Jibin Zhou

The genetic association of adakites and Cu–Au ore deposits

Adakites may form by partial melting of either the subducting oceanic lithosphere or the lower part of the continental crust. These two magma types can be discriminated geochemically using a combination of La/Yb, Sr/Y ratios, MgO and Na2O contents, and Sr–Nd isotopes. Given that the basaltic crust has Cu concentrations more than two times higher than the lower continental crust and the mantle wedge, ‘primitive’ adakites produced by oceanic slab melting should contain significantly higher Cu contents than adakites derived from the continental crust, as well as normal arc andesites. A globally compiled dataset shows that Cu concentrations in adakites are generally lower than that in normal arc rocks. We attribute this low copper content to loss of magmatic fluids as a result of sulphate reduction during adakitic magma differentiation, in turn induced by the crystallization of Fe–Ti oxides, essential to mineralization. Therefore, the underflow of oceanic-slab-derived adakites that can release larger amounts of Cu (presumably Au as well) by crystal fractionation leads to higher potential for Cu–Au mineralization along convergent margins, usually associated with ridge subduction. Such basaltic slab melts initially have considerably higher Cu contents and thus play a crucial role particularly in the relatively closed magma system responsible for generating porphyry Cu deposits.

Mesozoic large magmatic events and mineralization in SE China: oblique subduction of the Pacific plate

SE China is well known for its Mesozoic large-scale granitoid plutons and ore deposits. In SE China, igneous rocks with intrusion ages between 180 and 125 Ma generally become progressively younger towards the NE. More specifically, 180–160 Ma igneous rocks are distributed throughout a broad area, with mineralization ranging from Cu–Au and Pb–Zn–Ag to W–Sn; 160–150 Ma plutons are present mainly in the Nanling region and are associated with the large-scale W–Sn mineralization; younger igneous rocks occur in the NE area that has many fewer deposits. These can be plausibly interpreted as reflecting a southwestward subduction followed by a northeastward rollback of a subducted oceanic slab, in rough agreement with contemporaneous drift of the Pacific plate. Consistent with this scenario, SE China contains three Jurassic metallogenic belts distributed systematically from NE to SW: (1) a Cu–(Au) metallogenic belt in the NE corner of the South China Block, represented by the Dexing porphyry Cu deposits; (2) a Pb–Zn–Ag metallogenic belt in the middle, represented by the Lengshuikeng Ag and Shuikoushan Pb–Zn deposits; and (3) the famous Nanling W–Sn metallogenic belt in the SW. The distribution of these metallogenic belts is analogous to those in South America where Fe deposits are distributed close to the subduction zone, followed by porphyry Cu–Au deposits and Pb–Zn–Ag deposits in a medial zone, and Sn–W deposits distant from the trench. Inasmuch as quite a few late Mesozoic Fe deposits occur in the Lower Yangtze River Belt to the NE of the Cu–Au deposits in SE China, the distribution of late Mesozoic deposit belts in SE China is identical to that in South America. Therefore, southwestward subduction of the Pacific plate and the corresponding slab rollback are proposed here to explain the distributions of the late Mesozoic (180–125 Ma) magmatism and the associated metallogenic belts in SE China.

Different origins of adakites from the Dabie Mountains and the Lower Yangtze River Belt, eastern China: geochemical constraints

Cretaceous adakites are widely distributed in the Lower Yangtze River Belt (LYRB) and the Dabie Mountains, east-central China. Adakites from the LYRB in general are closely associated with Cu–Au deposits, whereas Dabie adakites lack any mineralization. Based on geochemical characteristics, we show that these adakites have different origins; for example, adakites from the Dabie Mountains have more variable Sr/Y (6.47–1303) and systematically higher La/Yb (20.8–402), Th/U (2.28–50.6), and Nb/Ta (5.07–65.2) compared to adakites from the LYRB, Sr/Y (28.8–185), La/Yb (14.1–49), Th/U (0.33–8), and Nb/Ta (7.5–23). The systematically higher La/Yb of Dabie adakites supports their continental origin, because the La/Yb of the lower continental crust is more than 10 times higher than that of mid-ocean ridge basalt (MORB). Moreover, the lower continental crust is also highly enriched in Sr, with Sr/Y > 10 times that of MORB. Interestingly, with the exception of those from Fuziling, most Dabie adakites have Sr/Y comparable to normal adakites, suggesting the presence of residual plagioclase. Because Th and U do not fractionate significantly from each other during magmatism, the high but variable Th/U suggests that the protolith of Dabie adakites underwent subduction. The LYRB adakites can be plausibly interpreted as being a result of Early Cretaceous partial melting of a young, hot, descending oceanic slab during ridge subduction. By contrast, Dabie adakites were likely formed by partial melting of the lower continental crust attending ridge subduction.

Geochemical and zircon U–Pb study of the Huangmeijian A-type granite: implications for geological evolution of the Lower Yangtze River belt

The Early Cretaceous Huangmeijian Pluton is an A-type granite located on the northern bank of the Lower Yangtze River in Anhui Province, east-central China. It intruded the SE edge of the Early Cretaceous Luzong volcanic basin. The moderate- to coarse-grained granite is mainly composed of alkali feldspar, plagioclase, and quartz and has a typical A-type geochemical signature. LA-ICP-MS zircon dating yielded a weighted mean 206Pb/238U age of 127.1 ± 1.4 Ma, similar to other A-type granites in the Lower Yangtze River belt, indicating an Early Cretaceous extensional environment. Temperatures calculated using the Ti-in-zircon thermometer suggest that the magma formed under high-temperature conditions (720–880°C). The low calculated Ce(IV)/Ce(III) ratio based on zircon rare earth element patterns indicates low oxygen fugacity for this A-type magma. Previous studies suggested that eastern China was an active plate margin related to the Early Cretaceous subduction of the Pacific and Izanagi plates. The ridge between these two plates probably passed under the Lower Yangtze River belt, forming A-type granites and adakites. The Huangmeijian Pluton is roughly the same age within error but is marginally older than the Baijuhuajian A-type granite in the eastern part of the Lower Yangtze River belt. A-type granite genesis in the Lower Yangtze River belt only lasted for 2–3 million years and slightly predates the transition from regional extension to compression. All these can be plausibly interpreted by the ridge subduction model, that is, A-type granites formed because of mantle upwelling through the slab window during subduction of the ridge separating the Pacific and Izanagi plates.

Formation of the world's largest molybdenum metallogenic belt: a plate-tectonic perspective on the Qinling molybdenum deposits

Qinling ore belt is the largest known molybdenum belt in the world with a total reserve of >5 Mt of Mo metal. Based on the geochemical behaviour of Mo, the structural settings of the Qinling orogenic belt, and geological events in eastern China, we propose that tectonic settings are of critical importance to the formation of these ore deposits. Molybdenum is very rare in the earth with an abundance of ∼0.8 ppm in the continental crust. Both surface- and magmatic-hydrothermal enrichment processes are required for Mo mineralization. It can be easily oxidized to form water-soluble MoO4 – in the surface environment, especially in the Phanaerozoic, and then precipitated under anoxic conditions. Therefore, closed or semi-closed water bodies with large catchment areas and high chemical erosion rates are the most favourable locations for Mo-enriched sediments. The Qinling orogenic belt was located in the tropics during crustal collisions, such that the chemical erosion was presumably intense, whereas the Erlangping back-arc basin was probably a closed or semi-closed water body as a result of plate convergence. More than 90% of the Mo reserves so far discovered in the Qinling molybdenum belt are associated with the Palaeozoic Erlangping back-arc basin. Compiled Re–Os isotopic ages for porphyry deposits (including several carbonate vein deposits) that have been dated show peaks during 220 million years (>0.32 Mt), 145 million years (> 3.5 Mt), and 115 million years (> 0.84 Mt), which correlate well with the three major episodes of granitoid magmatism since the Triassic. The ∼220 million year episode of mineralization, represented by the Huanglongpu carbonate vein-type deposit and the Wenquan porphyry deposit, coincided with the formation of the South Qinling syn-orogenic granites as well as the Dabie ultrahigh-pressure metamorphic rocks, suggests a genetic relationship with the collision between South and North China Blocks. The ∼145 Ma porphyry Mo deposits, representing the main mineralization, are attributed to reactivation by ridge subduction along the lower Yangtze River belt to the east of the Qinling orogen ∼150–140 Ma. The ∼115 Ma Mo deposits likely reflect slab rollback of the northwestwards subducting Pacific plate ∼125–110 Ma.

A Novel Mass Spectrometry System for Helium-4 Measurement

Here we report a novel mass spectrometry measurement system (MSHE4) developed at State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences for quantifying helium-4 contents of geological mineral samples. The MSHE4 system consists of seven units, a high vacuum level generating unit, a gas purified unit, a laser heating unit, a pneumatic valve unit, an automatic control unit, a gas standard unit and a quadropole mass spectrometer unit. The unique software package, developed using LabVIEW for embedded system, allows users to control the full measurement sequences. Results from a test of the Durango apatite sample are used to illustrate the performance of the new MSHE4. We show that the constructed MSHE4 system is able to accurately measure helium-4 contents of apatite mineral, which in turn can be used to quantify the low temperature thermochronology of geological minerals in combination with measurements of uranium and thorium by LA-ICP-MS. The configuration of the MSHE4 system has following advantages: smaller volume of pipeline for gas purification, easier operation of graphic user interface for measurements, and more compact design of sample holder for single grain measurements. The separate step-heating unit can be mounted to the system smoothly. The MSHE4 system can be used for measurement of noble gases from minerals, and in turn thermochronology applications for geochemists.