Estimating the Parental Magma Composition and Temperature of the Xiaohaizi Cumulate-Bearing Ultramafic Rock: Implication for Magma Evolution of the Tarim Large Igneous Province, Northwestern China

The ultramafic dikes in the Tarim large igneous province（Tarim LIP）, exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo（Mg/（Mg＋Fe）） numbers ranging from 73 to 85, which are not in equilibrium with the olivine（Mg# of 89） from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid（parental magma） composition of SiO2 of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%– 18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al2O3 of 8.70 wt.%–11.62 wt.% and TiO2 of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike（ca. 45%–60% in the whole rock）. The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 oC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.

Porphyry Cu deposits linked to episodic growth of an underlying parental magma chamber

Saindak is one of the typical porphyry Cu deposits(PCDs)in the Chagai magmatic arc in Pakistan.Ore-forming porphyries at Saindak PCD are mainly composed of tonalite.Here,we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization.Although apatite inclusions have homogeneous intra-grain S and Cl compositions,there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase.Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An,FeO and Mg contents of the hosting plagioclase phenocryst.It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber,rather than by one major injection or by addition of mafic melt derived from different source region.Each primitive melt injection introduced essential ore-forming materials such as S and Cl,which were rapidly and effectively released to the coexisting fluids,causing mineralization.Once primitive melt injection stops,signaling the end of growth of underlying magma chamber,mineralization will cease quickly although the hydrothermal system can still survive for a long time.However,the later released fluids are relatively depleted in ore-forming materials,and thus have lower capability to generate mineralization.Accordingly,predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system.

Hydration vs.oxidation:Modelling implications for Fe-Ti oxide crystallisation in mafic intrusions,with specific reference to the Panzhihua intrusion,SW China

Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion：（1） low-Ti,high CaO and low H2O（0.5 wt.%） parent magma（equivalent to Emeishan low-Ti basalt） at FMQ;and（2） high-Ti,low CaO and higher H2O（＆gt;1.5 wt.%） parent magma（equivalent to Emeishan high-Ti basalt） at FMQ ＋ 1.5.Modelling of these parent magma compositions produces significantly different results. We present here detailed f（O2） and H2O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultra-mafic -mafic layered intrusions.Modelling is consistent with numerous experimental studies on ferro-basaltic magmas from other localities（e.g.Skaergaard intrusion）.Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe-Ti oxides ore layers.We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H2O contents（0.5-1 wt.%） but at the lower end of TiO2 content for typical high-Ti basalts（2.5 wt.%TiO2）. Distinct silicate disequilibrium textures in the Fe-Ti oxide ore layers suggest that an influx of H2O may be responsible for changing the crystallisation path.An increase in H2O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid.Continued cooling of the magma with high H2O then results in precipitation of Mt-Uv alone. The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable.H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f（O2） Mt -Uv crystallises earlier than plagioclase and clinopyroxene.Furthermore,the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe-Ti oxide ore layers.