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Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma?1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events. 相似文献
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秦祁结合部位王家岔石英闪长岩体锆石LA-ICPMS定年及地质意义 总被引:3,自引:0,他引:3
出露于秦岭、祁连造山带结合部位的王家岔石英闪长岩体,侵入于变质基底陇山岩群之中。岩石学、岩石化学、地球化学特征显示石英闪长岩侵入体具有俯冲花岗岩特征。利用LA-ICPMS(激光剥蚀等离子体质谱)测年技术,对石英闪长岩进行单颗粒锆石微区U-Pb同位素测定,获得加权平均年龄为(454.7±1.7) Ma,代表了岩石的形成年龄。这一新的成果资料,对研究秦祁结合部位早古生代构造演化、造山过程及秦、祁造山带的关系具有重要意义。 相似文献
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Re-Os and U-Pb Geochronology of the Erlihe Pb-Zn Deposit,Qinling Orogenic Belt,Central China,and Constraints on Its Deposit Genesis 总被引:4,自引:0,他引:4
ZHANG Fan LIU Shuwen LI Qiugen SUN Yali WANG Zongqi YAN Quanren YAN Zhen Key Laboratory of Orogenic Belt Crustal Evolution Ministry of Education Peking University Beijing China Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou China Institute of Mineral Resources Chinese Academy of Geological Sciences Beijing China Institute of Geology China 《《地质学报》英文版》2011,(3)
The Erlihe Pb-Zn deposit is an important mine of the Pb-Zn metallogenic zone in the South Qinling Orogen.It has been considered a sedimentary exhalative deposit in previous investigations because the ore body occurs concordantly at the transitional location of an upright fold.Re and Os isotopic analyses for paragenetic pyrites with sphalerite and galena from the ore body have been used to determine the timing of mineralization and to trace the source of metallogenic materials.The Re-Os isotopic data of f... 相似文献
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本文通过对分布于胶西北地区南部的秦姑庵花岗岩体开展详细的岩石学、岩石地球化学、锆石UPb年代学和锆石LuHf同位素地球化学研究,识别出粗中粒二长花岗岩、斑状中粒二长花岗岩和细粒黑云母二长花岗岩,它们具有相同的地球化学特征和同位素组成,呈连续演化关系,为同源岩浆活动的产物。岩石地球化学研究表明,花岗岩具有高的SiO2(68.73%~75.27%)、全碱(K2O+Na2O=8.39%~8.93%),低的Al2O3(13.38%~15.84%)、MgO(0.14%~0.98%)、P2O5(0.02%~0.13%)、Sr(118×10-6~520×10-6)、Y(7.03×10-6~12.30×10-6)、Yb(0.78×10-6~1.34×10-6)含量特征,轻稀土(LREE)富集、重稀土(HREE)相对亏损(LREE/HREE=18.39~26.72),轻、重稀土元素分馏明显[(La/Yb)N=17.13~42.58],Eu中等负异常(δEu=0.49~0.76),富集大离子亲石元素(LILE),亏损高场强元素(HFSE)。锆石SHRIMP定年结果表明,斑状中粒二长花岗岩形成于(119.9±1.3)Ma,秦姑庵岩体形成于早白垩世晚期,时代上与胶东地区伟德山型花岗岩一致。锆石原位LuHf同位素分析表明,εHf(t)为24.07~19.27,二阶段模式年龄TDM2为2398~2696 Ma,岩浆源区主要来源于古元古代物质的部分熔融,少部分为新太古代基底岩石。岩体形成于加厚下地壳的下部,其残留相可能由斜长石+辉石+角闪石+石榴石组成,相当于麻粒岩相或石榴石麻粒岩相。花岗岩的同位素年龄与胶西北金矿床的成矿时代一致,指示120Ma左右的岩浆活动为金成矿提供了流体和热动力条件。 相似文献
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古亚洲洋通过板块俯冲而闭合后,形成规模巨大的中亚造山带,石尖山花岗岩体位于中亚造山带东段中部,是石尖山地区出露面积最大的侵入岩体,为了研究该地区构造演化特征,探讨该地区古亚洲洋的闭合时间,本次研究应用激光剥蚀-电感耦合等离子体质谱法,对石尖山花岗岩体进行了锆石U-Pb同位素测年,获得岩体同位素年龄(230.9±2.6)Ma和(231.5±2.5)Ma;同时,作者对该花岗岩体的地球化学特征及形成的构造环境进行了分析探讨,分析表明石尖山花岗岩形成于造山晚期-造山后过渡的拉伸环境。综合上述研究,石尖山花岗岩体为中三叠世结晶成岩,是造山活动末期的拉伸环境产物,反映了内蒙古石尖山地区古亚洲洋最可能闭合于晚二叠世—早三叠世。 相似文献
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内蒙古柴胡栏子金矿区断裂构造发育,岩浆岩类型复杂。本文在野外地质工作的基础上,通过锆石LA-ICP-MSU-Pb同位素年代学研究,得到了矿区岩浆岩的形成时代,并对海西期的岩浆活动进行了讨论。闪长玢岩样品中存在(2447±13)Ma(MSWD=0.0108)的继承性岩浆锆石和(296.6±3.5)Ma(MSWD=0.58)的结晶岩浆锆石。认为结晶锆石的年龄代表了闪长玢岩的形成年龄;(2447±13)Ma的继承性岩浆锆石的存在说明了闪长玢岩在侵位过程中存在壳源物质的重熔和加入。 相似文献
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对胶东崮庄岩体黑云母闪长岩进行了岩石学、岩石地球化学和年代学研究。研究表明:崮庄岩体黑云母闪长岩为钙碱性系列岩石,岩石富集大离子亲石元素(如K,Ba,Rb)、LREE和活泼的不相容元素(如Th,U),相对亏损高场强元素(如Nb,Ta,Ti,P),崮庄闪长岩属壳幔混合成因,即由交代地幔部分熔融形成的基性岩浆与地壳物质熔融形成的酸性岩浆混合形成的。构造环境判别显示崮庄闪长岩属火山弧花岗岩,暗示伟德山序列花岗岩与古太平洋板块向欧亚板块俯冲密切相关。崮庄闪长岩LA-ICP-MS锆石U-Pb年龄值为(111.7±0.6)Ma,属燕山期早白垩世。 相似文献