Zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system

This study documents the chemical and textural responses of zircon in the Elba igneous complex, with particular reference to the 7- to 7.8-Ma-old Monte Capanne pluton in relation to its coeval volcanic counterpart (Capraia), using BSE imaging and quantitative electron microprobe analyses. The Monte Capanne pluton displays multiple field and geochemical evidence for magma mixing. The samples we have investigated (including monzogranitic, mafic enclave and dyke samples) display similar zircon textures and are associated with an extremely large range of trace and minor element (Hf, Y, HREE, Th, U) compositions, which contrast with relatively simple textures and zoning patterns in zircons from a Capraia dacite. We have used a relatively simple textural classification (patchy zoning, homogenous cores, oscillatory zoning and unzoned zircon) as the basis for discussing the chemical composition and chemical variation within zircons from the Monte Capanne pluton. Based on these data and other works (Dini et al. 2004 in Lithos 78:101–118, 2004) , it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the Monte Capanne magma chamber. In particular, mixing was responsible for the development of the patchy-zoning texture in the zircon cores, which was associated with reactions between other accessory phases (including monazite, apatite, allanite), which we infer to have significantly affected the Th distribution in zircon. Zircons from the MC pluton displaying “homogeneous cores” have chemical affinities with zircons in the coeval Capraia volcanic system, consistent with the participation of a Capraia-like mantle end-member during mixing. Further zircon growth in the MC pluton produced the oscillatory zoning texture, which records both long-term (crystal fractionation) and transient (recharge with both silicic and mafic magmas) events in a hybrid magma chamber. It is inferred that Hf and the Th/U ratio cannot be used alone to infer magmatic processes due to their dependency on temperature, nor are they a diagnostic feature of xenocrystic grains. This study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system.     

南秦岭迷魂阵岩体LA-ICP-MS锆石U-Pb年代学和Lu-Hf同位素特征

位于陕西省的姜家沟-磨沟峡-小岭镇地区的迷魂阵岩体,为南秦岭构造带中一个古老地块内的深成侵入体。该岩体主要由闪长岩、石英闪长岩和花岗闪长岩组成。根据野外地质关系和LA-ICPMS锆石U-Pb定年结果,可将迷魂阵岩体的岩浆作用分为两个阶段:早期岩浆作用阶段主要形成闪长岩,其侵位时代为885±4Ma;晚期岩浆阶段主要形成石英闪长岩-花岗闪长岩,其侵位时代为~737±4Ma。定年锆石原位Lu-Hf同位素分析揭示早期闪长质岩浆主要形成于亏损地幔的部分熔融,晚期石英闪长岩-花岗闪长岩岩浆主要来源于早期闪长质岩浆的结晶分异,并经历了地壳物质的混染或者壳幔岩浆混合作用。     

Geochronology and geochemical properties of the large-scale graphite mineralization associated with the Huangyangshan alkaline pluton,Eastern Junggar,Xinjiang, NW China

The Huangyangshan alkaline pluton is located within the southern part of the Eastern Junggar orogenic belt in Xinjiang Province, and forms part of the Kalamaili alkaline granite belt. The pluton hosts the Huangyangshan super-large graphite deposit, which develops unique spherical structure and coexists with metal sulfides. This study examines the genetic relationship between the alkaline magmatism that formed the pluton and the graphite mineralization using zircon LA–ICP–MS U–Pb dating, geochemical analysis for representative rock types in the Huangyangshan pluton, and new Re–Os isotope dating for the graphite in the Huangyangshan graphite deposit. Zircons from medium-grained arfvedsonite granite, medium–fine-grained amphibole granite, medium-grained biotite granite, and fine-grained biotite granite phases of the Huangyangshan pluton yield weighted mean U–Pb ages of 322.7 ± 4.5, 318.3 ± 4.0, 303.9 ± 2.1, and 301.1 ± 3.6 Ma, respectively, indicating that all of the granite phases were emplaced during the Late Carboniferous over a period of around 20 Myr. Six graphite samples from the deposit yield a Re–Os isochron age of 332 ± 53 Ma. Combining these ages with the genetic relationship between the graphite mineralization and magmatism in the study area and the relatively large uncertainty on the Re–Os isochron age for the graphite suggests that the mineralization formed at ca. 320 Ma. The graphite samples yield an initial ¹⁸⁷Os/¹⁸⁸Os value of 0.38 ± 0.2, indicative of carbon derived from a mixture of organic and mantle-derived sources. The different granite phases in the Huangyangshan pluton are geochemically similar with relatively high SiO₂ (75.6–78.2 wt%) and Na₂O + K₂O (8.01–9.04 wt%) and relatively low CaO (0.18–0.7 wt%), MgO (0.06%–0.13 wt%) and Fe₂O₃ (T_Fe2O3 = 1.08–2.06 wt%) contents. The granites are enriched in light rare earth elements (LREE), large-ion lithophile elements (LILEs) (e.g. Rb, Th, and K), and high field strength elements (HFSEs) (e.g. Zr and Hf), depleted in heavy rare earth elements (HREEs), and have negative Ba, Sr, P, Ti, and Eu anomalies. These geochemical characteristics are indicative of derivation from juvenile basaltic oceanic crustal materials in the lower crust. This suggests that the Huangyangshan pluton formed from magmas generated by partial melting caused by mantle-derived magma underplating, with the magmas then undergoing mixing, separation, and significant fractional crystallization. Diorite enclaves within the granites have weaker trace element anomalies that are indicative of magma mixing. In addition, the mantle-derived intermediate–basic end-member involved in the magma mixing is likely one of the important carriers of carbon and metal. In summary, the Late Carboniferous Huangyangshan pluton and its associated graphite mineralization formed in a post-collision extensional tectonic setting in the Kalamaili area.     

湖南锡田花岗岩锆石U-Pb年代学及钨锡成矿时代的探讨

华南是世界上最大的花岗岩省之一,其中中生代花岗岩最为发育,与之相伴生的是大量钨锡多金属矿床,花岗岩的成因演化因与这些矿床的成矿作用密切相关而备受关注。湖南锡田花岗岩体是该区的一个典型岩体,主要由黑云母花岗岩、黑云母二长花岗岩和细粒花岗岩组成,并伴生有钨锡矿床。本文以湖南锡田花岗岩体为研究对象,对其中不同类型的岩石进行了详细的岩石学和锆石SIMS与LA-ICP-MS U-Pb定年工作。分析结果表明,锡田花岗岩体存在晚三叠世(227~233Ma)和晚侏罗世(150~154Ma)两期岩浆活动,早期的岩浆活动主要分布在岩体北部和中部,晚期岩浆活动仅在岩体中部及东部矿体附近可见,两期岩浆活动具有相同的岩性组合。另外,对含矿花岗岩的锆石U-Pb定年结果表明该地区可能存在晚三叠世的成矿作用,结合前人的工作推断锡田地区钨锡矿的形成受晚三叠世和晚侏罗世两期岩浆事件的影响。     

U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of Early Indosinian intrusive rocks in West Qinling, central China: petrogenesis and tectonic implications

The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.     

Geochemistry and geodynamic implications of the Triassic bimodal magmatism from Western Kunlun Orogen,northwest China

The Western Kunlun Orogen occupies a key tectonic position at the junction between the Tarim block and the Tethyan domain. However, the late Paleozoic to early Mesozoic, especially the middle to late Triassic tectonic evolution history of the Western Kunlun Orogen remains controversial. This study reports SHRIMP zircon U–Pb ages and geochemical as well as Sr–Nd–Hf isotopic data for middle to late Triassic Taer pluton in Western Kunlun Orogen, Northwest China. The Taer pluton shows a strong bimodal distribution of compositions, with the felsic rocks dominant and the mafic rocks subordinate. Zircon U–Pb dating reveals that the coexisting mafic and felsic rocks are coeval, both emplacing in a period between 234 and 225 Ma. Most of the studied rocks are potassium rich and can be classified into high-K calc-alkaline to shoshonitic series. They are also strongly enriched in LREE, LILE and depleted in HFSE with strong negative Ti and Nb anomalies, and characterized by enriched Sr–Nd–Hf isotopic signatures. Detailed geochemical and isotopic studies indicate that the Taer pluton was emplaced in a post-collisional extensional setting, with the mafic rocks derived from partial melting of the enriched continental lithospheric mantle in the spinel facies field, and the felsic rocks formed by anatexis of newly underplated basaltic rocks. The existence of middle to late Triassic post-collisional magmas in Western Kunlun region suggests that the final closure of Paleo-Tethys and the initial collision between the Western Kunlun and the Qiangtang terranes may have happened before ~234 Ma, most probably in late Permian, rather than in late Triassic or early Jurassic. In assistance with other geological evidences, such as the presence of early Triassic to late Triassic/early Jurassic S-type magmatism, terrestrial molasse depositions, regional unconformities, and strong deformation, we propose that the Western Kunlun Orogen may have undergone a long post-collisional intracontinental process from early Triassic to late Triassic/early Jurassic.     

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.707069–0.707138, and ε_Nd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive ε_Hf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.7041–0.7061, and ε_Nd(t) = −3.1 to −4.3] and high contents of K₂O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative ε_Hf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive ε_Hf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive ε_Hf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.     

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。     

湘东邓阜仙花岗岩成因及对成矿的制约:锆石/锡石U-Pb年代学、锆石Hf-O同位素及全岩地球化学特征

南岭地区晚三叠世成矿规模较小且零散,而晚侏罗世成矿规模巨大,以往的研究较少关注两期花岗岩之间的成因联系及与成矿的关系。湘东邓阜仙花岗岩体位于南岭地区中段,是具有晚三叠世、晚侏罗世两期岩浆活动的复式岩体,并发育有钨矿床。本文对区内花岗岩及蚀变岩型矿体进行了锆石/锡石U-Pb定年、锆石Hf-O同位素与全岩主微量分析,以期揭示两期花岗岩的地球化学特征、成因联系和与成矿的关系。锆石SIMS U-Pb定年结果表明,区内花岗岩的成岩年龄分别为~228Ma和~153Ma。湘东钨矿的锡石U-Pb定年结果为154.4±2.1Ma,表明钨锡矿化形成于晚侏罗世。两期花岗岩均具有高硅、高钾、弱过铝质至强过铝质的特征,结合锆石的Hf-O同位素组成,指示它们具有相似的成因,均起源于中-古元古代基底地壳物质的部分熔融,晚三叠世花岗岩混入了更多的成熟地壳物质,区域的富矿地层对于湘东钨矿形成的直接影响不大。邓阜仙晚侏罗世花岗岩更高的Fe含量和Rb/Sr、Rb/Ba比值,更显著的Eu、Ce负异常,指示其经历了更高程度的分异演化,两期成岩过程中的差异可能与区域上的晚侏罗世发生的太平洋俯冲作用相关。     

Dioritic intrusions of the Slavkovský les (Kaiserwald), Western Bohemia: their origin and significance in late Variscan granitoid magmatism

Mafic and intermediate intrusions occur in the Slavkovsky les as dykes, sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late Variscan granites near the SW contact of the Western Krušné hory/Erzgebirge granite pluton. They are similar in composition and textures to the redwitzites defined in NE Bavaria. Single zircon Pb-evaporation analyses constrain the age of a quartz monzodiorite at 323.4 ± 4.4 Ma and of a granodiorite at 326.1 ± 5.6 Ma. The P–T range of magma crystallization is estimated at ~1.4–2.2 kbar and ~730–870°C and it accords with a shallow intrusion level of late Variscan granites but provides lower crystallization temperatures compared to the Bavarian redwitzites. We explain the heterogeneous composition of dioritic intrusions in the Slavkovsky les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization. Increased K, Ba, Rb, Sr and REE contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle, the melts being close to lamprophyre or alkali basalt composition. Diorites and granodiorites originated from mixed magmas derived by addition of about 25–35 and 50 vol.%, respectively, of the acid end-member (granite) to lamprophyre or alkali-basalt magma. Our data stress an important role of mafic magmas in the origin of late Variscan granitoids in NW Bohemian Massif and emphasize the effect of mantle metasomatism on the origin of K-rich mafic igneous rocks.     

Magma Replenishment as Revealed by Textural and Geochemical Features of Plagioclase Phenocrysts in Subduction-Related Lavas

Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO~T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.     

Chronology of multiphase emplacement of the Salmi rapakivi granite-anorthosite complex, Baltic Shield: implications for magmatic evolution

The U-Pb dating of 18 samples, representing the principal rock types of the 4000 km² Salmi anorthosite-rapakivi granite complex and its satellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield, reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years. From oldest to youngest they are: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 1543.4 Ma; (3) early wiborgite and pyterlite at 1540.6–1537.9 Ma; (4) biotite granite and more evolved granite at 1538.4–1535 Ma; (5) late pyterlite at 1535.2 Ma; (6) olivine gabbro and biotite-amphibole granite at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5–5.0 million years. Early wiborgite and pyterlite (3, above) and biotite granite (4, above) probably crystallized from multiple magma intrusions. Age differences of 3.4±1.5 million years between zircon and baddeleyite in olivine gabbro (6, above) are probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the Salmi complex. The ages and chemical features of early and late zircon populations, together with our modeling of magma crystallization and zircon growth, show that the duration of magma crystallization and Pb-diffusion in zircon was short lived and insignificant compared to the precision of dating of about ±1–2 million years. Hence, the range of U-Pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas. Average rate of magma emplacement was about 0.01 km³/year for the most voluminous phase of early biotite-amphibole rapakivi granite, and about 0.0024 km³/year for the Salmi complex as a whole. Compositional changes of the Salmi magmas over time are in agreement with the model of magmatism related to lithospheric extension. Received: 2 August 1996 / Accepted 19 December 1996     

Timing of granitic magma mixing in the southeastern Gyeongsang Basin,Korea: SHRIMP-RG zircon data

Late Cretaceous calc-alkaline granites in the Gyeongsang Basin evolved through the mixing of mafic and felsic magmas. The host granites contain numerous mafic magmatic/microgranular enclaves of various shapes and sizes. New SHRIMP-RG zircon U–Pb ages of both granite and mafic magmatic/microgranular enclaves are 75.0?±?0.5 Ma and 74.9?±?0.6 Ma, respectively, suggesting that they crystallized contemporaneously after magma mixing. The time of injection of mafic melt into the felsic magma chamber can be recognized as approximately 75 Ma by field relations, petrographic features, geochemical evolution, and SHRIMP-RG zircon dating. This Late Cretaceous magma mixing event in the Korean Peninsula was probably related to the onset of subduction of the Izanagi (Kula)–Pacific ridge.     

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。     

藏南冈底斯带中段始新世岩浆作用的厘定及其大地构造意义

本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICPMS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(～50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。     

藏南冈底斯岩基日多地区花岗岩体形成时代和地球化学特征

冈底斯岩基广泛发育三叠纪-中新世的岩浆岩,是研究与新特提斯洋北向俯冲和印度-欧亚大陆碰撞相关的构造岩浆作用特征的天然实验室。日多地区花岗岩体位于藏南墨竹工卡县东侧日多乡附近,其主体为花岗岩,被花岗闪长玢岩脉侵入。锆石U-Pb地质年代学表明:主体花岗岩形成于62.7±0.5Ma,侵入其中的花岗闪长玢岩脉形成于59.5±1.5Ma,并捕获了大量的侏罗纪岩浆岩锆石(155.4±1.8Ma)。日多地区花岗岩体的全岩地球化学特征为:(1)高Si O_2、Na_2O、Al_2O_3,低Fe O~T、MgO、Ti O_2;(2)富集轻稀土(LREE),亏损重稀土(HREE)及高场强元素Nb、Ta、Ti、P元素;(3)具有Eu负异常,总体显示高钾钙碱性、过铝质花岗岩和岛弧型岩浆岩特征。锆石Hf同位素特征暗示其岩浆源区为基性下地壳物质。花岗闪长玢岩脉裹挟大量侏罗纪岩浆型锆石,表明冈底斯岩基拉萨以东地区可能经历了较广泛的晚侏罗世岩浆作用。     

Isotopic variations in S-type granites: an inheritance from a heterogeneous source?

Inherited zircons from S-type granites provide exceptionally good insight into the isotopic heterogeneity of their sources. Zircons from four samples (one granite, two granodiorites, one granodioritic enclave) of Pan-African S-type granite of the Cape Granite Suite (c. 540 Ma) have been the subject of a laser LA-ICP-MS zircon U/Pb study to determine emplacement ages and inheritance. Zircons from three of these samples (2 granodiorites and 1 granodioritic enclave) were also analysed for Hf isotopes by LA-MC-ICP-MS. Ages of inherited cores range from 1,200 to 570 Ma and show Hafnium isotope values (ε_Hf,t ) for the crystallisation age (t) of the different cores that range from −14.1 to +9.1. Magmatic zircons and magmatic overgrowth with concordant spot ages between ca. 525 and ca. 555 Ma show a similar range of ε_Hf,t , between −8.6 and +1.5, whilst ε_Hf values calculated at 540 Ma (ε_Hf,540) for inherited cores range from −15.2 to +1.7. Thus, our results show that the time evolved ε_Hf arrays of the inherited cores overlap closely with the ε_Hf range displayed by the magmatic rims at the time of crystallisation of the pluton. These similarities imply a genetic relationship between magmatic and inherited zircons. Within the inherited cores, four main peak ages can be identified. This, coupled with their large Hf isotopic range, emphasises that the source of the granite is highly heterogeneous. The combination of the U/Pb zircon ages ranges and Hf isotope data implies that: (1) The source of S-type granite consists of crustal material recording several regional events between 1,200 and 600 Ma. This material records the recycling of a much older crust derived from depleted mantle between 1.14 and 2.02 Ga. (2) The homogenisation of Hf isotopic variation in the magma acquired through dissolution of the entrained zircon, via mechanical mixing and/or diffusion between within the granite was particularly inefficient. (3) This evidence argues for the assembly of the pluton through many relatively small magma batches that undergo rapid cooling from their intrusion temperature (ca. 850°C) to background magma chamber temperature that is low enough to ensure that much of the magmatic zircon crystallised rapidly (>80% by 700°C). (4) There is no evidence for the addition of mantle-derived material in the genesis of S-type Cape Granite Suite, where the most mafic granodiorites are strongly peraluminous, relatively low in CaO and K₂O rich. Interpreted more widely, these findings imply that S-type granites inherit their isotopic characteristic from the source. Source heterogeneity transfers to the granite magma via the genesis of discrete magma batches. The information documented from the S-type CGS zircons has been recorded because the individual batches of magma crystallised the bulk of their magmatic zircon prior to mechanical or diffusional magma homogenisation. This is favoured by zirconium saturation in the magma shortly after emplacement, by partial dissolution of the entrained zircon fraction, as well as by the intrusion of volumetrically subordinate magma batches into a relatively cool pluton. Consequently, evidence recorded within inherited cores will most likely be best preserved in S-type granite plutons intruded at shallow depths. Other studies that have documented similar ε_Hf arrays in magmatic zircons have interpreted these to reflect mixing between crustal- and mantle-derived magmas. This study indicates that such arrays may be wholly source inherited, reflecting mixing of a range of crustal materials of different ages and original isotopic signatures.     

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.     

Trace elements and Hf isotope composition as indicators of zircon genesis due to the evolution of alkaline-carbonatite magmatic system (Ilmeny-Vishnevogorsky complex,Urals, Russia)

We present results of investigation of the trace-element (REE, HFSE) and Hf isotope compositions and U-Pb age of single zircons crystallized from alkaline-carbonatite magmas of the Ilmeny-Vishnevogorsky complex (IVC) (Urals, Russia). It has been established that the geochemical characteristics of the early zircon (U-Pb age of 430-410 Ma) from alkaline rocks and carbonatites of this complex are determined mainly by the magmatic evolution of parental fluid-saturated alkaline-carbonatite melts and are highly associated with the cocrystallization of zircon and uranium-containing rare-metal minerals (gatchettolite and pyrochlore) at the final stages of the magmatic-system activity. The early zircons have a moderately depleted Hf isotope composition (e_Hf from + 11.3 to + 4.7), confirming the mantle nature of the magma source and indicating the participation of DM-like and enriched-source (probably, lower-crust component) substances in the magma generation. The considerable variations in the initial Hf isotope composition of the early zircons testify to the multistage zircon crystallization involving new portions of melts with different isotope compositions controlled by mixing of substances at their source. Late IVC zircons (250-350 Ma) have strongly disturbed “rejuvenated” isotope systems and a geochemical composition different from that of the magmatic zircons. They formed apparently at the metamorphic stage of the IVC evolution without a significant input of additional material.     

内蒙古大青山地区新太古代晚期岩浆作用:来自锆石SHRIMP U-Pb定年的证据

本文报道了华北克拉通西部大青山地区新太古代晚期变质深成侵入岩的锆石SHRIMP年龄。本文涉及的3个样品取自大青山的石拐南部,岩性为黑云母二长花岗质片麻岩、紫苏石英闪长质片麻岩和闪长质片麻岩。根据矿物组合,黑云母二长花岗质片麻岩和紫苏石英闪长质片麻岩遭受高角闪岩相-麻粒岩相变质,闪长质片麻岩遭受角闪岩相变质。锆石具核-幔-边或核-边结构。岩浆锆石年龄为2484±7Ma、2494±12Ma和2495±10Ma,考虑到岩石遭受后期构造热事件强烈改造,岩浆锆石发生不同程度变质重结晶,振荡环带变得模糊,年龄很可能偏年轻,岩浆锆石真实的形成年龄应更大一些,形成于新太古代晚期。2个样品记录早期变质锆石年龄为2441±7Ma和2481±10Ma;2个样品记录晚期变质锆石年龄为1847±35Ma和为1919±73Ma。结合前人工作,可得出如下结论:1)石拐地区存在新太古代晚期二长花岗岩、石英闪长岩和闪长岩等不同类型变质深成侵入岩。新太古代晚期岩浆作用在整个大青山地区很可能广泛发育,古元古代晚期孔兹岩带是在太古宙基底之上或其邻区发展起来的。2)与大青山地区其它古元古代早期以前的地质体类似,新太古代晚期变质深成侵入岩普遍遭受古元古代早期和晚期构造热事件叠加改造。