浙西南遂昌—大柘地区八都岩群印支期变质变形序列

浙西南遂昌-大柘地区八都岩群在印支期变质事件影响下发生变质变形,通过详细野外调查和岩相学研究,可将其划分为3期变质变形序列：S₁变形期,NW向片麻理记录的残留紧闭褶皱,共生矿物组合为石榴子石变斑晶及其内部定向分布的包裹体矿物,石榴子石+黑云母+石英（泥质）和石榴子石+角闪石+斜长石+石英（长英质）;S₂变形期,区域性宽缓褶皱及NE向缓倾透入性片麻理,共生矿物组合为石榴子石变斑晶及定向分布的基质矿物,矽线石+石榴子石+黑云母+石英+斜长石±钾长石（泥质）和石榴子石+钾长石+斜长石+黑云母+石英（长英质）;S₃变形期,NE向陡倾透入性片麻理及韧脆性断裂大部分被花岗斑岩脉填充,共生矿物组合为石榴子石变斑晶及其周围退变矿物,石榴子石+矽线石+堇青石+斜长石+黑云母+石英±钾长石（泥质）和角闪石+斜长石+黑云母+钛铁矿（长英质）。结合前人研究成果,八都岩群印支期变质事件峰期变质程度达到麻粒岩相,显示顺时针近等温降压（ITD）型的p-T演化轨迹,S₁-S₃变质变形反映出从俯冲碰撞到快速折返冷却的演化过程,伴随S₃同期侵位的花岗斑岩锆石U-Pb定年结果,将该演化过程完成时间约束在229.7 Ma,可能是浙西南地区对印支期古特提斯洋域内印支-华南-华北板块之间俯冲-碰撞过程的响应。     

Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon

The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO₂, 17–13 wt.% Al₂O₃, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na₂O and 6.3–4.4 wt.% K₂O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K₂O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H₂O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).     

北秦岭蟒岭岩体的锆石U-Pb年龄、地球化学及其演化

蟒岭岩体位于北秦岭构造带北部,岩石类型主要为似斑状黑云母二长花岗岩、中粗粒黑云母二长花岗岩、中细粒二长花岗岩、含辉石黑云角闪闪长岩和黑云母钾长花岗岩。依据LA-ICPMS锆石U-Pb定年结果,结合前人测试的年龄,将蟒岭岩体的岩浆演化划分为晚侏罗世早期、晚侏罗世晚期—早白垩世早期和早白垩世中期3期。第一期为含辉石黑云角闪闪长岩,其LA-ICPMS锆石U-Pb年龄为（157±1） Ma,该期岩石SiO₂质量分数较低,富碱,属于准铝质,钾玄岩-高钾钙碱性系列;第二期二长花岗岩,侵位年龄为（148±1） Ma~（144±1） Ma,具有高硅、富碱的特征,属于准铝质-弱过铝质,钾玄岩-高钾钙碱性I-A过渡型花岗岩;第三期黑云母钾长花岗岩,侵位年龄为（124±2） Ma,具高硅、富碱、低镁、铝饱和指数偏高的特征,属过铝质,高钾钙碱性I-A过渡型花岗岩。组成蟒岭岩体的花岗岩从早到晚,SiO₂质量分数逐渐升高,而Al₂O₃、TiO₂、MgO、CaO、P₂O₅、TFe₂O₃质量分数逐渐降低;稀土元素总量具有由高到低的变化趋势,第一期和第二期岩石的稀土元素配分曲线为轻稀土元素相对富集的右倾型,而第三期的稀土元素配分曲线呈两边高中间低的不对称弧形,整体上负铕异常不明显或呈微弱正铕异常;微量元素上,这3期岩石均富集K、Rb、Ba、Sr等大离子亲石元素,而相对亏损P、Nb、Ta、Ti等高场强元素。与区域上同时代成矿花岗岩体对比,两者均具有高硅、富碱的特征,稀土元素球粒陨石标准化曲线呈轻稀土元素富集的右倾斜型,但蟒岭岩体中二长花岗岩没有明显的Eu异常,且Ba、P、Ti亏损及Ta、Nb富集没有含矿花岗岩明显。     

Origin and evolution of the calcic and magnesian skarns hosting the El Valle-Boinás copper–gold deposit, Asturias (Spain)

The El Valle-Boinás copper–gold deposit is located in the southern part of the Rio Narcea Gold Belt 65 km west of Oviedo (NW Spain), within the Cantabrian Zone (Iberian Hercynian Massif). The deposit is related to the Boinás stock, which ranges from quartz-monzonite to monzogranite and intruded (303 Ma) the carbonated Láncara Formation (early Cambrian) and the siliciclastic Oville Formation (middle-late Cambrian).A copper–gold skarn was developed along the contact between the igneous rock and the carbonated sedimentary rocks. The skarn distribution and mineralogy reflects both structural and lithologic controls. Two types of skarn exists: a calcic skarn mainly developed in the upper calcic member of the Láncara Formation, and a magnesian skarn developed in the lower dolomitic and organic-rich member. The former mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of K-feldspar, epidote, quartz, calcite, magnetite, ferroactinolite, titanite, apatite, chlorite and sulfides. Magnesian skarn mainly consists of diopside with interbedded forsterite zones. Pyroxene skarn is mainly altered to tremolite, with minor phlogopite and serpentine. Olivine skarn is pervasively altered to serpentine and magnetite, and is commonly accompanied by high sulfide and gold concentrations. This altered skarn results in a very dark rock, referred to as “black skarn”, which has great importance in gold reserves. Sulfide mineralization mainly consists of chalcopyrite, bornite, arsenopyrite, pyrrhotite and pyrite, while wittichenite, sphalerite, digenite, bismuthinite, native bismuth and electrum occur as accessory minerals.After extensive erosion, reactivation of the northeast-trending fracture zone provided conduits for the subsequent emplacement of porphyritic dikes (285±4 Ma) and diabasic dikes (255±5 Ma). Alteration, characterized by sericitization, silicification, carbonatization and hypogene oxidation took place, as did sulfide mineralization (pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, bournonite, and Fe–Pb–Sb sulfosalts). Veins with quartz, carbonate, adularia and sulfide minerals crosscut all previous lithologies. Jasper and jasperoid breccias developed at the upper parts of the deposits.The fluid inclusion and stable isotope studies suggest a predominantly magmatic prograde-skarn fluid characterized by high-salinity (26–28 wt.% KCl and 32–36 wt.% NaCl) and high temperature, above 580°C. This fluid evolved into two immiscible fluids: a CO₂- and/or CH₄-rich, high-salinity aqueous fluid. Temperatures for the first retrograde-stage are between 350 and 425°C. A second stage is related to a more diluted aqueous fluid (3–6.2 wt.% NaCl eq.) and temperatures from 280 to 325°C. The fluid inclusion study performed on quartz from low-temperature mineralization indicates a very low salinity (0.2–6.2 wt.% NaCl eq.), low-temperature aqueous fluid (from 150 to 250°C), and trapping pressure conditions less than 0.2 kbar. In addition, the stable isotope study suggests that an influx of metamorphic waters derived from the country rocks produced these lower temperature fluids. The last control for the Au mineralization is the Alpine tectonism, which developed fault breccias (cataclasites to, locally, protomylonites) and gold remobilization from previous mineralization.     

SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China: Geochronological constraints for the evolution of Paleo-Tethys

SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic mélange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic mélange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of the Jinshajiang belt, a granitoid batholith (268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic mélange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the mélange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.     

Pan-African Magmatism, and Sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.     

Trans-alkaline magmatism in the Serrinha–Pedro Velho Complex,Borborema Province,NE Brazil and its correlations with the magmatism in eastern Nigeria

The Solânea Complex (572 ± 8 Ma) comprises various intrusions that total 200 km². It cuts Palaeoproterozoic migmatized orthogneisses of the Serrinha–Pedro Velho Complex, which is part of the São José do Campestre Massif, North Tectonic Domain of the Borborema Province, NE Brazil. It is a post-collisional intrusion constituted of coarse-grained biotite–amphibole monzogranites, fine-grained leucogranites and diorites. Major, trace and rare earth elements data, used to constrain the origin and nature of these rocks, indicate that the monzogranites display chemical features of weakly peraluminous ferro-potassic trans-alkaline granitoids, similar to coeval occurrences described in eastern Nigerian. Sm–Nd data together with the whole rock geochemical data suggest that: (I) the monzogranites were generated by partial melting of tonalitic gneiss from the lower crust, leaving behind a granulitic residue; (II) the diorites originated by partial melting of an enriched lithospheric mantle; (III) the evolution of the Solânea Complex involved magma mixing between the monzogranites and diorites. K-feldspar and amphibole fractionation appear to have occurred during the evolution of the monzogranites.Leucogranites constituting an elliptical shaped intrusion (80 km²), the Dona Inês Pluton, are intruded to the north of the Solânea Complex. They also occur as small intrusions cross-cutting the border of the Solânea Complex. Geochemical and isotopic data suggest that the leucogranites were generated by melting of crustal Palaeoproterozoic rocks. Rb–Sr data gave an age of 544 ± 16 Ma.Trans-alkaline magmatism is associated with ductile–brittle deformation in the Serrinha–Pedro Velho Complex of the São José do Campestre Massif. This event follows the peak of an extensional-related high-temperature metamorphism (upper amphibolite to granulite facies), with estimated age of 578 to 574 Ma. If a pre-Gondwana reconstruction is considered, this event is seen to extend to the African side of Gondwana in eastern Nigeria.     

The petrogenesis of an I-type volcanic-plutonic suite: The St. Marys Porphyrite,Tasmania

The Devonian I-type St. Marys Porphyrite (388±1Ma) comprises two petrographically similar units, an 800 m thick pyroclastic sheet (compositionally dacite and rhyolite) and a subvolcanic feeder dyke. The pyroclastics are crystal-rich and contain (in order of decreasing abundance) plagioclase, quartz, biotite, augite, hypersthene and sanidine phenocrysts in an aphanitic groundmass.The early phenocryst assemblage clinopyroxene+orthopyroxene+plagioclase was followed by crystallisation of less magnesian pyroxene, more sodic plagioclase and biotite, quartz and K-feldspar. The phenocrysts crystallised at high temperature, between 1,000°-850° C, and at a pressure of 2.5±1 kb from a water undersaturated (<2.5 wt.%) magma in a chamber underlying the intrusive centre.At least two eruptive phases are present in the pyroclastic pile, each commencing with rhyolite. Bulk chemical variation probably reflects a zonation in the magma chamber prior to eruption. The low pressure phenocryst crystallisation conditions and the pyroxene Fe-enrichment trend with falling temperature support a fractional crystallisation model. The chemical variation can be explained by 20% fractional crystallisation involving plagioclase, quartz, biotite and pyroxene in proportions similar to modal phenocryst abundances.Volcanics like the St. Marys Porphyrite preserve evidence of their early magmatic history by quenching of mineral phases. Textural relationships and physico-chemical parameters deduced from the St. Marys Porphyrite are applicable to the interpretation of I-type granitoids in eastern Australia and elsewhere and constrain petrogenetic models for their genesis. Pyroxene cores of hornblende grains, pyroxene inclusions in plagioclase and corroded cores of plagioclase crystals may be formed through magmatic crystallisation and need not represent restite.     

Chemical composition of rock-forming minerals in granitoids associated with Au–Bi–Cu,Cu–Mo,and Au–Ag mineralization at the Freegold Mountain,Yukon, Canada: magmatic and hydrothermal fluid chemistry and petrogenetic implications

The chemical compositions of rock-forming minerals have been determined for both altered and least-altered igneous rocks spatially associated with numerous mineralized zones (Nucleus Au–Bi–Cu–As deposit, Revenue Au ± Cu and Stoddart Cu–Mo ± W mineral occurrences, and Laforma Au–Ag deposit) across the Freegold Mountain area, Yukon, Canada. Within the study area, K-feldspar has a narrow compositional range (89.4–91% Or), whereas plagioclase spans a wide range (4.4–70.07% An). In all of the investigated samples, T _Ab = T _An = T _Or, suggesting that magmatic equilibrium between the coexisting plagioclase and K-feldspar was maintained. Igneous amphibole phenocrysts from hypabyssal dikes are typically calcic, whereas the Stoddart Cu–Mo ± W, Laforma Au–Ag, and Goldy Au mineralization are associated with Mg-enriched primary amphibole of edenite composition, and Au–Bi–Cu–As mineralization from Nucleus is related to Al-enriched primary amphibole of ferropargasite composition. Primary biotite phenocrysts across the Freegold Mountain area re-equilibrated with oxidized magma (f(O₂) values between 10–13 and 10–11.5 bars, lying between the Ni/NiO and the magnetite/haematite buffers). However, biotite and amphibole phenocrysts from Stoddart, Goldy, Laforma, and the Highway zones crystallized from a more oxidized magma, as indicated by their elevated X _Mg up to 0.65, relative to biotite and hornblende from Nucleus and Revenue characterized by a lower X _Mg (typically < 0.50). This suggests that various sources and (or) rapid emplacement were involved in magma genesis, as further supported by the considerable variation of pressure (1.8–7.3 kb) of amphibole crystallization and of the total Al content in least-altered biotite (2.6–2.9 afu) within the Freegold Mountain area. Biotite and apatite equilibrated within the T range of 520–780°C, consistent with temperatures of equilibration between ilmenite and magnetite, and their compositions indicate that they formed from an oxidized I-type magma. Magma differentiated by fractional crystallization (indicated by the presence of normally zoned plagioclase with Ca-rich cores and Na-enriched outer rims) and multiple magma mixing (supported by the presence of reversed zoned plagioclase and coexistence of normally and reversely zoned plagioclase). Lower X _Mg biotite associated with the mineralized (Cu–Mo ± W) potassic alteration incorporated more F and Cl relative to least-altered biotite with higher X _Mg. In both Nucleus and Revenue Au–Cu mineralizations, secondary biotite composition varies with respect to the associated alteration mineral assemblages. Although secondary biotite in the skarn re-equilibrated with F-poor fluids, secondary biotite from the pervasive biotitization is related to F- and Cl-enriched fluids, and secondary biotite from the phyllitic zone is related to F-, Cl-, and Mg-depleted fluids, thus consistent with a change in mineralizing fluid composition during mineralization.     

Metallogenic and Metamorphic Age of the Hongtoushan Copper–Zinc Massive Sulfide Deposit,Liaoning Province,China

The Hongtoushan copper–zinc deposit is the only large Archean volcanogenic massive sulfide (VMS) deposit in China. This paper presents new information on the timing of metallogenesis and metamorphism of the deposit, including new cathodoluminescence (CL) images, and Th U^?1 and LA–ICP–MS U–Pb dating on zircons of the biotite–plagioclase gneiss and plagioclase–amphibole gneiss that host the deposit. The CL images and Th U^?1 ratios indicate that the zircons within the gneiss are of metamorphic origin. LA–ICP–MS dating of the plagioclase–amphibole gneiss yielded the upper intercept ages of 2549 Ma, and the biotite–plagioclase gneiss yielded the upper intercept ages of 2552 Ma and 2550 Ma correspondingly, indicating that the mineralization of the deposit occurred at around 2550 Ma. The rims of zircons within these host rocks yielded younger U–Pb ages at 2529, 2520 and 2515 Ma, identical to the age of felsic intrusive rocks in the study area. This suggests that the Hongtoushan VMS deposit underwent initial metamorphism and deformation at around 2520 Ma by the intrusion of granite magmas.     

塔里木巴楚小海子正长岩杂岩体的岩石成因探讨

巴楚小海子正长岩杂岩体是二叠纪塔里木大火成岩省的重要组成部分.SIMS锆石U-Pb定年显示其形成于279.7±2.0Ma,与本区辉绿岩脉和石英正长斑岩岩脉近于同时侵位.根据矿物学特征,小海子正长岩体可分为铁橄榄石正长岩和角闪正长岩两类.前者主要由碱性长石、铁橄榄石、单斜辉石、角闪石和少量石英、斜长石组成,后者主要由碱性长石、角闪石、黑云母和少量的石英、斜长石组成.小海子正长岩体为铁质、碱性系列,轻稀土相对富集,重稀土亏损,具有明显的Eu正异常,无Nb、Ta负异常,相对低的(87Sr/86Sr);(0.7033 ～0.7038)和正的εNd(t)值(+3.1～+3.8),暗示它们来自亏损的地幔源区,没有地壳物质的加入.主微量和同位素地球化学分析,暗示巴楚小海子正长岩的母岩浆为碱性的幔源玄武质岩浆经橄榄石、单斜辉石分离结晶后的残余熔体,并且含有堆晶的碱性长石.这种含有碱性长石堆晶的熔体,在相对还原的条件下结晶,形成铁橄榄石正长岩;在相对氧化的条件下结晶,并经过不同程度斜长石的分离结晶形成角闪正长岩.     

大陆碰撞过程中的巴罗式变质作用及原地深熔作用:以南阿尔金为例

南阿尔金吐拉地区所出露的变质泥质岩和变质基性岩普遍经历了中压麻粒岩相变质作用,其中变泥质岩以出现石榴子石+夕线石+长石+黑云母+石英为特征,而基性麻粒岩则以石榴子石+单斜辉石+紫苏辉石+斜长石+石英为特征,具有典型中压相系的麻粒岩相变质作用矿物组合,即显示"巴罗式"变质作用特征。野外宏观特征显示这套变泥质岩普遍经历了原地深熔作用,并局部发生混合岩化作用。岩相学观察结果显示泥质片麻岩保留了关键的深熔作用显微结构证据:(1)石榴子石内部发育有钾长石、石英和斜长石组成的矿物集合体,可能代表了早期熔体的假象;(2)黑云母颗粒边界发育尖锐的、不规则的微斜长石,而且黑云母边界溶蚀明显,形成锯齿状不规则的边界,指示深熔作用可能与黑云母的分解密切相关,即黑云母可能为深熔作用的主要反应相;(3)石英、斜长石或石榴子石颗粒边界发育圆珠状不规则的钾长石,而且颗粒边界或三联点中尖锐状钾长石与周围矿物的形成较小的二面角,有些甚至相互连通呈网络状,这也与它们继承了熔体结构特征一致;(4)不规则钾长石(或微斜长石)分布在石榴子石和夕线石附近,指示石榴子石和夕线石可能为深熔作用的残留相。锆石U-Pb定年结果显示麻粒岩相变质作用和相关深熔作用时代基本一致,主要发生在~450Ma。因此,吐拉地区的中压麻粒岩相变质作用和深熔作用明显要晚于南阿尔金地区榴辉岩和高压麻粒岩的峰期变质时代40~50Myr,而是与榴辉岩折返过程中麻粒岩相叠加变质作用的时代较为接近。但南阿尔金~450Ma的变质作用、深熔作用和岩浆作用是否为独立的构造热事件抑或深俯冲板片折返阶段的产物,这还需要今后进一步的工作验证。     

Early cretaceous granitoids and their implications for the collapse of the Dabie orogen, eastern China: SHRIMP zircon U–Pb dating and geochemistry

Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen, eastern China, which recorded processes of extensional collapse of the orogen. The early stage granitoids ( 132 Ma) are foliated hornblende quartz monzonites and porphyritic monzogranites. They are of high-K calc-alkaline series and metaluminous to weakly peraluminous, with high K₂O and low MgO contents (Mg^# values: 32.0–46.0), they contain high Sr, low Y and heavy rare earth elements (HREE), and have high Sr/Y and (La/Yb)_N ratios, without clear negative Eu, Sr and Ti anomalies. The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase, and thus indicate the existence of an over-thickened (> 50 km) crustal root beneath the orogen at  132 Ma. The later stage granitoids ( 128 Ma) are undeformed fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry. They belong to a peraluminous and high-K calc-alkaline to shoshonite series, and display a flat HREE pattern and have strong negative Eu, Sr and Ti anomalies, with low Sr/Y and (La/Yb)_N ratios. The late stage granitoids are equilibrated with residues rich in anorthite-rich plagioclase, hornblende, ilmenite/titanite and poor in garnet, indicating that the crust of the Dabie orogen became thinner (< 35 km) at  128 Ma. SHRIMP zircon U–Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous, and collapsed quickly in a few million years during the early Cretaceous.     

Experimental studies of granitoids from the Central and Southern Coast Ranges,California

Summary The mineral phase relationships have been determined in the presence of excess water to a water pressure of 10 kb for three quartz diorites, a granodiorite, and a quartz monzonite from the Central and Southern Coast Ranges of California. Water pressure-temperature curves were constructed to locate the beginning of melting and the disappearance of K-feldspar, quartz, biotite, plagioclase and hornblende. Results indicate that plagioclase is the silicate liquidus phase in all granitoids studied at low water pressures, while hornblende or biotite is the silicate liquidus phase at high water pressure. New data illustrate the potent effect of water pressure on magma composition at temperatures 50° to 100°C above the solidus. At a water pressure of 1 kb, magmas are granitic to quartz monzonitic, while at water pressures of 10 kb, they are granodioritic to quartz dioritic.

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Des études expérimentales des granitoids des chaînes du centre et du sud de la côte de la Californie

Résumé On a determiné en présence d'un excès d'eau jusqu'à une pression d'eau de 10 kb les corrélations entre les phases minérales de trois diorites quartziques, une granodiorite, et une monzonite quartzique obtenues des chaînes du centre et du sud de la côte de la Californie. On a utilisé des courbes de pression d'eau contre température afin de fixer le commencement de fusion et la disparition de K-feldspath, quartz, biotite, plagioclase, et hornblende. Les résultats indiquent que plagioclase est la phase liquidus de silicate pour tous les granitoids examinés aux pressions d'eau basses, en même temps que soit hornblende, soit biotite la phase liquidus de silicate aux pressions d'eau hautes. Des données nouvelles manifestent l'effet puissant de la pression d'eau sur la composition de magma aux températures élevées 50° ou 100°C plus que le solidus. Avec une pression d'eau de 1 kb les magmas sont granitiques à quartz monzonitiques; avec une pression d'eau de 10 kb ils sont granodioritiques à quartz dioritiques.

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With 8 Figures     

Ni-Co sulphide segregation in the Mamb pyroxenite intrusion, Cameroon

The amphibole metapyroxenite intrusion from Mamb (Cameroon) consists of enstatite, diopside-augite, edenite, and traces of plagioclase, biotite and rutile. It contains notable amounts of sulphides (pyrrhotite, pentlandite, chalcopyrite, pyrite and Co-rich violarite). The sulphide phase segregated during magma crystallization as immiscible droplets (≤ 1 mm) included in growing ferromagnesian minerals or aggregated in the silicate crystal mush to form an interstitial phase. However, observed sulphide assemblages are likely to result from low-T (≤ 300 °C) re-equilibration of high-T monosulphide solid solutions, more particularly during the Pan-African metamorphic event. The Mamb amphibole pyroxenite intrusion is tentatively considered as a potential source for Ni and Co mineralization.     

哈萨克斯坦阿克斗卡特大型斑岩铜矿床成矿时代与剥露历史研究

哈萨克斯坦阿克斗卡特大型斑岩型铜矿床产在中亚成矿域巴尔喀什成矿带阿克斗卡矿田。本文根据花岗岩类岩石的锆石U-Pb、40Ar/39Ar和裂变径迹(FT)热年代学研究,结合前人研究成果,给出了阿克斗卡斑岩铜矿床从深成岩浆活动、成矿作用、区域冷却到剥露作用的全过程。阿克斗卡矿床及附近花岗岩类锆石SHRIMPU-Pb定年结果,给出科尔达尔岩体早期英云闪长岩的结晶年龄为335.7±1.3Ma;主成矿期的含矿二长花岗斑岩结晶年龄为327.5±1.9Ma,反映了阿克斗卡矿床斑岩型铜成矿作用的年龄。花岗岩类角闪石、黑云母、钾长石40Ar/39Ar热年代学分别给出矿物冷却年龄为310.6Ma、271.5Ma和274.9Ma,进一步限定了深成斑岩型铜成矿作用的时代和区域冷却的历史。磷灰石FT测年数据揭示,受区域构造作用的影响,阿克斗卡铜矿田在晚白垩世(91～68.0Ma)发生地块的差异隆升和剥露作用。     

Chemical and isotopic compositions of Absaroka granitoids,Southwestern Montana

Contents of major and most trace elements of granitoids in three intrusions associated with the Cretaceous Independence volcanic complex, Montana, correlate well with SiO₂. Major-element contents in granitoids in each intrusion are accurately modeled as mixtures of minimum melts and phenocryst assemblages (presumably restite). Restite assemblages are hypersthene+augite+plagioclase, hornblende+plagioclase, and biotite+plagioclase+quartz. Residues of melting are granulite or amphibolite. Melts in two of the bodies were LREE-enriched but unfractionated in MREE and HREE. REE patterns are consistent with residues dominated by pyroxene or amphibole and feldspar. Initial ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb of granitoids define a line interpreted as a secondary isochron established during crustal homogenization 3.3 Ga ago. The relatively low of source rocks (8.25) suggests that they did not spend long in U-rich environments. Source regions had variable trace element patterns; Th/Pb and U/Pb were correlated, Rb/Sr and Sm/Nd moderately well correlated, but Rb/Sr and U/Pb were decoupled. This is consistent with poor correlation of Rb, Sr and Ba with SiO₂ in some granitoids and may suggest that minor phases that concentrate these elements were inhomogeneously distributed in source regions. The source probably consisted of LREE-rich, Rb-poor metamorphic rocks. Archean amphibolites, exposed in the Beartooth Mountains, are similar to the postulated source materials. They contain plagioclase, hornblende, minor quartz, biotite, and muscovite, and have low Rb/Sr and high LREE/HREE. Certain trace-element characteristics of the granitoids indicate that the deep crust in this part of Montana may be dominated by metamorphosed mafic-intermediate lavas that formed on the sea-floor. Metapelites, intercalated with amphibolites at the surface, were rare in granitoid source regions. This buried supracrustal pile was isotopically homogenized 3.3 Ga ago. Although some material melted 2.7 Ga ago to form granites that dominate the exposed basement, enough remained fertile that heating by mantle-derived magmas 85–90 Ma ago produced the granitic rocks at Independence.     

陕西秦岭梁岩体LA ICP MS锆石U Pb年龄及其地质意义

三叠纪花岗岩在秦岭造山带广泛发育,直接记录了秦岭造山带演化过程中的重要信息。秦岭梁岩体位于秦岭造山带商丹缝合带北侧,岩性主要为石英二长斑岩,斑晶为钾长石,部分斑晶具有白色斜长石环边;基质主要由石英(25%)、斜长石(45%)、钾长石(10%)、黑云母(15%)、角闪石(5%)组成。秦岭梁岩体中的锆石自形程度好,具环带结构,显示岩浆锆石的特征,两个样品的LA ICP MS锆石U Pb同位素年龄分别为(2165±21) Ma和(2194±26) Ma,二者在误差范围内一致,表明该岩体的岩浆结晶年龄为晚三叠纪。秦岭梁岩体的岩浆结晶年龄与勉略洋壳向北俯冲的时间(248~200 Ma)一致,表明岩体的形成与勉略洋壳向北俯冲有密切联系;同时秦岭梁石英二长斑岩的地球化学特征显示其是洋壳俯冲时弧岩浆活动的产物。因此北秦岭地区勉略洋闭合时间和陆陆碰撞拼合时间应不早于216~219 Ma。     

Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Rind River Range, Wyoming, USA

We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U–Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170±8 Ma (95% confidence). An ⁴⁰Ar/³⁹Ar amphibole date from the same sample yields a similar apparent age of about 2124±30 Ma (2σ), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355°, Inc.=65°, k=145, α₉₅=5°) with a paleomagnetic pole at 84°N, 215°E (δm=6°, δp=7°). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20°, N20°E), gives a tilt corrected pole of 75°N, 58°E (δm=4°, δp=6°), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic.     

K–Ar and Ar/Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation

New K–Ar and ⁴⁰Ar/³⁹Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO₂ contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb)_N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb)_N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate–K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64–59 Ma for felsic dikes. Because Jurassic–Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Resumo

Novos dados K–Ar e ⁴⁰Ar/³⁹Ar de enxames de diques toleíticos e alcalinos localizados no embasamento onshore da Bacia de Santos (SE Brasil) apontam para diferentes pulsos magmáticos ocorridos entre o Jurássico e o Terciário. Os diques de rochas toleíticas (basalto, diabásio e microgabro), são mais velhos, exibem altos teores de TiO₂ (3,65% peso na média) e podem ser subdivididos em dois grupos magmáticos. O Grupo A aflora a norte da Bacia de Resende, compreende diques orientados na direção NW, com razões (La/Yb)_N entre 15 e 32,3, e idades entre 192.9±2.2 e 160.9±1.9 Ma. O Grupo B aflora a sul e a leste da Bacia de Resende, engloba diques orientados na direção NNW e NNE, com razões (La/Yb)_N entre 7 e 16, e idades obtidas entre 148.3±3 e 133.9±0.5 Ma. As rochas alcalinas (sienito, traquito, fonolito, basalto alcalino e lamprófiro) possuem teores médios de K, e afloram como diques, plugs e stocks.. As idades obtidas são de ca. 82 Ma para os diques lamprofíricos, de ca. 70 Ma para os plugs sieníticos, e entre 64 e 59 Ma para os diques félsicos. Como estas idades Jurássicas para diques toleíticos ainda não foram descritas para a região sudeste do Brasil, pode-se especular que durante o emplacement dos diques do GrupoA o cenário tectônico indicaria esforços extensionais anteriores à abertura do Oceano atlântico Sul, e contemporânea ao derrame basáltico do Karoo na África do Sul. Já os diques do Grupo B são contemporâneos ao magmatismo Serra Geral e ao enxame de diques de Ponta Grossa, e portanto este episódio está diretamente relacionado à separação entre o Brasil e África no Cretáceo. O magmatismo alcalino está associado a diversos episódios tectônicos que sucedem à abertura do Oceano Atlântico Sul e que resultaram no desenvolvimento das Bacias terciárias do sudeste brasileiro. Está provavelmente relacionado à chegada da Pluma de Trindade e as idades obtidas para a região em estudo indicam que o magmatismo lamprofírico é mais antigo (ca. 82 Ma), seguido pelos plútons sieníticos (ca. 70 Ma) e diques associados (64 a 59 Ma.)