Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7057 to 0.7114; ε_Nd values from −2.7 to −7.4; ∂¹⁸O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial ⁸⁷Sr/⁸⁶Sr=0.7143–0.7167; initial ε_Nd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.     

Contexte lithostructural, âges 40K40Ar et géochimie du volcanisme calco-alcalin tertiaire de Cap-d'Ail dans le tunnel ferroviaire de Monaco

Lithostratigraphic and structural framework, chronology and geochemistry of the Tertiary calc-alkaline volcanism of Cap-d'Ail (French Maritime Alps) are specified according to the new data collected during the earthworks of the recent railway tunnel built between Cap-d'Ail and Monaco. Two different magmatic events were dated: the first one, to Palaeogene, the other one, to Neogene. To cite this article: J.-P. Ivaldi et al., C. R. Geoscience 335 (2003).     

Les granitoı̈des hercyniens post-collisionnels du Maroc oriental : une province magmatique calco-alcaline à shoshonitique

The post-collisional Hercynian granitoids crop out in the easternmost part of the Moroccan Hercynian belt. Petrographical and geochemical studies show a composition similarity in the various granitoids. The granitoids belong to per-aluminous and metaluminous magmatic associations. They have evolved according to a scheme similar to high-K calc-alkaline to shoshonitic associations. To cite this article: H. El Hadi et al., C. R. Geoscience 335 (2003).     

四川理塘地区花岗闪长岩特征及其增生楔弧岩浆活动

【研究目的】通过查明理塘地区拉扎嘎山花岗闪长岩的年龄、地球化学特征,探讨花岗闪长岩形成的时代、成因及构造背景,为研究甘孜—理塘洋盆俯冲增生构造演化过程提供依据。【研究方法】选取甘孜—理塘蛇绿混杂岩带俯冲增生楔内花岗闪长岩,系统开展岩相学、LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究。【研究结果】花岗闪长岩含有大量的角闪石、黑云母等铁镁矿物,局部见大量的闪长质包体和围岩捕掳体。岩体形成于晚三叠世（（207.2±1.5） Ma）,岩石属I型钙碱性准铝质花岗岩类,具富集大离子亲石元素Rb、Ba、K、Th、U,亏损高场强元素Nb、Ta、P、Zr、Ti,显示轻稀土富集、重稀土亏损的右倾式配分模式,具有Eu的负异常,是典型的火山弧型花岗岩。【结论】结合区域地质资料及本文研究成果,认为四川理塘地区拉扎嘎山花岗闪长岩与甘孜—理塘洋向西俯冲致使中咱地块东缘增生楔不断扩大密切相关,是增生楔杂岩熔融成不同类型岩浆混合的产物。创新点：四川理塘地区拉扎嘎山花岗岩形成于晚三叠世,具典型的火山弧型花岗岩地球化学特征,形成于甘孜—理塘洋西向俯冲致使增生楔杂岩熔融,为甘孜—理塘洋俯冲增生构造演化提供了新的证据。     

铜陵中酸性侵入岩成因及锆石SHRIMP定年

铜陵地区高钾钙碱性系列侵入岩包括辉长闪长岩+石英二长闪长岩+花岗闪长岩,橄榄安粗岩系列侵入岩包括辉石二长闪长岩+二长岩+石英二长岩。高钾钙碱性系列侵入岩中含有富云母包体和微粒闪长质包体以及镁铁质石英二长闪长质包体,与区内的铜(金)矿化关系密切;橄榄安粗岩系列岩体中含多种深源堆积岩包体,与金(铜)、银、铅、锌等矿化关系密切。锆石SHRIMP定年结果表明,石英二长闪长岩为140Ma左右,花岗闪长岩为142～146Ma,辉长闪长岩为143Ma,辉石二长闪长岩为138～142Ma,二长岩为143Ma,可见两个系列侵入岩的年龄既有一定的差别,也有一些重叠。岩石地球化学及包体岩石学研究表明,橄榄安粗岩系列岩石可能为幔源碱性玄武质岩浆分异后形成,高钾钙性系列岩石可能为分异的幔源岩浆与壳源岩浆混合后形成。     

内蒙古色日崩地区白音高老组流纹岩岩石地球化学特征及地质意义

内蒙古西乌旗色日崩地区早白垩世白音高老组流纹岩为高硅（SiO₂=78.32%~86.80%）、富碱（K₂O+Na₂O=3.84%~7.63%）、高铝（Al₂O₃=8.59%~13.70%）,低MgO、CaO、TiO₂、MnO,稀土元素与微量元素地球化学特征显示其类似于A型花岗岩特征;低Ti/Y、Nb/Y与Ti/Zr比值,高Rb/Nb与Rb/Y比值以及Sr、Eu强烈亏损,表明白音高老组流纹岩为上地壳物质部分熔融成因,形成于早白垩世非造山板内伸展环境.     

云南腾冲火山岩岩石学特征

腾冲火山岩群是我国著名的新生代火山岩群之一。岩相学特征表明,该火山群为典型的钙碱性系列玄武岩安山岩英安岩弧火山岩组合。它喷发于晚上新世-晚更新世,但这一时期腾冲地区已为大陆板内环境,腾冲弧火山岩的形成至少在俯冲作用停止以后60Ma,属于一种滞后型弧火山。     

铜陵矿集区燕山期中酸性侵入岩地球化学特征及其地质意义

铜陵矿集区燕山期中酸性侵入岩由橄榄玄粗质系列和高钾钙碱性系列岩石组成,两者的岩石化学和地球化学特征存在差异,相互间不存在岩浆分异演化关系.稀土和微量元素以及O、Pb、Sr、Nd同位素研究表明,高钾钙碱性系列侵入岩的原始岩浆以下地壳深变质岩部分熔融岩浆为主,在有幔源玄武质岩浆注入的情况下先于橄榄玄粗质系列岩浆侵位形成;起源于富集地幔的碱性玄武岩与有限的地壳物质发生同化混染并通过结晶分异作用(AFC)形成了橄榄玄粗质系列侵入岩.下地壳或岩石圈地幔拆沉继而环流热幔上涌是本区侵入岩形成的直接起因;同时,侵入岩浆的强烈活动也是本区构造环境由挤压向伸展转换的标志.     

富金斑岩铜矿床研究进展

富金斑岩型铜矿床作为斑岩型矿床的一类,自20世纪70年代起逐渐引起了人们的重视.近年来又取得了很多重要进展,主要体现在以下5个方面:①富金斑岩型铜矿不仅在全世界范围内大量发现,而且部分矿床规模巨大;②绝大多数富金斑岩型矿床集中在新生代和中生代产出,尤以第三纪最为普遍;③富金斑岩型铜矿床不仅仅发育于汇聚板块边缘的岩浆弧环境,在大陆碰撞带甚至是陆内环境也发现了大量此类矿床;④含矿斑岩绝大多数为钙碱性岩浆系列,但部分矿床与高钾钙碱性(甚至钾玄质)岩浆密切相关;⑤富金斑岩型矿床金的富集,与大地构造背景、成矿时代、含矿斑岩性质、围岩性质、蚀变与矿化类型等因素关系不大,而主要受地幔岩浆过程、岩浆-热液过程及热液过程控制.