Zircon U-Pb and Hf isotopes of volcanic rocks from the Batamayineishan Formation in the eastern Junggar Basin

Abstract

An internal structural study was conducted to investigate U-Pb age, trace elements and Hf isotopes of basaltic zircons from the Batamayineishan Formation. The basalt was obtained from drill well San-Can 1 on the eastern Luliang uplift within the Junggar Basin. Trace element data of zircons show that all samples are magmatic, with similar REE patterns, including positive Ce (δCe=5.06-134), but negative Eu (δEu=0.06-0.55) anomalies and enrichment in heavy rare earth elements. Among 25 grains, the concordant ages were subdivided into three groups; ages of 300.4±1.3 Ma (n=11), 339.2±2.7 Ma (n=3) and 392.0±1.7 Ma (n=8). Three remaining grains were nearly concordant, with 206 Pb/ 238 U ages of 510±7, 488±6 and 453±6 Ma, respectively. The youngest concordant age (i.e., 300.4±1.3 Ma) could be interpreted as the formation age of the studied basaltic rock; this is consistent with the sampling position at the upper part of the Batamayineishan Formation. On the other hand, ages such as Ordovician and early Devonian are consistent with the ages of island-arc volcanic rocks (enrichment in Pb) or ophiolites around the basin. Moreover, the positive ε Hf (t) values of the early and middle Paleozoic zircons (+3.6-+10.5) may suggest that the basement traversed by the studied volcanic rocks may be Paleozoic in age, formed from the residual oceanic crust and island-arc complex. The ε Hf (t) values (+4.2-+17.1) of the late Paleozoic (~300.4 Ma) zircons suggest that the basaltic magmas were derived from partial melting of the asthenospheric mantle or depleted lithospheric mantle. These magmas were slightly contaminated by the existence of early-middle Paleozoic materials. The late Carboniferous basalts represent direct eruption of mantle-derived magmas at the upper crustal level during a post-collisional tectonic setting. We therefore consider that extensive vertical growth of the continental crust to have occurred before the late Carboniferous. Batamayineishan Formation, zircon U-Pb ages, Hf isotopes, continental crust growth, eastern Junggar Basin Citation:Su Y P, Zheng J P, Griffin W L, et al. Zircon U-Pb and Hf isotopes of volcanic rocks from the Batamayineishan Formation in the eastern Junggar Basin.

Geochemistry and geochronology of Carboniferous volcanic rocks in the eastern Junggar terrane, NW China: Implication for a tectonic transition

“…Three inherited zircons from sample 09LSG-6 and seven inherited grains from sample 09BJE-79 yielded weighted mean 206 Pb/ 238 U ages of 337.0 ± 6.6 Ma and 337.3 ± 5.9 Ma, respectively. These early Carboniferous zircons also have been recently reported from late Carboniferous granitoids in the Karamaili area and basalts within the Junggar Basin (Su et al, 2010;Yang et al, 2010). Xiao et al (2011) acquired zircon U-Pb ages of 332-336 Ma for early Carboniferous volcanic rocks.…”

“…The late Carboniferous felsic volcanic rocks have eruption ages and Sr-Nd isotopic compositions (Table 3) similar to those of the coeval basic rocks (Su et al, 2010), which could indicate that they were differentiation products of the basic magmas. However, there is an obvious "Daly gap" between the basic and felsic rocks (Fig.…”

“…Our results are consistent with fossil data (Zhu et al, 2005) and zircon U-Pb ages of 315-323 Ma for the rhyolites . The volcanic and sedimentary rocks obtained from drill holes within the Junggar Basin have zircon U-Pb ages of 300.4 ± 1.3 Ma for the basalt (Su et al, 2010), 300.0± 5.0 Ma for the basaltic andesite and 314.0 ± 1.0 Ma for the dacite (authors' unpublished data), and 314.6 ± 1.2 Ma for the sandstone (Yang et al, 2011b), further indicating the late-Carboniferous formation of the BN Fm. rocks.…”

Tectonic framework of the northern Junggar Basin part I: The eastern Luliang Uplift and its link with the East Junggar terrane

“…However, the time of BS Formation is not well constrained. A 207 Pb/ 206 Pb apparent age of 405 ± 10 Ma obtained from rhyolite in the well SN1 (Zheng et al, 2007) is significantly different from a U-Pb zircon age of 300.4 ± 1.3 Ma obtained from basalt in the well SC1 (Su et al, 2010). The tuff and dacite, respectively from the well LD1 and well SX4, are dated at 313.3 ± 4.6 Ma and 299.8 ± 5.2 Ma, whereas the tuffaceous sandstone in the BS Formation of well DZ1 yielded LA-ICP-MS zircon U-Pb age of 314.6 ± 1.2 Ma (X.F.…”

“…The recent information on Carboniferous strata from the Junggar Basin mainly comes from the Luliang Uplift in the Junggar Basin, especially from the eastern Luliang Uplift (Zheng et al, 2000;Wang et al, 2002;Zheng et al, 2007;X. Zhao et al, 2008;Wu et al, 2009;Su et al, 2010;Li et al, 2012a,b;X.F. Yang et al, 2012a,b).…”