The isotope composition of Hf in zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Sharyzhalgai uplift (southern Siberian craton): Implications for the continental-crust growth

This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.     

Trace element composition and Lu-Hf isotope systematics of zircon from plagiogneisses of the Kola superdeep well: Contribution of a Paleoarchean crust in Mesoarchean metavolcanic rocks

Zircons have been studied in three samples of Archean plagiogneisses from the Kola superdeep well (SG-3). The crystals consist of cores, magmatic shells, and metamorphic rims. The cores and shells are characterized by similar lowered concentrations of most trace elements, which is typical of zircons from plagiogranitoids, rocks of elevated basicity, and basites. At a wide range of Hf isotope characteristics, the cores and shells have similar average ¹⁷⁶Hf/¹⁷⁷Hf_i, which determines the close composition of their sources. The metamorphic rims have close ¹⁷⁶Hf/¹⁷⁷Hf_i ratio. The minimum age of the crustal contaminant of parental melts is estimated at 3.4 and 3.3 Ga for cores and 3.3–3.2 Ga for shells at almost equal proportions of mantle and crustal components in them. The contribution of Paleoarchean crust established in zircons from plagiogneisses of SG-3 using Lu-Hf isotope systematics is confirmed by the presence of 3.3and 3.4-Ga old zircons in surrounding TTG.     

Origin and Geological Significance of TTG Gneisses from the Maevatanana Greenstone Belt in North–Central Madagascar, and A Comparison with India

The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite(TTG) gneisses,and is important for its concentrations of various metal deposits(e.g.,chromium,nickle,iron,gold).In this paper we report on the petrography,and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area,as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses.The gneisses consist mainly of granitoid gneiss and biotite(±hornblende) plagiogneiss,and analysis of thin sections provides evidence of crushing,recrystallization,and metasomatism related to dynamic metamorphism.Samples have large variations in their major and trace element contents,with SiO_2 = 55.87-68.06 wt%,Al_2O_3 = 13.9-17.8 wt%,and Na_2O/K_2O =0.97-2.13.Geochemically,the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend,while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage.Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at2.5-2.4 Ga.Most ε_(Hf(t)) values of zircons from the biotite(± hornblende) plagiogneisses are positive,while most ε_(Hf(t)) values from the granitoid gneisses are negative,suggesting a degree of crustal contamination.Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca.3.4-2.6 Ga,representing a period of paleocontinent formation in the Mesoarchean.Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃.Based on these data,the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt(mostly lower amphibolite facies) in the Maevatanana area,and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent.The lithological similarity of Precambrian basement,the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India,which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics,and also important constraints on the correlation of the two continental fragments.     

Petrogenesis and zircon LA-ICP-MS U–Pb dating of newly discovered Mesoarchean gneisses on the northern margin of the North China Craton

Geological mapping and zircon U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating has identified a Mesoarchean (2857 ± 17 Ma) geological unit in the Luanjiajie area of the northern margin of the North China Craton, within the northern part of Liaoning Province, China. This unit is dominated by tonalitic and trondhjemite gneisses that form part of a typical tonalite–trondhjemite–granodiorite (TTG) rock assemblage. These Mesoarchean gneisses are enriched in Na and depleted in K, yield K₂O/Na₂O ratios of 0.34–0.50, have Rittmann index (σ) values of 1.54–3.04, and are calc-alkaline. They have Eu_N/Eu_N* values of 0.77–1.20 (average of 1.03), indicating that these samples have negligible Eu anomalies, and yield high La_N/Yb_N values (4.92–23.12). These characteristics indicate that these Mesoarchean gneisses have fractionated rare earth element (REE) compositions that are enriched in the light REE (LREE) and depleted in the heavy REE (HREE), with steeply dipping chondrite-normalized REE patterns. These gneisses are also enriched in Rb, Th, K, Zr, and Hf, and are relatively depleted in Ta, Nb, P, and Ti. In summary, the magma that formed these tonalitic and trondhjemite gneisses was most likely derived from the partial melting of lower-crustal basaltic rocks during subduction. The timing of formation (2.85 Ga) of the Luanjiajie tonalite and trondhjemite gneisses probably represents the timing of initiation of plate tectonics within the LongGang Block during a SE-directed subduction event. The presence of inherited zircons with ages of >3.0 Ga within the Luanjiajie gneisses suggests that this area may contain as yet undiscovered rocks that formed before 3.0 Ga.     

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~ 1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)_n ratios, which is typical of igneous zircon. The weak positive correlation between ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~ 2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of ε_Hf values (from + 9.8 to –3.3) and model age T ^C 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive ε_Hf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower ε_Hf and higher T ^C values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~ 2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (ε_Hf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.     

Sapphirine-Bearing Ultrahigh-Temperature Granulites of the Anabar Shield: Chemical Composition,U–Pb Zircon Ages,and <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> Conditions of Metamorphism

The results of thermobarometry yielded the P–T parameters of formation and evolution of sapphirine- bearing granulites in the Anabar shield with peak values of UHT metamorphism in the range of T = 920–1000°C at P = 9–11 kbar. Isotope–geochronological data indicate a polymetamorphic evolution of these rocks. Detrital zircon cores in the center of crystals yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later, superimposed metamorphic transformations of the detrital zircon formed rims dated to 2.4, 2.3, 2.2, and 1.83 Ga. A potential provenance source of the detrital zircons could be hypersthene plagiogneisses and metabasics of the Daldyn Group with a premetamorphic age no less than 3.32 Ga and products of their metamorphism of about 2.7 Ga old.     

Age and chemical characteristics of some pre-Pan-African rocks in the Egyptian Shield

UPb ages on zircons from several types of sialic rocks, together with a variety of geochemical data on one sample of early plutonic activity within the Egyptian Shield are presented.The earliest autochthonous granitic rocks in the Egyptian Shield are probably quartz diorites (tonalites) emplaced just prior to the main peak of Pan-African igneous activity (550–650 Ma). The studied example has an age of 711 Ma ± 7 Ma, and is characterized by very low initial Sr ratio (⁸⁷Sr/⁸⁶Sr = 0.7026) and very low abundances of K₂O and related large-ion lithophile trace elements. REE contents are low (less than 20 × chondritic abundances). Abundance patterns show only moderate light rare-earth enrichment (La/Yb = 4.5). These characteristics are inconsistent with any models requiring fractional fusion of pre-existing continental crustal material.Granitic and arkosic cobbles occur in rare conglomerate beds within volcanoclastic and greywacke sedimentary sequences. The cobbles show a wide range in ages (1.1–2.3 Ga) and have no obvious source within the Egyptian Shield. It is suggested that they are not indicative of any sialic plutons in the Egyptian Shield which might predate deposition of the enclosing immature sediments. More probably, they were derived from adjacent continental areas such as the Uweinat region to the west, and were deposited in an evolving arc-ocean basin complex.     

Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks,Kyushu, Japan

On the basis of internal structures, laser ablation U–Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K‐feldspar, yield ²³⁸U–²⁰⁶Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48–1.64. The rims contain inclusions of jadeite, yield ²³⁸U–²⁰⁶Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite‐normalized REE patterns of the cores indicate higher Sm_N/La_N ratios, lower Yb_N/Gd_N ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different ²³⁸U–²⁰⁶Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the ²³⁸U–²⁰⁶Pb ages of the rims are consistent with the reported ⁴⁰Ar/³⁹Ar spot‐fusion ages of matrix muscovite in the jadeitite, the ²³⁸U–²⁰⁶Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.     

Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong (SE Tibet): Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (ε_Hf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high ε_Hf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon ε_Hf and zircon δ¹⁸O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high ε_Hf and low δ ¹⁸O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-ε_Hf and low- δ ¹⁸O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.     

Characteristics of Rare Earth Elements,Zr, and Hf in Ore‐Bearing Porphyries from the Western Awulale Metallogenic Belt,Northwestern China and their Application in Determining Metal Fertility of Granitic Magma

The western Awulale metallogenic belt in northwestern China hosts a number of small‐ to medium‐sized porphyry Cu deposits that are associated with albite porphyry. The common presence of plagioclase (albite) as phenocrysts and the absence of hydrous minerals (amphibole and biotite) indicate that the water content of albite porphyry is low. Trace‐element compositions of whole rocks and zircon grains from these ore‐bearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce⁴⁺/Ce³⁺ ratios ranging from 7.75 to 95.1, which indicate that these porphyries have a low oxygen fugacity. Trace element compositions of ore‐bearing porphyries exhibit (La/Yb)_N ratios ranging from 1.09 to 11.1 and Eu/Eu^* ratios ranging from 0.10 to 0.66. These ore‐bearing porphyries have Zr values ranging from 171 to 707 ppm and Hf values ranging from 8.30 to 18.9 ppm. Combining these porphyries with other ore‐bearing porphyries that formed in the Central Asian Orogenic Belt (CAOB) and the Pacific Rim metallogenic belt, we found that the (La/Yb)_N and Eu/Eu^* ratios of ore‐bearing porphyries in western Awulale are low, while the Zr and Hf values are high. Specifically, REEs can be used to evaluate the degree of differentiation and degree of partial melting, and Zr and Hf can be used to evaluate the redox conditions and water content of magmatic rocks. Our findings indicate that ore‐bearing porphyries in western Awulale have a lower oxygen fugacity, degree of differentiation, and water content than do others in the CAOB and the Pacific Rim metallogenic belt. Compared to those of ore‐bearing porphyries with lower zircon Ce⁴⁺/Ce³⁺ ratios, the (La/Yb)_N ratios of ore‐bearing porphyries in our study are low, and the Zr and Hf values are high. This finding indicates that, under reducing conditions, the degree of evolution and water content may have an important influence on the metal abundance in magmas. There is also a clear relationship between (La/Yb)_N, Eu/Eu^*, Zr, Hf, and the size of the deposits. Large‐ (>4 Mt Cu) and intermediate (1.5–4 Mt Cu)‐sized porphyry Cu deposits are associated with granitic intrusions that have higher (La/Yb)_N and Eu/Eu^* ratios and lower Zr and Hf values. This finding indicates that, in addition to oxygen fugacity, the degree of evolution and water content are controlling parameters for metal abundance in magmas, especially in low oxygen fugacity porphyry Cu deposits. Such a conclusion may be useful in the exploration for other concealed porphyry Cu deposits.     

华北克拉通怀安陆块新太古代低铝和高铝TTG片麻岩的地球化学特征与成因

英云闪长岩-奥长花岗岩和花岗闪长岩(简称TTG)是太古宙高级变质地体的主要物质组成,对深入理解早期大陆生长及其机制具有重要的科学意义。目前,人们对其成因过程与机制仍有不同认识。本文以怀安陆块中广泛分布的TTG片麻岩为例,探讨其成因演化和机制。研究区位于华北克拉通中北部,主要由新太古代英云闪长岩及少量奥长花岗岩、花岗闪长岩组成。我们从该区识别出富硅富重稀土和负铕异常的低铝奥长花岗质片麻岩,形成时代与广泛分布的高铝TTG质片麻岩一致(锆石SHRIMP U-Pb年龄2.53Ga)。岩石地球化学数据显示,低铝奥长花岗质片麻岩的主量元素具有富SiO 2(76%~79%),低Al2O3(11.01%~12.61%)、CaO(1.27%~1.59%)、MgO(0.74%~0.24%)和Mg#(18~53)等特征,而广泛分布的高铝TTG岩系的主量元素含量变化大,例如,SiO 2=63%~77%、Al2O3=13.2%~17.77%、CaO=1.8%~5.78%、MgO=0.18%~3.84%和Mg#=35~64。微量元素方面,低铝奥长花岗质片麻岩具有Eu/Eu*负异常(除1个样品为弱正异常1.38外,其余样品分布在0.59~0.44),富集重稀土((La/Yb)N=4~7,(Gd/Yb)N=0.36~1.27),而高铝TTG岩系从弱负铕异常到正异常(Eu/Eu*=0.8~5.35),轻重稀土分馏明显((La/Yb)N=10~103、(Gd/Yb)N=1.97~5.72)。在微量蛛网图中二者的区别除重稀土明显存在区别外,低铝奥长花岗质片麻岩显示出Ba、Sr的相对亏损,而高铝TTG岩系则相反。二者Lu/Hf比值差异明显,低铝奥长花岗质片麻岩变化在0.1~0.16,而高铝TTG岩系变化在0.01~0.07。在Lu/Hf与相关元素以及SiO 2与相关元素哈克图解中,二者差异更加明显,表明它们之间不存在直接的成因联系。综合锆石U-Pb、Lu-Hf同位素特征以及岩石地球化学特征,我们认为低铝奥长花岗质片麻岩是低压下由新太古代新生基性地壳物质低程度部分熔融而成,源区残留矿物相以辉石+斜长石为主,岩浆可能存在过独居石的分异作用。高铝TTG岩系主要由新生基性地壳在相对高压下部分熔融而成,源区残留相以石榴石+辉石+角闪石以及无或少量斜长石为特征。岩浆经历过角闪石和辉石分离结晶作用,铕正异常增大的现象可能与斜长石堆晶有关。本区同时形成高铝和低铝TTG岩系的机制还需深入研究。俯冲机制、地幔柱机制以及二者共同作用等机制均能解释TTG的成因。依据本区同期还形成大量辉长质-闪长质岩浆和稍晚(2.5~2.45Ga)形成的钾质花岗岩类岩浆的侵入活动,我们认为本区高铝和低铝TTG岩系分别来自底侵作用导致的下地壳不同深度不同程度的部分熔融有关。引起底侵作用的机制可能与地幔柱或地幔柱与板块俯冲共同作用有关。     

The Sr,Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in ε_Nd(T) = 8.6–9.7 and ε_Hf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: ε_Nd(T) = 3.7–6.0, ε_Hf(T) = 11.1–12.7, and T _DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary ⁸⁷Sr/⁸⁶Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.     

Geochemical constraints on the petrogenesis of the Paleozoic granitoids of the Sierra de San Luis,Sierras Pampeanas,Argentina

The Paleozoic granitoids of the Sierra de San Luis comprise the Ordovician tonalite suite (OTS; metaluminous to mildly peraluminous calcic tonalite–granodiorites) and granodiorite–granite suite (OGGS; peraluminous calcic to calc-alkaline granodiorite–monzogranites), as well as the Devonian granite suite (DGS; peraluminous alkali-calcic monzogranites) and monzonite–granite suite (DMGS; metaluminous alkali-calcic quartz monzonite–monzogranite ± granodiorite, mildly peraluminous alkalicalcic monzogranites). The OTS has relatively high K₂O, CaO, and Yb_N and low Cr, Ni, Ba, Sr, Rb/Sr, Sr/Y, and (La/Yb)_N, as well as negative Eu/Eu¹, high ⁸⁷Sr/⁸⁶Sr (0.70850–0.71114), and unradiogenic ε_Nd(470Ma) (−5.3 to −6.0), which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. The OGGS consists of two granitoids: (1) high-temperature characterized by low Al₂O₃/TiO₂, Rb/Sr, and (La/Yb)_N, a smooth negative Eu/Eu¹, and relatively high CaO and (2) low-temperature with high Al₂O₃/TiO₂ and Rb/Sr, low CaO, (La/Yb)_N, and Sr/Y, and negative Eu/Eu¹. Melting of metagreywackes at pressures below 10 kbar with a variable supply of water could account for the chemistry of the high-T OGGS, whereas dehydration melting of biotite-bearing metasedimentary sources at low pressures is proposed for the low temperature OGGS. Melting of crustal sources relates to a contemporaneous mafic magmatism.Devonian magmatism is characterized by high Ba, Sr, K₂O, Na₂O, Sr/Y, and (La/Yb)_N. Sources for the DGS include metasedimentary or metatonalitic protoliths. Biotite dehydration melting triggered by the addition of heat, supplied by mantle-derived magmas, is proposed. High Ba, Sr, LREE, MgO, Cr, Ni, Zr, and V of the monzonites suggest an enriched lithospheric mantle source. Low Yb and Y and high Sr and (La/Yb)_N indicate a garnet-rich residual assemblage (P ⩾ 10 kbar). Melts for the peraluminous rocks may have derived from a metasedimentary or metaigneous source at lower pressures in a process dominated by biotite consumption and plagioclase in the residue.The Ordovician granitoids are synkinematic with compressive deformation related to the early stages of Famatinian convergence. The Devonian magmatism is synkinematic with a system of shear zones that were active during the Achalian cycle.     

Geochemical and Lu/Hf isotopic (LA–ICP–MS) systematics of detrital zircons from the Upper Ordovician sandstones of the Bashkir Uplift (Southern Urals)

Analyses of trace elements and Lu/Hf isotopes have been carried out in already U–Pb dated detrital zircons from the Upper Ordovician sandstones of the southern part of the Bashkir Uplift. The concentrations of trace elements in the zircons suggest that they were derived from rocks of intermediate (62%), basic (24%), and felsic (9%) compositions as well as alkaline rocks (2%). The Lu/Hf systematics of the zircons demonstrated a wide variation of both ε_Hf (from +9.5 to–8.7) and model ages of the parental rocks (1.60 Ga < T _DM ^C < 3.28 Ga). Comparison of the isotopic–geochemical characteristics of the detrital zircons from different levels of the Riphean–Paleozoic sequence of the Bashkir Uplift against those from the Early Cambrian Brusov Formation of the Mezen Basin and Early Neoproterozoic Dzhezhim sandstones of Timan Ridge suggests that at the end of the Late Precambrian near the Uralian margin of Baltica a large block of Late Mesoproterozoic–Early Neoproterozoic crust existed, comprising a significant proportion of melanocratic rocks.     

华北克拉通胶东地区～2.7Ga TTG岩石的成因及地质意义

胶东是华北克拉通保存~2.7Ga岩石的主要地区之一。通过地质调查和锆石SHRIMP测年,我们在胶东5个地方识别出2.7Ga的岩石,这些岩石包括片麻状英云闪长岩、片麻状花岗闪长岩以及片麻状高硅奥长花岗岩。SHRIMP测年结果显示,除样品QX12126外,其他样品中的锆石大多具有清晰的岩浆环带,207Pb/206Pb加权平均年龄为2718~2743Ma,指示这些样品的原岩结晶年龄为~2.7Ga。样品QX12126中锆石大多遭受后期改造,核部获得的最老的207Pb/206Pb年龄(2710±11Ma)应最接近原岩的结晶年龄。根据地球化学结果,胶东~2.7Ga岩石可以分为两组。组1以高的(La/Yb)N、Sr/Y,低的Yb、Y为特征,并表现出无Eu异常或正Eu异常;组2则具有低的(La/Yb)N和Sr/Y,高Yb、Y的特征,并具有负的Eu异常。这些地球化学特征指示组1可能来自以榴辉岩或斜长石很少的石榴角闪岩为残留的源区,而组2源区残留可能为几乎不含石榴石的角闪岩。结合放射性同位素数据,我们认为组1可能起源于加厚的中太古代玄武质下地壳,榴辉岩下地壳发生拆沉作用,上涌的热的软流圈加热浅部残留的地壳物质,形成高HREE(组2)的TTG岩石。胶东与霍邱地区新太古代早期岩石成因、更早期地质记录存在相似性,两者可能为同一古老陆块。出露~2.7Ga岩石的地区在华北分布分散,不同地区~2.7Ga岩石的成因可能存在差异,在探讨华北克拉通新太古代早期构造演化过程时应充分考虑这些因素。     

挥发份对高硅岩浆演化趋势的制约:以东南沿海白垩纪晚期花岗岩类岩石为例SCIEI北大核心CSCD

东南沿海分布大面积的白垩纪晚期侵入岩。这些岩石可分为两期:其中115~100Ma以钙碱性系列岩石为主,岩石组合为辉长岩-闪长岩-花岗闪长岩-二长花岗岩-碱性长石花岗岩;而100~86Ma的岩石为碱性系列,岩石组合为石英二长斑岩-正长斑岩-碱性长石花岗岩。115~100Ma的辉长岩以角闪辉长岩为主,具有极高的CaO、MgO和Al_(2)O_(3)含量,具有极低的SiO_(2)(42.9%~53.8%)、全碱(K_(2)O+Na_(2)O:0.86%~5.28%)、Ba、Nb、Th、Rb和Zr含量,也具有极低的FeO^(T)/MgO、La/Yb和Zr/Hf比值,较高的Eu/Eu^(*)、Sr/Y比值和Sr含量,为基性-超基性堆晶岩。与辉长岩同期的闪长岩和细粒暗色包体具有较高的SiO_(2)(50.34%~63.68%),较低的CaO、P_(2)O_(5)、MgO、Al_(2)O_(3)含量,相对低的Eu/Eu^(*)和Sr/Y比值,变化较大的La/Yb和Zr/Hf比值,代表了从基性岩浆储库中抽取的富硅熔体。115~100Ma的花岗闪长岩和二长花岗岩类岩石为准铝质岩石,SiO_(2)含量变化较大(61.7%~75.3%),具有较低的FeO^(T)/MgO、Ga/Al比值和Nb、Zr及Nb+Zr+Ce+Y元素含量,显示出典型I型花岗岩的特征。这些花岗岩具有相对高的La/Yb、Eu/Eu^(*)和Zr/Hf比值和高的Sr、Ba和Zr含量。结合岩相学特征,这些花岗岩为堆晶花岗岩。而115~100Ma的碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),低的Eu/Eu^(*)、La/Yb、Zr/Hf和Sr/Y比值,具有低的Ba、Sr和Zr含量和高的Rb、Nb、Y和Th含量和Rb/Sr比值,表明这些花岗岩是由富硅岩浆储库中抽离的高硅熔体侵入地壳形成。100~86Ma期间形成的二长斑岩和正长斑岩具有极高的全碱含量,可以达到8%~12%,其SiO_(2)主要集中在60%~70%,具有极高的Zr、Sr和Ba含量和Eu/Eu^(*)、La/Yb和Sr/Y比值,显示出堆晶花岗岩的特征。而100~86Ma期间形成的大部分碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),并显示出A型花岗岩的特征,具有高的Rb/Sr比值和高的Rb、Y和Th和低的Ba、Sr含量和低的Zr/Hf、La/Yb、Eu/Eu^(*)和Sr/Y比值,表明它们是由富硅岩浆储库抽离的高硅熔体侵入浅部地壳形成。东南沿海高硅花岗岩的形成和穿地壳岩浆系统密切相关,高硅花岗岩是由浅部地壳内晶体-熔体分异产生的熔体侵入地壳所形成,而高硅花岗岩的地球化学特征与岩浆储库的水及挥发份含量密切相关。115~100Ma期间,从富水的岩浆储库抽离的熔体形成具有低高场强元素含量和低Rb/Sr比值的高硅花岗岩,这一过程与古太平洋板块俯冲有关;100~86Ma期间,从富挥发份的岩浆储库抽离的熔体形成碱性特征、富含高场强元素和具有高的Rb/Sr比值的高硅花岗岩,这一过程和古太平洋板块回撤软流圈上涌有关。     

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen: Implications for Crustal Evolution of the South China Craton

The Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China. As a volcanic-sedimentary sequence developed in the Nanhua rift, the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent. In this study, we report the results from whole-rock major-and trace-element concentrations, with zircon LA-(MC)-ICP-MS U-Pb ages, trace elements and Lu-Hf isotopic comp...     

Zircon U–Pb Geochronology,Elemental Geochemistry and Geological Implications of the Andesite of the Yixian Formation in the Tiejiangyingzi Basin in Southeastern Chifeng,China

The Early Cretaceous andesite of the Yixian Formation is an ore‐host rock in the Tiejiangyingzi basin in southeastern Chifeng, China, and bears several meso‐epithermal Au deposits, such as the Nalingou and Yaogou Au deposits. The geochronology and elemental geochemistry of the Yixian Formation were investigated to reveal its metallogenic context, petrogenesis, and mineralization contribution. The ²⁰⁶Pb/²³⁸U ages of zircons reveals that the andesite formed at 125.5 ± 0.9 Ma. This rock belongs to calc‐alkali rock series according to its elemental geochemical analysis results (i.e., SiO₂ = 63.63%–67.27%, MgO = 1.17%–1.43%, K₂O = 4.07%–4.67%, Mg^# = 35–41, Al₂O₃ = 17.16%–18.63%, Na₂O = 3.35%–4.08%, Sr = (240.1–585.6) × 10^‐6, Yb = (1.83–2.22) × 10^‐6, Y = (13.9–23.36) × 10^‐6, Na₂O/K₂O = 0.7–0.9, Eu/Eu* = (0.75–0.79) × 10^‐6, Sr/Y = 13–27, La_N/Yb_N = 11–19 ). The chondrite‐normalized trace element plot shows that the andesite is enriched in light rare earth elements (LREEs) and Yb. Thus, the residual phase of andesite volcanic rocks is a granulite suite that contains amphibole and little remaining garnet in the Tiejiangyingzi basin. It can be concluded that andesite volcanic rocks distributed in the Tiejiangyingzi basin were formed in an extensional tectonic environment during the Mesozoic orogen, and the lower crust had experienced extensive high‐temperature metamorphism and melting. This gold‐bearing lower crust is closely related to mineralization. Therefore, the conclusions of this research have an important implication for further prospecting in southeastern Chifeng, China.     

辽宁本溪歪头山条带状铁矿的成因类型、形成时代及构造背景

辽宁鞍本地区位于华北克拉通东北缘,分布有诸多大型-特大型条带状铁矿床。本文对该区歪头山铁矿进行了岩石学、矿物学及年代学研究。歪头山铁建造以条带状铁矿石为主,兼含有少量的块状矿石,其顶底板围岩及矿体夹层主要为太古界鞍山群斜长角闪岩。元素地球化学分析表明,铁矿石富集重稀土[(La/Yb)_PAAS=0.24~0.33],具La正异常(La/La^*=1.43~1.61)、Eu正异常(Eu/Eu^*=2.40~4.54)及Y正异常(Y/Y^*=1.10~1.30),Y/Ho值平均30.59,Sr/Ba值平均17.62,Ti/V值平均19.45,反映成矿物质可能来源于由海底火山活动带来的高温热液与海水的混合溶液。铁矿石无明显Ce负异常(Ce/Ce^*=0.92~1.06),暗示BIF沉积时海水处于缺氧环境。除Fe₂O₃^T与SiO₂外,铁矿石中其它氧化物含量均非常低,且贫Th、U、Zr等具有陆源性质的元素,表明大陆碎屑物质对BIF贡献极少。斜长角闪岩稀土元素配分型式近于平坦[(La/Yb)_N=0.80~1.10],无明显Ce异常(Ce/Ce^*=0.95~0.99)与Eu异常(Eu/Eu^*=0.88~1.16);其大离子亲石元素富集,高场强元素无明显亏损。地球化学分析表明,斜长角闪岩原岩可能为产于弧后盆地的玄武质火山岩。锆石形态与微量元素分析显示,斜长角闪岩中的锆石均属岩浆成因。SIMS锆石U-Pb定年显示斜长角闪岩原岩形成于2533±11Ma,代表了歪头山BIF的成矿年龄;在玄武质岩浆喷发过程中,还捕获了一组年龄为2610±5Ma的锆石。电子探针分析显示磁铁矿成分纯净(FeO^T=92.04%~93.05%),其标型组分特征暗示歪头山BIF属沉积变质型铁矿。综合分析认为,歪头山铁矿属Algoma型BIF,成矿与弧后盆地岩浆活动密切相关,指示了新太古代末华北克拉通普遍发育的一期BIF成矿事件。     

Change of Conditions of the Formation of the Karelian Province of the Baltic Shield Continental Crust during Transition from Meso- to Neoarchean: Geochemical Study Results

The compositions of the tonalite–trondhjemite–granodiorite (TTG) assemblage and volcanic rocks of the Archaean greenstone belts from different domains of the Karelian province of the Baltic Shield are compared. Neoarchean medium felsic volcanic rocks and TTG of the Central Karelian domain drastically differ from analogous Mesoarchean rocks of the neighboring Vodlozero and West Karelian domains in higher Rb, Sr, P, La, and Ce contents and, correspondingly, values of Sr/Y, La/Yb, and La/Sm, and also in a different REE content distribution owing to different rock sources of these domains. This fact is confirmed by differences in the composition and the nature of the REE distribution in the basic and ultrabasic volcanic rocks making up the greenstone belts of these domains. It is established that the average compositions of Mesoarchean TTG rocks and volcanic rocks of the Karelian province differ markedly from those of plagiogranitoids and volcanic rocks of the recent geotectonic environments in high Mg (mg#) and Sr contents. Neoarchean volcanic rocks of Karelia differ from recent island-arc volcanic rocks, but are similar in composition to recent volcanic rocks of the continental arcs. On the basis of the cumulative evidence, the Karelian province of the Baltic Shield was subject to dramatic changes in the crust formation conditions at the beginning of the Neoarchean at the turn of about 2.75–2.78 Ga. These changes led to formation of volcano-sedimentary and plutonic rock complexes, different in composition from Mesoarchean rocks, and specific complexes of intrusive sanukitoids and granites. Changes and variations in the rock composition were related to the mixing of plume sources with continental crust and/or lithospheric mantle material, likely as a result of the combined effect of plumes and plate tectonics. This process resulted in formation of a younger large fragment of the Archean crust such as the Central Karelian domain which factually connected more ancient fragments of the crust and likely contributed to development of the Neoarchean Kenorland Supercontinent.