西藏羌塘中部才多茶卡蓝闪石40Ar/39Ar年代学及地质意义

本文报道了藏北羌塘中部才多茶卡地区晚三叠世蓝闪石片岩及蓝闪石矿物的新发现及地质意义。通过对才多茶卡构造混杂岩地质剖面的实测,在剖面第6层灰绿色变基性火山岩(绿片岩)中新发现以产蓝闪石类矿物为特征的中—高压变质岩系——蓝闪石片岩。对蓝闪石片岩的岩石学和矿物学特征进行了详细室内研究。从蓝闪石片岩中挑选出蓝闪石单矿物进行了40Ar/39Ar同位素年龄测定,结果表明,蓝闪石单矿物的坪年龄值为209±4Ma,等时线年龄值为216±4Ma,坪年龄与等时线年龄基本一致,属于晚三叠世诺利期,这与区域上羌塘中部发育的红脊山—双湖蓝闪石片岩的中—高压变质事件相吻合。由于蓝闪石类矿物的产出具有特殊的构造机制和地质意义,因此双湖以东的才多茶卡地区蓝闪石片岩的新发现及蓝闪石矿物的40Ar/39Ar同位素年代学研究进一步补充和丰富了青藏高原腹地羌塘中部龙木错-双湖构造混杂岩带的物质组成和同位素年代学信息,同时也为解决羌塘地区"龙木错-双湖构造混杂岩带是否向东延伸"等重大基础性地质问题,提供了新的重要资料。研究资料表明,羌塘中部自西向东从冈玛错、红脊山、绒马,经嘎尔错到才多茶卡地区,沿构造线方向断续分布的长达600km的含蓝闪石片岩的中—高压变质岩系,都是龙木错-双湖构造混杂岩带印支期构造活动事件的产物。     

藏北羌塘中部绒玛地区蓝片岩岩石学、矿物学和40Ar/39Ar年代学

藏北羌塘中部沿龙木错-双湖-线出露一条低温高压变质带,目前已有多处蓝片岩的报道.然而,除冈玛错地区产有典型的蓝闪石外,多数地区并没有典型蓝闪石的报道.绒玛蓝片岩位于羌塘中部高压变质带的中段,是该带中规模最大、保存最好的蓝片岩,对蓝片岩进行了详细的岩石学和矿物学研究,钠质角闪石主要为蓝闪石、青铝闪石、钠闪石和镁钠闪石.对蓝片岩中蓝闪石和多硅白云母进行了40Ar/39Ar定年,获得了227.3±3.8Ma和215±1.5Ma的坪年龄,分别代表蓝片岩快速俯冲消减和俯冲作用结束开始折返抬升的时代.绒玛蓝片岩岩石学、矿物学和40Ar/39Ar年代学研究为羌塘中部高压变质带的研究提供了新的资料.     

藏北羌塘菊花山那底岗日组火山岩锆石SHRIMP定年及其意义

那底岗日组主要为一套中酸性火山岩、火山碎屑岩,夹砂砾岩的岩石组合,主要分布在藏北羌塘中部地区,龙木错-双湖缝合带的北侧。对那底岗日组底部安山岩进行锆石SHRIMP测年,时代为219±4Ma,确定那底岗日组火山岩早期喷发时间为晚三叠世而不是早侏罗世,与龙木错-双湖一线的低温高压变质带的时代基本一致。结合区域地质资料,那底岗日组火山岩可能形成于板块消减的火山弧构造环境,为龙木错-双湖缝合带晚三叠世向北俯冲消减的产物。     

青藏高原中部中生代OIB型玄武岩的识别:年代学、地球化学及其构造环境

目前对青藏高中部的蛇绿岩类型、形成环境及其深部地幔源区特征还缺乏很好的约束。在区域地质调查基础上,本文展示了青藏高原中部龙木错—双湖缝合带嘎错玄武岩、班公湖—怒江缝合带多玛、塔仁本玄武岩及那曲盆地西侧中生代玄武岩的单斜辉石Ar-Ar测年、锆石SHRIMP定年和地球化学及Sr,Nd,Pb同位素数据,以约束形成这些玄武岩的时代、构造环境和地幔源区特征。目前的数据表明:1羌塘双湖嘎错枕状玄武岩单斜辉石的中温坪年龄为232.5±2.4Ma,可能指示嘎错玄武岩浆活动发生于中三叠世晚期,班公湖—怒江缝合带多玛枕状玄武岩、塔仁本玄武岩浆活动时代大约在早白垩世中晚期(110Ma左右);2在这些蛇绿混杂岩带中的玄武岩显示OIB而不是MORB型地球化学特征,双湖嘎错玄武岩的地球化学特征介于峨眉山高Ti玄武岩与夏威夷碱性玄武岩之间;中晚三叠世那曲嘎加组玄武岩的地球化学特征非常类似于夏威夷碱性玄武岩;班公湖—怒江缝合带内的早白垩世多玛玄武岩和塔仁本玄武岩的地球化学特征在很大程度上可比于夏威夷碱性玄武岩;3双湖嘎错OIB型玄武岩可能形成于以增生楔为基底的裂谷环境而不是以洋壳为基底的大洋板内环境,那曲嘎加组OIB型玄武岩很可能形成于以弧内—弧前沉积物为基底的陆棚—陆坡环境下的裂谷背景,塔仁本和多玛OIB型玄武岩形成于以洋壳为基底的洋岛环境,这表明班公湖—怒江洋壳在大约110Ma时尚未彻底消亡,可能暗示班公湖—怒江洋盆的关闭时间明显晚于晚侏罗世—早白垩世早期闭合的早期认识;4地球化学指标显示青藏高原中部中生代玄武岩未受到地壳物质或很少受到陆下岩石圈物质改造,一些相对新鲜样品的Nd,Pb组成似乎可以用来代表其地幔源区的成分特点,其高206Pb/204Pb比值(>18.5)指示羌塘双湖中晚三叠世嘎错玄武岩、班公湖—怒江缝合带早白垩世洋岛玄武岩所代表的中生代特提斯地幔很可能不具“Dupal”异常。然而,由于研究程度的限制和缺乏更多的可靠数据,这种观察还需要进一步确认。     

青藏高原羌塘高压变质带的特征及其构造意义

羌塘高压变质带分布于龙木错-双湖板块缝合带的南侧．西起红脊山．经片石山、蓝岭、角木查尕日、纳若．到双湖以东的才多茶卡,长约500km。向东已经延伸到巴青以北和昌都的吉塘地区,再向南进入滇西与澜沧江蓝片岩带共同构成一条断续延伸近2000km的高压变质带。羌塘高压变质带主要由蓝片岩和榴辉岩构成,蓝闪石片岩中蓝闪石和多硅白云母同位素定年结果为223-215Ma,榴辉岩的变质年龄为243-217Ma。估算蓝片岩的变质温度为410-460℃,变质压力为0．67-0．75GPa：榴辉岩相的变质作用温度不超过5000C,压力为1．56-2．35GPa。羌塘高压变质岩定年研究结果确定龙木错-双湖板块缝合带最终碰撞闭合发生在晚三叠世早期。羌塘高压变质带是冈瓦纳与欧亚大陆汇聚事件的重要记录。     

青藏高原龙木错-双湖板块缝合带闭合的年代学证据——来自果干加年山蛇绿岩与流纹岩Ar-Ar和SHRIMP年龄制约

青藏高原龙木错-双湖板块缝合带从碰撞缝合何时转为羌塘盆地的沉积基底并接受沉积,一直受到地学界密切关注。羌塘中部的果干加年山主脊首次发现未变质的沉积岩以角度不整合覆盖于蛇绿混杂岩岩之上,上覆地层底部流纹岩夹层锆石SHRIMP U-Pb年龄为214±4Ma,为沉积盖层提供了可靠的年龄依据;不整合面之下强烈变形的阳起片岩(变质玄武岩)中阳起石Ar-Ar年龄为219.7±6.5Ma,与羌塘中部龙木错-双湖高压变质带获得的榴辉岩多硅白云母、蓝闪石等Ar-Ar变质年龄一致。Ar-Ar和SHRIMP定年结果表明该角度不整合的时限为220～214Ma之间。角度不整合上下岩石的定年研究,为龙木错-双湖缝合带的闭合提供了确切的时间约束:羌塘地区冈瓦纳与欧亚(扬子)板块在214Ma以前实现了闭合,古特提斯消亡,进入陆表海演化阶段。     

西藏双湖纳若地区花岗闪长岩LA-ICP-MS 锆石U-Pb 年龄与地球化学特征

龙木错—双湖缝合带内呈东西向断续分布大小不等的花岗岩,1∶25 万区域地质调查研究结果显示,该花岗岩时代为早—晚侏罗世。本文对双湖纳若地区原为早侏罗世的花岗闪长岩2 件样品进行了LA-ICP-MS 锆石U-Pb 测年,获得年龄为215.1±1.8Ma、217.0±1.5Ma,属晚三叠世。岩石地球化学研究表明,花岗岩闪长岩属偏铝质—过铝质高钾钙碱性花岗,轻、重稀土元素分馏较明显,高场强元素Nb、Ta、Ti 强烈的亏损。形成于龙木错—双湖古特提斯洋闭合后陆—陆碰撞晚期的后碰撞阶段的伸展环境,以龙木错—双湖缝合带所代表的特提斯洋岩石圈向北俯冲,伴随之后的俯冲板片的断离,软流圈物质上涌,压力减低,温度升高,促使地壳物质发生了部分熔融,花岗闪长岩就是地壳物质部分熔融的产物。     

青海玉树哈秀岩体成因及^40Ar/^39Ar年代学研究

哈秀石英闪长岩体出露于西金乌兰金沙江缝合带西段,对其进行成因和侵位时代研究可以对沿西金乌兰金沙江缝合带发生的板块俯冲、碰撞事件提供同位素年代制约。用主元素判别法判定其为造山花岗岩类(IAG型),形成于板块俯冲阶段。角闪石的40Ar/39Ar测年结果表明其侵位时代约为216.4Ma,是印支晚期岩浆活动的产物。哈秀岩体的成因和时代研究表明,沿金沙江缝合带,迟至晚三叠世,板块碰撞作用仍然没有开始,洋壳的俯冲作用仍在继续。     

新疆东昆仑野牛泉石英闪长岩与英安斑岩的^40Ar／^39Ar同位素年龄

对出露在新疆东昆仑落雁山—阿尔喀山一带闪长岩-花岗闪长岩带中的野牛泉石英闪长岩中的角闪石和次火山岩英安斑岩中的黑云母2种单矿物进行40Ar39/Ar同位素定年,分别获得角闪石(219±1.4)Ma坪年龄和(220±11)Ma等时年龄,黑云母(218.59±0.94)Ma坪年龄和(226±11)Ma等时年龄,证实它们形成于晚三叠世。由于它们具有弧侵入体的岩石学和地球化学特征,因此该年龄的确定对于以木孜塔格-鲸鱼湖蛇绿混杂带为标志的南昆仑造山带构造演化研究具有重要意义。     

藏北南羌塘陆块北缘晚三叠世弄佰组火山岩的特征及构造环境

藏北南羌塘陆块北缘发育一套晚三叠世火山岩,命名为弄佰组。该组火山岩以玄武岩为主。岩石化学成分以低TiO_2、FeO Fe_2O_3、K_2O为特征。∑REE介于162.91×10~(-6)383.4×10~(-6)之间,∑Ce／∑Y介于1.83～3.00之间,δEu介于0.47～0.97之间,平均为0.73,负Eu异常较明显。微量元素Sr、K、Rb、Ba、Th、Ce、Sm强烈富集,Ta、Nb、P、Ti、Cr相对亏损。岩石学和岩石地球化学特征表明弄佰组火山岩形成于消减带岛弧区。弄佰组火山岩出露于龙木错-双湖结合带的南侧,K-Ar同位素年龄值为223Ma±5Ma.与双湖带中蓝片岩和绿片岩相变质岩系的形成年龄(220～235Ma)基本一致,时空关系的紧密相连,说明其形成可能与晚三叠世龙木错-双湖古特提斯洋的南向俯冲消减事件有关。     

40Ar‐39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang,Northern Tibet,Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China (~84°30′ E, 34°24′ N), provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite- and glaucophane-bearing assemblages equilibrated at 375–400 °C and ~11 kbar. 40Ar-39Ar analysis on mineral separate from one blueschist sample yielded a well-defined plateau age of ~242 Ma. Geochemical studies show the blueschist is metamorphosed within-plate basalts. The high pressure-low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in?situ suture between eastern and western Qiangtang.     

北秦岭二郎坪岩群~(40)Ar/~(39)Ar坪年龄特征及其地质意义

对来自二郎坪岩群的白云母、角闪石用阶段升温法进行了~(40)Ar/~(39)Ar年龄谱研究,结合二郎坪岩群地质特征,认为白云母近似直线年龄港的评年龄（111．2Ma）及角闪石稳定评年龄（121．5Ma）代表了二郎坪岩群最后一次热事件的时代;角闪石与白云母封闭温度的不同以及角门石的成分与结构环带导致二者坪年龄的差异,二郎坪岩群在121．5～111．2Ma曾发生区域变质作用,整个秦岭造山带在此期间仍处于构造活动期。     

Collision Event during 177-135 Ma on the Eastern Marginof the Qinghai-Tibet Plateau: Evidence from ~(40)Ar/ ~(39)Ar Dating for Basaltson the Western Margin of the Yangtze Platform

Geochronology of continental flood basalts sampled from the Emei large igneous province (LIP) on the western margin of the Yangtze platform was investigated by the laser microprobe 40Ar/39Ar dating technique. These basalts yield a fairly wide range of 40Ar/39Ar ages, varying from 259 to 135 Ma. One basalt sample, at least altered, recorded the oldest 40Ar/39Ar age of about 259 Ma, corresponding to a peak eruption age of the Emei LIP continental flood basalts. Most of the samples yield much younger ages from 135 to 177 Ma, which are consistent with the K-Ar ages for the same samples (122.8-172.1 Ma). The dating data suggest that these Permian basalts had been widely affected by the regional tectonothermal event at 177-135 Ma. The event was probably caused by the convergence and collision among the Laurasia, Yangtze and Qiangtang-Qamdo continental blocks on the eastern margin of the Qinghai-Tibet plateau after the late Triassic. The age of the event reflects the timing of the peak collisional orogeny.     

Geochronology of the Duguer range metamorphic rocks,Central Tibet: implications for the changing tectonic setting of the South Qiangtang subterrane

The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected for in situ zircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments.     

锆石La-ICP-MS U-Pb与白云母~(40)Ar/~(39)Ar年代学及其对中国东南部早燕山事件的制约

起始于中、晚侏罗世之交的燕山运动使中国东南部上三叠统至中侏罗统发生强烈的褶皱和逆冲变形,形成北北东向构造带,并诱发广泛的深熔作用和岩浆活动。迄今为止,对该事件的形成时代尚缺乏精确的年代学制约。基于武夷山地区的野外调查,选取挤压活动前形成的正长岩和挤压过程中形成的片麻状花岗岩进行锆石La-ICP-MSU-Pb定年,并对逆冲剪切过程中形成的云母片岩进行40Ar/39Ar年代学测试分析,结合区域上后构造期的伸展型花岗岩对早燕山事件的形成时代进行制约。结果表明,形成于伸展构造背景的福建政和县铁山正长岩,其结晶年龄为(169.3±1.6)Ma,提供了早燕山事件的下限;形成于挤压环境的粤北平远县八尺片麻状花岗岩,其结晶年龄为(165.4±1.2)Ma,指示早燕山事件的主变形时代;同变形期的闽西长汀县濯田云母片岩,其40Ar/39Ar坪年龄为(162±2)Ma,代表逆冲剪切的冷却年龄,可视作早燕山事件的上限。结合区域上侵入北北东向褶皱的花岗岩年龄,认为早燕山挤压事件发生在(165±4)Ma,其动力作用与古太平洋板块向中国东南大陆之下俯冲有关。     

40Ar/39Ar Geochronology of Volcanic and Intrusive Rocks in the Papandayan Metallic Prospect Area,West Java,Indonesia

This study presents new ⁴⁰Ar/³⁹Ar ages on the volcanic and intrusive rocks from the Papandayan metallic district in West Java, Indonesia. The vein system in the Arinem area, one of the prospective areas in the district, has been considered as an epithermal gold–silver–base metal deposit, however, no published age results are available for the host volcanic rocks in the district. The ages of these rocks are critical in terms of their association with mineralization and are important to understand the evolution of volcanism in the region, which has implications for mineral exploration in the district. ⁴⁰Ar/³⁹Ar plateau ages of two typical basalt and one andesite samples of the Jampang Formation volcanic rocks yielded ages of 11.65 ± 0.52 Ma, 18.15 ± 0.46 Ma and 7.69 ± 0.05 Ma, respectively. ⁴⁰Ar/³⁹Ar ages of three intrusive rock samples from Gunung Halang diorite, Gunung Lingga diorite, and Gunung Buligir fine‐grained quartz diorite yielded ages of 12.98 ± 0.20 Ma, 10.81 ± 0.15 Ma, and 7.37 ± 0.05 Ma, respectively. The age of the youngest fine‐grained diorite (Gunung Wayang dike) is 3.95 ± 0.03 Ma. An ⁴⁰Ar/³⁹Ar age obtained from adularia in the Arinem mineralized vein (18.30 ± 0.20 Ma) is older than the age of altered basalt sample of this study (11.65 ± 0.52 Ma) and the K–Ar illite ages of the Arinem vein (9.4 ± 0.3 Ma and 8.8 ± 0.3 Ma) which resulted from a previous study. The age results suggest that the Papandayan district may have experienced multiple hydrothermal and mineralization events. This study, therefore, provides crucial age data to support future mineral exploration in the district.     

北羌塘地区北缘上三叠统若拉岗日岩群地层特征及沉积环境

北羌塘地区北缘上三叠统若拉岗日岩群分布于北羌塘陆块与拉竹龙-金沙江缝合带之间的若拉岗日冲断带,以砂泥质复理石、中基性-超基性火山岩及大理岩组合为特征,夹晚二叠世灰岩岩片及蛇绿岩残块。岩石低-中级变质,构造变形强烈,顶底均被断层切割断失,为总体无序、局部有序的构造-地层体。若拉岗日岩群中基性火山岩具有洋岛和岛弧型成因,它是金沙江洋盆在晚三叠世向南俯冲,而在其南缘形成的岛弧带沉积。在若拉岗日岩群采获大量上三叠统常见的孢粉、腕足、双壳类生物化石,其玄武岩年龄值为201±4Ma(Ar-Ar法),时代属诺利期。     

Late Palaeozoic 40Ar/39Ar ages of the HP-LT metamorphic rocks from the Kekesu Valley,Chinese southwestern Tianshan: new constraints on exhumation tectonics

abstract

Although numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining ⁴⁰Ar/³⁹Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for ⁴⁰Ar/³⁹Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year^–1) uplift to ~10 km by ~293 Ma.     

40Ar/39Ar Ages of the Da Lien Granite Related to the Nui Phao W Mineralization in Northern Vietnam

⁴⁰Ar/³⁹Ar dating was conducted on the Da Lien granite related to greisen‐skarn type polymetallic (W‐CaF₂‐Cu‐Bi‐Au) mineralization in Nui Phao, northern part of Vietnam in the South China Plate. Biotite and muscovite separates from the biotite‐muscovite granite and greisenized granite indicate four plateau ages: 82.2 ± 0.4 Ma, 82.8 ± 0.3 Ma, 81.5 ± 0.3 Ma and 82.5 ± 0.4 Ma. The plateau ages were not significantly influenced by excess ⁴⁰Ar in dated minerals or by loss of radiogenic ⁴⁰Ar due to hydrothermal activities. The results indicate that solidification of granite related to the polymetallic mineralization occurred in the Late Cretaceous between 82.8 Ma and 81.5 Ma.     

地幔包体金云母40Ar/39Ar年龄对玄武岩浆喷发结束时间的约束

山东临朐-昌乐地区新生代岩浆活动强烈,以形成含大量地幔包体的玄武岩为特征.作者在考察与研究昌乐北岩古火山口玄武岩中地幔包体的过程中,发现一些包体中发育有地幔钾交代成因的金云母细脉.鉴于金云母的封闭温度远低于玄武岩浆喷发时的温度以及金云母可以记录喷发的玄武岩浆冷却通过金云母封闭温度的时间,本文尝试通过金云母所记录的~(40)Ar/~(39)Ar年龄来推断玄武岩浆喷溢的结束时间.该火山口玄武岩不同部位的三个地幔岩包体其金云母的~(40)Ar/~(39)Ar坪年龄分别为18.42±0.21Ma、18.65±0.27Ma和18.39±0.36Ma,年龄结果具有很好的一致性,充分说明了该定年手段的有效性.因此可以确定该火山口玄武岩浆喷溢活动约在18.5Ma前结束.由此推测山旺盆地中不整合发育在源于该火山口喷溢的玄武岩之上的化石群的形成时代下限约为18.5Ma.用玄武岩地幔包体金云母的~(40)Ar/~(39)Ar年龄来确定玄武岩年龄是玄武岩定年一个有效的新方法.