松辽盆地北部上二叠统林西组古生物年代学、地球化学特征及地质意义     

张海华  李晓海  张健  郑月娟  陈树旺  张德军  苏飞  卞雄飞  孙雷 《现代地质》2021,35(2):568-578

对松辽盆地北部地区黑富地1井钻遇的上二叠统林西组碎屑岩样品进行微体化石以及主量元素、微量元素和稀土元素测试分析,进而探讨该区林西组时代、地球化学特征、沉积环境及构造背景。获得的Protohaploxypinus fertilis-Piceaepollenites opimus-Alisporites communis孢粉化石组合表明地层时代为晚二叠世。样品主量元素平均质量分数:SiO₂为60.41%,CaO为3.65%,Al₂O₃为16.36%,K₂O为2.69%, Na₂O为3.25%, K₂O/Na₂O值介于0.40~7.01之间,A1₂O₃/(CaO+Na₂O)值介于0.92~6.89之间。稀土元素ΣREE为63.20×10^-6~208.05×10^-6,平均值147.56×10^-6;δEu介于0.56~1.34之间,平均为0.73,绝大部分表现为Eu负异常;δCe介于0.90~0.98之间,为弱的Ce负异常;呈现LREE富集、HREE亏损。微量元素以亏损Nb、Ta、Sr和相对较富集Rb、Ba、La、Sm、Pb、Nd为特征。碎屑岩元素分析和物源La/Th-Hf、TiO₂-Ni等判别图表明,林西组具有多样性沉积物源,其碎屑岩沉积物主要源于长英质物质,少量来自基性岩。样品稀土元素标准化配分模式图、微量元素标准化蛛网图、K₂O/Na₂O-SiO₂、构造背景判别函数(F₁-F₂)等分析结果及地球化学参数对比表明,松辽盆地北部黑富地1井林西组物源区构造背景主要为活动大陆边缘和大陆岛弧,推测为形成于靠近大陆岛弧的活动大陆边缘构造背景,所形成的沉积环境应为海陆过渡相逐渐转变为陆相。  相似文献   

松辽盆地北部上二叠统林西组地球化学特征及地质意义     

张健  张海华  陈树旺  郑月娟  张德军  苏飞  黄欣 《吉林大学学报(地球科学版)》2020,50(2):518-530

为探讨松辽盆地北部林西组地球化学特征、沉积环境及构造背景，对该区黑富地1井所钻遇的上二叠统林西组暗色泥质板岩进行主量元素、微量元素和稀土元素测试分析，结果表明：样品主量元素平均质量分数SiO₂为65.05%、Al₂O₃为17.23%、CaO为2.58%、Na₂O为2.84%、K₂O为3.29%，K₂O/Na₂O值为0.20~3.48，A1₂O₃/（CaO+Na₂O）值为0.41~8.01；稀土元素w（∑REE）值介于80.68×10^-6~215.59×10^-6之间，平均值为173.41×10^-6，δEu值介于0.61~0.88之间，平均值为0.70，表现为Eu的负异常，δCe值介于0.91×10^-6~1.01×10^-6之间，为微弱的Ce负异常，LREE富集，HREE亏损；微量元素以亏损Nb、Ta、Sr，相对富集Rb、Ba、La、Ce、Pb、Nd、Sm为特征。样品元素分析和物源w（TiO₂）-w（Ni）、La/Th-w（Hf）等判别图解表明，林西组沉积物源具有多样性，主要来源于上地壳长英质物源，还有少量来自中、基性岩火成岩物源区。样品微量元素标准化蛛网图，稀土元素球粒陨石标准化配分模式图，K₂O/Na₂O-w（SiO₂）、Th-Co-Zr/10、F₂-F₁构造背景判别图及地球化学参数对比结果表明，松辽盆地北部林西组物源构造背景主要为活动大陆边缘和大陆岛弧，可能形成于靠近大陆岛弧的活动大陆边缘构造背景。林西组应形成于海陆过渡相到陆相环境，水体由半咸水逐渐转变为淡水。  相似文献   

贺兰山—六盘山地区中侏罗统直罗组地球化学特征及其地质意义^*          下载免费PDF全文

罗伟  刘池洋  张东东  王建强  牛海青  郭佩 《古地理学报》2016,18(6):1030-1043

贺兰山—六盘山地区是研究华北板块西部大地构造背景的关键区域。在研究区内中侏罗统直罗组地层采集了20件样品,应用全岩分析和ICP-MS方法对采集的样品进行主、微量及稀土元素测试并对其地球化学特征做出分析。测试结果显示,样品中Al₂O₃/SiO₂值介于0.17~0.31之间,平均值为0.25;K₂O/Na₂O值介于0.77~73.77之间,平均值为9.78;Al₂O₃ /(Na₂O+CaO)值介于0.49~96.00之间,平均值为15.68。上述含量和比值与样品成分成熟度、元素迁移、热液流体影响和风化程度有关。部分元素之间存在良好的相关性,能够反映物源及环境的变化。稀土元素值总和(ΣREE)变化较大(132.16~394.47μg·g^-1),其球粒陨石标准化配分模式与大陆上地壳(UCC)极为相似,表现为轻稀土富集、重稀土平坦、Eu负异常,Ce异常不明显等特征。结合UCC标准化曲线、La/Yb值、Gd_N/Yb_N值、La/Th值和Hf含量,认为直罗组源岩主要为长英质岩石,混杂了少量古老沉积岩和基性岩或者特殊矿物,且后太古界地层提供主要物源。微量元素构造背景判别图揭示研究区中侏罗统直罗组地层的物源区具有大陆岛弧和活动大陆边缘的特征,物源可能来自于兴蒙造山带和祁连造山带。化学蚀变指数(CIA)值和A-CN-K图反映源岩经历了较强的风化作用,但不同区域之间的风化程度存在差异。  相似文献   

鄂尔多斯盆地延长组长7段沉积岩元素地球化学特征及沉积环境分析     

马奂奂  刘池洋  张龙  张东东  王文青  赵岩  高明  全晓园 《现代地质》2019,33(4):872-882

沉积岩的地球化学特征可以反映盆地沉积环境。为了探讨鄂尔多斯盆地延长组沉积时期古气候、氧化还原条件、古盐度等特征,对鄂尔多斯盆地东南部铜川延长组长7段的5个砂岩、8个页岩样品进行主量、微量以及稀土元素地球化学分析。结果显示,长7段细粒沉积物Al₂O₃/SiO₂值与K₂O/Na₂O值指示样品成分成熟度差别不大,黏土矿物含量较高。∑REE平均值为132.65×10^-6,明显低于北美页岩平均稀土元素含量,也低于全球平均大陆上地壳值;LREE分异程度较高,呈微弱的Eu正异常,无Ce异常。“C值”、Sr/Cu、Rb/Sr、Mg/Ca等指标显示延长组沉积期整体处于温暖潮湿的气候区。研究区样品富集微量元素U、Sc、Cs(平均20.11×10^-6、9.66×10^-6、6.08×10^-6),结合V、δU、Th/U、V/Cr、V/Sc、V/(V+Ni)等指标分析,结果指示延长组沉积期以底层水体呈强分层的还原环境为主。Li、Sr、Ni、Sr/Ba、(MgO/Al₂O₃)×100等值显示沉积期水体总体为淡水-微咸水。  相似文献   

北祁连山镜铁山地区镜铁山群岩石地球化学特征及其地质意义     

赵建仓  吴雯辉  何谋惷  丁振举  胡新露  宫勇军 《地质论评》2023,69(2):727-746

镜铁山铁(铜)矿床是我国西北地区发现和勘探最早的大型铁(铜)矿床。对于铁矿床的成因,目前还存在不同的观点。笔者等对镜铁山铁铜矿床赋矿地层开展了岩石学和岩石化学成分分析,结果显示：变质石英砂岩的SiO₂平均含量为96.95%、Al₂O₃平均为1.36%、Fe₂O₃平均为0.31%,SiO₂/Al₂O₃平均值为73.26,K₂O/Na₂O平均值为12.54;除Sn以外,其他微量元素平均含量均低于后太古宙澳大利亚页岩(PAAS)值,稀土元素总量显著低于PAAS值,具有弱的Eu正异常,Ce异常不明显;铁碧玉岩SiO₂平均含量为85.67%、Fe₂O₃平均含量为10.19%、SiO₂/Al₂O₃平均为430.47、K₂O/Na<...  相似文献   

四川省西昌盆地上三叠统白果湾组地球化学特征及其意义   总被引：2，自引：2，他引：0       下载免费PDF全文

白宪洲  文龙  王玉婷  伏国通 《中国地质调查》2017,4(2):51-58

对四川省西昌盆地上三叠统白果湾组碎屑岩的主量元素、微量元素、稀土元素进行综合研究,得出其主量元素具有富SiO₂、CaO、P₂O₅,贫Al₂O₃、TFe₂O₃、MgO、Na₂O、K₂O的特征; 微量元素具有富Zr、Cs、Th、U等特征; 稀土元素总量相对较高,轻重稀土元素分馏明显,轻稀土较重稀土相对富集,具有中等负铕异常。白果湾组物源区为长英质岩区,属于被动大陆边缘区; 白果湾期的气候经历了湿润→干旱→湿润→干旱的变化过程; 成分成熟度总体偏低,呈现高→低→高→低的变化规律,与CIA变化趋势呈负相关。本次研究为该区晚三叠世环境演化研究提供了基础资料。  相似文献   

下扬子地区下寒武统幕府山组黑色岩系地球化学特征及其地质意义     

侯阳红  康志宏  赵晨君  余旭东  王恩博 《沉积学报》2020,38(4):886-897

为探讨下扬子地区下寒武统幕府山组底部富有机质黑色岩系的物源属性及源区构造背景特征，在南京幕府山剖面幕府山组底部系统的采集样品，并对其进行地球化学测试与分析。Th/Sc?Zr/Sc图解表明幕府山组黑色岩系未曾经历沉积再循环；通过分析主量元素比值特征、稀土元素（REE）配分模式、δEu、（La/Yb）_N及La/Th?Hf和La/Yb?∑REE判别图解，认为幕府山组黑色岩系源岩主要是花岗岩和富含长英质矿物的沉积岩等上地壳长英质岩石，存在基性岩的混入。从物源属性、地质年代关系、华南大陆构造演化历程等多方面考虑，源岩中花岗岩成分主要为新元古时期岩浆活动形成的岩浆岩，物源区为江南古陆（江南造山带）。通过分析K₂O+Na₂O?SiO₂、K₂O/Na₂O?SiO₂/Al₂O₃、La?Th?Sc、Th?Co?Zr/10和Th?Sc?Zr/10图解，认为幕府山组黑色岩系源区构造背景为被动大陆边缘。  相似文献   

甘肃景泰崔家墩下奥陶统阴沟组砂岩化学组分特征及物源区构造背景判别   总被引：2，自引：0，他引：2  

杨江海  杜远生  朱杰 《古地理学报》2007,9(2):197-206

甘肃景泰一带早奥陶世沉积了以浊流砂岩夹板岩为主的阴沟组。崔家墩剖面阴沟组较为发育，总体分为两个岩性段，但东西向逆冲断层造成地层垂向不连续。北段（下段）为粉-细砂岩、板岩夹硅质岩；南段（上段）为岩屑含量较多的岩屑砂岩夹板岩，硅质岩少或无。对阴沟组砂岩进行主化学组分特征分析，其特征指标：Fe₂O*3+MgO值为503%～17.63%，Al₂O₃/SiO₂值为0.12%～0.28%，K₂O/Na₂O值为0.25%～1.26%及 Al₂O₃/（CaO+Na₂O）值为1.01%～3.53%。利用砂岩的化学组分特征结合岩相学分析，可以判断物源区特征及大地构造背景。研究表明:该套浊积砂岩下部物源主要来自北部的阿拉善地块及该地块上的古老造山带，为被动大陆边缘背景；上部碎屑主要来自南侧岛弧中基性火山岩，为活动大陆边缘初始弧后盆地构造背景。  相似文献   

皖北新元古代刘老碑组页岩的地球化学特征及其地质意义   总被引：16，自引：0，他引：16  

李双应  岳书仓  杨建  贾志海 《地质科学》2003,38(2):241-253

从安徽北部新元古代刘老碑组页(泥)岩中采集了11个样品。地球化学分析表明,研究区南部的淮南和凤阳页岩样品SiO₂、TiO₂、Al₂O₃、K₂O、TFeO和CaO含量分别为55.28%、0.87%、15.60%、2.99%、4.01%和5.83%,接近页岩平均值,只是CaO含量稍高。北部宿县地区页岩样品SiO₂、TiO₂、Al₂O₃、K₂O、TFeO和CaO含量分别为57.01%、1.82%、20.97%、4.85%、3.31%和0.57%,与南部地区相比,TiO₂和K₂O含量显著偏高,是其2.1倍和1.6倍。刘老碑组页岩REE总量平均为219×10^-6,LREE富集,La/Yb为14.2(9.8～18.6),Eu负异常,Eu/Eu^*为0.60(0.54～0.63),但是北部页岩有更高的REE总量(295.8×10^-6)和La/Yb比值(17.8)。微量元素分析表明,北部页岩中Sc、V、Zr、Nb和Th较高,南部Co和Pb较高。但是南部和北部却有比较一致的La/Sc(2.29±0.15)、Th/Sc(0.69±0.06)和La/Th(3.29±0.20)比值。La-Th-Sc和Th-Sc-Zr/10判别图清楚地表明,刘老碑组页岩形成于大陆岛弧构造环境,岛弧可能位于研究区的南部边缘。北部页岩明显偏高的TiO₂和Al₂O₃含量以及化学风化指数(CIW)表明北部沉积物的源区经历了更强烈的风化作用。  相似文献   

小兴安岭北部八里桥地区碱长花岗岩成因与地质意义     

苏航  刘旭光  郑涛  刘颖新 《矿物学报》2023,(5):665-673

小兴安岭八里桥地区位于兴蒙造山带东部兴安地块与松嫩—张广才岭之间的构造带上,区内构造演化复杂,岩浆活动频繁。本文通过对八里桥地区晚古生代碱长花岗岩的年代学和岩石地球化学研究,旨在为兴安地块与松嫩—张广才岭地块碰撞拼合时限提供新的约束。LA-ICP-MS锆石U-Pb定年结果显示,八里桥地区发育的中粒碱长花岗岩形成于(295.3±0.7) Ma,属早二叠世侵入体。碱长花岗岩w(SiO₂)为75.27%～77.71%,w(Al₂O₃)为11.48%～13.28%,w(Na₂O+K₂O)为6.96%～8.72%,Na₂O/K₂O比值为0.27～1.05,A/CNK值在1.00～1.51之间,属过铝质、高钾钙碱性系列岩石。稀土元素总量为116.46×10^-6～321.52×10^-6,ΣLREE/ΣHREE和(La/Yb)_N比值分别为3.07～9.65和1.70～6.91,显示轻稀...  相似文献   

北京周边二叠系山西组泥岩地球化学特征及其地质意义     

刘清俊  林海亮  柯柏林  林天懿  尹腾宇  付昌鸿  赵连海  邢罡 《岩石矿物学杂志》2014,33(3):491-502

对采自北京周边的二叠系山西组露头及钻井泥岩样品进行了岩石学和地球化学研究,结果表明：岩石主要由粘土矿物和石英组成,兼有少量碳酸盐岩和长石;岩石中碳酸盐岩含量的高低与CaO、MgO含量有很好的对应关系;SiO₂、Na₂O和K₂O亏损,TiO₂和Fe₂O₃^T富集,反映了基性物源的存在;微量元素特征比值显示源区母岩为非单一物源,稀土元素总量较高,轻稀土元素富集,重稀土元素平坦,铕大部分具明显负异常、少部分微弱正异常,Ce基本无异常,显示为多物源,各样品稀土元素配分模式与大陆上地壳一致,显示了沉积物具有同源性。源岩应为来自于华北地块北缘的大陆上地壳的沉积岩、花岗岩和碱性玄武岩的混合。二叠系山西组泥岩沉积于覆水较深的还原环境,源区构造背景为大陆岛弧与活动大陆边缘。  相似文献   

羌北-昌都地块那益雄组火山岩锆石U-Pb年代学及地球化学特征          下载免费PDF全文

张锦  程鑫  高扬  李玉玉  朱亚林  周亚楠  吴汉宁 《岩石矿物学杂志》2019,38(1):77-89

本文对青藏高原羌北-昌都地块阿布日阿加措地区的晚二叠世那益雄组火山岩进行了年代学和地球化学研究。该火山岩主要由玄武岩、安山玄武岩、安山岩、流纹岩、凝灰岩组成,具有碱性玄武岩到酸性熔岩的特征。锆石U-Pb年代学研究表明该火山岩的形成时代为251. 1±4. 8～249. 6±1. 3 Ma之间。地球化学分析结果表明,该地区的流纹岩具有高的Si O2(74. 85%～77. 55%)和Na2O+K2O(5. 40%～6. 61%)含量,较低的MgO、K2O和Ca O含量,Al2O3含量低且稳定,里特曼指数平均为1. 15,小于3. 3。安山岩Si O2含量55. 13%～56. 28%,Na2O+K2O含量4. 13%～6. 15%,里特曼指数平均为2. 20,小于3. 3,属于钙碱性安山岩。碱性玄武岩Si O2含量51. 49%,Na2O+K2O含量6. 34%,里特曼指数为4. 73,属于碱性系列。稀土元素配分曲线为富集LREE的右倾型。另外,富集大离子亲石元素(LILE) Th、U,亏损高场强元素(HFSE) Nb、Ta等特征,均说明羌北-昌都地块阿布日阿加措地区的火山岩形成于陆缘岛弧环境。  相似文献   

Andesites from northeastern Kanaga Island,Aleutians   总被引：1，自引：0，他引：1  

James G. Brophy 《Contributions to Mineralogy and Petrology》1990,104(5):568-581

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO₂ contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO₂ content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO₂ contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO₂ contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO₂ content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO₂) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO₂ that increase the thickness and lowers the overall SiO₂ content of the upper silicic zone. It is suggested that the maximum SiO₂ content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.  相似文献   

藏北阿里地区改则县雀康一带纳丁措组火山岩锆石U-Pb年龄和地球化学特征   总被引：2，自引：2，他引：0       下载免费PDF全文

褚慧力 《中国地质调查》2016,3(6):37-48

纳丁措组火山岩分布于班公湖—怒江结合带以北,属于羌塘—三江复合板片内多玛地层分区雀康地层小区的火山岩地层,代表了羌南地区古近纪的火山活动,为一套粗安岩-粗面岩的岩石组合。在地层区采集了3件岩石年龄样品,测年结果分别为(35.99±0.29) Ma、(37.2±0.65) Ma、(35.23±0.37) Ma。样品中SiO₂含量均较高,介于58.7%~65.51%之间; 铝饱和指数A/NFC为0.046~1.029,普遍较低,显示偏铝岩石的特性; MgO含量普遍略高,介于1.10%~2.95%之间。稀土元素(rare earth element,REE)特征显示,样品中REE总量为243.38×10^-6~313.48×10^-6,说明岩石属于稀土元素偏高型,其中∑[w(Ce)]/∑[w(Y)]为15.24~20.61,属于轻稀土富集型; [w(La)/w(Yb)]_N为26.34~52.01,说明轻、重稀土分异明显; δEu为0.77~0.91,属于弱负异常。Rb、Th等大离子亲石元素富集度高,而Hf和重稀土元素等高场强元素丰度值较低,绝大部分样品分析结果显示Ti、Ta、Nb亏损明显,w(La)/ w(Sm)值较为稳定,w(Sm)/ w(Yb)为4.3~6.7,比值较低。根据岩石地球化学特征,判断纳丁措组火山岩原始岩浆来源于青藏高原加厚陆壳之下与岩石圈地幔之上的特殊壳幔过渡带,形成环境为造山带附近的大陆拉张的板内初始裂谷环境。  相似文献   

Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough Island lavas: variation with fractional crystallization and evidence for assimilation     

Chris Harris  H. Stuart Smith  Anton P. le Roex 《Contributions to Mineralogy and Petrology》2000,138(2):164-175

We have measured the δ¹⁸O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore, prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the δ¹⁸O values of olivine, pyroxene and feldspar in the Gough lavas show that the phenocrysts in the mafic Gough Island lavas are in oxygen isotope equilibrium. The olivines in lavas with SiO₂ <50 wt% have a mean δ¹⁸O value of 5.19‰, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts in all the Gough lavas show a systematic increase in δ¹⁸O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment in δ¹⁸O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with SiO₂ >48 wt% contain phenocrysts which have δ¹⁸O values that are systematically ∼0.3‰ lower than their counterparts from Gough. We suggest that the parental mafic Tristan magmas were contaminated by material from the volcanic edifice that acquired low δ¹⁸O values by interaction with water at high temperatures. The highly porphyritic SiO₂-poor lavas show a negative correlation between olivine δ¹⁸O value and whole-rock silica content rather than the expected positive correlation. The minimum δ¹⁸O value occurs at an SiO₂ content of about 45 wt%. Below 45 wt% SiO₂, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt% SiO₂, magmas appeared to have evolved via closed-system fractional crystallization. Received: 23 November 1998 / Accepted: 27 September 1999  相似文献   

Mid-Pleistocene lavas from the Seguam volcanic center,central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite   总被引：7，自引：0，他引：7  

Bradley S. Singer  James D. Myers  Carol D. Frost 《Contributions to Mineralogy and Petrology》1992,110(1):87-112

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO^*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at 1160°C between about 3–5 kb (±1–2% H₂O), andesites crystallized Plag+Cpx+Opx±Mt at 1000°C between 3–4 kb with 3–5% H₂O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H₂O. All lavas crystallized at f _{o 2} close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al₂O₃, Cr, Ni, and Sr abundances decrease and K₂O, Na₂O, Rb, Ba, Zr, and Pb increase with increasing SiO₂ (50–71%). LREE enrichment [(Ce/Yb)_n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO₂. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K₂O, Na₂O, TiO₂, Rb, Ba, Sr, Zr, Y, and LREE abundances. ⁸⁷Sr/⁸⁶Sr ratios (0.70361–0.70375) and ratios of ²⁰⁶Pb/²⁰⁴Pb (18.88–18.97), ²⁰⁷Pb/²⁰⁴Pb (15.58–15.62), ²⁰⁸Pb/²⁰⁴Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, Nd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.  相似文献   

Late Vendian unfolded Molasse in the northeastern, eastern, and southwestern peripheries of the east European platform: Distinctive lithogeochemical features     

A. V. Maslov  V. P. Podkovyrov 《Geochemistry International》2013,51(1):44-64

The paper reports data on the lithogeochemistry of sandstones and silty mudstones from Upper Vendian sedimentary sequences in the northeastern, eastern, and southwestern peripheries of the East European Platform belonging to the so-called unfolded molasse. The sequences are dominated by wackes, arkoses, subarkoses, litharenites, and sublitharenites, i.e., chemically immature and moderately mature psammites, and can be classed with rocks produced by clastic material brought from orogens surrounding the platform. The higher TiO₂, Al₂O₃, FeO_tot, MgO, Na₂O, and K₂O concentrations of the psammites than those in the average cratonic Phanerozoic sandstone (APhSa) testify that the chemical maturing of the rocks was not completed. The silty mudstones accompanying the sandstones have a composition closer to those of the average cratonic Phanerozoic shale (APhSh), but this is likely explained by the fact that the rocks were produced of material brought from erosion territories of much greater area. The lithogeochemical data generally indicate that these territories were dominated by acid and intermediate magmatic rocks with variable fraction of sedimentary rocks when the Late Vendian sedimentary associations in question were produced. The distribution of certain indicator trace elements in the sandstones and silty mudstones show that the average composition of the eroded complexes was close to the composition of the post-Archean upper continental crust, but the erosion areas occasionally (in the Vychegorskii trough and the Shkapovsko-Shikhanskaya depression) contained relatively primitive source rocks. The data points of the great majority of the sandstones and silty mudstones plot in the SiO₂-K₂O/Na₂O and F1–F2 diagrams in the fields of sediments typical of the environments of active continental margins, which is consistent with the arrangement of the data points of these rocks in the La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 diagrams. All of these features confirm that the sedimentary rocks in question affiliate with rock associations produced at an active tectonic regime.  相似文献   

The Geochemical and Zircon Trace Elements Characteristics of A-type Granitoids in Boziguoer,Baicheng County,Xinjiang   总被引：1，自引：0，他引：1  

LIU Chunhu  YIN Jingwu  WU Cailai  SHAO Xingkun  YANG Haitao  XU Haiming  WANG Jun 《《地质学报》英文版》2013,87(6):1585-1603

The Boziguoer A-type granitoids in Baicheng County,Xinjiang,belong to the northern margin of the Tarim platform as well as the neighboring EW-oriented alkaline intrusive rocks.The rocks comprise an aegirine or arfvedsonite quartz alkali feldspar syenite,an aegirine or arfvedsonite alkali feldspar granite,and a biotite alkali feldspar syenite.The major rock-forming minerals are albite,K-feldspar,quartz,arfvedsonite,aegirine,and siderophyllite.The accessory minerals are mainly zircon,pyrochlore,thorite,fluorite,monazite,bastnaesite,xenotime,and astrophyllite.The chemical composition of the alkaline granitoids show that SiO2 varies from 64.55％ to 72.29％ with a mean value of 67.32％,Na2O＋K2O is high （9.85％-11.87％） with a mean of 11.14％,K2O is 2.39％-5.47％ （mean =4.73％）,the K2O/Na2O ratios are 0.31-0.96,Al2O3 ranges from 12.58％ to 15.44％,and total FeOT is between 2.35％ and 5.65％.CaO,MgO,MnO,and TiO2 are low.The REE content is high and the total SREE is （263-1219） ppm （mean =776 ppm）,showing LREE enrichment and HREE depletion with strong negative Eu anomalies.In addition,the chondrite-normalized REE patterns of the alkaline granitoids belong to the ＂seagull＂ pattern of the right-type.The Zr content is （113-1246） ppm （mean =594 ppm）,Zr＋Nb＋Ce＋Y is between （478-2203） ppm with a mean of 1362 ppm.Furthermore,the alkaline granitoids have high HFSE （Ga,Nb,Ta,Zr,and Hf） content and low LILE （Ba,K,and Sr） content.The Nb/Ta ratio varies from 7.23 to 32.59 （mean =16.59） and the Zr/Hf ratio is 16.69-58.04 （mean =36.80）.The zircons are depleted in LREE and enriched in HREE.The chondrite-normalized REE patterns of the zircons are of the ＂seagull＂ pattern of the left-inclined type with strong negative Eu anomaly and without a Ce anomaly.The Boziguoer A-type granitoids share similar features with A1-type granites.The average temperature of the granitic magma was estimated at 832-839℃.The Boziguoer A-type granitoids show crust-mantle mixing and may have formed in an anorogenic intraplate tectonic setting under high-temperature,anhydrous,and low oxygen fugacity conditions.  相似文献   

Petrochemical and thermodynamic evidence on the origin of kimberlites     

L. L. Perchuk  V. I. Vaganov 《Contributions to Mineralogy and Petrology》1980,72(2):219-228

The petrochemistry of kimberlites from Yakutia and Lesotho has been studied using a silicate melt model with the SiO₂, CO₂ and H₂O derivatives as the main anions.A model has been developed, according to which the dissolution of H₂O in an ultramafic melt results in orthosilicates (H₂SiC ₄ ^-2 , H₃SiO ₄ ^– , H₄SiO₄ etc.) rather than metasilicates, while the dissolution of CO₂ produces additional hydrocarbonate complexes. It suggests that at high PCO ₂ ¹ , and where the orthosilicic calcium salt clusters are likely to be present in the magma, the kimberlite melt can break down into carbonate and silicate liquids. Therefore, the composition of kimberlite magma will be determined by the H₂O/CO₂ ratio under the relatively constant fluid pressure. This can be seen from the distinct fluidrs trend in the H₂O-CO₂-SiO₂ diagram for the Yakutia and Lesotho diamond-bearing kimberlites. The H₂O/CO₂ ratio changes with the liquidus temperature along this trend (Perchuk and Vaganov 1977) which suggests that liquid immiscibility predominates over the simple CO₂ solubility in the melts of kimberlite composition. The well-known Boyd's diagrams for the equilibrium PT-conditions in peridotites have been applied along with new experimental data to natural Cpx and Opx, and the PT-parameters were correlated for peridotite inclusions in kimberlite pipes in Yakutia and Lesotho. The liquidus temperatures for the extrapolated area of these correlations gave depths (pressures) at which kimberlite magmas are formed (200–250 km).The hypothesis on SiO₂ partitioning between the melt and the fluid was used to calculate the composition of dry initial kimberlite which characterised the average mantle composition: SiO₂ — 45.12; TiO₂ — 2.49; Al₂O₃ — 3.58; Cr₂O₃ — 0.12; FeO — 9.32; MnO — 0.16; CoO — 0.11; MgO — 23.47; CaO — 13.44; Na₂O — 0.20; K₂O — 1.12; P₂O₅ — 0.69; S — 0.18; sum — 100 wt.%. This kimberlite is close to wehrlite in composition.  相似文献