塔里木二叠纪基性-中性-酸性岩浆岩的年代学及其地质意义

塔里木西部柯坪地区出露的早二叠纪玄武岩与盆地内其他地区广泛分布的玄武岩共同构成一个面积约0.25Mkm²的溢流玄武岩省。巴楚地区出露的早二叠纪辉绿岩、正长岩、石英正长岩与石英正长斑岩等多种岩浆岩构成了连续的基性-中性-酸性岩浆序列。野外露头观察表明,巴楚麻扎尔塔格地区的基性-中性-酸性岩浆岩为近同时侵位。柯坪塔格开派兹雷克组顶部层位玄武岩的锆石SHRIMP U-Pb年龄为279.0±4.5Ma, 该年龄值可限定该区二叠纪溢流玄武岩喷发的最晚时限。巴楚麻扎尔塔格石英正长斑岩脉的锆石SHRIMP U-Pb年龄为273.0±3.7Ma, 与前人测定的巴楚辉绿岩脉和正长岩的锆石年龄在误差范围内一致,表明巴楚地区的基性-中性-酸性岩浆岩为近同时侵位,时间在275Ma左右。综合已发表的塔里木二叠纪岩浆岩高精度年代数据,本文提出：整个塔里木大火成岩省岩浆作用的持续时间约为20Myrs; 291~287Ma喷发的巨量溢流玄武岩可能构成塔里木大火成岩省的主体,是地幔柱头部熔融的直接产物; 283~272Ma侵位的巴楚、一间房和塔北地区的其它类型岩浆岩体积较小,为塔里木大火成岩省晚期岩浆作用的产物。     

An early Paleoproterozoic high-K intrusive complex in southwestern Tarim Block, NW China: Age, geochemistry, and tectonic implications

Systematic geochronologic, geochemical, and Nd isotopic analyses were carried out for an early Paleoproterozoic high-K intrusive complex exposed in southwestern Tarim, NW China. The results provide a better understanding of the Paleoproterozoic tectonic evolution of the Tarim Block. Zircon U–Pb age dating indicates two Paleoproterozoic magmatic episodes occurring at ca. 2.41 Ga and ca. 2.34 Ga respectively, which were followed by a ca. 1.9 Ga metamorphic event. The 2.41 Ga granodiorite–adamellite suite shares characteristics of late to post-orogenic metaluminous A-type granites in its high alkalinity (Na₂O + K₂O = 7.6–9.3%), total REE (410–788 ppm), Zr (370–660 ppm), and Y (21.7–58.4 ppm) contents. εNd(t) values for the suite range from − 3.22 to − 4.71 and accordingly the Nd modal ages (T_2DM) vary between 3.05 Ga and 3.17 Ga. Based on geochemical data, the 2.34 Ga suite can be subdivided into two sub-suites, namely A-type and S-type. However, both types have comparable Nd isotope compositions (εNd(t) ≈ − 0.41 to − 2.08) and similar narrow T_2DM ranges (2.76–2.91 Ga).Geochemical and Nd isotopic data for the high-K intrusive complex, in conjunction with the regional geological setting, suggest that both the 2.41 Ga suite and the 2.34 Ga A-type sub-suite might have been produced by partial melting of the Archean mafic crust in a continental rift environment. The S-type sub-suite is thought to have formed by partial melting of felsic pelites and/or metagreywackes recycled from Archean crust (TTG?). Gabbro enclaves with positive εNd(t) value (2.15) have been found to be intermingling within the 2.34 Ga suite; ca. 2.34–2.36 Ga gabbroic dykes and adamellites have previously been documented in eastern Tarim. These observations indicate that the high-K intrusions may reflect the emergence of depleted mantle upwelling beneath the Tarim Block at that time. We suggest a three-stages model for the Precambrian crustal evolution in the Tarim Block: (1) the formation of proto-crust (TTG) by ca. 2.5 Ga, (2) episodes of felsic magmatism possibly occurring in continental rift environments at ca. 2.41 Ga and ca. 2.34–2.36 Ga, and (3) ca. 1.9 Ga metamorphism that may represent the solidification of the Precambrian basement of the Tarim Block.     

Geochemistry and Nd–Sr isotopic studies of Late Mesozoic granitoids in the southeastern Hubei Province, Middle–Lower Yangtze River belt, Eastern China: Petrogenesis and tectonic setting

A geochemical and isotopic study was carried out for the Mesozoic Yangxin, Tieshan and Echeng granitoid batholiths in the southeastern Hubei Province, eastern China, in order to constrain their petrogenesis and tectonic setting. These granitoids dominantly consist of quartz diorite, monzonite and granite. They are characterized by SiO₂ and Na₂O compositions of between 54.6 and 76.6 wt.%, and 2.9 to 5.6 wt.%, respectively, enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), and relative depletion in Y (concentrations ranging from 5.17 to 29.3 ppm) and Yb (0.34–2.83 ppm), with the majority of the granitoids being geochemically similar to high-SiO₂ adakites (HSA). Their initial Nd (ε_Nd = − 12.5 to − 6.1) and Sr ((⁸⁷Sr/⁸⁶Sr)_i = 0.7054–0.7085) isotopic compositions, however, distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the Yangtze Craton in the Late Mesozoic. The granitoid batholiths in the southeastern Hubei Province exhibit very low MgO ranging from 0.09 to 2.19 wt.% with an average of 0.96 wt.%, and large variations in negative to positive Eu anomalies (Eu/Eu = 0.22–1.4), especially the Tieshan granites and Yangxin granite porphyry (Eu/Eu = 0.22–0.73). Geochemical and Nd–Sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent. Late Mesozoic granitoids in the southeastern Hubei Province of the Middle–Lower Yangtze River belt were dominantly emplaced in an extensional tectonic regime, in response to basaltic underplating, which was followed by lithospheric thinning during the early Cretaceous.     

Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China

The Late Middle Permian ( 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma.The oxide-bearing intrusions have higher Al₂O₃, FeO (as total iron) and total alkalis (Na₂O + K₂O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of Nd(t) values. All the intrusions, however, have Nd(t) values ranging from − 3.9 to + 4.6, and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe₂O₃ (2.5 to 13 wt.%) with initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.     

塔里木盆地中部辉绿岩脉LA-ICP-MS锆石U-Pb年龄及其地质意义

塔里木盆地塔中地区地震资料显示,寒武系、奥陶系、二叠系中发育大量辉绿岩岩脉,在塔中隆起断裂带附近采集了侵入到志留系与奥陶系中的辉绿岩脉样品(中1井、中16井岩心)。用LA-ICP-MS测定技术,测定了其中锆石的U-Pb年龄和微量元素。锆石U-Pb年龄显示,这些辉绿岩岩脉形成于早二叠世(273~296Ma);测得的700~930Ma(新元古代)、396~544Ma(古生代)年龄显示,盆地早期有构造岩浆活动;121.0±8.5Ma(早白垩世)锆石存在,显示其形成与盆地内部同期热液活动有关。     

The 2.40 Ga Ringvassøy mafic dykes, West Troms Basement Complex, Norway: The concluding act of early Palaeoproterozoic continental breakup

A north to northwest trending mafic dyke swarm of gabbronoritic and gabbroic composition makes up a significant part of the Archean basement on the island of Ringvassøy in northern Norway. U–Pb geochronology of zircon and baddeleyite in a gabbronorite provides an age of emplacement of 2403 ± 3 Ma. Metamict zircon in a plagioclase phyric dyke yield data that are discordant but consistent with the age of the gabbronoritic dyke. Titanite indicates a metamorphic overprint at 1768 ± 4 Ma. The two types of dyke show some distinct chemical characteristics. They are both tholeiitic, enriched in LREEs and LILE elements but depleted in HFS elements including Nb. Their Nd isotopic composition yields _Nd values of −1.5 to −1.8 for gabbronorites and −0.4 to +1.3 for the plagioclase phyric dykes. The chemical and isotopic constraints are typical of continental basalts.The Ringvassøy mafic dykes correlate broadly with a global Palaeoproterozoic magmatic event that formed extensive bimodal intrusive and extrusive suites in most Archaean cratons, including the northeastern Fennoscandian Shield. In detail, the 2403 ± 3 Ma Ringvassøy dykes postdated most episodes of magmatism at this time. On the regional scale there is a distinct trend from a 2505–2490 Ma suite present in the Kola Peninsula, over a second 2460–2440 Ma suite present both in Kola and further south in Karelia, to the 2403 Ma dykes on Ringvassøy. This variation suggests that the locus of maximum extension and magmatic activity may have been shifting with time.     

Ages and geodynamic settings of Xilamulun Mo–Cu metallogenic belt in the northern part of the North China Craton

The Xilamulun belt along the northern part of the North China Craton is located in eastern segment of the Central Asian Orogenic Belt and has great economic potential for Mo–Cu mineralization. More than ten medium to large ore deposits have been discovered in this region in the recent years. The major types of mineralization type include porphyry (Chehugou Mo–Cu, Kulitou Mo–Cu, Xiaodonggou Mo and Jiguanshan Mo), quartz vein (Nianzigou Mo, Xinjing Mo), epithermal (Hongshanzi Mo–U) and alteration assemblage (Liulingou Mo). The timing of mineralization was previously thought to be Yanshanian (208–290 Ma), however, Indosinian (260–208 Ma) ages for intrusions and mineralization have been recognized in recent years. Based on geochronologic data and regional geological evidence, it is suggested that the mineralization in the Xilamulun belt was formed during multiple events. The mineralization processes are related to a post-collisional extension stage (~ 258–210 Ma) with the generation of the porphyry molybdenum–copper deposit, a tectonic stress transformation from NS to EW (~ 185–150 Ma) that gave rise to vein or porphyry molybdenum deposit, and a lithospheric thinning stage (~ 140–110 Ma) with porphyry molybdenum deposit.     

Postorogenic carbonatites at Eden Lake, Trans-Hudson Orogen (northern Manitoba, Canada): Geological setting, mineralogy and geochemistry

The Eden Lake pluton in the Trans-Hudson Orogen is the first known occurrence of carbonatites in Manitoba. The pluton is largely made up of modally and geochemically diverse syenitic rocks derived from postorogenic magma(s) of shoshonitic affinity. Their diversity can be accounted for by a combination of crystal fractionation and fluid release in the final evolutionary stage (crystallization of quartz alkali-feldspar syenite). At Eden Lake, carbonatites, represented predominantly by coarse-grained massive to foliated sövite, occur as branching veins and lenticular bodies up to 4 m in thickness showing crosscutting relations with respect to all of the syenitic units. The host rocks are intensely fenitized at the contact, and there is also abundant mineralogical and textural evidence for assimilation of silicate material by carbonatitic magma through wallrock reaction and xenolith fragmentation and digestion. The bulk of the carbonatites are composed of (in order of crystallization): Sr–REE-rich fluorapatite, aegirine–augite, and coarse calcite crystals surrounded by fine-grained calcite (on average,  90 vol.% of the rock). Noteworthy accessory constituents are celestine, bastnäsite-(Ce) (both as primary inclusions in calcite), Nb–Zr–rich titanite, low-Hf zircon, allanite-(Ce) and andradite. The calcite is chemically uniform (Sr-rich, Mg–Mn–Fe-poor and low in ¹³C), but shows clear evidence of ductile deformation and syndeformational cataclasis. Geochemically, the carbonatites are enriched in Sr, Ba, light rare-earth elements, Th and U, but depleted in high-field-strength elements (particularly, Ti, Nb and Ta). The stable-isotope composition of coarse- and fine-grained calcite from the carbonatites and interstitial calcite from syenites is remarkably uniform: ca. − 8.16 ± 0.27‰ δ¹³C (PDB) and + 8.04 ± 0.19‰ δ¹⁸O (SMOW). The available textural and geochemical evidence indicates that the Eden Lake carbonatites are not consanguineous with the associated syenites and may have been derived from a Nb–Ti-retentive and ¹³C-depleted source such as the subducted crustal material underlying the Eden Lake deformation corridor.     

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).     

Porphyry Cu–Au and associated polymetallic Fe–Cu–Au deposits in the Beiya Area, western Yunnan Province, south China

The Alkaline porphyries in the Beiya area are located east of the Jinshajiang suture, as part of a Cenozoic alkali-rich porphyry belt in western Yunnan. The main rock types include quartz-albite porphyry, quartz-K-feldspar porphyry and biotite–K-feldspar porphyry. These porphyries are characterised by high alkalinity [(K₂O + Na₂O)% > 10%], high silica (SiO₂% > 65%), high Sr (> 400 ppm) and ⁸⁷Sr/⁸⁶Sr (> 0.706)] ratio and were intruded at 65.5 Ma, between 25.5 to 32.5 Ma, and about 3.8 Ma, respectively. There are five main types of mineral deposits in the Beiya area: (1) porphyry Cu–Au deposits, (2) magmatic Fe–Au deposits, (3) sedimentary polymetallic deposits, (4) polymetallic skarn deposits, and (5) palaeoplacers associated with karsts. The porphyry Cu–Au and polymetallic skarn deposits are associated with quartz–albite porphyry bodies. The Fe–Au and polymetallic sedimentary deposits are part of an ore-forming system that produced considerable Au in the Beiya area, and are characterised by low concentrations of La, Ti, and Co, and high concentrations of Y, Yb, and Sc.The Cenozoic porphyries in western Yunnan display increased alkalinity away from the Triassic Jinshajiang suture. Distribution of both the porphyries and sedimentary deposits in the Beiya area are interpreted to be related to partial melting in a disjointed region between upper mantle lithosphere of the Yangtze Plate and Gondwana continent, and lie within a shear zone between buried Palaeo-Tethyan oceanic lithosphere and upper mantle lithosphere, caused by the subduction and collision of India and Asia.     

Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution

Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean ²⁰⁶Pb/²³⁸U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO₂ contents (45.2–52.7 wt.%) and total alkaline (K₂O + Na₂O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/Yb_N = 0.86–1.1), together with their high ε_Nd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/Yb_N = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low ε_Nd(t) values (+ 3.4 to + 5.4) and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give ε_Hf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai.     

Two Cu–Co sulfide phases and contrasting fluid systems in the Katanga Copperbelt, Democratic Republic of Congo

The Katanga Copperbelt is the Congolese part of the well-known Central African Copperbelt, the largest sediment-hosted stratiform Cu–Co province on Earth. Petrographic examination of borehole samples from the Kamoto and Luiswishi mines in the Katanga Copperbelt recognized two generations of hypogene Cu–Co sulfides and associated gangue minerals (dolomite and quartz). The first generation is characterized by fine-grained Cu–Co sulfides and quartz replacing dolomite. The second generation is paragenetically later and characterized by coarse-grained Cu–Co sulfides and quartz overgrown and partly replaced by dolomite. Fluid inclusion microthermometric data were collected from two different types of fluid inclusions: type-I fluid inclusions (liquid + vapor) in the quartz of the first generation and type-II fluid inclusions (liquid + vapor + halite) in the quartz of the second generation. The microthermometric analyses indicate that the fluids represented by type-I and type-II fluid inclusions had very different temperatures and salinities and were not in thermal equilibrium with the host rock.Petrographic and microthermometric data indicate the presence of at least two main hypogene Cu–Co sulfide phases in the Katanga Copperbelt. The first is an early diagenetic typical stratiform phase, which produced fine-grained sulfides that are disseminated in the host rock and frequently concentrated in nodules and lenticular layers. This phase is related to a hydrothermal fluid with a moderate temperature (115 to 220 °C, or less if reequilibration of inclusions has occurred) and salinity (11.3 to 20.9 wt.% NaCl equiv.). The second hypogene Cu–Co phase produced syn-orogenic coarse-grained sulfides, which also occur disseminated in the host rock but mainly concentrated in a distinct type of stratiform nodules and layers and in stratabound veins and tectonic breccia cement. This second phase is related to a hydrothermal fluid with high temperature (270 to 385 °C) and salinity (35 to 45.5 wt.% NaCl equiv.).A review of available microthermometric and ore geochronological data of the Copperbelt in both the Democratic Republic of Congo and Zambia supports the regional presence of the two Cu–Co phases proposed in our study. Future geochemical analyses in the Copperbelt should take into account the presence of, at least, these two Cu–Co phases, their contrasting fluid systems and the possible overprint of the first phase by the second one.     

Early Paleoproterozoic–Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield—New paleomagnetic,isotope age and geochemical investigations

We present here new palaeomagnetic, isotopic age and geochemical data from Archean and Early Palaeoproterozoic rocks in the eastern Fennoscandian Shield. We have studied NE–SW trending gabbronorite dyke sets and their host Archean basement rocks in the Vodlozero block near the 2449 Ma Burakovka layered intrusion in southern Russian Karelia. Both dyke sets are genetically related to the Burakovka intrusion. The other, ca. 25 km long Avdeev dyke, locating a few kilometers south from the Burakovka intrusion, yields a stable single component remanence direction that is in agreement with the direction previously obtained from the Burakovka intrusion. Another NE–SW trending dyke, 0.8 m wide Shalskiy diabase dyke, about 30 km south of the Burakovka intrusion yields a similar remanence direction as the Avdeev dyke. The overall mean remanence direction has a palaeopole at Plat = −12.3°N, Plong = 243.5°E (A95 = 15.4°, 4 sites, 28 samples). The thin Shalskiy diabase dyke transects a similarly NE–SW trending 500 m wide coarse grained gabbronorite dyke which has now been dated by Sm–Nd method as 2608 ± 56 Ma. Geochemically all the dykes are quite similar showing slight calc-alkaline affinity and low TiO₂ and high SiO₂ with moderate MgO and low Cr and Ni. Furthermore, the dykes are geochemically identical to the 2.45 Ga dyke swarm in the northern Karelian Province.The remanence direction of the thin Shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked Archean gabbronorite dyke which yields a palaeopole at Plat = 22.7°N, Plong = 222.1°E (dp = 8.2°, dm = 16.2°, five samples). On the basis of different remanence directions of the diabase dyke and the unbaked Archean gabbronorite dyke, the baked contact test for the diabase dyke is positive. In addition to the high temperature and high coercivity component of the baked and unbaked Archean gabbronorite dyke, in low temperatures and coercivities we isolated a similar component as in the diabase dyke. A comparable remanence component was also obtained from the Archean basement at ca. 8 km from the dykes. We propose that in the studied area, the Archean basement and the Archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large Burakovka layered intrusion and related dykes at about 2.40 Ga ago.This interpretation lends support from a new ⁴⁰Ar/³⁹Ar dating of hornblende from another area, Lake Paajarvi area, in northern Karelia. There, a negative baked contact test was previously obtained for the remanence of the dated ca. 2.45 Ga dyke rocks related to the ca. 2.45 Ga Oulanka layered intrusion. The ⁴⁰Ar/³⁹Ar dating of the unbaked Archean basement which yields the same remanence component as the dykes, shows a plateau age of ca. 2.6 Ga, but in addition, it also shows resetting of the basement at ca. 2.4 Ga ago. The dating thus supports reactivation and partial remagnetization of the Archean basement at ca. 2.4 Ga ago.Our new palaeomagnetic results from the Burakovka dykes and the new ⁴⁰Ar/³⁹Ar dating from the Lake Paajarvi area give support to our previous interpretation that at Lake Paajarvi area the remanence component suggested to be 2.4 Ga, despite to negative baked contact test, is indeed of this age. Therefore, it is implied that the results can be used for continental reconstructions.     

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ¹⁸O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648.

The initial ¹⁴³Nd/¹⁴⁴Nd ratios, expressed by ε_Nd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ¹⁸O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of ¹⁸O/¹⁶O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ¹⁸O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ¹⁸O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.     

Geochemistry of Late Mesozoic mafic dykes in western Fujian Province of China： Sr-Nd isotope and trace element constraints

The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%–17.54% and K2O/Na2O=0.65–0.77; as for the Bali dyke, Al2O3=16.89%–17.81% and K2O/Na2O=0.93–0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low εNd (t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their εNd (t) values vary between -6.8 and -6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their εNd (t) values are -4.7– -4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.     

Zr-rich accessory minerals (titanite, perrierite, zirconolite, baddeleyite) record strong oxidation associated with magma mixing in the south Peruvian potassic province

The Oroscocha Quaternary volcano, in the Inner Arc Domain of the Andean Cordillera (southern Peru), emitted peraluminous rhyolites and trachydacites that entrained decimetric to millimetric lamprophyric blobs. These latter show kersantite modal compositions (equal proportion of groundmass plagioclase and K-feldspar) and potassic bulk-rock compositions (1<K₂O/Na₂O<2; 6.7–7.2 wt.% CaO). Kersantite blobs have shapes and microstructures consistent with an origin from a mixing process between mafic potassic melts and rhyolitic melts. Both melts did exchange their phenocrysts during the mixing process. In addition to index minerals of lamprophyres (Ba–Ti–phlogopite, F-rich apatite, andesine and Ca-rich sanidine), the groundmass of kersantite blobs displays essenite-rich diopside (up to 22 mol%), Ti-poor magnetite microlites, Ti-poor hematite microlites and a series of Ca–Ti–Zr- and REE-rich accessory minerals that have never been reported from lamprophyres. Titanite [up to 5.3 wt.% ZrO₂ and 5.2 wt.% (Y₂O₃ + REE₂O₃)] and Zr- and Ca-rich perrierite (up to 7.2 wt.% ZrO₂ and 10.8 wt.% CaO) predate LREE- and iron-rich zirconolite and Fe-, Ti-, Hf-, Nb- and Ce-rich baddeleyite (up to 5.3 wt.% Fe₂O₃, 3.2 wt.% TiO₂, 1.5 wt.% HfO₂, 1.2 wt.% Nb₂O₅, 0.25 wt.% CeO₂) in the crystallization order of the groundmass. Isomorphic substitutions suggest iron to occur as Fe³⁺ in all the accessory phases. This feature, the essenitic substitution in the clinopyroxene and the occurrence of hematite microlites, all indicate a drastic increase of the oxygen fugacity (from FMQ − 1 to FMQ + 5 log units) well above the HM synthetic buffer within a narrow temperature range (1100–1000 °C). Such a late-magmatic oxidation is ascribed to assimilation of water from the felsic melts during magma mixing, followed by rapid degassing and water dissociation during eruption of host felsic lavas. Thus, magma mixing involving felsic melt end-members provides a mechanism for mafic potassic melts to be oxidized beyond the HM synthetic buffer curve.     

Dissolved air flotation (DAF) of fine quartz particles using an amine as collector

Experimental studies concerning the dissolved air flotation (DAF) of fine (d_p < 100 μm) quartz particles, using two different flotation cells (setups), are presented. Pure and well characterised quartz samples were treated with a commercial amine as collector prior to flotation and bubbles were characterised by the LTM-BSizer technique. Bubble size distribution showed 71% (by volume) and 94% (by number) of the bubbles having sizes (d_b) lower than 100 μm (i.e. microbubbles). The Sauter and arithmetic mean diameters were 79 μm and 56 μm, respectively, for the bubbles generated at 300 kPa (gauge) saturation pressure (after 30 minute saturation time). Quartz particle size distribution (obtained by laser diffraction) showed a volume-moment diameter of 13 μm. The Rosin–Rammler–Bennett, Gates–Gaudin–Schumann and log-normal distribution functions were well fitted (R² > 0.96) to the bubble size distribution and quartz particle size distribution data. Values of total quartz recovery ranging from 6% to 53% (by mass) were obtained for the DAF experiments under different collector concentrations (up to 2 mg g^− 1), with an optimal collector concentration found at 1 mg g^− 1. These results are significant considering that 27% (by volume) of the quartz particles are ultrafine (d_p < 5 μm), demonstrating the widely-known efficiency of DAF to remove small particles when applied in the field of water and wastewater treatment. The true flotation behaviour, as a function of particle diameter (d_p), exhibits a local minimum when particles are approximately 3–5 μm in size. The results contribute to the discussion in the literature about the existence of such a minimum, which is generally interpreted as a change in the mechanism of particle collection from convection (collision) to diffusion at lower particle sizes.     

Neoproterozoic adakitic plutons in the northern margin of the Yangtze Block, China: Partial melting of a thickened lower crust and implications for secular crustal evolution

Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of  735 Ma. The rocks have high K₂O (0.8–3.6 wt.%) and Na₂O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)_n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from  1.0 to  0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the  735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at  735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region.     

Ordovician ferrosilicic magmas: Experimental evidence for ultrahigh temperatures affecting a metagreywacke source

Peculiar magmatic rocks were erupted and emplaced at depth at the margin of the Gondwana supercontinent during the Cambro-Ordovician transition. These rocks are characterized by high contents in silica and iron but they do not have equivalents in the high-silica members of the calc-alkaline series. They have particular geochemical signatures, with Al saturation index, ASI > 1, FeO > 2.5 wt.%, MgO > 0.8 wt.% for very low contents in calcium (CaO < 2.0 wt.%), supporting a derivation from near-total melting (> 80 vol.% melt) of metagreywackes. The results from inverse experiments indicate that the most plausible conditions are within the range 1000 °C (excess water) and 1100–1200 °C (subsaturated and dry) at pressures of 1.5 to 2.0 GPa. A tectonic scenario implying melting of subducted sediments within an ascending mantle-wedge plume is suggested for the generation of primary ferrosilicic melts at the Gondwana continental margin during Upper Cambrian to Lower Ordovician times.     

Alkaline-ultrabasic mantle-derived magmas, their sources, and crystallization features: Data of melt inclusion studies

To find out the reasons responsible for the diversity of igneous rocks forming the alkaline-ultrabasic carbonatite Krestovskiy massif (the Maimecha–Kotui province, Russia) we have studied melt inclusions in clinopyroxene of trachydolerites, porphyric melanephelinites, and tholeiites. It was established that the homogenization temperatures of inclusions in these rocks are rather close: 1140–1180 °C, 1190–1230 °C, and 1150–1210 °C, respectively. Compositions of melt inclusions in clinopyroxenes from different rocks are significantly different. The chemical composition of clinopyroxene of trachydolerites corresponds to that of trachybasalts and their derivatives. The inclusions are enriched in Sr, Ba, P, and S and their total sum of alkalies (at K ≥ Na) is never less than 5–6 wt.%. Inclusions from the rims of clinopyroxene phenocrysts in porphyric melanephelinites are similar in composition also to inclusions in trachydolerites. But in the cores of clinopyroxene phenocrysts the composition of inclusions corresponds to nephelinite melt. The composition of some melt inclusions in the intermediate and cores zones of clinopyroxene from porphyric melanephelinite has high SiO₂ (53–55 wt.%), MgO (8–9 wt.%) and a low (1–2 wt.%) total sum of alkalies (at Na ≥ K) and is depleted in Al₂O₃ (6–7 wt.%), which is similar to the composition of basaltic komatiites. The composition of inclusions in tholeiites is also basic, highly magnesian, and low-alkaline, Sr and Ba are rare to absent. Compared to the inclusions of basaltic komatiite composition, the inclusions in tholeiites are enriched in Al and depleted in Ca, Ti, and P. The melts trapped in clinopyroxenes from different rocks contain low (0.014–0.018 wt.%) water but they are enriched in F: from 0.37 wt.% in nephelinite melts to 0.1–0.06 wt.% in tholeiite and basaltic komatiite melts. Inclusions in all the rocks under study, host clinopyroxene, and the rocks themselves are significantly enriched in incompatible elements (1–2 orders of magnitude relative to the mantle norm). In tholeiites, the partitioning of these elements is rather uniform, while in trachydolerites and especially in melanephelinites it is contrasting with a drastic depletion in HREE relative to LREE, MREE, and HFSE. A conclusion is made that the Krestovskiy massif was formed by no less than three mantle-derived magmas: melanephelinite, tholeiite and basaltic komatiite. Magmas were generated in different magma sources at different depths with various degrees of enrichment in incompatible elements. These magmas were, most likely, dominated by melanephelinite magma. In intermediate chambers this magma differentiated to form derivative melts of nephelinite, trachydolerite–trachyandesite–trachyte compositions. Komatiite-basalt melts were, most likely, derivatives of primitive meimechite magmas.