Magma Mixing Genesis of the Mafic Enclaves and Related Granitoids in the Kan Granite-Gneiss Complex of Central Côte d’Ivoire: Evidence from Geology, Petrology and Geochemistry

The mafic enclaves from Paleoproterozoic domain are considered to be the results of large-scale crust-mantle interaction and magma mixing. In this paper, petrography, mineralogy and geochemistry were jointly used to determine the origin of the mafic enclaves and their relationship with the host granitoids of the Kan granite-gneiss complex. This study also provides new information on crust-mantle interactions. The mafic enclaves of the Kan vary in shape and size and have intermediate chemical compositions. The diagrams used show a number of similarities in the major elements (and often in the trace elements) between the mafic enclaves and the host granitoids. Geochemical show that the Kan rock are metaluminous, enriched in silica, medium to high-K calc-alkaline I-type granite. The similarities reflect a mixing of basic and acid magma. Mafic enclaves have a typical magmatic structure, which is characterized by magma mixing. The genesis of these rocks is associated with the context of subduction. They result from the mixing of a mafic magma originating from the mantle and linked to subduction, and a granitic magma (type I granite) that arises from the partial melting of the crust.

Mineral Chemistry,Trace Elements,Isotopic Analysis and Zircon U-Pb Dating in the Hesar Pluton,Northern UDMA,Iran:Implications for Pre-Collisional Magma Mixing

The Hesar pluton in the northern Urumieh-Dokhtar magmatic arc hosts numerous mafic-microgranular enclaves(MMEs).Whole rock geochemistry,mineral chemistry,zircon U-Pb and Sr-Nd isotopes were measured.It is suggested that the rocks are metaluminous(A/CNK=1.32-1.45),subduction-related I-type calc-alkaline gabbro to diorite with similar mineral assemblages and geochemical signatures.The host rocks yielded an U-Pb crystallization age of 37.3±0.4 Ma for gabbro-diorite.MMEs have relatively low SiO_(2) contents(52.9-56.6 wt%)and high Mg^(#)(49.8-58.7),probably reflecting a mantle-derived origin.Chondrite-and mantle-normalized trace element patterns are characterized by LREE and LILE enrichment,HREE and HFSE depletion with slight negative Eu anomalies(Eu/Eu^(*)=0.86-1.03).The host rocks yield(^(87)Sr/^(86)Sr)_(i) ratios of 0.70492-0.70510,positive ε_(Nd)(t)values of+1.55-+2.06 and T_(DM2)of 707-736 Ma,which is consistent with the associated mafic microgranular enclaves((^(87)Sr/^(86)Sr)_(i)=0.705014,ε_(Nd)(t)=+1.75,T_(DM2)=729 Ma).All data suggest magma-mixing for enclave and host rock formation,showing a complete equilibration between mixed-mafic and felsic magmas,followed by rapid diffusion.The T_(DM1)(Nd)and T_(DM2)(Nd)model ages and U-Pb dating indicate that the host pluton was produced by partial melting of the lower continental crust and subsequent mixing with injected lithospheric mantlederived magmas in a pre-collisional setting of Arabian-Eurasian plates.Clinopyroxene composition indicates a crystallization temperature of~1000℃ and a depth of~9 km.

Petrology,geochemistry,and crystal size distribution of the basaltic andesite-dacite association at Mt.Sumbing,Central Java,Indonesia:Insights to magma reservoir dynamics and petrogenesis

Ten rock samples consisting of one pyroclastic density current(PDC1)deposit,seven lava flows(LF1–7),and two summit lava domes(LD1,2)were studied to understand the petrogenesis and magma dynamics at Mt.Sumbing.The stratigraphy is arranged as LF1,PDC1,LF2,LF3,LF4,LF5,LF6,LF7,LD1,and LD2;furthermore,these rocks were divided into two types.TypeⅠ,observed in the oldest(LF1)sample,has poor MgO and high Ba/Nb,Th/Yb and Sr.The remaining samples(PDC1–LD2)represent typeⅡ,characterized by high MgO and low Ba/Nb,Th/Yb and Sr values.We suggest that type I is derived from AOC(altered oceanic crust)-rich melts that underwent significant crustal assimilation,while typeⅡoriginates from mantle-rich melts with less significant crustal assimilation.The early stage of typeⅡmagma(PDC1–LF3)was considered a closed system,evolving basaltic andesite into andesite(55.0–60.2 wt%SiO_(2))with a progressively increasing phenocryst(0.30–0.48φ_(PC))and decreasing crystal size distribution(CSD)slope(from-3.9 to-2.9).The evidence of fluctuating silica and phenocryst contents(between 55.9–59.7 wt%and 0.25–0.41φ_(PC),respectively),coupled with the kinked and steep(from-5.0 to-3.3)CSD curves imply the interchanging condition between open(i.e.,magma mixing)and closed magmatic systems during the middle stage(LF4–LF6).Finally,it underwent to closed system again during the final stage(LF7–LD2)because the magma reached dacitic composition(at most 68.9 wt%SiO_(2))with abundant phenocryst(0.38–0.45φ_(PC))and gentle CSD slope(from-4.1 to-1.2).

The Applicability and Productiveness of a Systematic Literature Review Methodology in Volcanology Research: Case of the Magma Pathway at the Mount Cameroon Volcano

The presented research illustrates the applicability and productiveness of the systematic literature review methodology, a non-empirical methodology in the geological sciences, particularly volcanology. The systematic literature review methodology is a replicable, rigorous, and transparent methodology for synthesizing existing literature to answer questions on a specific topic. The synthesis allows for knowledge consolidation, such as identifying knowledge gaps. In our illustration of this methodology, we focused on the expanding knowledge about the magma pathway at Mount Cameroon, one of Africa’s active volcanoes. Our synthesis of the relevant international geoscience research literature is based on the framework of knowledge about the magma pathway beneath a typical basaltic volcano. The framework has three primary components: magma supply, storage, and transport to erupting vents. Across these components is a total of twelve secondary components. The result is a previously non-existent and fragmented overall understanding of the magma pathway at Mount Cameroon. The gaps in the understanding (such as in the magma supply rates, timescales of chamber processes, and magma ascent rates) may be addressed in future research. Another key implication of the presented research lies in the proof of concept of the systematic literature review methodology as an applicable qualitative research methodology in the study of volcanoes.

Source Enrichment Control on the Scale of Magmatic-Hydrothermal W-Sn Mineralization:Insights from Triassic and Jurassic Magma Reservoirs in the Continental Crust,Xitian,South China

There are two factors,source composition and magmatic differentiation,potentially controlling W-Sn mineralization.Which one is more important is widely debated and may need to be determined for each individual deposit.The Xitian granite batholith located in South China is a natural laboratory for investigating the above problem.It consists essentially of two separate components,formed in the Triassic at ca.226 Ma and Jurassic at ca.152 Ma,respectively.The Triassic and Jurassic rocks are both composed of porphyritic and fine-grained phases.The latter resulted from highlydifferentiated porphyritic ones but they have similar textural characteristics and mineral assemblages,indicating that they reached a similar degree of crystal fractionation.Although both fine-grained phases are highly differentiated with elevated rare metal contents,economic W–Sn mineralization is rare in the Triassic granitoids and this can be attributed to less fertile source materials than their Jurassic counterparts,with a slightly more enriched isotopic signature and whole-rockεNd(226 Ma)of−10.4 to−9.2(2σ=0.2)compared withεNd(152 Ma)of−9.2 to−8.2(2σ=0.2)for the Jurassic rocks.The initial W-Sn enrichment was derived from the metasedimentary rocks and strongly enhanced by reworking of the continental crust,culminating in the Jurassic.

基于MAGMA的铝合金发动机缸体低压铸造工艺优化

以铝合金发动机缸体的低压铸造过程为研究对象,应用MAGMA铸造模拟软件对充型及凝固过程进行模拟,将固相分数、充型温度、虚拟实验设计以及非热平衡状态的模拟结果和实际生产情况相结合,制定工艺参数,预测缺陷产生的位置,确定最佳工艺参数组合,优化生产工艺,保证质量稳定。

Magmatic Evolution of the Western Branch of the East African Rift System Melts: Evidence by Silicate Melt Inclusions, Rock Petrography and Geochemistry of the Nyiragongo 1977 and 2002 Lavas in DRC

Nyiragongo volcanic eruptions of 1977 and 2002 emitted silica-undersaturated lavas named melilite-nephelinites with microlithic to sub-porphyritic textures, and consisted of olivine, clinopyroxene (augite), phlogopite, melilite, magnetite, and rare plagioclases. This melilite-nephelinite as an evolved rock, shows low SiO2 (38.40 - 39.52 wt%) and MgO (3.10 - 4.01 wt%), and relatively high FeOt (13.76 - 14.10 wt%), Al2O3 (15.01 - 16.48 wt%), CaO (11.00 - 12.29 wt%) and Na2O + K2O (10.34 - 11.85 wt%). Unlike LA-ICP-MS on silicate melt inclusions (SMIs) hosted in augite show a pristine melt of picrobasaltic (low Ti-picrite) rock poor in SiO2 (31.14 - 32.26 wt%), FeOt (2.19 - 2.79 wt%), Al2O3 (8.01 - 9.57 wt%), and Na2O + K2O (2.34 - 3.05 wt%), while enriched in MgO (20.27 - 28.63 wt%), and CaO (24.95 - 33.17 wt%). The sums (∑REEs) for lavas and SMIs are ranging 712 - 799 and 43 - 119 ppm respectively. REE contracted multi-element patterns showed a W-feature for most lavas except for SMIs. High Rb/Sr, and low Ba/Rb, Zr/Nb, and Sm/Hf ratios of lavas suggest a phlogopite-rich source of materials. .

Magma source and tectonic setting of the Dunde granite in the Western Tianshan:constraints from geochronology,geochemistry,and Sr–Nd–Hf isotopes

The Dunde iron-zinc polymetallic deposit is one of large iron deposits occurred in the Awulale Metallogenetic Belt,Western Tianshan(NW-China).This study reports new geochronology and geochemistry for granite in the Dunde mining area in order to constrain the tectonicmagmatic activities and metallogenesis of this region.Granites in the southwest of Dunde mining area are mainly syenogranites intruded into volcanics of the Dahalajunshan Formation in the Early Carboniferous,and they are far from the area where ore bodies and mineralized altered rocks are widely developed.LA-ICP-MS U–Pb zircon dating indicates that Dunde syenogranite was at 306.8±1.0 Ma,which could constrain the upper limit of metallogenic age for this deposit.The Dunde granites are high SiO_(2)(73.41–80.07 wt%),high differentiation index(D.I.=89.7–95.0),weakly peraluminous to metaluminous(A/CNK=0.94–1.08),and they are enriched in LILE and LREE and depleted in Eu,Ba,Sr and P_(2)O_(5),indicating that they belong to highly fractionated Ⅰ-type granite.Based on εHf values(+9.2 to+10.5)for zircon and high εNd(-t)values(+4.7 to+5.8)for whole-rock,and the two-stage model ages for 601–735 Ma,suggest that the magma source could be the juvenile lower crust.Combined with regional geological setting,the 306.8 Ma Dunde granites are formed in post-collision extensional tectonic setting.

Peraluminous A-type granites formed through synchronous fractionation,magma mixing,mingling,and undercooling:evidence from microgranular enclaves and host Mesoproterozoic Kanigiri granite pluton,Nellore Schist Belt,southeast India

The field and microstructural features coupled with mineral chemical data from microgranular enclave(ME)and host Mesoproterozoic Kanigiri granite(KG)pluton of Nellore Schist Belt(NSB),Southeastern India,have been documented in order to infer the likely processes responsible for the origin and evolution of ME and host KG magma.The ME and host KG bear the same mineral assemblages barring the KG which does not contain amphibole;however,they are modally disequilibrated.The ME in KG is originated due to multiple intrusions of ME magmas into the crystallizing host KG magma chamber.Field and textural features indicate the dynamic magma flow,mingling,and undercooling of the ME against a relatively cooler surface of host KG magma.The presence of NSB country rock xenoliths and its diffuse boundaries suggest the intrusive relation and marginal assimilation by the intruding KG magma.The occasional cumulate texture in the ME appears to have formed by the accumulation of early-formed minerals that crystallized rapidly in the ME magma globules.The ME shows the magmatically deform features developed due to the flowage and erosion by the subsequent intrusions of ME magma pulses into the crystallizing host KG magma chamber.The ME amphiboles show unusual composition as ferro-edenitic hornblende to potassian-hastingsitic hornblende,that crystallized in the subalkaline-alkaline transition,low fO_(2)(reducing to mildly oxidizing)magma.The unusual extremely low Mg/Mg+Fe^(t)=0.015(avg.)of ME amphiboles may be related to the changing physico-chemical(P,T,fO_(2),and H_(2)O)condition of the ME magma or they might have crystallized in equilibrium with more evolved KG magma.The KG(FeOt/MgO=37.04,avg.)and ME(FeO~t/MgO=77.72,avg.)biotites are siderophyllite,and buffered between QFM and NNO syn-crystallizing in the water undersaturated(H_(2O)≈3.58 wt.%in KG;≈3.53wt.%in ME),alkaline anorogenic(A-type)host magmas that were emplaced at mid-crustal(4–5 kbar;17 km)depth.Field,microtextural and mineral chemical evidences suggest that the alkaline KG magma originated from crustal source and evolved through synchronous fractionation,mixing,and mingling with coeval ME magmas in the KG magma chamber.

Decoding the nature of interaction between felsic clasts and mafic magma in a subvolcanic magma chamber from amphibole–titanite transformation and chemistry

The Ghansura Rhyolite Dome of Bathani vol-cano-sedimentary sequence,eastern India,represents a subvolcanic felsic magma chamber that was invaded by crystal-rich mafic magma during its cooling phase to form an assortment of hybrid rocks.A prominent solidified portion of the magma reservoir was embedded in the intruding mafic magma as fragments or clasts that pro-duced mafic rocks with felsic clasts.Two distinct compo-sitional zones could be identified in the mafic rocks containing felsic clasts-(a)medium-grained mafic zones that are dominated by amphiboles,and(b)fine-grained felsic zones consisting primarily of quartz and feldspar.Amphiboles occur in most of the felsic clasts suggesting the mechanical transfer of crystals from the mafic to the felsic zones.Compositions of amphiboles were determined from both the mafic and felsic zones that show linear compositional variation from actinolite to ferro-hornblende through magnesio-hornblende,suggesting the interplay of complex substitutions in individual amphibole sites.Cationic schemes have confirmed the role of pargasite(Pg)-type substitution,which is a combination of edenite(Ed)-and tschermakite(Ts)-type substitutions.Moreover,amphibole has been extensively replaced by titanite in the studied rock.Titanite produced in the mafic zones due to the destabilization of amphiboles was observed migrating from the mafic to the felsic zones through mineral-trans-porting veins.Compositions of titanite were determined from grains that occur in association with amphiboles and those which are present as individual entities in the felsic zones.Similar to amphiboles,titanite also displays cationic substitutions in the studied rock.From the results presented in this work,we infer that extensive replacement of amphibole by titanite and cationic substitutions in amphi-boles,and also titanite,may be considered important pet-rogenetic indicators to decipher magma mixing events.

Textural and compositional zoning in plagioclase phenocrysts:implications for magma chamber processes in the Emeishan large Igneous Province,SW China

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes,such as magma differentiation,magma recharge and mixing,and crustal contamination.The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province(LIP)show complex textural and compositional zoning patterns,e.g.,normal,reverse,oscillatory,and patchy zoning patterns.Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern(Pl-A)with euhedral high-An cores(An=76–78%,in mole fraction)and low-An rims(An=68–72%),indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores.Some phenocrysts have a core–rim reverse zoning pattern(Pl-B)with irregular ovaloid cores,characterized by extremely low An(60–61 mol%)and Ba(84–88 ppm)contents and extremely high87Sr/86Sr ratios(0.7120–0.7130).The rims of the Pl-B have relatively high An(69–72%),Ba(~160 ppm)contents,and low87Sr/86Sri(~0.7056).These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas.Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern(Pl-C)with multiple oscillations of An(70–80%),Ba(88–147ppm)from core to rim,revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions.87Sr/86Sr ratios of the Pl-C do not vary significantly(0.7050–0.7054).A small portion of phenocrysts has patchy patterns in the cores(Pl-D),where the low-An patches(72–75%)in form of elliptical or irregular elongated shapes were enclosed by the high-An domains(80–87%).These features can be attributed to crystal dissolution and regrowth processes during the reaction between earlyformed low-Cumulates and recharged hot primitive melts.The cores,mantles,and rims of different types of plagioclase phenocrysts(except the core of Pl-B)commonly display nearly constant Sr isotopic compositions,implying insignificant wall-rock assimilation at shallow-level magma reservoir(s)during the growth of these plagioclase phenocrysts.In conclusion,the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP.

淮北石台煤矿接触变质煤速热碳化的微组构解译

作为岩浆吞噬煤层的信息记录者和物质承载者,接触变质煤是揭示煤层速热碳化过程和机理的重要研究对象。为此,本研究采集了淮北石台煤矿不同热变质程度的接触变质煤样品,利用镜质组反射率测试、元素分析、工业分析、偏光显微镜(PLM)和扫描电镜(SEM)等实验手段,表征其微组构特征,以期揭示煤层速热碳化的机理和过程。结果显示,从未受影响煤、浅热变质煤到天然焦,样品最大镜质组反射率、灰分产率升高,氢、氮元素以及挥发分含量降低。未受影响煤显微组分以胶质结构体为主,局部发育丝质体;浅热变质煤中裂隙发育、少见脱挥发孔;靠近岩体的块状天然焦中镶嵌结构炭和脱挥发孔发育,孔径多介于20~150μm;岩体内的细脉状天然焦,主要由多孔炭和炭微球组成,富含圆形—椭圆形气孔,孔径多介于1~3μm。分析表明,趋近岩体,接触变质煤热变质程度连续增加:浅热变质煤是煤层受较弱热变质而脆性断裂的结果;天然焦是浅热变质煤热解脱挥发分和中间相化的产物;至岩体附近,天然焦被液化成多孔炭和炭微球,后二者最终被氧化为气态碳氧化物而消失。我们认为,岩浆接触变质煤速热碳化的实质就是固态煤岩被岩浆热解而中间相化、液化和氧化气化的过程。

中国花岗岩型锂矿床:重要特征、成矿条件及形成机制

随着新能源汽车产业的快速发展和可控核聚变技术的不断革新,锂产业已成为各国竞相发展的新兴朝阳产业,全球锂资源争夺态势愈发激烈。中国是全球拥有花岗岩型锂矿床数量最多的国家,目前该类型锂矿占全国锂总供应量的1/4,进一步加强对该类型锂矿的研究和勘查对提升我国锂资源的自给能力至关重要。通过对全国40个主要花岗岩型锂矿床的时空分布、岩石序列、矿物演变、地球化学和锂独立矿物等特征的梳理,发现该类矿床主要形成于中生代晚侏罗世-早白垩世期间,主要集中在华南地区尤以华南腹地的南岭地区最为密集;近1/3的成矿岩体隐伏于地下,出露的岩体面积普遍较小,为0.07~5.7km^(2);花岗岩体垂直方向从下至上逐渐从黑云母二长花岗岩演变为锂云母钠长石花岗岩,其上通常发育云英岩和伟晶岩;锂及其他稀有元素(铷铯铍铌钽钨锡)一般在强钠长石花岗岩和云英岩中高度富集;黑云母二长花岗岩→含锂云母钠长花岗岩→云英岩,岩浆分异程度逐步增加且逐渐从熔体过渡到熔体-流体共存体系,全岩SiO_(2)含量、Nb/Ta和Zr/Hf比值、A/CNK和δEu值发生规律性变化;多数情况下岩体伴生煌斑岩脉、辉长岩脉、辉绿岩脉、花岗斑岩脉、细晶岩脉、霏细岩脉等各类脉体。总结规律提出:(1)伟晶岩壳是较早进入岩浆房内的岩浆与上覆围岩接触而冷却固结的产物,其构成岩浆房与外界的隔绝屏障,使Li、F、P、H_(2)O等易挥发组分无法逃逸;(2)较晚进入岩浆房的岩浆经历了长时间晶粥体与残留熔体的分凝过程,锂等稀有金属元素愈来愈富集于残余熔体中,最终形成富锂花岗岩和云英岩;(3)各类岩脉是岩浆房底部有持续热补给从而使残留含矿熔体能够逐步抽离上升的证据:基性岩脉来源于下伏高温地幔岩浆,不含矿中酸性斑岩脉是岩浆房底部堆晶体重熔的产物,含矿酸性斑岩脉和细粒岩脉来自岩浆房顶部富矿熔体-流体,三者沿地壳裂隙快速上升至地壳浅部就位。正是由于上覆伟晶岩壳形成的封闭体系和下伏幔源岩浆提供的充足热量,才使得岩浆房在热驱动机制下长期保持原地分异状态,最终形成花岗岩型锂矿床。

液压后盖铸造工艺优化及MAGMA模拟分析

以液压后盖铸件为研究对象,材质为QT450-10。针对液压后盖铸件在实际生产中的铸造缺陷,采用MAGMA模拟软件对液压后盖生产工艺进行模拟分析,分析缺陷位置,并进行工艺优化。研究结果表明:采用顶冒口补缩方案,可以解决铸件缩松问题,有效降低铸件废品率,铸件废品率整体从8%降低为0.8%。

从长英质侵入体揭示火山喷发过程与岩浆通道系统

汇聚板块边缘的大型长英质火山喷发是地球上最具破坏性的自然灾害之一。然而,与这些火山相关的岩浆通道系统的结构及其对火山活动的控制方式迄今仍知之甚少。基于在青藏高原北部东昆仑造山带三叠纪火山岩与侵入体关系的观察以及全球岩浆通道系统的研究进展,本文提出要将火山机构与侵入体结合起来开展研究,通过重建岩浆通道系统来揭示侵入体(古岩浆储库)与火山活动的潜在联系。本文指出,地球上只有少部分侵入岩与火山活动存在密切关系,这些侵入岩可能是抽取火山喷发物质后留下的岩浆储库残余物质。有些岩浆储库不仅为火山喷发提供物质,还在提供火山喷发动力、控制火山喷发方式在溢流式与爆炸式之间的转换以及影响火山机构定位等方面发挥着关键作用。富挥发分的流体大量溢出地表会减少大型伟晶岩型和斑岩型矿床的形成机会,特别是在爆炸式的长英质火山喷发过程中。本文将岩浆通道系统划分为5个区域,即部分熔融区、岩浆聚集区、岩浆迁移区、岩浆侵位区和火山喷口区,这些区域构成了一个复杂的纵向递进、横向连接的岩浆网络系统,是构造环境(构造阶段)、地壳状态和岩浆动力学属性的耦合产物。为了深入理解长英质岩浆通道系统结构和动力学演化及其对火山活动机制的控制,本文建议要将火山机构、侵入体和围岩系统作为岩浆通道系统的组成部分,开展构造地质学、岩石学、矿物学、岩浆动力学、地球化学、地球物理学等多学科的综合研究,以精细解剖典型岩浆通道系统的组成和结构,重建其岩浆动力学过程。

花岗伟晶岩分异演化与含矿性评价——从造岩矿物组构视角

高演化稀有金属伟晶岩的成矿过程是复杂和苛刻的,涉及到岩浆-热液过程热历史、岩浆迁移路径与成矿空间、侵位环境的过冷程度以及岩浆组分中稀有金属元素的初始富集等条件。而伟晶岩的分带特征是其形成过程的一种综合体现,不论是伟晶岩区域分带还是伟晶岩内部结构分带,都反映了伟晶岩岩浆结晶分异过程中岩浆的性质与形成环境,而这同时也赋予了造岩矿物在组构上的特殊性,形成遗传代码,对其进行解密,从而能够用于示踪与评估花岗伟晶岩的演化程度与成矿潜力。对于区域伟晶岩群的成矿潜力评估,由于含矿熔体在组分上的跨度较大,随着远离母体花岗岩体距离的增大,伟晶岩群演化程度增加,熔体中铁镁质组分以及Ca、Ba、Sr等碱土金属含量迅速降低,而挥发分、助熔剂组分、碱金属含量在残余熔体中逐渐增加,长石、云母与石英在类别和组构上产生显著变化;其中长石向钾-钠端元演化,且钠长石相对钾长石占据主导地位,云母由黑云母向白云母以及锂白云母转变,石英阴极发光特征与晶体结构及微量元素成分也表现出规律性的变化。受矿物晶格的限制,长石的K/Rb、K/Cs以及云母的K/Rb、K/Cs、Nb/Ta等比值特征能有效区分不同矿化潜力的伟晶岩群。石英的阴极发光特征能够揭示其生长环境与历史,石英的Li、Al、Ti、Ge等微量元素组成可以用来区分不同类型的伟晶岩矿床,并提供关于岩浆演化和成矿过程的重要线索。伟晶岩内部分带的形成主要受成矿熔体规模、过冷度以及成矿空间的封闭性等因素的控制,但在初始熔体成分上可以具有较大的变化。其中强分带型稀有金属伟晶岩记录了完整的岩浆-热液演化过程,不同阶段石英、长石和云母在晶体形态、微观结构以及微量元素组成上都具有显著变化。弱分带型稀有金属伟晶岩(钠长石-锂辉石伟晶岩)的造岩矿物组分在伟晶岩内部变化不大,其受构造控制明显,在空间上与贫矿伟晶岩之间可具有较大的矿物组成差异,表明二者之间存在流动分异过程。而长距离(以更厚的地壳或大型拆离断层为标志)的熔体迁移正是形成超大型伟晶岩稀有金属矿床的有利要素。

南秦岭早古生代碱性岩地球化学特征及其成因机制:研究进展与展望

南秦岭地区在早古生代经历了大规模且持续性的碱性岩浆活动,其丰富的碱性岩石记录为探索深部地幔物质组成、壳内岩浆演化过程以及地球动力学演化机制提供了珍贵的地质载体。文章基于团队近期对于区域上典型碱性岩的地球化学分析结果和成因机制探讨,旨在对南秦岭早古生代碱性岩浆的源区和演化过程进行全面地约束。南秦岭早古生代碱性岩石类型主要包括一套成分从碱性玄武质向粗面质变化,呈双峰式分布的碱性火山-侵入岩组合,以及少量与碱性硅酸岩(角闪辉石岩、正长岩)-碳酸岩共生杂岩体。岩相学、年代学和地球化学证据显示这些碱性岩具有相同的地幔源区,其中演化程度较低的镁铁质端元记录了南秦岭早古生代交代岩石圈地幔的部分熔融事件,交代介质主要为硅酸盐熔体。演化程度较高的碱性岩端元(粗面-正长岩、碳酸岩)来源于初始镁铁质组分的岩浆分异过程,其中粗面-正长岩类主要受到以长石和单斜辉石为主的分离结晶作用控制。中生代热液交代过程主要记录在北大巴山东部和武当地块西南缘的早古生代碱性岩体中,热液交代作用促进了碳酸岩杂岩体中稀土元素的富集成矿。副矿物年代学和独居石Nd同位素特征反映了热液可能形成于岩体本身的再活化事件,晚三叠世秦岭地区的造山运动可能对此过程具有促进作用。

宁芜矿集区多阶段火山作用岩浆源区的转变:来自侵入岩锆石Hf-O同位素特征的记录

宁芜玢岩型铁矿矿集区是中国东部重要的铁矿矿集区之一。宁芜矿集区在早白垩世135~126 Ma间连续发育了成分和源区相似的4组火山作用旋回,并伴随有两大类侵入岩发育:一类为辉石闪长玢岩-闪长玢岩,与玢岩型铁矿床的形成密切相关,主要发育于大王山火山旋回晚期(约131 Ma);另一类为花岗岩类侵入岩,形成于铁矿化之后,成岩时代与姑山和娘娘山火山旋回相近(130~126 Ma)。两类侵入岩源区的差异与联系目前尚未揭露。本次研究通过对两类侵入岩锆石年代学、Hf-O同位素特征的研究,区分了不同侵入岩源区差异。结果表明,与成矿母岩同旋回侵入岩锆石的δ^(18)O集中于6.0‰~6.5‰,εHf(t)集中于-6.0~-5.0;成矿后形成的花岗岩类侵入岩锆石的δ18O集中于7.0‰至8.0‰,ε_(Hf)(t)分布于-8.7~-1.2。结合前人研究,区内岩浆活动存在有富集地幔、太古代地壳和新元古代地壳3个岩浆源区,4个火山作用旋回的岩浆源区在130 Ma左右发生了改变。130 Ma之前的龙王山和大王山火山旋回的岩浆岩源区主要为受太古代地壳混染的富集地幔,而130 Ma之后的姑山和娘娘山火山作用旋回的岩浆岩源区主要为受新元古代地壳混染的富集地幔。多阶段火山作用中只有大王山旋回的闪长玢岩与玢岩型铁矿的形成有关。相比较其他火山旋回,大王山旋回具有更高的富集地幔组分以及更少的新元古代地壳的混染。因而,岩浆中较高比例的富集地幔含量是控制玢岩型铁矿形成的关键因素之一。

还原性矽卡岩型金矿床基本特征、研究现状及在中国前景

还原性矽卡岩型金矿以独立金矿形式赋存于钙质沉积岩中,以金品位高(5~15 g/t)著称。目前,该类型金矿的总体研究程度较低,在成矿背景、还原性岩浆源区及成因、金高效富集机制等方面仍缺乏理解。笔者对该类型金矿的地质特征、时空分布规律、成矿机理、找矿标识等方面进行了系统梳理,探讨了该类金矿的研究难点及其在中国的成矿潜力和找矿前景。还原性矽卡岩型金矿具有以下特征:(1)独特的Au-Bi-Te-As±Co元素组合,缺乏Cu等贱金属;(2)矽卡岩以钙铁辉石为主,其次为石榴子石;(3)成矿岩体属于还原性钛铁矿系列,包括辉长闪长岩、闪长岩和花岗闪长岩等;(4)代表性低硫逸度的金属矿物,包括斜方砷铁矿、黑铋金矿、贫S的Te-Bi矿物。关键问题或薄弱环节主要有:(1)还原性岩浆存在温度、盐度、氧逸度、硫逸度、含水量、还原性组分类型及含量等多种属性,这些属性对还原性矽卡岩型金矿的形成有哪些影响?(2) Au可能以氯络合物、硫络合物、Bi-Te熔体、纳米絮状物等多种方式运移,还原性岩浆流体中Au的高效迁移及富集方式有哪些?(3)还原性成矿岩浆的氧化-还原性质有争议,部分学者认为岩浆始终为还原性,另一些学者认为母岩浆为氧化性,后期混入还原性物质;(4)存在深部W-Mo矿化、浅部Au-Sb-Bi矿化的金属元素分带现象,这种分带规律受哪些因素控制?总之,还原性矽卡岩型金矿在中国有着良好的成矿潜力和找矿前景,其中中国东部W/Sn成矿区(带)和Au成矿区(带)的叠加区域、西部还原性岩体分布区是该类金矿的有利勘探区。

岩浆通道成矿核心内涵“两深一浅一通道”及其找矿意义

岩浆通道成矿模型的建立深刻改变了岩浆铜镍硫化物矿床成因研究的范式,成为该领域的研究前沿,而且已经成功地应用于找矿勘查中。该模型揭示了硫化物熔离与聚集成矿在空间上的不一致性以及成矿与相关玄武岩铂族元素强烈亏损的成因联系,很好地解释了为什么很多含矿岩体围岩贫硫化物的现象。笔者将该模型的核心内涵概括为“两深一浅一通道”,“两深”指成矿物质来源于地幔、硫化物熔离发生在深部,“一浅”指硫化物的聚集成矿主要发生在较浅的岩浆房,“一通道”指较稳定的岩浆通道更有利于成大矿。结合对典型矿床和成矿带的分析,笔者提出侵位较浅的、规模相对较大的岩体,其成矿潜力好于深部的岩浆通道相。因此,认为地表矿化并不是唯一可靠的、甚至不能作为最重要的找矿标志,要提高找矿效率(特别是隐伏矿床),首先要全面分析成矿带岩浆通道系统在不同区域的埋藏、剥蚀和暴露状况,目标岩体地质产状及其与围岩时代差异等宏观地质要素,查明深部通道相与浅部岩体空间分布和形成深度的差异,在此基础上,判断浅部岩体的位置、合理部署重点勘查区域、选择有效的勘查技术、定位目标岩体,进行钻探验证。