THREE Fe-Ti OXIDE ORE-BEARING GABBRO-GRANITOID COMPLEXES IN THE PANXI REGION OF THE PERMIAN EMEISHAN LARGE IGNEOUS PROVINCE, SW CHINA

摘要

The Permian (～260 Ma) Emeishan large igneous province of SW China contains three nearly identical gabbro-granitoid complexes that host giant Fe-Ti oxide deposits. The Fe-Ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with A-type granitic plutons. The 264 ± 3Ma Taihe layered gabbroic intrusion hosts a large magmatic Fe-Ti oxide deposit and is coeval with the Taihe peralkaline, A-type granitic pluton, which is dated at 261 ± 2 Ma. Within the A-type granitic pluton are microgranular enclaves, which have compositions intermediate between the gabbro and host granite. Primitive mantle-normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks. The chondrite-normalized REE patterns show Eu-anomalies changing from positive (Eu/Eu~* -=1.5 to 5.9) in the gabbroic intrusion to negative in the enclaves (Eu/Eu~* = 0.4 to 0.6) and granites (Eu/Eu~* = 0.2 to 0.5). Whole rock -Nd(T) values of the gabbroic intrusion (εNd_(T) =2.5 to +3.3) are similar to those of the enclaves (εNd_(T) =1.0 to 2.0) and granite (εNd_(T)=1.5 to +1.9) whereas the zircon -Hf(T) values of the gabbro (εHf_(T)=8.1 + 0.8) are indistinguishable from those of the granites (εHf_(T)=9.2 +1.0), suggesting that the parent magmas for all rock types originated from the same mantle source. Geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high-Ti Emeishan flood basalt. The compositional jump from the gabbro to the enclaves is attributed to the crystallization of Fe-Ti oxide minerals. The results of this study and other studies suggest that the magmatic conditions (for example, pressure, composition, fO_2), which led to the formation of at least three Fe-Ti oxide bearing gabbro-granitoid complexes, were relatively common during the development of the Emeishan large igneous province.