A geochemical perspective on the genesis of Cenozoic basic volcanism in northeastern Turkey:an overview of metasomatism and heterogeneity of the sub-continental lithospheric mantle in a post-collisional setting

The post-collisional Cenozoic basic volcanic rocks in NE Turkey show temporal variations in whole-rock lithophile element and highly siderophile element(HSE)systematics that are mainly associated with the nature of sub-continental lithospheric mantle(SCLM)sources and parental melt generation.So far,the traditional whole-rock lithophile geochemical data of these basic volcanic rocks have provided important constraints on the nature of SCLM sources.Integrated lithophile element and HSE geochemical data of these basic volcanic rocks also reveal the heterogeneity of the SCLM source,which is principally related to variable metasomatism resulting from previous subduction(s)and post-collisional mantle-crust interactions in an extensional setting.Lithophile element geochemical features suggest that the parental magmas have derived from metasomatized spinel-to garnet-bearing SCLM sources for Eocene and Miocene basic volcanic rocks with subduction signatures whereas originated from spinel-to garnet-bearing SCLM sources for Mio-Pliocene and Plio-Quaternary basaltic volcanic rocks without the subduction signature.Lithophile element and HSE geo-chemistry also reveal that Eocene and Miocene basic vol-canic rocks were affected by more pronounced crustal contamination than the basaltic volcanic rocks of Mio-Pliocene and Quaternary.Furthermore,the integrated lithophile element and HSE compositions of these basic volcanic rocks,together with the regional asymmetric lithospheric delamination model,reveal that the compositional variation(especially due to metasomatism)was significant temporally in the heterogeneity of the SCLM sources from which parental magmas formed during the Cenozoic era.

Hydro-uvarovite from Mantle Peridotites of Naga Hills Ophiolite: A Mineral Tracer for Neo-Tethyan Mantle Wedge Metasomatism

Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydrous uvarovite garnet having Cr_(2)O_(3) upto 19.66 wt% and CaO of 32.12–35.14 wt% in the serpentinized mantle peridotites of Naga Hills Ophiolite(NHO), India. They occur in association with low-Cr diopsides. They are enriched in LILE(Ba, Sr), LREEs, with fractionating LREE-MREE [avg.(La/Sm)_(N) = 2.16] with flat MREE/HREE patterns [avg.(Sm/Yb)_(N) = 0.95]. Raman spectra indicate the presence of hydroxyl(OH^(–)) peaks from 3500 to 3700 cm^(-1). Relative abundances in fluid mobile elements and their close association with clinopyroxenes are suggestive of the formation of uvarovite garnets through low temperature metasomatic alteration of low-Cr diopsides by hydrothermal slab fluids. The high LREE concentration and absence of Eu anomaly in the garnet further attest to alkaline nature of the transporting slab dehydrated fluid rather the involvement of low-p H solution. The chemical characteristics of the hydroxyl bearing uvarovite hosted by the mantle peridotite of NHO deviate from the classical features of uvarovite garnet, and their origin is attributed to the fluid-induced metasomatism of the sub arc mantle wedge in a suprasubduction zone regime.

Fluid-induced high-temperature metasomatism at Rundv?gshetta in the Lützow-Holm Complex, East Antarctica: Implications for the role of brine during granulite formation

We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundv?gshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex(LHC),East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite(K-feldspar+quartz+sillimanite+garnet+ilmenite) with foliation-parallel bands and/or layers of mafic granulite(plagioclase+orthopyroxene+garnet+ilmenite+quartz+biotite). The boundary between the two lithologies is defined by thin(about 1 -20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet+plagioclase+quartz+ilmenite+biotite ? orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe+Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850 -860℃, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapeliteemetabasite boundary(up to 40%), which is consistent with the field and thin section observations. The occurrence of brine(7.0 -10.9 wt.% Na Cleqfor ice melting or 25.1 -25.5 wt.% NaC leqfor hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite(Cl = 0.22 -0.60 wt.%) in the metasomatic rock compared to that in pelitic(0.15 -0.24 wt.%) and mafic(0.06-0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation.

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths (garnet peridotite,eclogite and clinopyroxene-phlogopite rocks) and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation (Phl1) occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phl1,within the garnet peridotite and clinopyroxeneephlogopitexenoliths,is characterised bylow Ti and Cr contents (TiO2<1 wt.%,Cr2O3<1 wt.% andMg#=100×Mg/(MgtFe)>92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr2O3 despite a wider range of TiO2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations (TiO2 > 2 wt.%,Cr2O3 > 1 wt.% and Mg#=100 × Mg/(Mg + Fe)< 92) and compositionally overlaps with phlogopite from polymict breccia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phl1 grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.

Early Devonian Ultrapotassic Magmatism in the North China Craton: Geochemical and Isotopic Evidence for Subcontinental Lithospheric Mantle Metasomatism by Subducted Sediment–Derived Fluid

The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.

The Sachakou Deposit in West Kunlun of Xinjiang: A Pb-Zn Polymetallic Deposit Associated with Magmatic Metasomatism of Carbonate Rock

Objective The Sachakou Pb-Zn polymetallic deposit is located in Hetian County, Xinjiang （geographical coordinates of E78° 57＇ 54.30＂-78°59＇ 53.63＂, N34° 39＇ 27.50＂-34° 40＇ 57.21＂）. It belongs to the West Kunlun orogenic belt on the northwest edge of the Qinghai-Tibet Plateau and is connected to the Sanjiang orogenic belt to the south （Spurlin et al., 2005）. In recent years, a series of Pb-Zn mineralized spots and deposits have been discovered in this area one after another, which is called the Huoshaoyun ore concentration area. Among them, the Sachakou Pb-Zn deposit has reserves up to140 Mt, which has reached a large scale. However, the study on the genesis of deposits in this area has only just begun. This work studied the genesis ofthis Pb-Zn deposit in order to provide new ideas for the genesis of regional deposits and regional prospecting.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

Geological Relationships Between the Intrusions,Country Rocks,and Ni-Cu-PGE Sulfides of the Kharaelakh Intrusion,Noril′sk Region:Implications for the Roles of Sulfide Differentiation and Metasomatism in Their Genesis

The Ni-Cu-platinum group element sulfide ore deposits of the Kharaelakh Intrusion,Noril′sk Region,Siberia,represent a large concentration of sulfides associated with a small differentiated intrusion formed at the edge of the Siberian Craton in the roots of the Siberian Trap flood basalt.The deposit is associated with an intrusion that occupies a flanking periclinal structure adjacent to the Noril′sk-Kharaelakh Fault.The intrusion is strongly differentiated and comprises taxitic gabbrodolerites,picritic gabbrodolerites,and gabbrodolerites within the main body which in turn forms a chonolith within a sheet-like intrusion that extends laterally to form extensive undifferentiated sills of gabbrodolerite.The intrusion substantially replaces the stratigraphy of the country rocks,and although it appears to have exploited the axis of structures developed in response to transtension,the intrusion has created space by both mechanical dilation of stratigraphy and magmatic replacement of pre-existing sedimentary rocks.The frontal lobes of the main intrusion have complex apophyses of gabrodolerite on a range of scales that demonstrate replacement of the sedimentary rocks and link to the development of an extensive metamorphic halo in the country rocks.This halo is much narrower over the main body of the intrusion,and these observations have implications for the thermal history of the intrusion.Mg-skarns and breccias are developed in the roof of the main body of the intrusion.Within the intrusion,the taxitic rocks contain vesicles and the blebby sulfides developed in the picritic and taxitic gabbrodolerites appear to have a linkage to volatile phases.Cuprous sulfide mineralization developed at the roof of the Kharaelakh Intrusion is associated with metamorphosed and skarn-bearing country rocks,and appears to have been generated by a combination of sulfide fractionation and associated metasomatism.The geological relationships appear consistent with a chonolith model for the development of the differentiated intrusion and mineralization,but the extent of metasmorphism of the country rocks appears to be related to the unusual thickness of gabbrodolerite apophyses at the flanks of the intrusion rather than metamorphism produced by the passage of mafic magma through the intrusion.Variations in disseminated sulfide compositions and metasomatic textures in the skarns are described,and a model is proposed which balances traditional views on the evolution of the magma conduits with the impact of magmatic fluids transported through the magma column(i.e.transmagmatic fluids).The importance of structures in controlling the nature of the conduit,and the resultant small intrusions with excess sulfide is a feature of many other Ni-Cu sulfide deposits including Voisey′s Bay,and it is suggested that the sulfides are more likely to have beentransported from depth into their final resting place rather than developed by in-situ equilibration of sulfide with fresh magma in the chonolith.

Metasomatism-induced wehrlite formation in the upper mantle beneath the Nógrád-G?m?r Volcanic Field(Northern Pannonian Basin):Evidence from xenoliths

Clinopyroxene-enriched upper mantle xenoliths classified as wehrlites are common(~20% of all xenoliths) in the central part of the Nograd-G(o| ")m(o|")r Volcanic Field(NGVF),situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia.In this study,we thoroughly investigated 12 wehrlite xenoliths,two from each wehrlite-bearing occurrence,to determine the conditions of their formation.Specific textural features,including clinopyroxene-rich patches in an olivine-rich lithology,orthopyroxene remnants in the cores of newlyformed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock.Based on the major and trace element geochemistry of the rock-forming minerals,significant enrichment in basaltic(Fe,Mn,Ti) and high field strength elements(Nb,Ta,Hf,Zr) was observed,compared to compositions of common lherzolite xenoliths.The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved.The composition of the metasomatic agent is interpreted to be a mafic silicate melt,which was further confirmed by numerical modelling of trace elements using the plate model.The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation.The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before xenolith entrainment in the host mafic melt.We suggest that the wehrlitization and the Plio-Pleistocene basaltic volcanism are related to the same magmatic event.

Geochemical signatures of potassium metasomatism in anthracite from the Himalayan fold‑thrust belts of Sikkim,India

The present study focuses on the inorganic geochemical features of the bituminous coal samples from the Raniganj and the Jharia Basins,as well as the anthracite samples from the Himalayan fold-thrust belts of Sikkim,India.The SiO_(2)content(48.05 wt%to 65.09 wt%and 35.92 wt%to 50.11 wt%in the bituminous and anthracite samples,respectively)and the ratio of Al_(2)O_(3)/TiO_(2)(6.97 to 17.03 in the bituminous coal samples and 10.34 to 20.07 in the anthracite samples)reveal the intermediate igneous source rock composition of the minerals.The ratio of the K_(2)O/Al_(2)O_(3)in the ash yield of the bituminous coal samples(0.03 to 0.09)may suggest the presence of kaolinite mixed with montmorillonite,while its range in the ash yield of the anthracite samples(0.16 to 0.27)may imply the presence of illite mixed with kaolinite.The chemical index of alteration values may suggest the moderate to strong chemical weathering of the source rock under sub-humid to humid climatic conditions.The plot of the bituminous coal samples in the A–CN–K diagram depicts the traditional weathering trend of parent rocks,but the anthracite samples plot near the illite feld and are a bit ofset from the weathering trend.This may imply the plausible infuences of the potassium-metasomatism at post coalifcation stages,which is further supported by high K_(2)O/Na_(2)O ratio(29.88–80.13).The Fourier transform infrared spectra further reveal the hydroxyl stretching intensity of illite in the anthracite samples substantiating the efect of the epigenetic potassium-metasomatism.The decrease in total kaolinite intensity/compound intensity of quartz and feldspar may provide additional evidence towards this epigenetic event.

K-and Si-Metasomatism, Mineral Transformation and Formation of Granitoids f rom Basic Rocks in Qooshchi Area,NW Iran:A Mineralogical Context

The Qooshchi area lies to northwest of Orumieh Lake in western Azerb ai jan, NW Iran. A basement metamorphic complex, consisting of Precambrian schists and gneisses, has been intruded by gabbros and diorites. Granitoids are grouped into five suites according to their mineralogy, texture and exposed features. Th e main body, pink Qooshchi granite, and apophyse like, myrmekite bearing grani toids are discussed in this paper. On the basis of field observations and micros copic studies, an intensive metasomatism has overprinted the country rocks, espe cially gabbros, transforming them into a more felsic composition. A prior ev ent of intensive deformation and cataclasis preceded the metasomatism, allowing the introduction of hydrothermal fluids. K metasomatism converted plagioclase i nto K feldspar (microcline), myrmekite, and sodic plagioclase as Si metasomati sm replaced the ferromagnesian silicates by quartz. Apophyse like bodies within gabbros, called leucometasomatites, are formed during this process.

Recycling process and proto-kimberlite melt metasomatism in the lithosphere-asthenosphere boundary beneath the Amazonian Craton recorded by garnet xenocrysts and mantle xenoliths from the Carolina kimberlite

Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite,Pimenta Bueno Kimberlitic Field,which is located on the southwestern border of the Amazonian Craton.We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite.Mantle xenoliths are mainly clinopyroxenites and garnetites.Some of the clinopyroxenites were classified as GPP–PP–PKP(garnet-phlogopite peridotite,phlogopite-peridotite,phlogopite-K-richterite peridotite)suites,and two clinopyroxenites(eclogites)and two garnetites are relicts of an ancient subducted slab.Temperature and pressure estimates yield 855–1102℃ and 3.6–7.0 GPa,respectively.Clinopyroxenes are enriched in light rare earth elements(LREE)(La_(N)/Yb_(N)=5–62;Ce_(N)/Sm_(N)=1–3;where N=primitive mantle normalized values),they have high Ca/Al ratios(10–410),low to medium Ti/Eu ratios(742–2840),and low Zr/Hf ratios(13–26),which suggest they were formed by metasomatic reactions with CO_(2)-rich silicate melts.Phlogopite with high TiO_(2)(>2.0 wt.%),Al_(2)O_(3)(>12.0 wt.%),and FeOt(5.0–13.0 wt.%)resemble those found in the groundmass of kimberlites,lamproites and lamprophyres.Conversely,phlogopite with low TiO_(2)(<1.0 wt.%)and lower Al_(2)O_(3)(<12.0 wt.%)are similar to those present in GPP-PP-PKP,and in MARID(mica-amphibole-rutile-ilmenite-diopside)and PIC(phlogopite-ilmenite-clinopyorxene)xenoliths.The GPP-PP-PKP suite of xenoliths,together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton.The Sr-Nd isotopic ratios of pyrope xenocrysts(G3,G9 and G11)from the Carolina kimberlite are characterized by high ^(143)Nd/^(144)Nd(0.51287–0.51371)and eNd(+4.55 to+20.85)accompanied with enriched ^(87)Sr/^(86)Sr(0.70405–0.71098).These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites.Based on Sr-Nd whole-rock compositions,the Carolina kimberlite has affinity with Group 1 kimberlites.The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9±5.4 Ma(2σ),which represents the cooling age after the proto-kimberlite melt metasomatism.Therefore,we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle(garnetites and eclogites);with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.

Phlogopite-Amphibole-Pyroxenite Xenoliths in Langao, Shaanxi Province, China: Evidence for Mantle Metasomatism

Phlogopite-amphibole-pyroxenite xenoliths contained in the alkali basic-ultrabasic subvolcanic complex in Langao, Shaanxi Province, are composed of diopside, Ti-rich pargasite, phlogopite apatite, sphene and ilmenite, which have subsolidus metamorp hism- de formation textures such as triple-points, cataclastic boundaries and kink-bands. Mineral chemical characteristics show that the diopside, Ti-rich pargasite and phlogoPite are derived from the mantle and are the products of mantle metasomatism. Compared with normal mantle-derivedspinel-lherzolites, the xenoliths are enriched in TiO2, Fe2O3, CaO, Na2O and K2O, with apparent depletion in MgO. Chondrite-normalized REE patterns and primordial-mantle normalized trace elements data show that they are enriched in REE (especially LREE) and incompatible trace elements. The petrographic, mineralogical and petrochemical characteristics indicate that the xenoliths are metasomatized mantle xenoliths, which offers the evidence for mantle metasomatism and represents the anomalous mantle beneath the Early Paleozoic rift in northern Daba Mountains. The agents of mantle metasomatism are probably derived from the rising of mantle hot plumes. The processes of metasomatism varied from limited-range fluid metasomatism in deep mantle (＞90 km) to pervasive metasomatism of silicate melt.

Sodic metasomatism in a dacite weathering profile in Pinxiang, Guangxi, China

The Pinxiang weathering profile is well developed on Early Triassic dacite lavas of the Baisi Formation. At the top of the profile is developed a red clay zone which is characterized mineralogically by kaolinite, iron oxide minerals, quartz, and a small amount of illite, montmorillonite and vermiculite. In going downwards the red clay zone gives way to a saprolite zone in which plagioclase pseudomorphs have been well preserved although replaced by kaolinite. Beneath the saprolite zone is the saprock zone characterized by less weathering for dacite. At the bottom of the weathering profile is the parent material, dacite, which is composed mainly of plagioclase, quartz, K-feldspar and biotite which have been largely altered into chlorite owing to submarine extrusion of dacite lavas. Some layers in the weathering profile show obvious sodium enrichment and potassium depletion relative to others. In the Al2O3-(CaO*+Na2O)-K2O triangular diagram, the weathering trends of these layers in the middle stage are remarkably deviated from normal ones. Both mineralogy and micro-morphology of these layers indicate such deviation resulted from sodic metasomatism of orthoclase.

Mantle metasomatism for metaperidotite from Shuanggou ophiolite of Yunnan Province by proton microprobe

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甘肃北山地区晚三叠世煌斑岩地球化学特征及构造意义

甘肃北山地区位于中亚造山带南缘,是研究中亚造山带构造演化的关键区域。煌斑岩多为岩石圈地幔在伸展背景下低程度部分熔融形成的碱性岩石,确定其形成时代和岩石成因可为区域构造演化提供新的依据。笔者对北山地区柳园南煌斑岩进行了系统的岩石学、地球化学和锆石U-Pb年代学及Hf同位素研究。柳园南煌斑岩的锆石U-Pb年龄为(228.2±1.1)Ma(晚三叠世)。煌斑岩中富含金云母和角闪石等富挥发性组分的矿物。岩石地球化学分析表明:柳园南煌斑岩属于超钾质煌斑岩,母岩浆为碱性岩浆系列;全岩微量元素具有明显的Nb-Ta和Zr-Hf负异常,锆石εHf(t)值为0.5~4.9,平均值为+2.8,具俯冲洋壳板片流体交代特征。柳园南煌斑岩中橄榄石Mn/Fe与Ca/Fe值对比表明,柳园南煌斑岩源区为富金云母的方辉橄榄岩地幔;微量元素模拟计算表明,地幔源区具有富集的特征。因此,柳园煌斑岩岩浆源区为被俯冲板片流体交代的岩石圈地幔。结合前人对中亚造山带南缘构造演化的研究,北山南部地区在晚三叠世时已进入陆内伸展阶段,减压作用促使被俯冲流体交代的岩石圈地幔发生低程度部分熔融,形成柳园煌斑岩脉。

交代地幔源区与造山带铜镍成矿作用

造山带具有复杂的动力学环境、俯冲交代历史和热力学状态,其地幔源区物质组成、熔融机制和岩浆产物明显不同于地幔柱和大陆裂谷带,因此造山带环境中与镁铁-超镁铁质岩浆有关的铜镍成矿作用有别于板内环境。本文综述了交代地幔源区与造山带铜镍成矿关系的新进展,重点聚焦于:(1)岩石圈与软流圈的贡献。成矿岩浆来源于上涌的软流圈驱动交代岩石圈地幔不同程度的混合熔融,其中后俯冲阶段岩浆产物的成矿作用最为普遍、规模最大;岩石圈地幔经历不同比例俯冲板片沉积物熔体(富钙沉积物)和低温蚀变洋壳流体交代,具有显著的富挥发分和碳酸盐特征。(2)幔源岩浆氧化还原状态对成矿的约束。显生宙造山带成矿初始岩浆(硫化物未发生饱和)可能普遍为氧化性岩浆,岩浆体系硫化物饱和前后存在氧逸度骤降,导致岩浆体系硫化物饱和时硫的溶解度呈指数级下降,直接触发硫化物熔离,或者为达到熔离创造低的硫溶解度条件,其中岩浆还原作用由碳质物在岩浆期的有效混染造成。(3)挥发分对岩浆硫化物的运载汇聚。挥发分显著降低熔体粘度,从而减小粘滞阻力对岩浆超压的消耗;岩浆在上侵减压过程中挥发分能够发生出溶-成核作用,形成大量低密度气泡并与硫化物液滴形成复合液滴,促进硫化物液滴聚结,提高其成矿元素含量,并向上输运硫化物。总之,全球部分典型铜镍成矿省与交代地幔时空耦合关系表明,地幔具交代属性可能一定程度上促进了岩浆硫化物成矿。尽管交代地幔对造山带铜镍成矿的贡献方面取得了诸多重要进展,但前寒武造山带幔源岩浆氧化还原状态对成矿的影响、造山带镍铜钴成矿多样性、经历不同交代作用的地幔源区熔融过程中亲铜元素的行为机制、壳幔圈层内挥发分循环对亲铜元素迁移-富集的控制作用等领域仍需深入研究。

钠交代作用对碱性花岗岩体系中稀土富集成矿的贡献

碱性岩是全球稀土资源的主要来源之一,特别是中-重稀土资源。针对碱性花岗岩体系中稀土(REE)和部分高场强元素(HFSE)超常富集机制及其控制因素等关键科学问题,目前尚缺乏深入系统的研究工作。本文重点关注碱性花岗岩成矿体系中的钠交代作用,对常见的钠长石化、钠铁闪石化和钠辉石化等热液交代过程中的矿物交代反应过程及其稀有稀土元素再活化-迁移过程开展分析和讨论。在碱性岩中,碱性角闪石和辉石被认为是REE和HFSE的重要载体,同时也是岩浆分异演化的有效示踪剂。以中国黑龙江碾子山碱性花岗岩为例,岩浆晚期的热液交代会导致碱性角闪石和辉石从富钙的矿物相向富钠的矿物相转变,在这个相转变过程中会导致REE和HFSE的活化和迁移。特别是岩浆晚期热液富钠霓辉石的形成过程中REE和HFSE会大规模释放到交代流体中,可能是导致这些关键金属元素在岩浆晚期超常富集和成矿的关键控制因素。因此,碱性角闪石和辉石的钠指数[Na^(#)=Na/(Na+Ca)摩尔比]可能是一个潜在的矿物地球化学指标,或将为碱性岩型稀土资源勘查提供新的方法。

希夏邦马峰地区矽卡岩型稀有金属矿化:岩石学特征、成矿时代及其地质意义

近年来,喜马拉雅稀有金属找矿工作和成矿研究不断取得突破,喜马拉雅已成为我国新的战略性矿产资源基地。本次研究对希夏邦马峰地区的吉隆岩体北部扎龙沟、岗布以及聂拉木北部等多处的矽卡岩进行了岩相学、矿物学和年代学研究,指出该区域广泛发育石榴子石、透辉石、符山石、绿帘石和角闪石等典型矽卡岩矿物。研究结果显示,扎龙沟矽卡岩具有明显的Li-Be-Sn矿化现象,成矿元素主要赋存在符山石(Li:469×10^(-6)~3162×10^(-6);Be:47.9×10^(-6)~2010×10^(-6);Sn:58.1×10^(-6)~1767×10^(-6))和云母(Li:2900×10^(-6)~3570×10^(-6))中,且云母中Rb(1572×10^(-6)~2364×10^(-6))、Cs(390×10^(-6)~749×10^(-6))、Ga(68.2×10^(-6)~111×10^(-6))元素含量较高;岗布矽卡岩以Li-Be矿化为主,赋存矿物为符山石(Li:833×10^(-6)~1624×10^(-6);Be:401×10^(-6)~1664×10^(-6);Sn:87.8×10^(-6)~181×10^(-6))和榍石(Sn:30340×10^(-6)~48600×10^(-6));聂拉木矽卡岩以Sn矿化为主,赋存矿物为石榴子石(92.3×10^(-6)~571×10^(-6))和绿帘石(86.0×10^(-6)~2003×10^(-6))。榍石U-Pb定年结果显示,扎龙沟矽卡岩形成于19~18Ma,岗布和聂拉木矽卡岩形成于24~23Ma,分别与已报道的钠长石花岗岩、电气石白云母淡色花岗岩时代一致,是岩浆出溶的热液流体交代围岩的产物,反映喜马拉雅稀有金属阶段性成矿特点。榍石Zr和黑云母Ti等温度计结果结合矿物成分表明,矽卡岩化过程中,热液流体温度从~680℃降低至~520℃,晚期均以氧化环境为主。进变质过程中脱CO 2导致原岩孔隙度增加,为后续成矿流体的加入和热量传输提供通道;退化蚀变过程中体积增大,岩石产生构造裂隙,控制成矿矽卡岩最终产出形态。综合研究表明,淡色花岗岩在藏南拆离系驱动下高度分异演化造成Li、Be、Sn等元素富集,晚期出溶的热液流体与碳酸盐岩围岩发生接触交代变质和矽卡岩化反应,是吉隆-聂拉木一带矽卡岩型Li-Be-Sn矿化的主要机制。我们指出,喜马拉雅造山带具有很好的矽卡岩型稀有金属成矿前景,未来应加强对接触变质带和围岩蚀变的研究,以寻找更多稀有金属矿床。

东天山维权银铜多金属矿控矿因素及成因分析

维权银铜多金属矿床位于新疆东天山地区,是具有典型特征的大型银铜多金属矿,大地构造划属觉罗塔格沟弧带中的雅满苏岛弧带。该矿床的地质特征、控矿因素、矿床成因长期存在争议。本次研究建立在多年野外地质勘查的基础上,对矿床的地质特征、控矿因素、矿床成因等进行了系统研究,认为银铜、铅锌矿体均分布于火山碎屑岩中,受地层控制的铅锌矿体被断裂构造控制的银铜矿体所切穿;铅锌矿成矿严格受地层层位控制,主要的蚀变类型为矽卡岩化,为接触交代成因类型,成矿时代为早石炭世晚期;银铜矿体形成于晚石炭世中晚期,矿体就位空间为断裂构造,矿体形态明显受断裂形态和性质所控制,有着明显的矿化边界,即断层面,认为其属岩浆期后远程中低温热液成因,成矿晚于铅锌成矿。