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

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 Fo71-73,Mg^# = 76 to 79 and An65-75 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 εNd（t）（-0.1 to ＋0.9） and initial ^87Sr/^86Sr values（0.7044 to 0.7068）. The slightly lower εNdt）, initial ^206Pb/^204Pb and higher initial ^87Sr/^86Sr values of the intrusion compared to those of the least contaminated dykes[εNdt） =＋2.8 to ＋3.4;^206Pb/^204Pb）i = 18.516-18.521;（^87Sr/^86Sr）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 Cr2O3 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.

Crystal Growth and Crystallization Time Scales of the Panzhihua Layered Intrusion:Constraint from Crystal Size Distribution

The Panzhihua layered intrusions is generated closely related to the Emeishan LIPs.This paper analyzes the spatial distribution of plagioclase and pyroxene.The quantitative texture analysis of 2209 plagioclase shows that the characteristic length of plagioclase is 0.54 to 0.96 mm,the intercept variation range is large,from-0.67 to 0.96,and the slope is-1.85 to-1.04,the Aspect Ratio shows from 1.84 to 2.59 and fractal dimension D is 1.908–1.933.The quantitative texture analysis of 2342 pyroxene shows that the characteristic length of pyroxene is 0.38–0.64 mm,the intercept shows from 0.46 to 2.26,The slope ranges from-2.6 to-1.47,the Aspect Ratio value varies from 1.53 to 1.71,the fractal dimension D is 0.93 to 1.13.All the CSDs results of the Panzhihua intrusions indicate that plagioclase and pyroxene form in an open magma system and undergo four replenishment of magma injection.The plagioclase crystals do not grow as the lathlike shape,and the fractal growth leads to complex crystal surface.The plagioclase undergoes deformation compaction during the crystal process,and then is oriented.The pyroxene crystals grow along an approximately triaxial ratio and undergo texture adjustment and small crystal dissolution reabsorption.When all crystals in magma system grows up to 2 mm,the pyroxene undergoes cumulation in the Panzhihua layered intrusions.The plagioclase crystallization time scale is 171.23–304.41 years,representing that the crystallization is the more uniform in central part of the melt.The nucleation density continuously increases during the crystallization process of the magma system.The time scale to reach the final maximum crystal nucleation density is 15.28–58.98 years.

SHRIMP Zircon U-Pb Age of the Piqiang Layered Intrusion in the Tarim Large Igneous Province and Subducted Slab-Plume Interaction in Its Petrogenesis

Objective During the Permian, at least four mafic continental large igneous provinces （LIPs） were tbrmed in eastern Asia, i.e., the Siberian traps （-251 Ma）, Emeishan LIP （-260 Ma）, Tarim LIP （-290-270 Ma） and Panjal traps （-290 Ma） （Shellnutt et al., 2015）. The Emeishan and Tarim LIPs in China are both known for the presence of several magmatic Fe-Ti-V oxide deposits hosted in layered mafic- ultramafic intrusions. The origin of such magmatic Fe-Ti- V oxide deposits is enigmatic. One of the long-lasting debates is the mechanism by which large amounts of Fe-Ti oxides accumulated in the layered intrusions. Regardless of mechanism, there is still considerable debate regarding the mantle source compositions of the Fe-Ti-V oxide ore- bearing intrusions, in the Tarim LIP, a giant Fe-Ti-V oxide deposit is hosted by the Piqiang layered intrusion at the northern margin of the Tarim block. This intrusion consists mainly of gabbro and minor plagioclase-bearing clinopyroxenite and anorthosite （Fig. l a）. For this study we present new SHRIMP zircon U-Pb age and whole-rock geochemical data for the Piqiang layered gabbroic intrusion to evaluate the nature of its possible source compositions, which in turn aids in understanding the formation of the giant Fe-Ti-V oxide deposit in the plume- related LIPs.

Crystallization of Hydrous Ti-Rich Basaltic Magma and Its Implication for the Origin of Fe-Ti Oxide in Layered Intrusions of the Emeishan Large Igneous Province

A series of crystallization experiments have been carried out by using natural Emeishan Ti-rich hydrous basalts as starting materials at a pressure of 0.5 GPa and temperatures of 800-1000℃to constrain the origin of Fe-Ti-V oxide ore deposits.Our experimental results demonstrate that the sandwich-and trellis-type ilmenite lamellae in titanomagnetite of layered intrusions can be formed by the reaction of earlier crystallized ilmenite and the evolved parental magma.During evolution of parental basaltic magma,the Fe-Ti oxide should be composed of titanomagnetite+ilmenite in the earlier stage,but changed to titanomagnetite+titanomagnetite-ilmenite intergrowth±ilmenite at the later stage.Accordingly,the Panzhihua Fe-Ti oxide ores,which are mainly composed of titanomagnetite,should be formed earlier than the adjacent gabbro,in which titanomagnetite-ilmenite intergrowth is the major form of the Fe-Ti oxide.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block,southern India:From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean Northern Madurai block,Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block.The margins of these blocks are well-known for the occurrence of ultrahigh-temperature(UHT)granulite facies rocks mostly represented by Mg-Al metasediments.Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block.The major rock types of the layered intrusion include spinel orthopyroxenite,garnet-bearing gabbro,gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture.The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel,cordierite and orthopyroxene.The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions.Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980℃.We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases,which yield temperature above 1000℃.FMAS petrogenetic grid,Al-in-orthopyroxene isopleth,conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature(P–T)path and near isothermal decompression.The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca.2.0 Ga and the metamorphic overgrowths yield weighted ^(206)Pb/^(238)U mean ages ca.520 Ma.Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded ^(206)Pb/^(238)U weighted mean ages of ca.532 Ma and 523 Ma marking the timing of metamorphism.We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting,defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks.The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Petrology and geochemistry at the Lower zone-Middle zone transition of the Panzhihua intrusion,SW China:Implications for differentiation and oxide ore genesis

A sequence of gabbros showing isotropic,layered and fine-grained textures is exposed in the Nalaqing mine at the southern tip of the～260 Ma Panzhihua intrusion,SW China.The field relations,structure,texture and mineralogy of the rocks indicate that the sequence represents the transition between the Lower zone and Middle zone of the intrusion.Isotropic gabbros characteristic of the Lower zone pass upward to layered gabbros of the Middle zone through a～5 m-thick microgabbro sheet,within and close to which small-scaled, concordant Fe-Ti oxide ore horizons are identified.Strong fractionation between HFSE and REE in a subset of samples is ascribed to cumulus titanomagnetite into which HFSE are preferentially incorporated over REE,as reflected in the parallel relations between Nb/La,Hf/Sm and Ti/Ti＊.Both the isotropic and layered gabbros display cumulate textures and have similar mineral compositions（Mg# of clinopyroxene =～76-79 and An59-61）,isotopic compositions[（87Sr/86Sr）i = 0.7044-0.7045 andεNd（t） = ＋2.4 to ＋3.9]and trapped liquid contents inferred from Zr abundance（～17-34 ppm）.However,there are substantial variations in elemental abundances（V,Cr and PGE） and ratios（Ti/V,La/Yb,Ba/Y and Cu/Pd） between the two types of gabbros,features that cannot be explained by cumulate formation from a common magma in a closed system.The microgabbros generally resemble high-Ti Emeishan basalts in major element compositions,but their low trace element abundances indicate some lost of residual liquid is inevitable despite rapid nucleation and cooling.Combined with available data and observations,we propose a model involving in-situ crystallization,followed by magma recharge and closed-system fractionation to explain the formation of texturally distinctive gabbros at Nalaqing and the evolution of the lower part of the Panzhihua intrusion.

Phase Equilibria Constraints on Relations of Ore-bearing Intrusions with Flood Basalts in the Panxi Region,Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province （LIP）; namely, small ultramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element （PGE）-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis （Q-PI-OI-Opx-Cpx system） has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts （including high-Ti, low-Ti and alkali basalts）, respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basalts. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.

Two stages of immiscible liquid separation in the formation of Panzhihua-type Fe-Ti-V oxide deposits,SW China

Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province（ELIP）,SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-（P） oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.

Formation of transgressive anorthosite seams in the Bushveld Complex via tectonically induced mobilisation of plagioclase-rich crystal mushes

The formation of anorthosites in layered intrusions has remained one of petrology＇s most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a ＂pothole＂ in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates.

Physicochemical Control of the Early Permian Xiangshan Fe-Ti Oxide Deposit in Eastern Tianshan(Xinjiang),NW China

The Xiangshan mafic-ultramafic complex is one of the major Early Permian maficultramafic intrusions in eastern Tianshan （Xinjiang, NW China）, and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian （280.1 and 279.2 Ma, respectively）. Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce^4＋/Ce^3＋ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, HEO-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization.