Xueyuan Yu

Geochemistry of late Paleozoic mafic igneous rocks from the Kuerti area, Xinjiang, northwest China: implications for backarc mantle evolution

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high eNd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian.

MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China: implications for the source of the low 206Pb/204Pb and high 143Nd/144Nd mantle component in the Indian Ocean

Abstract Samples from a basalt and gabbro section of the Paleo-Tethyan (∼350 Ma) Mian-Lue northern ophiolites (MLNO) in the Qinling Mountains of central China display sub-parallel and relatively smooth incompatible trace element-depleted patterns and have high ϵ Nd (350 Ma) (8.1–11.3) and low 206 Pb/ 204 Pb (350 Ma) (16.90–17.25). The MLNO basalts and gabbros are compositionally similar to normal mid-ocean ridge basalts (MORB), particularly to those from the Carlsberg Ridge and Indian Ocean Ridge Triple Junction. The basalts and gabbros also have high Δ7/4 and Δ8/4 isotopic values characteristic of the Dupal isotopic anomaly in the southern hemisphere. Although the MLNO is presently in the northern hemisphere, it was previously located within the southerly location of the Indian Ocean based on paleomagnetic data. Thus, assuming that the low 206 Pb/ 204 Pb ratio of the MLNO basalts and gabbros is not due to seawater alteration or continental contamination, the unique isotopic signature of both the Paleo-Tethyan oceanic igneous crust and the modern Indian MORB may have come from a very similar, if not identical mantle reservoir. This indicates that a portion of the modern Indian MORB mantle isotopic domain could have been in existence for at least ∼350 Ma. We propose that the low 206 Pb/ 204 Pb and high 143 Nd/ 144 Nd isotopic character of the MLNO basalts and gabbros as well as similar Indian MORB originated either from a low μ sub-domain of the depleted asthenospheric mantle in the southern hemisphere or due to contamination of the depleted asthenosphere by deep-rooted plumes carrying a low 206 Pb/ 204 Pb mantle component. In contrast, the origin of the more common Indian MORB with low 206 Pb/ 204 Pb and low 143 Nd/ 144 Nd is most probably associated with the delamination of the Gondwanan continental lithosphere during formation of the Indian Ocean.

Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay, North Xinjiang

Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

Adakites related to subduction in the northern margin of Junggar arc for the Late Paleozoic: Products of slab melting

Volcanic rocks with adakitic compositional signature have been recognized in the northern margin of ancient Junggar island arc for the Late Paleozoic. These adakites for the early Devonian from the Tuoranggekudouke Group (D1t) are characteristic of high Sr, Sr/Y and (La/Yb)N but low Y, Yb and HREE. Their compositional characteristics are much similar to those of the typical adakites in the world but distinct from those of the normal arc volcanic rocks from the same Group. We conclude that these adakitic volcanic rocks were produced by slab melting during the early period of Paleoasia-ocean lithosphere subduction. This infers that the Paleoasia Ocean in the north Junggar area began a new subduction process in the early Devonian.

Discovery of Mg-righ volcanic rock series in western Altay area, Xinjiang and its geologic significance

A series of Mg-rich volcanic rocks, including rare Mg-rich dacites in the Asherle copper mine, western Altay Area, has been found. This suite of Mg-rich volcanic rocks is characterized by high SiO2, MgO and low Ti contents. Compared with the island arc tholeiite and MORB, these rocks are depleted in HFSE and are therefore similar to boninite. The dacites show V-shaped distribution REE patterns whereas the basalts display flat or LREE-depletton patterns. Based on their geochemical features and occurrence, it is suggested that they were formed in oceanic island arc setting. Its petrogenesis is related to the slab movement. At the early stage, the peridotite of the ocean upper mantle diapirically uprised to the shallow level and started to melt owing to subduction of the ocean slab, forming the island arc tholeiite with pillow structure. At the late stage, the residual refractory peridotites melted again, giving rise to the formation of the parent magma of Mg-rich basalt.

Isotopic evidence for continental ice sheet in mid-latitude region in the supergreenhouse Early Cretaceous

Cretaceous represents one of the hottest greenhouse periods in the Earths history, but some recent studies suggest that small ice caps might be present in non-polar regions during certain periods in the Early Cretaceous. Here we report extremely negative δ18O values of −18.12‰ to −13.19‰ for early Aptian hydrothermal zircon from an A-type granite at Baerzhe in northeastern China. Given that A-type granite is anhydrous and that magmatic zircon of the Baerzhe granite has δ18O value close to mantle values, the extremely negative δ18O values for hydrothermal zircon are attributed to addition of meteoric water with extremely low δ18O, mostly likely transported by glaciers. Considering the paleoaltitude of the region, continental glaciation is suggested to occur in the early Aptian, indicating much larger temperature fluctuations than previously thought during the supergreenhouse Cretaceous. This may have impact on the evolution of major organism in the Jehol Group during this period.

Discovery of the highly depleted N-MORB-type volcanic rocks: new evidence for the Mianlue paleo-ocean

The highly depleted N-MORB-type volcanic rocks have been discovered in Mianlue mélange of Qinling orogenic belt, central China. These mafic rocks are associated with the meta-peridotites, showing LREE-depleted pattern, similar to N-MORB and typical ophiolites. It is indicated that an ancient ocean existed in the Mianlue area of Qinling orogenic belt during the late Paleozoic.