Western Central Orogenic Belt Research Articles

Determination of Fe2+/Fe3+ Ratios of Magnetite using Different Methods: A Case Study from the Qimantag Metallogenic Belt

AbstractDetermination of Fe2+/Fe3+ ratios from metallogenic belts to explore controlling physical and chemical conditions of rock formation is of great significance. In order to explore magnetite Fe2+/Fe3+ ratios of the Qimantag metallogenic belt, part of the Eastern Kunlun orogenic belt in the northeastern margin of the Qinghai–Tibetan plateau, western Central Orogenic Belt of China, and overcome the limitation of the traditional electronic probe, five different measurement methods are proposed and their respective advantages and disadvantages evaluated, with the composition data of the magnetite obtained using electron probe microanalysis (EPMA). The direct oxygen measurement method has a significant impact on the determination results of FeO and Fe2O3, but the accuracy and uniformity of the results are low. The valence method (Flank method) based on the spectral intensity ratio of Lα to Lβ for iron is also unreliable for FeO and Fe2O3 measurements because it is difficult to establish a relationship between Lβ/Lα, the spectral intensity ratio, and the Fe2+/Fe3+ content ratio. In comparison, the charge difference method, the surplus‐oxygen method and the Mössbauer spectrum method are still the most favorable. Mössbauer spectroscopy, with its isomer movement particularly sensitive to the oxidation state of iron, yields results closer to 0.5, which is relatively reliable. Earlier magnetite deposits are located in intrusions or contact zones and formed by magmatic fluids with high Fe2+/Fe3+ ratios, whereas later magnetite deposits are farther away from intrusions and have low Fe2+/Fe3+ ratios. The transformation mechanism of hematite and magnetite in the Qimantage metallogenic belt is also studied. No large volume changes, such as pore filling and shrinkage fracture, were detected in the metallogenic belt, and the transformation mechanism is more similar to a reoxidation and reduction mechanism.

Geology of China and adjacent regions: An introduction

This introduction outlines the papers collected in the special issue “Geology of China and Adjacent Regions”. The papers focus on scientific issues from Archean to Cenozoic geology in the areas including Northwest Pacific to western Central Orogenic Belt concerning tectonics, geophysics, petrology, geochemistry, mineral deposit, and energy resources. A tectonic framework of China and adjacent regions is briefly introduced.

Multiple episodes of gold mineralization in the East Kunlun Orogen, western Central Orogenic Belt, China: Constraints from Re-Os sulfide geochronology

The Gouli goldfield (>110 t Au), located in the East Kunlun Orogen, western Central Orogenic Belt of China, is one of the most important goldfields in this area. In the last decade, a number of orogenic gold deposits (e.g., Guoluolongwa and Annage) have been shown to be hosted by rock units of different lithology and ages. Rhenium-osmium (Re-Os) geochronology of sulfides from gold-bearing veins was performed to define the chronologic relationships between gold mineralization present in the metamorphic rocks (Proterozoic and Silurian) of the East Kunlun Orogen. Sulfides (pyrite and chalcopyrite) from pyrite-quartz vein and polymetallic sulfides-quartz vein in the Guoluolongwa gold deposit yield Re-Os isochron dates of 374 ± 15 Ma (MSWD = 4.6; initial 187Os/188Os ratio (Osi) = 0.06 ± 0.22) and 354 ± 7 Ma (MSWD = 0.18; Osi = 0.13 ± 0.01), respectively. Similar ages are also revealed by the pyrite mineral separates from the Annage gold deposit (383 ± 8 Ma and 349 ± 6 Ma). These ages are interpreted to record the timings of the formation of the two vein types in these deposits, which are nominally separated by ~20 Ma.The new Re-Os ages presented here identify the first two Late Paleozoic (Devonian and Early Carboniferous) gold-mineralizing events in the East Kunlun Orogen and thus indicate at least two mineralization epochs in this area given ages (Late Triassic) of other gold systems and field observations. Considering the geological background and temporal distribution of gold deposits in adjacent areas (western Qinling and Qaidam-Qilian), we suggest that gold deposits in the western Central Orogenic Belt were formed in collisional/post-collisional settings being controlled by common tectonic-magmatic activities related to the evolution of both the Prototethys Ocean (Proterozoic – Paleozoic) and Paleotethys Ocean (Paleozoic – Early Cenozoic).Further, the initial Os (Osi) obtained from the Re-Os isochron suggest that for the two vein types in the Guoluolongwa gold deposit the Os and by inference the ore metal (Au) were derived from a mantle-like source (Osi values = ~0.12–0.13), which should be related to the contemporaneous mantle-like magmatism. In contrast, the pyrite-quartz vein in the Annage gold deposit possesses a significantly radiogenic Osi value (3.65 ± 0.51). Given the similar timing of mineralization between the Guoluolongwa and Annage deposits, it is considered that the ore metal likely has a similar origin, i.e., a mantle-like source, however at Annage the hydrothermal fluid interacted with the Proterozoic metamorphic host rocks and leached radiogenic Os that masks any evidence of a mantle-like source.

The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions

The pressure–temperature–time evolution of the UHP eclogites of Dabie–Sulu, in the eastern sector of the Central Orogenic Belt of China shows a complex pattern of predominantly Triassic, and to a lesser extent Early Paleozoic ages. 40Ar/ 39Ar stepwise-crushing analysis of eclogitic garnets yielded consistently Paleozoic ages. Because our initial results from the Bixiling eclogite were of limited areal extent, their significance required further testing. Data for garnet-bearing gneiss and eclogites from new locations in northern and southern Dabieshan (Shuanghe, Ganghe and Hongmiao), confirm the Paleozoic garnet results obtained from Bixiling eclogite. We also tested the technique on a garnet sample of known Paleozoic age from Altyn Tagh in the western Central Orogenic Belt. These new results indicate that light-colored garnets from Dabieshan and Altyn Tagh yield concordant 40Ar/ 39Ar ages ranging from 445–465 Ma by in vacuo crushing, and confirm that a Paleozoic (U)HP metamorphic event took place in the Central Orogenic Belt from Altyn Tagh in western China to Dabieshan in eastern China. In contrast, dark-colored garnets from mafic metavolcanics do not yield 40Ar/ 39Ar plateaux or isochrons ages.

Two Ultrahigh-Pressure Metamorphic Events Recognized in the Central Orogenic Belt of China: Evidence from the U-Pb Dating of Coesite-Bearing Zircons

A ~4000 km long ultrahigh-pressure metamorphic (UHPM) belt in northern China has been documented on the basis of the discovery of coesite-bearing rocks in the Altun-North Qaidam terrane in the western Central Orogenic Belt (COB), and diamond-bearing rocks in Qinling in the central and Dabie-Sulu terrane in the east. New SIMS and SHRIMP U-Pb dates of zircons from coesitebearing UHPM rocks indicate two UHPM events: one in the early Paleozoic and the other in the Triassic. Coesite-bearing zircons from a North Qaidam gneiss yielded UHP metamorphic ages of 452 ± 13.8 Ma and retrograde ages of 419 ± 6.7 Ma. A diamond-bearing gneiss from Qinling gave a lower intercept age of 502 ± 45 Ma, and an upper intercept age of 1545 ± 100 Ma, whereas a Qinling eclogite sample gave a lower intercept age of 493 ± 170 Ma and an upper intercept age of 1381 ± 82 Ma. The lower and upper intercept ages of the Qinling samples are interpreted as UHPM and protolith ages of the rocks, respectively. Coesite-bearing zircons from a Qinglongshan eclogite in the south Sulu belt yielded early Paleozoic UHPM ages of 441 ± 9 Ma, 449 ± 9 Ma, and 442 ± 9 Ma, whereas the core of a zircon containing plagioclase and apatite inclusions gave a protolith age of 761 ± 13 Ma. These age data suggest that the early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB, whereas the Triassic UHPM belt extends across the Dabie-Sulu region for about 1000 km. Based on available geochronological and geochemical data, we suggest the following tectonic model for evolution of the COB. At about 1000 Ma, the area was amalgamated to form the Rodinian continent, which contained ophiolitic fragments of oceanic affinity. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and intruded by granitic plutons. Closure of this ocean basin produced Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks, along with subduction-related island-arc volcanics and granites of early Paleozoic age suggest a second cycle of rifting and subduction along the COB, whereas the Triassic UHPM rocks record a final subduction and collision event between the North China and South China blocks.