July 2009 LIP of the Month

OIB-like, heterogeneous mantle sources of Permian basaltic magmatism in the western Tarim Basin, NW China: Implications for a possible Permian Large Igneous Province

Mei-Fu Zhou, Sheng-Hong Yang
Department of Earth Sciences, University of Hong Kong, Hong Kong, China
mfzhou@hkucc.hku.hk (for Mei-Fu zhou),
shyang81@gmail.com (for Sheng-Hong Yang)

[Corresponds to event 40 in LIP Record database]

Extracted mainly from Zhou et al. (2009)


Permian basalts are widely distributed in the Tarim Basin and surrounding areas of NW China (Fig. 1) (Yang et al., 2006c, 2007). Paleontological data and stratigraphic correlations suggest that the basalt Formations are lower to middle and middle Permian, respectively (Zhang, 2003).


Figure 1: A simplified geological map showing the distribution of Permian basalts underlying the Tarim Basin and outcrops in the Keping region, western Tarim. A Permian dyke swarm in Bachu is also shown. Note that the basalts shown represent the minimum distribution based on available oil company drill holes in the basin (Yang et al., 2006, 2007). The lower right inset shows the locations of the Siberia Traps to the north and the ELIP (Emeishan LIP) to the south.

The magmatism is represented by basaltic flows in Keping and mafic and ultramafic dykes in Silurian-Devonian strata in Bachu, southwestern Tarim Basin (Fig. 2). The basalts in Keping have SiO2 (44.1%–55.5 wt.%) and total alkalis (Na2O+K2O =3.20-7.79 wt.%) similar to the mafic dykes in Bachu, but with much higher TiO2 (3.53-4.33 wt.%). An ultramafic dyke has relatively low SiO2 (44.6-43.2 wt.%) and high MgO (19.0-20.2 wt.%), reflecting the abundance of cumulate olivine.

Figure 2: Field photos showing the flood basalts in the Permian strata near Keping (A and B). The flood basalts are interlayered with Permian sandstones and include several flows. The basalts are well exposed along the margin of the Tarim Basin and are visible for at least 40 km along strike. Near Keping, mafic dyke swarms (dark in color) in the Devonian strata (C and D) are abundant and remarkably well preserved.

All the rocks, including both lavas and dykes, have parallel, mantle-normalized trace element patterns enriched in Rb, Ba, Th, Nb, Ta, Zr, Hf, and light rare earth elements (LREE) (Fig. 3). The basalts have higher initial 87Sr/86Sr ratios (0.7064 to 0.7080) and lower εNd(t) values (-2.66 to -9.27) than the dykes (initial 87Sr/86Sr ratios range from 0.7048 to 0.7052 and εNd(t) values from +1.64 to +5.16). Both the basalts and dykes show a narrow range of 206Pb/204Pb (17.87-18.77), 207Pb/204Pb (15.52-15.58) and 208Pb/204Pb (38.38-39.04) ratios. The dykes do not show significant crustal contamination and were derived from an OIB-like, asthenospheric mantle source. In contrast, the basaltic flows show variable degrees (up to 10%) of crustal contamination and were derived from an OIB-like, but isotopically more enriched, asthenospheric mantle source. Olivine from the ultramafic dyke has Fo values up to 85, corresponding to a melt temperature of 1300°C and a melt Mg# of 63.

Figure 3: Primitive mantle-normalized trace element patterns for the basalts and dykes in the Tarim Basin. The normalizing values are from Sun and McDonough (1989).

Recent geochronological and stratigraphic studies of the Permian basalts in Tarim and the surrounding areas show that a large volume of basalt was erupted between 280 and 270 Ma (Chen et al., 1997, 1999, 2006; Jia et al., 2004; Jiang et al., 2004a; Zhou et al., 2006; Borisenko et al. 2006; Zhang et al., 2008). The Tarim Basin magmatism may be unrelated to the northward subduction of the paleo-Tethyan oceanic lithosphere (Yang et al., 1996, 2005, 2006a, b), but are quite likely related to a mantle plume as proposed recently by various authors (Jiang et al., 2004a, b, 2006b; Zhang et al., 2008). The Tarim Basin igneous event reflects partial melting of heterogeneous mantle sources related to a major mantle plume. Spatially and temporally associated mafic-ultramafic and syenitic intrusions and volcanic rocks form the ~275 Ma Tarim large igneous province.

Indeed, Middle and Late Permian mantle plumes were very active in the Eurasian continent, as evidenced by the well-known ~250 Ma Siberian Traps and the ~260 Ma Emeishan LIP. Latest Permian volcanic ashes also occur widely across Eurasia (Yin et al., 1992). The wide distribution and the range of ages of these Permian plumes in Eurasia (Tarim LIP, ELIP and Siberian Traps) may suggest a single protracted thermal anomaly caused by heat loss from the core, which may have contributed collectively to the Permian mass extinction.


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Zhou, M.F., Zhao, J.H., Jiang, C.Y., Gao, J.F., Wang, W., Yang, S.H. (2009) OIB-like, heterogeneous mantle sources of Permian basaltic magmatism in the western Tarim Basin, NW China: Implications for a possible Permian Large Igneous Province, Lithos, doi: 10.1016/j.lithos.2009.06.027