KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 中国西天山南部晚古生代花岗岩的成因及地质意义 | |
| Alternative Title | Petrogenesis and tectonic implication of the late Paleozoic granitic rocks in the southern part of the Chinese Western Tianshan |
| 包子鹤 | |
| Subtype | 博士 |
| Thesis Advisor | 蔡克大 |
| 2019-06-30 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Discipline | 工学博士 |
| Keyword | 花岗岩 晚古生代 中亚造山带 伊犁地块 南天山洋 起始俯冲 后碰撞 Granitoids Late Paleozoic CAOB Yili Block South Tianshan Ocean subduction initiation post-collision |
| Abstract | 花岗质岩石广泛分布于地球上，是大陆地壳的重要组成部分。花岗岩是特定地质背景的产物，记录了大陆地壳演化的丰富信息， 被用来探讨成矿过程、地球的深部物质组成、大地构造背景以及地球动力学机制。中亚造山带是显生宙以来全球陆壳增生与改造最显著的大陆造山带，可分为南蒙古拼贴系统、塔里木-华北拼贴系统以及哈萨克斯坦拼贴系统。哈萨克斯坦拼贴系统由多个岛弧、微陆块拼贴形成，这些构造单元在早古生代拼合形成一个狭长的复合大陆，并最终在晚古生代弯曲成现今的哈萨克斯坦山弯构造。中国西天山南部构造带位于哈萨克斯坦山弯构造南翼的外侧，该地区出露有大量的晚古生代花岗质岩石，其形成与南天山洋的俯冲消减及后续的碰撞和后碰撞演化有着密切关联， 记录了南天山洋盆的演化历史。然而，这些花岗岩的成因一直存有争议，有关南天山洋壳俯冲的起始时间与俯冲极性、南天山洋盆的闭合时限以及南天山洋是否存在等众多科学问题也尚需进一步讨论。基于此，本论文选择位于中国西天山南部构造带四个大型复式花岗岩体——昭苏岩体、夏特岩体、科克苏岩体和卡特巴阿苏岩体作为研究对象， 在详细野外地质考察工作的基础上， 进行了岩相学、全岩地球化学、同位素年代学和同位素地球化学的研究，综合区域上的研究资料，探讨了研究区内花岗岩的成因、时空分布和演化历史，以讨论其形成的构造环境，并进一步探讨南天山洋盆晚古生代的构造演化历史以及与哈萨克斯坦山弯构造的联系。本文初步得到认识如下：（1） 对四个岩体进行 LA-CP-MS 锆石 U-Pb 定年，得出四期年龄范围，分别为：泥盆世末期-早石炭世（365~350 Ma）、晚石炭世（313 Ma）、早二叠世（293Ma）和晚二叠世（268~265 Ma），这分别代表了四期重要的构造-热事件。（2）昭苏岩体中，花岗岩样品的铝指数 A/CNK < 1.1，普通刚玉含量 < 1.2%，A/NK > 1.0，同时，它们的 P2O5 含量随 SiO2含量的增大而减少，而 Pb 含量则随SiO2 含量的增大而增加，指示其均为 I 型花岗岩。其中，钾长花岗岩的分异指数极高（DI > 92%），且具有较低的锆石饱和温度（713~727℃）、较低的 Zr 含量（75.39~91.42 ppm）与较低的 Zr/Hf（22.57~27.22）和 Nb/Ta 比值（4.1~8.33）指示其为高分异 I 型花岗岩，而二长花岗岩（DI = 73~79%， TZr = 760~782℃，Zr = 136.70~178.66 ppm， Zr/Hf = 34.87~39.30， Nb/Ta = 9.77~14.54）则为未分异I 型花岗岩。较为一致的 Nd 同位素组成（εNd(t) =-3.5~-1.8）指示二者来自于同一岩浆房。二长花岗岩的 Cr、Co、Ni 含量（Cr = 6.59~12.17 ppm， Co = 6.89~8.77ppm，Ni = 5.3~8.01 ppm）与大陆地壳元素含量相似， Mg#值 < 47，在源区判别图解中落在变质英云闪长岩源区，表明其源区可能来自于中下大陆地壳。然而与该地区前寒武纪变质基底（(87Sr/86Sr)i = 0.7128, εNd(t) = -5.2）相比，二长花岗岩样品具有明显更高的 εNd(t)值（-3.5 ~ -3.0），因此在母岩浆形成过程中需要有较高 εNd(t)值的端元加入，而亏损地幔源区玄武质岩浆混合/底侵模型可以满足这一要求。将该地区同时代的玄武岩（(87Sr/86Sr)i =0.705, εNd(t) =5.8） 作为参与岩浆混合模型的另一个端元，通过计算可知在成岩过程中约有 20 - 40%的幔源物质加入。（3） 科克苏岩体中，与其他样品（Eu/Eu* = 0.34~0.73）相比，钾长花岗岩具有更强烈的负 Eu 异常（Eu/Eu* = 0.11~0.14）。加之它们具有更高的 FeO*/MgO值（8.67~13.35）与更低的 Zr/Hf 值（22.08~25.78），综合推断钾长花岗岩为高分异 I 型花岗岩，而二长花岗岩、花岗斑岩和黑云二长花岗岩均为低分异的 I 型花岗岩。 晚泥盆世-早石炭世二长花岗岩与花岗斑岩的 Mg#值较大（40.80 ~ 53.40），指示熔体起源于地壳物质部分熔融且有地幔组分的加入；而二叠纪黑云二长花岗岩的 Mg#值偏小，（35.77~36.37），指示样品的母岩浆可能主要来源于玄武质下地壳物质的部分熔融。晚泥盆世-早石炭世样品中，与花岗斑岩（εHf(t) = -6.25~5.29，TCDM = 1.03~1.76Ga）相比，二长花岗岩具有较高的 εHf(t)值（3.42~9.31）与较年轻的 Hf 同位素两阶段模式年龄（TCDM = 0.78 ~ 1.15Ga）；同时二长花岗岩也具有较高的 εNd(t)值（3.1）与较年轻的 Nd 同位素模式年龄（TDM = 782Ma）（花岗斑岩 εNd(t) = 1~3.4， TDM = 768 ~ 1340 Ma）。 二叠纪黑云二长花岗岩的 εHf(t)值（-10.05~2.71）与 εNd(t)值（-4.2）比较负， Hf 同位素两阶段模式年龄（TCDM = 1.12~ 1.93 Ga）和亏损地幔 Nd 模式年龄（1470 Ma）都比较古老。两组不同时代的花岗岩的 Nd-Hf 同位素可能指示了样品源区的物质混合比例的差异，因此，我们使用 Sr-Nd 同位素两端元混合模型来计算各端元的混合比例，两个端元分别为大哈拉军山组玄武岩和伊犁地块新元古代结晶基底岩石。模拟计算结果指示，晚泥盆世-早石炭世花岗岩的源区主要为年轻的玄武质下地壳（75~85%），而二叠纪黑云二长花岗岩的母岩浆则更多来自于伊犁地块古老的陆壳基底岩石的部分熔融（~75%）。（4） 中天山卡特巴阿苏钾长花岗岩的地球化学特征符合 I 型花岗岩的特点。夏特花岗闪长岩颜色较浅，结晶较细，其野外特征与 A 型花岗岩相符。而在 CIPW标准矿物计算结果中，刚玉的出现则反映了样品具有准铝质到弱过铝质的性质，其微量元素特征为具有较高的 Zr（211~322 ppm）含量与 Nb（27.7 ~ 30.3 ppm）含量，以及较高的 Ga/Al 比值（10000Ga/Al = 2.3~2.4），指示其为铝质 A 型花岗岩。卡特巴阿苏钾长花岗岩的 Mg#值（41.53~43.36）较低， εNd(t)值（1.5~3.2）与 εHf(t)值（0.8~5.9）均为正值，且具有较年轻的 Nd 模式年龄（740 ~ 898 Ma）和 Hf 同位素两阶段模式年龄（0.97 ~ 1.28 Ga），指示其母岩浆主要来自于年轻的玄武质下地壳的部分熔融，并有少量古老地壳物质的加入。 夏特花岗闪长岩比较负的 εNd(t)值（-4.3 ~ -4.9）和 εHf(t)值（-5.9 ~ -2.1）以及相对古老的 Nd 模式年龄（1.31 ~ 1.32 Ga）和 Hf 同位素两阶段模式年龄（1.45 ~ 1.69 Ga）指示其母岩浆主要来自于元古代古老的大陆地壳的部分熔融。样品的 Mg#值（42.09~47.58）较高，则指示其熔体可能有地幔组分的加入，幔源岩浆为其形成提供了热源和物源。（5） 伊犁地块南缘的双变质带的空间展布特征指示了南天山洋在石炭纪存在向北的俯冲。科克苏岩体伊犁地块南缘部分处于伊犁-中天山地块南部岩浆岩带，样品的稀土配分模式与微量元素组成均与该地区同时代的岛弧型玄武岩高度一致。在花岗岩构造判别图解中，样品投于火山弧花岗岩区域。我们推断伊犁-中天山南缘在石炭纪为火山岛弧的构造环境。而乌孙山岩体与伴生的火山岩共同构成乌孙山岩浆带，考虑到现今挤压缩短后距伊犁-中天山南缘岩浆带的距离约100km，这种两条独立的岩浆带的分布格局符合环太平洋造山带弧-弧后系统的岩浆带分布格局。因此推断，伊犁-中天山地块南缘可能在南天山洋的俯冲作用下构成一个晚泥盆世火山弧；而乌孙山岩浆带则是在与之相对应的弧后部位中形成的，此构造格局类似现今北美的科迪勒拉弧后地区。将伊犁地块南缘与中天山地块火成岩的锆石 U-Pb 年龄进行统计，结果显示了该地区存在着 500-400 Ma 与 365-320 Ma 的两个呈幕式爆发的岩浆活动事件。人们普遍认为俯冲过程的开始，标志着被动大陆边缘向汇聚型板块边界的转变，伴随着大洋板片后撤，将会导致俯冲洋壳板片的脱水和上覆的大陆板块的伸展，从而引起交代地幔楔部分熔融形成弧岩浆活动；还会在弧后地区引发地幔减压熔融上涌，并与高地温梯度下造成的地壳部分熔融一起形成弧后岩浆活动。而在~365 Ma 所形成的“爆发式”的岩浆活动，则可能是与南天山洋向北开始俯冲相响应的构造-岩浆事件。在整个弧-弧后地区，样品的 εNd(t)值随年龄的年轻而逐渐增高，说明地壳物质的参与减少，幔源物质的参与增多，指示了一个地壳逐渐拉张减薄的构造环境。（6） 南天山高压-超高压变质的峰期年龄主要集中于 321~305 Ma，指示南天山洋进入了俯冲消亡的晚期，而卡特巴阿苏钾长花岗岩（313 Ma）具有富钾、LREE 富集、 HFSE 负异常的岛弧岩浆的地球化学特征，稀土配分模式与微量元素组成也与新源地区石炭纪岛弧形玄武岩高度一致，结合该地区同时代的岩浆岩性质，推断南天山洋的北向俯冲在晚石炭世（~313 Ma）依然存在。（7） 夏特花岗闪长岩在地球化学特征上兼有 A 型花岗岩和板内花岗岩的一些特征，符合后碰撞花岗岩的特点。而南天山分布的二叠纪低压变质岩和后碰撞花岗岩（298~260 Ma）也指示西天山南部构造带在~300 Ma 发生了一次由聚敛型向拉张型的构造转换，进入了后碰撞演化阶段。样品中的继承锆石年龄为 293Ma，推断在后碰撞阶段早期（~298 Ma），俯冲洋壳板片发生断离，引起软流圈地幔上涌，使上覆板片发生部分熔融，从而在该地区形成了低压-高温变质岩与后碰撞花岗岩。而夏特花岗闪长岩（268 Ma）与科克苏黑云二长花岗岩的年龄（265Ma）则指示后碰撞演化阶段可能持续至晚二叠世（~263 Ma）。 |
| Other Abstract | Granitoid rocks are widely distributed on Earth, and represent a majorcomponent of the continental crust. Granite is product of specific geologicalbackground, which can provide important constraints on the evolution of continentalcrust and thus, is used to study the mineralization process, deep material compositionof the Earth, tectonic setting and geodynamic mechanism.The Central Asian Orogenic Belt (CAOB), represents one of the largestaccretionary orogens on Earth and is known as a major site of continental crustgrowth. The CAOB primarily consists of three main collage systems: southernMongolia collage system in the north, Tarim-North China collage system in the south,and the intervening Kazakhstan collage system. The Kazakhstan collage systemincludes diverse arcs and microcontinents, and these separate components aredocumented to amalgamate and weld together into a long, single composite continentin the Early Paleozoic, and then finally bent to form the present Kazakhstan oroclinein the Late Paleozoic.The southern part of the Chinese Western Tianshan is located in outer side of thesouth limb of the Kazakhstan orocline. A large number of Late Paleozoic graniticintrusions are exposed in this area. Their formation is closely related to the subductionand subsequent collision and post-collision evolution of the South Tianshan Ocean(STO). These granites record the evolution history of the STO, but their petrogenesishas been controversial, such as the starting time and polarity of the subduction of theSouth Tianshan oceanic crust, the closing time of the STO and even the existence ofthe STO. To delineate of the evolution process of the STO, we conducted thegeochronlogical, geochemical, and isotopic studies on the Zhaosu batholith, theKekesu batholith, the Katebaasu batholith, and the Xiate batholith in the southern partof the Chinese Western Tianshan, for the sake of investigating their petrogensis,temporal-spatial distribution, and associated geodynamic processes, identifying the tectonic evolution and implication for continental crust growth, and discussing theirlink to the Kazakhstan orocline. The major conclusions are summarized as follows:(1) LA-ICP-MS zircon U-Pb dating result shows four groups of the studiedgranites, includes the Latest Devonian - the Early Carboniferous (365~350 Ma), theLate Carboniferous (313 Ma), and the Permian (293~265 Ma), which represent fourimportant tectonic-thermal events.(2) Granites in the Zhaosu batholith are metaluminous to weak peraluminous.Most of them have A/CNK < 1.1, normative corundum < 1.2% and A/NK > 1.0.Meanwhile, their P2O5 content decreases with increasing SiO2, whereas Pb contentmoves in the opposite direction, indicating that they are all I-type granites. TheK-feldspar granites are characterized by lower zircon saturation temperature(713~727 °C), lower Zr contents (75.39~91.42 ppm) and lower Zr/Hf (22.57~27.22)and Nb/Ta (4.1~8.33) relative to the monzogranites (TZr = 760~782 °C, Zr =136.7~178.7 ppm, Zr/Hf= 34.87~39.30, Nb/Ta= 9.77~14.54). According todifferentiation index (DI, sum of normative minerals Q + Ab + Or) values of theK-feldspar granites (> 92%) and the monzogranites (73-79%), it is reasonable toconclude that the K-feldspar granites are highly fractionated I-type granites, while themonzogranites have features of unfractionated I-type granites. Undistinguishablewhole-rock Sr-Nd isotopes suggest that the monzogranites and the K-feldspar granitesmight have been derived from common magma sources. Cr, Co and Ni contents of themonzogranites (Cr = 6.59–12.17 ppm, Co = 6.89–8.77 ppm, Ni= 5.3–8.01 ppm) areclose to those of continental crust. Their low Mg# (< 47) values and the plot in thefield of partial melts from metatonalitic sources implyed a derivation from themiddle-lower continental crust. However, as for whole-rock Sr-Nd isotopes, themonzogranites have obviously high εNd(t) values (-3.5 to -3.0) relative to the regionalPrecambrian metamorphic basement rocks (εNd(t) = -5.2). Therefore, the othercomponent with high radiogenic Nd isotopes should be required during the genesis ofthe granites. This acquisition can be satisfied by mixing and/or underplating or thedepleted mantle-derived basaltic magmas. Taking the basement rocks of the Yili Block (YB) ((87Sr/86Sr)i = 0.7128, εNd(t) = -5.2) and the Dahalajunshan Formationbasalt as two end-members, the magma mixing modeling results suggest that ca.20-40% of mantle-derived magma should be acquired to generate the parentalmagmas.(3) In the Kekesu batholith, the K-feldspar granites show stronger negative Euanomalies (Eu/Eu* = 0.11~0.14) than other samples (Eu/Eu* = 0.34~0.73). They alsohave higher FeO*/MgO values (8.67-13.35) and lower Zr/Hf values (22.08-25.78),which are consistent with the characteristics of highly fractionated granites. All theresults indicate that the K-feldspar granites are highly fractionated I-type granites,while the monzogranites, the granite-porphyries and the biotite monzogranites areunfractionated I-type granites. The Latest Devonian-Early Caboniferous samples havehigher Mg# (40.8~53.4), implying that the melt originated from partial melting ofcrustal materials and the addition of mantle components; while the Permian sampleshave relatively low Mg# (35.77~36.37), indicating the parental magma may be mainlyderived from partial melting of basaltic lower crustal materials. In the LatestDevonian-Early Caboniferous samples, the monzogranites have higher εHf(t) value(3.42~9.31) and younger TCDM ages (0.78~1.15Ga) relative to the granite-porphyries(εHf(t) = -6.25~5.29， TCDM = 1.03~1.76Ga). Meanwhile, the monzogranites also have ahigher εNd(t) value (3.1) and younger depleted mantle Nd model ages of 782Ma,relative to the granite-porphyries (εNd(t) = 1~3.4, TDM = 768 ~ 1340 Ma). The Permianbiotite monzogranites samples have negative εNd(t) values (-10.05 ~ 2.71) and εHf(t)value (-4.2), and accordingly old TCDM ages (1.12~1.93 Ga) and depleted mantle Ndmodel age (1470Ma). The Nd-Hf isotopes of the granite samples of different agesmay indicate the difference of mixing ratios of materials in the magma source.Therefore, we use the Sr-Nd isotope mixing model to calculate the mixing ratio ofeach end-member. Taking the Dahalajunshan Formation basalt and the metamorphicbasement rocks of the YB as two end-members, the magma mixing modeling resultssuggest that the juvenile basaltic crustal melts might participate more in the LatestDevonian-Early Caboniferous samples’ panrental magma (75~85%) than in the Permian samples’ (20~30%), while the panrental magma of the Permian biotitemonzogranites might mainly come from paritial melting of ancient metamorphicbasement rocks of the YB.(4) The Central Tianshan Katebaasu K-feldspar granites have a chemicalcharacristic of I-type granite. The Xiate granodiorites are shallow in color and fine incrystallization, the existence of anhedral hornblende correspond to the characteristicsof A-type granite. Corundum occurs in the CIPW norm mineral calculation, whichreflects the properties of the granites from metaluminous to weak peraluminous. Thegranodiorites have ralitively high Zr (211~322 ppm) and Nb (27.7~30.3 ppm)contants, and high Ga/Al (10000Ga/Al = 2.3~2.4) values, indicating that they arealuminous A-type granites (ALAG). The Katebaasu K-feldspar granites have low Mg#(41.52 ~ 43.36), positive εNd(t) values (1.5~3.2) and εHf(t) values (0.8~5.9), andaccordingly depleted mantle Nd model ages of 740 ~ 898 Ma and TCDM ages of 0.97 ~1.28 Ga, indicating that their parental magma might be mainly partial melting ofjuvinile basaltic lower crust, with a small amount of melts from ancient crustalmaterials. The negative εNd(t) values (-4.3 ~ -4.9), εHf(t) values (-5.9 ~ -2.1), and theold depleted mantle Nd model ages (1.31~1.32Ga) and TCDM ages (1.45 ~ 1.69Ga) ofthe Xiate biotite monzogranites indicate that their parental magma were mainly partialmelting of Proterozoic continental crust, while the high Mg# (42.09 ~ 47.58) indicatethat the melts may have the addition of mantle components. The mantle-derivedmagma provides both heat source and material source for their formation.(5) The spatial distribution of the paired metamorphic belt in the southern YiliBlock indicates the northward subduction of the South Tianshan oceanic plate in theCarboniferous. The Kekesu granites are located in the southern YB-CTB magmaticbelt, while their REE pattern and trace element composition are highly consistent withthose of the island-arc volcanic rocks; the samples plot in the field of volcanic arcgranite (VAG) in the tectonic setting distinguish diagram. Thus, we conclude that thesouth margin of the Yili-Central Tianshan Block should be a volcanic island arc in theCarboniferous. The intrusive-volcanic rock associations constitute a Wusun Mountain magmatic belt, and it locates about 100 km north to the YB convergent magmatic belt.Such distribution pattern of two separate magmatic belts is ubiquitous in an arcback-arc system of the circum-Pacific orogens. Therefore, it is concluded that thesouthern margin of the Yili-Central Tianshan Block may form a late Devonianvolcanic arc under the subduction of the South Tianshan oceanic plate, while theWusun Mountain magmatic belt was formed in the corresponding back-arc tectonicsetting. This tectonic framework is similar to that of the present-day North AmericanCordillera back-arc regions.Our compilation of zircon U-Pb ages for igneous rocks in the south domain ofthe YB and the CTB reveals two magmatic episodes at time intervals of 500-400 Maand 363-320 Ma, with an obvious magmatic transient in the period from ca. 400 Mato 363 Ma. The magmatic transient was suggested to be the result of the developmentof the STO. It is widely accepted that the initiation of the subduction process marksthe transition from passive continental margin to convergent plate boundary. Suchtectonic shift, followed by oceanic slab rollback, will result in dehydration ofsubducted oceanic slab and the extension of the overlying continental plate, whichgive rise to arc magmatism by partial melting of the metasomatized mantle wedge andback-arc magmatism due to mantle decompression melting and upwelling as well asthe crustal melting under the resultant high geothermal gradient. Thus we suggest thatthe “flare-up” magmatic event of ∼365 Ma was likely to be the tectono-magmaticevent in response to a nascent northward subduction of the South Tianshan oceanicplate.In the whole arc-back-arc region, the εNd(t) value of the samples increases withage, which indicates that the participation of crustal materials decreases and that ofmantle-derived materials increases, indicating a tectonic environment in which thecrust is gradually stretched and thinned.(6) The peak metamorphism age of the HP/UHP rocks in the South Tianshancomplex is mainly concentrated on the 321~305 Ma, indicating that the SouthTianshan Ocean entered the latest stage of subduction. The Katebaasu K-feldspargranites (313 Ma) have charactristics of island arc magma, e.g. low K2O contant,LREE enrichment and negative HFSE anomlies, and their REE and trace elementpatterns are also highly consistent with the Carboniferous island arc basalt in Xinyuan.In combination with the coeval magmatic rocks in this area, it is inferred that thenorthward subduction of the South Tianshan oceanic plate still existed in the LateCarboniferous (~313 Ma).(7) The Xiate granodiorites with inherited zircon age of ~293 Ma havegeochemical charactristics of both A-type and within-plate granites and it conforms tothe characteristics of post-collisional granites. Permian LP metamorphic rocks andpost-collisional granites in the South Tianshan complex (360~298 Ma) indicate that inthe southern part of the Western Tianshan, a tectonic transition from convergent todivergent type has occurred in ~300 Ma, and the studied area entered into thepost-collisional evolution stage. It is concluded that during the early stage of thepost-collision (~298 Ma), the subduction of oceanic crust plate broke off, causing theupwelling of the asthenosphere mantle, and resulting in partial melting of overlyingplates, and then caused the formation of LP-HT metamorphic rocks andpost-collisional granites in this area. The magmatic ages of Xiate granodiorites (268Ma) and the Kekesu biotite monzogranites (265 Ma) indicate that the post-collisionalstage may last to the Late Permian (~263 Ma) |
| Subject Area | 地球探测与信息技术 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/15309 |
| Collection | 中国科学院新疆生态与地理研究所 研究系统 |
| Affiliation | 中国科学院新疆生态与地理研究所 |
| First Author Affilication | 中国科学院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | 包子鹤. 中国西天山南部晚古生代花岗岩的成因及地质意义[D]. 北京. 中国科学院大学,2019. |
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