东昆仑土鲁音花岗岩的地球化学、锆石U-Pb年龄及地质意义

岩浆活动在板块构造机制中可以看作是大洋板块俯冲或者大陆板块碰撞等过程所伴随的深部作用响应,也是探索深部作用过程的"岩石探针"。土鲁音岩体位于东昆仑东部,紧邻昆中断裂南部区域。在详细野外调研基础上,主要对土鲁音花岗岩体进行岩石学、地球化学以及年代学的研究。结果显示,土鲁音岩体主要岩性为花岗闪长岩,其Si O2为68.49%~70.87%,全碱含量为5.94%~7.16%,K2O/Na2O比值为0.34~0.57,Mg#值中等(35.92~41.86),A/CNK值(1~1.07)均小于1.1,属弱过铝质钙碱性I型花岗岩。岩体稀土总量较低(118.91×10-6~143.97×10-6),(La/Yb)N为4.17~9.14,具有轻稀土元素富集、重稀土元素平坦分布的特征,弱负Eu异常,且显著亏损Nb、Ta、P、Ti等高场强元素,富集Rb、Ba、Th等大离子亲石元素。锆石LA-ICP-MS U-Pb测年结果显示土鲁音花岗岩体侵位时代(245.1±3.3)Ma,和区域内下得波利、埃坑德勒斯特花岗岩体具有一致的成岩年龄,同属印支早期产物。地球化学特征反映壳源部分熔融成因,并有幔源物质及热流参与,结合区域已有的年代学、地球化学以及地质资料,笔者认为土鲁音岩体形成于阿尼玛卿—古特提斯洋北向俯冲的地球动力学背景。     

西藏冈底斯带叶巴组火山岩地球化学及成因

叶巴组早侏罗世双峰式火山岩分布在拉萨、达孜至墨竹工卡之间, 岩性为浅变质玄武岩、玄武质熔结凝灰岩、英安岩、酸性凝灰岩及火山角砾岩等.火山岩SiO₂含量集中在41%~50.4%和64%~69%两个区间, 为钙碱性系列的玄武岩和英安岩2类.玄武岩的显著特征是TiO₂含量极低, 仅为0.66%~1.01%, 远低于大陆拉斑玄武岩.玄武岩的稀土总量∑REE=60.3~135μg/g, 英安岩的稀土总量∑REE=126.4~167.9μg/g.玄武岩和英安岩具有相似的稀土和微量元素特征, 两者均为轻稀土富集型, 分布特征相似, 轻、重稀土的分馏较明显, Eu异常均不显著; 均表现为LILE、LREE富集, HFS、HREE亏损的特点.玄武岩亏损Ti、Ta、Nb、Zr, Nb和Ta仅略负亏损, Nb*=0.54~1.17, 平均为0.84;英安岩亏损HFS中P、Ti, Ta、Nb略负异常, Nb*=0.74~1.06, 平均为0.86.玄武岩类的ε_Nd(t) =0.96~10.03、(87Sr/86Sr)_i=0.7043~0.7064, 英安岩的ε_Nd(t) =-1.42~1.08、(87Sr/86Sr)_i=0.7038~0.7049.从微量元素和同位素成分看, 玄武岩和英安岩浆起源于俯冲带之上的地幔楔不同程度的部分熔融, 源岩可能是亏损的尖晶石二辉橄榄岩.源区曾受到具地壳成分特征的流体不均匀交代.后期变质作用对岩石大离子亲石元素含量有影响.叶巴组双峰式火山岩形成于成熟岛弧后期的短暂拉张环境, 是印支期冈底斯岩浆弧演化的结果.      

Water as a Petrologic Catalyst Driving 18O/16O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ¹⁸O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ¹⁸O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable ¹⁸O/¹⁶O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of ¹⁸O/¹⁶O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ¹⁸O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in ¹⁸O/^l6O must be a result of exchange with magmatic or metamorphic H₂O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ¹⁸O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system.

LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H₂O, aided by internal muscovite dehydration at ?8 kbar and ?750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (?30°) partial melting of the metapelites produced magmas near H₂O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H₂O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H₂O. The released H₂O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H₂O was released and again re-used for a final episode of ¹⁸O/^l6O exchange with the unmelted metamorphic lithologies. In addition to the direct ^l8O/¹⁶O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H₂O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.     

西藏尕尔穷铜金矿床花岗岩类的地球化学特征及其地球动力学意义

尕尔穷铜金矿位于念青唐古拉板片与南羌塘板片之间的班公湖-怒江缝合带西段,区内花岗岩类形成于燕山晚期,主要为中酸性侵入岩,以岩基、岩株、岩枝、岩脉等形态产出。岩石具有较低的SiO2(52.92%～79.21%,多数集中在60%～65%,平均为63.64%)和较高的Al2O3(10.43%～16.49%,平均为15.23),高碱(ω(Na2O+K2O):4.86%～7.15%),ω(Na2O)/ω(K2O)比值变化较大(0.26～2.26),弱的负铕异常和负铈异常,相对富集不相容元素Rb、Th、K、Ce,而Ba、Ta、Nb、Ti亏损。这些特征表明花岗岩类是准铝质-弱过铝质钙碱性I型花岗岩,其源区物质可能是基性玄武质成分,形成于与板块俯冲有关的活动大陆边缘的陆缘弧环境,为深部地壳物质熔融后与幔源基性岩浆混合作用的产物。     

中国大陆印支碰撞造山系及其造山机制

古特提斯洋盆的闭合导致了诸多的微块体于晚三叠世至中侏罗世(T3-J2)碰撞,形成东亚大陆南部巨型印支造山系。中国大陆的印支碰撞造山系呈现巨大的"T"型复合造山系,位于西部的印支造山系(巴颜喀拉-北羌塘-南羌塘-拉萨印支造山带)形成于"多洋盆、多地体、多岛弧"的古特提斯的构造背景,伴随古特提斯多洋盆的俯冲和闭合,产生广泛的岛弧、增生楔和高压变质带的增生造山,以及多地体的碰撞造山作用,形成大型造山拼贴体,伴随以紧闭同劈理褶皱和逆冲、走滑断裂为特征的地壳变形。而位于中东部的印支造山系为北中国与扬子陆块之间的直接碰撞的产物,在扬子被动陆缘之上形成的松潘-甘孜、南秦岭造山带显示深层滑脱为造山机制的大规模地壳上部剪切应变;由于扬子地壳印支期深俯冲(>100km)和山根挤出机制,造成大别-苏鲁造山带中大规模高压-超高压变质带的出露。     

粤西高枨铅锌银矿区黑云母花岗岩的年代学及岩石成因

粤西高枨大型铅锌银矿赋存在黑云母花岗岩中,本文采用LA-ICP-MS锆石U-Pb测年获得黑云母花岗岩的结晶年龄为429±13Ma,为早志留世,属于加里东期岩浆活动的产物。详细的地球化学分析显示岩石的SiO₂含量为(64.33%~74.54%,平均为68.67%),Al₂O₃含量偏高(13.88%~16.61%),碱质含量高(K₂O+Na₂O=5.31%~7.89%),低P₂O₅(<0.25%)含量,强过铝质(A/CNK=1.07~1.32)。岩石富集LREE和Rb、K、U、Pb,贫Ba、Sr、Nb、P、Ti元素,铕呈负异常(Eu/Eu^*=0.38~0.90)。以上特征表明该岩体属于钙碱性强过铝质花岗岩。经年龄校正后的²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb 和²⁰⁸Pb/²⁰⁴Pb值分别为17.458~18.131、15.563~15.638和37.547~38.406。Sr初始值变化范围较大((⁸⁷Sr/⁸⁶Sr)_i=0.7091~0.7259),但ε_Nd(429Ma) 值相对较低 (-8.9~-6.1),暗示岩体来源于元古代(t_2DM=1.67~1.83Ga)的地壳组分,其源岩主要为含少量泥质的砂岩质组成。结合区域地质背景的综合分析,认为高枨黑云母花岗岩与华夏板块与扬子板块发生板内碰撞-拼合事件有关,由元古代地壳物质重熔形成的形成的S型花岗岩。     

湘中印支期关帝庙岩体地球化学特征及成因

关帝庙岩体位于湘中盆地,据年龄资料可分为中三叠世末和晚三叠世中期两个形成时期。中三叠世花岗岩中发育闪长质暗色微粒包体,自早至晚依次为细中粒角闪石黑云母花岗闪长岩、细中粒斑状角闪石黑云母二长花岗岩、细-细中粒(斑状)黑云母二长花岗岩。晚三叠世花岗岩自早至晚依次为细中粒-粗中粒斑状二云母二长花岗岩、细粒二云母二长花岗岩。岩石高硅、富铝、高钾,(Na2O+K2O)含量为6.80%~8.87%,平均7.74%;K2O/Na2O比值在1.35~2.66之间,平均为1.58;ASI值为0.99~1.40。总体属镁质、高钾钙碱性系列过铝质花岗岩类。中三叠世花岗岩Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集。中、晚三叠世花岗岩ΣREE含量为121.60~197.56μg/g,平均为158.70μg/g;δEu值0.28~0.68,平均为0.53;(La/Yb)N值为5.94~17.53,平均13.80。中三叠世花岗岩ISr值为0.71302~0.71758,εSr(t)值为121~186,εNd(t)值为-9.95~-8.74,t2DM为1.72~1.82Ga。C/MF-A/MF图解显示源岩主要为碎屑岩、少量基性岩和泥质岩。地质地球化学特征表明,印支期关帝庙花岗岩属S型花岗岩,形成于碰撞-后碰撞构造环境。     

西藏冈底斯西部地区印支期闪长岩的特征及其地质意义

对西藏冈底斯西部措勤县打加错地区辉石角闪二长闪长岩进行了LA-ICP-MS锆石U-Pb法定年及岩石地球化学研究,结果表明:辉石角闪二长闪长岩的结晶年龄为207.3±3.6Ma,其物质来源与下地壳基性熔岩及上地幔的部分熔融有关,是特提斯洋在晚印支期仍在俯冲的证据。该印支晚期侵入岩系首次在南冈底斯带西侧发现,对研究冈底斯带在晚印支期的构造岩浆演化具有重要意义。     

从中下三叠统膏盐特征看中下扬子早印支运动

中下扬子早-中三叠世膏盐层是对早印支运动的沉积响应,本文从膏盐的成因、中下扬子膏盐层位的差异、沉积环境的变迁、膏盐的厚度变化和产状特征、沉积中心和沉降中心的分布等方面,分析早印支运动的表现形式。中下扬子早-中三叠世依次出现陆棚台地(碳酸盐岩)、潟湖—潮坪(白云岩与膏盐)和滨海(碎屑岩),这种大规模海退,正是早印支运动表现为构造抬升的结果。抬升过程中,残留海水在炎热干燥的气候条件下蒸发形成膏盐层,而中扬子的膏盐层位比下扬子低,据此推测中扬子先于下扬子与华北板块碰撞而抬升。平面上,现今构造格局(主要由中印支—早燕山运动形成复向斜、复背斜和推覆构造)的坳陷区主要为白云岩与(硬)石膏互层或石膏夹少量白云岩或云质石膏,膏盐层厚度大,现今构造的隆坳过渡区主要为石膏假晶、含膏云岩、石膏质白云岩、白云岩夹石膏等,膏盐层厚度小,而膏盐的厚度和产状与沉积时的水深和地势直接相关;另外,地层等厚图显示膏盐层主要分布于当时的沉降中心。这些沉积特征显示膏盐沉积时的地势与现今隆坳相间的构造地貌存在一定程度的吻合,说明膏盐沉积时的地层已经发生了轻微的褶皱变形(未见角度不整合和地层缺失)。