Search

Neoproterozoic mafic dyke swarms are an important probe for reconstructing the evolutionary history of the Rodinia supercontinent. Here, we present detailed geochronological, geochemical, and Sr–Nd–Hf isotopic data to constrain the petrogenesis and geodynamic setting of Neoproterozoic mafic dyke swarms in the western Yangtze Block, South China. Petrological and zircon U–Pb dating studies reveal that the studied dykes mainly comprise subophitic-textured dolerite and intergranular dolerite with crystallization ages of ca. 823–817 Ma and ca. 795–794 Ma, respectively. The two groups of dykes exhibit varied geochemical and Sr–Nd–Hf isotopic compositions. Among them, the older group has moderate rare earth elements (∑REE = 71.0–94.9 ppm) with flat REE patterns ((La/Yb)N = 1.38–1.81) and exhibits enriched large ion lithophile elements and Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.704865–0.707641, εNd(t) = −6.15 to 1.26, εHf(t) = −2.59 to 2.11), resembling typical island arc basalts. Whereas, the younger group has high Nb contents (4–10 ppm) with high Nb/U values (6.93–29.3), high REE contents (∑REE = 123.3–180.8 ppm) with moderate (La/Yb)N values (1.84–4.51), and slightly depleted but variable Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.702280–0.706761, εNd(t) = −2.67 to 4.07, εHf(t) = −0.74 to 5.91), akin to Nb-enriched basalts in the subduction zone. Petrogenesis studies indicate that the ca. 823–817 Ma and ca. 795–794 Ma dykes may be derived from different degrees of melting of the enriched mantle wedge that was metasomatized by subducted slab-related fluids and melts, respectively. Varying degrees of the interaction of the slab-derived fluid/melts with mantle peridotite account for their elemental–isotopic heterogeneity. The geochemical database compilation of Neoproterozoic igneous rocks suggests that the Yangtze Block may be located on the periphery of the Rodinia supercontinent and slab tearing or break-off, rather than mantle plume, may account for the Rodinia break-up and associated transition of the mantle metasomatic mechanism at ca. 830–820 Ma. [ABSTRACT FROM AUTHOR]

Paleoproterozoic granitoids provide crucial constraints on the tectonic evolution of the Yangtze Block, South China, and its reconstructed position in the Columbia supercontinent. Here, we report zircon U–Pb age and Hf–isotope data coupled with whole-rock geochemistry for monzogranites from the Lengshui complex (LSC) of the Zhongxiang Terrane in the northern Yangtze Block. Zircon U–Pb dating yields crystallization ages of 2039 ± 36 Ma and 1950 ± 16 Ma for two monzogranite samples, respectively. Both samples show S-type granite affinities. The older S-type granites in the LSC are interpreted to record a subduction-related tectonic setting, whereas the younger ones are related to a syn-collisional setting. This scenario reflects a critical geodynamic transition during the assembly of the Columbia supercontinent at ca. 2.04–1.95 Ga within the Yangtze Block. The two granites have similar zircon Hf isotope ratios with εHf(t) values of −17.11 to −9.69 and TDM2 ages of ca. 3.67–3.22 Ga, suggesting that the monzogranites could have been generated by partial melting of Archaean crustal materials in the Zhongxiang Terrane. Widespread Paleoproterozoic magmatism and metamorphism in the Yangtze Block indicate a change in tectonic regime from ca. 2.15–2.04 Ga subduction to ca. 2.0–1.93 Ga collision followed by ca. 1.87–1.82 Ga post-collisional extension. Based on the similarity of ages and rock types with other blocks in the Columbia supercontinent, our results support that the Yangtze Block may be placed near Laurentia in the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

The 87Sr/86Sr minimum of the Capitanian seawater is one of the most significant features in the Phanerozoic seawater 87Sr/86Sr history. In order to assess possible contribution of the Emeishan large igneous provinces (LIPs) to strontium isotope evolution of the Capitanian seawater, 87Sr/86Sr ratios were measured from the Capitanian limestones which are locally interlayered with the Emeishan basalts. The limestones underlying the Emeishan basalts have high 87Sr/86Sr ratios (0.7070–0.7074). However, extremely low 87Sr/86Sr ratios (0.7068–0.7070) were identified in the late Capitanian Jinogondolella prexuanhanensis–J. xuanhanensis zones, which correspond to the eruption time of the Emeishan LIP. The temporal coincidence of these two phenomena supports the idea of a potential linkage between Capitanian 87Sr/86Sr minimum and eruption of this igneous province. The strong submarine hydrothermal activity and erosion of the Emeishan LIP could have released large amounts of non-radiogenic Sr to the oceans and play an important role in strontium isotope evolution of the seawater. [ABSTRACT FROM AUTHOR]

New zircon LA-ICP-MS U–Pb age, zircon Hf isotope, and whole-rock major and trace elemental data of the Late Cretaceous Ageledaban complex in the Karakorum Terrane (KKT), northwest Tibet, provide new constraints on the tectonic processes of the collision and thickening of the terrane between the Lhasa and Qiangtang terranes. The granitoids from the Ageledaban complex have a variable SiO2content, from 62.83 to 73.35 wt.% and A/CNK<1.1 (except for YM61-2). They have rare earth element and trace element patterns that are enriched in light rare earth elements, Rb, Pb, Th, and U, and are depleted in Ba, P, Sr, Ti, and Nb, indicative of weakly peraluminous-metaluminous I-type affinity. Zircon U–Pb dating reveals that the Ageledaban complex was emplaced at ca. 80 Ma. Zircons from the monzogranite and monzonite samples with concordant206Pb/238U ages about 80 Ma have a zirconεHf(t) of −6.6 to −1.1, corresponding to the Mesoproterozoic Hf crustal model ages (TDMC = 1.2–1.6 Ga); the remaining inherited zircons from the monzonite with concordant206Pb/238U ages of about 108.1 Ma haveεHf(t) values that range from −8.3 to −5.0, corresponding to the Mesoproterozoic Hf crustal model ages with an average of 1.6 Ga. These signatures indicate that the Ageledaban granitoids may have been derived from the partial melting of a mixed mantle-crust source. Together with the age and geochemical data in the literature, we propose that the collisional event in the KKT in northwestern Tibet would postdate the northern Lhasa–southern Qiangtang collision, which occurred first in the Amdo in the east and later in the Shiquanhe in central Tibet. Our results support the previous view that the collision of the Bangong–Nujiang suture zone (BNSZ) may be diachronous. [ABSTRACT FROM AUTHOR]