Occurrence of Mafic Rocks within Ediacaran Strata in the Aksu Region,NW Tarim Craton,and its Geological Implications

The Tarim Craton is an ancient Precambrian continental block,and detailed knowledge of its thermo-tectonic history is crucial for understanding the early history of continental evolution.Abundant layered mafic rocks,which have commonly been regarded as basalts,occur within the Ediacaran Sugetbrak Formation(Fm.)in the Aksu region of the northwestern Tarim Craton.Clear intrusive features have now been discovered,including mafic rocks truncating Ediacaran sedimentary layers,exhibiting an intrusion-baked margin where they interact with both the overlying and bottom wall rocks,and displaying a fine-grained transition zone from their interior to their margins.The new findings demonstrate that these mafic rocks within the Aksu Ediacaran strata were not erupted basalts but instead are intrusive diabase dykes.Therefore,these mafic rocks cannot be used to constrain the timing of the Sugetbrak Fm.in the Aksu area,nor as marker layers for regional stratigraphic correlation.Furthermore,the Ediacaran thermo-tectonic evolution in this region,deduced from the assumption that the mafic rocks are lavas,needs to be revised.

Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India:Implications for petrogenesis

The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory).

Baddeleyite and zircon U-Pb ages of the ultramafic rocks in Chigu Tso area,Southeastern Tibet and their constraints on the timing of Comei Large Igneous Province

A suite of ultramafic and mafic rocks developed in the Chigu Tso area,eastern Tethyan Himalaya.Baddeleyite and zircon U-Pb ages acquired by SIMS and LA-ICP-MS from olivine pyroxenite rocks in the Chigu Tso area are 138.9±3.0 Ma and 139.0±1.9 Ma,respectively.These two Early Cretaceous ages are similar with the ages of the more abundant mafic rocks in the eastern Tethyan Himalaya,indicating that this suite of ultramafic and mafic rocks in the Chigu Tso area should be included in the outcrop area of the Comei Large Igneous Province(LIP).These ultramafic rocks provide significant evidence that the involvement of mantle plume/hot spot activities in the formation of the Comei LIP.Baddeleyite U-Pb dating by SIMS is one reliable and convenient method to constrain the formation time of ultramafic rocks.The dating results of baddeleyite and zircon from the olivine pyroxenite samples in this paper are consistent with each other within analytical uncertainties,suggesting that baddeleyite and zircon were both formed during the same magmatic process.The consistency of baddeleyite U-Pb ages in the Chigu Tso area with zircon U-Pb ages for a large number of Early Cretaceous mafic rocks in the eastern Tethyan Himalaya further support that zircon grains from such mafic rocks yielding Early Cretaceous ages are also magmatic in origin.

The Evolution of Alpine Ultramafic Rocks and Partial Melting of the Upper Mantle

Ultramafic rocks of Tibet and Xinjiang are the products of partial melting of the upper mantle. The evolution of their mineral composition is marked by two parallel evolutionary series: one is the progressive increase of the 100 Mg/(Mg+Fe^(2+) ratio of silicate minerals in order of lherzolite→harzburgite→dunite, i.e. the increase in magnesium; the other is the increase of the 100 Cr/(Cr+Al) ratio of accessory chrome spinel in the same order, i. e. the increase in Chromium. The above-mentioned evolutionary trends are contrary to that of magmatic differentiation. The evolution of fabrics of ultramafic rocks is characterized by progressive variation in order of protogranular texture→melted residual texture, symplectic texture and clastophyritic texture→equigranular mosaic texture and tabular mosaic texture. Experiments of partial melting of lherzolite have convincingly shown that the evolution of Alpine ultramafic rocks resulted from the partial melting of pyrolite. Various subtypes of them represent different degrees of partial melting. The vertical zoning marked by more basic rocks in the upper part and more acid rocks in the lower actually belongs to the fusion zoning of pyrolite.

Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic(~1.1 Ga) magmatism of kimberlite and related rocks

Detailed mineralogy,bulk rock major,trace and Sr-Nd isotope compositions,and ^(40)Ar/^(39)Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India,is reported.Based on the presence of Ti-rich phlogopite,high Na/K content in amphibole,Al-and Ti-rich diopside,a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass,the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre(i.e.,aillikite).An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr-Nd isotope geochemistry.Sr-Nd isotope data are consistent with a common,moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India.A phlogopite-rich groundmass ^(40)Ar/^(39)Ar plateau age of 1115.8±7.9 Ma(2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053-1155 Ma emplacement ages of kimberlites and related rocks in India.The presence of ultramafic lamprophyres,carbonatites,kimberlites,and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle.The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic(ca.1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari,Australian,West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly.In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia.This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia.It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent,with the Indian block located on its periphery,could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic,volatile rich rocks in India due to comparatively low thermal effects from the distant plume.

Efficiency of a re-usable Carius tube for determination of platinum group elements in ultramafic rocks

Digestion with aqua regia in Carius tube is commonly employed for determination of PGEs in geological samples. However, silicates cannot be completely dissolved by aqua regia and PGEs might partially remain in silicate residue. In this study, an ultramafic reference material was used to investigate the efficiencies of a new re-usable Carius tube after digestion at 220 and 240℃, and an autoclave-lined Carius tube at 260 and 280℃. The results show that about 10% of PGEs retained in the silicate residues at 220 and 5% still r℃ emains even digestion temperature increases to 280℃. These results agree with previous works that increasing the digestion pressure and temperature can achieve more effective dissolution. Thus, a modified digestion procedure for determination of PGEs in ultramafic rocks by the re-usable Carius tube was proposed in this study.

KT Boundary Chromites Determined to be Terrestrial:Cr Isotopic Evidence for Excavation and Ejection of Mafic/Ultramafic Rocks by the KT Boundary Impact

Evidence for a mantle and/or basaltic component in KT boundary distal ejecta is apparently inconsistent with ejection from Chicxulub Crater since it is located on;5km thick continental crust（De Paolo et al.,1983;Montanari et al.,1983;Hildebrand and Boynton,1988,1990）.Evidence for mafic/ultramafic target rocks was reinforced by discovery of chromites,some with shock planar deformation features（PDF）,in impact layer samples from sites in southern Colorado and eastern Wyoming（Bohor et al.,1990）.However,until now it was unclear whether the chromites originated with an impactor or with terrestrial target rocks.To this end,high-precision 54Cr/52Cr isotope ratios were measured on KT boundary chromites along with known terrestrial chromites.We find a terrestrial 54Cr/52Cr ratio in KT boundary chromites from impact layer samples collected at the above sites over the last several years（Fig.1）.Ejected terrestrial chromites suggest the impact sampled terrestrial mafic and/or ultramafic target rocks not known to exist in the Chicxulub target area.

MINERALOGY AND GEOCHEMISTRY OF ULTRAMAFIC ROCKS FROM THE INDUS SUTURE OPHIOLITE IN SWAT, NW PAKISTAN

Genetically diverse lithologies, which include blueschist, greenschist, ultramafic, mafic plutonic, e.g. gabbros, and volcanic, e.g. pillowed basalts, and metasedimentary rocks, occur as a wedge\|shaped zone in the area to the north of the Mingora town in Swat, NW Pakistan. This zone of fault\|bounded rock bodies and blocks is distinguished into three principal groups, each of which is itself a composite of lithologies, or a melange—the blueschist melange, greenschist melange and ophiolitic melange. All these lie along the Main Mantle Thrust (MMT)—the westward extension of the Indus\|Tsangbo suture zone and the locus of collision between the Indo\|Pakistan plate and the Kohistan island arc—and, are therefore, collectively termed as the Main Mantle Thrust Melange Group (MMTMG).The ultramafic rocks, locally containing chromite pods and constituting a part of the ophiolitic member of the MMTMG, occur as small to large lenticular bodies, distributed along the marginal part of the Indo\|Pakistan plate. They are invariably serpentinized to varying degrees and, therefore, consist of abundant, fine grained serpentine which is mostly accompanied by subordinate amounts of completely bastitised, medium to coarse grained orthopyroxene, trace to accessory amounts of commonly altered (zoned with ferritchromite and/or magnetite) chrome spinel, variable proportions of olivine, and accessory amounts of diopside. In other words, the ultramafic rocks are mostly harzburgitic in mineralogy and, therefore, the ophiolite as a whole can be classified as the harzburgite sub\|type..Detailed field and mineralogical studies show that the present petrographic ch aracteristics and mineral\|chemical composition of these dominantly harburgitic rocks are the result of a number of phenomena involving partial melting, serpentinization, metamorphism and talc\|carbonate alteration.

Economical Potential of An Ultramafic Rock at The Kval ya Island, North Norway

A low-grade metamorphic ultramafic rock at the Kval ya Island, North Norway, shows Ni content up to 5 600 ppm, and an average content of 2 500 ppm Ni. Olivine is absent from the rock, and Ni is principally bonded in pentlandite, violarite and millerite. The rock might be considered as a low-grade ore, with 0.2wt%– 0.56wt% Ni. Other metals that might be of economic interest to extract from the rock include Co (from pentlandite and violarite), and Cr and Fe (from magnetite).

Genesis and Tectonic Implications of the Kabr El-Bonaya Ultramafic Rocks,Sinai Peninsula,Egypt:Constraints from Mineralogical and Geochemical Characteristics

The Kabr El-Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield(ANS).It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic complex.The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites(dunite,harzburgite and wehrlite)and pyroxenite.These rocks are generally unmetamorphosed and have intrusive contacts with the country rock.Mineral chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory suprasubduction tectonic settings.Based on the variations of the major elements,the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle,as it seems to have melted at 1–2 GPa and 1300–1450℃.Linear variations of Al2O3,Ca O,V and Ni with Mg O,coupled with incompatible and rare-earth-element depletion and mineral compositions,suggest prior events of partial melting in both wehrlites and harzburgites.The LREE enrichment in the harzburgite,as well as the development of Cr-rich spinel,is consistent with a history of melt–peridotite interaction.The calculated(Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite(~13–15 wt%of fluid)in the source,after initial spinel peridotite melting to the harzburgite(~20–25 wt%of fluid)in the same source,which is contrary to normal abyssal peridotites.The estimated equilibration temperature ranges from 1214 to 1321℃for the studied wehrlites and from 1297 to 1374℃for harzburgites.The Mg-rich nature of the analysed olivines from the studied ultramafic rocks(Fo=81.41 to 91.77)reflect their primary composition and are similar to olivines in Alaskan-type ultramafic rocks.The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite,reflecting a fractional crystallization trend.The high Cr#and very low TiO_(2)contents(0.03–0.12 wt%)of the Cr-spinels from the studied peridotites are mostly consistent with modern highly refractory fore-arc peridotites,indicating that these peridotites developed in a supra-subduction zone environment.

Olivine and Cr-spinel as indicators of the petrogenesis and partial melting conditions of the high-MgO ultramafic volcanic rocks from NW Ad Dhala Province—Yemen

The high-MgO ultramafic volcanic rocks in the NW Ad Dhala province are classified as meimechite according to the IUGS classification scheme.This province represents the southeastern outcrops of the Yemen Volcanic Group(YVG),which constitutes part of the AfroArabian continental large igneous province(LIP) and located within the boundary of the Afar mantle plume.In this study,we present the chemical compositions of olivine and Cr-spinel in meimechite rocks from Bagah Village in NW Ad Dhala province,aiming to characterize the genesis and partial melting conditions and to estimate the crystallization temperatures of these high-MgO rocks.Olivine crystals are characterized by high forsterite,ranges from Mg-rich core(up to Fo_(89.69)) to relatively Fe-rich rim(down to Fo_(78.57)),high CaO,MgO and MnO whereas Cr-spinel crystals have high TiO_(2) and Cr# values ranging from 0.49 to 0.63 which indicate that they are crystallized from primary magma and are typical of volcanic olivine and Crspinel that formed in intraplate tectonic setting.Olivine and Cr-spinel compositional data and primary melt composition(MgO ~ 23 wt.%) are compatible with the derivation of studied meimechite rocks from peridotite mantle source by small degrees of partial melting under conditions of high temperature and pressure at great depths,mostly within the garnet stability field.Also,these data provide a compelling indicator for the important role of upwelling Afar mantle plume in the genesis of these high-MgO ultramafic volcanic rocks.Finally,based on the different olivine-liquid equilibrium methods and Al-in-olivine thermometer approach the estimated crystallization temperature ranges from 1450 to 1490℃,and mantle potential temperature(Tp) ranges from 1617 to 1677℃,at high pressure(3-4.8 GPa).These high temperatures substantiate the existence of the Afar thermal mantle plume and its important role in the genesis of the studied meimechite rocks.

Geochemistry of Platinum Group Elements in Mafic-Ultramafic Rocks of Xinjie Intrusion

The platinum group elements (PGE) in the mafic ultramafic suite in the Xinjie layered intrusion and associated basalts and syenites were analyzed using neutron activation techniques after fire assay preconcentration. On this basis, the geochemistry of the platinum group during the magmatic stage is discussed. With respect to PGE distribution, the Xinjie layered intrusion is similar to the Bushveld ferruginous ultramafic series and is distinct from komatiite and Alpine type peridotite. It is also similar to the Emeishan basalt in PGE characteristics, implying that the original magmas of them may be of the same type.

The Fate of Chromium in Weathered Ultramafic Rocks and Their Derivitative Soils in Cuba: Clues from Spectroscopic Studies

In its cationic,trivalent form,Chromium(Cr)it is a micronutrient,and exhibits low environmental mobility.In hexavalent form,however,it is a human carcinogen and also highly mobile.Climate is a key environmental factor controlling weathering rates and stability of primary and secondary Cr-bearing minerals.Knowledge of Cr oxidation state and mineral residence is therefore essential to estimating the risk posed by Cr in serpentinites,chromite mine wastes,and soils developed on these parent materials.X-ray absorption spectroscopy(XAS)is currently the best available technique for determination of the relative abundance of Cr(III)and Cr(VI)in situ(that is,without digestion of solid phases).A brief review of relevant XAS studies of is presented below,focusing on studies in tropical climates1,as they will be most relevant to eastern Cuba’s extensively serpentinized ophiolite belt.Cr(III)-bearing spinels are usually the dominant and most refractory Cr host in ultramafic rocks.Previous XAS studies2 indicate that in tropical climates,Cr-spinels weather rapidly to form Cr(III)-bearing secondary Fe(III)(hydr)oxides(goethite,hematite).Manganese(Mn)is also enriched in ultramafic rocks2;as Mn(IV),it can also co-precipitate with Fe(III)(hydr)oxides,or form its own secondary(hydr)oxides.A previous study found up to 20%Cr(VI)in in a tropical,serpentine soil that contained substantial Mn,and a strong correlation between the*amounts of Cr(VI)and Mn(IV)in the soil profile2.Theresults of several XAS studies suggest that a close association of Mn(IV)and Cr(III)in secondary Fe(hydr)oxides is necessary for oxidation of Cr(III)to Cr(VI)via electron transfer reactions with Mn(IV);however,additional XAS studies have shown that organic matter3and Cr-bearing aluminosilicates4 may also be important sources of Cr(III)to the environment under specific conditions.The stability and fate of Cr has not been studied in detail for these two host phase types,to the best of our knowledge.Access to XAS facilities to perform Cr geochemical experiments is limited and will only become more so in the future.We are working to develop and apply(micro)Raman spectroscopy to evaluate Cr oxidation state and mineral residence(in crystalline and amorphous materials).In addition to standard Raman scattering,we are employing resonance Raman(785 nm laser)to enhance signal from Cr(VI)-bearing phases and laser-stimulated photoluminescence to identify Cr(III)associated with Al-rich alteration products

Petrochemistry and Mineral Chemistry Studies on Metamorphic Ultramafic Rocks in Yanghou Area, Northern Fujian, China

Petrochemical studies indicate that the Yanghou metamorphic ultramafic rocks are composed of metamorphosed harzburgite and ultramafic cumulate. Trace element geochemistry and mineral chemistry of the metamorphic harzburgite indicate that they are relicts of depleted mantle. Systematic petrochemical, mineral chemical and geochronological studies led to such a conclusion that the Yanghou metamorphic ultramafic rocks may be the components of Late Sinian Early Paleozoic ophiolites in South China.

Minerogenic Model of Chrysotile Deposits in Ultramafic Rocks

Most chrysotile deposits occur in ultramafic rocks of the ophiolite suite. The chrysotile deposits dis-cussed in the present paper were formed through metasomatism and infilling-crystallization in a continentalserpentinization environment after plate convergence, where ultramafic rocks were replaced byhydrothermal solutions consisting mainly of deep-circulating heated water derived from atmospheric precip-itation. The critical state for the formation of asbestos in ultramafic rock bodies might be reached bysuperposition of multiple stages of serpentinization. Favourable fracture systems and relatively stable geo-logical environment are important conditions for forming chrysotile deposits. Three subtypes of chrysotiledeposits could be formed in different tectonic settings and under different minerogenic geochemical condi-tions.

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block,China

The Neoproterozoic Sugetbrak Formation in the Aksu area,which is located at the northwest margin of Tarim Block,comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent.However,the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear.In this study,the petrology,geochemistry,and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections.The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase,containing a small amount of Fe-Ti oxides and with typical diabasic textures.All the mafic rocks in this region also showed similar geochemical compositions.They were characterised by high TiO_(2)contents(1.47%–3.59%)and low MgO(3.52%–7.88%),K_(2)O(0.12%–1.21%).Large ionic lithophile elements(LILEs)(Rb,Sr,and Cs)were significantly depleted.Meanwhile,high field strength elements(HFSEs)were relatively enriched.In the samples,light rare earth elements(LREEs)were enriched,while heavy rare earth elements(HREEs)were depleted.Based on the Zr/Nb,Nb/Y,and Zr/TiO_(2)ratios,the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks.The mafic rocks along the three typical sections showed similar initial values of^(87)Sr/^(86)Sr(I_(Sr))(0.7052–0.7097)and ε_(Nd)(t)(–0.70 to–5.35),while the Pb isotopic compositions with^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb and^(208)Pb/^(204)Pb values of 16.908–17.982,15.487–15.721,37.276–38.603,respectively.Most of the samples plot into the area near EM-Ⅰ,indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved.The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials.Combining with the regional geological background and geochemical data,we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment.Summarily,based on our study,the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source,and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.

Geology and Genesis of the Mafic-Ultramafic Complexes in the Huangshan-Jingerquan (HJ) Belt, East Xinjiang

More than twenty mafic-ultramafic complexes. which host several medium-or large-sized Cu-Ni deposits, occur along the Huangshan-Jingerquan (HJ ) belt in EastXinjiang. Rock types in these complexes are predominated by peridotite, pyroxene peri-dotite, olivine pyroxenite, gabbronorite, orthopyroxene gabbro, troctolite, gabbro anddiorite. The ultramafic rocks are relatively Fe-euriched and are characterised by an as-semblage of olivine + orthopyroxene + clinopyroxene + hornblend ±plagioclase withoutobvious metamorphic textures. Chemically these complexes are relatively Fe-enrichedand show a tholeiitic trend of evolution. The complexes in this belt are intruded underthe extensional environment in a Mid-Carboniferous back-arc basin. They call be consid-ered as a new type of mafic-ultramafic complexes in orogenic belts, as designated by thename of the East-Xinjiang-type complexes.

The Gaojiacun Mafic-Ultramafic Complex(Sichuan,SW China):Neoproterozoic Magmatic Activity at the Western Margin of the Yangtze Craton

The Neoproterozoic Gaojiacun intrusive complex （GIC） is composed of one big intrusion （ca. 9 × 7.5 km） and numerous satellite intrusions, emplaced in the low-grade schists of the Neoproterozoic Yanbian Group. The main intrusion is concentrically zoned, with peridotite bodies ＋ leucogabbroids in its core （inner zone） and the sequence： olivine hornblende gabbronorite - porphyric pyroxene hornblende gabbronorite （transitional zone） - pyroxene hornblende gabbronorite ＋ hornblende gabbro ＋ diorite （outer zone）, towards the margins. The satellite intrusions are composed either of a single rock type （peridotite, gabbro, diorite or granite） or of different rock types （peridotite ＋ diorite ＋ granite）. Upper crustal contamination is evidenced by the occurrence of numerous metamorphic enclaves, migmatites and mixed rocks in the outer zone of the main intrusion and in the satellite intrusions. Hornblende （often oikocrystic） is present in all mafic and ultramafic rocks. Cu-Ni （-PGE） deposits occur in several satellite intrusions, hosted in peridotite. The geochemical and mineralogical features of GIC support the presence of a subduction zone at the western edge of the Yangtze Craton. A feederrelated genesis of the satellite intrusions is inferred.

Geochemical Characteristics and Metallogenesis of the Qingkuangshan Ni-Cu-PGE Mineralized Mafic-Ultramafic Intrusion in Huili County, Sichuan Province, SW China

The Qingkuangshan Ni-Cu-PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni-Cu-PGE deposits in the Emeishan Large Igneous Province （ELIP）. The ore-bearing intrusion is a mafic-ultramafic body. This paper reports major elements, trace elements and platinum-group elements in different types of rocks and sulfide-mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore-forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle-normalized patterns of Ni-Cu-PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni-Cu-PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni-Cu-PGE deposits in the ELIP, the Qingkuangshan Ni-Cu-PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni-Cu deposit, and has similar PGE contents to the Yangliuping Ni-Cu-PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide-bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.

Late Carboniferous-Early Permian Mafic-Ultramafic Complexes in Beishan,Southwestern Central Asian Orogenic Belt and Their Significance

Numeral Permian mafic-ultramafic complexes occur in the Beishan terrane atthe northeastern margin of the Tarim craton,southwestern Central Asian Orogenic Belt,including the Pobei,Cantoushan,Bijiashan,Hongshishan,Xuanwoling,Zhongposhan and Luodong etc.,intrusions(Qin et al.,2011;Zhang et al.,2017;Xue et al.,2018).These Beishan mafic-ultramafic complexes are composed of peridotite,pyroxenite.