Hf isotope composition of zircons and implication for the petrogenesis of Yajiangqiao granite,Hunan Province,China

In the peralumineous granite of Yajiangqiao zircon population can be divided into two groups (i.e. Zircon Ⅰ and Zircon Ⅱ), which were formed in magmatic chamber and in emplacement place, respectively. The Hf isotope compositions of two stages of zircon show that the host magma was essentially derived from crustal material by melting. However, some higher 176Hf/177Hf ratios indicate that the granitic zircons should contain fine crystal of zircon formed in mantle-derived magma. In fact, the backscattered electron imaging and the electron microprobe analysis reveal that there is another type of zircon included within Zircon Ⅰ, which is quite different from Zircon Ⅰ and Zircon Ⅱ in morphology and chemistry. They are considered to be the product of the mantle-derived magma intruded into the granitic magma chamber at the beginning of anatexis. Thus,it is suggested that the formation of Yajiangqiao granite is related to the underplating of mantle magma.     

Zhang-Xuan thermal uplift and its genesis

Variousformsofcrustdeformationaccompany ingthelithospherethinninginNorthChinasincethe Mesozoichavedrawnattentionfrommanygeologists.Therecordsofdeformationandthermalevolutionary historyindifferentperiodsarequitecompletefortwo upliftbelts,whichlocatedrespectivelyatthenearE WstrikingYanshanMt.intersectingwiththeNNE strikingTaihangMt.andDaHigganMt.risingin theMesozoic,nowtheZhangjiakou Xuanhuaand Chifeng Harqinareas.Theformerareahasbeenafo cusofattentionforitsgoldfield[1],anditstectonic se…     

Post-collisional lithosphere delamination of the Dabie-Sulu orogen

The consistence between the first rapid cooling time (226-219 Ma) of the untrahigh pressure metamorphic (UHPM) rocks in the Dabie Mountains and the formation time (205-220 Ma) of the syncollisional granites in the Qinling and Sulu areas suggests that the first rapid cooling and uplift of the UHPM rocks may be related to breakoff of subducted plate. Therefore the second rapid cooling and uplift (180-170 Ma) of the UHPM racks needs a post-collisional lithosphere delamination which resulted in the granitic magmatism with an age of about 170 Ma. In addition, the rapid rising of the Dabie dome in the early Cretaceous (130-110 Ma) and the corresponding large-scale magmatism in the Dabie Mountains need another lithosphere delamination. The geochronology of the post-collis- ional mafic-ultramafic intrusions and geological relationship between the mafic-ultramafic intrusions and granites suggest that partial melting was initiated in the mantle, and then progressively developed in the crust, suggesting a mantle upwelling underneath the Dabie Mountains. The unusual fractional trend of the gabbros characterized by lower SiO2 content (46.24%) corresponding to lower MgO content (4.53%) and their typical geochemistry features of the lower crust suggest underplating of the mantle derived magma and interaction between the magma and lower crust before their intrusion. Lithosphere delamination could be the dynamic cause of the mantle upwelling and underplating. The seismic tomography results of the Dabie Mountains and adjacent areas clearly show lithosphere thinning below the north and south sides of the Dabie Mountains. Because there is no Cenozoic magma event in the Dabie Mountains, the lithosphere thinning may result from delamination of thickened lithosphere mantle after collision. In addition, both the lower velocity zone in the 40 km depth and the basin + dome + basin coupling relationship in the Dabie Mountains also suggest the lithosphere delamination and underplating on the two sides of the orogen.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China─U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China—U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Forma- tion in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma,which is consistent with the age of the Early Mesozoic underplating granulites,and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma,which is highly consistent with the Ar-Ar dating of the host volcanic rocks.Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites,indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China─U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97—158 Ma basaltic underplating and granulite-facies metamorphism

U-Pb zircon dating by LA-ICP-MS andSHRIMP for one olivine pyroxenite yields complex agepopulations including Mesozoic ages of 97-158 Ma and 228±8.7 Ma, Early Paleozoic ages of 418--427 Ma, Paleoprotero-zoic age of 1844±13 Ma, Neoarchean age of 2541±54 Ma andmiddle Archean age of 3123±4.4 Ma. The 97--158 Ma and228±8.7 Ma zircons show typical igneous oscillatory zona-tion in CL images, suggesting two episodes of magmaticevents. Overlapping of the 97-158 Ma ages with that ofgranulite xenoliths indicates that the Mesozoic granu-lite-facies metamorphism was induced by heating from thebasaltic underplating at the base of the lower crust. Bothprocesses lastcd at least from about 158 to 97 Ma. Ages of 418--427 Ma could be records of the subduction of Mongoliaoceanic crust under the North China craton. Ages of 1.84 Ga,2.54 Ga and 3.12 Ga correspond to the three importantcrust-mantle evolutionary events in the North China craton,and imply preservation of Precambrian lower crust in thepresent-day lower crust.     

Cumulate complex xenoliths in the Early Mesozoic in eastern Inner Mongolia

Mafic xenoliths occur in Mesozoic diorites from nine localities in eastern Inner Mongolia from the Da Hinggan Mts. to Huabei craton. Their petrologic and geochemical features suggest that they are an intact suite of cumulate complex whose mineral assemblages contain typical patent mantle metasomatism minerals such as pargosite, phlogopite and apatite. Their REEs and trace element characteristics suggest a comagmatic relationship in these cumulates with different compositions. Isotopic dating of the xenoliths indicates that they are products of the mantle magmatic underplating in the Early Mesozoic (237–224 Ma), and they may provide direct evidence for the crust-mantle interaction and vertical accretion of continental crust under a background of the mantle upwelling in the Early Mesozoic in Huabei region.     

Adakite-type sodium-rich rocks in Awulale Mountain of west Tianshan: Significance for the vertical growth of continental crust

The sodium-rich dacites and albite porphyries of Permian in the Awulale Mountain of west Tianshan have unique chemical and trace element signatures identical to adakite. These intermediate-acidic igneous rocks are characterized by high Na2O, Al₂O₃ and Sr contents and high Sr/Y and La/Y ratios (＞40 and ＞20, respectively), and low Y and Yb contents, and strong depletion in HREE, and positive Eu anomaly. The (^{143Nd/₁₄₄Nd)_i} is in the range from 0.51236 to 0.51248 and the e_Nd(t) is positive value (+0.79—+3.11); the (⁸⁷Sr/⁸⁶Sr)^_i is in the range from 0.7052 to 0.7054. These Nd and Sr isotopic composition features indicate that the source rocks of these adakite-type rocks are from a weakly depleted mantle, or a depleted mantle, but was contaminated by the crustal materials. These adakite-type rocks were most likely derived from the partial melting of new underplated basaltic rocks under the conditions of amphibolite to eclogite transition in the postcollisional environment of North Xinjiang during the Permian Period. They are not only the Phanerozoic juvenile crust materials, but also are probably animportant symbol of the underplating of mantle- derived basaltic magmas and the vertical growth of continental crust in the west Tianshan area during the postcollision of Late Paleozoic.     

乌拉特中旗二叠纪Ⅰ型花岗岩类地球化学特征及构造意义

内蒙古乌拉特中旗的华北板块北缘中段发育了大量的花岗岩类,主要由石英闪长岩和花岗闪长岩组成。岩石具有钙碱性I型花岗岩类特征:暗色矿物以黑云母为主,A/CNK为0.77～1.04。多数岩体地球化学特征类似于埃达克岩:SiO2含量为55.70%～68.65%,Al2O3含量为15.32%～22.87%,Y和Yb含量分别为2.82～20.89μg/g和0.25～2.24μg/g;大离子亲石元素(LILE)和轻稀土元素(LREE)富集,如Ba含量为627～2124μg/g,Sr含量为327～968μg/g,Sr/Y为32.2～211,(La/Yb)N为5.83～81.1;Eu多为正异常(δEu=0.63～2.25)。它们形成于后碰撞抬升的背景,主要由增厚的下地壳部分熔融形成,可能与来自EM1型富集地幔的玄武质岩浆底侵作用有关。北七哥陶岩体、塔拉布基岩体、阿格如岩体和保尔汗图庙岩体中黑云母的40Ar/39Ar年龄分别为260±3,258±1,260±3和258±2Ma,表明它们的侵位时代均为晚二叠世。