Pei-Rong Chen

Comparative study of ore-forming fluids of hydrothermal copper–gold deposits in the lower Yangtze River Valley, China

Fluid-inclusion and stable isotope studies were carried out on five types of Mesozoic (Yanshanian) hydrothermal copper–gold deposits in the lower Yangtze River Valley. Deposits include (1) copper in cryptoexplosive breccia pipes, (2) skarn copper, (3) porphyry copper, (4) high-temperature quartz vein-type copper and gold, and (5) medium–lower temperature fracture zone gold. This research has allowed a comparison between various types of ore-forming fluids. Melt-fluid inclusions in garnet from the matrix of the breccia pipe at the Shizishan copper deposit reveal the existence of a water-rich magma. In all deposit types, fluid temperatures and salinities were higher at early stages and generally decreased with time. Magmatic water is dominant in the high-temperature ore-forming fluids, whereas meteoric water was involved only in the medium–lower temperature Xiaomiaoshan gold deposit and in the post-mineralization stage of the Shaxi porphyry copper deposit. Fluid boiling played an important role in the mineralization of most deposits, particularly at Shizishan, where multi-stage boiling was associated with the formation of cryptoexplosive breccia, skarn, quartz-sulphide, and quartz-carbonate-sulphide stages. Boiling of an aqueous magmatic fluid system at high temperatures reflects the release of crystallization heat and increase of total volume of the magma–fluid system, and hence it can be referred to as active boiling. On the contrary, boiling of a fluid at lower temperatures is typically triggered by pressure release due to fracturing or dilation in the surrounding rocks, and is thus referred to as passive boiling. In general, passive boiling occurs more commonly at the higher levels of a hydrothermal mineral system and at later stages of the ore-forming process.

Multiple Mesozoic magma processes formed the 240–185 Ma composite Weishan pluton, South China: evidence from geochronology, geochemistry, and Sr-Nd isotopes

Mesozoic granitic intrusions are abundant in the South China Block (SCB). Understanding the generation of these granitic intrusions is important to better constrain tectonic regimes and mineralization processes. We studied the Weishan pluton, one of the largest Mesozoic composite granitic complexes in Hunan Province, to probe the relationship between multiple magma processes and tectonic events induced by plate interactions. The samples can be classified into three groups. Group 1 is highly fractionated and weakly peralumious, with flatter REE patterns and more negative Eu, Ba, Sr, P, and Ti anomalies compared with other groups. Group 2 is peralumnious-metaluminous while Group 3 is peraluminous, but with higher MgO, FeO, εNd(t), and lower initial 87Sr/86Sr in Group 2 than in Group 3. Whole-rock and biotite compositions from all groups define a good linear trend, with Group 1 and Group 2 samples defining two mixing end-members. Single-grain mica Rb-Sr isochron and LA-ICP-MS zircon U-Pb ages show that Group 1 formed at ca. 240–230 Ma, Group 2 at 220–215 Ma, and Group 3 during two episodes at 210–205 Ma and 185 Ma. Integrated geochemical studies suggest that precursor magmas of the three groups were emplaced under oxidizing conditions at relatively low temperatures (<830°C). These magmas were mainly derived by partial melting of Palaeoproterozoic psammites but subsequently differentiated by fractional crystallization, crustal contamination, and remelting. Considering regional tectonothermal events, it is proposed that the oldest Group 1 granitic melts were derived by low-degree partial melting of thickened Palaeoproterozoic psammitic materials during prograde metamorphism due to the collision of the Indochina Block (ICB) and the SCB. The slightly younger Group 2 granitic melts were generated by further partial melting in response to stress relaxation in the post-collision stage and conductive heating from underplating mafic magma. The youngest Group 3 melts represent remobilized buoyant magma pulses supplied from a mushy Group 2 magma source accompanying regional reheating, possibly associated with the uplift of the SCB following its deep subduction during the Dabie orogeny and the subduction of the Palaeo-Pacific oceanic plate beneath the Eurasia plate, respectively.

Cretaceous A-type volcanic–intrusive rocks and simultaneous mafic rocks along the Gan-Hang Tectonic Belt, Southeast China: petrogenesis and implications for the transition of crust–mantle interaction

ABSTRACT Abundant evidence points to the Cretaceous crust–mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125 Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 104*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (~137–125 Ma) A-type volcanic–intrusive rocks in the GHTB. Their εNd(t) and zircon εHf(t) values gradually increased through time from approximately −9.0 to −1.0 and −10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust–mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust–mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting.

Petrogenesis of Early Cretaceous adakitic granodiorite: Implication for a crust thickening event within the Cathaysia Block, South China

Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous (100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2 (mainly ranging from 64.4 to 68.9 wt.%) and Sr (624–894 ppm) contents, Sr/Y (49.9–60.8) and La/Yb (23.4–42.8) values, low Y (10.3–17.1 ppm), Ni (5.62–11.8 ppm) and MgO (mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86Sr ratios of 0.7086–0.7091, εNd(t) values of −6.2 to −5.9 and zircon εHf(t) values mostly of −10.1 to −7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar (or crust thickness ≥40 km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.