西藏南部印度-亚洲碰撞带岩石圈:岩石学-地球化学约束

拟以岩石学和地球化学的研究为基础, 结合地球物理与构造地质学的研究成果, 从一个侧面探讨青藏高原岩石圈、特别是印度-亚洲主碰撞带岩石圈结构、组成及今后进一步的研究方向.印度-亚洲主碰撞带具有青藏高原最厚的地壳, 由初生地壳及再循环地壳两类不同性质的地壳构成; 青藏巨厚地壳是由于构造增厚及地幔物质注入(通过岩浆作用) 增厚两种机制形成的.碰撞以来藏南地壳加厚主要发生在约50~25Ma期间.青藏岩石圈地幔在地球化学和岩石学上是不均一的, 至少存在3种地球化学端元: (1) 新特提斯大洋岩石圈端元; (2) 印度陆下岩石圈端元; (3) 新特提斯闭合前青藏原有的岩石圈端元.在青藏高原还发现了一批壳幔深源岩石包体及高压-超高压矿物, 对于认识青藏深部有重要的意义.可以识别出青藏高原现今存在3种岩石圈结构类型: 第1种, 增厚的岩石圈(帕米尔型); 第2种, 减薄的岩石圈(冈底斯型); 第3种, 加厚-减薄-再加厚的岩石圈(羌塘型).这3类岩石圈是否在时间上具有先后顺序, 尚无明确的证据, 需要在今后加以注意.研究表明, 沿冈底斯带后碰撞钾质-超钾质火山活动, 可能与新特提斯洋俯冲板片在后碰撞阶段的断离及印度大陆岩石圈向青藏的持续俯冲作用有关, 但西段、中段与东段的动力学机制不相同.在青藏高原北部地区(羌塘、可可西里等地区), 后碰撞钾质-超钾质火山活动, 可能与波状外向扩展式的软流圈上隆引起的减压熔融有关.在高原北缘西昆仑、玉门等地区, 其形成机制可能为大规模走滑断层引起的减压熔融.青藏高原后碰撞火成活动具有明显而有规律的时空迁移.同碰撞的林子宗火山活动在65Ma左右始于冈底斯南部, 标志印度-亚洲大陆碰撞的开始.于45Ma左右火山活动向北迁移到羌塘-“三江”北段, 开始了后碰撞火山活动; 然后自内向外迁移, 即北向可可西里、南向冈底斯(在冈底斯内部又自西向东)、东向西秦岭迁移; 最后(6Ma以来), 再分别向高原的西北、东北、东南三隅迁移.结合已有地球物理资料, 一种可能的解释是它可能暗示由印度和亚洲大陆板块碰撞所诱发的深部物质(如中-下地壳、软流圈地幔物质) 流动.      

Geochemical Constraints for the Genesis of Post-collisional Magmatism and the Geodynamic Evolution of the Northern Taiwan Region

Taiwan is an active mountain belt created by the oblique collisionof the northern Luzon arc with Asia. Late Pliocene extensionalcollapse of the northern Taiwan mountain belt (NTMB) was accompaniedby magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ;2·8–0·2 Ma). The geochemical characteristicsof the NTVZ magmas can thus provide constraints both for themantle source composition and the geodynamic processes operatingin the late orogenic stage of the region. The NTVZ volcanicrocks consist dominantly of calc-alkaline andesites and basalts,along with subordinate but heterogeneous lavas including low-K,shoshonitic and ultrapotassic magmas. From the NE to the SWin the NTVZ, the magmas show systematic compositional variationsfrom low-K to calc-alkaline and then shoshonitic. This spatialgeochemical variation, characterized by southwesterly increasein potassium and incompatible trace elements, appears to besubparallel to the southwestern part of the modern Ryukyu subductionsystem. Sr–Nd isotope ratios of the NTVZ volcanic rocks(⁸⁷Sr/⁸⁶Sr     

Nature of the Source Regions for Post-collisional, Potassic Magmatism in Southern and Northern Tibet from Geochemical Variations and Inverse Trace Element Modelling

Neogene potassic lavas in northern and southern Tibet have differentisotopic (     

Potassic Magmatism in Western Sichuan and Yunnan Provinces, SE Tibet, China: Petrological and Geochemical Constraints on Petrogenesis

Potassic volcanism in the western Sichuan and Yunnan Provinces,SE Tibet, forms part of an extensive magmatic province in theeastern Indo-Asian collision zone during the Paleogene (40–24Ma). The dominant rock types are phlogopite-, clinopyroxene-and olivine-phyric calc-alkaline (shoshonitic) lamprophyres.They are relatively depleted in Na₂O, Fe₂O₃, and Al₂O₃ comparedwith the late Permian–early Triassic Emeishan continentalflood basalts in the western part of the Yangtze craton, andhave very high and variable abundances of incompatible traceelements. Primitive mantle-normalized incompatible element patternshave marked negative Nb, Ta and Ti anomalies similar to thoseof K-rich subduction-related magmas, although the geodynamicsetting is clearly post-collisional. Spatially, some incompatibletrace element abundances, together with inferred depths of meltsegregation based on the Mg-15 normalized compositions of thesamples, display progressive zonation trends from SW to NE withincreasing distance from the western boundary of the Yangtzecraton. Systematic variations in major and trace element abundancesand Sr–Nd–Pb isotope compositions appear to havepetrogenetic significance. The systematic increases in incompatibletrace element abundances from the western margin to the interiorof the Yangtze craton can be explained by progressively decreasingextents of partial melting, whereas steady changes in some incompatibletrace element ratios can be attributed to changes in the amountof subduction-derived fluid added to the lithospheric mantleof the Yangtze craton. The mantle source region of the lamprophyresis considered to be a relatively refractory phlogopite-bearingspinel peridotite, heterogeneously enriched by fluids derivedfrom earlier phases of late Proterozoic and Palaeozoic subductionbeneath the western part of the Yangtze craton. Calculationsbased on a non-modal batch melting model show that the degreeof partial melting ranges from 0·6% to 15% and the proportionof subduction-derived fluid added from0·1% to 0·7%(higher-Ba fluid) or from 5% to 25% (lower-Ba fluid) from theinterior to the western margin of the Yangtze craton. Some pre-existinglithospheric faults might have been reactivated in the areaneighbouring the Ailao Shan–Red River (ASRR) strike-slipbelt, accompanying collision-induced extrusion of the Indo-Chinablock and left-lateral strike-slip along the ASRR shear zone.This, in turn, could have triggered decompression melting ofthe previously enriched mantle lithosphere, resulting in calc-alkalinelamprophyric magmatism in the western part of the Yangtze craton. KEY WORDS: Tibet; potassic magmatism; lithospheric mantle; metasomatism     

青藏东缘新生代高钾岩浆活动的热年代学制约

热年代学研究表明 ,青藏东缘存在两期新生代高钾岩浆活动 ,其活动时限分别为 40～ 2 8Ma和 16～ 0Ma。两期高钾岩石在地质背景、岩石类型和地球化学特征上明显不同 ,表明早期高钾岩石是在转换压缩过程中产生的 ,起源于富集地幔的部分熔融 ,而晚期高钾岩石则与东亚和青藏地区构造伸展有关 ,起源于新近交代富集的亏损地幔。     

西藏甲玛埃达克质斑岩的地球化学特征及意义

分析了西藏甲玛石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩岩相学、年代学及地球化学特征,研究其形成演化及构造环境,并建立岩浆成因模式。甲玛石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩是具弧岩浆地球化学特征的准铝质高钾钙碱性岩岩石系列,同时具有O型(向C型过渡)埃达克质岩和I型(或磁铁矿型)花岗岩的特征。其成因与新特提斯洋板片脱水熔融形成的流体与地幔楔发生地幔交代作用形成的EMⅡ型富集地幔有关。中新世兰格期(15 Ma左右),由于拉萨地体东西向的"崩塌",导致拉萨地体开始伸展减薄,从而使交代地幔(EMⅡ型富集地幔)减压熔融,形成了富含Cu、Au等成矿物质的具I型(或磁铁矿型)弧岩浆地球化学特征的准铝质富钾O型埃达克质铁镁质岩浆,并在沿该时期的正断层及裂谷系统侵位过程中,不断与下地壳高钾及中—高Fe/Mg的花岗质岩浆发生不同程度的岩浆混合作用,伴随着相对较弱的岩浆结晶分异过程不断侵位浅地表,进而形成了独特的石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩岩石组合。     

大陆俯冲带超高压变质岩部分熔融与壳幔相互作用研究进展

自20世纪80年代在大陆地壳岩石中发现柯石英和金刚石等超高压变质矿物以来,大陆深俯冲和超高压变质作用就成为了固体地球科学研究的前沿和热点领域之一.经过三十余年的研究,已经在大陆地壳的俯冲深度、深俯冲岩石变质P-T-t轨迹、俯冲地壳岩石的折返机制、深俯冲岩石的原岩性质、大陆碰撞过程中的熔/流体活动与元素活动性、俯冲隧道内...     

西藏乃东县扎嘎当O型埃达克岩成因——来自地球化学和Sr-Nd-Pb同位素的制约

埃达克岩是具有特定地球化学性质的一套中酸性火成岩组合,具有多种成因模式和特殊的构造意义。位于西藏泽当一带的泽当蛇绿岩对研究新特提斯洋的演化具有重要的意义,选取泽当蛇绿岩内出露的奥长花岗岩为研究对象,开展了岩石学、岩石地球化学、Sr-Nd-Pb同位素等研究。研究显示,扎嘎当奥长花岗岩地球化学特征表现为高硅、高铝、富钠（Na2O/K2O>2）、低镁,准铝质钙碱性,同时具高Sr和Sr/Y值,低Y、Yb,属O型埃达克岩。岩石稀土元素总量偏低,富集轻稀土元素和大离子亲石元素Rb、K、Ba,亏损高场强元素Nb、Ti、P,Eu具微弱的正异常。岩石具有较高的Sr、Sm、Nd、Pb含量,ISr=0.704251~0.704329,INd=0.512776~0.512807,εNd（t）=6.61~7.22。岩石铅同位素组成较均一,206Pb/204Pb=18.378~18.495,207Pb/204Pb=15.542~15.554和208Pb/204Pb=38.336~38.406。研究表明,扎嘎当奥长花岗岩源区来源于具有地幔特征的洋壳,由消减大洋板片在一定深度部分熔融形成。综合泽当地区的地质特征和前人研究成果,认为泽当洋内弧是存在的。     

Geochemical Constraints on the Role of Oceanic Lithosphere in Intra-Volcano Heterogeneity at West Maui, Hawaii

Stratigraphically well-constrained sequences of late shield-buildingstage lavas from West Maui volcano, Hawaii, show age-dependentcompositional variability distinct from that seen in shield-stagelavas from any other Hawaiian volcano. These distinctions aredefined by ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb variation as well as⁸⁷Sr/⁸⁶Sr correlation with ²⁰⁶Pb/²⁰⁴Pb and trace element compositions.The West Maui lavas from stratigraphically higher in the sequencehave major and trace element and Sr–Pb–Hf–Ndisotopic compositions similar to Kea-type lavas sampled at theyounger Mauna Kea and Kilauea volcanoes, indicating that theKea compositional end-member of Hawaiian lavas has remainedhomogeneous over     

太行山南段中生代杂岩体的岩石成因:元素和Nd-Sr-Pb同位素地球化学证据

通过太行山南段三个中生代杂岩体(西戌、武安和洪山)的元素和同位素地球化学特征的研究。讨论其成因和地球动力学环境。结果表明，西戌和武安杂岩体主要由从二长辉长岩到二长岩的一系列岩石组成，其地球化学性质相似(高Mg^s，具微弱至正Eu异常的REE模式等)。西戊杂岩体的εNd(135Ma)=-12．3～-16．9，Isε=0．7056～0．7071，与武安杂岩体稍有不同。西戌杂岩体的(^206Pb／^204Pb)i=16．92～17．3，(^207Pb／^204Pb)i=15．32～15．42，(^208Pb／^201Pb)i=37．16～37．63，较武安杂岩体的略高。西戊-武安杂岩体都起源于EM1型富集地幔，但被下地壳物质不同程度混染。洪山杂岩体(正长岩-花岗岩)也来自EM1型富集地幔的部分熔融，但属于不同的岩浆事件，并仅受轻微的下地壳混染。太行山岩浆作用的发生可能与古太平洋板块的水平俯冲消减而形成的弧后伸展环境有关。     

西藏纳如松多铅锌矿床石英闪长岩成因:地球化学及Sr-Nd-Pb同位素证据

纳如松多矿区石英闪长岩具有低SiO_2、富MgO、CaO的特征,含10%～15%的角闪石,为准铝质、分异程度较低的高钾钙碱性-钾玄岩系列I型花岗岩。岩体发育较弱的Eu负异常,富集强不相容元素Rb、Th、U,而亏损Nb、Ta、Ti等高场强元素。同位素测试结果显示,纳如松多石英闪长岩具有相对较高的~(87)Sr/~(86)Sr初始比值(0.709 13～0.709 68)与负的ε_(Nd)(t)值(-5.8～-5.2),Pb同位素的~(206)Pb/~(204)Pb为18.6091～18.6438,~(207)Pb/~(204)Pb为15.6900～15.6986,~(208)Pb/~(204)Pb为39.2116～39.2225,指示岩浆可能为特提斯洋俯冲消减阶段产生的岛弧地幔楔部分熔融的产物。一种可能的成因模式为:晚白垩世,随着雅鲁藏布江板块向拉萨地块之下俯冲,俯冲板片流体交代上覆地幔楔,形成幔源基性岩浆,上侵到地壳后,诱发了岛弧基底物质的部分熔融,形成了中基性的纳如松多石英闪长岩,其形成可能是晚白垩世北向俯冲的新特提斯洋板块在回转初期的岩浆活动响应。     

Early Devonian Post-collisional Granitic Magmatism in the North Qilian Orogenic Belt, Western China: Insights into Lithospheric Delamination and Orogenic Collapse

Post-collisional magmatism contains important clues for understanding the reworking and growth of continental crust, as well as lithospheric delamination and orogenic collapse. Early Devonian magmatism has been identified in the North Qilian Orogenic Belt (NQOB). This paper reports an integrated study of petrology, whole-rock geochemistry, Sm-Nd isotope and zircon U-Pb dating, as well as Lu-Hf isotopic data, for two Early Devonian intrusive plutons. The Yongchang and Chijin granites yield zircon U-Pb ages of 394–407 Ma and 414 Ma, respectively. Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium-rich minerals, LREE-enriched patterns with negative Eu anomalies and a negative correlation between P2O5 and SiO2 contents, consistent with geochemical features of I-type granitoids. Zircons from the studied granites display negative to weak positive εHf(t) values (?5.7 to 2.1), which agree well with those of negative εNd(t) values (?6.4 to ?2.9) for the whole-rock samples, indicating that they were derived from the partial melting of Mesoproterozoic crust. Furthermore, low Sr/Y ratios (1.13–21.28) and high zircon saturation temperatures (745°C to 839°C, with the majority being >800°C) demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source. Taken together, the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle-derived magmas at high-temperature and low-pressure conditions during post-collisional extensional collapse. The data obtained in this study, when viewed in conjunction with previous studies, provides more information about the tectonic processes that followed the closure of the North Qilian Ocean. The tectonic transition from continental collision to post-collisional delamination could be constrained to ~430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon εHf(t) values for granitoids. A two-stage tectonic evolution model from continental collision to post-collisional extensional collapse for the NQOB includes (a) continental collision and crustal thickening during ca. 455–430 Ma, characterized by granulite-facies metamorphism and widespread low-Mg adakitic magmatism; (b) post-collisional delamination of thickened continental crust and extensional collapse of orogen during ca. 430–390 Ma, provided by coeval high-Mg adakitic magmatism, A-type granites and I-type granitoids with low Sr-Y ratios.     

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

中祁连党河南山碱性系列岩浆岩埃达克岩成因及其对俯冲带壳幔相互作用的启示

碱性系列岩浆岩和埃达克岩是通常产生于汇聚板块边缘的特殊岩石类型,记录了俯冲物质与地幔橄榄岩相互作用的过程。笔者对中祁连南缘党河南山地区贾公台杂岩体和鸡叫沟岩体进行了岩石学、地球化学和锆石U-Pb年代学研究。LA-ICP-MS锆石U-Pb定年表明,鸡叫沟岩体中的二长闪长岩形成于（467±4.7）Ma,贾公台岩体花岗闪长岩形成于（445±4.3）Ma。地球化学分析结果表明,鸡叫沟杂岩体中角闪辉石岩、辉长岩和二长闪长岩整体上具有富K、富Na的特征,均属于碱性系列岩浆岩,具有钾玄岩特征;岩体中不同岩石类型具有相似的微量元素配分型式,富集Sr、Nb、LILE和LREE等,岩浆源区存在金红石残留,其形成过程与弧地幔楔底部富碱的俯冲沉积物和地幔橄榄岩混合、熔融有关。贾公台花岗闪长岩属于中钾钙碱性系列,Na2O/K2O值大于2.5,样品具有高Sr（>400×10-6）、低Y（<7×10-6）的特征,高Sr/Y值（>70）和La/Yb值（>22）具有高硅埃达克岩特征,与俯冲洋壳的榴辉岩相部分熔融有关,源区存在石榴子石残留特征。党河南山碱性岩浆岩-埃达克岩组合指示了俯冲物质深部熔融的特点（大于50~100 km）,揭示了该地区强烈的壳幔相互作用过程,表明俯冲沉积物和俯冲洋壳可以为弧岩浆侵入体形成提供直接的物质来源,进一步说明俯冲物质和地幔橄榄岩相互作用可能是岛弧地壳生长的方式之一。     

Heterogeneity of the Lithosphere in Tibetan Plateau on the Constraints of Magmatism

HETEROGENEITY OF THE LITHOSPHERE IN TIBETAN PLATEAU ON THE CONSTRAINTS OF MAGMATISM     

峨眉地幔柱-岩石圈的相互作用:来自低钛和高钛玄武岩的Sr-Nd和O同位素证据

蛾眉山玄武岩总体具有较高的^87Sr／^86Sr比值和较低的εNd(t)值，并具有富集地幔源区的特点。而低钛玄武岩(LT)与高钛玄武岩(HT)间又表现出一定的差异性，即早期低钛玄武岩(LTl)的^87Sr/^86Sr比值最高(0．7063—0．7078)，而其εNd(t)最低(—6．74-—0．34)：晚期高钛玄武岩(HT)具有最低的^87Sr/^86Sr比值(0．7049—0．7064)和最高的εNd(t)值(—0．71—1．5)。蛾眉山低钛玄武岩中单斜辉石的氧同位素变化范围为6．2‰-7．86‰，高于洋岛拉斑玄武岩的平均值5．4‰。研究样品较地幔岩石偏高的δ^18O值说明在其形成和演化过程中有壳源物质的参与。结合前人的研究成果和对元素地球化学的研究认为，壳源物质可能主要来自于新元古代富集的扬子西缘次大陆岩石圈地幔。地幔柱—岩石圈的相互作用过程中表现在时间和空间的系统变化，即早期西岩区形成含大量壳源组分的低钛玄武岩，晚期为壳源组分相对较少的高钛玄武岩。空间上低钛玄武岩仅分布在西岩区，而中、东岩区皆为高钛玄武岩。壳源组分随着时间演化逐渐减少，在空间上由西而东也逐渐减少。表明蛾眉山火成岩省形成早(主)期地幔柱头卷入并熔融了较多交代富集的次大陆岩石圈物质，晚期则有较少的壳源物质参与。建立了蛾眉地幔柱与大陆岩石圈作用的工作模型。     

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     

Late Triassic granites from the northwestern margin of the Tibetan Plateau,the Dahongliutan example: petrogenesis and tectonic implications for the evolution of the Kangxiwa Palaeo-Tethys

The Dahongliutan granitic pluton consists of two-mica granites and is located in the eastern part of the Western Kunlun Orogen, northwestern Tibetan Plateau. Zircon separates from the pluton yield a SIMS U–Pb age of 217.5 ± 2.8 Ma. Rocks from the pluton contain relatively high and uniform SiO₂ (72.32–73.48 wt%) and total alkalis (Na₂O + K₂O = 8.07–8.67 wt%) and are peraluminous and high-K calc-alkaline to shoshonitic in composition. The Dahongliutan granites are relatively depleted in the high-field-strength elements and the heavy rare earth elements (HREEs) and have relatively high Rb, and low Ba and Sr concentrations. They contain low total rare earth element (REE) concentrations. The light REEs are strongly enriched relative to the HREEs, with (La/Yb)_N values of 28.56–37.01. The ε_Nd(t) values range from ?10.6 to ?8.8, and (⁸⁷Sr/⁸⁶Sr)_i = 0.7142–0.7210. Zircons from the pluton yield ε_Hf(t) values of ?13.8 to ?1.6, and δ¹⁸O = 10.5–11.6‰. Petrographic and geochemical features of the pluton indicate that the granites are S-type and were derived from parting melting of a mixture of metasedimentary and minor metaigneous sources in the middle–lower crust. Magmatic differentiation was dominated by the fractional crystallization of plagioclase, K-feldspar, muscovite, biotite, and accessory monazite, allanite, and Fe–Ti oxides. Regional granitoids were emplaced in the Early-to-Middle Triassic. Other younger granitoids, with ages of 240–200 Ma, are mostly I-type in character and were likely derived from multiple types of source rock, suggesting the source was heterogeneous Triassic crust. Such a scenario is consistent with their formation in a post-collisional setting. Our new data, combined with other geological evidence, suggest that the collision between the Tianshuihai and southern Kunlun terranes occurred between ca. 250 and 240 Ma, resulting in the closure of the Palaeo-Tethys. Post-collisional tectono-magmatic events may have occurred between 240 and 200 Ma.     

苏北中生代碰撞后花岗岩的地球化学特征及其地质意义

苏鲁超高压变质带南部的花岗岩体为中生代的Ⅰ型花岗岩，侵入到围岩片麻岩中，属碰撞后花岗岩。SiO2含量为56．15％～66．67％，Na2O含量为3．58％～4.54％，K2O含量为2．16％～4．58％，Alk为6．93～8．62％。里特曼指数为2．7～3．1，属于高钾钙碱性岩石。δEu=0．76～0．82，为弱的负铕异常。围岩片麻岩SiO2含量较高，为70．55％～77．28％，Na2O为3．12％～5．39％，K2O为1．78％～6．22％，δEu为0．38～0．59，具有明显的负铕异常。花岗岩与片麻岩的地球化学特征有明显差别，表明两者在成因上关系不大，花岗岩的源岩为未出露地表的中下地壳岩石。拆沉模式可较好地解释碰撞后花岗岩与超高压变质岩的“共生”现象。