青藏高原拉萨地块西部中新世赛利普超钾质岩石的地球化学与岩石成因

西藏拉萨地块西部赛利普中新世碰撞后超钾质火山岩由中国地质调查局地质填图首次发现,露头呈残丘状集中分布于中新世赛利普盆地,为一套含地幔包体的粗面岩,SiO_2含量中等(55.36%～6.70%),高K_2O含量(6.70%～7.50%)和K_2O/Na_2O比值(3.34～4.93)。岩石高MgO(6.4%～7.95%)、Cr(174×10~(-6)～421×10~(-6))、Ni(268×10~(-6)～337×10~(-6))和Mg~#(68～72),岩石为地幔部分熔融的原始岩浆。岩石高度富集大离子亲石元素(LILE)和轻稀土元素(LREE)、具有明显的Nb、Ta、Ti的负异常、富集放射性成因Sr、Pb及Nd同位素(~(87)Sr/~(86)Sr=0.727327～0.727803,~(206)Pb/~(204)Pb=18.705～18.779,~(207)Pb/~(204)Pb=15.731～15.761,~(208)Pb/~(204)Pb=39.775～39.919,~(143)Nd/~(144)Nd=0.511848～0.511861)、较低的ε_(Nd)值(≈15)和古老的Nd模式年龄(t_(DM)=2.2～2.4 Ga),这些地球化学特征揭示出赛利普的岩浆源区为富集地幔(EMⅡ)。将赛利普与拉萨地块西部其他地点和青藏高原北部的北羌塘和西昆仑地区出露的超钾质岩石进行综合对比表明,赛利普超钾质岩石可能为尖晶石相含金云母橄榄岩及少量石榴石相含金云母橄榄岩地幔的低度部分熔融的产物。拉萨地块超钾质岩石的形成可能与印度大陆岩石圈俯冲,或是俯冲的印度大陆地壳前缘撕裂和分段俯冲有关。     

藏北羌塘地块新生代火山岩中麻粒岩捕虏体的岩石学和地球化学研究:对青藏高原新生代火山岩成因及下地壳性质的约束

青藏高原北部羌塘地块的新生代高钾钙碱性火山岩中含有很多下地壳捕虏体,这些捕虏体的主要岩石类型为二辉石麻粒岩和单斜辉石麻粒岩。本文对鸟兰乌拉湖南侧枕头崖地区新生代高钾钙碱性火山岩中的9个麻粒岩样品(6个基性麻粒岩和3个酸性麻粒岩)进行了系统的岩石学,矿物化学,地球化学和Sr-Nd-Pb同位素地球化学研究。其中,紫苏辉石具有高MgO FeO,低Al_2O_3的特征,单斜辉石具有低Al_2O_3和TiO_2的特征,黑云母具有高TiO_2的特征,这些特征都表明这些矿物为变质成因。矿物温压计算表明二辉石麻粒岩形成的平衡温度为783℃～818℃。单斜辉石麻粒岩形成压力在0.845～0.858GPa之间,来源深度约28km。表明它们可能是来自青藏高原加厚陆壳中部的岩石样品。基性麻粒岩的SiO_2=48.76%～58.61%,Al_2O_3=18.34%～24.50%,Na_2O=3.16%～5.41%,K_2O=1.58%～3.01%,低Mg~#(30～67),富集轻稀土(LREE)和大离子亲石元素(LILE),其具有较高的Rb/Sr(0.09～0.21)和(La/Yb)_N(17.32～49.35),具有较低的Nb/Ta (9.76～14.92),其Eu异常变化于0.19～0.89之间。基性麻粒岩的Sr-Nd-Pb同位素地球化学表现为~(87)Sr/~(86)Sr=0.710812～0.713241,ε_(Sr)=169.13～203.88,~(143)Nd/~(144)Nd=0.512113～0.512397;ε_(Nd)=-4.70～-10.05,~(206)Pb/~(204)Pb=18.7000～-18.9565,~(207)Pb/~(204)Pb=15.7135～-15.7662,~(208)Pb/~(204)Pb=39.1090～39.4733。和基性麻粒岩类似,酸性麻粒岩也表现出富集轻稀土(LREE)和大离子亲石元素(LILE)的特征,它们的~(87)Sr/~(86)Sr=0.712041～0.729088,ε_(Sr)=180.71～430.59,~(143)Nd/~(144)Nd=0.512230～-0.512388;ε_(Nd)=-4.74～-7.96,~(206)Pb/~(204)Pb=18.9250～-19.1717,~(207)Pb/~(204)Pb=15.7662～-15.7720,~(208)Ph/~(204)Pb=39.2109～39.6467。上述地球化学特征表明这些基性麻粒岩的源岩是下地壳岩石,而非地幔岩或玄武质堆晶岩,而酸性麻粒岩的源岩极有可能是准铝质的花岗质岩石。这就表明青藏高原新生代下地壳的地温梯度很高,并含有部分沉积岩,而非典型的辉长质下地壳。而且,详细的研究表明,这种特殊的下地壳可能对青藏高原新生代高钾钙碱性和橄榄粗安质岩浆的起源有重要作用。因此目前所认为的超钾质-橄榄粗安质岩浆源于富集岩石圈地幔在对流减薄作用下发生部分熔融的观点值得重新考虑。     

汝城地区晚中生代火山岩地球化学特征及其对源区属性的指示

湘东南汝城地区发育一套由基性玄武岩和中酸性安山质-英安质岩石组成的火山岩建造,属于低钾拉斑系列,该火山岩系中两个玄武岩的K-Ar年龄分别为124.5±2.5Ma和127.6±1.9Ma,属晚侏罗—早白垩世产物。在主、微量元素上两者成分存在明显差异。其中安山质-英安质岩石具有高MgO特征,属高MgO岩石,LILE富集、Nb-Ta、Sr-P亏损强烈,(La/Yb)N=6.7～7.9,Eu*/Eu=0.74～0.85,具岛弧型微量元素配分型式,87Sr/86Sr(t)=0.71079～0.71118,εNd(t)=-7.64～-8.16,与adakites高Mg岩石有着明显的差别,可能是富集岩石圈地幔熔融后直接分异的产物;玄武岩LILE富集,Nb-Ta富集,(La/Yb)cn=4.0～4.3,Eu*/Eu=1.00～1.16,具OIB型微量元素配分型式,87Sr/86Sr(t)=0.70812～0.70832,εNd(t)=0.48～1.03,其源区具二元混合趋势,其源区可能是富集型岩石圈地幔端员与亏损的软流圈地幔端员的混合产物。汝城地区晚中生代玄武岩和高Mg安山质-英安质岩石源区属性的限定及其相互的空间依存关系表明该区晚中生代时有着较薄的岩石圈厚度,处于岩石圈伸展减薄的大地构造背景。     

藏南特提斯喜马拉雅带中段中侏罗统遮拉组OIB型玄武岩浆的识别及其意义

对特提斯喜马拉雅带中段中侏罗统遮拉组玄武岩进行了系统的岩石学、地球化学和同位素地球化学研究.结果表明:①遮拉组玄武岩以高钛[平均w(TiO2)=2.57%]、高铁[平均w(TFe2O3)=11.58%]、低镁[平均w(MgO)=4.06%]、富集LREE和高场强元素{[w(La)/w(Yb)]N、w(Nb)、w(Zr)平均分别为8.61,32.67×10-6,331.33×10-6}为特征,w(Ti)/w(V)(62.74～80.26)、w(Ti)/w(Y)(218.71～665.20)和w(Zr)/w(Y)(10.89～11.09)比值高,具有与OIB相似的主量元素和微量元素组成特征,同位素组成以高Sr{[n(87Sr)/n(86Sr)]i为0.712 763～0.714 765}、低Nd{[n(143Nd)/n(144Nd)]i为0.512 417～0.512 458}为特点;②遮拉组玄武岩为一套产于大陆边缘裂谷背景下的碱性玄武岩,没有经历地壳混染;③遮拉组玄武岩是地幔热柱或热点物质与岩石圈地幔物质相互作用的结果,对了解与雅鲁藏布江洋盆构造演化有关的一些关键地质问题可能具有重要意义.     

琼河坝矿集区花岗岩体的Sr-Nd,Pb同位素特征及地质意义

琼河坝地区在志留纪—泥盆纪频繁的岩浆活动与成矿关系密切。矿集区中花岗岩ΣREE含量23.16×10-6~110.24×10-6,平均值68.19×10-6,总体上稀土含量较低。(La/Yb)N=3.23~10.97平均值为5.74,显示轻、重稀土分馏不明显,且轻稀土元素较重稀土元素富集。蒙西、琼河坝、和尔赛三个花岗岩体Sr-Nd、Pb同位素值很近,表明琼河坝花岗岩具有相近的成因。(87Sr/86Sr)i值为0.704 01~0.704 42,平均值为0.704 19;(143 Nd/144 Nd)i值为0.512 300~0.512 478,平均值为0.512 415;(208Pb/204Pb)i值17.600~17.910,平均值为17.738,(207Pb/204Pb)i值为15.410~15.547,平均值为15.459,(206Pb/204Pb)i值为37.250~38.019平均值为37.534。表现出壳幔混染的特征。εSr(t)值为-0.3~7.8,εNd(t)值为2.9~7.2,为较小的正值,表现出洋壳的特征。花岗岩Al2O3,Sr,Y,Yb和δEu的特征与埃达克岩相近,可能为俯冲洋壳熔融的产物。因此该地区众多铜矿点的出现,应该与埃达克岩有关。     

吉林东南部晚中生代中酸性火山作用成因的地球化学制约

详细的主、微量元素和Sr-Nd-Pb同位素研究结果显示,延吉地区晚中生代(早白垩世)中酸性火山岩具有岛弧型微量元素特征(富集轻稀土(LREE)、大离子亲石元素(LILE)和亏损高场强元素(HFSE)),和中等放射成因Sr、Pb及类似于硅质地球的Nd同位素组成(~(87)Sr/~(86)Sr(i)=0.70437～0.70525;~(206)Pb/~(204)Pb(i)=18.19～18.37,~(207)Pb/~(204)Pb(i)=15.54～15.57,~(208)Pb/~(204)Pb(i)=38.13～38.22;ε_(Nd)(t)=-2.75～+1.61)。其Sr-Nd-Pb同位素组成特征类似于区域同期基性火山岩和大兴安岭地区同时代火山岩。这些广泛分布于吉林南部的晚中生代中酸性火山岩是岩石圈地幔来源岩浆经分离结晶或地壳物质混染/AFC过程的产物。吉林省东南部晚中生代不同区域火山岩的Sr-Nd-Pb同位素对比结果显示,辽源-延吉与通化地区分属不同构造单元,其中辽源-延吉一带则为中亚造山带的东段,而通化地区为华北克拉通的一部分,夹皮沟-松江断裂可能是华北克拉通东北缘的边界。     

大别山北部条带状片麻岩及其伴生的斜长角闪岩的裂谷成因:岩石地球化学论据

岩石地球化学特征表明:大别山北部条带状片麻岩及其伴生的斜长角闪岩的原岩分别是碱性的I型花岗岩类和碱性的玄武岩,属于一套双峰式火山岩。Hf Rb/10 Ta×10和Hf Rb/10 Ta×30判别图解指示条带状片麻岩原岩属于板内花岗岩;斜长角闪岩的87Sr/86Sr=0.722345,143Nd/144Nd=0.511919,εNd(0)=-14.0,fSm/Nd=-0.15和εNd(800Ma)=-11.0,具有大陆玄武岩特征;同时,206Pb/204Pb=18.10,207Pb/204Pb=15.41和208Pb/204Pb=38.16,也接近于陆壳成分而且更接近下地壳成分,它的原岩有可能来源于古老的陆壳,推测其原岩形成时,其所在地区已经发育了比较成熟的大陆地壳。因此,它们应形成于新元古代(大约700Ma～800Ma左右)由扬子大陆发生裂解而形成的裂谷环境中。     

壳幔混染作用——下扬子区早中生代玄武岩同位素地质特征

下扬子区中生代（231Ma）玄武质火山岩全岩εNd（t）值-8.62～1.92,87Sr/86Sr 值0.7074～0.7179，206Pb/204Pb值为17.786～18.323。岩石中含有前寒武纪捕虏晶锆石。研究表明玄武质岩石可能是由亏损地幔与元古宙地壳岩石混染作用的结果。     

西秦岭新生代钾霞橄黄长岩的地球化学及其岩浆源区性质

西秦岭礼县地区新生代钾霞橄黄长岩具有贫SiO_2和Al_2O_3,富CaO、Mgo、TiO_2及K_2O Na_2O的特征,矿物组合中除橄榄石、辉石外,普遍含有霞石、黄长石、白榴石和磁铁矿等矿物。火山岩的∑REE为97.82×10~(-6)～639.1×10~(-6)。岩石以强烈富集大离子亲石元素(LILE,如Rb、Ba、K、Sr)和高场强元素(HFSE,如Nb、Th、Ta、P、Zr、Hf等)为特征具有Pb和Ti的负异常。火山岩具有相对高的~(143)Nd/~(144)Nd比值(0.512007～0.512130,ε~(Nd)= 3.4～ 5.8)和相对低的~(87)Sr/~(86)Sr比值(0.703374～0.704726)。~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Ph/~(204)Ph的比值范围分别为18.088～19.112,15.476～15.626和38.071～39.680。岩石的微量元素和Sr、Nd和Pb同位素特征显示其与洋岛玄武岩(OIB)的地球化学特征相似。源区组成显示了亏损地幔端元DM、HIMU端元和富集地幔端元EMII这三种地球化学端元混合的特征。     

华北龙岗第四纪玄武岩：岩石成因和源区性质

华北克拉通东北缘龙岗第四纪玄武岩的地球化学研究为大陆碱性玄武岩的成因以及源区的性质提供了重要的依据。龙岗第四纪玄武岩为碱性玄武岩,具有类似 OIB 的 REE 和微量元素分配特征。岩石的 Sr-Nd 同位素轻度亏损(~(87)Sr/~(86)Sr=0.7044～0.7048,ε_(Nd)=0.6～2.1),具有 Dupal 异常的高放射性成因 Pb 同位素组成(~(206)Pb/~(204)Pb:17.734～18.194,~(207)Pb/~(204)Pb=15.553～15.594,~(208)Pb/~(204)Pb=38.322～38.707)。这种地球化学特征指示了原始岩浆起源于<70km 深度的地幔,并经历了一定程度的橄榄岩、单斜辉石和钛-铁氧化物的结晶分异。岩浆源区中以来类似 MORB 软流圈物质的熔体为主,另外有少量来自 EM Ⅰ性质的富集岩石圈地幔以及俯冲流体/熔体的物质贡献,显示了深部岩石圈-软流圈一定程度的相互作用以及太平洋板块俯冲的影响。岩浆源区多种端元组分的存在表明该地区岩石圈的减薄/置换受到多种因素的影响。     

西天山喇嘛苏岩体年代学、地球化学及成矿意义

文中主要对西天山喇嘛苏岩体进行SHRIMP锆石U-Pb年龄、主微量及Sr-Nd-Pb同位素测定,阐明岩体成因及形成构造背景。喇嘛苏岩体主要由石英二长闪长岩、花岗闪长斑岩和英云闪长斑岩组成。其中,石英二长闪长岩形成于(394.8±4.9)Ma,花岗闪长斑岩和英云闪长斑岩形成于(380.9±3.9)Ma,略晚于石英二长闪长岩。岩体具有埃达克质岩的特征,且显示从钙碱性向高钾钙碱性演化的趋势,稀土元素配分曲线显示相对富集轻稀土((La/Yb)N:3.55～15.52)及中等的负或正Eu异常(δEu:0.53～1.12)。岩体具有较高的Sr含量((322～808)×10-6)和较低的Y含量((12.90～18.86)×10-6)。微量元素特征显示岩体富集LILE亏损HFSE,并具有Nb、Ta和Ti负异常。岩体初始Sr-Nd同位素组成为εNd(t)=-4.29～+0.75和ISr=0.706 052～0.708 263,Nd模式年龄为1.03～1.46Ga。花岗闪长斑岩和英云闪长斑岩的铅同位素特征为206Pb/204Pb=18.500～19.044,207Pb/204Pb=15.575～15.626,208Pb/204Pb=38.443～38.864;石英二长闪长岩为206Pb/204Pb=18.694～18.711,207Pb/204Pb=15.622～15.630,208Pb/204Pb=38.648～38.660。所有地球化学特征显示喇嘛苏岩体是俯冲洋壳部分熔融形成的熔体,上升过程中与受俯冲带沉积物交代的地幔楔相互作用,且有少量古老地壳的混染而形成。岩体形成于晚泥盆世准噶尔残余洋盆向伊犁—中天山地块俯冲的大陆弧背景,与该区Cu(Au)矿化有较密切的联系。     

Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution

Late Triassic granitoids in the Songpan-Garzê Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr > 400 ppm, Y < 11 ppm, strongly fractionated REE patterns ((La/Yb)_N = 32–105) and high K₂O/Na₂O (≈ 1). Group 2 are ordinary high-K calc-alkaline I-types with lower Sr (< 400 ppm), higher Y (> 18 ppm) and weakly fractionated REE patterns ((La/Yb)_N < 20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu = 0.50 to 0.94) and initial ⁸⁷Sr/⁸⁶Sr (0.70528 to 0.71086), but group 1 rocks have higher ε_Nd(t) (− 1.01 to − 4.84) than group 2 (− 3.11 to − 6.71). Calculated initial Pb isotope ratios for both groups are: ²⁰⁶Pb/²⁰⁴Pb = 18.343 to 18.627, ²⁰⁷Pb/²⁰⁴Pb = 15.610 to 15.705 and ²⁰⁸Pb/²⁰⁴Pb = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval ( 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction.     

青藏高原东北缘多福屯第三纪钠质基性火山岩及构造启示

位于青藏高原东北缘的多福屯第三纪火山岩,为钠质基性火山岩系。该火山岩(La/Yb)N值在6~11,ΣREE平均为117,样品普遍具有不相容元素富集、Pb亏损以及弱的Nb、Ta正异常,Nb/U和Ce/Pb平均值分别为30和17;样品的87Sr/86Sr值为0.7041~0.7069,Nd初始值为0.5129(εNd(t)=6),Δ207Pb/204Pb和Δ208Pb/204Pb分别在11~19和73~84之间,结合高的Sr初始值,表现了特征的Dupal(高放射成因铅)异常。地球化学特征表明,该火山岩属于似OIB性质的陆内火山岩,源区具有DM与EMⅡ混合特点。个别岩石表现出明显陆壳混染的迹象。火山岩源区特征反映了OIB类岩石的复杂性,可能由区域原、古特提斯地幔继承而来,因而是原地的和固有的,并非与青藏高原物质向东挤出有关。通过与邻区同时代火山岩的对比推断,整个高原东北缘新生代火山岩幔源区具有小尺度的不均一性。高原东北缘新生代火山岩的发育可能与较大规模的贺兰-川滇南北复合构造有关。     

Adakite-like porphyries from the southern Tibetan continental collision zones: evidence for slab melt metasomatism

We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dy_n/Yb_n and La_n/Yb_n. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, and the highest ¹⁴⁴Nd/¹⁴³Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.     

胶东范家庄地区晚侏罗世低镁埃达克质花岗岩成因及构造背景

胶东地区广泛发育一系列晚侏罗世岩浆作用产生的埃达克质岩,其成因机制及构造背景研究为揭示胶东地区中生代构造演化提供了重要证据.选取出露于胶东苏鲁地区的范家庄花岗岩进行锆石U-Pb年龄、全岩主微量元素和Sr-Nd-Pb同位素组成分析,探讨了岩石成因及成岩构造背景.锆石U-Pb年龄结果表明范家庄花岗岩侵位于晚侏罗世（161±2 Ma）.岩石主微量数据具有富硅（SiO₂=68.94%~71.00%）、高铝（Al₂O₃>15.17%）、低镁（MgO=0.32%~0.41%）;高Sr、低Y、Yb含量以及高（La/Yb）_N（>38.59）比值的特点,同位素测试结果显示相对高的（87Sr/86Sr）_i比值（0.709 28~0.711 41）、相对较低的ε_Nd（t）值（-20.5~-14.1）和高放射性Pb同位素组成（206Pb/204Pb）_t=16.853~17.207,（207Pb/204Pb）_t=15.436~15.495,（208Pb/204Pb）_t=37.340~37.629.综合分析认为,范家庄岩体属于低镁埃达克质岩,产于增厚下地壳部分熔融,源区以扬子板块下地壳组分为主,混合有华北板块下地壳成分.晚侏罗世伊泽奈奇板块俯冲形成的弧后拉张环境诱发重力不稳定或者岩石圈伸展造成加厚的造山带垮塌,软流圈上涌的导致加厚地壳部分熔融可能是形成胶东范家庄低镁埃达克岩的地球动力学背景.      

Petrochemistry of granulite xenoliths from the Cenozoic Qiangtang volcanic field,northern Tibetan Plateau: implications for lower crust composition and genesis of the volcanism

High-K calc-alkaline magmas from the Cenozoic Qiangtang volcanic field, northern Tibetan Plateau, contain lower crustal two-pyroxene and clinopyroxene granulite xenoliths. The petrology and geochemistry of six mafic and three felsic xenoliths from the Hol Xil area south of Ulan Ul Lake are discussed. Mafic granulites (Pl, Opx, Cpx, Ksp, and Bt) contain 48.76–58.61% SiO₂, 18.34–24.50% Al₂O₃, 3.16–5.41% Na₂O, 1.58–3.01% K₂O, low Mg# (30–67), LREE and LILE enrichment, high Rb/Sr (0.09–0.21), (La/Yb)_N (17.32–49.35), low Nb/Ta (9.76–14.92), and variable Eu anomalies (Eu?=?0.19?0.89). They also have more evolved Sr-Nd-Pb isotopic compositions in comparison with the host dacites ⁸⁷Sr/⁸⁶Sr (0.710812 vs. 0.713241), ?_Sr (+169.13 vs.?+203.88), ¹⁴³Nd/¹⁴⁴Nd (0.512113 vs. 0.512397), ?_Nd (?4.70 to??10.05), ²⁰⁶Pb/²⁰⁴Pb (18.7000 vs. 18.9565), ²⁰⁷Pb/²⁰⁴Pb (15.7135 vs. 15.7662), and ²⁰⁸Pb/²⁰⁴Pb (39.1090 vs. 39.4733). Felsic granulites (Qtz, Pl, Ksp, Bt, and Cpx) show enrichment of LREE and LILE and have evolved Sr-Nd-Pb isotopic compositions with (La/Yb)_N (2.04–10.82), ⁸⁷Sr/⁸⁶Sr (0.712041–0.729088), ?_Sr (+180.71–+430.59), ¹⁴³Nd/¹⁴⁴Nd (0.512230–0.512388); ?_Nd (?4.74 to??7.96), ²⁰⁶Pb/²⁰⁴Pb (18.9250–19.1717), ²⁰⁷Pb/²⁰⁴Pb (15.7662–15.7720), and ²⁰⁸Pb/²⁰⁴Pb (39.2109–39.6467). These geochemical data suggest that the protolith of the mafic granulites could have been a hybrid mafic magma (e.g. enriched mantle type II) or metasomatized restite derived from the partial melting of metamafic-intermediate rocks rather than basaltic cumulates, whereas the felsic granulite protolith was a quartzofeldspathic S-type granitic rock. We argue that the lower crust of the northern Tibetan Plateau is hot and heterogeneous rather than wholly gabbroic. Interaction between the mantle-derived magma and the metasedimentary/granitic lower crust of the Tibetan Plateau may have played an important role in the generation of shoshonitic and high-K calc-alkaline andesite-dacite rocks.     

Petrogenesis of Early Cretaceous low-Mg adakitic rocks along the southernmost margin of the North China Craton: implications for late Mesozoic crustal evolution

This article reports systematic zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotopic data for the Early Cretaceous Jialou granitoids along the southernmost margin of the North China Craton (NCC), adjacent to the Tongbai Orogen. These results will provide significant constrains on the crustal evolution of the southern margin of the NCC. Zircon U–Pb analyses, using laser ablation–multicollector–inductively coupled plasma–mass spectrometry, indicate that the Jialou granitoids were emplaced at ~130 Ma. The granitoids have high SiO₂, K₂O, Al₂O₃, Sr, and Ba contents, high Sr/Y and (La/Yb)N ratios, and low concentrations of MgO, Y, and heavy rare earth elements, indicating a low-Mg adakitic affinity. They have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707464–0.708190) and negative εNd(t) values (–11.8 to –15.2), similar to those of the Palaeoproterozoic lower crust in the NCC. These geochemical and isotopic features indicate that the Jialou low-Mg adakitic rocks were derived by partial melting of mafic Palaeoproterozoic lower crust of the NCC at >50 km depth, leaving behind a garnet amphibolite residue. The petrogenesis of the Jialou low-Mg adakitic rocks, plus the petrogenesis of Mesozoic granitoids and lower crustal xenoliths entrained in the Late Jurassic Xinyang volcaniclastic diatreme, suggests that the continental crust along the southern margin of the NCC was thickened during the Middle Jurassic to Early Cretaceous, but thinned after 130 Ma. We propose that crustal thickening was caused by a late Middle Jurassic to Early Cretaceous intra-continental orogeny, rather than continent–continent collision between the NCC and the Yangtze Craton. We also suggest that crustal thinning and Early Cretaceous magmatism were related to subduction of the palaeo-Pacific plate, rather than post-orogenic collapse of the Qinling–Tongbai–Dabie Orogen.     

The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios. The Cyclades samples, however, have higher²⁰⁶Pb/²⁰⁴Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.     

Early Cretaceous adakitic granites in the Northern Dabie Complex, central China: Implications for partial melting and delamination of thickened lower crust

To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143-129 Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg^# and Ni values, and Nd-Sr isotope compositions (ε_Nd(t) = −14.6 to −19.4 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7067-0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie-Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118-105 Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic-ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic-Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic-ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.     

Zircon LA-ICP MS U-Pb Age, Sr-Nd-Pb Isotopic Compositions and Geochemistry of the Triassic Post-collisional Wulong Adakitic Granodiorite in the South Qinling, Central China, and Its Petrogenesis

The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite. Geochemistry, zircon LA-ICP MS （laser ablation inductively-coupled plasma mass spectrometry） U-Pb chronology and Sr- Nd-Pb isotope geochemistry of the pluton are reported in this paper. The biotite granodiorite shows close compositional similarities to high-silica adakite. Its chondrite-normalized REE patterns are characterized by strong HREE depletion （Yb = 0.33--0.96 10-6 and Y = 4.77-11.19 ×10^-6）, enrichment of Ba （775-1386 x 10-6） and Sr （643-1115 × 10^-6） and high Sr/Y （57.83-159.99） and Y/Yb （10.99-14.32） ratios, as well as insignificant Eu anomalies （6Eu = 0.70-0.83）, suggesting a feldspar-poor, garnet±amphibole-rich residual mineral assemblage. The mafic enclaves have higher MgO （4.15- 8.13%）, Cr （14.79-371.31 × 10-6）, Ni （20.00-224.24× 10^-6） and Nb/Ta （15.42-21.91） than the host granodiorite, implying that they are mantle-derived and might represent underplated mafic magma. Zircon LA-ICP MS dating of the granodiorite yields a ^206pb/^238U weighted mean age of 208±2 Ma （MSWD=0.50, 1σ）, which is the age of emplacement of the host biotite granodiorite. This age indicates that the Wulong pluton formed during the late-orogenic or post-collisional stage （〈242±21 Ma） of the South Qinling belt. The host biotite granodiorite displays ^87Sr/^86Sr = 0.7059-0.7062, Isr = 0.7044-- 0.7050,^143Nd/^144Nd = 0.51236-0.51238, εNd（t）= -2.26 to -2.66 to ^206Pb/^204pb = 18.099-18.209, ^207pb/^204pb = 15.873-15.979 and ^208pb/^204pb = 38.973-39.430. Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling. Furthermore, its Nd isotopic model age （-1.02 Ga） is consistent with the age （-1.1 Ga） of the Yaolinghe Group. Based on the integrated geological and ge