Internal Structures Of Zircons Research Articles

Timing of deposition and tectonothermal events of banded iron formations in the Anshan–Benxi area, Liaoning Province, China: Evidence from SHRIMP U-Pb zircon geochronology of the wall rocks

Most of the Algoma-type BIFs and associated volcanic suites in the North China Craton (NCC) have undergone high-grade metamorphism during the ∼2.5Ga tectonothermal event. Isotopic systems in high-grade metamorphic rocks are often reset, thereby losing the earlier records, whereas zircons in low-grade metamorphic rocks are potential candidates to evaluate earlier thermal events. Detailed textural relationship and internal structures of zircons and their age data for eight wall-rock samples with metamorphic grades from greenschist to upper amphibolite facies of the Algoma-type BIFs in the Anshan–Benxi area are used to constrain the age of protolith. Zircons in the greenschist to lower amphibolite facies are generally metamorphism-resistant, whereas those in the upper amphibolite facies rocks show variable recrystallization. SHRIMP zircon U–Pb dating of magmatic zircons in six samples constrains the peak of BIF-deposition at 2.56–2.53Ga, which is consistent with ∼2.5Ga crustal growth in the NCC. In addition, the greenschist-facies metamorphic rocks in the Anshan area also record an event of volcanic activity at 2.72–2.65Ga. We consider that low-grade metamorphic wall rocks were not significantly affected by the ∼2.5Ga tectonothermal event, and the 2.72–2.65Ga age group records another important magmatic event corresponding to ∼2.7Ga crustal growth. The metamorphic zircons from these rocks record a thermal event during 1.88–1.84Ga.

Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North China Craton: implications for the interleaving of the Dabie–Sulu orogenic crust

Lamprophyre dikes emplaced in a Jurassic granite at the southeastern margin of the North China Craton (NCC) carry different types of xenoliths. Here, we report a combined study of zircon U–Pb ages and whole-rock geochemistry of the xenoliths, as well as an Ar–Ar age of the lamprophyre, providing constraints on the sources of the magmatism and tectonic evolution in the southeastern margin of NCC. Phlogopite from the lamprophyre dike gave a 40Ar/39Ar plateau age of 116.15 ± 0.33 Ma. The felsic xenoliths can be classified into three groups: monzogranite, banded biotite granitic gneiss, and garnet-bearing gneiss. The internal structures of zircons from the banded biotite granitic gneiss xenolith show complex growth patterns. The mantles of these zircons display low luminance with a concordant SHRIMP U–Pb age of 227 ± 10 Ma. The rims with lighter luminance provide a SHRIMP U–Pb age of 213 ± 10 Ma, both ages are identical within error and significantly different from the ages of the basement rocks in the surrounding Bengbu uplift. However, the age is identical to those of the ultrahigh-pressure metamorphic rocks in the adjacent Dabie–Sulu orogen. In addition, zircon mantle domains of the banded biotite granitic gneiss xenolith have low Th/U ratios (>0.01) and flat HREE patterns ((Yb/Dy)n < 10), which suggest growth in an assemblage with garnet during HP metamorphism. The rim domains show very low Th/U ratios (<0.01) and steep HREE patterns ((Yb/Dy)n > 10), implying growth during exhumation in the absence of a garnet. Our studies show that the banded biotite gneiss represents vestiges of the subducted South China crust injected into or thrust below the North China Craton and provides constraints on the process of underthrusting in a continental collision zone, as well as the Mesozoic tectonic history of the southeastern margin of the NCC.

Zircon crystal morphology and internal structures as a tool for constraining magma sources: Examples from northern Portugal Variscan biotite-rich granite plutons

In northern Portugal, large volumes of granitoids were emplaced during the last stage (D3) of the Variscan orogeny and display a wide range of petrological signatures. We studied the morphologies and internal structures of zircons from syn-, late- and post-D3 granitoids. The sin-D3 granitoids include the Ucanha–Vilar, Lamego, Felgueiras, Sameiro, and Refoios do Lima plutons, the late- and post-D3 granitoids are represented by the Vieira do Minho and the Vila Pouca de Aguiar plutons, respectively. Typological investigations after Pupin (1980) along with scanning electron microprobe imaging reveal that the external morphology of zircon changes consistently with a decrease in the crystallization temperature. Zircon populations from the Refoios do Lima and the Vieira do Minho granites show gradual changes in the internal morphologies and their typologic evolution trends are consistent with their mainly crustal origin. The Sameiro, Felgueiras, Lamego and Ucanha-Vilar granites have more complex internal and external morphology and typological evolution trends that cross the domain of the calc-alkaline to the aluminous granites compatible with a mixing process. Finally, the morphological types of the Vila Pouca de Aguiar granites are found both in calc-alkaline and sub-alkaline granites and their typological evolutionary trends follow the calc-alkaline/sub-alkaline trend, suggesting crustal sources with some mantle contribution.

명창-원주 지역의 경기육괴 기반암 편마암 복합체에 대한 SHRIMP 저어콘 연대 측정

영남육괴에 발달한 영월-태백지역의 고생대 퇴적분지의 서쪽에 위치한 평창-원주 일대의 선캠브리아 편마암 복합체는 경기육괴의 일부로 간주되고 있으나 정확한 생성사기 및 변성시기에 대하여는 잘 연구되지 않았다. 이 연구에서는 이 지역의 편마암 복합체에 대한 SHRIMP 저어콘 U-Pb 연대측정을 수행하였다. 분석 자료들을 저어콘의 내부 구조 특성과 연관지어 해석한 결과 약 1960 Ma의 화성연대와 약 1860Ma의 변성연대를 구분하여 구하였다. 이 연대들은 지금까지 한반도에서 보고된 경기육괴, 영남육괴, 낭림육괴 등의 선캠브리아 화강편마암류들의 주 관입시기 빛 주 변성작용의 시기와 일치한다. 저어콘의 상속핵에서 얻어진 3200~3300, 2900, 2660, 2430, 2260, 2080~2070 Ma 등의 연령 역시 경기육괴 또는 영남육괴 기반암류에서 자주 보고되고 있는 연대들과 유사하다. 변성주변부에서는 고생대 후기의 변성작용(옥천조산운동)으로 해석될 수 있는 270Ma 부근의 하부교점 연대를 구하였지만 이를 확인하기 위해서는 더 적절하고 충분한 자료가 요구된다. 이 연구의 연대측정 결과는 편마암 복합체위에 부정합으로 놓이는 방림층군의 시대는 고원생대 화성활동(약1960 Ma)과 변성작용(약 1860 Ma) 이후임을 지시하고 있다. Precambrian gneiss complex in the Pyeongchang-Wonju area, which lies west of the Paleozoic sedimentary basin of the Yeongwol-Taebaek area, is being considered as a part of the Gyeonggi massif, but its ages of formation and metamorphic events are not well defined yet. In this study, SHRIMP zircon U-Pb ages were determined from the gneiss complex in the area, We obtained the discrete ages of magmatic (ca. 1960 Ma) and metamorphic (ca. 1860 Ma) events through the interpretation of the SHRIMP data based on the internal structures of zircons. These are almost the same to the ages of main intrusion and metamorphism reported from the Precambrian basements of Gyeonggi, Yeongnam and Nangnim massifs of the Korean Peninsula, Ages of 3200~3300 Ma, 2900 Ma, 2660 Ma, 2430 Ma, 2260 Ma, and 2080~2070 Ma obtained from inherited cores of studied zircons are also very similar to the frequently reported ages from the basement rocks of the Gyeonggi and Yeongnam massifs, Lower intercept age of about 270 Ma calculated from the rim data seems to indicate that the study area suffered from a late Paleozoic metamorphism (Okcheon Orogeny), but we need more reasonable and sufficient data to confirm it. According to the results of this study, it is suggested that the Bangnim group unconformably overlying the gneiss complex was deposited after the Paleoproterozoic granitic magmatism (ca. 1960 Ma) and metamorphism (ca. 1860 Ma).

Sensitive high‐resolution ion microprobe U‐Pb ages of the Latest Oligocene‐Early Miocene rift‐related Hidaka high‐temperature metamorphism in Hokkaido, northern Japan

Abstract In order to define the timing of granulite facies metamorphism, sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206Pb/ 238U ages (n = 31) with low Th/U ratios, mostly &lt;0.1). The internal structure of zircons and their SHRIMP U‐Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene‐Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene‐Early Miocene Hidaka high‐temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back‐arc rifting of the Kuril and Japan basins.

The Rand Granite in the southern Schwarzwald and its geodynamic significance in the Variscan belt of SW Germany

The Rand Granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the Moldanubian Zone at the southern margin of the Central Schwarzwald Gneiss Complex. It is a largely mylonitized elongated body and is thrust over the Badenweiler-Lenzkirch Zone forming a nappe with shear zones along its northern and southern boundaries. It comprises meta-granites, meta-trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites. Mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions. Nappe thrusting, which also affected the South Schwarzwald Gneiss Complex, occurred in Visean time during high-temperature / low-pressure metamorphism. Kinematic indicators in the mylonites document E- to ESE-directed nappe transport, highly transpressive relative to the trend of the nappe boundaries and the foliation. The trondhjemites formed at 351 +5/-4 Ma, predating the Variscan HT metamorphism. They have initial eNd-values of +6.6 to +6.7 and relatively low initial 87Sr/86Sr ratios (0.7042 to 0.7063), indicating a predominant mantle origin. The granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 Ma, suggested by U-Pb zircon model ages. They are different from the trondhjemites with low initial eNd-values (−4.7 to −3.3) and higher initial 87Sr/86Sr ratios (0.7068–0.7077), indicating that large part of the Rand Granite originated from anatexis of continental crust. Internal structure of zircons from the Rand Granite reveals mixing of magmas derived from both mantle and crust sources. These new data support an interpretation that the Rand Granite developed along an active continental margin and therefore represents a possible root of a Variscan magmatic arc.

In-situ trace element analyses and Pb-Pb dating of zircons in granulite from Huangtuling, Dabieshan by LAM-ICP-MS

It is revealed by CL images that there are multi-stage growth internal structures of zircons in the Huangtuling granulite, including the inherited zircons, protolith zircons, sector and planar zone zircons and retrograde zircons. In-situ trace element compositions and Pb-Pb ages have been analyzed by LAM-ICP-MS. The sector and the planar zone domains show typical trace element characteristics of granulite zircon (low Th, U, Th/U, total REEs, clear negative Eu anomalies, relatively depleted HREE and small differential degree between MREE and HREE, etc.), indicating that they formed during granulite-facies metamorphism. The protolith zircons have trace element characteristics of crustal zircon (high Th, U, Th/U, total REEs and enriched HREEs, etc.). 12 analyzed spots on granulite-facies domains give a weighted mean 207Pb/206Pb age of (2154±26) Ma (MSWD = 3.8), which is the best estimated age of granulite-facies metamorphism of this sample. The weighted mean 207Pb/206Pb age of 5 analyzed spots on protolith zircon domains is (2714 ± 22) Ma (MSWD = 1.4), which represents the protolith forming time. The discovery of ca. 3.4 Ga inherited zircon indicates that there are Palaeoarchean continental materials in this area. The interpretation of formation conditions and the ages of zircons can be constrained by simultaneous in-situ analysis of trace elements and ages.

In-situ trace element analyses of zircons from Dabieshan Huangzhen eclogite: Trace element characteristics of ec-logite-facies metamorphic zircon

The internal structures of zircons in eclogite from Huangzhen have been studied by cathodoluminescence (CL) microscopy. Two growth stages were distinguished: protolith magmatic cores and metamorphic overgrowth rims. These different domains were analyzed for trace elements using LAM-ICP-MS. The protolith and the overgrowth zircons have different trace elements characteristics. The trace element contents of protolith zircons are high and very variable. The overgrowth zircons show a typical trace element feature of equilibrium with garnet, e.g. low contents of HREE (132.2–197.6 μg/g) and small differential degree of HREE ((Yb/Gd)CN=8.6–11.9). The contents of Nb, Ta and the ratio of Nb/Ta are lower in the metamorphic domains (0.5–1.4 μg/g, 0.7–1.5 μg/g, 0.3–1.3, respectively) than in the protolith domains (3.8–19.7 μg/g, 2.7–12.1 μg/g, 1.0–4.6, respectively). This is the first time to give the evidence that the metamorphic zircon equilibrates with the rutile, which formed during the peak metamorphic stage. The REEs and other trace elements data demonstrate that the metamorphic zircons overgrow in the eclogite-facies conditions. The trace element composition of zircon can therefore give new ways to constrain their formation conditions.

Internal structures of zircons from Archaean granites from the Darling Range batholith: implications for zircon stability and the interpretation of zircon U-Pb ages

Internal structures in zircons from granitoids from the late Archaean Darling Range Batholith show secondary features revealed by HF etching, which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid. Zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation. The most striking feature is the development of high trace element concentration areas found in zircons from a number of granites. These structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band, about half way between the outer edge and the centre of the zircon. In any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim. This suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones. This explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger SHRIMP ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres. Also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon. This structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands. All structures can be transgressed by generally rounded lobes and patches of low U, weakly nebulously zoned zircon. This is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid, resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon.

Fracture sealing in zircon as evaluated by electron microprobe analyses and back-scattered electron imaging

The internal structure of zircon is commonly revealed by back-scattered electron (BSE) images. Grains from a 750-700 Ma old talc-alkaline mafic metatonalite from southernmost Brazil show relict igneous faces, irregular and diffuse metamorphic overgrowths, and fracture sealing. Sealing of fractures is observed as light grey patches in BSE images. EPMA profiles show strong but differential increases in Hf and U in the sealed fractures, unlike the common assertion in the literature that fractures are only pathways of leaching. Sealing of fractures occurs near borders of grains as well as in their cores and is not visible during standard petrographic examination; thus careful BSE image evaluation is needed before either in situ or whole grain isotopic determinations are made.

Petrology and U/Pb geochronology of the Telohat migmatites, Aleksod, Central Hoggar, Algeria

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth's, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material