Origin Of The Fe-ti Oxide Mineralization And The Host Mafic-ultramafic Layered Intrusions In The Tarim Large Igneous Province, NW China

As an important manifestation of the thermal transfers in the Earth as well as the transfer of mantle materials from great depth to shallow level, mantle plumes are not only responsible for the generation of large igneous provinces, but are also associated with many economically important ore deposits. The Wajilitag and Piqiang layered intrusions, located in the Tarim large igneous province(LIP), NW China, host the giant Fe-Ti-V oxide ore deposits. Major Fe-Ti-V oxide ore layers occur within the upper part of the Wajilitag intrusion and the middle part of the Piqiang intrusion. So far, researches on the metallogenesis of the mafic-ultramafic layered intrusions in the Tarim LIP are scarce. In this paper, we use zircon U-Pb geochronology, mineral compositions, PGE geochemistry and Sr-Nd-Os isotopic compositions to understand the petrogenesis and the mechanism of the enrichment of Fe-Ti oxides, and to elucidate the genetic relationship bwtween the layered intrusion and silicic pluton.This study allows us to reach the conclusions as follows:(1) Relatively low olivine forsterite content(Fo= 76 to 78) and abundant ilmenite exsolution lamellae in clinopryroxene indicate that the Wajilitag and Piqiang parental magmas are highly evolved Fe-Ti-rich magmas. Calculations using average experimental Mg-Fe exchange coefficient and partition coefficients of trace elements between clinopyroxene and magmas show that the Wajilitag parental magma has higher Ti O2, Mg O/Fe O ratios and more fractionated mantle-normalized trace element patterns than the Piqiang parental magma. The estimated compositions of the parental magmas for the Wajilitag and Piqiang ore-bearing lithologies resemble those of the Bachu and Piqiang mafic dykes, respectively.(2) In comparison with the Tarim flood basalts, the Wajilitag and Piqiang intrusions have higher εNd(t)(+1.0 ~ +3.9 for Wajilitag and-3.1~ +1.0 for Piqiang)values and display OIB-like trace element signatures, are consistent with a derivation from a convective mantle source. Also, the least contaminated samples possess γOs(t)of 14 ~ 23, overlapping with the field of OIB. The Sr-Nd isotope variations between the Wajilitag and Piqiang intrusions are attributed to variable degrees of contamination with the upper crust. In this case, the Piqiang magma is thought to have experienced significantly higher degrees of contamination(~ 15-25%) than the Wajilitatg magma(< 10%).(3) The Mazaertag, Wajilitag and Piqiang layered intrusions in the Tarim LIPhave low PGE concentrations(<0.4 ppb Os, <0.7 ppb Ir, <1 ppb Ru, <0.2 ppb Rh, <5ppb Pt, <8 ppb Pd) and elevated Cu/Pd ranging from 2.2×104to 5.1×106. The low PGE concentrations of the rocks are attributed to PGE-depleted, parental magma that was produced by the low degrees of partial melting of the mantle. Strong positive correlations between IPGE and PPGE suggest that magmatic sulfide is the dominant mineral controlling the distribution of PGE. The sulfide saturation was likely triggered by late-stage magnetite precipitation.(4) There are two types of titanomagnetite in the Wajilitag intrusion: Cr-rich titanomagnetite commonly enclosed in either olivine or clinopyorxene, and Cr-poor titanomagnetite interstitial to silicate minerals. The titanomagnetites in the Piqiang intrusion are comparable with the Cr-poor titanomagnetites in the Wajilitag intrusion.It is possible that the Wajilitag Fe-Ti oxide ores may have been formed in the same fashion as the Fe-Ti oxide ores in the Piqiang intrusion. The primitive magma produced by the mantle plume underwent different degrees of evolution in deeper magma chambers, and thereafter formed different primitive Fe-, Ti-enriched parental magmas. After it intruded in shallow magma chamber, interstitial liquid immiscibility triggered by prolonged fractional crystallization facilitates the formation of Fe-Ti oxide ores.(5) The Wajilitag syenitic plutons are spatially associated with the maficultramafic layered intrusion. The SHRIMP zircon U-Pb dating shows that the Wajilitag mafic-ultramafic rocks and the syentitic rocks were formed at the virtually same time(282 ± 4 Ma and 278 ± 3 Ma). Linear trend between Y/Zr and Nd/Zr indicate the syenitic rocks probably share a common parental magma with the mafic-ultramafic rocks.