐Pb ages and Hf‐isotopic compositions of riverine detrital zircons have been published. Here a new model of the Nd‐Hf isotopic compositions of the weathered zircon‐free part of the upper continental crust is presented, which is based on published Hf isotope compositions and formation ages of modern riverine detrital zircons combined with Nd isotopic compositions of rocks from the upper continental crust. Our model results indicate that the Nd‐Hf isotopic composition of the weathered zircon‐free part of the upper continental crust is not consistent with the seawater isotopic compositions. This suggests that the elevated seawater Hf isotope compositions for given Nd isotope compositions cannot be fully explained by incongruent zircon weathering of the continents, which is also supported by a recent study demonstrating incongruent weathering of other minerals than zircon.

Highly metalliferous carbonaceous shale and Early Cambrian seawater: COMMENT and REPLY COMMENT

[Lehmann et al. (2007)][1] reported trace, rare earth element (REE), and Mo-isotope data from the Ni-Mo sulfide layer and host black shale of the Lower Cambrian Niutitang Formation in southern China. They argue that the metals in the sulfidic rocks originated purely from Early Cambrian seawater.