This study describes a previously unidentified Neoproterozoic mafic dyke emplaced in the northern flank of the Langshan Tectonic Belt. This dyke intruded into the micaquartz schist of the Zhaertaishan Group, and yielded an age of 908 ± 8 Ma. The youngest U–Pb ages of micaquartz schist from the Zhaertaishan Group in the Langshan area were 1118 ± 33 Ma, 1187 ± 3 Ma and 1189 ± 39 Ma, suggesting that the depositional age of the protolith of the schist was between 908 ± 8 Ma and 1118 ± 33 Ma. In addition, 436 U–Pb age data and 155 Lu–Hf isotopic data from six samples in the Langshan Tectonic Belt and one Permian greywacke from the Wuhai area show distinct differences between the northern and southern flanks of the Main Langshan area. The U–Pb ages of the northern flank are primarily Meso-Neoproterozoic; similar ages have not been identified in the southern flank to date. Moreover, two-stage Hf model ages of the northern flank feature three age peaks at ∼900 Ma, ∼1700 Ma and ∼2600 Ma; this differs from Hf model ages of the southern flank, which feature one strong age peak at ∼2700 Ma. These results suggest that the northern and southern flanks of the Main Langshan area have different geochronologic characteristics and should be divided further. Based on the U–Pb ages and Hf model ages, the northern and southern flanks of the Main Langshan area are named the North and South Langshan Tectonic Belts. Comparison of the U–Pb age and two-stage Hf model age distributions from the North Langshan Tectonic Belt, South Langshan Tectonic Belt, Alxa Block and the North China Craton (NCC) reveal that the North Langshan Tectonic Belt is similar to the Alxa Block and that the South Langshan Tectonic Belt is similar to the NCC. In addition, the zircon U–Pb age of 860 ± 7 Ma commonly observed in the Alxa Block was detected in the Permian greywacke from the Wuhai area of the NCC, which suggests that the amalgamation of the North and South Langshan Tectonic belts (i.e., the amalgamation of the Alxa Block and the NCC), occurred between Devonian and late Permian.
In this paper, 219 concordant detrital zircons from the main stem and tributary rivers of the Central Qilian Block (CQB) have been analyzed using excimer laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine their U-Pb ages and Lu-Hf isotopic compositions. The detrital zircons from three samples show four major age groups: 246–509 Ma, 899–1176 Ma, 1620–2089 Ma, and 2131–2610 Ma. These age populations indicate that prominent magmatic events occurred at 0.5 Ga, 0.9 Ga, 1.8 Ga, and 2.5 Ga. Archean basement components are rare in these river sands and likely derived from Paleoproterozoic metamorphic complexes. The crustal model ages from Hf isotopic analyses show age peaks at ~2.5 and 1.8 Ga. The crustal accretion of material derived from the depleted mantle of the CQB occurred in two stages at 3.2–1.8 Ga and 1.8–0.6 Ga. The crustal accretion curve based on the Hf model ages indicates that approximately 10% of the present crustal volume of the CQB formed at 3.2 Ga, while ~60% formed at 1.8 Ga. Few crustal components have accreted from the depleted mantle since 0.6 Ga in the CQB. The reworking rate calculation shows that continental crustal accretion also occurred at 2.5 Ga in the CQB; the most intensive crustal reworking occurred at 0.8 Ga. Comparisons of Precambrian continental growth patterns of the African continent and CQB indicate that the zircon Hf isotopic compositions can be used to accurately constrain the growth patterns of continental crust and that a microcontinent can serve as window into the crustal growth of supercontinents during certain periods.
Mechanisms for the enrichment and re-precipitation of gold in the giant Jiaodong gold deposits (eastern North China Craton) remain poorly constrained. To better understand the mineralization mechanism, we did in situ analyses of S isotopes on sulfides such as pyrite, pyrrhotite, galena and chalcopyrite from the disseminated (altered-rock type) and quartz-vein type gold deposits by femtosecond laser ablation coupled multi-collector inductively coupled plasma mass spectrometry. Pyrites from the altered-rock type gold deposit show δ34S values in the range from 7.4 to 11.3 ‰, which is obviously heavier than the quartz-vein type gold deposits with δ34S = 6.2 ∼ 8.8 ‰. Traditionally, the difference of sulfur isotopic compositions between the two types of gold deposits was attributed to the change in oxygen fugacity. However, we found that, from early to late metallogenic stage, sulfur isotopes of pyrites from the altered rock type gold deposits tend to decrease gradually and pyrrhotites can always be observed in the third stage. Moreover, the S isotopic compositions (δ34S = 7.9 to 8.2 ‰) of the pyrites coexisting with magnetite are comparable with those (δ34S = 6.2 to 8.0 ‰) of the pyrites coexisting with pyrrhotite in the quartz vein type gold deposits. These features indicate that the decrease of sulfur isotopes in pyrites was not caused by increase of oxygen fugacity. We suggest that the S isotopic and fO2 variation could be ascribed to an increase of pH of the ore-forming fluid, which is supported by the typically quartz dissolution and common occurrence of calcite and pyrrhotite in the late metallogenic stage (the third stage) and an overall decrease of aluminum contents of quartz from core to rim. We further proposed that the variation of pH of ore-forming fluids is probably related to a process of decompression due to development and enlargement of fractures filled with ore-forming fluids. Gold enrichment in the main ore-forming stage of the northwest Jiaodong gold deposit probably was realized by multiple phases of fluid pressure fluctuation, which subsequently led to repeatedly dissolution and re-precipitation of Au from pyrites due to decreasing oxygen fugacity and increasing pH values of the ore-forming fluids.
Abstract Rutile grains often occur in different types of gold deposits, and their U-Pb ages have been widely used to determine the formation time of gold mineralization. However, the origin of rutile grains in the gold deposits remains controversial. In this paper, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of U-Pb ages and trace elements on rutile grains were applied to investigate the metamorphic and hydrothermal processes of the Baiyun gold deposit (70 t Au, avg grade: 3 g/t) in the Liaodong Peninsula in the northeastern part of the North China craton. Rutile grains in the hydrothermal altered gold schist from the Baiyun deposit yielded two group ages of 1924 ± 18 and 237.0 ± 1.8 Ma, respectively. Combined with our systematic U-Pb zircon geochronological results of the ore-hosting schists and post-ore dikes, we suggest these rutile ages record a Paleoproterozoic metamorphic event and a Triassic hydrothermal gold mineralization event, respectively. The metamorphic and hydrothermal rutile grains have no obvious textural differences, but they show distinct trace element contents of Zr, W, Nb, and Ta. Combined with previous published data, we propose that high W (>1,000 ppm) and low Zr (<200 ppm) contents in rutile can be used to distinguish hydrothermal rutile from metamorphic and magmatic rutile. The newly identified ca. 237 Ma hydrothermal event is much older than the ca. 227 to 210 Ma Triassic magmatic rocks in the region, which precludes a temporal and genetic link between the Baiyun gold mineralization and the regional Mesozoic magmatism. Rather, the ca. 237 Ma gold mineralization may be associated with the Triassic orogenic metamorphism, and Baiyun is an orogenic gold deposit. The Triassic gold deposits in the northern margin of the North China craton formed by orogenesis between the Siberian craton and the North China craton. After a hiatus, the large-scale gold deposits formed during the Early Cretaceous in the North China craton due to a westward subduction of the paleo-Pacific plate beneath the craton since the Early Jurassic. Our study highlights that rutile in gold deposits may be inherited from the host rocks and/or formed by hydrothermal fluids. Distinguishing between these two different rutile generations requires a combination of in situ age dating and trace element geochemistry in petrogenetic context.
The North Qilian Orogenic Belt (NQOB) is an important element of the northern edge of the Tibetan Plateau, and several rivers run along it. Zircon U–Pb dating, trace elements, and Hf isotopic data derived from the sediments carried by the rivers have been used to deduce the crustal evolution of the NQOB. U–Pb ages obtained from detrital zircons found in the sediments of the Zamu, Zhuanglan, and Beida rivers can be divided into five major age groups, that is, 2,600–1,500, 1,500–1,100, 1,100–650, 550–400, and 360–150 Ma, and the corresponding peaks occur at ~2,450, ~1,750, ~950, ~450, and ~250 Ma. Archean components, which may be derived from the ancient continental nucleus in the region, are rare in the river sediments. The zircon grains with ages of 360–150 Ma are most likely related to crustal thickening and decompression melting in a post‐collisional tectonic setting. The age distribution patterns indicate that the sediments carried by the Zamu, Zhuanglang, and Beida rivers are derived from the NQOB. Furthermore, the Hf isotopic compositions of Meso‐ and Neoarchean, Mesoproterozoic, Grenvillian, Pan‐African, Caledonian, and Hercynian zircon age groups exhibit a wide range of ε Hf ( t ) values, suggesting diverse sources. The existence of strongly negative ε Hf ( t ) values among the Grenvillian and Paleoproterozoic zircons indicates that the source magma included reworked Palaeoarchean crustal materials. New results indicate that continental rifting and the opening of the Qilian Ocean occurred at 775–520 Ma; the subduction and closure of the Qilian Ocean occurred at 520–440 Ma; arc–continent collision and continental subduction occurred at 440–420 Ma; and orogen collapse and extension occurred at 400–360 Ma. Our study indicates that the formation of the NQOB was mainly related to the evolutionary history of the Proto‐Tethys Ocean.
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