Petrogenesis and Metallogenesis of the Mazaertag Layered Intrusion in the Tarim Large Igneous Province,NW China

摘要

The Mazaertag layered intrusion is located in the northwestern part of the Tarim large igneous province where several early Permian layered mafic-ultramafic intrusions host important Fe-Ti oxide deposits. The intrusion covers an area of ～0.13 km^2 and has a vertical stratigraphic thickness of at least300 m. It consists chiefly of olivine clinopyroxenite, and is cut through by the nearby mafic-ultramafic dykes. In this paper, we report new mineral chemistry data and whole-rock chemical and isotopic compositions for the Mazaertag intrusion along with whole-rock isotopic compositions for the nearby mafic dykes. The averaged compositions of cumulus olivine, clinopyroxene and intercumulus plagioclase within individual samples range from Fo_(71-73),Mg~# = 76 to 79 and An_(65-75) but they do not define sustained reversals. The observed mineral compositions are consistent with the differentiation of a single batch of magma in a closed system. Rocks of the Mazaertag intrusion are characterized by enrichment in light REE relative to heavy REE, positive Nb and Ta anomalies and a small range of age-corrected ε_(Nd)(t)(-0.1 to +0.9) and initial ^(87)Sr/^(86)Sr values(0.7044 to 0.7068). The slightly lower ε_(Nd)(t), initial ^(206)Pb/^(204)Pb and higher initial ^(87)Sr/^(86)Sr values of the intrusion compared to those of the least contaminated dykes[ε_(Nd)(t) =+2.8 to +3.4;(^(206)Pb/^(204)Pb)_i = 18.516-18.521;(^(87)Sr/^(86)Sr)_i = 0.7038-0.7041] imply that the Mazaertag magma was subjected to small to modest degrees of contamination by the upper crust. The Sr-Nd isotopic compositions of the least contaminated dykes are consistent with derivation from a FOZO-like mantle source. The parental magma of the Mazaertag intrusion, estimated from clinopyroxene compositions using mineral-melt partition coefficients, has trace element compositions similar to some of the most primitive mafic dykes in the same area. This suggests that the Mazaertag intrusion and mafic dykes shared a similar mantle source. Therefore, the parental magma of the Mazaertag intrusion was interpreted to have originated from a mantle plume. Based on the Cr_2 O_3 contents in titanomagnetite and less-evolved characteristics of the Mazaertag intrusion compared to the Wajilitag Fe-Ti oxide deposit in Bachu, it is speculated that there might not be a potential to find economic Fe-Ti oxide mineralization in the intrusion.