阿尔金英格利萨依超高压岩石中锆石的成因矿物学与年代学研究

【作者】 张安达；

【导师】 刘良； 陈丹玲；

【作者基本信息】 西北大学 ， 矿物学、岩石学、矿床学， 2003， 硕士

【摘要】 阿尔金断裂构造带位于我国新疆、青海、甘肃、西藏4省区交界地带，是青藏高原的西北部边界，分隔了塔里木与柴达木盆地。近年来的地质研究新进展表明，阿尔金构造带不仅是一个由中新生代的巨型走滑断裂系所造就的断块或断隆，而且是一个由不同层次、不同时期和形成于不同构造环境地质体所组成的造山带，尤其是其中不同类型高压—超高压变质岩的陆续发现和深入研究，该区已成为中、外地学界瞩目的超高压变质岩及其相关的大陆深俯冲作用研究的新的热点地区之一。 本文以前人研究为基础，选定阿尔金英格利萨依—带呈互层状产出的超高压石榴石二辉橄榄岩的直接围岩—超高压变质的花岗质片麻岩和中基性片麻岩的锆石为研究对象，主要采用阴极发光、拉曼光谱和微量元素分析等多种手段综合对锆石进行了成因矿物学研究，并在此基础上采用LA-ICP-MS和SHRIMP（部分样品）方法对其进行了同位素年代学测定及其地质意义的初步研究，取得的重要成果和认识为： （1）阴极发光图像显示，超高压花岗质片麻岩锆石内部结构相当复杂，大多数锆石发育三层结构，其相对均匀的亮色残核为具有振荡生长环带的岩浆锆石，而绕残核分布的宽窄有变化的幔部暗色环状层可能为变质期变质增生锆石，边部很窄的亮色环状边缘层则可能是后期地质作用或退变质过程中新生成的锆石。 中基性片麻岩和石榴石二辉橄榄岩锆石内部结构可区分出核-幔-边三层或核-边两层结构，核部或幔部都不具岩浆生长环带，而具有扇形或面形分带区域的特征，所以其核部和幔部总体可能属于变质成因；其亮色边缘层宽窄有变化，有些颗粒边部呈现港湾状，指示其形成可能与后期交代或退变质作用有关。 （2）锆石拉曼谱图特征研究表明，超高压花岗质片麻岩锆石脱晶化程度不大，故其不会造成严重的Pb丢失，因而可用于锆石测年；对其中包裹体的拉曼测试发现主要有绿辉石、金红石、磷灰石、石英和长石等矿物，其中长石主体分布在锆石的核部，而绿辉石、金红石主要分布在锆石的幔部，磷灰石和石英在锆石颗粒从核部到幔部都有分布。综合锆石的阴极发光图像分析认为，绿辉石+金红石+磷灰石十石英为幔部变质矿物组合，而磷灰石+石英+长石为核部原岩浆矿物包裹体。 （3）超高压花岗质片麻岩锆石微区微量元素分析表明，锆石微区微量元素特征表现出三种类型：一种微量元素特征为∑REE、∑EHREE都很高，其值分别为1224.956～1891.820ppm、1177.290～1849.70ppm，Th/U比值变化于0.661～0.801之间，代表核部岩浆锆石的微量元素特征：一种表现为∑REE、∑HREE都很低，其值分别为112.441～297.140ppm、97.960～277.440ppm，Th/U比值为0.077～0.013，代表幔部变质锆石的微量元素特征：还有一种微量元素特征，其∑REE为（381.188～790.180）ppm、∑HREE为（335.160～739.850）ppm，Th/U比值变化于0.191～0.605之间，显然，无论是∑REE还是Th/U比值都介于前二者之间，可能是前二者混合信息的一种结果，因此，对应于这种微量元素特征的微区的测年所得年龄值可能不具有真实的地质意义。 中基性片麻岩锆石微区微量元素明显分为两种类型，第一种类型的（Yb/Gd）_CN、Nb/Ta20（）3.4阿尔金英格利萨依超高岩石中铬石的成因矿物学与年代学研究分别为一435弓一09、一3一756一343.043，其T扮U比值主体大于0.679，并具有明显的ee正异常（8 Ce二17.76吞礴9.292）;第二种类型的口粉〔d如、Nb门阻分别为7.284-14.17、44.379¹ 11.772，其口11订U比值主体小于0.1而Ce正异常不明显或为Ce负异常（8 ce=。.202² .250），可能代表了错石变质成因的部分。 （4）在错石成因矿物学研究的基础上，分别采用LA-ICP一MS和SH阳MP两种高精度的定年方法对超高压花岗质片麻岩中的错石进行了微区原位U一Pb年龄测定，LA一IcP一MS方法获得错石慢部变质成因微区的年龄为495士6.SMa，SH租MP方法给出的变质年龄为48社10Ma，两个年龄值非常一致;同时，利用LA一IcP一MS获得超高压中基性片麻岩和石榴石二辉橄榄岩的峰期变质年龄分别为501月1 Ma、509士12Ma。超高压花岗质片麻岩、中基性片麻岩和石榴石二辉橄榄岩分别所获得的变质年龄值在误差范围内是基本一致的，从而确证它们是同一超高压变质事件的产物。结合野外露头上三种超高压变质岩石呈互层状或间层状产出的地质产状特征综合分析，一致表明阿尔金英格利萨依一带不同类型的超高压岩石共同经历了超高压变质作用，峰期变质时代为487一509Ma。 （5）阿尔金西段江格萨依一带榴辉岩的矿物对Sm一d等时线年龄为50肚1 oMa和单颗粒错石u一Pb年龄为503巧3Ma，该年龄值被认为代表了区内高压变质岩石的峰期变质时代（张建新等，]999）。新近，江格萨依一带的榴辉岩中也陆续发现超高压变质的新证据（张建新等，2002:Chen D L etal，2003）。因此，阿尔金英格利萨依一带超高压变质岩487一50一Ma峰期变质时代的确定，进一步说明从西部江格萨依一带的榴辉岩到东部英格利萨依一带的超高压变质的石榴二辉橄榄岩及其围岩花岗质片麻岩和中基性片麻岩共同组成了沿阿尔金南缘断续分布着的一条超高压变质岩带。参更多还原

【Abstract】 The Altyn orogenic belt, locating at the border of four provinces of Tibet, Sinkiang, Qinghai and Gansu, is the natural northwest margin of the Qinghai-Tibet plateau and separates Qaidam basin from Tarim basin. The latest progresses in the regional geology in this area show that the belt is not only a Mesozoic-Cenozoic strike-slip system, but also a complicated orogenic residue composed of blocks formed at different levels and tectonic setting. The discoveries and further studies of the different HP-UHP metamorphic rocks in Altyn mountains have attracted many attention of geologists all over the world.Based on the existing studies of HP-UHP rocks in this region, the author focus his studies mainly on the minerageny and chronology of zircons from granitoid gneiss and basic-intermediate gneiss, which are the direct country rocks of garnet Iherzolite.(1) Cathode-luminescence investigations reveal complicated internal structures of zircons from UHP granitoid gneiss, mainly consisted of core-mantle-rim frames. The cores show the oscillatory bands of magmatic feature.Zircons in UHP basic-intermediate gneiss and garnet Iherzolite have core-mantle-rim structures or mantle-rim structures. The cores and mantles of zircons have the sector or planar zone.(2) Ranman microprobe analyses for zircons in granitoid gneiss show that the metamictization extent of zircons is so little that there is Pb enough for dating. Numerous mineral inclusions, such as omphacite, rutile, apatite, quartz and feldspar, have been found in the zircons. The feldspar occurs mainly in the core, and the omphacite and rutile in the mantle. Combined with the Cathode-luminescence images, we argue that the mineral assemblages of omphacite + rutile + apatite + quartz and apatite + quartz -t-feldspar are formed under the metamorphic and magmatic conditions, respectively.(3) Characteristics of trace element for zircons from the UHP granitoid gneiss can be divided into three different groups. 1) The core part, ΣREE=1224.95~1891.82ppm, EHREE=1177.29 ~1849.70ppm and Th/U=0.661~ 0.801, show the characteristic of magmatic zircon. 2) The mantle region, ΣREE=297.14ppm, Σ HREE=277.44ppm and Th/U=0.013, show the characteristic of metamorphic zircon. 3) values of ?REE, Σ HREE and Th/U between that of in group 1) and 2) may resulted from the mix of the core and mantle of zircon.Characteristics of trace element for zircons from the UHP basic-intermediate gneiss can be divided into two different groups. The first group have the high abnormal value of Ce (17.766-49.292) and the high Th/U(>0.679); the ratios of (Yb/Gd)cN and Nbrta are 1.435-3.109 and 131.786~343.043 respectively; the second group have the low abnormal value of Ce(0.202-2.250) and the low Th/U(<0.1), and (Yb/Gd)CN =7.284-14.17, NbHa= 44.379- 111.772, , showing the characteristics of typical metamorphic zircon.(4) In-situ U-Pb dating was realized by LA-ICP-MS and SHRIMP respectively. LA-ICP-MS dating give the peak metamorphic age of graniteod gneisss is 495?.5Ma, which is close to the result of 487±10Ma gained by SHRIMP dating for the same rock. The peak metamorphic ages of basic-intermediate gneiss and garnet Iherzolite dating by LA-ICP-MS, assumed to be 501 ±31 Ma and 509±12Ma respectively, which is consist with the age of graniteod gneiss within the error, suggest that they are the products of the same UHP metamorphism. Combined with the occurrence of these three rocks interbeds in the outcrops , the three UHP rocks in this region all underwent the UHP metamorphism, and the peak metamorphic age is 487-509 Ma.(5) The Sm-Nd isochronal age and single-zircon U-Pb age of eclogites in Jianggasayi, western Altyn, is 500±10Ma and 503±5.3Ma respectively, which was considered as the peak metamorphic age of HP metamorphism. Some evidences of UHPM have been found recently in these eclogites (Zhang Jianxin et al., 2002; Chen D L et al, 2003). The determination of peak metamorphic age (487-501 Ma) of the UHP metamorphic rocks in Yingelisayi, give further evid更多还原