南太行山闪长岩的SHRIMP锆石U-Pb年龄及岩石成因研究

南太行山符山和东冶角闪闪长岩岩体的精细SHRIMP锆石U-Pb定年和元素-同位素地球化学研究表明:符山角闪闪长岩体形成于126.7±1.1Ma,东冶角闪闪长岩体形成于125.9±0.9Ma,与区内基性侵入岩和北太行地区侵入杂岩具有相似的形成年龄,表明晚中生代(±130Ma左右)太行山地区经历了与华北陆块同期的重要构造岩浆事件。区内闪长质岩石SiO_2=54.84％～65.75％,MgO=1.31％～3.89％,K_2O Na_2O=6.53％～11.40％,mg值=0.36～0.58,(La/Yb)_(cn)=9.86～22.77,(Gd/Yb)_(cn)=1.51～2.00;Eu/Eu=1.00～1.23,以富集LREE、LILE元素和明显亏损Nb-Ta、Zr-Hf-Ti等高场强元素为特征。~(87)Sr/~(86)Sr(t)=0.705363～0.706165,ε_(Nd)(t)=-13.8～-16.8,源自于EMI型富集岩石圈地幔,可解释为新生地幔底侵物质熔融后经过结晶分异作用的产物。与华北克拉通内部其他地区一样,早白垩世南太行山地区处于软流圈上涌的岩石圈伸展构造背景。     

扬子地台西南缘传统型铂族矿产地质特征

传统型铂族矿产,系指与镁铁质岩浆成矿作用有关的铂族矿产资源。华力西运动时期,扬子地台西南缘沿超壳深断裂带发生的大陆裂谷作用,为来自上地幔的镁铁质(拉斑玄武岩质)岩浆的上涌和侵位提供了极为有利的前提条件。含铂基性超基性岩的时空分布,受到大陆裂谷作用的主要发生发展时期和裂谷活动带的控制。通过对典型矿床特征及其成矿作用的探讨,论述了扬子地台西南缘主要的铂族矿床类型;并从四维成矿的角度,阐述了对区域成矿规律的一些基本认识。     

Petrogenesis of the ü?kap?l? Granitoid and its Mafic Enclaves in Elmal? Area (Ni?de, Central Anatolia, Turkey)

The Late Cretaceous ükapili Granitoid including mafic microgranular enclaves intruded into metapelitic and metabasic rocks, and overlain unconformably by Neogene ignimbrites in the Ni de area of Turkey. It is mostly granite and minor granodiorite in composition, whereas its enclaves are dominantly gabbro with a few diorites in composition. The ükapili Granitoid is composed mainly of quartz, K-feldspar, plagioclase, biotite, muscovite and minor amphibole while its enclaves contain mostly plagioclase, amphibole, minor pyroxene and biotite. The ükapili Granitoid has calcalkaline and peraluminous (A/CNK= 1.0-1.3) geochemical characteristics. It is characterized by high LILE/HFSE and LREE/HREE ratios ((La/Lu) N = 3-33), and has negative Ba, Ta, Nb and Eu anomalies, resembling those of collision granitoids. The ükapili Granitoid has relatively high 87Sr/86Sr (i) ratios (0.711189-0.716061) and low εNd (t) values (-5.13 to -7.13), confirming crustal melting. In contrast, the enclaves are tholeiitic and metaluminous, and slightly enriched in LILEs (K, Rb) and Th, and have negative Ta, Nb and Ti anomalies; propose that they were derived from a subduction-modified mantle source. Based on mineral and whole rock chemistry data, the ükapili granitoid is H-(hybrid) type, post-collision granitoid developed by mixing/mingling processes between crustal melts and mantle-derived mafic magmas.     

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone), I. Chemistry of MORB-like magmas

Geochemical data are presented for the meta-igneous, mafic-ultramafic complex near Finero. This complex is in contact with a phlogopite-bearing mantle peridotite and is subdivided into the Internal Gabbro unit, the Amphibole Peridotite unit, and the External Gabbro unit. The Internal Gabbro and the Amphibole Peridotite units consist of coarse-grained, chemically heterogeneous cumulates, whereas the External Gabbro unit is generally massive, chemically more uniform and approximately representative of the residual melt with MgO contents between 6.6 and 9.1% and Mg numbers between 38 and 58. Both whole-rock and mineral contents of Ni and Cr are significantly higher (at similar Mg numbers) in the Amphibole Peridotite unit than in the Internal Gabbro unit. The most straightforward interpretation of this is that the Amphibole Peridotite unit accumulated after the influx of fresh mafic (or ultramafic) magma into the magma chamber. Major-element chemical trends are continuous from the Amphibole Peridotite unit to the External Gabbro unit and are consistent with closed-system fractionation with no further addition of magma or contamination by wall or roof rock assimilation. In the External Gabbro unit, total FeO and TiO₂ contents are strongly correlated with each other (and with P₂O₅ and Zr) and reach values as high as 19 and 4%, respectively, indicating an advanced degree of crystal fractionation along a tholeftic trend. The External Gabbro samples have generally smooth normalized trace element patterns, which are consistent with being representative of a liquid composition. The residual nature of the External Gabbro magma is also indicated by negative Eu and Sr anomalies, clear evidence for prior feldspar fractionation. REE patterns are otherwise indistinguishable from N-type MORB, but Th and U are significantly more depleted than in MORB. This Th and U depletion is similar to that found in olivine basalts and picrites on Iceland and Hawaii; its origin is not well understood. No evidence is seen for any assimilation of crystal material, in sharp contrast with the situation of the igneous complex in Val Sesia near Balmuccia, where the magma composition is dominated by assimilation of crust. We suggest that the heat provided by at most two injections of magma near Finero was insufficient to induce crystal anatexis, in contrast with the excess heat supplied by multiple magma injections at Balmuccia.     

Petrogenesis of mid-Carboniferous volcanics and granitic intrusions from western Junggar Basin boreholes: geodynamic implications for the Central Asian Orogenic Belt in Northwest China

The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high ε_Nd(t) values (5.3–5.6) and initial ⁸⁷Sr/⁸⁶Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)_N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted ε_Nd(t) (5.6–6.4) and ε_Hf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.     

大别山北部蛇绿岩的地球化学制约

常量元素,微量元素,稀土元素,Ｎｄ同位素及氧同位素地球化学特征表明,太别山北部变质镁铁－超镁铁质岩带中存在变质的蛇绿岩,它主要由变质橄榄岩,辉长岩（－辉绿岩？）和基性熔岩三部分组成,其中,变质的基性熔岩的亏损地幔模式年龄ｔＤＭ／Ｍａ＝１０３６．８～１２９３．８,εＮｄ（ｔ）＝７．２～７．７,表明它可能代表１０００Ｍａ左右形成于中等扩张速率（２ｃｍ／ａｄ左右）洋盆条件下的洋壳残片。     

Discriminating characters of ore-forming intrusions in the super-large Chalukou porphyry Mo deposit,NE China

The Chalukou porphyry Mo deposit, located in the Great Hinggan Range, is the largest Mo deposit in northeast China, although the age and genesis of the associated magmatic intrusions remain debated.Here we report zircon U-Pb ages and trace elements, whole rock geochemistry and Sre Nd isotope data with a view to understand the relationship between the magmatism and molybdenum mineralization.Zircon U-Pb analysis yield an age of 475 Ma for rhyolite in the older strata, 168 Ma for the premineralization monzogranite, and 154 Ma for the syn-mineralization granite porphyry. The granite porphyry and quartz porphyry are considered as the ore-forming intrusions. These rocks are peraluminous, alkali-calcic, and belong to high-K to shoshonitic series with a strong depletion of Eu. They also display characteristics of I-type granites. The rocks exhibit wide variations of(87 Sr/86 Sr)iin the range of 0.705426 -0.707363, and ε_(Nd)(t) of -3.7 to 0.93. Zircon REE distribution patterns show characteristics between crust and the mantle, implying magma genesis through crust-mantle interaction. The Fe_2O_3/FeO values(average 1) for the whole rock and EuN/Eu*Nvalues(average 0.45), Ce~(4+)/Ce~(3+) values(average 301)for zircon grains from the granite porphyry are higher than those from other lithologies. These features suggest that the ore-forming intrusions(syn-mineralization porphyry) had higher oxygen fugacity conditions than those of the pre-mineralization and post-mineralization rocks. The Chalukou Mo deposit formed in relation to the southward subduction of the Mongol-Okhotsk Ocean. Our study suggests that the subduction-related setting, crust-mantle interaction, and the large-scale magmatic intrusion were favorable factors to generate the super-large Mo deposits in this area.     

Identifying gravity anomalies caused by granitic intrusions in Nanling mineral district,China: a multifractal perspective

Several power‐law relationships of geophysical potential fields have been discussed recently with renewed interests, including field value–distance () and power spectrum–wavenumber () models. The singularity mapping technique based on the density/concentration–area (C–A) power‐law model is applied to act as a high‐pass filter for extracting gravity and magnetic anomalies regardless of the background value and to detect the edges of gravity or magnetic sources with the advantage of scale invariance. This is demonstrated on a synthetic example and a case study from the Nanling mineral district, Southern China. Compared with the analytic signal amplitude and total horizontal gradient methods, the singularity mapping technique provides more distinct and less noisy boundaries of granites than traditional methods. Additionally, it is efficient for enhancing and outlining weak anomalies caused by concealed granitic intrusions, indicating that the singularity method based on multifractal analysis is a potential tool to process gravity and magnetic data.     

Dating of altered mafic intrusions by applying a zircon fission track thermochronometer to baked country rock,and implications for the timing of volcanic activity during the opening of the Japan Sea

Once a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area.