The Qinzhou Bay-Hangzhou Bay Suture Zone (QHSZ) lies between the northwestern Yangtze Block and the southeastern Cathaysia Block in South China. An integrated zircon U-Pb age, whole-rock and mineral geochemical analyses have been enabled for understanding the Neoproterozoic and early Paleozoic geological evolution of the QHSZ. Zircon U-Pb dating reveals that the Guizi metabasite and Zhuya gabbro were formed at 948 ± 11 Ma and 450.8 ± 2.9 Ma, respectively. The Guizi metabasites are characterized by Nb enrichments (8.18–16.11 ppm) with high Nb/La (0.76–0.98) and Nb/U (17.86–25.65) ratios. The samples are also LREE/HREE ([La/Yb]N = 2.66–3.29) and LILEs (Rb, Th, U and Pb) enriched with weakly depleted HFSEs (Nb, P and Ti), resembling the Nb-enriched basalts. These geochemical signatures were likely to be originated from a newly subducted slab. Elemental contrasts illustrate that the Guizi metabasites are geochemically consistent with the Neoproterozoic Nb-enriched basalts along the Wuyi-Yunkai Domain, South China. Age and elemental characteristics of the Guizi samples suggest an earliest Neoproterozoic subduction within the QHSZ. The Zhuya gabbro samples have high contents of MgO (11.23–14.48%), Cr (520.8–782.7 ppm) and Ni (126.1–203.7 ppm), (La/Yb)N = 1.89–2.41, Eu/Eu* = 0.84–0.90, positive LILEs (Rb, Th, Sr and Pb) anomalies and negative HFSEs (Nb, P and Ti) anomalies. Clinopyroxene geochemical data show a significant arc cumulates trend. They were considered to be derived from an E-MORB-like source modified by subduction-related fluids. Geochemical modelling indicates that a mixture of the Neoproterozoic mafic magma and SCB sediments (20–40%) can match the elemental and Sm-Nd isotopic compositions of the Zhuya gabbros, which suggests the involvement of Neoproterozoic subduction-modified mantle. Combined with regional geological and geochemical data, it is concluded that the Yangtze and Cathaysia blocks were likely sutured along the QHSZ forming an earliest Neoproterozoic arc-back-arc system, and the early Paleozoic mafic rocks are dominantly remelting products of the Neoproterozoic subduction modified mantle.
This study includes 3 localities from the Jáchal area in the Central Precordillera of San Juan Province, from north to south: the Oculta creek, Las Aguaditas creek, and Cerro La Chilca sections. We deal with the graptolite faunas and conodont index species recorded from units that overlie the carbonate San Juan Formation, spanning the lower part of the Los Azules, Las Aguaditas, and Gualcamayo formations in their respective areas. The index graptolites and associated species are reported, which enable the recognition of the Levisograptus dentatus Zone in the Central Precordillera. The presence of graptolites in the limestones from the top of the San Juan Formation at the Cerro La Chilca section is documented for the first time. The record of representatives of the Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus, Histiodella, and Periodon lineages recognized in these units, linked with respective graptolite zones, provide precise information for global correlation purposes. The use of conodont and graptolite zones from different areas of the Central Precordillera enables the verification of the diachronous contact between the San Juan Formation and overlying units, which spans the lower to middle Darriwilian, in our investigated sections and classical localities previously documented.
Geochronological, geochemical, and isotopic studies were carried out on the Paleoproterozoic granitic rocks, widely exposed on the southeastern Liaodong Peninsula in Northeast China, in order to determine their ages and petrogenesis and further to provide constraint on the tectonic nature of the north segment of Paleoproterozoic Jiao–Liao–Ji Belt. The zircons from the monzogranitic gneisses, magnetite-bearing granitic gneisses, biotite-monzogranitic gneisses, and amphibole-bearing granitic gneisses fall into two groups, namely, magmatic and metamorphic. U–Pb and Lu–Hf isotopic data show that the magmatic zircons have peak ages of ~2,194, 2,485–2,603, and 2,951 Ma, and these age groups yield εHf values of +0.86 to +9.29, −3.04 to +3.70, and −2.86 and TCDM model ages of 2.19–2.67, 2.76–3.21, and 3.56 Ga, respectively, whereas the metamorphic zircons have a peak age of 1,910 Ma, εHf values of −7.71 to −2.02 and +0.69 to +7.63, with the corresponding TCDM model ages of 2.72 to 3.01 Ga and 2.08 to 2.52 Ga, respectively. Most of these granitic rocks are characterized by depletion in elements such as Nb, Ta, P, Ti, and peraluminous and belong to the high- to medium-K calc-alkaline series, suggesting I-type granites, whereas a few samples have high contents of SiO2, alkalis, and TFe2O3 and are metaluminous to weakly peraluminous, which show typical features of A-type granites. We consider these granitic rocks were emplaced at 2,194 Ma and modified by a regional metamorphic event at 1,910 Ma. The parental magma was originated mainly from the partial melting of Neoarchean–Paleoproterozoic juvenile crustal materials. Taking into account the regional geology, we consider these granitic rocks were formed in relation to subduction at an active continental margin. A crustal growth event took place at 2.5–2.2 Ga and a metamorphic event at ~1.9 Ga.
Mineralogical, geochemical, and isotopic (Sr and Nd) studies of 14 ferromanganese encrustations on basaltic substrates from Central Indian Ridge at 06°38.5′S indicate that except for two (NC3 and NC4), the samples are hydrogenous type. NC3 and NC4 have slightly higher growth rates, lower bulk trace metal contents, and low ∑REE. They have a lower content of high field strength (e.g., Hf, Ta, Zr, and Nb), incompatible (e.g., Pb, Th, and U), and lithophilic elements (e.g., Sr and Ba) than others. In addition, NC3 has a lower 87Sr/86Sr ratio (0.708825) than the remaining samples (range 0.709169–0.709253) but higher 143Nd/144Nd ratio (0.512296) than others (range 0.512258–0.512279). Both NC3 and NC4 have a higher radiogenic component indicated by their low εNd value (−6.67) than the remaining samples (range −7.00 to −7.41). These evidences together indicate a possible contribution of trace metals by a local mixed source of hydrothermal and hydrogenous origin during their growth history.
This study presents new information on the petrogenesis and tectonic setting of a hornblende gabbro in the northern Great Xing’an Range of northeastern China using new whole-rock geochemical, mineral geochemical, and in situ zircon U–Pb and Hf isotopic data obtained for samples taken from near the town of Tayuan. Zircon U–Pb dating indicates that the hornblende gabbro was emplaced during the late Carboniferous (~311 Ma). The hornblende gabbros are alkaline with high K2O + Na2O (4.25–6.42 wt.%), low SiO2 (41.69–50.22 wt.%), and variable MgO (4.49–7.16 wt.%) and TiO2 (1.23–2.77 wt.%) contents. The pressure and temperature conditions of gabbro crystallization were determined using amphibole–plagioclase and amphibole thermobarometry, which indicate that the Tayuan hornblende gabbro formed at pressures of 4.1–6.9 kbar and temperatures of 686–762 °C, respectively. The hornblende gabbros are enriched in light rare earth elements and large ion lithophile elements, are depleted in the heavy rare earth elements and high field strength elements, and have positive zircon εHf(t) values (+3.9 to +9.9), all of which are indicative of formation from magmas generated by the partial melting of a depleted region of the lithospheric mantle that was previously metasomatized by subducted slab-derived fluids and/or melts just before the generation of the hornblende gabbro magmas. The field observational, geochronological, geochemical, mineral geochemical, and zircon Hf isotopic data presented here are indicative of a complex petrogenetic history that involved crystal fractionation and magma mixing. These hornblende gabbros were emplaced in an extensional setting associated with the collision of the Xing’an and Songnen massifs.
The East China Sea Shelf Basin (ECSSB) lies at the south-eastern margin of the Eurasian Plate and was affected by the subduction of the Pacific Plate and the Philippine Plate. It experienced and recorded multistage tectonic inversions in the Cenozoic, especially in the Xihu Sag. In an attempt to investigate the evolution and mechanism of tectonic inversion, this paper presents numerical simulation results by the finite element method to the Xihu Sag. Combined with comprehensive structural analyses of seismic profiles, this paper determines the structural geometry of the sag for establishing a viscoelastic geologic model including six-layer strata and nine major faults. Simulation results show that the boundary conditions of transtension and transpression control the inversion process that propagates from east to west, and the distribution of low compressive stress displays certain correlations with the distribution of oil deposits. Based on quantitative analysis of the vertical displacement field of the Xihu Sag, this paper identifies a tectonic inversion process, which indicates that the western part of the sag uplifts and the eastern part subsides during the first-stage inversion; whereas the western part subsides and central-eastern parts uplift during the second and third stages. The formation of the tectonic inversion is controlled by the adjustment of the stress field from dextral transtension to sinistral transpression caused by the change of subduction rates and direction of the Pacific Plate and the Philippine Sea Plate.
There are several Precambrian microcontinents in the Central Asian Orogenic Belt (CAOB). Across these microcontinents, there are many metasedimentary rocks which were not all deposited in the Precambrian. We report on a field-based petrological and zircon geochronological study of metasedimentary rocks from the Tianshan and Beishan orogens, southern CAOB, in order to reveal the Precambrian affinity and Palaeozoic reworking. Most metasedimentary rocks in the Central Tianshan Block (CTB) and Dunhuang arc accretionary system reveal Palaeozoic records of arc accretion, collision, and postcollisional processes. The zircon age patterns of these metasedimentary rocks in the CTB show strong affinity to the Tarim Craton, whereas those of the Dunhuang arc accretionary system suggest a Mongolian affinity. The CTB constituted an independent microcontinent during the Palaeozoic accretionary process and was transformed into a Japan-type arc until it became amalgamated with the Tarim Block in the late Permian. In contrast, the Dunhuang Block shows a probable Mongolian affinity and was a stable unit during the late Mesoproterozoic to Neoproterozoic until the beginning of CAOB orogenesis.
To explain the roughly contemporary magmatic activities between Cathaysia Block and both sides of the Jiangnan Orogenic Belt, we discussed the magmatic-ore related and closely compressive tectonism in the Gan-Hang Belt. Also, we compared the Yongping Pluton and Yinshan volcanic-plutonic rocks in northeastern Jiangxi, and Huangshitan and other plutons in the northern Zhejiang, which were produced in the setting of compressive tectonism. We compared bimodel dikes and Sanqingshan A-type granite in the northeastern Jiangxi, and Huanshitan and other A-type granites in the northern Zhejiang, which formed in the setting of extension. We proposed that approximately in Middle Jurassic time (175 ± 5 Ma), South China entered into the tectonic system, roughly from south to north oblique subduction of the Paleo-Pacific Plate. The intermission period of the magmatic activities in Japan and South Korea (~170–120 Ma) was actually the period of the large-scale magmatic-ore forming activities in South China. Thus, this oblique subduction resulted in the dynamic imbalance and mutual movement from the deep part to the shallow part of the crust among surrounding blocks of South China and the multiple blocks of South China including those orogens between the North and South China blocks, between the Yangtze Craton and Cathayasian Block, and between the South China Block and Indosinian Block, as well as microcontinental blocks within Cathaysia Block. At present, we cannot rule out the influence of compressional–extensional tectonism for the Gan-Hang Belt controlled by the Paleo-Asia tectonic system.
Orebodies in the Xiadian gold deposit in the Jiaodong Peninsula, China are mainly hosted in the Mesozoic granitoids, controlled structurally by the Zhaoyuan–Pingdu Fault Zone, and occur as disseminated and cataclastic altered type. Four mineralization stages were identified as follows: quartz–pyrite stage (I), gold-bearing fine-grained pyrite–quartz stage (II), polymetallic sulfide–quartz stage (III), and quartz–carbonate stage (IV). Quartz was classified as including quartz granules with dentation boundaries (I), cataclastic quartz grain assemblages (II and III), and rod-like quartz grains (IV). Petrography, laser Raman analysis, and microthermometry of fluid inclusions in these stages (in both tunnel and borehole samples) reveal (a) CO2–H2O fluid inclusions (C–H type), (b) CO2–H2O ± CH4 fluid inclusions (C–H–CH4 type), and (c) aqueous fluid inclusions (H type). Fluid immiscibility caused by fluid mixing caused rapid precipitation of gold. The ore-forming fluid of the Xiadian gold deposit evolves from an H2O–CO2–NaCl ± CH4 system with medium temperature and salinity to an H2O–NaCl system with low temperature and salinity, from CO2-rich to CO2-poor in composition and from a mixture of magmatic water with increasing meteoric water as δ18OH2O values. Sulphur isotope compositions suggest a mixed source of ore metal, and the Jiaodong Group may be a major source for sulphur. Fluid parameters of borehole samples indicate that there is the same fluid system for Au precipitation at different depths and fault gouge with poor permeability may play a crucial role in forming a relatively closed semi-open space for Au precipitation. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusions and stable isotope geochemistry, the Xiadian gold deposit is concluded as an orogenic-type gold deposit formed in the tectonic transition from compression to extension.
The Dabie–Sulu orogen is located in the eastern segment of the Central China Orogen between the North China Block (NCB) and the South China Block (SCB). The complex processes of break-up and assemblage among the continental blocks of East Asia caused them to undergo multiple-stage continental collisions. It is extremely important for the final kinematics and dynamics of convergence and collision among continental blocks to explore the exhumation mechanism of the HP (high pressure)-UHP (ultra-high pressure) metamorphic rocks. Therefore, this paper focuses on the final collisional process and tries to find a reasonable exhumation mechanism of the HP–UHP rocks. On the basis of the structural geology, petrology, seismic tomography, palaeomagnetism, and mathematical calculation, we propose a south-eastward subduction of the NCB under the SCB in the Indosinian, and the Sulu Orogen actually became an orocline. This was caused by continuous south-eastward subduction and the subsequent westward-retreating delamination of the NCB slab. Coevally, it produced a two-stage exhumation process, a vertical extrusion during the first stage (240–220 Ma) and an eastward extrusion controlled by an oroclinal contraction at the second stage (220–200 Ma).