豆荚状铬铁矿:古大洋岩石圈残片的重要证据

豆荚状铬铁矿为蛇绿岩的特征性矿产 ,保留了上地幔岩浆构造作用、高温变形以及岩石成因的重要信息。它们常见于方辉橄榄岩内 ,位于大洋岩石圈莫霍面下 1～ 2km的古深度范围内。豆荚状铬铁矿常被纯橄岩薄壳围限 ,保留特征的豆状、豆壳状等构造。豆荚状铬铁矿的TiO2 含量较低 ,铂族元素 (PGE)的分布模式显示特征的负斜率。普遍认为 ,豆荚状铬铁矿形成于部分熔融条件下 ,涉及原始地幔熔体与亏损地幔橄榄岩的相互作用 ,伴随复杂的岩浆混合及结晶过程。狭窄的上地幔岩浆通道或孔穴为豆荚状铬铁矿理想的堆积部位。超俯冲带 (弧后盆地、岛弧、弧前 )、大洋中脊、转换断层均可能是豆荚状铬铁矿形成的理想环境。其中 ,洋脊扩张模式及大洋上俯冲带模式较好地解释了豆荚状铬铁矿成因。对于经历高级变质及多期变形的华北大陆基底 ,豆荚状铬铁矿是研究古老蛇绿岩最直接而有效的地质标志 ,对于研究古大洋岩石圈增生过程 ,上地幔演化 ,探索早期板块构造意义重大。     

西藏南部桑日群火山岩的时代：来自晚期马门侵入体的约束

位于拉萨地块南缘的桑日群火山岩通常被认为是新特提斯洋沿拉萨地块南缘向北俯冲消减的产物,但其喷发持续时间一直没有得到有效约束。在桑日群火山岩系中识别出一套稍晚于桑日群火山岩的侵入岩脉, 脉岩中的锆石用U-Pb LA-ICPMS 法测得年龄为（93.4±1.1） Ma。据此认为桑日群火山岩的喷发时间应不晚于93.4 Ma。前人的研究表明桑日群的活动时限可以追溯到晚侏罗世-早白垩世, 因此其所代表的新特提斯洋的俯冲消减至少从晚侏罗世延续到了93.4 Ma 左右。另一方面, 这些侵入岩脉的地球化学特征明显不同于俯冲成因的桑日群弧火山岩, 如埃达克岩（麻木下组）和钙碱性的火山岩（比马组）, 而与东部朗县和里龙一带 的侵入岩非常相似, 它们很可能是板片俯冲导致的加厚下地壳部分熔融的产物。这种差异暗示在新特提斯洋 俯冲消减过程中其深部动力学环境发生了较大的改变。     

Two-episode Tectono-thermal Events of the Heyuan Fault in Late Cretaceous and Oligocene and their Tectonic Implications, Southernmost South China Block

The Heyuan Fault, is one of the main NE-trending faults in the southernmost South China Block and is close to the northernmost South China Sea to the south. The fault features multi-stage deformation and controlled the formation of late Cretaceous to Cenozoic basins. Based on detailed field investigations and comprehensive geochronological research, including zircon U-Pb, Rb-Sr isochron, zircon U-Th-He dating, two episodic tectono-thermal events are recognized. The first occurred during ~79 Ma to 66 Ma, which is characterized by large-scale quartz-fluid emplacement. The other occurred at ~34 Ma, which features the eruption of a set of basalts. The two events show a changing on material source from siliceous hydrotherm to basalt magma, supporting obvious lithospheric thinning of the southernmost South China Block, which shed light on the geological evolution and the interaction mechanisms between the SCB and the northern South China Sea since the Late Cretaceous.     

中拉萨地块南缘孔隆晚白垩世火山岩成因及对地壳演化的约束

拉萨地块晚白垩世岩浆岩的研究集中于南北两缘,中部的岩浆-构造演化对班公湖—怒江洋或新特提斯洋演化过程的响应及地幔贡献度仍然缺乏精确约束。本次研究报道了中拉萨地块南缘孔隆地区发现的一套流纹质火山岩。锆石U-Pb年代学、全岩地球化学及Hf同位素分析结果显示,孔隆火山岩LA-ICP-MS锆石U-Pb年龄为(88.0±1.7)Ma,形成于晚白垩世早期。主量元素SiO₂及Al₂O₃含量较高,Mg^#较低;轻、重稀土分馏明显,表现为轻稀土相对富集,重稀土相对亏损;相对富集Rb、Th、K,相对亏损Ba、Eu、Sr、P、Ti;与周缘埃达克岩区别明显。锆石ε_Hf(t)在-10.2~-5.3之间,T_DM2模式年龄为1 492~1 804 Ma,结合Nb/La平均值(0.21)及Nb/Ta平均值(10.1)分析,认为孔隆火山岩可能直接源于古老的下地壳深熔,未经幔源物质混染而形成,其主要驱动力为新特提斯洋壳北向俯冲。本项研究为印亚大陆碰撞前拉萨地块演化提供了新的约束。     

拉萨地区地下水水化学特征及形成机制研究

西藏是我国重要的生态安全屏障,拉萨地区地下水化学特征及形成机制研究,对揭示青藏高原现代表生过程变化机理具有重要作用,对服务国家生态安全建设具有重要意义。本文通过拉萨地区地下水调查、水样采集与分析,综合运用Gibbs模型模拟、水化学分析方法分析了地下水化学特征及水岩作用机理。结果表明:地下水电导率介于38.801 193.00 μS/cm,平均值为123.99 μS/cm,总溶解性固体(total dissolved solids,TDS)含量介于44.051 050.55 mg/L,平均值为150.75 mg/L,pH值大于7,属弱碱性水,地下水化学类型为HCO₃-Ca型和Cl-Na型,其中Cl-Na型水为地下温泉水。地下水形成过程主要是碳酸盐岩和硅酸盐岩的溶解、阳离子交换等作用下的结果,且受到一定程度的人为因素影响;地下水中的Na⁺、K⁺和Cl^-主要来自盐岩矿物的风化溶解,过量的Na⁺、K⁺来源于钠长石和钾长石等硅酸盐矿物的溶解, {\mathrm{HCO}}_3^{-}、Ca²⁺、Mg²⁺和 {\mathrm{SO}}_4^{2-}主要来自方解石、白云石、石膏以及其他含钙镁矿物的溶解。     

西藏阿翁错地区早白垩世花岗闪长岩——班公湖—怒江洋壳南向俯冲消减证据

对拉萨地块北部阿翁错地区花岗闪长岩进行了年龄分析、岩石地球化学研究。锆石LA-ICP-MS定年测得花岗闪长岩U-Pb年龄为(114.4±1.9) Ma,属于早白垩世晚期岩浆活动的产物。花岗闪长岩地球化学特征表明,其具有典型镁安山岩/闪长岩(MA)的地球化学特征,所有样品均具有较高Mg~#值(45.42～54.29),低的TFeO~*/MgO值(1.58～2.26);所有样品都显示轻稀土元素富集,富集大离子亲石元素,亏损高场强元素的特征。研究表明,阿翁错花岗闪长岩是班公湖—怒江洋壳在俯冲消减背景下,由俯冲洋壳脱水熔融产生的溶体与地幔橄榄岩发生交代作用的产物,为晚中生代班公湖—怒江洋盆的南向俯冲消减提供了直接的岩石学、地球化学、年代学证据。     

祁连造山带东端张家川地区长宁驿中元古代花岗质片麻岩LA-ICP-MS锆石U-Pb年龄及其构造意义

在祁连造山带东端古元古界陇山岩群中新识别出中元古代长宁驿花岗质片麻岩。为了精确确定长宁驿花岗质片麻岩的形成时代,对其进行了激光等离子体质谱LA-ICP-MS单颗粒锆石微区U-Pb同位素测年,花岗质片麻岩中锆石Th/U比值介于0.50～2.09,阴极发光图像显示锆石内部发育典型的岩浆生长震荡环带结构,属于岩浆结晶的产物。测得花岗质片麻岩的原岩岩浆结晶年龄为1765Ma±57Ma,表明该花岗质片麻岩原岩的形成时代为中元古代早期,可能形成于华北板块经吕梁运动后的后造山环境中,同时还从捕获的继承性岩浆锆石中得到了时代为1.95Ga、2.20Ga左右的继承年龄。结合区域地质资料,综合分析研究后认为,在中元古代祁连地块仍为华北板块西南缘的一部分,两者之间具有相同的结晶基底和构造属性,它们共同经历了吕梁运动后的后造山和陆内伸展裂解阶段,很可能与Columbia超大陆的聚合和裂解相关。     

Petrogenesis of Early Carboniferous Alkaline Basalt from the Wusun Mountain: Implications for Tectonic Evolution of the Western Yining Block,NW China

Late Paleozoic volcanic rocks are well exposed in the Yining Block, NW China, and are predominately composed of andesites, rhyolites and volcaniclastics as well as minor basalts. Study of the petrology, whole-rock geochemistry and zircon U-Pb dating for the Early Carboniferous alkaline basalts from Wusun Mountain, western Yining Block, constrains their petrogenesis and tectonic evolution. The alkaline basalts consist mainly of plagioclases, mostly albite and labradorite, as well as clinopyroxenes and olivines; zircon U-Pb dating indicates their formation at ca. 350 Ma. Geochemically, the basaltic samples have low SiO2 contents, and high TiO2, Al2O3 and alkaline contents, coupled with high Na2O/K2O ratios, displaying an alkaline basalt affinity. They show remarkable LILE enrichment and HFSE depletion. Meantime, these samples have relatively high TFe2O3, MgO, and Mg# values as well as Ni and Cr, relatively high Sm/Yb and U/Th, suggesting origination from a mantle source metasomatized by slab fluids. They formed in a transitional tectonic setting from arc to intraplate, showing a typical affinity of back-arc basin basalts. The alkaline basalts were likely generated in a nascent back-arc extension setting resulting from slab rollback of the southern Tianshan oceanic lithosphere. A bi-directional subduction model seems more reasonable for the evolution of the southern Tianshan Ocean. These new data will provide a new tectonic model for Late Paleozoic tectonic evolution of the western Yining Block.     

Triassic magmatism in the eastern part of the South China Block: Geochronological and petrogenetic constraints from Indosinian granites

The granitic dykes in the Badu Group,Zhejiang Province,South China provide important insights on tectonic setting and crustal evolution of the South China Block(SCB) and the Indochina Block during Triassic.Here we report LA-ICP-MS U-Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma,respectively,representing their timing of emplacement.The dyke rocks are enriched in K,Al,LREE,Rb,Th.U,and Pb.and are depleted in Nb,Ta,Sr,and Ti.The rocks are characterized by highly fractionated REE patterns with(La/Yb)N ratios of 28.46-38.07 with strong negative Eu anomalies(Eu/Eu* = 0.65-0.73).In situ Hf isotopic analyses of zircons from the Hucun granite yielded ε_(Hf)(t) values of-13.9 to-6.4 and two-stage depleted mantle Hf model ages of 1.68-2.15 Ga,which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block.The zircons from the Kengkou granite have ε_(Hf)(t) values ranging from 40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99-2.49 Ga,indicating magma origin from a mixed source.The Hucun and Kengkou dykes,together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust-mantle decoupling along the convergent plate boundary between SCB and the Indochina Block.     

贺西地区板块构造分区及晚古生代盆地形成与演化

贺西地区以阿拉善陆块为主体,太古界裸露地表、花岗岩发育,在南北缝合带之间还有六条形成期早、演化时期长的深大断裂,对早古生代壳块分区,晚古生代盆地形成、分布起着控制和分割作用。晚古生代在前陆盆地率先接受了上泥盆统、下石炭统沉积;步入晚石炭世、沉积范围继续扩大至碰撞裂谷盆地后,又扩大到鄂尔多斯西缘并与华北海(陆表海)连为一体。进入海西旋回中晚期,受蒙兴海槽、秦祁海槽会聚、收缩、对接碰撞构造背景和构造运动的影响,使曾经是石炭纪南海北陆的古构造格架、古地理面貌,被全面海退后二叠纪的陆相沉积所代替。晚海西—早印支运动,使上古生界全面发生挤压变形,形成多种类型断裂、褶皱构造与圈闭,并受到了普遍的隆起剥蚀和缺失。晚印支期断裂活动,构造将晚古生代形成的两类盆地解体为八个断陷和五个隆起区,又使新的盆岭构造格局在此基础上形成、发育和演变。