Mantle‐derived and crustal melts dichotomy in northern Greece: spatiotemporal and geodynamic implications

Two distinct groups of subduction‐related (orogenic) granitoid rocks, one Jurassic and the other Tertiary, occur in the area between the Vardar (Axios) Zone and the Rhodope Massif in northern Greece. The two groups of granitoids differ in many respects. The first group shows evolved geochemical characters, it is not associated with mafic facies, and evidence of magmatic interaction between mantle‐ and crustal‐derived melts is lacking. The second group has less evolved geochemical characters, it is associated with larger amount of mafic facies, and magmatic interaction processes between mantle‐derived and crustal melts are ubiquitous as evidenced by mafic microgranular enclaves and synplutonic dykes showing different enrichment in K₂O, Ti, and incompatible elements. This kind of magmatism can be attributed to the complex geodynamic evolution of the area. In particular, we suggest that two successive subduction events related to the closure of the Vardar and the Pindos oceans, respectively, occurred in the investigated area from Late Jurassic to Tertiary. We relate the genesis of Jurassic granitoids to the first subduction event, whereas Tertiary granitoids are associated with the second subduction. Fluids released by the two subducted slabs induced metasomatic processes generating a ‘leopard skin’ mantle wedge able to produce mafic melts ranging from typical calc‐alkaline to ultra‐potassic. Such melts interacted in various amounts with crustal calc‐alkaline anatectic melts to generate the wide spectrum of Tertiary granitoids occurring in the study area. Copyright © 2004 John Wiley & Sons, Ltd.     

Rb/Sr isotope studies of Cretaceous granitoids in the central Chukchi region: correctness of geochronological interpretation of results

The validity of the Rb/Sr isotopic ages of Cretaceous granitoid complex in the central Chukchi region has been verified. The performed studies showed that the Rb/Sr isotopic ages of late-orogenic granitoids are not reliable. Their variation can be explained in terms of the mixing of primary mantle magmas with continental-crust matter. Most probably, the variation is due to the constant compositions of mixing components and their proportions.     

雪峰弧形构造带南段牛角界钨矿区花岗岩地球化学特征

湖南牛角界钨矿区花岗岩体位于雪峰弧形构造带南段。岩体主要由细粒黑云母二长花岗岩、中细粒黑云母二长花岗岩、中粗粒黑云母二长花岗岩组成,其中钨矿化与细粒、中细粒花岗岩有关。岩石地球化学特征表明,富碱[ω(Na2O+K2O)=7.58~8.23wt%)],富钾(K2O/Na2O=1.41~1.56),A/CNK=1.45~1.55,属过铝质高钾“S”型花岗岩。稀土元素总量较低(66.29×10^-6~107.78×10^-6),富集轻稀土,LREE/HREE为4.59~8.51,呈右倾型。δEu含量为0.13~0.48,远小于1,显示明显的负异常。在微量元素配分模式中,岩石富集K、W、Ba、Cs、Rb等大离子亲石元素,亏损Th、Nb、Ta、P、Zr、Yb等高场强元素。岩体形成于碰撞造山后环境,并且对牛角界钨矿的形成具有一定的贡献。     

内蒙古苏尼特左旗南两类花岗岩同位素年代学及其构造意义

内蒙古北部索伦缝合带（索伦-苏尼特左旗-锡林浩特）被多数中外学者认为是西伯利亚板块南缘和华北板块北缘的最终缝合带,本文选择该缝合带上苏尼特左旗南两类花岗岩-与俯冲有关和与碰撞有关的岩浆岩（分别叫弧岩浆岩和碰撞岗岩）进行同位素年代学研究,结果表明：（1）弧岩浆活动有两期,分别约为490Ma和310Ma(锆石U-Pb,SHRIMP);碰撞花岗岩的侵位年代在230-250Ma(Rb-Sr全岩和锆石U-Pb);（2）根据本文新的年代学数据,索伦缝合带的最终缝合时间可能是在230-310Ma,这显然不同于国内多数学者坚持的“晚泥盆世碰撞”模式;而Sengor等推测的“晚二又开展碰撞”模式与本文数据一致。     

阿尔金断裂带东段古生代花岗岩浆作用及其大陆动力学意义

造山带花岗岩浆作用一直是地学的重要研究方向, 它记录了地球动力学深部过程的信息, 开展深入的研究工作可以更好的了解板块汇聚环境的陆壳生长和再造以及壳幔之间的相互作用。北祁连造山带是一典型的早古生代造山带, 先后经历了洋盆的打开到闭合, 敦煌地块则是主要由前寒武纪TTG片麻岩和变质表壳岩组成。北祁连造山带和敦煌地块分别位于阿尔金断裂带东段的东南侧和西北侧, 且均出露有大面积的古生代花岗岩体。本文以阿尔金主断裂两侧产出的花岗岩类为研究对象, 涉及北祁连造山带中的赵家庄二长花岗岩, 石包城复式岩体(花岗岩、正长花岗岩和花岗闪长岩)和红柳河花岗岩, 敦煌地块中的党河水库花岗闪长岩、沙枣园二长花岗岩、安盆沟复式岩体(正长花岗岩和花岗岩)以及小草湖似斑状花岗岩。通过对上述花岗岩体的岩相学、锆石U-Pb年代学、地球化学和锆石Hf同位素的研究, 取得了新的认识:     

俄罗斯远东地区晚中生代花岗岩类的时空分布 及其地质意义

在俄罗斯远东地区晚中生代花岗岩类年龄和相关地球化学数据的基础上,初步建立了该区晚中生代花岗岩类的年代学格架：大致以145Ma为界,分为侏罗纪(178~151Ma)和早白垩世(142~122Ma)2期。侏罗纪的花岗岩类主要为花岗岩－花岗闪长岩－石英二长岩组合,总体上为准铝质—强过铝质高钾钙碱性系列;早白垩世的花岗岩类主要为花岗岩－石英闪长岩－石英二长岩组合,主要为过铝质钙碱性—高钾钙碱性系列—钾玄岩系列。2期花岗岩稀土元素配分曲线均呈右倾型,重稀土元素曲线较平坦,都富集大离子亲石元素(如U、K)和轻稀土元素。与中国东北地区晚中生代花岗岩类对比,中国东北地区总体以兴安岭为中心,中间为早白垩世的花岗岩类,两侧为侏罗纪花岗岩类对称分布。境内外的侏罗纪花岗岩类构造背景不同,其分布与鄂霍次克洋和太平洋板块的俯冲有关,早白垩世花岗岩类可能形成于鄂霍次克带挤压造山后的伸展垮塌和太平洋板块的俯冲弧后伸展阶段。     

Jurassic–Cretaceous tectonic evolution of Southeast China: geochronological and geochemical constraints of Yanshanian granitoids

We report results of laser ablation inductively coupled plasma-mass spectrometry-based dating, as well as the analysis of bulk-rock major and trace elements, and Sr–Nd isotopes to address the genesis and tectonic settings of the Yanshanian granitoids in neighbouring sections of Zhejiang, Jiangxi, and Anhui provinces (the WZG region) within the Yangtze block. Geochronological results indicate that intense magmatic activity took place during Jurassic to Cretaceous time in the WZG region. Three episodes can be clearly distinguished by their bulk-rock geochemistry. (1) Early–Middle Jurassic granitoids (180–170 Ma) have high Sr and low Yb content, high ?_Nd(t) and low initial ⁸⁷Sr/⁸⁶Sr ratios, and weakly negative Eu anomalies. These granitoids are strongly enriched with LREE, Rb, K, and Th but are depleted of HREE, Nb, and Ta. (2) Late Jurassic to Early Cretaceous granitoids (165–140 Ma) have relatively low Sr and low Yb contents, as well as low ?_Nd(t) and high initial ⁸⁷Sr/⁸⁶Sr ratios, with characteristics similar to those of the Early–Middle Jurassic granitoids in terms of the rare earth element and trace element patterns. (3) Early Cretaceous granitoids (140–120 Ma) have extremely low Sr and high Yb concentrations, as well as high SiO₂ but low MgO, CaO, and Al₂O₃ content, with strong negative anomalies in Eu, Ba, Sr, P, and Ti. These characteristics indicate that the WZG Jurassic granitoids were related to northwestward subduction of the Izanagi plate, whereas the Early Cretaceous granitoids formed in a within-plate extensional setting. The time of transition between the two tectonic environments can be constrained to ～140 Ma. This tectonic transition may be attributed to progressive slab roll-back of the Izanagi plate. The presence of two A-type granite belts in the WZG region probably reflects lithospheric thinning. The NE trend of the A-type granite belts indicates that this extension in Southeast China was controlled by underflow of the Izanagi plate.     

Deciphering the geochronology of a large granitoid pluton (Karkonosze Granite,SW Poland): an assessment of U–Pb zircon SIMS and Rb–Sr whole-rock dates relative to U–Pb zircon CA-ID-TIMS

Granitoid plutons are often difficult to radiometrically date precisely due to the possible effects of protracted and complex magmatic evolution, crustal inheritance, and/or partial re-setting of radiogenic clocks. However, apart from natural/geological issues, methodological and analytical problems may also contribute to blurring geochronological data. This may be exemplified by the Variscan Karkonosze Pluton (SW Poland). High-precision chemical abrasion (CA) ID-TIMS zircon data indicate that the two main rock types, porphyritic and equigranular, of this igneous body were both emplaced at ca. 312 Ma, while field evidence points to a younger age for the latter. This is in contrast to the earlier reported SIMS (SHRIMP) zircon dates that scattered mainly between ca. 322 and 302 Ma. In an attempt to overcome this dispersion, at least in part caused by radiogenic lead loss, the CA technique was used before SHRIMP analysis. The ²⁰⁶Pb/²³⁸U age obtained in this way from a sample of porphyritic granite is 322 ± 3 Ma, ~16 Ma older than the untreated zircons; another porphyritic sample yielded a mean age of 319 ± 3 Ma, and the mean age was 318 ± 4 Ma for an equigranular granite sample – all three somewhat older than the age obtained by ID-TIMS. Older SIMS dates of ca. 318–322 Ma might indicate either faint inheritance or that zircon domains crystallized during earlier stages of Karkonosze igneous evolution. The ID-TIMS results have been used to re-assess the whole-rock Rb–Sr data. Excluding a porphyritic granite with excess radiogenic ⁸⁷Sr, it appears that isotopic homogeneity was achieved for most samples during the 312 Ma event, as shown by a pooled 21-point isochron with an age of 311 ± 3 Ma and an initial ⁸⁶Sr/⁸⁶Sr of 0.7067 ± 4. Local crustal contamination by stopping of metapelitic material might account for the more radiogenic Sr isotope signature observed in biotite-rich schlieren. A critical re-evaluation of all available SHRIMP data using the ID-TIMS age of 312 Ma as a benchmark suggests that the observed scatter may be partly attributed to analytical and methodological problems, in particular failing to distinguish subtly discordant spots from truly concordant ones, which is a serious limitation of the microbeam analytical approach. Other likely pitfalls contributing to geochronological scatter are identified in the published Re–Os ages on molybdenite and the ⁴⁰Ar/³⁹Ar data on micas. A scenario postulating a 15–20 milliion year evolution of the Karkonosze Pluton cannot be established on the basis of available geochronological data, which rather supports a brief igneous event, although a more protracted pre-emplacement evolution is possible. A short timescale for crystallization of large igneous bodies, as suggested by the ID-TIMS data from the Karkonosze Granite, is in line with models of transport of granitic magmas through dikes to form large plutons.     

新疆东天山花岗岩类岩石形成的物理化学条件

通过对新疆东天山花岗岩类岩石中的角闪石、斜长石、碱性长石、磷灰石、黑云母成分的系统研究,确定花岗岩类岩石在11—17km深处结晶;无矿化、蚀变的二长花岗岩结晶温度约550℃,花岗闪长岩、石英二长岩的结晶温度为552—650℃,钠长石化的含矿二长花岗岩其二长石平衡温度均低于510℃,蚀变愈强烈,平衡温度愈低。二长花岗岩的起源温度为780—840℃,石英二长岩的起源温度为860—880℃,碱长花岗岩的起源温度最低。推算得到二长花岗岩的极限起源深度为28—32km,石英二长岩的极限起源深度为33—34km。HF,HCI逸度计算表明,它们趋向于岩浆晚阶段富集,但无论是早结晶的磷灰石,还是晚结晶的黑云母,总是钠长石化的含矿花岗岩低于正常无蚀变、无交代现象花岗岩的逸度值,正常花岗岩类岩石由黑云母指示的f_(HC(?)),f_(HF)分别为36.60—51.10kPa,1.20—1.60 kPa。花岗岩类岩石形成的fo_2为1.05×10~(-11)—3.11×10~(-11)Pa,而钠长石化的含矿二长花岗岩具最低的氧逸度,为1.48×10~(-17)—2.45×10~(-15)Pa。     

Phase Equilibria of the Lyngdal Granodiorite (Norway): Implications for the Origin of Metaluminous Ferroan Granitoids

The Proterozoic (950 Ma) Lyngdal granodiorite of southern Norwaybelongs to a series of hornblende–biotite metaluminousferroan granitoids (HBG suite) coeval with the post-collisionalRogaland Anorthosite–Mangerite–Charnockite (AMC)suite. This granitoid massif shares many geochemical characteristicswith rapakivi granitoids, yet granodiorites dominate over granites.To constrain both crystallization (P, T, fO₂, H₂O in melt) andmagma generation conditions, we performed crystallization experimentson two samples of the Lyngdal granodiorite (with 60 and 65 wt% SiO₂) at 4–2 kbar, mainly at fO₂ of NNO (nickel–nickeloxide) to NNO + 1, and under fluid-saturated conditions withvarious H₂O–CO₂ ratios for each temperature. Comparisonbetween experimental phase equilibria and the mineral assemblagein the Lyngdal granodiorite indicates that it crystallized between4 and 2 kbar, from a magma with 5–6 wt % H₂O at an fO₂of NNO to NNO + 1. These oxidized and wet conditions sharplycontrast with the dry and reduced conditions inferred for thepetrogenesis of the AMC suite and many other rapakivi granitesworldwide. The high liquidus temperature and H₂O content ofthe Lyngdal granodiorite imply that it is not a primary magmaproduced by the partial melting of the crust but is derivedby the fractionation of a mafic magma. Lyngdal-type magmas appearto have volcanic equivalents in the geological record. In particular,our results show that oxidized high-silica rhyolites, such asthe Bishop Tuff, could be derived via fractionation of oxidizedintermediate magmas and do not necessarily represent primarycrustal melts. This study underlines the great variability ofcrystallization conditions (from anhydrous to hydrous and reducedto oxidized) and petrogenetic processes among the metaluminousferroan magmas of intermediate compositions (granodiorites,quartz mangerites, quartz latites), suggesting that there isnot a single model to explain these rocks. KEY WORDS: ferroan granitoids; crystallization conditions; experiments; Norway; Sveconorwegian; Bishop Tuff