中国东北地区中生代构造体制的转变:来自火山岩时空分布与岩石组合的制约

东北地区中生代经历了蒙古-鄂霍茨克构造体系向太平洋构造体系的转换,形成了不同期次火山活动。本文归纳总结了露头区与覆盖区中生代火山岩的年代学、空间分布、岩石组合以及地球化学特征,揭示了两个构造域的时空分布范围。该区火山岩锆石U-Pb年龄统计结果表明中生代存在五期火山活动:早-中侏罗世(190~160Ma)、晚侏罗世(160~145Ma)、早白垩世早期(145~120Ma)、早白垩世晚期(120~100Ma)、晚白垩世早期(100~90Ma)。早-中侏罗世火山岩分布较少,火山岩仅分布在大兴安岭西部满洲里地区和东部张广才岭以及南侧辽宁北票-朝阳地区,火山岩属于高钾钙碱性系列,为蒙古-鄂霍茨克海闭合和法拉隆板块双俯冲作用的产物。晚侏罗世东北地区火山活动明显增强,主要分布在大兴安岭地区,张广才岭以及小兴安岭也有少量分布。西部大兴安岭地区以粗面安山岩、粗面岩为主,属于同碰撞造山成因,为蒙古-鄂霍茨克海闭合造山环境产物。东部以中酸性、酸性岩为主,为法拉隆板块背离欧亚大陆,岩石圈伸展引起的壳源物质熔融产物。早白垩世早期火山活动最为强烈,火山岩主要分布在大兴安岭地区。岩性以高钾钙碱性系列的粗面玄武安山岩、粗面安山岩、安山岩、粗面岩为主,为蒙古-鄂霍茨克海闭合造山后伸展环境产物。早白垩世晚期火山岩主要分布在松辽盆地内部。火山岩以中酸性岩为主,属于中钾-高钾钙碱性系列,为伊泽奈崎板块俯冲引起的弧后拉张,软流圈上涌导致年轻地壳熔融的产物。晚白垩世早期火山岩仅分布在小兴安岭及吉林、黑龙江省东部地区。火山岩为一套玄武岩、玄武安山岩、安山岩和英安岩组合,属于中钾钙碱性系列,是伊泽奈崎-库拉板块高角度俯冲的大陆边缘岩浆活动产物。东北地区中生代不同期次火山岩记录了蒙古-鄂霍茨克构造域向太平洋构造域转换过程及其时空影响范围。     

滇西马登地区晚二叠世-早三叠世地层组合及年代学:火山岩锆石U-Pb定年证据

在西南"三江"造山带中段的兰坪盆地内,由于露头状况不好,盆地基底岩石出露状况不详,导致地层划分、归属相当混乱。详细的野外地质调查揭示,盆地东缘马登地区出露的基底岩石主要由2个构造地层单元组成,上部为火山-沉积序列,下部为浅海相泥岩、灰岩及生物碎屑灰岩。上部火山-沉积序列出露厚约1200m,可分为4个喷发-沉积韵律,由英安质熔岩、流纹质熔岩与晶屑凝灰岩、火山集块岩、火山角砾岩、流纹质凝灰岩、火山碎屑岩及少量泥岩相间组成。火山岩锆石LAICP-MS U-Pb法测年数据显示,岩浆活动始于250Ma,持续至244Ma,总体处于早三叠世,构成江达-维西-云县弧火山岩带的一部分。强烈变形的海相地层与火山岩二者呈断层接触,其时代老于250Ma。结合砂岩中碎屑锆石年龄结果(大于260Ma)判定,这套沉积岩应属于晚二叠世,其与早三叠世-中三叠世火山岩一起组成兰坪盆地的基底岩石。     

中国火山温泉气体地球化学特征及其在火山监测中的应用

中国火山温泉主要分布在吉林长白山、云南腾冲和黑龙江五大连池等火山区。这些火山虽然处于休眠状态,但大面积的温泉分布指示着岩浆房存在的可能性。本文总结了前人研究成果,分析了中国主要火山区温泉气体地球化学特征,并探讨了温泉气体在火山监测中的应用。长白山、腾冲和五大连池火山区温泉气体地球化学性质类似,都以CO₂为主要气体,含量在80%以上,最高可达99%以上,其它气体组分包括CH₄、N₂、O₂、SO₂、H₂S、He和H₂等。长白山火山温泉气体中氦同位素比值（3He/4He）最高,约为4—6R_A,CO₂中碳同位素比值（δ13C）为-7.9‰—-1.3‰,CH₄中碳同位素为-48.0‰—-28.7‰;腾冲火山温泉气体氦同位素比值为3—5.5R_A,CO₂中的碳同位素为-6.49‰—-2.07‰,CH₄中碳同位素为-23.5‰—-9.3‰;五大连池火山温泉气体氦同位素比值约为3R_A,CO₂中的碳同位素比值为-9.6‰—-3.1‰,CH₄中碳同位素为-47.2‰—-44.4‰。3个火山区的温泉气体均显示地幔来源的岩浆气体特征,并在上升运移过程中受地壳或古俯冲物质的影响。     

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran:Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

The Nain and Ashin ophiolites consist of Mesozoic melange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent(CEIM).They largely consist of serpentinized peridotites slices;nonetheless,minor tectonic slices of sheeted dykes and pillow lavas-locally stratigraphically associated with radiolarian cherts-can be found in these ophiolitic melanges.Based on their whole rock geochemistry and mineral chemistry,these rocks can be divided into two geochemical groups.The sheeted dykes and most of the pillow lavas show island arc tholeiitic(IAT)affinity,whereas a few pillow lavas from the Nain ophiolites show calc-alkaline(CA)affinity.Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting.Petrogenetic modeling shows that these components were represented by pure aqueous fluids,or sediment melts,or a combination of both,suggesting that the studied rocks were formed in an arc-forearc tectonic setting.Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous.Previous tectonic interpretations suggested that the Nain ophiolites formed,in a Late Cretaceous backarc basin located in the south of the CEIM(the so-called Nain-Baft basin).However,recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic,which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location.This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction,below the CEIM,of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia.As a major conclusion of this paper,a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.     

长白山火山活动历史、岩浆演化与喷发机制探讨

广义的长白山火山在我国境内包括著名的天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。图们江火山和望天鹅火山活动都始于上新世,喷发活动分别介于上新世—中更新世（5.5~0.19 Ma）和上新世—早更新世（4.77 ~2.12 Ma）。天池火山和龙岗火山属于第四纪火山,喷发活动从早更新世（~2 Ma）持续到全新世。图们江火山岩为溢流式喷发的拉斑玄武岩,望天鹅火山、天池火山和龙岗火山母岩浆都是钾质粗面玄武岩,但经历了不同的演化过程。天池火山和望天鹅火山都经历了钾质粗面玄武岩造盾、粗面岩造锥和晚期碱性酸性岩浆（碱流岩和碱性流纹岩）的喷发;龙岗火山来自地幔的钾质粗面玄武岩浆则未经演化和混染直接喷出地表。图们江火山岩以溢流式喷发的拉斑玄武岩为主,少量玄武质粗安岩等。天池火山造盾之后,地壳岩浆房和地幔岩浆房具互动式喷发特点,来自地幔的钾质粗面玄武岩浆一方面在天池火山锥体内外形成诸多小火山渣锥,另一方面持续补给地壳岩浆房发生岩浆分离结晶作用和混合作用,分别导致双峰式火山岩分布特征和触发千年大喷发。火山岩微量元素和Sr-Nd-Pb同位素示踪揭示,长白山东（图们江火山、望天鹅火山和天池火山）、西（龙岗火山）两区显示地幔非均一性,东区岩浆源区具有软流圈地幔与富集岩石圈地幔混合特征,西区岩浆源区具有相对亏损的较原始地幔特征。西太平洋板块俯冲—东北亚大陆弧后引张是长白山火山活动的动力学机制。     

老黑山火山碎屑降落堆积物研究

由中心式喷发形成的老黑山火山碎屑降落堆积物，主要是玄武质火山渣、火山弹、火山饼和少量岩屑。在火山爆发自身能量和北北西风的影响下，形成高耸的火山渣锥和向南南东展布的低缓的火山碎屑席地貌景观。区域上系统测量了火山碎屑席中火山碎屑的厚度、最大平均粒径、分选系数和中值粒径，均呈现规律性变化；剖面层序则反映老黑山火山经过两个大喷发阶段，多次脉动式喷发。经估算，老黑山火山碎屑席中火山碎屑量比火山渣锥中要少得多，老黑山火山碎屑属“建造火山锥”火山碎屑沉积。据投点，老黑山火山喷发属斯通博里型。老黑山、火烧山火山仍有再次爆发的可能。未来火山喷发的潜在危险主要来自火山喷发空落堆积物。老黑山、火烧山火山的南东、东和南，包括五大连池市在内有必要对未来火山爆发加以防范。     

个旧东区变基性火山岩型铜矿成因分析

有证据表明,个旧矿区矿产除与花岗岩密切相关外,基性火山或喷流沉积作用也较明显,变基性火山岩型铜矿的发现及研究增添不少理由。该型Cu矿层状、似层状,赋存于个旧组下段卡房段,矿层多,具同生沉积构造,从而对个旧矿区成矿理论增添新看法。     

西藏措勤晚白垩世英安岩的厘定与地球化学特征

野外地质调查和SHRIMP锆石U/Pb地质年代学研究表明,拉萨地块中带措勤地区发育一套形成于晚白垩世中期的英安岩(88.5±1.2 Ma)。该套火山岩具有岛弧火山岩的地球化学特征,为地壳深部岩浆岩部分熔融形成的Ⅰ型岩浆岩,经历了明显的斜长石分离结晶作用。与同时代的侵入岩相比,该套火山岩为高度演化的岩浆,Rb/Sr比值较高,但Sr/Y比值较低。在拉萨地块内部,广泛发育的90～85 Ma的岩浆作用与新特提斯洋的北向俯冲作用相关。     

Lead isotope evolution of the Central European upper mantle:Constraints from the Bohemian Massif

The Pb isotope composition of the upper mantle beneath Central Europe is heterogeneous due to the subduction of regionally contrasting material during the Variscan and Alpine orogenies.Late Variscan to Cenozoic mantlederived melts allow mapping this heterogeneity on a regional scale for the last ca.340 Myr.Late Cretaceous and Cenozoic anorogenic magmatic rocks of the Bohemian Massif(lamprophyres,volcanic rocks of basanite/tephrite and trachyte/phonolite series) concentrate mostly in the Eger Rift.Cretaceous ultramafic lamprophyres yielded the most radiogenic Pb isotope signatures reflecting a maximum contribution from metasomatised lithospheric mantle,whereas Tertiary alkaline lamprophyres originated from mantle with less radiogenic ~(206)Pb/~(204)b ratios suggesting a more substantial modification of lithospheric source by interaction with asthenosphericderived melts.Cenozoic volcanic rocks of the basanite/tephrite and trachyte/phonolite series define a linear mixing trend between these components,indicating dilution of the initial lithospheric mantle signature by upwelling asthenosphere during rifting.The Pb isotope composition of Late Cretaceous and Cenozoic magmatic rocks of the Bohemian Massif follows the same Pb growth curve as Variscan orogenic lamprophyres and lamproites that formed during the collision between Laurussia,Gondwana,and associated terranes.This implies that the crustal Pb signature in the post-Variscan mantle is repeatedly sampled by younger anorogenic melts.Most Cenozoic mantle-derived rocks of Central Europe show similar Pb isotope ranges as the Bohemian Massif.