Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan foldbelt

The potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns. In the Lachlan fold belt, southeastern Australia, granites and associated high-grade metasedimentary rocks intrude low-grade Ordovician country rock. This relationship is well-exposed in the Tallangatta region, northeast Victoria (part of the Wagga-Omeo Metamorphic Complex). In this region granites (two I-types and two S-types) have intruded during the mid-late Silurian between approximately 410–430 Ma based on the ages of magmatic zircons. The age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low- and high-grade metasediments. In broad terms, both for detrital zircons in all four sediments and for inherited zircons in three of the four granites, the dominant ages are early Paleozoic and Late Precambrian, with sporadic older Precambrian ages extending up to 3.5 Ga. The ages of the youngest detrital zircons from the low-grade Lockhart and Talgarno terranes limit the time of sedimentation to ca. 466 Ma or younger. The youngest detrital zircons from two samples of the high-grade Gundowring terrane are 473 Ma, making these sediments Ordovician or younger, not Cambrian as originally suggested. However, the individual age spectra for the four selected metasediments are not well matched when closely examined. The age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments. Thus, the metasediments might not be representative of the actual source rocks of the granites. While the exact source of the granites cannot be identified from the analysed samples, the existence of a large population of ca. 495 Ma inherited zircon grains in the S-type granites requires that the granite source contains a significant proportion of Cambrian or younger material. This does not preclude the existence of a Precambrian basement to the Lachlan fold belt but indicates that at the level of S-type magma generation, a Cambrian and/or younger protolith is required. Received: 28 August 1998 / Accepted: 7 July 1999     

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.     

Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

 Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher ⁸⁷Sr/⁸⁶Sr_i and lower ɛ_Nd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high ⁸⁷Sr/⁸⁶Sr, low ɛ_Nd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale. Received: 28 September 1998 / Accepted: 29 December 1999     

A zircon U-Pb study of metaluminous (I-type) granites of the Lachlan Fold Belt,southeastern Australia: implications for the high/low temperature classification and magma differentiation processes

Following seminal studies in the Lachlan Fold Belt (southeastern Australia), it has become almost axiomatic that metaluminous granites derive from infracrustal precursors, whereas strongly peraluminous plutons have metasedimentary or supracrustal sources, as reflected in the I- and S-type designation. Recently, zircon saturation thermometry has been used to further subdivide I-type granites into high- and low-temperature categories. That low-temperature I-type granites evolved by restite separation from magmas generated in the zircon stability field is implicit in this classification. To explore this hypothesis, we report an ion microprobe U-Pb (zircon) study into three hallmark ‘low-temperature’ Lachlan Fold Belt I-type suites. The combined patterns of zircon age inheritance and bulk rock Zr trends suggest that each suite formed from magmas that were initially zircon-undersaturated, and that fractional crystallisation, not restite unmixing, was the dominant differentiation process. The low temperature status presently applied to these rocks cannot therefore be justified. The inherited zircons in these I-type granites reflect melting and assimilation of metasedimentary rock, and testify to a supracrustal source component. Electronic Supplementary Material Supplementary material is available for this article at     

Petrogenesis of the microcrystalline-dioritic enclaves from Jiuling granitoids in the eastern segment of Jiangnan Orogen and constraints on magma source materials

Numerous dark enclaves with different shapes are found in Jiuling Neoproterozoic granitoids. Precise LA-ICP-MS U-Pb dating was conducted on zircons extracted from two microcrystalline enclave samples, yielding crystallization ages of 822.6±5.8 Ma and 822.2±6.2 Ma, respectively. The consistent ages within analytical errors with the host granitoids suggested that they were the products of the same magmatism. The microcrystalline-dioritic enclaves commonly show plastic forms and contain similar plagioclase megacrysts to the host rocks, and both of the enclaves and host granitoids showed a complex composition and structural imbalance in plagioclases. Furthermore, the apatites with a euhedral acicular shape occurred widely in the microcrystalline-dioritic enclaves. All of these petrographic features above imply magma mixing is involved in their diagenesis. The enclaves and host granitoids show a marked zircon trace element difference and Hf isotopic signatures without correlation in zircon trace element pairs but form their own system between enclaves and host granitoids. Additionally, most of the zircons show extremely high ε_Hf (t) with ε_Hf (t) =3.54–11.94 from the southern samples, and ε_Hf (t) =1.0–9.09 from the central region. Some zircons with the higher ε_Hf (t) are similar to the zircons from the juvenile island arc in the eastern segment of Jiangnan Orogen. Integrated geological and Hf isotopic characteristics suggest microcrystalline-dioritic enclaves were derived from the partial melting process of the Mesoproterozoic crust which enriched juvenile island arc materials and mixed with the granitic magma that remelted from the Mesoproterozoic continental crust which relatively enriched ancient sediments and mixed with the host granitoid in diagenesis.     

Origin of biotite-apatite-rich enclaves, Achala batholith, Argentina

The Achala batholith of Argentina contains very unusual layered enclaves containing up to 30% apatite and 50% biotite in some layers. This modal mineralogy produces bulk-rock compositions that cannot represent liquids, having as little as 29% SiO₂ and up to 8% P₂O₅. Nor can the enclaves represent metasedimentary xenoliths because: (1) none of the Precambrian wall rocks has these compositions; (2) none of the metasedimentary xenoliths present within the batholith shows any degree of transition to the mica-apatite-rich enclaves; (3) the compositions and textures in the enclaves are inconsistent with metasediments; (4) a geochronological study of zircon from an enclave gives an age of 368 ± 2 Ma, the exact age of zircons in the granitic host rocks. For these reasons, we conclude that the enclaves are neither xenoliths of Precambrian wall rocks nor restite of a Precambrian source. The identical age of the enclave and the host granites, coupled with textural, mineralogical, and bulk-rock characteristics of the enclaves, indicates that the enclaves are magmatic segregations, i.e., cumulates. The F-rich nature of the stubby-shaped apatites and biotites indicates a high F content of the magma parental to the enclaves. We infer that the viscosity of the melt was lowered sufficiently to allow cumulates to form in spite of the granitic composition of the melt. Received: 12 December 1996 / Accepted: 11 August 1997     

Do S-type granites commonly sample infracrustal sources? New results from an integrated O, U–Pb and Hf isotope study of zircon

In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons, and high mean zircon δ¹⁸O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ¹⁸O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ¹⁸O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ¹⁸O. Mixing trends in plots of δ¹⁸O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.     

U-Pb Zircon Studies of Mid-crustal Metasedimentary Enclaves from the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia

High-grade metasedimentary enclaves in granites can be usedto directly characterize the lower and mid crust. Enclaves inthe Silurian S-type Deddick Granodiorite, SE Australia, havechemical and Nd–Sr isotopic compositions broadly similarto Ordovician–Silurian clastic sedimentary rocks throughoutthe fold belt. SHRIMP U–Pb ages of detrital zircons inthe enclaves indicate Early Ordovician maximum depositionalages for their psammitic–pelitic precursors. Patternsof zircon inheritance show the same     

Evidence for hybridisation in the Tynong Province granitoids,Lachlan Fold Belt,eastern Australia

The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ?_Hf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ?_Hf in two of the three analysed samples (8.8 and 10.1 ?_Hf units) exceeds that expected from a single homogeneous population (～4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ?_Nd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for ⁸⁷Sr/⁸⁶Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.     

冈底斯岩基中包体的初步研究

本文综合国内外研究成果对花岗岩中岩石包体进行成因归类,指出对其研究的理论和实际意义。以岩相学为基础,讨论冈底斯岩基中镁铁质包体的类型及形成机理,揭示冈底斯各单元岩浆作用初期的演化过程。     

An integrated zircon geochronological and geochemical investigation into the Miocene plutonic evolution of the Cyclades, Aegean Sea, Greece: Part 1: Geochronology

We use 369 individual U–Pb zircon ages from 14 granitoid samples collected on five islands in the Cyclades in the Aegean Sea, Greece, for constraining the crystallisation history of I- and S-type plutons above the retreating Hellenic subduction zone. Miocene magmatism in the Cyclades extended over a time span from 17 to 11 Ma. The ages for S-type granites are systematically ~2 million years older than those for I-type granites. Considering plutons individually, the zircon data define age spectra ranging from simple and unimodal to complex and multimodal. Seven of the 14 investigated samples yield more than one distinct zircon crystallisation age, with one I-type granodiorite sample from Mykonos Island representing the most complex case with three resolvable age peaks. Two samples from S-type granites on Ikaria appear to have crystallised zircon over 2–3 million years, whereas for the majority of individual samples with multiple zircon age populations the calculated ages deviate by 1–1.5 million years. We interpret our age data to reflect a protracted history involving initial partial melting at deeper lithospheric levels, followed by crystallisation and cooling at shallower crustal levels. Our study corroborates published research arguing that pluton construction is due to incremental emplacement of multiple magma pulses over a few million years. Assuming that multiple age peaks of our 14 samples can indeed serve to quantify time spans for magmatic emplacement, our data suggest that Aegean plutons were constructed over a few million years. Our tectonic interpretation of the U–Pb ages is that the S-type granites resulted from partial melting and migmatisation of the lower crust, possibly starting at ~23 Ma. The I-type granites and associated mafic melts are interpreted to reflect the magmatic arc stage in the Cyclades starting at ~15 Ma.     

Zircon Crystal Morphology, Trace Element Signatures and Hf Isotope Composition as a Tool for Petrogenetic Modelling: Examples From Eastern Australian Granitoids

In situ laser ablation inductively coupled plasma mass spectrometryanalysis of trace elements, U–Pb ages and Hf isotopiccompositions of magmatic zircon from I- and S-type granitoidsfrom the Lachlan Fold Belt (Berridale adamellite and Kosciuskotonalite) and New England Fold Belt (Dundee rhyodacite ignimbrite),Eastern Australia, is combined with detailed studies of crystalmorphology to model petrogenetic processes. The presented examplesdemonstrate that changes in zircon morphology, within singlegrains and between populations, generally correlate with changesin trace element and Hf-isotope signatures, reflecting the mixingof magmas and changes in the composition of the magma throughmingling processes and progressive crystallization. The zircondata show that the I-type Kosciusko tonalite was derived froma single source of crustal origin, whereas the S-type Berridaleadamellite had two distinct sources including a significantI-type magma contribution. Complex morphology and Hf isotopevariations in zircon grains indicate a moderate contributionfrom a crustal component in the genesis of the I-type Dundeerhyodacite. The integration of data on morphology, trace elementsand Hf isotope variations in zircon populations provides a toolfor the detailed analysis of the evolution of individual igneousrocks; it offers new insights into the contributions of differentsource rocks and the importance of magma mixing in granite petrogenesis.Such information is rarely obtainable from the analysis of bulkrocks. KEY WORDS: granite source origins; zircon Hf isotopes; zircon petrogenesis; zircon morphology; zircon U–Pb ages     

U–Pb geochronology and zircon composition of late Variscan S- and I-type granitoids from the Spanish Central System batholith

The Spanish Central System (SCS) batholith, located in the Central Iberian Zone, is one of the largest masses of granite in the European Variscan Belt. This batholith is a composite unit of late- and post-kinematic granitoids dominated by S- and I-type series granite, with subordinate leucogranite and granodiorite. Zircon trace element contents, from two representative S-type and three I-type granitoids from the eastern portion of the SCS batholith, indicate a heterogeneous composition due to magma differentiation and co-crystallisation of other trace element-rich accessory phases. In situ, U–Pb dating of these zircons by SHRIMP and LA-ICP-MS shows 479–462-Ma inherited zircon ages in the I-type intrusions, indicating the involvement of an Ordovician metaigneous protolith, while the S-type intrusions exclusively contain Cadomian and older zircon ages. The zircon crystallisation ages show that these granites have been emplaced at ca. 300?Ma with a time span between 303?±?3?Ma and 298?±?3?Ma. Precise dating by CA-ID-TIMS reveals a pulse at 305.7?±?0.4?Ma and confirms the major pulse at 300.7?±?0.6?Ma. These ages match the Permo-Carboniferous age for granulite-facies metamorphism of the lower crust under the SCS batholith and coincide with a widespread granitic event throughout the Southern Variscides. Ti-in zircon thermometry indicates temperatures between 844 and 784°C for both the S- and I-type granites, reinforcing the hypothesis that these granites are derived from deep crustal sources.     

江西印支期蔡江岩体中暗色微粒包体的发现及其地质意义

文章报道了江西蔡江花岗质岩体中发现暗色微粒包体,以及这些包体的地质、岩相学、LA-ICP-MS锆石U-Pb年代学和元素地球化学特征。包体多呈椭圆状,显示淬冷边和反向脉,具有典型的岩浆结构并含有针状磷灰石,有的包体含有长石捕虏晶。包体具有相对较低的SiO₂（低至57.05 wt%）和较高的MgO+Fe₂O₃（高达14.21 wt%）含量。LA-ICP-MS锆石U-Pb定年数据表明,包体形成于晚三叠世（224 Ma）,与寄主花岗岩（230~228 Ma）在误差范围内基本一致。上述特征表明,包体是离散的幔源偏基性岩浆团或者是幔源与寄主岩浆混合的产物。原始包体岩浆属于超钾质岩浆,可能是通过岩石圈地幔中交代成因的金云母辉石岩脉发生部分熔融而形成的。暗色微粒包体的发现为幔源岩浆底侵提供了直接证据,从而为蔡江花岗质岩石形成于较高温度提供佐证。该研究对于进一步探讨华南印支期花岗岩形成的热源机制具有意义。     

Petrogenesis of granitoids from the Lachlan Fold Belt,southeastern Australia: The role of disequilibrium melting

S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial εHf compositions of magmatic zircon populations and the host bulk-rock (ΔεHf^blk-zrc) range from −0.6 to +2.5 ε units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples.Assuming that magmatic zircon records the melt composition, differences between the bulk-rock εHf and εHf of magmatic zircon (ΔεHf^blk-zrc values) measured for I-type granites (0.4–2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4–1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions.By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.     

Igneous origin of K-feldspar megacrysts in deformed granite of the Papoose Flat Pluton, California, USA

Many points of evidence, especially igneous microstructures and structures resulting from solid-state deformation, indicate that K-feldspar megacrysts in deformed granites of the Papoose Flat pluton are residual phenocrysts, not porphyroblasts. Evidence of an igneous origin includes features such as crystal shapes, simple twinning, zonally arranged euhedral plagioclase inclusions, oscillatory compositional zoning, and local occurrence in microgranitoid enclaves. Evidence of solid-state deformation of the megacrysts (which is consistent with their existence prior to the mylonitic deformation) includes marginal recrystallization and neocrystallization, microcline twinning, marginal replacement by myrmekite, and recrystallized/neocrystallized “tails”. Evidence of porphyroblastic growth, such as overgrown inclusion trails, is absent. This appears to be the situation in most felsic augen gneisses and mylonites.     

浙东天台地区早白垩世花岗岩及暗色包体锆石U-Pb年龄、地球化学及其成因

在华南东部浙闽沿海一带普遍发育有大量晚中生代花岗质岩体及其中的暗色包体,这些岩体被认为是大规模壳幔相互作用和岩浆混合作用的产物。本文对浙东天台地区白鹤岩体中的寄主花岗岩及其中发育的暗色包体分别进行了LA-ICP-MS锆石U-Pb定年和详细的岩石地球化学研究,其锆石U-Pb年龄分别为(120.4±1.2)Ma和(120.6±1.1)Ma,属浙东燕山期侵入活动集中的早白垩世中晚期产物。岩石地球化学特征显示,寄主花岗岩为高硅、富碱、弱过铝质的高钾钙碱性花岗岩,具有较强的Eu负异常,富集Rb、Th、U、K,并有Sr、Ba、P、Ti、Nb、Ta等元素的亏损,岩石成因为高分异I型花岗岩;暗色包体多为低硅、富钠、偏铝质低钾拉斑玄武系列岩石,轻稀土富集、重稀土亏损,并具有弱的Eu正异常。锆石Hf同位素组成表现出不同物质来源(壳幔混源)花岗岩类岩石的特点。综合年代学及岩石地球化学特征,认为浙东地区早白垩世I型花岗岩及其暗色包体是在燕山期弧后碰撞伸展引张的构造背景下,由底侵的幔源岩浆与其诱发熔融的深部壳源岩浆经混合后,经过一定程度的分异演化,最后定位于浅成环境的产物。     

Residence, Resorption and Recycling of Zircons in Devils Kitchen Rhyolite, Coso Volcanic Field, California

Zircons from the Devils Kitchen rhyolite in the PleistoceneCoso Volcanic field, California have been analyzed by in situPb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescenceimaging. The zircons yield common-Pb-corrected and disequilibrium-corrected²⁰⁶Pb/²³⁸U ages that predate a previously reported K–Arsanidine age by up to 200 kyr, and the range of ages exhibitedby the zircons is also approximately 200 kyr. Cathodoluminescenceimaging indicates that zircons formed in contrasting environments.Most zircons are euhedral, and a majority of the zircons areweakly zoned, but many also have anhedral, embayed cores, witheuhedral overgrowths and multiple internal surfaces that aretruncated by later crystal zones. Concentrations of U and Thvary by two orders of magnitude within the zircon population,and by 10–20 times between zones within some zircon crystals,indicating that zircons were transferred between contrastingchemical environments. A zircon saturation temperature of 750°Coverlaps within error a previously reported phenocryst equilibrationtemperature of 740 ± 25°C. Textures in zircons indicativeof repeated dissolution and subsequent regrowth are probablycaused by punctuated heating by mafic magma input into rhyolite.The overall span of ages and large variation in U and Th concentrations,combined with calculated zircon saturation temperatures andresorption times, are most compatible with crystallization inmagma bodies that were emplaced piecemeal in the crust at Cosoover 200 kyr prior to eruption, and that were periodically rejuvenatedor melted by subsequent basaltic injections. KEY WORDS: zircon geochronology; residence time; rhyolite; ion microprobe; California     

Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I‐type plutons of the Lachlan Fold Belt,southeast Australia

New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher ¹⁴³Nd/¹⁴⁴Nd_(i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.     

U–Pb zircon geochronology of Silurian–Devonian granites in southeastern Australia: implications for the timing of the Benambran Orogeny and the I–S dichotomy

Despite extensive geochemical study and their importance to granite studies, the geochronology of Silurian to early-Devonian granitic rocks of southeastern Australia is poorly understood. In order to provide an improved temporal framework, new ion microprobe U–Pb zircon ages are presented from these rocks, and previous work is critically reviewed. Geochronological control is best in the Berridale Batholith, where S- and I-type granites have a close spatial relationship. In this region, there is a small volume of I-type granite that crystallised at 436 Ma, followed closely by a large volume of S-type granite at 432 Ma. I-type granite is abundant in a second peak at ca 417 Ma, although the Jindabyne pluton from the Kosciuszko Batholith is slightly older, at 424 Ma. A broader survey of S-type granite throughout the eastern Lachlan Orogen shows that the 432 Ma event is ubiquitous. There is no temporal overlap between S- and I-type granites in the Kosciuszko and Berridale Batholiths, which suggests that factors other than variations in degree of crustal contamination (which may include variation in tectonic setting, heat-flow, mass transfer across the crust–mantle boundary and/or availability in source materials) contribute to the diversity in granite types. The S-type granitic rocks occupy an aerial extent of greater than 28 000 km², and geochronological constraints suggest that the crystallisation of these granites took place over a relatively small interval, probably less than 10 m.y. This implies a magmatic flux of over 64 km³/Ma per km strike length, comparable to other high-flux granitic belts. Previous work has linked the Benambran Orogeny to the generation of the S-type granites, and so the age of these granites constrains the age of Benambran Orogenesis