Geochemical Characteristics And Significance Of Apatite From The Baima Intrusion In The Emeishan Large Igneous Province,SW China

The Emeishan Large Igneous Province（ELIP）in southwestern China hosts large areas of high and low-titanium basalts,which are closely associated with the genesis of Fe-Ti-V magnetite oxide deposits and Cu-Ni sulfide deposits.The Baima intrusion hosts a typical Fe-Ti-V magnetite deposit,which is located in the central Tibetan Plateau,surrounded by syenite and granite of the same period,and is a typical vanadium-titanium magnetite deposit in the western part of the Panxi region.The study of the Baima intrusion can provide important information on the genesis of Fe-Ti-V deposits in the Emeishan large igneous province,and reveal its magmatic origin and evolutionary process.Since the geochemical characteristics of apatite are extremely sensitive to changes in mineralization processes and are resistant to physical weathering,the major and trace elements,and Sr-Nd isotopes of apatite in troctolites are used to explore the genesis mechanism of the Baima intrusion.（1）It is shown that the apatite in the troctolite samples of the Baima intrusion is crystallized from silicate magma,and the volatile F and Cl contents range from 0.99 to 1.62 wt.%and 0.21 to 0.73 wt.%,respectively,which is typical of fluorapatite.The calculation reveal that the F content of the melt in equilibrium with apatite is less than the Cl content,since presumably F preferentially enters the apatite crystal,and the SO₃ content of the melt is not enriched,and the sulfur content of the melt has little influence on the mineralization of the Baima intrusion.（2）The relationships between the apatite total REE+Y content,Sr content,and Eu/Eu*values indicate that the apatite samples were all derived from Fe-Ti-P-rich（mineralised）layers.The Eu/Eu*and Ce/Ce*ratio of apatite from the Baima intrusion shows a negatively correlation,and a comparison with the Panzhihua,Hongge and Taihe deposits reveals that their parent magma is more reduced.（3）The apatite（La/Yb）_N,（La/Sm）_N,and（Sm/Yb）_N of the Baima intrusion show weak positive correlations with Sr content,reflecting that the fractional crystallization of plagioclase,and Sr elements in the inversed liquids show pronounced negative anomalies,indicating that plagioclase crystallized before apatite,and a large amount of plagioclase crystallization did not cause a significant loss of Eu in the melt and it is speculated that the Baima intrusion may have experienced multiple magma recharge.（4）The values of apatite(⁸⁷Sr/⁸⁶Sr)_i range from 0.7049 to 0.7055,with a mean value of 0.7052,and the values ofε_Nd（t）range from 0.27 to 2.95,with a mean value of 1.64.The results of mixing simulations show that the parent magma of the Baima intrusion experienced 20～40%contamination with marbles and less than 10%contamination with Precambrian metamorphic rocks.The isotopic heterogeneity between apatite and whole-rock Sr-Nd isotopes is likely to be related to the mingling of the siliceous marble.The relationship between Sr-Nd isotopes and（Sm/Yb）_N indicates that the Baima intrusion originated at the shallowest depth among Fe-Ti oxide-bearing intrusions including Panzhihua,Hongge and Taihe in Panxi.Therefore,it is hypothesized that the formation of Baima intrusion associated with Fe-Ti-V oxide ore deposit is in an open magmatic system and is constrained by various factors such as the crystallization of the parent magma,crustal contamination and multiple recharges of magma.