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1.
甘肃北祁连毛藏寺埃达克岩及其成因类型   总被引:1,自引:0,他引:1       下载免费PDF全文
李大民 《地质科学》2012,47(2):347-359
埃达克岩(Adakite)的特殊性及其科学和现实意义广泛地吸引着地球科学工作者。位于青藏高原东北缘的祁连山早古生代造山带是新中国地学界认识板块构造的摇篮。本文研究了北祁连造山带中段毛藏寺地区侵入岩的岩石学和地球化学特征,结果表明:毛藏寺地区加里东期中酸性侵入岩具有较高的SiO2、Al2O3、Na2O含量及Na2O/K2O值,MgO较低; LREE强富集,HREE明显亏损,L/H比值>15; 岩石Th、Sr、Ba具显著的正异常,Sr含量一般在507×10-6 以上,具有高Sr/Y、La/Yb比值和较高的Cr、Ni含量; 岩石σ值一般为1.45~2.76,A/CNK值为0.83~1.15,多数<1.0,主体属于钙碱性岩系列,偏铝质花岗岩类。岩石化学特征指向埃达克岩。在(La/Yb)N-YbN和Sr/Y-Y判别图上,样品落入埃达克岩范围; 在SiO2-Mg#图解上,岩体处在"板片来源的埃达克岩"区域,表明毛藏寺地区侵入岩属于"O"型埃达克岩,岩浆起源于俯冲洋壳的部分熔融。毛藏寺"O"型埃达克岩的识别表明,在早古生代中期,北祁连中段经历了洋壳俯冲、岛弧发育和壳幔共同作用的构造演化过程,为祁连山造山带的板块构造演化历史提供了新资料,为祁连山地区寻找和勘查与埃达克岩相关的矿产资源提供了一个可能的方向。  相似文献   

2.
吉林中部晚三叠世南楼山组埃达克岩与官马金矿床   总被引:2,自引:0,他引:2  
南楼山组火山岩为一套中酸性钙碱性岩系,属三叠世,Rb-Sr等时线年龄222±10Ma.其SiO2含量多大于57%,Al2O3多大于15%,MgO<6%,岩石富Na2O,Na2O/K2O>2者占半数,Yb为1.74μg/g,(La/Yb)N=14.2,LREE富集,δEu为0.80~0.92,弱负铕异常,Sr含量达405~1128μg/g,强烈富集,Sr/Y=44.5~48.9,SrI值为0.70533和0.7046,显示出埃达克岩的特征.官马金矿床产在南楼山火山岩系的隐爆热液角砾岩及其与围岩接触带中.火山岩浆为成矿热源和物源,属火山热液矿床.  相似文献   

3.
谢士稳  刘福来  王舫 《岩石学报》2021,37(2):481-496
本文对西南三江地区景洪南部新发现的晚泥盆世英安质火山岩进行了系统的锆石U-Pb年龄、Hf-O同位素以及全岩地球化学研究。测年结果显示,该英安质火山岩形成于362.3±3.4Ma。火山岩的SiO2含量为62.87%~66.29%,MgO含量为2.15%~2.49%,Mg#值为44~47,富集Na2O(4.77%~5.51%),高Na2O/K2O比值(2.1~3.3),低Y(12.5×10-6~15.3×10-6)和Yb(1.38×10-6~1.70×10-6)具有高硅埃达克岩的地球化学特征,表明它们由俯冲的洋壳部分熔融而成。低的锆石εHf(t)(+0.87~+3.27),相对较高的锆石δ18O(6.31‰~7.64‰),以及高的全岩Th/Yb(4.86~7.78)和(La/Sm)N(3.62~4.56)比值指示岩浆中混染了大洋沉积物或混入了大洋沉积岩起源的熔体。综合区域岩浆岩和沉积岩资料本文认为南澜沧江带晚泥盆世时经历了板片的俯冲,该俯冲作用很可能代表的是由大中河、大平掌火山岩以及火山块状硫化物矿床(VHMS)指示的弧后洋盆的俯冲消减过程。  相似文献   

4.
本文以贺根山缝合带呼都格奥长花岗岩体为研究对象,通过野外地质调查和岩石学、地球化学、锆石U-Pb年代学研究,讨论岩石成因、构造环境、TTG岩浆事件及古亚洲洋俯冲消亡过程。岩石地球化学研究表明,呼都格岩体富硅(SiO2=66.27%~71.59%)、高铝(Al2O3=15.23%~15.94%)、富钠(Na2O=4.13%~6.59%)、低钾(K2O=1.72%~2.53%),相对高锶(Sr=196.60×10-6~465.40×10-6)、低钇(Y=5.70×10-6~12.63×10-6),富集Ba、Sr等大离子亲石元素和LREE,亏损Nb、Ta、Ti、P等高场强元素和HREE,无明显Eu异常。岩石学和岩石地球化学特征表明,呼都格岩体属于以奥长花岗岩为主的英云闪长岩-奥长花岗岩-花岗闪长岩TTG岩石组合。这套TTG组合除Sr、Mg、Ni和Cr含量相对较低之外,与高Si埃达克岩的地球化学特征相类似,形成于大洋俯冲带岛弧环境,可能为俯冲洋壳脱水熔融成因。锆石LA-ICP-MS U-Pb测年获得两组年龄为306.3±1.9Ma和315.5±1.9Ma,表明该岩体侵位于晚石炭世,反映了贺根山缝合带晚石炭世大洋俯冲带TTG岩浆事件。结合其与梅劳特乌拉-高力罕蛇绿岩-TTG岩带前弧玄武岩、高镁安山岩/高镁闪长岩、埃达克岩、TTG、富铌弧玄武岩/辉长岩的岩石构造组合,认为古亚洲洋二连-贺根山洋盆在晚石炭世可能处于洋壳俯冲消减、TTG岩浆活动和新生陆壳生长洋陆转换过程中。  相似文献   

5.
地苏嘎印支期斑岩体位于中甸岛弧东斑岩带的北端,岩性为石英闪长玢岩,与普朗超大型斑岩铜矿的成矿岩体同属于甘孜—理塘洋壳俯冲作用晚期岩浆活动的产物。全岩地球化学特征显示,地苏嘎石英闪长玢岩具有中等的SiO2含量,较高的Al2O3及全碱(K2O+Na2O)含量。全岩亏损高场强元素,富集大离子亲石元素,稀土元素配分曲线呈典型的右倾状,显示出轻稀土富集、重稀土相对亏损的特点。主量元素和微量元素成矿指示方面的研究结果表明:与普朗38个与成矿有关的石英闪长玢岩的全岩数据相比,地苏嘎石英闪长玢岩具有相对较低的SiO2和Y含量、较高的∑REE和(La/Yb)N比值,但二者的Al2O3/(CaO+Na2O+K2O)比值及MnO含量差别不大。在成矿与非成矿斑岩的SiO2- Al2O3/(CaO+Na2O+K2O)和Y-MnO图解中,地苏嘎石英闪长玢岩与普朗石英闪长玢岩数据投图结果类似。此外,地苏嘎斑岩体在物探异常、化探异常及遥感影像特征上具有明显的矿化标志,同时岩体的剥蚀深度小于侵位深度,有利于矿床的保存。以上研究结果表明地苏嘎斑岩体具有成为斑岩型铜矿床的潜力。  相似文献   

6.
内蒙古苏左旗白音宝力道闪长质岩类的地球化学特征类似典型的埃达克岩,其SiO3含量≥56%(61.1-61.6%),Al2O3含量均大于15%(16.6-17.1%),MgO<3%(2.3%),Na2O>3%,Na2O/K2O比值较高(1.8-2.7,平均2.2),亏损HREE,Yb含量<1.9μg/g(1.61μg/g-1.64μg/g),岩体富Sr(604-649μg/g,>400μg/g)、贫Y(14.5-16.2μg/g,<18μg/g),Sr/Y>40(40-42),在微量元素蛛网图上具明显的Sr正异常。表明其成因与板块的俯冲作用有关,可能是消减的MORB板片部分熔融的产物。精确的SHRIMP皓石U-Pb年龄测定显示,内蒙古苏左旗白音宝力道英云闪长岩的结晶年龄为464±8Ma和479 8Ma,结合前人的SHRIMP定年资料,该闪长质岩体的年龄范围可能在464-490Ma之间。表明内蒙古中部的板块俯冲事件发生在早奥陶世。  相似文献   

7.
秦岭造山带是一条复合型大陆碰撞造山带,存在若干新元古代构造岩浆事件的遗迹,它们对深化认识南北秦岭汇聚-碰撞过程和Rodinia超大陆聚合具有重要意义。本文对南秦岭东部豆腐尖岩体英云闪长岩开展LA-ICP-MS锆石U-Pb年代学和全岩主微量元素地球化学研究。代表性样品的206Pb/238U加权平均年龄为860.7±6.0Ma,表明其形成时代为新元古代。岩石地球化学特征表现为高SiO2(62.41%-68.89%)、高Al2O3(15.33%-17.33%),富Na2O(4.23%-5.80%)和高Na2O/K2O比值(1.11-2.41),富Sr(>400×10-6),低MgO(0.55%-2.08%),低Y(7.40×10-6-18.20×10-6)、Yb(0.63×10-6-1.62×10-6),高Sr/Y比值(31.49-78.22),轻稀土元素显著富集[(La/Yb)N>20],弱Eu正异常,具埃达克质岩特征。较高的K2O含量(2.00%-4.31%)和低MgO以及显著的高La/Yb比值等特征指示,其具有典型高钾钙碱性埃达克质岩特征,很可能源于加厚下地壳的部分熔融,推测该岩体形成时南秦岭地壳厚度可能达到65 km。结合区域地质资料,认为豆腐尖岩体形成于陆-陆碰撞环境,是新元古代松树沟洋盆闭合后北秦岭和南秦岭碰撞造山的产物,是Rodinia超大陆聚合事件在该地区的岩浆响应。新元古代早期商南豆腐尖高钾钙碱性埃达克质岩的首次识别为限定南-北秦岭碰撞事件提供了有力约束。  相似文献   

8.
内蒙古索伦山地区朱恩道欧恩呼尔新识别出一套早中生代侵入岩,主要由角闪二长岩和二长闪长岩组成.通过LA-ICP-MS锆石U-Pb定年确定,角闪二长岩的结晶年龄为248±1 Ma(MSWD=1.1),表明该岩体形成于早三叠世.它们的SiO2含量介于51.82%~60.17%之间,具有高镁(Mg#=53.17~59.03)、富碱(K2O+Na2O=7.36%~9.83%)、富钠(K2O/Na2O=0.25~0.53)特征,属于准铝质碱性岩.样品稀土总量变化范围为117.52×10-6~160.89×10-6,表现出一致的稀土元素和微量元素配分型式,即呈现出轻稀土元素相对于重稀土元素富集((La/Yb)N=9.57~15.42)的右倾曲线,且无明显Eu异常(δEu=0.76~1.00),并富集Ba、Sr等大离子亲石元素,亏损Nb、Ta、Ti、Y等高场强元素,高Sr、低Y、高Sr/Y比值,具有高镁埃达克质闪长岩的地球化学特征.综合岩石地球化学特征及区域构造背景,朱恩道欧恩呼尔早中生代高镁埃达克质闪长岩类是由残余俯冲洋壳部分熔融形成的熔体与地幔橄榄岩相互作用而形成.朱恩道欧恩呼尔富碱侵入岩的形成与华北板块与西伯利亚板块碰撞后的伸展作用有关,这些新的年龄和岩石地球化学数据表明古亚洲洋闭合早于248 Ma.   相似文献   

9.
由于缺少对SSZ型蛇绿岩和洋内弧火成岩的系统研究,制约了古亚洲洋东段古生代洋内俯冲过程的进一步认识。本文报道了内蒙古迪彦庙SSZ型蛇绿岩带北部新发现的巴嘎哈尔早石炭世闪长岩。LA-ICP-MS锆石U-Pb定年显示,巴嘎哈尔闪长岩的侵位年龄为324.2±1.8 Ma,其形成时代为早石炭世末期。巴嘎哈尔闪长岩SiO2含量为57.71%~61.24%;高铝(Al2O3含量为15.58%~16.68%);具有相对富钠(Na2O含量为3.29%~4.15%)、低钾(K2O含量为1.05%~1.69%)的特征,Na2O/K2O比值为2.18~3.95;MgO含量较高(3.30%~4.23%),Mg#为47.20~51.82;贫TiO2(0.65%~0.76%)和P2O5(0.14%~0.15%);稀土元素总量(ΣREE为85.19×10-6  相似文献   

10.
青海玉树县南洞卡组粗面安山岩的SiO2含量为55.92%~57.85%,高Na2O、MgO和Al2O3,低K2O、TiO2和 P2O5,铝饱和指数A/CNK=0.98~1.08,具有准铝-弱过铝质的特点。岩石K2O含量低,Na2O/K2O=12.60~15.97,显示出钠质粗面安山岩特征。SiO2-K2O图解和里特曼指数(2.89~3.25),说明岩石属于钙碱性系列。稀土配分模式为轻稀土弱富集右倾型,铕弱负异常(σEu=0.90~0.96)。微量元素蛛网图显示,岩石富集Rb、Th和U等元素,亏损高场强元素Nb、Ta和大离子亲石元素Ba,样品Ce/Pb、Nb/U 并未随着MgO含量的升高而升高,样品的La/Nb、Zr/Nb、Ba/Nb比值均表现出良好的稳定性,表明岩浆在上升过程中遭受地壳物质混染的可能性不大,可能是其源区特征的反映。研究表明岩浆经历了橄榄石和单斜辉石的结晶分异,且受到俯冲板片熔体对源区的改造作用。结合234 Ma的形成年龄,说明该粗面安山岩是金沙江洋盆向西俯冲消减和斜向碰撞的地质记录,其形成时代和地球化学特征也为江达-维西火山弧的形成演化及成矿作用研究提供依据。  相似文献   

11.
埃达克岩的研究现状及其趋势   总被引:2,自引:0,他引:2       下载免费PDF全文
达克岩(adakite)是一种中酸性富钠火成岩(安山岩、英安岩、钠质流纹岩及相应的侵入岩),其突出的地球化学特征就是SiO2≥56%,Al2O3≥15%,低重稀土元素(HREE)与Y(如Yb≤1.9×10-6,Y≤18×10-6),高Sr(大多数>400×10-6,La/Yb(≥10.0)与Sr/Y(>20.0~40.0),一般具有正铕异常(少数具有极弱负铕异常).埃达克岩存在两种成因类型:一种由俯冲的年轻(≤25~30Ma)大洋板片熔融形成(O型埃达克岩);另一种由增厚地壳环境中的玄武质下地壳熔融形成(C型埃达克岩).文章从埃达克岩的特征、形成机制、成矿作用、构造背景、动力学意义等几个方面论述了埃达克岩的研究现状,并指出了未来埃达克岩研究的趋势.  相似文献   

12.
北山牛圈子地区的骆驼圈西埃达克岩为过铝质钙碱性岩浆系列,具有高的SiO_2(57.94%)、Al_2O_3(16.66%)和低的MgO(4.32%)含量,较高的Na_2O/K_2O比值(3.13),显示富钠的特征;而在微量元素上,高Sr(589×10~(-6)~1170×10~(-6)),低Y(4.60×10~(-6)~13.90×10~(-6))和Yb(0.43×10~(-6)~1.32×10~(-6));轻重稀土分异明显(La/Dy)N=4.60~13.87、(La/Yb)N=7.17~20.58,Eu异常不明显(Eu/Eu*=0.85~1.32);富集轻稀土和大离子亲石元素,而亏损高场强元素Nb,Ta,Ti和重稀土HREE,与世界上典型的俯冲洋壳熔融形成的埃达克岩特征相似。结合区域上埃达克岩的研究成果和岀露的构造位置,以及与牛圈子蛇绿岩的关系,认为骆驼圈西埃达克岩是志留纪(U-Pb年龄为428.9 Ma)时期热的洋壳向公婆泉—东七一山早古生代活动陆缘带俯冲过程中部分熔融的产物。对该岩体的研究,为进一步研究北山造山带早古生代北山洋盆闭合和晚古生代北山洋盆重新裂开提供新的依据。  相似文献   

13.
寨北山矿区海相火山岩为一套富钠的玄武安山玢岩、安山岩、英安岩、流纹岩组合,属于钙碱性系列岩石,具有低MgO(0.51%~5.93%,平均2.54%)、FeO(0.54%~6.39%,平均2.84%)和钛(TiO2=0.09%~1.10%,平均0.58%),富铝(Al2O3=12.23%~17.75%,平均15.20%,A/CNK=0.79~1.42,平均1.11)以及富钠(Na2O/K2O平均为7.30)、富水的特征。火山岩中斜长石主要为钠长石,少量更长石。轻、重稀土分馏较明显((LREE/HREE)N=3.68~9.00),微量元素显示大离子亲石元素(如Th、U、Rb)、轻稀土的富集和高场强元素(如Nb、Ta、Ti、P)相对亏损的特征。获得矿区雅满苏组钠质玄武安山玢岩SHRIMP锆石U-Pb谐和年龄为(337.6±3.3)Ma,为早石炭世火山活动的产物。火山岩岩石学及地球化学特征表明研究区钠质火山岩可能形成于俯冲带近大陆方向的岛弧构造环境,是早石炭世洋壳俯冲熔融产生的岩浆在海底喷发过程中与海水相互反应后,经低变质相作用产生的。成矿元素在钠长石化过程中可能被淋滤出来进入含矿热液,后期在适当的温压等条件下沉淀形成本区的矿床。  相似文献   

14.
Triassic granodiorites in South China (SC) provide an opportunity to examine crust–mantle interactions that may have been caused by a mantle plume. Here we present a combined study of chronological, geochemical, and Sr–Nd–Hf isotopic compositions for Dashenshan granodiorites. These are high-K, calc-alkaline, I-type granodiorites that yield a U–Pb zircon age of 211 ± 3 Ma. They are metaluminous to weakly peraluminous (A/CNK < 1.1), with 3.04–3.89 wt.% Na2O and 3.24–3.86 wt.% K2O, and Na2O/K2O ratio ranging from 0.79 to 1.11. These granodiorites contain 67.7–72.6 wt.% SiO2 but show moderate Mg# values (44.2–57.8) and variable contents of Ni (3.6–29.9 ppm) and Cr (7.6–53.5 ppm). They exhibit light rare earth element (REE) enrichment and flat, heavy REE patterns with negative Eu anomalies (Eu/Eu* = 0.52–0.87). They also display strongly negative Ba, Sr, Nb, Ta, P, and Ti anomalies and positive Rb, Th, K, and Pb anomalies. Dashenshan granodiorites have high whole-rock initial 87Sr/86Sr ratios (0.7121–0.7172), negative εNd (t) values (–8.8 to –6.8), and negative zircon εHf (t) values (–6.6 to –3.3). These results suggest that the Dashenshan granodiorites were generated by a mixing between crustal melt and mantle-derived magma in an extensional setting. We conclude that generation of the Dashenshan pluton may reflect an interaction between a mantle plume and the overlying SC crust.  相似文献   

15.
Precise Fe/Mn ratios and MnO contents have been determined for basalts from the Hawaiian shields of Ko’olau and Kilauea by inductively coupled plasma mass spectrometry. It is well known that the youngest Ko’olau (Makapu’u-stage) shield lavas define a geochemical endmember for Hawaiian lavas in terms of CaO and SiO2 contents and isotopic ratios of O, Sr, Nd, Hf, Pb, and Os. We find that their MnO content is also distinct. Despite the small range in MnO, 0.146 to 0.176 wt%, the precision of our data is sufficient to show that among unaltered Ko’olau lavas MnO content is correlated with Nd-Hf-Pb isotopic ratios, La/Nb and Al2O3/CaO elemental ratios, and contents of SiO2, MgO and Na2O + K2O adjusted for olivine fractionation. These trends are consistent with two-component mixing; one endmember is a SiO2-rich, MnO-, and MgO-poor dacite or andesite melt, generated by low degree (10-20%) partial melting of eclogite. Since this low-MgO endmember (dacite or andesite melt) has very low FeO and MnO contents, mixing of high Fe/Mn dacite or andesite melt with a MgO-rich picritic melt, the other endmember, does not significantly increase the Fe/Mn in mixed magmas; consequently, Ko’olau and Kilauea lavas have similar Fe/Mn. We conclude that the high Fe/Mn in Hawaiian lavas relative to mid-ocean ridge basalt originates from the high MgO endmember in Hawaiian lavas.  相似文献   

16.
Dikes, stocks and/or sheet flows of felsic volcanic and subvolcanic rocks are typically observed in the vicinity of rare-metal Li-F granite massifs. Their ubiquitous spatial association to rare-metal granites and, often, geochemical affinity to them suggest their certain petrological relation. Compositionally unique ultrapotassic trachydacites enriched in many rare elements were found among these rocks within the Khangilay complex of ore deposits in Eastern Transbaikalia. Melt inclusions in rock-forming quartz were studied to reconstruct the composition and evolution of parent melt. The obtained data demonstrated the existence of a super-potassic peraluminous melt (K2O = 6.12 wt %, Na2O = 1.08 wt %) having elevated contents of rare lithophile elements (730 ppm Rb2O and 900 ppm BaO). The ion-microprobe content of Li is 354.23 ppm at a relatively low F content (up to 0.5 wt %). The residual melt is characterized by the most unusual composition: extremely low contents of mafic components and basicity (< 0.5 wt % femic oxides), a high Al index (A/CNK = 1.53) at comparatively low SiO2 (60 wt %), and high total sodic alkalinity (more than 10 wt % K2O + Na2O; 6.11 wt % Na2O). Such a composition corresponds to ongonite magma. However, the melt contains no F but has a high Cl content (0.34 wt %), which corresponds to the limit Cl saturation of haplogranite melt. SHRIMP-II U-Pb zircon dating showed significant difference between rare metal granites and trachyrhyodacites of the Khangilay complex of ore deposits: 139.9 ± 1.9 Ma and 253.4 ± 2.4 Ma, respectively. The geochemical similarity of these rocks, primarily in terms of abundance of refractory elements, REE distribution patterns, and initial Sr ratio, indicates their derivation from similar protolith.  相似文献   

17.
车路沟岩体位于北祁连造山带最西端,岩石类型以英云闪长玢岩和闪长玢岩为主。本次对英云闪长玢岩进行U-Pb年代学研究,获得加权平均年龄为(462.1±3.8)Ma,表明岩体形成于中奥陶世。岩石地球化学分析结果显示,英云闪长玢岩中SiO_2含量60.37%~66.73%,Al_2O_3含量15.65%~16.45%,Na_2O/K_2O值7.47~9.96,A/CNK为0.90~1.00,属中酸性岩系列,显示相对富钠、低钾的特征, A/CNK=0.90~1.00,属于准铝质低钾钙碱性系列;稀土总量较低(33.40×10~(-6)~39.27×10~(-6)),LREE/HREE值为3.95~6.51,在稀土元素配分图解上呈现出右倾特征;δEu值1.23~1.49之间,呈现Eu呈正异常特征;在微量元素原始地幔标准化图解上,显示富集Ba、Sr,亏损Nb、Ti、Yb高场强元素的特征。对车路沟岩体地质、地球化学特征与产出背景的全面分析,认为其为奥陶纪祁连洋持续向华北板块俯冲,诱使俯冲洋壳部分熔融形成的埃达克岩。  相似文献   

18.
New geochemical and isotopic data for post-collisional Early Eocene and Late Miocene adakitic rocks from the eastern part of the Sakarya Zone, Turkey, indicate that slab and lower crustal melting, respectively, played key roles in the petrogenesis of these rocks. The Early Eocene Yoncal?k dacite (54.4 Ma) exhibits high Sr/Y and La/Yb ratios, low Y and HREE concentrations, moderate Mg# (44–65), and relatively high εNd and low ISr values, similar to adakites formed by slab melting associated with subduction. Geochemical composition of the Yoncal?k dacite cannot be explained by simple crystal fractionation and/or crustal contamination of andesitic parent magma, but is consistent with the participation of different proportions of melts derived from subducted basalt and sediments. Sr/Y correlates horizontally with Rb/Y, and Pb/Nd correlates vertically with Nd isotopic composition, indicating that Sr and Pb budgets are strongly controlled by melt addition from the subducting slab, whereas positive correlations between Th/Nd and Pb/Nd, and Rb/Y and Nb/Y point to some contribution of sediment melt. In addition to low concentrations of heavy rare earth elements (~2–3 times chondrite), a systematic decrease in their concentrations and Nb/Ta ratios with increasing SiO2 contents suggests that slab partial melting occurred in the garnet stability field and that these elements were mobilized by fluid flux. These geochemical and isotopic signatures are best explained by slab breakoff and fusion shortly after the initiation of collision. Although the Late Micone Tavda?? rhyolite (8.75 Ma) has some geochemical features identical to adakites, such as high Sr/Y and La/Yb ratios, low Y and HREE concentrations, other requirements, such as sodic andesite and/or dacite with relatively high MgO and Mg# (>50), relatively high Ni and Cr, low K2O/Na2O (<0.4), high Sr (>400 ppm), for slab-derived adakites are not provided. It is sodic in composition and shows no traces of fractionation from dacitic parent magma. Low Nd and high Sr isotope ratios suggest derivation by partial fusion of calc-alkaline, juvenile crust with high Sr/Y and La/Yb ratios.  相似文献   

19.
A change in the liquidus mineralogy from plagioclase-quartz-biotite to plagioclase-quartz-K feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of and interrelationships between Ba, Rb, Sr, K2O and Na2O. During plagioclase fractionation, Ba and Rb enrich in successive solids and melt, while Sr is depleted. The K2O content of the solid phase (around 1%) is very different from that of the melt (greater than about 3%) while Na2O contents are similar (about 4–5%) so that variations in the amount of intercumulus melt result in wide variation in the Na2O/K2O ratio (from about 5 in cumulus-rich to about 1 in intercumulus-rich samples). The incoming of K feldspar as a cumulus phase causes Ba to be depleted along with Sr in successively formed solids, while Rb continues to be enriched. A pronounced compositional hiatus with respect to trace elements therefore results. The K2O and Na2O contents of melt and cumulate are now similar (around 5% K2O and 4% Na2O), so that little scatter in Na2O/K2O ratio (around 1) occurs as a result of variation in the amount of intercumulus melt. In general, trace element data from a natural example in the Barberton Mountain Land, South Africa, fit the models well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions—possibly the result of the non-uniform distribution of biotite in the samples.  相似文献   

20.
This paper reports petrography, geochemistry and Rb-Sr age data on the rare metal bearing Neoarchean fertile (Nb-Ta) granite at Allapatna and elucidates its petrogenesis and role in Nb-Ta-Li-Be mineralization. The Allapatna granite (AG) intrudes the Tonalitic-Trondhjemitic - Granodioritic (TTG) Peninsular Gneiss and analysed SiO2 (72.3-75.6 wt%), K2O (4.0-5.7wt%), Na2O (3.0-4.4wt%), CaO (0.7-1wt%), MgO (0.13-0.25wt%) and K2O/Na2O (>1) indicating evolved nature. The presence of muscovite, biotite and garnet in the mode, peraluminous nature and high initial 87Sr/86Sr ratio (0.7284±0.0083) attest to their S-type characteristics. Varying Nb/Ta ratio and high Li with moderate abundance of Cs further indicate affinity to Li-Cs-Ta (LCT) type granite-pegmatite system. TheAG showing whole rock Rb-Sr isochron age of 2803± 68 Ma, is the oldest reported fertile granite in India parental to rare metal pegmatites hosting Nb-Ta, Be, and Li resources. Partial melting of a mixed source consisting of both basement TTG rocks and metapelites has generated such type of granitic magma. Fractionation of such granitic magma possibly has given rise to the rare metal (Ta-Nb-Li-Be) bearing pegmatites intruding the nearby schist belt.  相似文献   

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