五台山新太古代绿岩带中高镁安山岩（HMA）的化学特征

采用高镁安山岩（HMA）新的分类方法,描述了五台地区新太古代绿岩带中HMA的化学特征,并与实验的HMA以及自然界中典型的HMA（日本的Chichijima地区HMA和Setouchi地区HMA）进行了对比,显示该区HMA具有与上述HMA类似的化学特征。初步认为五台新太古代HMA产于洋俯冲带的环境,进而表明板块构造理论的适用时间范围可推演到新太古代。     

龙门山断裂带南段地壳一维P波速度结构

基于2009年1月1日至2013年5月6日四川地震台网、重庆地震台网记录的龙门山断裂带南段587个地震的5 012个P波到时数据,利用最小一维速度模型方法反演了龙门山断裂带南段地壳一维P波速度模型及台站校正值,并将其应用到龙门山地区地震重定位中。结果表明,台站校正值表征出龙门山断裂带南段地表速度结构的横向不均匀性,青藏高原的彭灌杂岩体及宝兴杂岩体在近地表表现为高速异常,而四川盆地的第四纪沉积表现为低速异常。重定位后地震震源在北西向的剖面上呈明显的条带状并向北西倾斜,该地震带与宝兴杂岩下方的滑脱带延伸趋势一致。此外,该地震带上方分布着一条反冲地震带,两地震带呈"y"型分布,这可能是宝兴杂岩上方的岩层为调节逆冲过程受阻而产生的反冲运动所致。     

Petrogenesis and geochemical characteristics of plagiogranites from Naga Ophiolite Belt,northeast India: Fractional crystallization of MORB-type magma

The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO₂ (51.81–56.71 wt.%), low K₂O (0.08–1.65 wt.%) and high Na₂O (4.3–5.03 wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K₂O, high Na₂O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO₂ concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1 wt% of TiO₂ and the ternary plots of A (Na₂O + K₂O) F(FeO^T) M(MgO) and TiO₂-K₂O-SiO₂/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (La_N/Sm_N: 2.37–3.62) and flat HREE (Eu/Eu*: 0.90–1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50–100 times chondrite with slight enrichment of LREE (La_N/Sm_N = 2.21–3.13) and flat HREE (Eu/Eu*: 0.94–1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge.     

Dating the giant Zhuxi W–Cu deposit (Taqian–Fuchun Ore Belt) in South China using molybdenite Re–Os and muscovite Ar–Ar system

The recently discovered Zhuxi W–Cu ore deposit is located within the Taqian–Fuchun Ore Belt in the southeastern edge of the Yangtze Block, South China. Its inferred tungsten resources, based on new exploration data, are more than 280 Mt by 2016. At least three paragenetic stages of skarn formation and ore deposition have been recognized: prograde skarn stage; retrograde stage; and hydrothermal sulfide stage. Secondly, greisenization, marmorization and hornfels formation are also observed. Scheelite and chalcopyrite are the dominant metal minerals in the Zhuxi deposit and their formation was associated with the emplacement of granite stocks and porphyry dykes intruded into the surrounding Carboniferous carbonate sediments (Huanglong and Chuanshan formations) and the Neoproterozoic slate and phyllites. The scheelite was mostly precipitated during the retrograde stage, whereas the chalcopyrite was widely precipitated during the hydrothermal sulfide stage. A muscovite ⁴⁰Ar/³⁹Ar plateau age of about 150 Ma is interpreted as the time of tungsten mineralization and molybdenite Re–Os model ages ranging from 145.9 ± 2.0 Ma to 148.7 ± 2.2 Ma (for the subsequent hydrothermal sulfide stage of activity) as the time of the copper mineralization. Our new molybdenite Re–Os and muscovite ⁴⁰Ar/³⁹Ar dating results, along with previous zircon U–Pb age data, indicate that the hydrothermal activity from the retrograde stage to the last hydrothermal sulfide stage lasted up to 5 Myr, from 150.6 ± 1.5 to 145.9 ± 1 Ma, and is approximately coeval or slightly later than the emplacement of the associated granite porphyry and biotite granite. The new ages reported here confirm that the Zhuxi tungsten deposit represents one of the Mesozoic magmatic–hydrothermal mineralization events that took place in South China in a setting of lithospheric extension during the Late Jurassic (160–150 Ma). It is suggested that mantle material played a role in producing the Zhuxi W–Cu mineralization and associated magmatism.     

The oldest Mo porphyry mineralization in the Yangtze Valley Metallogenic Belt of eastern China: Constraints on its origin from geochemistry,geochronology and fluid inclusion studies at Matou

The Matou Mo(-Cu) deposit, located in the Yangtze Valley Metallogenic Belt of central-eastern China, is a typical porphyry-type Mo deposit. The orebodies at the deposit are hosted by Matou porphyritic granodiorite, which is the largest intrusive in the area. Quartz vein-type and disseminated sulfide mineralization are well developed in the porphyry and near its contact with Silurian sandstone. Crosscutting relationships indicate that porphyritic granodiorite is the oldest phase in the pluton, which is crosscut by a porphyritic diorite containing traces of chalcopyrite, and later dolerite dykes. These phases have U-Pb zircon dates of 147 ± 3, 140 ± 1 and 135 ± 1 Ma, which confirms the cross-cutting relationships observed in the field. A Re-Os molybdenite isochron age of 147 ± 4 Ma indicates that the porphyritic granodiorite is the source of the oldest Mo mineralization in the metallogenic belt and was formed during a change of the tectonic setting in the area, from an intracontinental orogeny to extensional tectonics. From 147 to 135 Ma, crust-mantle interaction played an important role in the formation of magmatic rocks at Matou. Systematic petrological and geochemistry investigations reveal that the three phases have a crust source with minor input from the mantle. Investigation of ore-forming fluid, H-O isotopes, S isotopes, and the Re content of molybdenite indicate that the ore-forming fluid and metals were derived from the lower crust. During the evolution of fluid from initial magmatic fluids (stage I) to ore-forming fluids (stage II), fluid boiling accompanied by the input of relatively cooler meteoric water led to the deposition of the Mo mineralization.     

Relative contribution of magmatic and post-magmatic processes in the genesis of the Thompson Mine Ni-Co sulfide ores,Manitoba, Canada

The Ni-Co-(PGE) sulfide deposits of the Thompson Nickel Belt (TNB) in Northern Manitoba, Canada are part of the fifth largest nickel camp in the world based on contained nickel; past production from the TNB deposits is 2500 kt Ni. The Thompson Deposit is located on the eastern and southern flanks of the Thompson Dome structure, which is a re-folded nappe structure formed during collision of the Trans-Hudson Orogen with the Canadian Shield at 1.9–1.7 Ga. The Thompson Deposit is almost entirely hosted by P2 member sulfidic metasedimentary rocks of the Paleoproterozoic Ospwagan Group. Variably serpentinised and altered dunites, peridotites and pyroxenites contain disseminated sulfides and have a spatial association with sediment-hosted Ni sulfides which comprise the bulk of the ore types. These rocks formed from rift-related komatiitic magmas that were emplaced at 1.88 Ga, and subsequently deformed by boudinage, thinning, folding, and stacking.Disseminated sulfide mineralization in the large serpentinised peridotite and dunite intrusions that host the Birchtree and Pipe Ni-Co sulfide deposits typically has 4–6 wt% Ni in 100% sulfide. The disseminated sulfides in the less abundant and much smaller boudinaged serpentinised peridotite and dunite bodies associated with the Thompson Deposit have 7–10 wt% Ni in 100% sulfide. The majority of Thompson Mine sulfides are hosted in the P2 member of the Pipe Formation which is a sulfidic schist developed from a shale prololith; the mineralization in the schist includes both low Ni tenor (<1 wt% Ni in sulfide) and barren sulfide (<200 ppm Ni) and a Ni-enriched sulfide with 1–18 wt% Ni in 100% sulfide. The semi-massive and massive sulfide ores show a similar range in Ni tenor to the metasediment-hosted mineralization, but there are discrete populations with maximum Ni tenors of ∼8, 11 and 13 wt% Ni in 100% sulfide. The variations in Ni tenor are related to the Ni/Co ratio (high Ni/Co correlates with high Ni tenor sulfide) and this relationship is produced by the different Ni/Co ratios in sulfides with a range in proportions of pyrrhotite and pentlandite. Geological models of the ore deposit, host rocks, and sulfide geochemical data in three dimensions reveal that the Thompson Deposit forms an anastomosing domain on the south and east flanks of a first order D3 structure which is the Thompson Dome. In detail, a series of second order doubly-plunging folds on the eastern and southern flank control the geometry of the mineral zones. The position of these folds on the flank of the Thompson Dome is a response to the anisotropy of the host rocks during deformation; ultramafic boudins and layers of massive quartzite in ductile metasedimentary rocks control the geometry of the doubly-plunging F3 structures. The envelope of mineralization is almost entirely contained within the P2 member of the Pipe formation, so the deposit is clearly folded by the first order and second order D3 structures. The sulfides with highest Ni tenor (typically >13 wt% Ni in sulfide) define a systematic trend that mirrors the configuration of the second order doubly-plunging F3 structures on the flanks of the Dome. Although moderate to high Ni tenor mineralization is sometimes localized in fold hinges, more typically the highest Ni tenor mineralization is located on the flanks of the fold structures.There is no indication of the mineralogical and geochemical signatures of sedimentary exhalative or hydrothermal processes in the genesis of the Thompson ores. The primary origin of the mineralization is undoubtedly magmatic and this was a critical stage in the development of economic mineralization. Variations in metal tenor in disseminated sulfides contained in ultramafic rock indicate a higher magma/sulfide ratio in the Thompson parental magma relative to Birchtree and Pipe. The variation in Ni tenor of the semi-massive and massive sulfide broadly supports this conclusion, but the variations in metal tenor in the Thompson ores was likely created partly during deformation. The sequence of rocks was modified by burial and loading of the crust (D2 events) to a peak temperature of 750 °C and pressure of 7.5 kbar. The third major phase of deformation (D3) was a sinistral transpression (D3 event) which generated the dome and basin configuration of the TNB. These conditions allowed for progressive deformation and reformation of pyrrhotite and pentlandite into monosulfide solid solution as pressure and temperature increased; this process is termed sulfide kinesis. Separation of the ductile monosulfide solid solution from granular pentlandite would result in an effective separation of Ni during metamorphism, and the monosulfide solid solution would likely be spread out in the stratigraphy to form a broad halo around the main deposit to produce the low Ni tenor sulfide. Reformation of pentlandite and pyrrhotite after the peak D2 event would explain the broad footprint of the mineral system. The effect of the D3 event at lower pressure and temperature would have been to locally redistribute, deform, and repeat the lenses of sulfide.The understanding of the relationships between petrology, stratigraphy, structure, and geochemistry has assisted in formulating a predictive exploration model that has triggered new discoveries to the north and south of the mine, and provides a framework for understanding ore genesis in deformed terrains and the future exploration of the Thompson Nickel Belt.     

Magnetite geochemistry of the Longqiao and Tieshan Fe–(Cu) deposits in the Middle-Lower Yangtze River Belt: Implications for deposit type and ore genesis

Magnetite is a common mineral in many ore deposits and their host rocks, and contains a wide range of trace elements (e.g., Ti, V, Mg, Cr, Mn, Ca, Al, Ni, Ga, Sn) that can be used for deposit type fingerprinting. In this study, we present new magnetite geochemical data for the Longqiao Fe deposit (Luzong ore district) and Tieshan Fe–(Cu) deposit (Edong ore district), which are important magmatic-hydrothermal deposits in eastern China.Textural features, mineral assemblages and paragenesis of the Longqiao and Tieshan ore samples have suggested the presence of two main mineralization periods (sedimentary and hydrothermal) at Longqiao, among which the hydrothermal period comprises four stages (skarn, magnetite, sulfide and carbonate); whilst the Tieshan Fe–(Cu) deposit comprises four mineralization stages (skarn, magnetite, quartz-sulfide and carbonate).Magnetite from the Longqiao and Tieshan deposits has different geochemistry, and can be clearly discriminated by the Sn vs. Ga, Ni vs. Cr, Ga vs. Al, Ni vs. Al, V vs. Ti, and Al vs. Mg diagrams. Such difference may be applied to distinguish other typical skarn (Tieshan) and multi-origin hydrothermal (Longqiao) deposits in the MLYRB. The fluid–rock interactions, influence of the co-crystallizing minerals and other physicochemical parameters, such as temperature and fO₂, may have altogether controlled the magnetite trace element contents of both deposits. The Tieshan deposit may have had higher degree of fO₂, but lower fluid–rock interactions and ore-forming temperature than the Longqiao deposit. The TiO₂–Al₂O₃–(MgO + MnO) and (Ca + Al + Mn) vs. (Ti + V) magnetite discrimination diagrams show that the Longqiao Fe deposit has both sedimentary and hydrothermal features, whereas the Tieshan Fe–(Cu) deposit is skarn-type and was likely formed via hydrothermal metasomatism, consistent with the ore characteristics observed.     

Chronological constraints on the tectonic evolution of the Chinese Tianshan Orogen through detrital zircons from modern and palaeo-river sands

The Chinese Tianshan Orogen marks prolonged and complicated interactions between the southwestern Palaeo-Asian Ocean and surrounding blocks. New and previously published detrital zircon chronological data from modern and palaeo-river sands were compiled to reveal its tectonic evolution. It is characterized by predominant Palaeozoic as well as minor Mesozoic and Precambrian detrital zircon ages with a multimodal characteristic. The oldest Phanerozoic zircon population (peaking at 475 Ma) is a result of subduction and closure of the early Palaeozoic Terskey Ocean. However, the absence of this peak in the Chinese North and southern South Tianshan suggests that subductions of the North and South Tianshan oceans may not have initiated until the Late Ordovician with subsequent 460–390 and 360–320 Ma arc magmatism. Similar to the magmatic suite in classic collisional orogens, the youngest massive 320–270 Ma magmatism is suggested to be post-collisional. The North and South Tianshan oceans therefore probably had their closure to form the Chinese Tianshan Orogen during the late Carboniferous. The weak Mesozoic intra-plate magmatism further rejects a late Permian–Triassic Tianshan Orogen due to a lack of extensive syn- and post-collisional magmatism. Moreover, diverse Precambrian detrital zircon age patterns indicate that the surrounding blocks have distinct evolutionary processes with short-term amalgamation during the Meso- to Neoproterozoic.     

Reconstructing the Cryogenian–Ediacaran evolution of the Porongos fold and thrust belt,Southern Brasiliano Orogen,based on Zircon U–Pb–Hf–O isotopes

The Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with T_DM model ages between 2.5 and 1.3 Ga. The reworked signature (ε_Hf values = ?34 to ?4) and the characteristic crustal magma reservoirs (δ¹⁸O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.     

Geochemistry of late Palaeozoic granitoids of the Balkhash metallogenic belt,Kazakhstan: implications for crustal growth and tectonic evolution of the Central Asian Orogenic Belt

ABSTRACT

The Balkhash metallogenic belt (BMB) in Kazakhstan is a famous porphyry Cu–Mo metallogenic belt in the Central Asian Orogenic Belt (CAOB). The late Palaeozoic granitoids in the BMB are mainly high-K calc-alkaline and I-type granites, with shoshonite that formed during a late stage. Geochemical analyses and tectonic discrimination reveal a change in the tectonic environment from syn-collision and volcanic arcs during the Carboniferous to post-collision during the Permian. The late Palaeozoic granitoids from the Borly porphyry Cu deposit formed in a classical island-arc environment, and those from the Kounrad and Aktogai porphyry Cu deposits and the Sayak skarn Cu deposit are adakitic. The ε_Nd(t) values for the late Palaeozoic granitoids are between ?5.87 and +5.94, and the ε_Sr(t) values range from ?17.16 to +51.10. The continental crustal growth histories are different on either side of the Central Balkhash fault. On the eastern side, the ε_Nd(t) values of the granitoids from the Aktogai and Sayak deposits are very high, which are characteristic of depleted mantle and suggest that crustal growth occurred during the late Palaeozoic. On the western side, the ε_Nd(t) values of the granitoids from the Borly and Kounrad deposits are slightly low, which suggests the presence of a Neoproterozoic basement and the mixing of crust and mantle during magmatism. The granitoids have initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.335–20.993, 15.521–15.732, and 38.287–40.021, respectively, which demonstrate an affinity between the late Palaeozoic magmatism in the BMB and that in the Tianshan, Altai, and Junggar orogens.