Differentiation, crustal contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China

Abstract

The Nantianwan mafic intrusion in the Panxi region, SW China, part of the *260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17-151 ppb) and variable Cu/Pd ratios (1,500-32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (-1.3 to -0.1) and positive cOs(t) values (5-15), consistent with low degrees of crustal contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9-4.1 wt % sulfide and 37-160 ppb PGE and high Cu/Pd ratios (54,000-624,000). Sulfide-poor gabbronorites have 0.1-0.6 wt % sulfide and 0.2-15 ppb PGE and very high Cu/Pd ratios (16,900-2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (-0.9 to -2.1) similar to those for olivine gabbros, but their cOs(t) values (17-262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187 Os of the gabbronorites. Processes such as magma differentiation, crustal contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor crustal rocks and reached sulfide saturation, forming an olivine-and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have Communicated by T. L. Grove. Electronic supplementary materialThe online version of this article (

First Identification of Late Permian Nb‐Enriched Basalts in Ailaoshan Region (SW Yunnan, China): Contribution From Emeishan Plume to Subduction of Eastern Paleotethys

“…Therefore, minor contamination of these sedimentary rocks could give rise to the xenocrystic zircons but with insignificant effect on the geochemical compositions of the NEB magmas. Generally, significant crustal contamination would have caused Nb‐Ta depletion (H. Liu et al, ; C. Y. Wang et al, ; Q. Wang et al, ), rather than Nb‐Ta enrichments in the Ailaoshan NEBs (Figure b). In addition, magmas from a relatively depleted mantle source have high ε Nd( t ), but low Th/Nb and La/Sm ratios, whereas magmas from the continental crust have low ε Nd( t ), but high Th/Nb and La/Sm values (e.g., Condie, ; DePaolo, ).…”

“…Therefore, minor contamination of these sedimentary rocks could give rise to the xenocrystic zircons but with insignificant effect on the geochemical compositions of the NEB magmas. Generally, significant crustal contamination would have caused Nb-Ta depletion (H. Liu et al, 2017;C. Y. Wang et al, 2012;Q.…”

Geochemistry of the Abulangdang intrusion: Cumulates of high-Ti picritic magmas in the Emeishan large igneous province, SW China

“…The various rock types all have transitional boundaries. Zircon grains from the lherzolite yielded a weighted mean 206 Pb/ 238 U age of 262 ± 2 Ma using the SHRIMP zircon U-Pb technique (Electronic Supplementary material 1), similar to the ages of the mafic-ultramafic intrusions elsewhere in the ELIP (i.e., Zhou et al, 2002aZhou et al, , 2005Zhong and Zhu, 2006;Zhou et al, 2008;Wang et al, 2012).…”

“…Low-Ca olivine grains are in fact common in the mafic-ultramafic intrusions of the ELIP, such as those in the Ni-Cu-(PGE) sulfidebearing Jinbaoshan, Limahe and Nantianwan intrusions and the Fe-Ti oxide-bearing Panzhihua and Hongge intrusions Pang et al, 2008a;Tao et al, 2008;Pang et al, 2009;Wang et al, 2012) and mafic dikes in the Funing region (Wang et al, 2011). Olivine crystals in these mafic-ultramafic intrusions/dikes have b0.21 wt.% CaO (mostly b0.15 wt.%, i.e.,~1050 ppm Ca), whereas the olivine phenocrysts in the picrites of the ELIP commonly have N0.25 wt.% CaO (~1785 ppm Ca) at similar Fo contents (Li et al, 2012) (Fig.…”

Paleomagnetic Secular Variation Constraints on the Rapid Eruption of the Emeishan Continental Flood Basalts in Southwestern China and Northern Vietnam

“…In the past 300 Myr, many volcanic eruptions associated with large igneous provinces (LIPs) were synchronous with mass extinctions, and thus, the relationship between mass extinction events and the near‐simultaneous formation of the LIPs has generated much interest (Bond & Wignall, ; Burgess & Bowring, ; Courtillot & Renne, ; Kelley, ; Rampino & Caldeira, ; Rampino & Self, ; Saunders, ; Wignall, ). The Emeishan LIP (ELIP), which has a huge melt volume (1–3.8 × 10 6 km 3 ) and is associated with numerous large‐scale mineral deposits, is generally believed to have been emplaced in the late Middle Permian, at about 260 Ma (Fan et al, ; Guo et al, ; H. Huang et al, ; Shellnutt et al, ; C. Y. Wang et al, ; Y. G. Xu et al, ; Zhong et al, ; M. F. Zhou et al, , ). It is supposed to be coeval with the Capitanian mass extinction (259.8 ± 0.4 Ma, Henderson et al, ); however, although much effort has been expended to assess whether there was a causal relationship between them, it is still in dispute (Ganino & Arndt, ; Isozaki, ; Isozaki et al, ; Kofukuda et al, ; Sun et al, ; Wignall, Sun, et al, ; Wignall, Védrine, et al, ).…”