Nature and significance of the Early Cretaceous giant igneous event in eastern China

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

The golden transformation of the Cretaceous plate subduction in the West Pacific

Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes

Cretaceous tectonic evolution of South China: A preliminary synthesis

Nd and Sr isotopic data are compiled from literatures for Mesozoic igneous rocks in the Lower Yangtze region, eastern China. These Mesozoic igneous rocks have been classified into three associations: Na-enriched (alkaline mafic) association, K-enriched association which includes shoshonitic series and ultrapotassic rocks, and high potassium calc-alkaline (HKCA) association. The HKCA association has been further divided into three geographical sub-groups: the rocks north to, along and south to the Yangtze River.

The Na-enriched mafic rocks show eNd(T) of 1.4 to −9.9 and 87Sr/86Sr(T) of 0.7047 to 0.7077. The K-enriched rock association shows eNd(T) of −1.8 to −9.3 and 87Sr/87Sr(T) of 0.7053 to 0.7092. Low SiO2 in the rocks suggests that they are mantle-derived. These Nd-Sr isotopic variations may represent isotopic feature in the mantle source of these rocks. The Mesozoic mantle of this region is thus enriched to slightly depleted and highly heterogeneous in terms of Sr and Nd isotopic characteristics. The isotopic characters of the Mesozoic mantle of the region can be modeled by mixing between DMM component representing asthenospheric mantle and EM II representing lithospheric mantle. These characters closely resemble those of mafic and ultramafic rocks in the North Dabie terrane north to the region studied. This suggests that processes operating during continental collision and deep subduction in the Dabie orogenic belt might also operate in petrogenesis of mantle-derived rocks of the Lower Yangtze region. However, since the range of initial Sr and Nd isotopic ratios for the mantle-derived rocks in the Lower Yangtze region overlaps that of Mesozoic basalts from the eastern coast of China, it is yet not known if this is a prevalent phenomenon in the Mesozoic basalts from eastern China.

The intermediate and acid rocks of the HKCA association were most likely derived from mixing between the mantle-derived mafic magma and different crustal sources. The HKCA rocks occurring in the area north to the Yangtze River have eNd(T) of −13.5 to −15.5 and 87Sr/86Sr(T) of 0.7058 to 0.7067. The low eNd(T) and 87Sr/86Sr(T) suggest that a crustal source of old and depleted in Rb, such as the source of the Mesozoic granites from the Dabie terrain, was involved in the formation of this sub-group of rocks. The HKCA rocks along the Yangtze River show eNd(T) of −7.0 to −16.5 and 87Sr/86Sr(T) of 0.7061 to 0.7101. Old crustal source, such as Archaean Kongling group and Paleoproterozoic Dongling group rocks, was involved in the formation of these rocks. The granitic rocks of the HKCA association in the area south to the Yangtze River show relatively constant eNd(T) of −4.9 to −6.8 but variable 87Sr/86Sr(T) of 0.7081 to 0.7104. The eNd(T) values of this sub-group rocks are very similar to those of the Neoproterozoic Shangxi group of low-grade metamorphic rocks and volcanic rocks in the region, suggesting that such a component is dominant in the source of this sub-group rocks. These data further suggest that the basement rocks possessing different Sr, Nd isotopic ratios and Nd model ages existed in different locations of the region. The basement similar to that of the Dabie terrain underlays in the depth of the area north to the Yangtze River. Existence of the old basement similar to the Kongling and Dongling groups in the depth of the area along the Yangtze River is very likely although only a few outcrops of the old metamorphic rocks were found in the area. However, the basement like the Neoproterozoic Shangxi group is dominant in the depth of the area south to the Yangtze River.

Nd and Sr isotopic compositions of igneous rocks from the Lower Yangtze region in eastern China: constraints on sources

The (186)Os-(187)Os and (188)Os-(187)Os dating techniques were modified and applied to molybdenites from two Cu vein deposits and three Mo skarn deposits along the middle and lower reaches of the Yangtze River, China. The results show that these deposits formed at 138 to 133 Ma, identical to the ages of 140 to 130 Ma for the host granitoids. Re-Os and Os-Os ages of two of the samples agree within analytical uncertainties. The close temporal relationship between the granitoid intrusions and the formation of the deposits supports a magmatic model for the origin of these deposits. The results show that the novel Os-Os dating technique is capable of resolving geologic problems, although the accuracy obtained by the present work (1-2%) is lower than that of the Re-Os method.

Os-Os dating of copper and molybdenum deposits along the middle and lower reaches of the Yangtze River, China

The age of the alkali basalt from Daxizhuang in Jiaozhou, eastern Shandong, was determined to be 73.5±0.3 Ma by 40Ar-39Ar technique. The basalt gave high eNd(t) values of +7.5 and +7.6, suggesting that the primitive magma was derived from depleted asthenospheric mantle with a formation depth of about 65–95 km. Spinel-lherzolite xenoliths have been discovered in the basalt. The Fo values of olivine from the xenoliths range from 88 to 89. The estimated equilibrium P-T conditions of spinel-lherzolite xenoliths are about 2.0 GPa and 1010–1140°C, suggesting an equilibrium depth of about 65 km. Geochemical characteristics of the Late Mesozoic (125-115 Ma) mantle-derived rocks in Shandong Province suggest an enriched lithospheric mantle along the southern margin of the North China block. However, geochemical characteristics of the Late Cretaceous basalts in Daxizhuang and the equilibrium P-T condition estimated from the xenoliths suggest that the lithosphere at 73 Ma ago was as thick as that of the Cenozoic in the region. Additionally, the xenoliths are rich in basaltic-component, suggesting a derivation from a newly-formed lithosphere. Therefor, lithospheric thinning took place at a time interval between about 120 and 73 Ma along the southern margin of the North China block.

Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province: New constraint on the timing of lithospheric thinning in eastern China

Six pyrite samples from the giant Bayan Obo REE-Nb-Fe deposit are dated by Re-Os technique. Pyrite studied is associated with barite and separated from a vein cutting REE mineralization. Pyrites analyzed contain 16—30 ng Re and 0.10—0.16 ng Os, and yield a Caledonian isochron age of 439 ± 86 Ma. High Re/Os ratio, low Os concentration and highly radiogenic Os isotopic ratios of these samples suggest that they are of crustal origin. The northern margin of the North China Block was a passive continental margin, but not a subduction zone with enormous volcanic activities in the Early Paleozoic Era. Our Re-Os result provides new evidences, showing that Bayan Obo deposit experienced a thermal disturbance of crustal origin in the Caledonian time and some isotopic systems recorded only the time of the dis- turbance, not the mineralization age.

Re-Os dating of pyrite from Giant Bayan Obo REE-Nb-Fe deposit

Formation of positive εNd(T) granitoids from the Alataw Mountains, Xinjiang, China, by mixing and fractional crystallization: implication for Phanerozoic crustal growth

Zhi Xie

Papers

Nd and Sr isotopic compositions of igneous rocks from the Lower Yangtze region in eastern China: constraints on sources

Os-Os dating of copper and molybdenum deposits along the middle and lower reaches of the Yangtze River, China

Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province: New constraint on the timing of lithospheric thinning in eastern China

Re-Os dating of pyrite from Giant Bayan Obo REE-Nb-Fe deposit

Formation of positive εNd(T) granitoids from the Alataw Mountains, Xinjiang, China, by mixing and fractional crystallization: implication for Phanerozoic crustal growth