北大别早白垩纪花岗岩类的Sm-Nd和锆石Hf同位素及其构造意义

大别造山带产出两期早白垩纪造山后花岗岩类:早期(≈132Ma)变形的角闪石英二长岩和斑状二长花岗岩具高钾的类埃达克岩的地球化学特征,形成于增厚地壳(>50km)的下地壳部分熔融;晚期(≈128Ma)未变形的花岗岩包括细粒二长花岗岩和钾长花岗(斑)岩,属于正常安山岩-英安岩-流纹岩系列岩石,它们形成于相对薄的地壳(132Ma),增厚地壳(>50km)的下地壳存在双层结构:锆石Hf模式年龄约为3.0Ga的基性下地壳基底和约2.7Ga的中酸性(英云闪长质.花岗质)上部下地壳.在大另q造山带伸展垮塌的晚期阶段(≈128Ma),减薄的下地壳(<35km)主要为锆石Hf模式年龄约2.6～2.2Ga的中酸性(英云闪长质-花岗质)岩石,并夹杂少量新元古代Rodinia超大陆裂解时形成的新生陆壳.     

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t_DM model ages of 2.17–1.91 Ga and ε_Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t_DM model ages of 2.13–1.42 Ga and ε_Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.     

西大别山早白垩世谭冲和陈冲花岗岩体U- Pb年龄、地球化学和Sr- Nd- Hf同位素特征：对岩石成因和下地壳拆沉时限的约束

谭冲和陈冲岩体出露于西大别造山带新县岩基北缘,主要岩性分别为二长花岗岩和花岗斑岩。为理解其岩石成因和构造属性,对两个岩体开展了激光等离子质谱(LA-ICP-MS)锆石U-Pb测年、元素地球化学和Sr-Nd-Hf同位素研究。结果表明,谭冲和陈冲岩体锆石U-Pb年龄分别为133.5±1.1Ma(MSDW=0.63)和132.9±1.1Ma(MSDW=1.30),暗示其形成于早白垩世。岩石均具有较高的SiO_2(69.40%～77.82%)、Al_2O_3(11.72%～15.26%)和总碱(Na_2O+K_2O=6.40%～8.70%)含量,K_2O/Na_2O比值大于1,过铝质(A/CNK=1.14～1.66,一个样品值0.97除外)等特点,总体归为高钾钙碱性系列。岩石富集轻稀土((La/Yb)_N=22.98～28.64),负Eu异常不明显,亏损Ba、Nb、Ta、P、Ti和Y元素。岩石锶同位素初始比值I_(Sr)为0.707220～0.707557,钕同位素ε_(Nd)(t)值为-17.7～-18.1,两阶段Nd模式年龄T_(DM2)=2.36～2.40Ga。锆石ε_(Hf)(t)值集中于-21.4～-25.8,两阶段Hf模式年龄T_(DM2)=2.24～2.48Ga。详细的矿物组成、岩石地球化学和Sr-Nd-Hf同位素分析揭示,两个岩体成因类型均为分异的I型花岗岩,起源于残留相含有石榴子石的部分熔融岩浆。岩浆房形成压力和深度较大,可能是在加厚下地壳环境扬子陆块北缘古老陆壳物质重熔的产物。结合区域同期岩浆岩对比研究认为,西大别地区与东大别地区加厚下地壳拆沉时限基本一致,发生在133Ma左右。     

高黎贡西北缘早白垩世火山活动与怒江洋俯冲:来自流纹岩岩石地球化学、锆石U-Pb定年和Hf同位素的证据

高黎贡构造带西北缘早白垩世火山岩的厘定将为揭示腾冲与保山地块之间的关系补充强有力的证据。本文通过对构造带内高家寨和濮家寨一带的流纹岩进行岩石学、地球化学、锆石U-Pb定年和Hf同位素地质学等方面的研究,并结合前人对同时代侵入岩的研究成果,分析其形成的构造背景。锆石U-Pb定年结果表明高家寨和濮家寨流纹岩的形成时代相同,分别为122±2.2Ma和121±1.9Ma。岩石学和地球化学分析结果说明高家寨和濮家寨流纹岩都属于科迪勒拉型、镁质、弱过铝质-过铝质高钾钙碱性岩类,具有轻稀土富集、分馏程度高、重稀土相对亏损、Eu中度亏损及大离子亲石元素相对原始地幔强烈富集,高场强元素（HSFE）Nb、Ta、P、Ti及Sr强烈负异常的特点;流纹岩的Mg^#值较高,在SiO₂-Mg^#图解上分布于壳源物质部分熔融区上方;锆石ε_Hf（t）值变化范围大（-9.6~-2.6）和t_DM^C值高（1189~1572Ma）;在构造环境判别图解上所有样品落在大陆边缘弧区;流纹岩斑晶中普遍存在黑云母。这些特征与高黎贡构造带早白垩世岩浆岩带内壳幔混源的中酸性侵入岩一致,说明其岩浆主要来源于壳源物质的部分熔融,同时有少量幔源岩浆的混入,是怒江洋壳向腾冲地块下俯冲的结果。

     

Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt

K-Ar dating, major- and trace-element and Sr-Nd isotopic analyses were carried out for early Cretaceous (122-127 Ma) lamprophyres from the Sulu orogen in eastern China. The results show strong fractionation in rare-earth elements with (LREE) >100 times chondrite, but HREE <10 times chondrite, indicating the presence of residual garnet in the melting source. These rocks are characterized by significant LILE and LREE enrichment but Nb and Ta depletion with moderate Zr/Hf (39.8-50.8 with regard to 36 for primitive mantle) and Nb/Ta (17.8-23.0, compared with 17.5 for primitive mantle) fractionations, probably as a consequence of carbonate- and rutile-rich melt metasomatism induced by dehydration and/or melting of subducted continental slab at mantle depths. Age-correlated Sr-Nd isotope ratios show moderate ranges of ⁸⁷Sr/⁸⁶Sr(i) from 0.70787 to 0.70934 and −17.2 to −11.6 of ε_Nd(t). The lamprophyres from the Sulu orogen were derived from decompression melting of such a metasomatized lithospheric mantle that was mainly composed of phlogophite garnet peridotites and experienced crystal fractionation of a mineral assemblage of olivine+clinopyroxene±plagioclase en route to the surface. Such geochemical and isotopic signatures are also prevalent in the contemporaneous basaltic lavas in the Dabie-Sulu belt, suggesting predominant enrichment processes by carbonate- and rutile-rich metasomatic assemblage beneath the continental collisional belt.     

大别造山带古元古代黑云紫苏斜长片麻岩年代学、地球化学特征及其地质意义

为探讨大别造山带早期结晶基底物质组成及其演化特征,对大别造山带蕙兰山地区出露的黑云紫苏斜长片麻岩的岩石学、地球化学、锆石U-Pb年代学及Lu-Hf同位素组成进行研究,显示其原岩应为石英闪长岩,成岩时代为古元古代早期(2463±22 Ma),且经历了古元古代麻粒岩相变质作用,锆石Ti地质温度计显示其变质温度为767～827℃(平均799℃),变质时间为2002±11 Ma(MSWD=0.46)。岩石地球化学成分显示其SiO₂含量为55.16%～57.10%,MgO为4.74%～5.89%,Mg^#值为47.5～50.4,钙碱性系列,富Mg贫Si,稀土配分曲线呈现轻稀土相对富集的右倾分布特征((La/Yb)_N=19.3～34.1),无明显Eu异常((Eu/Eu^*)=0.9～1.2),且具较高的Sr、Ni、Cr含量,以及较低的Yb_N值及Y含量,与镁质闪长岩具相似特征;明显富集Rb、Ba、K等大离子亲石元素,相对亏损Nb、Ta、Zr、Hf等高场强元素;锆石Lu-Hf同位素显示具有较为均一...     

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.     

Episodic growth of zircon in UHP orthogneisses from the North Dabie Terrane of east‐central China: implications for crustal architecture of a collisional orogen

Zircon U‐Pb dating of three orthogneiss samples from the North Dabie terrane (NDT) is undertaken in order to reconstruct their formation and evolutionary histories, and also the crustal architecture of the Dabie orogen after Triassic subduction and exhumation. SHRIMP zircon U‐Pb dating, in combination with back scattered electron (BSE) imaging and Laser Raman spectrometry, provides accurate identification of the core, mantle and rim structure for zircon growth during protolith formation and overgrowth during subduction/exhumation and post‐collisional metamorphism. Concordant U‐Pb ages of 760–730 Ma and high Th/U ratios of >0.4 are obtained for relict oscillatory zoning fields of inherited cores that were not metamictized. These features suggest that these ages represent the time of magmatic protolith formation during the breakup of Rodinia. The overgrown mantle domains around the metamictized cores are clean with few mineral inclusions (e.g. quartz, garnet and apatite). Mantle domains have low Th/U ratios of <0.1 and yielded U‐Pb ages of 215–205 Ma, which are slightly younger than the known ages of peak ultrahigh‐pressure (UHP) metamorphism, suggesting that overgrowth took place during initial exhumation. The ages are similar to the time of retrograde metamorphism of the UHP orthogneisses in the Central Dabie terrane (CDT). Overgrown rims are also clean, with a few mineral inclusions of apatite and quartz. They yield two groups of U‐Pb ages, 138–137 Ma and 124–120 Ma. The former is considered to be the time of onset of orogenic extension and tectonic collapse, whereas the latter falls into the age range of widespread magmatism in the Dabie orogen, and is regarded as the time of extension climax that resulted in intensive anatexis of the crust. Whole‐rock Sr‐Nd isotope analyses of four orthogneisses show ε_Nd(t) values of ?1.2 to ?15 and I_Sr values >0.719, similar to the values obtained from UHP orthogneisses in the CDT. It is concluded that, as with the CDT, the orthogneisses with episodic zircon growths from the NDT should also be a part of the exhumed slice following the continental deep subduction. However, the orthogneisses in this study were buried at a lower level in the orogenic crust compared with those of the CDT prior to the Cretaceous magmatism. Therefore, the orthogneisses from the NDT were affected by the Cretaceous magmatism whereas the CDT orthogneisses were not affected.     

大别山造山带东南部天柱山花岗岩类侵入体的特征及其LA-ICP-MS锆石U-Pb年龄

大别山造山带内广泛发育晚中生代的花岗岩类侵入体,天柱山岩体是其中的一个典型代表,位于分隔南大别超高压变质带与北大别杂岩之间的五河-水吼断裂带附近.该岩体是个由多期次侵位构成的复式岩体,自早到晚依次为闪长岩→石英正长岩/花岗闪长岩→碱长花岗岩.整个杂岩体属高钾钙碱性系列,但碱长花岗岩的地球化学性质与构成杂岩体的其他类型岩...     

SHRIMP U–Pb zircon geochronology and geochemistry of metavolcanic and metasedimentary rocks in Northwestern Fujian, Cathaysia block, China: Tectonic implications and the need to redefine lithostratigraphic units

Northwestern Fujian Province is one of the most important Pre-Palaeozoic areas in the Cathaysia Block of South China. Metavolcano-sedimentary and metasedimentary rocks of different types, ages and metamorphic grades (granulite to upper greenschist facies) are present, and previously were divided into several Formations and Groups. Tectonic contacts occur between some units, whereas (deformed) unconformities have been reported between others. New SHRIMP U–Pb zircon ages presented here indicate that the original lithostratigraphy and the old “Group” and “Formation” terminology should be abandoned. Thus the “Tianjingping Formation” was not formed in the Archaean or Palaeoproterozoic, as previously considered, but must be younger than its youngest detrital zircons (1790 Ma) but older than regional metamorphism (460 Ma). Besides magmatic zircon ages of 807 Ma obtained from metavolcano-sedimentary rocks of the “Nanshan Formation” and 751–728 Ma for the “Mamianshan Group”, many inherited and detrital zircons with ages ranging from 1.0 to 0.8 Ga were also found in them. These ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break-up of the Rodinia supercontinent. The new zircon results poorly constrain the age of the “Mayuan Group” as Neoproterozoic to early Palaeozoic (728–458 Ma), and not Palaeoproterozoic as previously thought. Many older inherited and detrital zircons with ages of 3.6, 2.8, 2.7, 2.6–2.5, 2.0–1.8 and 1.6 Ga were found in this study. A 3.6 Ga detrital grain is the oldest one so far identified in northwestern Fujian Province as well as throughout the Cathaysia Block. Nd isotope t_DM values of eight volcano-sedimentary and clastic sedimentary rock samples centre on 2.73–1.68 Ga, being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation. These rocks underwent high grade metamorphism in the early Palaeozoic (458–425 Ma) during an important tectono-thermal event in the Cathaysia Block.     

Zircon U–Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan, central China

The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al₂O₃ content (15.73%), relatively high CaO (2.46%) and Na₂O contents (Na₂O/K₂O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)_N=46.8), similar to some high Al₂O₃ Archaean TTG gneisses. Conversely, the syenogranite is characterized by relatively low Al₂O₃ (14.05%) and CaO (0.82%) contents, and higher K₂O than Na₂O (Na₂O/K₂O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.     

北淮阳早白垩世金刚台组火山岩LA-ICP-MS锆石U-Pb年龄及其地质意义

金刚台组火山岩是大别造山带北缘北淮阳晚中生代火山岩带的重要组成部分,选取金刚台组的粗面安山岩、熔结凝灰岩以及紧邻火山岩的正长斑岩,用LA-ICP-MS锆石U-Pb法进行了年龄测定,结果显示:两个火山岩样品的年龄分别为128.8±0.7 Ma和127.6±0.5 Ma,紧邻火山岩的正长斑岩的年龄为129.8±0.7 Ma,这3组年龄值在误差范围内近于一致,说明金刚台组火山岩和紧邻火山岩的正长斑岩是在很短的时间内形成的。这些年龄与整个苏鲁-大别造山带内早白垩岩世岩浆活动年龄的峰值区间一致,可能意味着它们形成于相同的动力学条件下。     

北大别岳西岩体和飞旗寨岩体的LA-ICPMS锆石U-Pb定年及其地质意义

大别造山带发育大量早白垩世侵入体,前人对北大别岳西岩体和飞旗寨岩体的形成时代的认识存在差异。选取岳西岩体和飞旗寨岩体为研究对象,对其进行全岩主元素分析和LA-ICPMS锆石U-Pb定年。锆石的高Th/U比值特征及典型岩浆锆石环带显示其为岩浆成因。岳西岩体的26个测点的加权平均年龄为(128.5±1.4)Ma,飞旗寨岩体的15个测点的加权平均年龄为(127.4±1.7)Ma,说明岳西岩体和飞旗寨岩体均形成于早白垩世。结合目前大别造山带高精度火成岩年代学数据,大别造山带腹地侵入岩可以划分为早晚两期,分别为143~130 Ma和130~119 Ma。早期火成岩具有高钾钙碱性和埃达克质地球化学属性,而晚期火成岩具有向碱性岩过渡的性质,在120 Ma左右出现少量A型花岗岩,暗示130 Ma为本地区深部过程转换的重要时期。     

北大别的多期深熔作用及山根垮塌的新证据

大别山是由华南板块在245～210 Ma向华北板块之下俯冲并发生陆陆碰撞形成的。随着南、北板块的汇聚继续,地壳持续加厚。然而,加厚的下地壳岩石（特别是镁铁质下地壳岩石）在重力作用下密度增大、稳定性降低,在145～130 Ma 时发生深熔作用;130 Ma 左右加厚下地壳拆沉,引发软流圈上涌,产生了130～110 Ma的大规模镁铁质和花岗质岩浆作用以及北大别发生强烈的混合岩化作用。其中,北大别混合岩中不同类型浅色体（至少可以分为4种）和碰撞后变质闪长岩的甄别及其岩石地球化学和同位素年代学方面系统研究为大别山印支期深俯冲陆壳的折返以及燕山期镁铁质下地壳岩石拆沉和山根垮塌所引发的多期深熔作用提供了新的关键证据。山根垮塌诱发的地幔对流导致～145 Ma时岩石圈开始减薄,进而导致加厚镁铁质下地壳温度和地壳中下部地热增温率升高,并使其发生部分熔融;加厚下地壳的部分熔融导致造山带下地壳持续弱化,加剧其重力不平衡,从而引发深部俯冲的镁铁质下地壳岩石的大规模拆沉和山根垮塌。     

U–Pb zircon and monazite geochronology of post-collisional Hercynian granitoids from the Central Iberian Zone (Northern Portugal)

In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonça granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeirós plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Gerês massif (Gerês, Paufito, Illa and Carris granites). The Gerês granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma.     

SHRIMP U–Pb zircon dating of a metagabbro and eclogites from western Dabieshan (Hong''an Block), China, and its tectonic implications

The Tyrrhenian Sea is a Neogene back-arc basin formed by continental extension at the rear of the eastward migrating Apennine subduction system. Its central part, generated from Tortonian to Pliocene, includes the Sardinia rifted margin to the west, an area with large volcanoes in the deep central sector, and the Campania rifted margin to the east. A reprocessing of some 2000 km of MCS lines, a new swath bathymetric map, and a review of previous geological and geophysical data allow to analyse the nature and distribution of continental vs. oceanic crust in this area, which evolved in a short time span.The central portion of the southern Tyrrhenian Sea is characterized by MOHO at about 10 km depth. North of Magnghi and Vavilov Smts, this thinned crustal domain include a wide continent–ocean transition, with the occurrence of extensional allochthons and of serpenitinzed sub-continental mantle, recalling other well known rifted margins, as the Iberia one. Sectors floored by oceanic crust should occur, mainly in the southern part of the study area, but they do not appear related to discrete spreading ridges. The continent–ocean boundary cannot be drawn unequivocally in the area, due also to the occurrence of widespread and huge magmatic manifestations not related to oceanic spreading. These portions of the southern Tyrrhenian Sea represents therefore a complex oceanic back-arc basin surrounded by magma-rich rifted continental margins.The abundant igneous manifestations and the very high stretching rates observed in the area may be related to the fact that the present Tyrrhenian area was occupied by an orogenic domain affected by shortening until middle Miocene times, which is just before the Late Miocene onset of back-arc extension. The lithosphere in the region had then to be rheologically weak. Abundant generation and ascent of magmas, mostly of Ocean Island Basalt type, was favoured by the large lithospheric permeability induced by strong extensional deformations.     

Post-collisional granitoids from the Dabie orogen in China: Zircon U–Pb age, element and O isotope evidence for recycling of subducted continental crust

While recycling of subducted oceanic crust is widely proposed to be associated with oceanic island, island arc, and subduction-related adakite magmatism, it is less clear whether recycling of subducted continental crust takes place in continental collision belts. A combined study of zircon U–Pb dating, major and minor element geochemistry, and O isotopes in Early Cretaceous post-collisional granitoids from the Dabie orogen in China demonstrates that they may have been generated by partial melting of subducted continental crust. The post-collisional granitoids from the Dabie orogen comprise hornblende-bearing intermediate rocks and hornblende-free granitic rocks. These granitoids are characterized by fractionated REE patterns with low HREE contents and negative HFSE anomalies (Nb, Ta and Ti). Although zircon U–Pb dating gives consistent ages of 120 to 130 Ma for magma crystallization, occurrence of inherited cores is identified by CL imaging and SHRIMP U–Pb dating; some zircon grains yield ages of 739 to 749 Ma and 214 to 249 Ma, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and a Triassic tectono-metamorphic event in the Dabie–Sulu orogenic belt, respectively. The granitoids have relatively homogeneous zircon δ¹⁸O values from 4.14‰ to 6.11‰ with an average of 5.10‰ ± 0.42‰ (n = 28) similar to normal mantle zircon. Systematically low zircon δ¹⁸O values for most of the coeval mafic–ultramafic rocks and intruded country rocks preclude an AFC process of mafic magma or mixing between mafic and felsic magma as potential mechanisms for the petrogenesis of the granitoids. Along with zircon U–Pb ages and element results, it is inferred that the granitic rocks were probably derived from partial melting of intermediate lower crust and the intermediate rocks were generated by amphibole-dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement. The post-collisional granitoids in the Dabie orogen are interpreted to originate from recycling of the subducted Yangtze continental crust that was thickened by the Triassic continent–continent collision. Partial melting of orogenic lithospheric keel is suggested to have generated the bimodal igneous rocks with the similar crustal heritage. Crustal thinning by post-collisional detachment postdated the onset of bimodal magmatism that was initiated by a thermal pulse related to mantle superwelling in Early Cretaceous.     

Mafic dykes derived from Early Cretaceous depleted mantle beneath the Dabie orogenic belt: implications for changing lithosphere mantle beneath Eastern China

SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxene + hornblende + plagioclase and yield a SHRIMP zircon U–Pb age of 111.6 ± 5.3 Ma. They have a narrow range of SiO₂ from 46.16% to 49.78%, and relative low concentrations of K₂O (1.07−2.62%), Na₂O (2.45−3.54%), Al₂O₃ (13.04−14.07%), and P₂O₅ (0.42−0.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg^# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (⁸⁷Sr/⁸⁶Sr)_i ratios from 0.704 to 0.705, ε_Nd values from 3.36 to 4.33 and mantle‐depletion Nd model ages (T_2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative ε_Nd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back‐arc extension which resulted from the subduction of the Palaeo‐Pacific Plate towards the Asian Continent. Copyright © 2010 John Wiley & Sons, Ltd.     

Zircon U／Pb Dating of Cretaceous Adakitic Volcanic Rocks in the Eastern Part of North Dabie Mountains

1IntroductionMostgeologistshaveacceptedthattheQinling Dabieorogenicbeltistheproductofcontinentaldeepsubduction (Okayetal.,1993;Cong ,B .etal.,1995 ;Hackeretal.,1995 ;DongShuwenetal.,1993) .Butasthebiggest scaleUHPmetamorphicbeltintheworld ,thereactionbetweenthecrustandthemantleatthepost orogenicstageandtheexhumation ,upliftinganddenudationoftheorogenicbeltarethecomplexdynamicprocesses .ItwasreportedpreviouslythatsomeCretaceousvolcanicrocksexistinthemiddleoftheDabieMountains (GuanYuncaiet…     

北大别东部白垩纪埃达克质火山岩及其锆石U-Pb年代学

北大别东部中生代火山岩以富Na(Na2O=4.03%,Na2O/K2O=1.31)、高Al2O3(15.30%)、高Sr(865μg/g)和Ba(1361μg/g)含量及高Sr/Y比值(66.1),贫Nb、Y和HREE,轻、重稀土元素的分馏程度强犤(La/Yb)N=26.6犦为特征,岩石地球化学的特征类似于埃达克岩。火山岩中锆石的U-Pb年龄为(129.2±2.6)Ma,属早白垩世。这套火山岩与北淮阳地区中生代的火山岩不仅形成时代相近,地球化学性质相似,而且其中矿物形成的温度压力条件也很类似。结果表明:(1)大别造山带早白垩世前曾发生过山根的拆沉和镁铁质-超镁铁质岩浆的底侵作用;(2)北大别地区早白垩世岩浆作用比较强烈,不仅有超镁铁-镁铁质岩浆的侵入、片麻岩穹隆的侵位,还有一定规模的火山喷发和花岗质岩浆的侵入;(3)北大别东部中生代火山岩可能属北淮阳中生代火山-侵入岩带的一部分;(4)北大别中生代火山岩残留顶盖的存在说明至少早白垩世以来大别造山带的剥蚀规模不会很大。