藏北羌塘盆地基底高级变质岩LA-ICP-MS 锆石U-Pb年龄及其地质意义

俄久卖高级变质岩位于藏北羌塘盆地中央隆起带北缘的玛依岗日地区,是目前羌塘盆地基底高级变质岩石的唯一代表。该高级变质岩由正片麻岩和副片麻岩组成,本文以正片麻岩为研究重点。正片麻岩锆石CL图像显示出明显的核—幔—边结构。根据LA-ICP-MS锆石U-Pb测年结果,锆石核部年龄范围为242~2490Ma,记录了岩浆岩源岩的年代信息;锆石幔部具有典型的岩浆振荡环带结构,年龄为207Ma±2Ma,相应的Th/U值介于0.02~0.30之间,代表正片麻岩原岩的岩浆结晶时代,该年龄与羌塘中部地区晚三叠世高压变质作用和岛弧岩浆作用在时空上相对应。锆石增生边的年龄为161~197Ma,对应的Th/U值介于0.02~0.15之间,代表片麻岩发生主变质作用的时代,可能是班公湖-怒江洋盆向北的俯冲消减作用在羌塘中部地区的响应。地球化学资料显示,正片麻岩具有类似岛弧型火山岩的地球化学特征。综合区域地质资料,俄久卖高级变质岩原岩的形成与区域上广泛存在的晚三叠世构造、岩浆及角度不整合事件相对应,可能指示羌塘盆地统一基底的形成时代为晚三叠世。这对深入认识羌塘盆地基底的时代、性质及含油气盆地资源远景评价等具有重要意义。     

藏北羌塘盆地基底高级变质岩LA—ICP—MS锆石U—Pb年龄及其地质意义

俄久卖高级变质岩位于藏北羌塘盆地中央隆起带北缘的玛依岗日地区,是目前羌塘盆地基底高级变质岩石的唯一代表。该高级变质岩由正片麻岩和副片麻岩组成,本文以正片麻岩为研究重点。正片麻岩锆石CL图像显示出明显的核一幔一边结构。根据LA—ICP—MS锆石u—Pb测年结果,锆石核部年龄范围为242~2490Ma,记录了岩浆岩源岩的年代信息：锆石幔部具有典型的岩浆振荡环带结构,年龄为207Ma±2Ma,相应的Th／u值介于0．02-0．30之间,代表正片麻岩原岩的岩浆结晶时代．该年龄与羌塘中部地区晚三叠世高压变质作用和岛弧岩浆作用在时空上相对应。锆石增生边的年龄为161～197Ma．对应的Th／u值介于0．02-0．15之间,代表片麻岩发生主变质作用的时代,可能是班公湖一怒江洋盆向北的俯冲消减作用在羌塘中部地区的响应。地球化学资料显示,正片麻岩具有类似岛弧型火山岩的地球化学特征。综合区域地质资料,俄久卖高级变质岩原岩的形成与区域上广泛存在的晚三叠世构造、岩浆及角度不整合事件相对应,可能指示羌塘盆地统一基底的形成时代为晚三叠世。这对深入认识羌塘盆地基底的时代、性质及含油气盆地资源远景评价等具有重要意义。     

西藏羌塘中部亚丹高压变质岩年代学、地球化学特征及其构造意义

羌南和羌北地块沿龙木错-双湖缝合带碰撞对接,标志着古特提斯洋的最终闭合,然而古特提斯洋的构造演化,尤其它的打开时限一直争议不断。本文对龙木错-双湖缝合带以南荣玛地区的变质杂岩进行了岩相学、年代学和地球化学特征研究,进而约束古特提斯洋的开、合时限及其关闭后的构造演化特征。通过LA-ICP-MS锆石U-Pb定年,本文首次报道了1件新元古代斜长角闪岩,其加权平均年龄为717±7Ma(n=61,MSWD=1. 9),表明原岩形成于新元古代中期,反映羌南地区存在前寒武基底;另获得1件中侏罗世斜长角闪岩的锆石加权平均年龄为163±2Ma(n=26,MSWD=1. 9),表明其原岩形成于中侏罗世;获得2件石榴石多硅白云母石英片岩锆石边部谐和年龄范围分别为267～1349Ma和214～2050Ma;另对片岩中2组多硅白云母样品进行~(40)Ar/~(39)Ar定年,分别获得224. 2±1. 5Ma和223. 9±1. 5Ma的一致坪年龄,暗示区域变质作用可能从～224Ma持续到214Ma之后,且～214Ma的岩浆事件可能形成于碰撞过程中。新元古代和中侏罗世斜长角闪岩的岩石地球化学特征显示,前者原岩具E-MORB特征,后者原岩趋向于OIB特征。基于区域岩石组合与地球化学特征,推断二者均形成于伸展环境,前者可能形成于陆内伸展背景,响应Rodinia超大陆的裂解,后者可能形成于大陆裂谷环境,响应羌南、羌北地块碰撞结束后的裂解。综合前人与本文研究成果,认为新元古代中期(～717Ma),某未知陆块可能就已逐渐开始从羌南-印度大陆裂解,导致"古特提斯洋"在ca. 717～517Ma之间的某个时间点就已打开;古特提斯洋可能在～224Ma才完成闭合,羌南、羌北陆陆碰撞挤压持续到214Ma之后;中侏罗世区域为裂谷伸展环境,并发育OIB特征的碱性玄武岩和双峰式火成岩。     

Constraint on the eclogite age of the Sanbagawa metamorphic rocks in central Shikoku,Japan

ABSTRACT

To constrain the timing from the accretion to the subduction-related metamorphism of the protolith in the Sanbagawa eclogites, we performed zircon U–Pb datings and REE composition analyses on pelitic schist of the Seba eclogite-facies region in the Besshi area in central Shikoku, Japan. The detrital igneous cores of the zircons show ages from ca. 2000 to 100 Ma, and the metamorphic rims show ca. 90 Ma. These results show that the protolith was accreted at ca. 100–90 Ma, which is significantly younger than the previously reported accretion age of ca. 130 Ma of other eclogite-facies regions in this area. And, the metamorphic rim domains show HREE decrease without Eu anomalies, suggesting that they were formed at ca. 90 Ma eclogite-facies metamorphism. Our results combined with previous reports for the tectonics of the Sanbagawa metamorphic rocks suggest that there are at least two eclogite-facies units with different accretion ages in the Besshi area; ca. 130 Ma unit (Besshi unit) and ca. 100–90 Ma unit (Asemi-gawa unit), which structurally contact with each other. It is likely that the older unit was subducted into a depth of over 50 km and stagnated until the younger unit was subducted to the same depth. Probably, both units were juxtaposed at a mantle depth and began to exhume to the surface at the same timing after ca. 90 Ma. The juxtaposition and exhumation process might have relation to multi-factors such as tectonic erosion along the subduction zone, shallowing subduction angle of the hotter slab, backflow in the mantle and fluid infiltration along exhumation route.     

Ordovician sedimentation and bimodal volcanism in the Southern Qiangtang terrane of northern Tibet: Implications for the evolution of the northern Gondwana margin

This article reports our new interpretations of the depositional environment and provenance of the Dawashan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ), in the Southern Qiangtang terrane of northern Tibet, in order to gain a better understanding of the Ordovician tectonic evolution of the northern margin of Gondwana. The Dawashan Formation is dominated by greywacke and shale, with interlayered bimodal volcanic rocks that were deposited in a bathyal to abyssal marine basin. The detrital zircons in the greywacke of the Dawashan Formation have peak ages of 550, 988, 1640, and 2500 Ma, indicating a northern Gondwana margin provenance. The bimodal metavolcanic rocks from the Dawashan Formation are dominated by metarhyolite with subordinate metabasalt. The results of zircon LA-ICP-MS U–Pb dating indicate that the metarhyolite formed between 470 and 455 Ma. The metavolcanic samples are bimodal (SiO₂ = 45.27–55.05 and 66.09–74.59 wt.%). In comparison, the metabasalt has a wide range of MgO concentrations and Mg^# values, contains variable Cr and low Ni concentrations, is depleted in Rb, Ba, and Sr, and is enriched in TiO₂, Th, U, Nb, and Ta. Geochemical diagrams show that the metabasalt erupted in an intra-plate environment. The metarhyolites have high SiO₂, Th, and U concentrations, low concentrations of MgO, P₂O₅, Nb, Sr, and Ti, and negative Eu anomalies. The metarhyolites yield negative zircon ε_Hf(t) values (–2.08 to – 4.50) and T^C_DM model ages of 1436–1567 Ma. The metarhyolites formed from magma derived from the partial melting of old continental crust. These data indicate that the Dawashan Formation records Middle–Upper Ordovician bathyal to abyssal turbidite deposition in a deep-water rift basin at the northern margin of Gondwana.     

羌塘盆地变质岩锆石SHRIMP U-Pb年龄及其地质意义

笔者在位于北羌塘的玛尔果茶卡发现了具有区域变质特点的副变质岩组合,岩性为含蓝晶石矽线石长英质片麻岩、含蓝晶石夕线石绢云母石英片岩。1:25万区域地质调查将这套岩石命名为前奥陶纪齐陇乌如岩组,呈断续出露的小型穹隆分布在中央隆起带北缘附近。笔者对片麻岩中的锆石进行了SHRIMP年龄分析,获得锆石年龄347~2827Ma。本文对不同年龄段所反映的构造热事件及其地质意义进行讨论。     

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.     

藏北羌塘盆地基底变质岩的锆石SHRIMP年龄及其地质意义

羌塘盆地经受了青藏高原隆升事件等强烈的构造作用,盆地是否具有刚性结晶基底,对于油气的保存至关重要。长期以来对于该盆地是否具有前古生代的结晶基底,分歧较大。笔者新近在羌塘盆地中央隆起带北缘俄久卖发现的含夕线石和蓝晶石的片麻岩以及其附近发现的极浅变质的奥陶系地层,证实羌塘盆地具有变质结晶基底。本文进一步开展了片麻岩中锆石的SHRIMP年龄分析,获得了7组锆石年龄：2498~2374Ma 、1780 ~1666Ma、645~522Ma、465~420Ma、402~369Ma、270~233Ma和223~198Ma。通过CL图象对各组年龄锆石进行成因分析,并结合区域地质特征,认为1780 ~1666Ma为该片麻岩的主期变质年龄,羌塘盆地具有前寒武纪结晶基底,并对其余年龄组反映的构造热事件及其地质意义进行了讨论。     

Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks,Kyushu, Japan

On the basis of internal structures, laser ablation U–Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K‐feldspar, yield ²³⁸U–²⁰⁶Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48–1.64. The rims contain inclusions of jadeite, yield ²³⁸U–²⁰⁶Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite‐normalized REE patterns of the cores indicate higher Sm_N/La_N ratios, lower Yb_N/Gd_N ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different ²³⁸U–²⁰⁶Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the ²³⁸U–²⁰⁶Pb ages of the rims are consistent with the reported ⁴⁰Ar/³⁹Ar spot‐fusion ages of matrix muscovite in the jadeitite, the ²³⁸U–²⁰⁶Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.     

Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–TiO₂–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P?T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P?T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.     

Qiarbahete Sanukitoids and adakitic rocks in NW Tianshan: age and geochemical constraints on their petrogenesis and tectonic settings

The Qiarbahete complex in NW China consists of gabbroic diorite, granodiorite, and late-stage quartz diorite porphyry veins. Zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses show that the gabbroic diorite and granodiorite formed at 368 ± 5.2 Ma and 354 ± 4.1 Ma, respectively, indicating that the complex was emplaced in the Late Devonian–Early Carboniferous. The gabbroic diorites, characteristic of Sanukitoids, exhibit high Mg^# (62 average), MgO (6.84% average), Cr (195 ppm average), and Ni (61.4 ppm average) contents. The rocks show moderately fractionated rare earth element (REEs) patterns and weak negative Eu anomalies (δEu: 0.83–0.89), enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs), with low ?_Nd(t) values (1.46–1.73). The gabbroic diorites originated from partial melting of a hydrous mantle wedge followed by assimilation of crust during ascent. The granodiorites show a geochemical affinity with adakitic rocks, e.g. SiO₂ (64.95–67.87%) > 56%, Al₂O₃ (15.88–16.56%) > 15%, MgO (1.79–2.31%) < 3%, Sr (315–375 ppm) > 300 ppm, and Yb (1.84–2.06 ppm). They are enriched in light rare earth elements (LREEs) and LILEs and depleted in HFSEs, with weak negative Eu anomalies (δEu: 0.78–0.87). The granodiorites were mainly derived by the partial melting of a subducted oceanic slab, followed by subsequent melt–mantle interaction and crustal rocks contamination. All these indicate that the Qiarbahete complex was emplaced in a continental arc setting attending the southward subduction of the Junggar Ocean during the Late Devonian–early Carboniferous, generating the lateral accretion of continental crust in NW Tianshan.     

Late Cretaceous granitoids in Karakorum,northwest Tibet: petrogenesis and tectonic implications

New zircon LA-ICP-MS U–Pb age, zircon Hf isotope, and whole-rock major and trace elemental data of the Late Cretaceous Ageledaban complex in the Karakorum Terrane (KKT), northwest Tibet, provide new constraints on the tectonic processes of the collision and thickening of the terrane between the Lhasa and Qiangtang terranes. The granitoids from the Ageledaban complex have a variable SiO₂ content, from 62.83 to 73.35 wt.% and A/CNK<1.1 (except for YM61-2). They have rare earth element and trace element patterns that are enriched in light rare earth elements, Rb, Pb, Th, and U, and are depleted in Ba, P, Sr, Ti, and Nb, indicative of weakly peraluminous-metaluminous I-type affinity. Zircon U–Pb dating reveals that the Ageledaban complex was emplaced at ca. 80 Ma. Zircons from the monzogranite and monzonite samples with concordant ²⁰⁶Pb/²³⁸U ages about 80 Ma have a zircon ε_Hf(t) of ?6.6 to ?1.1, corresponding to the Mesoproterozoic Hf crustal model ages (T_DM^C = 1.2–1.6 Ga); the remaining inherited zircons from the monzonite with concordant ²⁰⁶Pb/²³⁸U ages of about 108.1 Ma have ε_Hf(t) values that range from ?8.3 to ?5.0, corresponding to the Mesoproterozoic Hf crustal model ages with an average of 1.6 Ga. These signatures indicate that the Ageledaban granitoids may have been derived from the partial melting of a mixed mantle-crust source. Together with the age and geochemical data in the literature, we propose that the collisional event in the KKT in northwestern Tibet would postdate the northern Lhasa–southern Qiangtang collision, which occurred first in the Amdo in the east and later in the Shiquanhe in central Tibet. Our results support the previous view that the collision of the Bangong–Nujiang suture zone (BNSZ) may be diachronous.     

Detrital provenance,depositional environment,and palaeogeographic implications of Lower Triassic marine sediments in central Tibet

We present our new investigation into the depositional environment and provenance of the Yingshuiquan Formation in the central Qiangtang region of northern Tibet, in order to further our understanding of the environment of the Longmu Co–Shuanghu Palaeo–Tethys during the Early Triassic. The Yingshuiquan Formation is composed of oolitic limestone, calcareous sandstone, calcarenite, thin-bedded ribbon limestone, bioclastic limestone, and coarse oolite limestone that were deposited in a shallow-marine basin and contain abundant Lower Triassic conodont fossils (e.g. Hadrodontina anceps, Pachycladina sp., gen. et sp, Pachyclaina oblique, Hibbardelloides sp). We selected detrital zircons from four calcareous sandstone samples for U–Pb dating, yielding minimum age peaks of 263, 269, 275, and 280 Ma, respectively, and a minimum age of 249 Ma, based on several zircons around the same age. Analysis of the conodont biofacies and zircon LA-ICP-MS dating of calcareous sandstone indicates that the data is consistent with deposition in the Early Triassic. The Yingshuiquan Formation records Early Triassic shallow-water sediment in the Longmu Co–Shuanghu Palaeo–Tethys, and has a Southern Qiangtang and Northern Qiangtang terranes provenance. During the Early Triassic, the carbonate sediments of the Yingshuiquan Formation were deposited in an active environment around the Longmu Co–Shuanghu Palaeo–Tethys, which has became a residual sea basin.     

藏北若拉岗日结合带中的浅变质地层及其锆石SHRIMP U-Pb年龄测定

通过对拉竹龙-西金乌兰湖-金沙江结合带西段若拉岗日一带的地层重新解体厘定,填绘出一套以白云母石英片岩、石英岩、变质石英砂岩为主的绿片岩相浅变质地层。该浅变质地层可与羌塘地块之上的浅变质岩系玛依岗日组对比。对浅变质地层的碎屑锆石进行U-PbSHRIMP年龄测定,认为所获得的最小年龄值524Ma代表了该套地层沉积时代的下限,再结合该地区出露未变质的泥盆纪地层这一事实,将这套浅变质岩系的形成时代置于早古生代。     

南秦岭城口火山岩锆石LA-ICP-MS U-Pb定年和地球化学研究

南秦岭大巴山城口断裂带出露一套玄武安山岩、安山岩组合,火山岩锆石LA-ICP-MS U-Pb定年测试结果为716±4Ma,表明其为新元古代岩浆产物;岩石地球化学研究表明火山岩富集轻稀土元素,原始地幔标准化微量元素蛛网图显示以富集大离子亲石元素Cs、Ba、Th、U及高场强元素分异为特征,Nb、Ta强烈亏损以及低的Ti(Ti O_20.85%)含量,与典型的岛弧火山岩相似;微量元素La/Nb、Th/Yb及Hf/Ta比值特征也显示岛弧岩浆属性,相对高的Zr/Y、Ta/Yb和低的Zr/Nb比值区别于大洋岛弧火山岩,具有明显的大陆亲缘性,表明城口火山岩形成于陆缘岛弧环境。综合已有的地质、地球化学及同位素年代学研究表明新元古代晚期扬子板块北缘及南秦岭地区为一活动陆缘岩浆杂岩弧,暗示中国华南板块很可能位于Rodinia超大陆的边缘部位。     

Dating of detrital zircons from the Dabure clastic rocks: the discovery of Neoproterozoic strata in southern Qiangtang,Tibet

ABSTRACT

This article reports the results of field mapping and the petrology of clastic rocks in the Dabure area, southern Qiangtang, Tibet, together with the results of U–Pb dating of detrital zircons from these rocks. The Dabure clastic rocks are characterized by low compositional and textural maturity, and they have been affected by lower greenschist facies metamorphism. The deposits exhibit the typical features of turbidites. Altogether, 279 detrital zircons were selected for U–Pb dating, and the ages fall into five groups: 550–650, ~800, 900–1100, 1600–1800, and 2300–2500 Ma. In general, the ages of the detrital zircons that are older than ~550 Ma are similar to those found elsewhere in the southern Qiangtang and Himalayan terranes. The most reliable youngest age of a detrital zircon from the Dabure clastic rocks is ~550 Ma. In the southern part of the Tibet Plateau, strata with the same ages and lithologies as the Dabure clastic rocks are widespread, especially in the Himalayan terrane. Combining our data with previous work on the basalts in the Dabure area (the Dabure basalts), we tentatively suggest that the Dabure clastic rocks represent the late Ediacaran (~550 Ma) sedimentary record for the Qiangtang terrane, and that before the late Neoproterozoic the southern Qiangtang terrane was possibly connected to the Himalayan terrane.     

浙江舟山群岛首次发现二叠纪变质岩

浙江舟山群岛大衢岛及周边若干小岛出露少量变质岩系,包括斜长角闪岩、片麻岩、片岩、大理岩等,以往被归为陈蔡岩群。LAICP-MS锆石U-Pb测年结果表明,黑云斜长片麻岩变质新生锆石或岩浆锆石的变质边年龄在260~270Ma之间,大理岩的变质年龄为261.1±1.3Ma,侵入变质岩的未变形伟晶岩脉年龄为258.0±1.6Ma,表明该套变质岩系的变质时代约为260Ma,不能归入加里东期变质的陈蔡岩群。这是在华南东北缘地区首次发现二叠纪变质岩,为探讨华南东吴运动和全球海西运动,甚至泛大陆的重建提供了重要新信息。     

Cambrian granitic gneiss within the central Qiangtang terrane,Tibetan Plateau: implications for the early Palaeozoic tectonic evolution of the Gondwanan margin

ABSTRACT

The Tibetan Plateau is located in the eastern Himalayan–Alpine orogen, an area where previous research has focused on ophiolites and a high-pressure metamorphic belt, whereas comparatively little research has been undertaken on the Tibetan basement. Cambrian granitic gneiss crops out in the Duguer area of the South Qiangtang terrane in northern Tibet and yields zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages of 502–492 Ma, providing insight into the possible existence of basement rocks within the South Qiangtang terrane. The granitic gneisses are geochemically similar to high-K, calc-alkaline S-type granites, and Hf isotopic analysis of zircons within the gneisses yields negative εHf(t) values (–7.4 to – 1.1) and old zircon Hf model ages (T_DM^C = 1757–1406 Ma). These granitic gneisses were generated by partial melting of ancient pelitic rocks, and the resulting melts were contaminated by a small amount of mantle-derived material. Combining our new data with previous research, we conclude that these Cambrian granitic gneisses developed in a post-collisional tectonic setting after Pan-African tectonism. This suggests that the South Qiangtang terrane might have the same early Palaeozoic crystalline basement as the Lhasa, Himalaya, Baoshan, Gongshan, and Tengchong terranes.     

滇西南大红山群变火山-沉积岩地球化学属性、年代格架及其构造意义

云南新平地区大红山群出露于扬子地块西南缘,主要由低绿片岩相-角闪岩相变质的火山-沉积岩组成。大红山群的岩石成因、年代格架及其形成的构造背景缺乏系统研究,制约了地质学家们全面认识和理解扬子西南缘<~1.75 Ga的构造演化历史。本文以大红山群底部老厂河组变沉积岩及其内部变火山岩夹层为重点研究对象,开展岩相学、全岩地球化学和锆石U-Pb定年等综合研究。岩石地球化学研究结果表明,变沉积岩的化学成分与大陆上地壳沉积物成分接近,原岩为成熟度较高的泥岩/页岩,未经历沉积再循环,形成于被动大陆边缘的构造背景;变火山岩原岩化学成分相当于钙碱性过铝质A型流纹岩,形成于造山后的大陆裂谷拉张环境。锆石U-Pb定年结果显示,老厂河组变沉积岩的碎屑锆石记录了2.3~2.2 Ga和1.9~1.75 Ga两个主年龄峰以及2.7~2.6 Ga次年龄峰。结合前人研究结果,表明大红山群物源主要来源于扬子地块西南缘的太古宙-古元古代基底岩石。变火山岩样品的岩浆锆石核部记录了1 713~1 711 Ma的年龄,应代表老厂河组原岩的形成时代,锆石的变质增生边限定峰期变质时代为约843 Ma。综合前人研究结果表明,大红山群普遍经历了849~837 Ma的新元古代变质事件。综上所述,扬子地块西南缘的大红山群完好记录了与Columbia超大陆裂解有关的非造山岩浆活动,新元古代变质事件可能与Rodinia超大陆裂解和聚合过程密切相关。     

西藏冈底斯东段日多地区典中组火山岩年龄、地球化学特征及其构造环境

林子宗群火山岩为古近纪岩浆活动的典型代表,记录了印度与欧亚大陆碰撞造山过程。以冈底斯带东段日多地区林子宗群典中组火山岩为研究对象,对其基性和中酸性火山岩进行岩石地球化学、锆石U-Pb年代学和Lu-Hf同位素研究。LA-ICP-MS锆石U-Pb测年结果表明,典中组玄武岩、安山质晶屑凝灰岩锆石U-Pb年龄分别为57.9±1.2 Ma和57.4±1.2 Ma。典中组火山岩为钙碱性-高钾钙碱性岩石,其中,基性火山岩表现为低SiO2（48.67%~49.34%）、富Al2O3（15.25%~18.59%）、MgO（3.76%~8.69%）,轻稀土元素相对富集和正Eu异常（δEu=1.15~1.37）特征;中酸性火山岩具有高SiO2（54.92%~64.16%）、富Al2O3（15.85%~16.72%）、K2O（0.65%~2.14%）,低MgO（1.34%~3.67%）的特征,轻稀土元素富集,呈现出弱的负Eu异常（δEu=0.77~0.92）。两者不同程度富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,球粒陨石标准化稀土元素配分模式图均呈现出右倾、轻稀土元素富集型曲线。锆石Lu-Hf同位素结果显示,基性火山岩（玄武岩）εHf（t）值为4.86~8.97,中酸性火山岩（安山质晶屑凝灰岩）εHf（t）值为0.26~6.37。结合前人研究成果,认为西藏日多地区典中组火山岩形成于古新世印度-欧亚大陆碰撞对接的同碰撞阶段,基性火山岩主要源自消减板片流体交代上覆地幔楔部分熔融的产物,中酸性火山岩主要为新生地壳的部分熔融,上升过程中均受到不同程度地壳物质的混染。