北太行紫荆关基性岩体的成因: 岩石学和地球化学证据

通过对北太行紫荆关晚中生代基性岩体的岩石学和地球化学特征研究, 讨论其岩浆源区特征及成因.结果表明, 紫荆关岩体主要由橄榄二辉石岩、角闪辉长岩和闪长岩等岩石组成.橄榄二辉石岩具有明显的堆晶结构, 可能是堆晶成因, 而角闪辉长岩和闪长岩可能是堆晶形成之后的残余岩浆的结晶.闪长岩中的斜长石显示复杂的结构和成分环带, 相对富钠的核部见富钙斜长石叠加, 后者又向富钠斜长石演化.闪长岩中的单斜辉石也显示复杂的成分环带, 主要是最边部的MgO含量比紧邻的内侧辉石更高.这表明在闪长岩形成过程中有新的幔源高温富镁的岩浆注入.以上各岩石类型具有类似的地球化学性质, 包括较强的稀土分异((La/Yb)_N比值为5.76~23.13), 高Sr/Y比值(50.95~146.13), 富集K、Sr、Ba等大离子亲石元素和LREE, 亏损Nb、Zr、Ti等高场强元素和HREE, Eu异常不明显.都显示富集的Sr-Nd同位素特征, I_Sr=0.7058~0.7066, ε_Nd(t) =-13.8~-18.2.这些特征表明紫荆关基性岩体的母岩浆来源于EM1型富集地幔的部分熔融.橄榄辉石岩和角闪辉长岩具有类似的Nd同位素成分(主要为ε_Nd(t) =-15.0~-16.6), 但形成较晚的闪长岩的ε_Nd(t) 值却比较高(-13.8~-14.6), 也支持新的幔源岩浆在晚期的注入.      

秦岭南召岩体中花岗岩脉的锆石U-Pb定年:对燕山期构造环境的约束

通常认为,花岗岩体中的花岗岩脉与岩体是同一次岩浆活动的产物,是深部演化的岩浆沿着岩体的裂缝侵入后冷凝形成的。南召岩体位于北秦岭东部,近年来在岩体中发现一条宽约5 m的花岗岩脉。岩脉呈紫红色,斑状结构,斑晶为石英。LA-MC-ICP-MS锆石U-Pb定年得出该岩脉的形成时间为(119.6±0.7)Ma,与岩体的年龄(452.3±6.2)Ma相差很大。锆石Hf同位素测试结果表明,岩脉的源岩可能主要是新生地壳,结合地球化学研究分析得出该岩脉呈现I型花岗岩的特性。该岩脉的年龄、Hf同位素和地球化学特征同邻近的伏牛山岩体相近,推测岩脉的源岩可能和伏牛山岩体的源岩相同。根据研究结果及区域地质构造分析,认为燕山期由于太平洋板块的俯冲作用和陆內拉张作用,导致扬子板块及华北板块沿秦岭造山带形成一系列断裂。岩浆沿着断裂上侵,大部分形成了伏牛山岩体,小部分沿着周围的裂隙侵入形成岩脉,并在侵位的过程中与寄主岩发生物质交换。     

The genetic relationship between mafic dike swarms and plutonic reservoirs in the mesozoic of central chile (30°–33°45′S): insights from AMS and geochemistry

Five mafic dike swarms between 30° and 33°45′S were studied for their geochemical signature and kinematics of magma flow directions by means of AMS data. In the Coastal Range of central Chile (33°−33°45′S), Middle Jurassic dike swarms (Concón and Cartagena dike swarms, CMDS and CrMDS, respectively) and an Early Cretaceous dike swarm (El Tabo Dike Swarm, ETDS) display the presence of dikes of geochemically enriched (high-Ti) and depleted (low-Ti) basaltic composition. These dikes show geochemical patterns that are different from the composition of mafic enclaves of the Middle Jurassic Papudo-Quintero Complex, and this suggests that the dikes were injected from reservoirs not related to the plutonic complex. The mantle source appears to be a depleted mantle for Jurassic dikes and a heterogeneous-enriched lithospheric mantle for Cretaceous dikes. In the ETDS, vertical and gently plunging magma flow vectors were estimated for enriched and depleted dikes, respectively, which suggest, together with variations in dike thickness, that reservoirs were located at different depths for each dike family. In the Elqui Dike Swarm (EDS) and Limarí Mafic Dike Swarm (LMDS), geochemical patterns are similar to those of the mafic enclaves of the Middle Jurassic plutons. In the LMDS, east to west magma flow vectors are coherent with injection from neighbouring pluton located to the east. In the EDS, some dikes show geochemical and magma flow patterns supporting the same hypothesis. Accordingly, dikes do not necessarily come from deep reservoir; they may propagate in the upper crust from coeval shallow pluton chamber. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Origin of nelsonite and Fe–Ti oxides ore of the Damiao anorthosite complex,NE China: Evidence from trace element geochemistry of apatite,plagioclase, magnetite and ilmenite

Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO₂-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.     

Multi-stage emplacement of the G?temar Pluton, SE Sweden: new evidence inferred from field observations and microfabric analysis, including cathodoluminescence microscopy

The emplacement of the Mesoproterozoic G?temar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The G?temar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8?km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx.     

新疆东准噶尔老鸦泉碱性岩及相关锡矿的岩石地球化学特征

老鸦泉富碱花岗岩杂岩体与侵入其中的富碱花岗斑岩岩体及云英岩型锡矿体、石英脉型锡矿体等REE配分曲线类似,均具有强烈的Eu亏损,它们的微量元素蛛网图等岩石地球化学特征也类似,是同源岩浆结晶分异演化的结果。锡矿体是老鸦泉富碱岩浆分异演化及以钠质为主的强烈碱交代自变质作用的最终产物。侵入于老鸦泉岩体内的花岗斑岩为锡矿体的直接围岩,它是岩浆结晶分异更晚期混入了更多地壳组分,并向酸性方向演化的产物,该花岗斑岩经自变质热液蚀变作用造成锡的富集并成矿。因此,碱性岩浆结晶分异晚期混有地壳组分,岩浆向酸性演化可能是碱性花岗岩形成锡矿的重要条件之一。     

佳木斯地块东北缘早二叠世六连岩体的岩浆混合成因：岩相学、年代学和地球化学证据

本文报道了佳木斯地块东北缘六连岩体中主岩花岗闪长岩和暗色微粒包体的岩相学、锆石U-Pb年代学、全岩地球化学以及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及其构造属性。主岩花岗闪长岩和暗色微粒包体中角闪辉长岩分别获得了284Ma 和278Ma 的成岩年龄,表明六连岩体形成于早二叠世而非前人认为的晚印支期。包体具有岩浆结构,部分包体存在塑性流变特征,包体中可见淬冷边、反向脉和针状磷灰石,包体和主岩中均发育矿物异常共生或不平衡结构,结合主岩和包体的年代学和地球化学特征可以判定六连岩体为早二叠世岩浆混合作用的产物。全岩地球化学和锆石Hf同位素特征揭示出,六连岩体中主岩和包体的原始岩浆分别起源于新元古代增生的深部陆壳基性火成岩和受俯冲流体交代的亏损地幔楔的部分熔融。结合同时代火成岩组合的空间变异特征以及区域构造演化历史,认为佳木斯地块东北缘早二叠世六连岩体形成于活动大陆边缘环境,其地球动力学机制与佳木斯地块东侧古洋板块的西向俯冲作用有关。     

Subducting sediment-derived arc granitoids: evidence from the Datong pluton and its quenched enclaves in the western Kunlun orogen, northwest China

This paper presents detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of the Datong pluton and its quenched enclaves from the western Kunlun orogen, northwest China, in an attempt to achieve a better understanding on the origin of diverse arc magmas. The Datong host granitoids are intermediate to acid in composition (SiO₂?=?57.5?~?73.1 wt.%), and exhibit high-K calc-alkaline to shoshonitic affinities. The quenched enclaves are silica-rich ultrapotassic rocks. Detailed SHRIMP zircon U–Pb dating indicates that the Datong pluton was emplaced in Ordovician time (473.4–447.7 Ma), which places the Datong pluton in an active continental margin setting, rather than a syn-collision setting of Early Silurian age. The Datong host granitoids were derived by partial melting of subducted sediments, with the subsequent melt interacting with the overlying mantle wedge during its ascent. Partial melting of the veined mantle wedge hybridized by sediment-derived melts generated the silica-rich ultrapotassic magma, which was injected into the Datong granitoid magma chamber and quenched, resulting in enclaves hosted by granitoids. This contribution provides evidence that arc magmas can be derived directly by partial melting of subducted sediments, which is helpful to further understand the origin of diverse arc magmas.     

Dike rock generation and magma interactions in the Bir Safsaf igneous complex,south-west Egypt: Implications for the Pan-African evolution in north-east Africa

The geological setting, ages, petrography and geochemistry of late Pan-African ( 580 Ma) calc-alkaline and tholeiitic dike rocks in the Bir Safsaf igneous complex of south-west Egypt are discussed. These basaltic to rhyolitic dikes intruded contemporaneously and shortly after the intrusion of granitoids. The major and trace element data, Sr and Nd isotope relations, in combination with textural observations, confirm complex interactions between most of the intermediate calcalkaline dike melts and plutonic melts, with different degrees of mixing, assimilation, replenishment and tapping of magma chambers. Trachytic and rhyolitic dikes are strongly differentiated melts from the granitic pluton. The tholeiitic dikes evolved dominantly by fractional crystallization processes. It is inferred that open system and closed system processes operated in calc-alkaline magma chambers, and that the calc-alkaline melts came from a garnet-and amphibole-bearing mantle, modified by a subduction component. Tholeiitic rocks were formed later by fractional crystallization and assimilation processes. Magma ascent of both dike types took place in an extensional environment and the presumed subduction zone has to be seen in connection with the Atmur-Delgo suture zone.     

Basites of the polychronous magmatic center with the Erdenetiyn-Ovoo porphyry Cu-Mo deposit (northern Mongolia): petrogeochemistry, 40Ar/39Ar geochronology,geodynamic position,and related ore formation

The Erdenetiyn-Ovoo magmatic center (EMC) with a porphyry Cu-Mo deposit includes the following intrusive complexes: Selenga, Shivota, ore-bearing porphyry, and post-ore dike. The EMC formed at 260–200 Ma. The geologic evolution of northern Mongolia in that period was much determined by the effect of a mantle plume, which showed two periods of activity: Late Paleozoic and Early Mesozoic. The long multistage evolution of the EMC was due to its localization on the periphery of the Late Paleozoic and Early Mesozoic areas of the plume’s influence. The Shivota and post-ore basites are considered to be comagmatic to the Late Permian–Early Triassic trachyandesite-basalt and Late Triassic–Early Jurassic trachyandesite series, respectively, which are similar to the products of Late Paleozoic and Early Mesozoic within-plate magmatism in northern Mongolia. The Selenga complex, which formed before the Shivota one, and the porphyry complex, which formed before the post-ore dike one, are differentiated gabbro-granite series. Gabbro-granitoid magmatism was initiated by the melting of rocks of continental lithosphere under the action of a plume. Later on, as the plume ascended to the surface and the lithosphere became thinner, the conditions were created favoring the lithosphere breakthrough and within-plate basaltoid magmatism.In geochemical features (high contents of LILE and LREE, low contents of HFSE and HREE) the studied basites are similar to the products of subduction magmatism. But this contradicts the geologic position of basites formed after the completion of subduction during the transition of the region to the rifting stage and during the rifting. The mantle metasomatized during the preceding subduction is regarded as the main source of basites. The high contents of alkalies and LREE in the volcanics of the post-ore dike complex and the REE patterns similar to the OIB ones evidence the influence of the plume on the magma formation. The high contents of incompatible trace elements and the Nd isotope composition corresponding to the weakly depleted mantle do not exclude a possible plume effect during the formation of the Selenga complex gabbroids. The geochemical features of the Shivota gabbros, comagmatic to volcanics produced during the Late Paleozoic within-plate activity, are partly transformed during the melt evolution in crustal chambers.The REE patterns of the EMC basites evidence that the evolution of ascending magma was accompanied by the fractionation of amphibole. During this process, ore elements were redistributed into mineral and concentrated in amphibole-containing rocks, from which metals were later mobilized by late melts and fluids. The evolution of basaltoid magmatism of the Selenga, Shivota, and porphyry complexes is regarded as a preliminary stage of ore formation, which was considerably responsible for the EMC productivity.     

Archean Enriched Mantle Beneath the Baltic Shield: Rare-Earth-Element Evidence from the Burakovsky Layered Intrusion,Southern Karelia,Russia

In order to better understand the origin and character of late-Archean mantle beneath the Baltic Shield, we have analyzed mafic-ultramafic rocks from one of the best-preserved, least-metamorphosed regions of Karelia, Russia. Trace-element data for samples from the ultramafic and gabbronorite zones of the large (700 km²) Burakovsky layered intrusion (BLI) are presented. Samples from the ultramafic zone are LREE enriched, indicating that they formed from a LREE-enriched parental magma. Indeed, a calculated parental magma for the ultramafic zone has a (Ce/Yb)_n ratio of 2.6, a (Nd/Sm)_n ratio of 1.1, and a (Dy/Yb)_n ratio of 1.6. The LREE enrichment in the parental magma suggests either that the source region was LREE enriched or that the melt was contaminated by crust en route to the BLI magma chamber. Samples from the gabbronorite zone also are LREE enriched and indicate two distinct parental magmas. Group-I magmas, from the lower part of the gabbronorite zone, have (Ce/Yb)_n ratios of 6.9 to 13.9, whereas Group-II magmas, from the upper portion, have (Ce/Yb)_n ratios of 15.8 to 27.3. Volcanic rocks in Karelia that are coeval to the Burakovsky layered intrusion, as well as volcanic rocks of a similar age in other parts of the Baltic Shield, also are LREE enriched. Furthermore, the BLI has an initial ε_Nd value of ?2.0, and other layered intrusions in the Baltic Shield of similar age also have negative initial ε_Nd values (e.g., ?1.8 to ?2.2). The consistency of these ε_Nd values for layered intrusions throughout Karelia precludes contamination as a controlling factor in their isotopic compositions. All of these data are most consistent with the development of LREE-enriched mantle beneath the eastern Baltic Shield, prior to the earliest Proterozoic.     

Mineralogy and petrography of a kimberlite-picrite dike in the northwestern Urik-Iya Graben,the eastern Sayan region

Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr₂O₃) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr₂O₃) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO₂, 0.61 Cr₂O₃, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.     

Petrogenesis of massif-type anorthosite complex, Gruber, Central Dronning Maud Land, East Antarctica： Implications for magma source and evolution

The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarctica. Field observations showed that the Gruber Complex is made up of gabbronorite-anorthosite pluton which was intruded by ferrodiorite dykes. Systematic samples collected from the Gruber Complex revealed significant geochemical variations within the region. Four rock types have been identified, based on modal proportions of mineral phases and their geochemistry data. Clinopyroxene-gabbronorite and plagioclase-gabbronorite are the two types of gabbronorite with the dominance of clinopyroxene and plagioclase, respectively. Anorthosite is represented by rocks having predominance of plagioclase with minor clinopyroxene. Ferrodiorite is characterized by modal abundance of orthopyroxene and Fe-Ti oxide. Major and trace element systematics showed that all the four rock types are co-magmatic and are related through fractional crystallization. Based on this study, it is reported that clinopyroxene was the first phase to crystallize followed by plagioclase and then Fe-Ti oxides. Furthermore, trace element composition of the parental melt was calculated using LA-ICPMS analysis of the most primitive, pure clinopyroxene found in the clinopyroxene gabbronorite. Our analyses suggested that the parental melt was similar to that of continental arc basalt and showed signatures of subduction-related metasomatism. Based on mineral chemical and geochemical data, it is interpreted that the parent melt went through changing sequence of crystallization which led to the formation of massive anorthosite.     

鄂东南地区存在古元古代—太古宙基底——来自铜鼓山岩体锆石U-Pb-Hf同位素的证据

对鄂东南地区位于毛铺—两剑桥断裂带上的铜鼓山岩体进行了野外地质及镜下显微研究及岩石化学分析, 重点分析了其中锆石U-Pb年龄和Hf同位素组成。结果表明铜鼓山岩体为石英闪长玢岩, 岩体形成于(147±2.6) Ma, 属晚侏罗世—早白垩世, 与鄂东南地区其它岩体年龄具有一致性。铜鼓山岩体中存在的大量继承锆石。分析的4个继承锆石形成于古元古代晚期1798~1888 Ma。继承锆石具有高的Th/U比值和极其相似的Lu-Hf同位素组成, 表明它们捕获于同一火成岩。这表明鄂东南地区存在古元古代基底。这些古元古代继承锆石低的εHf(t)值和冥太古代的Hf同位素两阶段模式年代暗示古元古代克拉通化是在太古宙基底上发展的。鄂东南地区和扬子陆块其它众多地区一样存在古元古代—太古宙基底。扬子陆块古元古代—太古宙基底极可能从四川盆地、鄂西崆岭和郧西地区东延至鄂东南地区。     

赣东北鹅湖岩体SHRIMP 锆石U-Pb 年龄、Sr-Nd-Hf 同位素地球化学与岩石成因

对赣东北鹅湖岩体进行了SHRIMP 锆石U-Pb 年代学、元素和Sr-Nd-Hf 同位素地球化学及岩石成因研究。SHRIMP 锆石U-Pb 定年结果表明,鹅湖岩体形成于早白垩世的（121.7±2.9） Ma;岩相学及元素和Sr-Nd-Hf 同位素特征表明鹅湖花岗岩属于S 型,主要是由地壳深处（至少40 km）的古元古代变质沉积岩发生部分熔融形成的,成岩过程中并没有发生强烈的分离结晶作用和幔源岩浆混合作用,赣东北地区早白垩世伸展构造背景造成的软流圈地幔上涌可能是下地壳岩石发生部分熔融的诱因。     

Morphology of the Nyurba kimberlite pipe and its relationship with the dolerite dike

Study of the magmatics in the Nakyn kimberlite field, with consideration of the isotope dating results, allowed us to establish a sequence of their formation. First, 368.5–374.4 Ma ago intrusions of the Vilyui–Markha dike belt formed. Then (363–364 Ma) intrusion of kimberlites took place. In the Early Carboniferous (338.2–345.6 Ma), alkaline basaltic magma intruded through faults controlling the kimberlites. The magmatic activity finished 331–324.9 Ma ago with the formation of explosive breccias. It has been found that the Nyurba kimberlite pipe consists of two bodies: their kimberlite melts have successively intruded through independent channels.     

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components.The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.     

Concentric zoning in the Tunk Lake pluton,coastal Maine

The Tunk Lake pluton of coastal Maine, USA is a concentrically zoned granitic body that grades from an outer hypersolvus granite into subsolvus rapakivi granite, and then into subsolvus non-rapakivi granite, with gradational contacts between these zones. The pluton is partially surrounded by a zone of basaltic and gabbroic enclaves, interpreted as quenched magmatic droplets and mushes, respectively, as well as gabbroic xenoliths, all hosted by high-silica granite. The granite is zoned in terms of mineral assemblage, mineral composition, zircon crystallization temperature, and major and trace element concentration, from the present-day rim (interpreted as being closer to the base of the chamber) to the core (interpreted as being closer to the upper portions of the chamber). The ferromagnesian mineral assemblage systematically changes from augite and hornblende with augite cores in the outermost hypersolvus granite to hornblende, to hornblende and biotite, and finally, to biotite only in the subsolvus granite core of the pluton. Sparse fine-grained basaltic enclaves that are most common in the outermost zone of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the upper portions of the magma chamber were occupied by a granitic crystal mush. However, the slight variations in initial Nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic melt played little role in generating the compositional gradation of the pluton. The zone of basaltic and gabbroic chilled magmatic enclaves, and gabbroic xenoliths, hosted by high-silica granite, that partially surround the pluton is interpreted as mafic layers at the base of the pluton that were disrupted by invading late-stage high-silica magma. These mafic layers are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse-grained gabbroic mush. Basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende-bearing, suggesting that their parent melts were relatively hydrous. The water-rich nature of the underplating mafic magmas may have prevented extensive invasion of the granitic magma by these magmas, owing to the much greater viscosity of the granitic magma than the mafic magmas in the temperature range over which magma interaction could have occurred.     

Geological and structural conditions of eclogitization of Paleoproterozoic basic dikes in the eastern Belomorian Mobile Belt

A new, Krasnaya Guba dike field that comprises more than 30 minor gabbronorite intrusions and dikes of “garnet” Fe-gabbro was revealed at the northeastern flank of the Belomorian Mobile Belt. Gabbronorite intrusions were emplaced along the faults that formed as a result of brittle failure under conditions of general extension. As follows from the structural relationships, dikes of Fe-gabbro are younger than gabbronorite. They were formed under compression and shearing. The mobile melt was pumped into gneisses that underwent ductile deformation. Eclogitization in the Krasnaya Guba dike field was selective and affected only dikes of Fe-gabbro and minor gabbronorite intrusions and did not develop in country rocks. The bodies of Fe-gabbro are eclogitized almost completely, whereas gabbronorite is transformed into eclogite only along shear zones. Eclogites were formed after the complete solidification of intrusive bodies, and their localization is controlled by the concentration of deformations owing to the different elastoplastic properties of gabbroic rocks and country gneisses and migmatites. The different degree of fracturing in basic dikes and country gneisses resulted in the concentration of fluid flows in fracture systems within competent rocks. Late Svecofennian pegmatites (1.86 Ga) distinctly crosscut eclogites that experienced retrograde metamorphism. The eclogitic rocks from the Krasnaya Guba are the youngest known in the Belomorye. The age of eclogitization is 2.12–1.86 Ga. A new finding of eclogitic rocks in the Belomorye, as well as previously known similar rocks near Gridino Village, emphasize tectonic activity of the northwestern Umba Collision Zone. The eclogitization of basic rocks in Krasnaya Guba is related to their desilification in shearing zones in the course of interaction of plagioclase with ortho-and clinopyroxene with the formation of garnet-omphacite aggregate and removal of silica. The currently observed texture, structure, and chemical and mineral compositions of eclogitic rocks are products of intense retrograde metamorphism.     

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling,China: insights from LA-ICP-MS zircon U–Pb ages,Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO₂ (63.59–76.22%) and low P₂O₅ (0–0.2%) concentrations, a high K₂O/Na₂O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous ε_Hf(t) (?4.425 to 1.067 for Xujiaba and ?4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.