Chronological and Geochemical Characterisitics of Volcanic Rocks of the Yingcheng Formation in the Southern Songliao Basin: Constraints on the Early Cretaceous Evolution of the Songliao Basin

The discoveries of oil and gas reservoirs in the volcanic rocks of the Songliao Basin(SB) have attracted the attention of many researchers. However, the lack of studies on the genesis of the volcanic rocks has led to different opinions being presented for the genesis of the SB. In order to solve this problem, this study selected the volcanic rocks of the Yingcheng Formation in the Southern Songliao Basin(SSB) as the research object, and determined the genesis and tectonic setting of the volcanic rocks by using LA-ICP-MS zircon U-Pb dating and a geochemical analysis method(major elements, trace elements, and Hf isotopes). The volcanic rocks of the Yingcheng Formation are mainly composed of rhyolites with minor dacites and pyroclastic rocks. Our new zircon U-Pb dating results show that these volcanic rocks were erupted in the Early Cretaceous(113–118 Ma). The primary zircons from the rhyolites have εHf(t) values of +4.70 to +12.46 and twostage model age(TDM2) of 876–374 Ma. The geochemical data presented in this study allow these rhyolites to be divided into I-type rhyolites and A-type rhyolites, both of which were formed by the partial melting of the crust. They have SiO2 contents of 71.62 wt.%–75.76 wt.% and Al2 O3 contentsof 10.88 wt.% to 12.92 wt.%. The rhyolites have distinctively higher REE contents than those of ordinary granites, with obvious negative Eu anomalies. The light to heavy REE fractionation is not obvious, and the LaN/YbN(average value = 9.78) is less than 10. The A-type rhyolites depleted in Ba, Sr, P, and Ti, with relatively low Nb/Ta, indicating that the rocks belong A2 subtype granites formed in an extensional environment. The adakitic dacites are characterized by high Sr contents(624 to 1,082 ppm), low Y contents(10.6 to 12.6 ppm), high Sr/Y and Sr/Yb ratios, and low Mg# values(14.77 to 36.46), indicating that they belong to "C" type adakites. The adakitic dacite with high Sr and low Yb were likely generated by partial melting of the lower crust under high pressure conditions at least 40 km depth. The I-type rhyolites with low Sr and high Yb, and the A-type rhyolites with very low Sr and high Yb, were formed in the middle and upper crust under low pressure conditions, respectively. In addition, the formation depths of the former were approximately 30 km, whereas those of the latter were less than 30 km. The geochemical characteristics reveal that the volcanic rocks of Yingcheng Formation were formed in an extensional environment which was related to the retreat of subducted Paleo-Pacific Plate. At the late Early Cretaceous Period, the upwelling of the asthenosphere mantle and the lithosphere delamination caused by the retreat of the subducted Paleo-Pacific Plate, had resulted in lithosheric extension in the eastern part of China. Subsequently, a large area of volcanic rocks had formed. The SB has also been confirmed to be a product of the tectonic stress field in that region.     

Geochronology and Geochemistry of the Xingxingxia Triassic A-type Granites in Central Tianshan, NW China: Petrogenesis and Tectonic Implications

The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt. In this study, we present SHRIMP zircon U-Pb ages, Hf isotopic and geochemical data on the Xingxingxia biotite granite, amazonite granite and granitic pegmatite in Central Tianshan, NW China. Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite. These granitoid rocks have high K2O with low MgO and CaO contents. They are enriched in Nb, Ta, Hf and Y, while being depleted in Ba and Sr, showing flat HREE patterns and negative Eu anomalies. They have typical A-type granite geochemical signatures with high Ga/Al (8–13) and TFeO/(TFeO + MgO) ratios, showing an A2 affinity for biotite granite and an A1 affinity for amazonite granite and granitic pegmatite. Zircon εHf(t) values of the granitoids are 0.45–2.66, with Hf model ages of 0.99–1.17 Ga. This suggests that these A-type granites originated from partial melting of the lower crust. We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.     

U and Th Contents and Th/U Ratios of Zircon in Felsic and Mafic Magmatic Rocks: Improved Zircon-Melt Distribution Coefficients

High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon-melt partition coefficients are 81 for DU and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zircon-melt partition coefficients are 169 for DU and 59 for DTh. The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.     

Petrology and Geochemistry of the Huangshan Granitic Intrusion in Anhui Province, Southeast China: Implications for Petrogenesis and Geodynamics

<正>The Huangshan granitic intrusion in Anhui province,SE China,is tectonically located at the southeastern boundary of the Yangtze Block.Based on the contact relation and the petrography,the Huangshan granitic intrusion can be divided into four stages,from early to late,medium-grained monzogranite,coarse-grained porphyric granite,fine-medium grained porphyric granite,and finecoarse grained granite.All rocks from the Huangshan granitic intrusion display similar petrological and geochemical characteristics,i.e.relatively high SiO_2(75%) and alkali(7.85%-8.59%),low CaO (1%),high Fe-number(FeO_T/(MgO+FeO_T) = 0.93-0.97) and A/CNK(atomic Al/(Ca+Na+K))=1.04- 1.19.They are also enriched in rare earth elements(REE,except for Eu,with a total REE contents ranging from 116 ppm to 421 ppm),high strength field elements such as Zr,Hf,Nb,but depleted in Ba,Sr and Ni.The 10 000×Ga/Al ratios are higher than 2.6,which are consistent with the A-type granitoids.Based on the classification diagrams proposed by Eby,the Huangshan granite can be classified into the A2 group,which is usually believed to be formed under an extensional tectonic setting.Their Nd isotopic compositions suggest that the primary magmas of the Huangshan granite are predominantly derived from the Proterozoic andesitic rocks in the region,and this conclusion is also supported by REE modeling.The systemic investigations on the geochemistry of the Huangshan granitic intrusion can provide significant implications for the understanding of the petrogenesis and the geodynamic regime of southeastern China during the Late-Mesozoic.     

Trace Elements and Sr, Nd Isotope Geochemistry of the Lajimiao Norite-Gabbro from the North Qinling Belt

The Lajimiao norite-gabbro complex, as a part of the ophiolites on the southern side of the North Qinling belt, consists of gabbro and norite-gabbro. They were derived from different magma series: the gabbro was derived from tholeiitic magma series with higher TiO2, REE abundance and Fe3+/Fe2+ ratio ; norite-gabbro was derived from calc-alkali magma series with lower TiO2, Fe3+/Fe2+ ratio and REE abundance and much lower HREE abundance, which suggests that the source of the norite-gabbro magma was deeper and controlled by eclogite facies. Geochemical characteristics of both plutonic rocks are similar to those of island-arc basalts, such as relatively high contents of Ba, Pb and Sr and relatively low contents of Nb, Zr and N j.The Sr, Nd isotopic characteristics of the Lajimiao norite-gabbro complex are similar to those of ophiolites. Its εNd values are constant, about+2; whereas εst values have wide variation from - 6.4 to +31.2 and positively correlate with Na2O, H2O+ and CO2 contents and the Fe3+/Fe     

Association of Late Paleozoic Adakitic Rocks and Shoshonitic Volcanic Rocks in the Western Tianshan, China

The late Paleozoic adakitic rocks are closely associated with the shoshonitic volcanic rocks in the western Tianshan Mountains, China, both spatially and temporally. The magmatic rocks were formed during the period from the middle to the late Permian with isotopic ages of 248-268 Ma. The 87Sr/86Sr initial ratios of the rocks are low in a narrow variation range (-0.7050). The 143Nd/144Nd initial ratios are high (-0.51240) with positive εND(t) values (+1.28-+4.92). In the εNd(t)-(87Sr/86Sr)i diagram they fall in the first quadrant. The association of the shoshonitic and adakitic rocks can be interpreted by a two-stage model: the shoshonitic volcanic rocks were formed through long-term fractional crystallization of underplated basaltic magma, while the following partial melting of the residual phases formed the adakitic rocks.     

The Petrological and Geochemical Evolution of Ediacaran Rare-Metal Bearing A-type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

New fieldwork, mineralogical and geochemical data and interpretations are presented for the rare-metal bearing A-type granites of the Aja intrusive complex(AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring-shaped outcrops cut by later faulting. The A-type rocks of the AIC are late Neoproterozoic post-collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A-type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A-type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A-type granites consist of K-feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe-Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved(71.3–75.8 wt% SiO_2) and display the typical geochemical characteristics of post-collisional, within-plate granites. They are rare-metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu-negative anomalies(Eu/Eu* = 0.17–0.37) of the A-type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interactions. The chemical characteristics indicate that the A-type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile-rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post-magmatic alteration related to the final stages of the evolution of the A-type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.     

Paleoproterozoic Nb-enriched meta-gabbros in the Quanji Massif,NW China:Implications for assembly of the Columbia supercontinent

Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of-1.71 Ga Nb-enriched meta-gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta-gabbroic rocks have Nb contents of 11.5-16.4 ppm with Nb/La ratios varying from 0.84 to 1.02((Nb/La)_N = 0.81-0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO_2(45.1-48.5 wt.%) and MgO(5.96-6.81 wt.%) and Mg#(Mg# = Mg/(Mg + Fe) = 43.5-47.7), high FeO~t(13.0-15.7 wt.%) and moderate Ti02(1.70-2.51 wt.%).with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies(Eu/Eu~* = 0.87-1.02). Their primitive mantle-normalized elements patterns display significant Zr-Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba,resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc-like geochemical signatures together with whole rock εNd(t) values of 0.4-2.1 and corresponding old T_(DM)(2.22-2.37 Ga)as well as(~(143)Nd/~(144)Nd)_t and(~(87)Sr/~(86)Sr)t(t = 1712 Ma) values of 0.5104-0.5105 and 0.7030-0.7058,respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ~1.7 Ga within the Columbia supercontinent.     

Sr and Nd Isotopic Characteristics and Magma Genesis of Mesozoic Volcanic Rocks Along the Coastal Region of Southeastern China

The Mesozoic volcanic rocks in the coastal region of southeastern China were superimposed on some different basement tectonic elements. The volcanic rocks developed in these different basement tectonic elements have great differences in Sr and Nd isotopic compositions. The rocks in western Zhejiang and northeastern Jiangxi Provinces which belong to the Lower Yangtze subplate have lower initial 87Sr/ 86Sr ratios, but are higher in initial Nd isotopic ratios. The initial 143Nd / 144Nd values of the volcanic rocks developed in the Cathaysian subplate increase clearly from early to late in time, and from the core of the Wuyishan uplift coastwards constantly, but the initial 87Sr/86Sr values tend to decrease. The isotopic characteristics and their spatial variations in Mesozoic volcanic rocks in the study region are, to a great extent, manifestations of the isotopic characteristics in basement metamorphic complexes, and the generation of the Mesozoic acid magma in this region is attributed to the recycling o     

Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei： Implications for Crustal Accretion

The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.     

A型花岗岩的微量元素地球化学

本文总结和评述了A型花岗岩典型的微量元素特征,如富集Ga、稀土元素（除Eu外）和高场强元素,亏损Ba、Sr和明显的Eu负异常。分别讨论了影响微量元素特征的多种制约因素,主要包括源区性质、岩浆的物理化学条件、岩浆作用过程和络合作用。通过对比世界范围内几个地区相伴生的碱性A型花岗岩和铝质A型花岗岩的微量元素地球化学特征,发现前者Ga、F含量更高,而轻重稀土比值小,Eu、Ba、Sr等元素含量更低,显示了前者的岩浆分异作用更强,同时说明了碱性A型花岗岩可以由与之伴生的铝质A型花岗岩分异而来。     

Petrology,geochemistry, and geochronology of Paleoproterozoic volcanic and granitic rocks (1.89–1.88 Ga) of the Pitinga Province,Amazonian Craton,Brazil

This paper presents geochemical, petrographic, and geochronological data on the Uatumã magmatism in the Pitinga Province, where it is represented by volcanic rocks from the Iricoumé Group and granitic rocks from the Mapuera Suite. The Iricoumé Group (1.89–1.88 Ga) is constituted of the Divisor Formation (intermediate volcanic rocks), Ouro Preto Formation (acid effusive rocks), and Paraiso Formation (acid crystal-rich ignimbrites, surge deposits, and basic rocks). The volcanic sequence is intruded by granitoids from the Mapuera Suite (1.88 Ga), mainly represented by monzogranites and syenogranites. Structural and field relations suggest that caldera complex collapse controlled the emplacement of volcanics and granitoids of the Mapuera Suite. Subsequent structure reactivations allowed the younger Madeira Suite (1.82–1.81 Ga) to be emplaced in the central portion of the caldera complex. The felsic Iricoumé magmatism is mainly composed of rhyolites, trachydacites and latites, with SiO₂ contents between 64 wt% and 80 wt%. The plutonic rocks from the Mapuera Suite present SiO₂ between 65 wt% and 77 wt%. Volcanic and granitic rocks present identical geochemical characteristics and that is attributed to their co-magmatic character. The felsic volcanic rocks and granites are metaluminous to slightly peraluminous and show affinity with silica-saturated alkaline series or with A-type magmas. They have Na₂O + K₂O between 6.6% and 10.4%, FeO^t/(FeO^t + MgO) varying between 0.76 and 0.99, Ga/Al ratios between 1.5 and 4.9, like typical A-type rocks; and plot in the within-plate or post-collisional fields in the (Nb + Y) vs. Rb diagram. The Nb/Y ratios indicate that these rocks are comparable to A2-type granites. This magmatism can be related to the (i) potassic alkaline series, with low Sr content in the felsic rocks explained by plagioclase fractionation at low pressure and high temperature or, alternatively, (ii) a bimodal association where magma had high crustal influence. The similarity of the Iricoumé felsic magmatism with A2-type granitoids and their high ETRL/Nb ratios suggest its relation with mantle sources previously modified by subduction, probably in a post-collision environment. Alternatively, this can be interpreted as bimodal within-plate magmatism with contamination by crustal melts. In this context, the extreme F, Nb and Zr enrichment of Madeira Suite could be explained by the presence of a thin crust which favored the presence and continuity of convective systems in the upper mantle.     

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。     

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group,Pitinga Mining District,Amazonian craton,Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO₂, FeO_t, K₂O, Rb, and alkalis; low TiO₂, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na₂O?+?K₂O averages of 8.8 wt.%, FeO_t/(FeO_t?+?MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb?+?Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO₂ and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.     

Early Cretaceous adakitic lavas and A-type rhyolites in the Songliao Basin,NE China: Implications for the mechanism of lithospheric extension

There is a broad consensus that the extensive late Mesozoic igneous rocks in NE China were generated in an extensional setting. However, the timing and mechanism of the lithospheric extension remain controversial. To address this, we carried out an integrated study involving LA–ICP–MS zircon U–Pb dating and geochemical analyses (major elements, trace elements, and Hf isotopes) for the Early Cretaceous adakitic lavas and A-type rhyolites of the Songliao Basin. The adakitic lavas are andesites and dacites. The U–Pb dating of zircons from the adakitic lavas and A-type rhyolites yielded ages between 115 and 102 Ma. Geochemically, the adakitic lavas are characterized by high Sr contents (515–1610 ppm) and low Y (0.98–17.58 ppm) and heavy rare earth element (HREE) contents, and they therefore have high Sr/Y (51–112) ratios. They also exhibit high Mg^# values (36–57) and high contents of MgO (0.56–3.53 wt%), Cr (15.7–87.3 ppm), and Ni (6.7–44.7 ppm) that are comparable with those of high-Mg adakitic rocks. The A-type rhyolites show an affinity with aluminous A-type magmatic rocks, and they are metaluminous to peraluminous (A/CNK = 0.98–1.35), enriched in alkalis, Ga, Zr, Nb, and Y, depleted in Sr and P, and exhibit fractionated REE patterns with negative Eu anomalies (Eu/Eu* = 0.05–0.77). All the primary zircons from the adakitic lavas and A-type rhyolites have positive ε_Hf(t) values of +3.6 to +12.1 and juvenile two-stage model (T_DM2) ages of 934–392 Ma. The adakitic lavas probably resulted from the partial melting of a delaminated region of the lower continental crust, with the magmas subsequently interacting with mantle materials upon ascent, while the A-type rhyolites were probably generated by the partial melting of a dehydrated charnockitic middle–lower crust. The data suggest that the adakitic lavas and the A-type rhyolites formed in an extensional environment related to the rollback of the subducting Paleo-Pacific Plate. The upwelling of asthenospheric mantle and local delamination of the lithosphere, which were induced by rollback of the subducting Paleo-Pacific Plate, extended from the Great Xing'an Range southeastward through the Songliao Basin to eastern Heilongjiang and Jilin provinces, giving rise to the southeastward migration of lithospheric extension and extension-related volcanism after ca. 140 Ma.     

Strontium isotopic and chemical variations of the granitic rocks in the Tsukuba district,Japan

Initial ⁸⁷Sr/⁸⁶Sr ratios, major and trace element compositions have been determined for the Paleogene granitic rocks in the Tsukuba district, Japan. Isotopic ages strongly suggest that the granitic rocks (seven units) were continuously emplaced and solidified during a short time interval. Initial ⁸⁷Sr/⁸⁶Sr ratios for seven granitic units vary from 0.7082 to 0.7132, while sedimentary and metasedimentary country rocks have ratios at the time of granitic magma emplacement ranging from 0.7149 to 0.7298. Continuous linear arrays for the granitic rocks in the diagrams of initial ⁸⁷Sr/⁸⁶Sr ratios versus some chemical parameters can be explained by either of following two processes. One is the assimilation — fractional crystallization (AFC) process between the parental magma (SiO₂ of 68% and initial ratio of 0.7078) and sedimentary country rocks, and the other is magma mixing process between above parental magma and sediment derived acidic magma (melt) (SiO₂ of 75%). The high initial ratios (0.7078–0.7098) for basic rocks such as gabbro or diorite in the Tsukuba district and the similar characteristics observed in the rocks of Ryoke belt (SW Japan) suggest that the parental magma had the same source region as the basic rocks, probably the lower crustal source.     

Petrochemical and Sr-Nd isotope investigations of A-type granites in the east of Misho, NW Iran

The Misho plutonic complex consists of a series of granitic bodies which range from syenogranite, alkali granite to monzogranites. They include metaluminous to peraluminous compositions. The garnitoid bodies are intruded into the unmetamorphosed late Paleozoic rocks and are located between two dextral, oblique-slip fault systems along which they have been uplifted as a major positive flower structure. The Misho granitoid belongs to the alkaline granitoid series that have been attributed to a Late Permian post-collisional setting. The studied granitoid displays high SiO₂ contents between 67.71 and 76.4 wt%. On both FeO/(FeO + MgO) and [(Na₂O + K₂O) ? CaO] vs. SiO₂ diagrams, the samples, plot in the ferroan and alkaline fields, therefore, show an A-type granitoid signature. Trace and rare earth elements distribution patterns for the Misho rocks indicate a distinctive depletion in Nb, Sr, Ba, P, and Ti relative to other trace elements and a greater enrichment in large-ion lithophile elements compared to high field strength elements that are also typical features of A-type granites. The granitoid samples are geochemically similar to typical A₂-type granites, e.g., high K₂O + Na₂O, FeO/MgO, Ga/Al, and Y/Nb values and low CaO, Ba, Sr, and Eu contents. They have initial Sr isotopic ratios in the range >0.712 and negative ε _Ndt values of ?1 to ?3.2 for a time of generation of 232 Ma. We suggest that shear zones play an important role in providing suitable conduits for ascending asthenospheric material and heat influx in the lower crust continental. It is proposed that the Misho parental granitoid magmas were produced by the partial melting of the lower crust continental at extensional setting in active continental margin setting that was formed after the collision of the Eurasia plate and Iranian plate following closure of paleo-Tethyan oceanic crust during Middle Jurassic times.     

Triassic magmatism in the eastern part of the South China Block: Geochronological and petrogenetic constraints from Indosinian granites

The granitic dykes in the Badu Group,Zhejiang Province,South China provide important insights on tectonic setting and crustal evolution of the South China Block(SCB) and the Indochina Block during Triassic.Here we report LA-ICP-MS U-Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma,respectively,representing their timing of emplacement.The dyke rocks are enriched in K,Al,LREE,Rb,Th.U,and Pb.and are depleted in Nb,Ta,Sr,and Ti.The rocks are characterized by highly fractionated REE patterns with(La/Yb)N ratios of 28.46-38.07 with strong negative Eu anomalies(Eu/Eu* = 0.65-0.73).In situ Hf isotopic analyses of zircons from the Hucun granite yielded ε_(Hf)(t) values of-13.9 to-6.4 and two-stage depleted mantle Hf model ages of 1.68-2.15 Ga,which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block.The zircons from the Kengkou granite have ε_(Hf)(t) values ranging from 40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99-2.49 Ga,indicating magma origin from a mixed source.The Hucun and Kengkou dykes,together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust-mantle decoupling along the convergent plate boundary between SCB and the Indochina Block.     

Elemental and Sr–Nd isotopic systematics of the early Mesozoic volcanic sequence in southern Jiangxi Province, South China: petrogenesis and tectonic implications

Elemental and Sr–Nd isotopic results are presented for the early Mesozoic volcanic sequence (~172 Ma) in southern Jiangxi Province, South China. The sequence is voluminously composed of ~45% subalkaline basaltic rocks (group 1), <5% high-mg andesite–dacites (group 2) and ~50% rhyolites (group 3). The group 1 rocks are characterized by (La/Yb)_cn = 3.8–7.2, Eu/Eu* = 0.65–1.15, Nb/La = 0.64–0.99, ⁸⁷Sr/⁸⁶Sr(t) = 0.70602~0.70822 and _Nd(t) = –1.63 to +0.11, similar to those of an EMII-like source. The group 2 rocks have mg=0.42~0.60, SiO₂=60.24~66.71%, MgO=2.65~ 5.54%, Ni=24~102 ppm and Cr=84~266 ppm, classified as high-mg andesitic rocks. These rocks are more enriched in LILEs and LREE with more significant negative Eu anomaly (0.63~0.79), are more depleted in HFSEs with Nb/La ratios of 0.40–0.56 and have lower _Nd(t) (–9.44 to –7.78) and higher ⁸⁷Sr/⁸⁶Sr(t) (0.70985~0.71016), in comparison with the group 1 rocks. They most likely originated from metasomatised veins in the lithospheric mantle. The origination of the group 1 and group 2 magma suggests the development of a peridotite-plus-vein lithospheric mantle during early Mesozoic era beneath the interior of the Cathaysian block. The group 3 rhyolites are characterized by high SiO₂ (72.75~77.97%), Zr (99~290 ppm), Hf (3.9~9.7 ppm) and Ga/Al (2.76~3.87) and significant Nb–Ta, Ba–Sr and P–Ti depletions. These rhyolites exhibit Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) = 0.70962~0.71104, _Nd(t) = –4.63 to –5.80) similar to the contemporaneous Zhaibei and Pitou A-type granites in the area. Such characteristics suggest that they might be derived from the underplating basaltic magma contaminated by crustal materials. Therefore, an early Mesozoic rifting model in response to intracontinental lithospheric extension is proposed to account for the early Mesozoic volcanism in southern Jiangxi Province, South China.     

新疆东天山白山钼矿深部岩体地球化学特征及成因意义

白山钼矿深部钻探表明矿体下部存在着矿化花岗斑岩体,所获岩芯显示其岩性主要为似斑状钾长花岗岩、黑云母斜长花岗岩和花岗斑岩.该岩体具有高Al、Na和Sr,低Mg、Y和Yb,以及高Sr/Y和La/Yb比值等特点,类似于埃达克质岩的地球化学特征,暗示来源于较深的含石榴子石的源区;岩石形成时代和地球化学构造判别图解表明,岩体形成于造山后的构造环境.因此,白山钼矿岩体可能为加厚下地壳熔融的产物.白山钼矿床的成矿作用可能与埃达克质岩浆活动有关,其成矿过程得益于高Mo丰度的地壳源区、成岩过程中提供流体和岩浆高氧逸度环境等.