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晚古生代伊犁地块北缘岩浆岩的成因及地质意义
王祥松
学位类型硕士
导师蔡克大
2017-05-01
学位授予单位中国科学院大学
学位授予地点新疆乌鲁木齐
学位专业工程硕士
关键词伊犁地块 中亚造山带 块体聚合 前进式造山 后撤式造山 陆壳生长 Yili Block Central Asian Orogenic Belt terrane assembly advancing orogeny retreating orogeny continental crust growth
摘要The Central Asian Orogenic Belt is one of the typical accretionary orogens on theEarth, and represents the major site of the continental crust growth during the Phanerozoic.The Central Asian Orogenic Belt can be devided into three collage systems, i.g., Siberiaarctive continental margin collage systems, Tarim and North China cratons collage systems,and Kazakhstan collage system. The Kazakhstan collage system is located at thecombinative region between the other two collage systems, and is characterized by thetypical oroclinal bending structure. The northern Yili Block is situated inboard in thesouthern limb of Kazakhstan collage system with numerous igneous rocks and ore deposits,and thus retains a record of the bending processes of the Kazakhstan Orocline and theassociated evolution of the Junggar Ocean in the late Paleozoic. However, the latePaleozoic evolution of Junggar ocean, the terrane assembly process and continental crustgrowth in the northern Yili Block have been poorly constrained. This thesis focuses on theLate Paleozoic igneous rocks from the Borohoro and Alataw area in the northern YiliBlock. Detailed geological field investigations combined with petrology, mineralogy,geochronlogy, major and trace element, Sr-Nd-Hf-O isotope data are reported in this study,for the sake of investigating the petrogensis of these rocks, their temporal-spatialdistribution and associated geodynamic processes, identify the tectonic evolution andimplication for continental crust growth and formation of ore deposits. The majorconclusions are summarized as follow:(1)These granitoids are mainly monzogranite, K-feldspar granite and granodiorite.On the basis of the chronological study, these samples are subdivided into Group 1 with theage of the Late Devonian (374-369 Ma) and Group 2 with the age of the LateCarboniferous (ca.304 Ma). They have similar major and trace elements contents, butdistinct Sr-Nd-Hf isotopic compositions. Most of these granitoids show high-Kcalc-alkaline and metalumious affinities, and enriched in large ion lithophile elements(LILE) and depleted in high field strength elements (HFSE), and contain low Sr/Y(3.4-20.7) and Sr (96-431 ppm) values, indicating typical arc geochemical affinities. They have variable SiO2 (62.6-72.9 wt.%) and Fe2O3T (2.54-5.21 wt.%) contents, and Mg#(33-51) values. The Group 1 samples display slightly higher initial 87Sr/86Sr ratios(0.7075-0.7089) but lower εNd(t) (+0.3 to +0.9) values than those of Group 2 samples(87Sr/86Sr(i)= 0.7060-0.7066 and εNd(t)= +1.7 to +2.0). The Group 1 samples show the lowerεHf(t) (-3.62 to +6.79) values and older TCDM (0.90-1.61 Ga) than those of Group 2 samples(εHf(t) of +1.21 to +11.3 and TCDM of 0.56 to 1.25 Ga). These features suggest that thegranitoids were derived from a hybridized source of a juvenile materials (includecrust-derived and mantle-derived magmas) and an old metamorphic component, while theGroup 2 samples have greater contribution of juvenile materials.(2)The Nd-Hf isotopic compositions of granitic rocks from northern Yili Blockshow comparable zircon εHf(t)-time and whole-rock εNd(t)-time patterns with an abruptchange at ca. 360 Ma. During ca. 400-360 Ma igneous isotopic features are characterizedby a large spread of zircon εHf(t) values (-12 to +10) and whole-rock εNd(t) values (-5 to+5), and an overall decreasing trend with time, whereas ca.360-300 Ma igneous isotopicfeatures have mostly positive zircon εHf(t) values (+2 to +12) and whole-rock εNd(t) values(-2 to +8) and gradually increasing with time, suggesting a subduction tectonic switch froman advancing subduction of the Junggar oceanic plate to the retreating subduction wasidentified occurred at ca.360 Ma.(3)Combing the data from this study and published in the study area, showing thatgranitic rocks have high whole-rock εNd(t) and zircon εHf(t) values with the affinity ofdepleted source, which indicate the continental crust growth event. In the Hf isotopesystem, the modeling results show that during the 400-300 Ma, about 15-100 % juvenilematerials with varying degrees involved in the formation of granitic rocks, while in the Ndisotope system, the modeling results show that about 5-60 % juvenile materials withvarying degrees involved in the formation of granitic rocks during the 400-300 Ma.(4)For the ca. 300 Ma volcanism, the ignimbrites have low SiO2 (65.8-71.5 wt.%)and Mg# (6-13) values, and exhibit arc affinity with significantly enrichment of large ionlithophile elements (LILE) and depletement of high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock εNd(t) and zircon εHf(t) values range from +6.9 to +7.0 and+9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyoliteshave slightly high SiO2 (72.7-74.0 wt.%) and K2O (3.86-4.53 wt.%) contents, high zirconδ18O (11.67-13.23‰) values, and low whole-rock εNd(t) (+2.9 to +3.8) and zircon εHf(t)(+2.8 to +10.0) values, which may suggest sediment involvements during magmageneration. In contrast, for the ca. 288 Ma plutonism, the biotite granites have obviouslyhigher SiO2 (74.7-75.5 wt.%) contents and whole-rock εNd(t) (+7.7 to +8.8), zircon εHf(t)(+9.8 to +12.7), and lower zircon δ18O (5.99-6.84‰) values, than those of the ca. 300 Mavolcanic rocks, which are consistent with signatures of juvenile magma source.(5)The ca.300 Ma ignimbrites and rhyolites have high zircon whole-rock saturationtemperature (888-938℃ and 815-832℃), and high εHf(t) values, indicating an hightemperature magmatic event at ca. 300 Ma, we interpret this magmatic episode having alink with arc-continent collision/amalgamation of the West Junggar Terrane and Yili Block,which intrigued the asthenospheric upwelling through slab break-off. The ca.288 Mabiotite granites have low zircon whole-rock saturation temperature (723-735℃), withresidual amphibole in the source. Thus, we infer the subsequent strike-slipping of NorthTianshan Fault that facilitated introduction of water-fluxes triggering hydrous partialmelting of juvenile lower crust.
其他摘要中亚造山带以显著的大陆地壳生长为主要特色,是全球典型的增生型造山带。目前的研究将中亚造山带划分为三大拼贴系统,即北部的西伯利亚南缘拼贴系统、南部的塔里木-华北北缘拼贴系统、以及介于二者之间的哈萨克斯坦拼贴系统。哈萨克斯坦拼贴系统位于关键的构造结合部位,发育典型的山弯构造。伊犁地块北缘位于哈萨克斯坦山弯构造南翼的内侧,出露大量的晚古生代岩浆岩,并伴生多金属矿床,这些岩浆-成矿事件是准噶尔地体与伊犁地块汇聚过程中的构造产物。然而,鉴于本区构造岩浆事件研究程度较低,有关晚古生代准噶尔洋的演化历史、伊犁地块北缘地体汇聚拼贴过程以及大陆地壳生长尚存在众多科学问题。本论文选择伊犁地块北缘博罗科努地区和阿拉套地区的晚古生代岩浆岩为研究对象,在详细野外地质考察工作的基础上,综合岩相矿物学、同位素年代学、全岩地球化学和同位素地球化学(Sr-Nd-Hf-O)等方面的研究成果,探讨了研究区岩浆岩的成因、时空分布与演化历史,揭示了岩浆作用的大地构造背景,以及对大陆地壳生长和成矿作用的指示意义。本论文取得了以下主要认识:(1)伊犁地块北缘博罗科努地区花岗岩类 SIMS 锆石 U-Pb 年代学研究显示:呼斯特花岗岩体、吉布克花岗岩体和冬图劲花岗岩体形成的时代分别为 374-369 Ma,303 Ma 和 304 Ma,指示两期岩浆事件。这些花岗岩体岩性主要分为二长花岗岩、钾长花岗岩以及花岗闪长岩,并有大量的基性包体产出。花岗岩类样品具有相似的主、微量元素含量,但 Nd-Hf 同位素组分表现显著差异性。花岗岩类具有变化较大的 SiO2(62.6-72.9 wt.%),Fe2O3T (2.54-5.21 wt.%)含量以及 Mg#(33-51)值。大部分花岗岩表现为高钾钙碱性系列以及准铝质成分(ASI<1.1),具有典型的大陆弧岩浆性质特征(富集轻稀土(LREE)以及大离子亲石元素(LILE),亏损高场强元素(HFSE))。同位素方面,对比吉布克和冬图劲花岗岩体的 Sr-Nd 同位素组分(87Sr/86Sr(i)=0.7060-0.7066 和 εNd(t)= +1.7-+2.0 ), 呼 斯 特 岩 体 显 示 较 高 的 初 始 87Sr/86Sr(0.7075-0.7089)值,较低的 εNd(t) (+0.3-+0.9)值。综合上述特征,确认两期花岗岩均形成于混合源区,包括古生代新生物质(新生下地壳和地幔起源岩浆)和元古代的变火成岩基底物质,其中吉布克和冬图劲花岗岩源区古生代年轻物质所占比例较大。(2)伊犁地块北缘花岗质岩浆岩的 Nd-Hf 同位素组成表明,花岗岩类具有相类似的锆石 εHf(t)-年龄和全岩 εNd(t)-年龄的同位素随时间演化趋势,并且在 ca.360 Ma表现为同位素演化趋势的明显差异。400-360 Ma 岩浆岩的 εHf(t)和 εNd(t)呈下降趋势,360-300 Ma 的岩浆岩的 εHf(t)和 εNd(t)呈升高趋势。结合伊犁地块北缘的泥盆-石炭纪岩浆岩性质以及时空分布,推断伊犁地块北缘在 ca.360 Ma 发生构造转换,可能表现为由前进式造山类型转变为后撤式造山类型。(3)综合本次研究和研究区已发表的同位素数据,明确了花岗岩样品均具有较高的全岩 εNd(t)和锆石 εHf(t)值,具有亏损的岩浆源区性质,指示了重要的陆壳生长事件。在 Hf 同位素体系中,模拟计算结果显示在 400-300 Ma 有 15-100 %不同程度的古生代新生物质参与了岩浆的形成;在 Nd 同位素体系中,模拟计算结果显示在400-300 Ma 有 5-60 %不同程度的古生代新生物质参与了岩浆的形成。(4)伊犁地块北缘阿拉套地区火山-侵入岩组合 SIMS 锆石 U-Pb 年代学揭示两期岩浆事件:ca.300 Ma 火山岩(熔结凝灰岩和流纹岩)和 ca.288 Ma 侵入岩(黑云母花岗岩)。熔结凝灰岩具有较低的 SiO2(65.8-71.5 wt.% )含量和 Mg#(5.93-12.57)值,同时表现出大离子亲石元素(LILE) 富集和高场强元素(HFSE) 亏损的特征,全岩εNd(t)和锆石 εHf(t)值分别为+6.9-+7.0 和+9.9-+14.1,指示年轻的玄武质的下地壳源区。流纹岩具有高的 SiO2(72.7-74 wt.%)和 K2O(3.86-4.53 wt.%)含量,高的锆石 δ18O(11.67-13.23‰)值,低的全岩 εNd(t)(+2.9-+3.8)和锆石 εHf(t)(+2.78-+9.97)值,指示源区有沉积物的加入。相比于火山岩,ca.288 Ma 花岗岩具有相当高的 SiO2(74.7-75.5 wt.%)含量,εNd(t)(+7.7-+8.8)和 εHf(t)(+9.81-+12.71)值,以及低的锆石 δ18O(5.99-6.84 ‰)值,指示母岩浆起源于古生代年轻的地壳源区。(5)ca.300 Ma 熔结凝灰岩和流纹岩具有较高的全岩锆石饱和温度(888-938℃和 815-832℃),以及高的 εHf(t)值,指示在 ca.300 Ma 阿拉套地区发生了一次高温岩浆事件,推断该事件可能与西准噶尔洋内弧系统和伊犁地块汇聚/拼贴过程中的俯冲板片断离,引起软流圈上涌有关。ca. 288 Ma 黑云母花岗岩具有相对较低的全岩锆石饱和温度(723-735 ℃),源区残余角闪石,指示源区富水熔融,推断与早二叠世北天山大规模的右旋剪切作用有关,剪切带被认为是变质流体运移和加入的通道。
学科领域测绘工程
语种中文
文献类型学位论文
条目标识符http://ir.xjlas.org/handle/365004/14901
专题研究系统_荒漠环境研究室
作者单位中国科学院新疆生态与地理研究所
第一作者单位中国科学院新疆生态与地理研究所
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王祥松. 晚古生代伊犁地块北缘岩浆岩的成因及地质意义[D]. 新疆乌鲁木齐. 中国科学院大学,2017.
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