西准噶尔晚泥盆世碱性玄武岩的地球化学特征及其与塔里木大火成岩省岩浆岩对比

西准噶尔是近年来中亚造山带的研究热点地区之一,发育多条蛇绿混杂岩带。随着研究的深入,蛇绿岩混杂岩带中不断有碱性洋岛玄武岩被识别出来。本文对克拉玛依及达尔布特蛇绿混杂岩中的晚泥盆世枕状玄武岩进行详细的岩石地球化学及Sr?Nd同位素组成研究。结果表明西准噶尔晚泥盆世枕状玄武岩属于碱性玄武岩系列,岩石具有高TiO₂ （2.3%～3.8%）及P₂O₅ （0.38%～0.91%）,低MgO （2.41%～4.97%）,轻、重稀土元素分异较为明显,(La/Yb)_N = 5.1～14.5,无明显Eu异常（Eu/Eu* = 0.96～1.1）,相对富集Rb、Th、U,亏损Ba、K、Sr,没有明显Nb、Ta负异常,这些地球化学特征与洋岛玄武岩（OIB）极其相似。克拉玛依及达尔布特蛇绿混杂岩中枕状玄武岩具正ε_Nd(t)值（3.3～4.4）及年轻的模式年龄（0.83～0.72 Ga）,表明其来源于亏损地幔源区,通过同位素及微量元素研究认为源区可能为富辉石的橄榄岩,或不含橄榄石的辉石岩和角闪石岩,克拉玛依及达尔布特蛇绿混杂岩中的晚泥盆世枕状玄武岩形成于大洋板内与地幔柱有关的海山/大洋岛屿环境。这些明显不同于大陆板内具有富集特征的塔里木大火成岩省,它们分别属于两个明显不同的同位素地球化学省,即以富集型地幔为同位素组成的南部省及以亏损型地幔为同位素组成的北部省。     

新疆卡拉麦里蛇绿混杂岩带北侧英云闪长岩年代学、地球化学特征及地质意义

以出露于卡拉麦里蛇绿混杂岩带北侧元湖一带英云闪长岩为研究对象,在野外地质调查基础上,结合锆石LA-LCP-MS U-Pb年龄测试、岩石地球化学分析,对该岩体侵入时代及成因进行讨论。英云闪长岩锆石LA-ICPMS U-Pb年龄为(342.3±2.8) Ma(MSDW=0.49)。岩石SiO₂含量73.21%～77.15%,Al₂O₃含量10.66%～13.00%,属偏铝质,A/CNK为0.89～0.98、富钠,Na₂O/K₂O值为6.99～22.31,低钾(拉斑)系列。稀土元素总量较低,∑REE=40.95～55.01,轻重稀土分馏较弱,(La/Yb)N=0.47～0.72,Eu负异常明显,δEu=0.15～0.25。Sr/Y比值为3.07～4.36,Nb/Ta比值为3.61～9.86。岩石具较低的Sr初始值,(⁸⁷Sr/⁸⁶Sr)i=0.704 485和正的Nd初始值,εNd(t)=9.45～10.295。认为卡拉麦里蛇绿混杂岩带北侧...     

大兴安岭北段龙江盆地中生代火山岩LA-ICP-MS 锆石U-Pb年龄、地球化学特征及其地质意义

大兴安岭北段龙江盆地中生代火山岩主要为一套中酸性岩石,岩石化学成分显示其主要为英安岩与流纹岩,少量安山岩.各类岩石的LA-ICP-MS锆石U-Pb定年结果显示,其形成于122.2～126.5Ma之间的早白垩世.火山岩为高钾钙碱性系列岩石,稀土元素总量较高(∑REE=103.22×10-6～249.29×10-6),轻重稀土元素分馏明显((La/Yb)N=6.29～25.89),负Eu异常(Eu/Eu* =0.48～0.84).富集大离子亲石元素Rb、Ba、Th、U、K和LREE,亏损高场强元素Nb、Ta、P、Ti;火山岩稀土元素与微量元素标准化配分曲线与地壳配分曲线近乎一致,为一套壳源岩浆系列火山岩;大部分岩石300＜Sr＜700;Yb平均为2.24,Y平均为18.36,符合埃这克岩型岩石标准;岩石Y/Yb平均为9.15,(Ho/Yb)N=0.98,而弱负Eu异常(平均值δEu=0.71).地球化学特征说明,岩浆主要是增厚(高压)基性麻粒岩下地壳部分熔融的结果.结合区域上同期火山岩的资料,龙江盆地中生代火山岩可能是蒙古-鄂霍茨克洋闭合碰撞造山构造背景下加厚下地壳部分熔融的产物.     

藏北羌塘比隆错一带新生代火山岩的成因:壳幔过渡带局部熔融的地球化学证据

羌塘比隆错新近纪火山岩主要岩石类型为安粗岩-粗面岩,为一套中基性-中性的碱性系列岩石组合,SiO2含量介于52%～62%之间,Al2O3＞15%,Na2O/K2O＞1,MgO＜3.30%.岩石轻稀土元素较强富集,LREE/HREE=10～13,(La/Yb)N=15～19,弱负Eu异常,δEu=0.71～0.89.Mg#与SiO2的相关关系和La/Sm-La、Cr-Tb等亲岩浆元素与超亲岩浆元素协变关系表明,该套岩石为共源岩浆分离结晶的产物.岩石组合类型和低的Sm/Yb值(3.23～3.97)表明,它们应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融;弱的负Eu异常和Nb、Ta、Ti的相对亏损,又反映了陆源岩浆的特征,可见,比隆错新近纪碱性火山岩应为青藏高原特殊的壳幔过渡带局部熔融的产物.     

藏北羌塘比隆错一带新生代火山岩的成因：壳幔过渡带局部熔融的地球化学证据

羌塘比隆错新近纪火山岩主要岩石类型为安粗岩-粗面岩,为一套中基性-中性的碱性系列岩石组合,SiO2含量介于52%～62%之间,Al2O3＞15%,Na2O/K2O＞1,MgO＜3.30%.岩石轻稀土元素较强富集,LREE/HREE=10～13,(La/Yb)N=15～19,弱负Eu异常,δEu=0.71～0.89.Mg#与SiO2的相关关系和La/Sm-La、Cr-Tb等亲岩浆元素与超亲岩浆元素协变关系表明,该套岩石为共源岩浆分离结晶的产物.岩石组合类型和低的Sm/Yb值(3.23～3.97)表明,它们应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融;弱的负Eu异常和Nb、Ta、Ti的相对亏损,又反映了陆源岩浆的特征,可见,比隆错新近纪碱性火山岩应为青藏高原特殊的壳幔过渡带局部熔融的产物.     

内蒙古乌日尼图花岗岩的年代学、地球化学及其地质意义

乌日尼图隐伏岩体位于内蒙古自治区苏尼特左旗查干敖包镇西北乌日尼图地区,主要岩石类型为细粒-中细粒花岗岩、二长花岗岩。岩相学及地球化学特征研究表明,乌日尼图花岗岩体属于高钾钙碱性岩系,SiO₂含量为74.85%～78.10%,K₂O/Na₂O比值介于1.45～3.08之间,铝指数A/CNK=1.00～1.21,属过铝质岩石。主微量稀土元素分析表明,该岩体富集Rb、Th、U等元素而Ba、Nb、Sr、P和Ti强烈亏损,Eu为负异常(δEu= 0.22～0.39),总体呈现轻稀土富集的右倾“V”形分布配分模式。LA-ICP-MS锆石U-Pb定年表明,乌日尼图花岗岩体形成于早白垩世(139 Ma),为燕山运动晚期的产物。乌日尼图花岗岩体属A型花岗岩,形成于伸展的大地构造环境,且源区成分不均一,源岩可能主要由富黏土的泥质岩夹杂少量砂屑岩变质形成。     

桂东北大宁岩体锆石SHRIMP年代学和地球化学研究

大宁岩体位于扬子板块与华夏板块结合带,岩性组合为二长闪长岩-花岗闪长岩-二长花岗岩-钾长花岗岩.岩体中广泛存在壳幔混合包体.本文利用锆石sHRIMPU-Pb法,获得大宁岩体成岩年龄上限为(419.1±6.4)Ma(MSWD=1.03),属加里东晚期.岩体的地球化学研究显示,大宁岩体属弱过铝高钾钙碱性岩石,富大离子亲石元素和轻稀土,显Nb、Ba、Sr、Eu组合异常.岩石具有较高的I_(Sr)值(0.7112～0.7196),较低的εNd(t)值(-6.77～-7.53)和T_(DM)值(1.40～1.71Ga).结合邻区加里东花岗岩壳幔混合特征,本文认为,大宁岩体为幔源岩浆底侵诱发的下地壳变砂屑岩部分熔融形成,形成过程中有显著的幔源组分加入.     

西藏谢穷地区晚白垩世淡色花岗岩体和岩脉年龄及地球化学特征：对班公湖-怒江带后碰撞背景的制约

藏东左贡地块的晚白垩世侵入岩鲜有报道。对谢穷地区侵入于新元古界酉西岩群中的花岗岩体和岩脉进行了岩石学、地球化学及锆石U-Pb测年研究,揭示其岩石成因及形成环境。岩体岩石类型为含电气石白云母二长花岗岩,岩脉岩石类型为细粒花岗质岩脉,二者的LA-ICP-MS锆石U-Pb年龄分别为88.5±0.7 Ma和85.8±2.3 Ma,均侵位于晚白垩世。岩体与岩脉含白云母,无角闪石,均具有高硅(SiO₂=74.17%～78.88%)、富碱(K₂O+Na₂O=5.92%～9.00%)和富铝(Al₂O₃=12.61%～15.13%)的特点,铝饱和指数(A/CNK=1.11～1.40)和里特曼指数(δ<3.3),指示二者均属于过铝质钙碱性系列;均富集Rb、U、K、P和Pb,亏损Nb、Ti和Ba,具强烈的负Eu异常(δEu=0.18～0.56),表明为一套强过铝质亚碱性S型花岗岩,具有电气石型淡色花岗岩的特征。其可能为地壳缩短增厚挤压环境向地壳伸展环境转换过程中,构造减压导致的上地壳变质泥岩中...     

泸定大河沟村辉长岩体岩石地球化学特征、锆石U-Pb定年及地质意义

大河沟村辉长岩体位于康定杂岩带泸定县大河沟村一带,岩性为辉长岩。LA-ICP-MS锆石U-Pb法测得辉长岩体的结晶年龄为782±6Ma,表明该辉长岩体形成时代为下南华世早期。岩石主量元素FeOT/MgO比值为1.03～2.35且富集轻稀土(La N/Yb N=4.19～6.07)和无明显Eu异常(δEu=0.93～0.97)等地球化学特征表明大河沟村辉长岩的源岩是受板片流体交代的地幔楔橄榄岩。在岩浆演化过程中经历了镁铁质矿物(橄榄石、辉石)的分离结晶和微不足道的地壳混染作用。按照地球化学判别标准,大河沟村辉长岩具有岛弧玄武岩和洋中脊玄武岩的双重地球化学特征,具有初始弧后盆地玄武岩的印记。     

库米什地区二云母花岗岩锆石LA-ICP-MSU-Pb定年及岩石地球化学特征

出露于南天山东段库米什地区的二云母花岗岩体侵入中元古界星星峡群,通过锆石LA-ICP-MS U-Pb法测得该岩体年龄为(293.3±1.3)Ma,侵位时代为早二叠世.岩石地球化学研究表明,岩石具高钾(K2O/Na2O=1.39～2.09)和强过铝(A/CNK=1.03～1.23)特征,为典型S型花岗岩.岩石普遍具较低稀土元素总量(TREE=89.99×10-6～105.11×10-6)和中等铕负异常(δEu=0.38～0.55),富集轻稀土和大离子亲石元素K,Rb,Th,U及亏损高场强元素Nb,Ta,Sr,Ba等特征.结合区域地质特征认为,库米什北二云母花岗岩源岩可能为中元古界星星峡群变质泥质岩,形成于碰撞晚期至后碰撞转换阶段,揭示了南天山东段库米什地区伊犁-中天山板块和塔里木板块于早二叠世碰撞造山结束,进入后碰撞演化阶段.     

Petrogenesis of mafic microgranular enclaves (MMEs) in the oligocene-miocene granitoid plutons from northwest Anatolia,Turkey

Mafic microgranular enclaves (MMEs) in host granitoids can provide important constraints on the deep magmatic processes. The Oligocene-Miocene granitoid plutons of the NW Anatolia contain abundant MMEs. This paper presents new hornblende Ar-Ar ages and whole-rock chemical and Sr-Nd isotope data of the MMEs from these granitic rocks. Petrographically, the MMEs are finer-grained than their host granites and contain the same minerals as their host rocks (amphibole + plagioclase + biotite + quartz + K-feldspar), but in different proportions. The Ar-Ar ages of the MMEs range from 27.9 ± 0.09 Ma to 19.3 ± 0.01 Ma and are within error of their respective host granitoids. The MMEs are metaluminous and calc-alkaline, similar to I-type granites. The Sr-Nd isotopes of MMEs are 0.7057 to 0.7101 for ⁸⁷Sr/⁸⁶Sr and 0.5123 to 0.5125 for ¹⁴³Nd/¹⁴⁴Nd, and are similar to their respective host granitoids. These lithological, petrochemical and isotopic characteristics suggest that the MMEs in this present study represent chilled early formed cogenetic hydrous magmas produced during a period of post-collisional lithospheric extension in NW Anatolia. The parental magma for MMEs and host granitoids might be derived from partial melting of underplated mafic materials in a normally thickened lower crust in a post-collisional extensional environment beneath the NW Anatolia. Delamination or convective removal of lithospheric mantle generated asthenospheric upwelling, providing heat and magma to induce hydrous re-melting of underplated mafic materials in the lower crust.     

Contributions of basaltic underplating to crustal growth in island arc and extensional tectonic settings in the Chinese Tianshan Orogenic Belt,NW China

A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB.     

Elemental and Nd-Sr isotopic geochemistry of granitoids from the West Junggar foldbelt (NW China), with implications for Phanerozoic continental growth

We report elemental and Nd-Sr isotopic data for two coeval postcollisional magmatic suites (∼300 Ma), the Miaogou and Karamay suites, from West Junggar foldbelt (NW China), aiming to determine their source regions and implications for continental growth. The Miaogou rocks, monzonitic to granitic in composition, show low Mg# (<0.30) and depletion of HFSE, and are highly depleted in isotopic compositions, with I_Sr ranging from 0.7035 to 0.7045, ε_Nd(300 Ma) from + 8.4 to + 6.6 and young Nd model ages (0.37-0.70 Ga) that coincide with the ages of the ophiolites in the area. These features suggest that the Miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle Palaeozoic. The Karamay suite contains a variety of rock types from gabbroic diorite to granite. These rocks show enrichment of LREE and LILE and depletion of HFSE, and have chemical composition (e.g., Mg# = 0.63-0.41) rather different from the Miaogou suite, though isotopically they are indistinguishable. This suggests that the parental magma of the Karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the Palaeozoic subduction. The West Junggar magmatic rocks represent production of juvenile continental crust in the Phanerozoic, triggered by upwelling of asthenophere in an extensional regime. The basement beneath the area is dominated by “trapped” Palaeozoic arc series and oceanic crust.     

Mineralogy, whole-rock and Sr-Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88-83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An₃₃-An₆₀), hornblendes (Mg#=0.77-1.0) and biotites (Mg#=0.61-0.63) of MMEs are slightly distinct or similar to those of host rocks (An_12-57; hbl Mg#=0.63-1.0; Bi Mg#=0.50-0.69), which suggest partial to complete equilibration during mafic-felsic magma interactions.The felsic host rocks have SiO₂ between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La_cn/Lu_cn=1.5-7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46-1.1). Initial ε_Nd(i) values vary between −3.1 and 1.6, initial ⁸⁷Sr/⁸⁶Sr values between 0.7056 and 0.7067.Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22-52, low contents of SiO₂ (53-63 wt%), low ASI (0.7-1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb)_cn=1.4-3.9; (Tb/Yb)_cn=0.9-1.5] and show small negative Eu anomalies (Eu/Eu*=0.63-1.01). Isotope signatures of these rocks (⁸⁷Sr/⁸⁶Sr_(i)=0.7054-0.7055; ε_Nd(i)=-1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO₂, ASI; high Mg#, CaO, Al₂O₃, TiO₂, P₂O₅, Sr and Nb concentrations compared to dioritic and tonalitic enclaves.The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3-3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.     

西秦岭金厂石英闪长岩的岩浆混合成因：岩相学和锆石U-Pb年代学证据及其构造意义

位于西秦岭南部的金厂石英闪长岩岩体内含有大量镁铁质暗色微粒包体,包体大多呈浑圆状和水滴状,部分呈不规则拉长状,与寄主岩的接触界线截然或呈渐变过渡关系。石英闪长岩中的磷灰石呈短柱状,而包体中的磷灰石则呈细长针状,反映基性岩浆的快速冷凝结晶。石英闪长岩中的斜长石发育振荡环带,核部的斜长石An低,而边部斜长石An先急剧上升,复又下降;核部与边部之间存在明显的间断,同时斜长石边部包裹有暗色矿物,指示其形成时可能有更基性的岩浆注入。寄主岩中的角闪石大多为普通角闪石和镁普通角闪石,属SiO2饱和型,而包体中角闪石一部分为镁普通角闪石,属SiO2饱和型,一部分为韭闪石、韭闪石质普通角闪石,属SiO2不饱和类型。包体中的角闪石自核部到边部,Al2O3与TiO2含量急剧下降,说明核部和幔部相对于边部形成于更高温的环境。寄主岩中黑云母部分为铁质黑云母,部分为镁质黑云母,而包体中黑云母均为镁质黑云母,在∑FeO/(∑FeO+MgO)对MgO图解上寄主岩与包体中黑云母均落入壳-幔混源区。寄主岩和包体中的锆石均为典型的岩浆锆石,LA-ICP-MS锆石U-Pb定年表明它们的形成年龄分别为212±2Ma及215±1Ma(2σ),在误差范围内基本一致,证明二者同时形成。综合以上岩相学和年代学证据认为,金厂石英闪长岩和镁铁质暗色微粒包体是幔源基性岩浆和壳源酸性岩浆混合作用的产物,形成于秦岭造山带中三叠世造山后伸展环境。结合区域上的研究结果认为,中—晚三叠世时期的幔源岩浆底侵和下地壳部分熔融在西秦岭广泛存在。     

东昆仑东段哈拉尕吐花岗岩基岩浆混合作用:来自岩石学和矿物学约束

哈拉尕吐花岗岩基位于东昆仑东段,其中花岗闪长岩岩浆混合作用明显,是研究岩浆混合作用的良好对象.从岩石学、岩相学和矿物化学等方面对哈拉尕吐花岗岩基进行了详细研究.电子探针结果显示:寄主岩斜长石的An值同相对应包体中斜长石捕掳晶近似;包体中基质斜长石大部分具核边结构,核部和边部An值存在间断;部分包体中浅色基质斜长石的An值与具核边结构斜长石的边部近似;辉长闪长岩中斜长石具较高的An值.寄主岩角闪石同相对应包体中角闪石捕掳晶的结晶温度、压力和氧逸度较为接近;包体中基质角闪石的结晶温度和压力低于寄主岩角闪石,氧逸度稍高于寄主岩角闪石;辉长闪长岩角闪石具有最高的结晶温度和压力及最低的氧逸度.哈图沟剖面和德福胜剖面寄主岩中的斜长石和角闪石的成分具有一定差别.岩浆不同期次侵入结晶和岩浆自身演化,使不同地点斜长石和角闪石的成分和物理化学特征具有一定变化.镁铁质岩浆位于地壳深部,氧逸度较低,使结晶的角闪石具有较高的形成压力和较低的氧逸度,斜长石具较高An值;随着镁铁质岩浆注入寄主岩,由于环境突变,使斜长石受到熔蚀;由于岩浆上侵以及两种岩浆物理化学性质差别较大,导致温度、压力和水饱和度降低,氧逸度升高,使包体中残留岩浆快速结晶,形成具核边结构、浅色均一的斜长石,以及结晶程度较差、较高氧逸度的角闪石.      

藏南冈底斯带中段始新世岩浆作用的厘定及其大地构造意义

本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICPMS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(～50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

松潘—甘孜造山带万里城花岗岩及其岩浆包体的成因与地球动力学意义

松潘—甘孜造山带广泛分布着三叠纪花岗岩体,其成因对正确认识研究区花岗岩浆的动力学背景具有重要意义。地球化学分析表明,万里城岩体寄主花岗岩具有高的SiO2含量（69.43%~73.10%）和较高的全碱含量,具弱过铝质（A/CNK=1.01~1.12）特征,属于高钾钙碱性—钾玄岩系列I型花岗岩类。暗色微粒包体具较低的SiO2含量（52.85%~59.50%）和较高的Mg#值（45~63）,为准铝质高钾钙碱性二长（闪长）岩。包体为典型的岩浆细粒结构,发育针状磷灰石、环带结构斜长石、瞳状石英、反鲍文序列的不平衡岩浆结构等。微量与稀土元素分析表明,包体起源于壳幔混合作用,是底侵的幔源玄武质岩浆与上覆壳源长英质岩浆混合的产物,混合的熔体经历了钛铁矿、黑云母等矿物的分离结晶,最终形成万里城暗色微粒包体。而寄主花岗岩则起源于纯的长英质陆壳,岩石具有较低的Mg#值(21~39)、中等的CaO/(MgO+TFeO)值、较高的K2O/Na2O和(Na2O+K2O)/(TFeO+MgO+TiO2)值等,指示源区主要为变杂砂岩类。综合区域地质资料,提出松潘—甘孜造山带内大规模花岗质岩体的形成主要受控于碰撞后伸展背景下的玄武质岩浆底侵加热。     

Late Cretaceous arc magma differentiation in southern Tibet: Evidence from zircon U-Pb chronology,mineralogy and geochemistrySCIEI北大核心CSCD

造山带岩浆作用记录了从大洋俯冲消减到陆陆碰撞的一系列地质过程,同时也蕴含了关于大陆地壳生成、生长的重要信息,其中,与俯冲相关的弧岩浆作用被认为是大陆地壳生长的主要机制。本文通过对西藏南部拉萨地体曲水县至贡嘎县一带出露的黑云母二长花岗岩、镁铁质微粒包体以及石英闪长岩开展同位素年代学、矿物学和岩石地球化学研究,探讨了该岩石组合的成因和弧岩浆的分异演化过程。锆石U-Pb定年结果表明,这些岩石的形成时代为91~88Ma。其中,黑云母二长花岗岩属中钾钙碱性偏铝质岩石系列(A/CNK=0.77~0.99),具高SiO 2(68.90%~69.18%)和Al_(2)O_(3)(15.21%~15.48%),低MgO(1.15%~1.16%)和Mg#值(~44),高Sr/Y比值(60~82),其地球化学特征与埃达克质岩石相似;镁铁质微粒包体的SiO 2含量为54.75%~54.96%,具有较高MgO含量(3.92%~5.40%)和Mg#值(52~61),并具弱的负Eu异常(δEu=0.79~0.82)。石英闪长岩为中钾钙碱性偏铝质岩石(A/CNK=0.84~0.88),其SiO 2含量为58.55%~63.32%,具有较高的Mg#值(44~48)和弱的负Eu异常(δEu=0.68~0.81)。另外,所有黑云母二长花岗岩、镁铁质微粒包体以及石英闪长岩样品的Sr-Nd同位素和锆石εHf(t)值相近((87 Sr/86 Sr)i=0.703594~0.703939,εNd(t)=+4.6~+4.8,εHf(t)=+10.9~+15.6)。矿物成分分析表明,黑云母二长花岗岩和镁铁质微粒包体中发育环带的斜长石An值由中心向边部逐渐下降,未发育反环带结构;石英闪长岩中的辉石为单斜辉石,所分析角闪石均为钙质角闪石,黑云母为镁质黑云母。综合上述特征,本文认为黑云母二长花岗岩、镁铁质微粒包体以及同生石英闪长岩分别代表了源自亏损地幔的母岩浆通过分离结晶作用所派生的不同阶段的产物:其中,石英闪长岩最先由基性岩浆发生以辉石、Ti-Fe氧化物和磷灰石为主的分离结晶作用形成,继而作为母岩浆进一步分异演化;镁铁质微粒包体是岩浆早期结晶的堆晶产物;而黑云母二长花岗岩代表了包体形成后残余熔体的组分。本文的研究表明,冈底斯岩浆岩带晚白垩世发生了富水条件下的岩浆分离结晶和堆晶作用,俯冲带弧岩浆分异对于大陆地壳的形成和演化发挥着重要的作用。