西天山吐拉苏盆地安山岩中发现岩石包体及地质找矿意义

新疆西天山斑岩型铜矿找矿前景良好,但面临攻坚;吐拉苏盆地是西天山浅成低温热液型金矿集区,晚古生代吐拉苏盆地是北天山洋向南向伊犁板块之下俯冲形成的“西南太平洋型”岛弧环境,在金矿深边部寻找斑岩型铜金矿值得关注。笔者最近在岛弧安山岩中发现硫化物矿化二长斑岩、细晶闪长岩等岩石包体。岩石包体发现于塔乌尔别克金矿区,多呈浑圆体或次棱角状,随机分布于安山岩中,与围岩界线截然,常见安山岩围绕岩石包体形成冷凝边。二长斑岩包体显示似斑状结构,斑晶由斜长石和钾长石组成;细晶闪长岩主要由斜长石和角闪石组成,细晶结构,块状构造;寄主安山岩为斑状结构,斑晶为斜长石和少量角闪石及单斜辉石,基质由短柱状斜长石及少量单斜辉石组成,玻基交织结构。二长斑岩包体、细晶闪长岩包体和寄主安山岩三者具有极为相似的微量元素和稀土元素组成,均富集大离子亲石元素、相对亏损Nb、Ta等高场强元素。这些岩石学和地球化学特点反映岩石包体是岛弧火山喷发过程中从下覆地质体捕掳成因,指示安山岩层之下存在同源斑岩体并可能已发生硫化物矿化。综合考虑吐拉苏盆地铜矿化点、酸性热液蚀变、高硫型和低硫型浅成低温热液金矿的组合关系,认为盆地内发育斑岩-浅成低温热液金铜成矿系统,塔乌尔别克是斑岩型铜金矿找矿重要靶区。     

Occurrence and significance of a quartz–amphibole schist xenolith within a mafic microgranular enclave in the Xiangshan volcanic-intrusive complex,SE China

The Xiangshan volcanic-intrusive complex is composed of rhyolitic crystal tuffs, welded tuffs, rhyodacite, porphyroclastic rhyolitic lava, subvolcanic rocks such as granite porphyry, and late quartz monzonitic porphyry and lamprophyre dikes. We report the first occurrence of a quartz–amphibole schist (QAS) xenolith enclosed within a mafic microgranular enclave (MME) in the Xiangshan volcanic-intrusive complex. The mineralogy of this xenolith consists of amphibole, biotite, quartz, and minor plagioclase. Petrographic and mineral composition studies indicate that the protolith of this xenolith likely originated from the metamorphic basement beneath Xiangshan. The amphibole (actinolite and magnesiohorblende) has been partially replaced by orthopyroxene at 800–1000°C and by diopside at <700°C, according to mineral thermometers; this replacement process may have taken place after the xenolith was trapped by the mafic magma host (now an MME). Studies of the QAS xenolith provide new information on the emplacement history of the mafic magma. The peak metamorphic temperature for amphibole replaced by pyroxene is higher than the crystallization temperature of the subvolcanic magma, which indicates that the heat of pyroxene formation must have been provided by the engulfing mafic melt. This magma must have emplaced to crustal level and trapped the QAS as a xenolith and then injected into the felsic magma. We suggested that the hybridization processes for the major elements of the pristine mafic magma may have been contaminated by crustal rocks to form its present composition of MME before mafic magma injection. However, the hybridization process appears not to have been formed via a single-stage process because various types of MMEs are presented in the Mesozoic magmatic rocks of SE China.     

Cryogenian magmatism and crustal reworking in the Southern Granulite Terrane,India

Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO₂-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (?5 to ?20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO₂ which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent.     

Geochemistry and C–O and Nd–Sr isotope characteristics of the 2.4 Ga Hogenakkal carbonatites from the South Indian Granulite Terrane: evidence for an end-Archaean depleted component and mantle heterogeneity

ABSTRACT

The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [δ¹³C_VPDB = ?6.7 to ?5.8‰ and δ¹⁸O_VSMOW = 7.5–8.7‰ except a single ¹⁸O-enriched sample (δ¹⁸O = 20.0‰)] represent unmodified mantle compositions. The εNd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive εNd values, close to CHUR (εNd = ?0.35 to 2.94) and named high-εNd group while the low-εNd group samples show negative values (?5.69 to ?8.86), corresponding to depleted and enriched source components, respectively. The ⁸⁷Sr/⁸⁶Sr_i ratios of the two groups also can be distinguished: the high-εNd ones have low ⁸⁷Sr/⁸⁶Sr_i ratios (0.70161–0.70244) while the low-εNd group shows higher ratios (0.70247–0.70319). We consider the Nd–Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.     

怀安麻粒岩锆石U-Pb年代学及Hf同位素：华北北缘下地壳增生再造过程研究

怀安杂岩体位于华北克拉通中北部,出露太古代-元古代的角闪岩相-麻粒岩相高级变质岩,记录了华北克拉通复杂的地壳形成和演化史.本文对其中的镁铁质麻粒岩和长英质麻粒岩进行了岩石学及锆石U-Pb年代学和Hf同位素组成研究,并在此基础上对其形成过程及所反映的华北北缘古老下地壳的增生再造过程进行讨论.镁铁质麻粒岩主要矿物组合为Cpx+ Pl+ Qz±Opx±Hb±Grt,粒状变晶结构,局部发育石榴子石的“白眼圈”现象,表明经历了高压麻粒岩相及后期近等温减压退变质作用过程.选择其中矿物组成为Cpx、Pl和少量Qz的单斜辉石麻粒岩进行锆石定年获得2461±20Ma的原岩形成年龄.这些岩浆锆石的εHf(t)为+2.1 ～ +9.7,最高值接近当时的亏损地幔值,其Hf亏损地幔模式年龄(tDM)集中在2.6～2.7Ga之间,最小为2412Ma和2461 Ma,接近原岩形成年龄.长英质麻粒岩主要组成矿物为Pl+ Kfs+ Cpx+ Qz+ Opx,粒状变晶结构.锆石测年获得2458±46Ma的原岩形成年龄.这些锆石εHf(t)范围为+2.9～ +5.3,锆石Hf两阶段模式年龄(t.ust)集中在2.8 ～2.7Ga之间.怀安麻粒岩的这些特征显示怀安地区在～2.5Ga发生亏损地幔的部分熔融,为单斜辉石麻粒岩提供了原岩物质,同时可能引起了先存(≥2.7Ga)地壳的重熔,即存在地壳再造,进一步为长英质麻粒岩提供了原岩物质.单斜辉石麻粒岩和长英质麻粒岩原岩都形成于～2.5Ga.     

Reintegrating nanogranitoid inclusion composition to reconstruct the prograde history of melt-depleted rocks

A recent fascinating development in the study of high-grade metamorphic basements is represented by the finding of tiny inclusions of crystallized melt(nanogranitoid inclusions) hosted in peritectic phases of migmatites and granulites. These inclusions have the potential to provide the primary composition of crustal melts at the source. A novel use of the recently-published nanogranitoid compositional database is presented here. Using granulites from the world-renowned Ivrea Zone(NW Italy) on which the original melt-reintegration approach has been previously applied, it is shown that reintegrating melt inclusion compositions from the published database into residual rock compositions can be a further useful method to reconstruct a plausible prograde history of melt-depleted rocks. This reconstruction is fundamental to investigate the tectonothermal history of geological terranes.     

Hot granulite nappes — Tectonic styles and thermal evolution of the Proterozoic granulite belts in East Africa

A section through the Neoproterozoic Mozambique Belt of Tanzania exposes western foreland (Archaean Tanzania Craton and Palaeoproterozoic Usagaran Belt), marginal (Western Granulites) and eastern, internal (Eastern Granulites) portions of the orogen. The assembly of granulite nappes at ca. 620 Ma displays westward emplacement along an eastward deepening basal decollement and forward propagation of thrusts, climbing from the deep crust to the surface. This goes along with eastward increase of syntectonic temperatures, derived from prevalent deformation mechanisms, and eastward decrease of the kinematic vorticity number. Distinctly different pressure - temperature paths with a branch of isothermal decompression (ITD) in Western Granulites and isobaric cooling (IBC) in Eastern Granulites reflect residence times of rocks within lower crustal levels. Western Granulites, exhumed rapidly at the orogen margin, display ITD and non-coaxial fabrics. Eastern Granulites in the internal orogen portions escaped from rapid exhumation and show IBC and co-axial flow fabrics. The vertical variation of structural elements, i.e. basement — cover relations within the Eastern Granulites, shows decoupling between lower and middle crust with horizontal west — east stretching in the basement and horizontal west — east shortening in the cover.A model of hot fold nappes [Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot orogens. In: Law, R.D., Searle, M.P., Godin, L., (eds). Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications. vol. 268, 91–145] is adopted to explain flow diversity in the deep crust. The lower crust represented by Eastern Granulite basement flowed coaxially outwards (westward) in response to thickened crust and elevated gravitational forces, supported by a melt-weakened, viscous channel at the crustal base. Horizontal flow with rates faster than thermal equilibration gave rise to isobaric cooling. Simultaneously the mid crust (Eastern Granulite cover) was shortened when hot fold nappes moved along upward climbing thrust planes. Western Granulites preserved isothermal decompression through exhumation by thrusting and coeval erosion at the orogen front.Two different styles define the Neoproterozoic East African Orogen between northern Egypt and southern Mozambique. The Arabian Nubian Shield in the north is classified as small and cold orogen in which thin — skinned thrusting was associated with lateral extrusion. The Central Mozambique Belt in Tanzania/Southern Kenya is classified as large and hot orogen characterized by thick-skinned thrusting and assembly of large granulite nappes.     

C-axis fabrics of quartz-ribbons during high-temperature deformation of syn-tectonic granitoids (Sila Massif, Calabria, Italy)

Late-Hercynian, high temperature (HT) shear deformation affected the zone straddling the contact between the Sila Massif batholith (above) and the host migmatitic paragneisses (below). A segment of the regional HT shear zone was investigated close to the town of Mesoraca, where a natural section allows one to analyse the solidus deformation of syn-tectonic granitoids. Shearing favoured the formation of polycrystalline quartz ribbons in deformed granodiorite and tonalite. Two main c-axis fabrics were found in the quartz-ribbons, different from each other by opening angles around Z between strong point maxima, in accordance with deformation under granulite to amphibolite facies conditions. These fabrics, along with microstructural observations, suggest that temperature of deformation played a key role and that ribboning was accompanied by the activation of (i) prism [c] slip and prism [c] + prism <a> slips, under granulite facies conditions, followed by (ii) basal <a> and prism <a> slips under amphibolite facies conditions.     

小兴安岭东南金山屯一带晚三叠世二长花岗岩成因及其地质意义

在小兴安岭东南端金山屯一带,广泛发育近南北向展布的晚三叠世似斑状二长花岗岩岩石组合,岩石中普遍见有暗色微细粒闪长质包体和中-基性脉岩群。对岩体及其包体、中-基性脉岩的宏观、微观特征和地球化学特征的研究表明,包体形态多呈浑圆的外形,显示出明显的塑性流变特点,具典型的岩浆结构包体中见针状磷灰石和含寄主岩钾长石、石英巨晶,具有明显的岩浆混合成因的包体特征;岩体的中-基性脉岩（群）形态多样,与花岗岩的界面或呈小波浪状或呈平直状,并见寄主岩长石斑晶,表现出壳幔混合作用形成的同深成岩墙群特征。该区晚三叠世二长花岗岩具显著的岩浆混合成因特征,其形成可能与碰撞后伸展动力学机制下的基性岩浆底侵作用有关。     

广西米场花岗岩及其暗色微粒包体的地球化学特征和成因分析

本文对博白-岑溪断裂带西南段的米场黑云母花岗岩体及其包体进行了岩石学和地球化学研究.结果表明,米场黑云母花岗岩体暗色包体广泛分布,暗色包体中发育有碱性长石斑晶、暗色镶边的眼球状石英、针状磷灰石和包裹有内环带的斜长石等.寄主岩及暗色包体两者都为准铝质、高钾钙碱性系列岩石,其主要氧化物含量在Harker图解中多呈直线变异趋...