Petrogenesis of pillow basalts in West Junggar, NW China: Constraints from geochronology, geochemistry, and Sr–Nd–Pb isotopes

Abstract

West Junggar is located at the southwest margin of the Central Asian Orogenic Belt and includes Silurian pillow basalts of the Mayilashan Formation. The petrogenesis and tectonic setting of these pillow basalts are important for the understanding of the tectonic evolution and metallogeny of the West Junggar area. This paper presents geochronological, geochemical, and whole‐rock Sr–Nd–Pb isotope data from the pillow basalts of the Mayilashan Formation. Zircon LA‐ICP‐MS U–Pb dating of a pillow basalt, which is in conformable contact with the chert, suggests that they were erupted at 437.2 ± 2.2 Ma marking the timing of generation of these rocks as Middle Silurian. Geochemically, all the pillow basalts bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of TiO2 (3.28–4.12 wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N = 5.5–7.3), with very weak Eu anomalies (Eu/Eu* = 0.96–1.06), and no obvious Nb, Ta, or Ti negative anomalies. Their Sr–Nd–Pb isotopic compositions ((87Sr/86Sr)I = 0.7037–0.7051, εNd(t) = 1.9–2.9, 206Pb/204Pbi = 17.74–18.22, 207Pb/204Pbi = 15.48–15.52, and 208Pb/204Pbi = 36.49–37.86) show Dupal‐like isotopic signature of ophiolites in the southern Paleo‐Asian Ocean. These characteristics indicate that the magmas were derived from a deep OIB reservoir, that is, a depleted but slightly heterogeneous asthenospheric mantle source with ~5–15% partial melting of garnet and spinel lherzolite. Our obtained results, in conjunction with previous published data, allow us to suggest that the alkaline pillow basalts formed in a seamount within an intraoceanic setting, where a larger number of seamounts with different ages occurred in the Paleo‐Asian Ocean.

The Ediacaran to Early Palaeozoic evolution of the Junggar–Balkhash Ocean: A synthesis of the ophiolitic mélanges in the southern West Junggar terrane, NW China

“…As mentioned before, the OIB‐type basalts are widely dispersed in the BMT areas, and the origins of these rocks are consistent with an oceanic intra‐plate seamount setting (Figure b; Wei, ; Yang, ; Yang, Li, Yang, et al, ; Yang et al, ; Yang et al, ; this study). However, the exact relationships between the OIB‐type basalts and SSZ‐type ophiolites are still inconclusive.…”

“…In general, the studied pillow basalts in the Tuomaqia segment of the BOM are compositionally identical to the pillow basalts ( Figures 6c,d, 7, and 8) from the Barleik Mountains , as well as those from the Mayile Mountains (Wei, 2010;Yang et al, 2018; Figure 2a). All these pillow basalts are suggested to form at seamounts/oceanic islands within an intra-oceanic setting, because (1) their OIB-type alkaline features are compatible with classic seamount basalts, such as the Hawaiian seamount lavas (Hauri, 1996); (2) the pillow basalts in the Barleik and Mayile Mountains are always covered by chert deposits (Wei, 2010;Yang et al, 2018; this study), which is similar to a typical seamount stratigraphic sequence (Safonova & Santosh, 2014;Yang et al, 2015). Additionally, some massive basalts from the Tangbale area also share the features of OIB and were suggested to occur at off-ridge seamounts (Yang, 1993).…”

“…In details, the pillow basalts exhibit higher Table 2). In Zr/TiO 2 versus Nb/Y diagram (Figure 7), they fall in the field of alkali basalt, comparable to the Mayile and Barleik pillow basalts Yang et al, 2018). Moreover, the pillow basalts have the highest total REE contents (109.44-120.94 ppm; Table 2) among studied samples, and their REE patterns show strong light REE enrichments (La N /Yb N = 7.84-9.03) with no Eu anomalies (Figure 6c).…”

“…The garnet amphibolites have metamorphic zircon U-Pb ages of 488-474 Ma, interpreted as the timing of metamorphism related to oceanic subduction (Ren, 2013). By contrast, the pillow basalts are characterized by OIB-type trace element patterns (Wei, 2010;Yang et al, 2015;Yang, Li, Tong, Wang, & Xu, 2018) and were dated at 437-433 Ma by zircon U-Pb and whole-rock Rb-Sr methods (Wei, 2010;Yang et al, 2018), which are obviously younger than the ophiolitic gabbros and garnet amphibolites. Furthermore, some Early Palaeozoic arc-related plutons (515-485, 444-439 Ma, Ren et al, 2014;Xu et al, 2012) occur in this area, and their temporal and spatial distributions reflect a south-dipping subduction (Xu et al, 2012;Z.…”

“…Although the published age data for the OIB-type basalts are very limited, an early Silurian age of whole-rock Rb-Sr and zircon U-Pb dating (Wei, 2010;Yang et al, 2018) is in good accordance with the geological features that the OIB-type basalts are atop covered by the clastic rocks with well-preserved middle Silurian radiolarian fossils (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region (BGMRXUAR), 1993; Li, 1994) and a Late Ordovician maximum depositional age of~452 Ma (Z. . This means that the juxtaposition of older ophiolite-arc rocks with younger OIB-type basalts in the southern WJT could result from seamount accretion during the late Silurian to Early Devonian, as indicated by an unconformity between the Middle Devonian Kulumudi Formation and underlying BMT ophiolitic mélange and Silurian OIB-type basalt-bearing strata (Bai et al, 2015;Ren et al, 2014;Wen et al, 2016).…”