Comparative paleofloristics of the Albian-early Paleocene in the Anadyr-Koryak and North Alaska Subregions,Part 1: The Anadyr-Koryak Subregion

A high-resolution phytostratigraphic scheme based on plant macrofossils is suggested for the first time for the Albian and Upper Cretaceous of the Anadyr-Koryak Subregion of the North Pacific. Seven phytostratigraphic horizons of the subregion are distinguished based on successive stages in flora evolution substantiated by comprehensive data on described floral assemblages.     

Comparative paleofloristics of the Albian-Early Paleocene in the Anadyr-Koryak and North Alaska Subregions,part 2: The North Alaska Subregion

The Albian-Paleocene phytostratigraphic scheme suggested in this work for the North Alaska Subregion of the North Pacific differs from the previous ones and includes three phytostratigraphic horizons of subregional extent and four plant-bearing beds. The scheme is based on distinguished successive stages in flora evolution, being substantiated by comprehensive data on the described floral (plant macrofossils) assemblages. As is established, sedimentation environments gradually changed during the Albian-Paleogene from the west of the subregion to the east so that the marine sedimentation and accumulation of sediments in general terminated in eastern areas later than western ones.     

Flora development in Northeastern Asia and Northern Alaska during the Cretaceous-Paleogene transitional epoch

Study of floral succession from the Cretaceous-Paleogene boundary interval in Russian Far East (Zeya-Bureya depression), Northeastern Russia (Koryak Upland), and Northern Alaska (Sagavanirktok River basin) is crucial for better understanding palaeoclimatic and palaeogeographic factors, which controlled events in vegetation evolution at that time. The succession of fossil floras in the Zeya-Bureya depression includes plant assemblages of the Santonian, Campanian, early Danian, Danian, and Danian-Selandian age. The early Danian Boguchan Flora keeps continuity in composition and dominating taxa with the Campanian Late Kundur Flora. The Koryak Flora of the Amaam Lagoon area (Northeastern Russia) is dated as late Maastrichtian based on correlation of plant-bearing beds with marine biostratigraphy, whereas the Early and Late Sagwon floras of Northern Alaska are dated back to the Danian-Selandian and early Paleocene based on palynological and macrofloristic data. The Early Sagwon Flora is most close to the late Maastrichtian Koryak Flora of the Amaam Lagoon area in composition and main dominants, while the Late Sagwon Flora is comparable with the Danian or Danian-(?) Selandian flora from the Upper Tsagayan Subformation of the Amur area. In a florogenic aspect, trans-Beringian plant migrations from northeastern Asia and southern palaeolatitudes of the Far East, which became possible due to Paleocene climate warming in Arctic, have played an important role in forming of the Paleocene floras of Northern Alaska. Floras of the Far East and high latitudes of Asia and North America show no evidence of catastrophic event at the Cretaceous-Paleogene boundary. Their development was most probably controlled by climate changes, plant evolution and migration.     

胶莱盆地及其邻区白垩纪－古新世沉积构造演化历史及其区域动力学意义

基于野外和钻孔测井资料分析、火山岩同位素年代学分析 (40Ar-39Ar and SHRIMP U-Pb)、地震剖面的构造解释、断层运动学的野外分析结果,综合研究了胶莱盆地及其邻区白垩纪－古新世沉积构造演化历史。岩性地层分析表明,胶莱断陷盆地由三套地层单元所充填：早白垩世莱阳群和青山群、晚白垩世－古新世王氏群。青山群火山岩的同位素年代学测试结果给出了该火山岩的喷发时代在120～105 Ma。地震剖面的构造解译结果揭示胶莱盆地伸展构造受到深部两个拆离构造系统控制：一个发育于盆地南部地区,拆离断面位于深部8~10 km,向南缓倾于苏鲁造山带之下;另一个拆离系统由一系列北倾的犁式断层组成、分布于宽阔的胶莱盆地北部地区,主拆离面向北倾。这两个拆离系统分别形成于早白垩世莱阳群和晚白垩世-古新世王氏群沉积阶段。通过对不同地层单元断层滑动矢量的野外测量和古构造应力场反演,以及地层时代和同位素年代学测试结果的制约,建立了白垩纪－古新世构造应力场演替的年代序列。结果表明,胶莱盆地在白垩纪-古新世之间经历了伸展－挤压应力体制的交替演化。早白垩世伸展作用经历了两个不同的阶段：早期NW-SE向伸展和晚期近W-E向伸展。在早白垩世末期至晚白垩世初期,盆地遭受NW-SE向挤压,导致了胶莱盆地的缩短变形和郯庐断裂带的左旋走滑活动。晚白垩世－古新世时期,构造应力场转变为N-S向伸展,直到古新世末期,构造应力场转换为NE－SW向挤压。胶莱盆地和沂沭裂谷系白垩纪－古新世沉积构造演化历史对华北地区岩石圈减薄过程的动力学背景提供了重要的构造地质学制约。笔者推断,早白垩世两期引张应力作用是分别对华北地区增厚地壳或岩石圈的重力垮塌和岩石圈拆沉的响应,而早白垩世末期NW-SE向挤压记录了古太平洋板块与亚洲陆缘俯冲碰撞产生的远程效应。晚白垩世-古新世的引张伸展作用完全不同于早白垩世伸展构造,它指示了沿NNE向郯庐断裂带的右旋走滑活动及其拉分作用,在动力学上受到青藏地区块体的陆－陆碰撞产生的远程效应和古太平洋板块向亚洲大陆俯冲作用的联合应力场控制。     

New records of coleoid cephalopod jaws from the Upper Cretaceous of Hokkaido,Japan, and their paleobiogeographic and evolutionary implications

Seven coleoid jaws recovered from Santonian to lower Campanian (Upper Cretaceous) strata in Hokkaido, Japan were taxonomically studied. Based on the comparison with the jaws of modern and fossil coleoids, six of the seven jaw fossils are referred to the following two genera and three species, including one possible new species: Nanaimoteuthis jeletzkyi and N. yokotai of the order Vampyromorpha, and Paleocirroteuthis sp. nov. (?) of the order Cirroctopodida. The other single lower jaw is seemingly similar to those of modern octopods and teuthids with respect to the shape of the inner lamella, but its order-level assignment could not be determined because of its imperfect preservation. N. jeletzkyi has been described in the Upper Cretaceous fore-arc basin deposits in Hokkaido (Yezo Group) and Vancouver Island, Canada (Nanaimo Group), whereas N. yokotai occurs only in the Yezo Group. These findings, complemented by previous reports of coleoid jaws, gladii, and phragmocones from the Yezo and Nanaimo Groups, demonstrate that a highly diversified, non-belemnitid coleoid fauna including large teuthids had already appeared during the post-Albian Late Cretaceous, in the North Pacific region.     

北淮阳东段同兴寺碱性岩体地球化学特征、LA-ICP-MS锆石U-Pb年龄及其地质意义

同兴寺岩体为北淮阳构造带内著名的碱性岩体。同兴寺碱性岩具SiO2、Al2O3含量中等,富碱、准铝质、Mg#小等特征。富集大离子亲石元素Rb、Ba、Sr,亏损高场强元素Ta、Ti。LaN/YbN与LREE/HREE比值均较大,具较弱的δCe负异常。稀土元素配分曲线表现为轻稀土分馏较明显,重稀土稍平坦,属轻稀土富集型。通过LA-ICP-MS锆石U-Pb定年,获得同兴寺二长闪长岩和角闪正长岩的锆石206 Pb/238 U年龄为126.1±1.1 Ma和124.4±1.3 Ma,均为同兴寺杂岩体早白垩世中期岩浆活动的产物。主、微量地球化学分析表明岩浆来源于地幔物质部分熔融,同时受到地壳物质的混染。基于前人关于该区区域地质背景方面的研究成果和本次地球化学构造环境判别方面的研究,认为同兴寺碱性岩形成于后碰撞的伸展环境。     

苏北东台坳陷古新统阜宁组三段微量、稀土元素地球化学特征及其地质意义

沉积岩的微量、稀土元素对沉积环境的水介质变化有着较高的敏感度,对于研究古环境、沉积物源性质和构造背景方面具有重要的意义。前人对于东台坳陷元素地球化学方面的研究相对薄弱,利用元素地球化学资料解释古环境和源区性质方面的研究尚属空白。本文以苏北东台坳陷古新统阜宁组三段20件泥岩样品为研究对象,对其进行微量、稀土元素测定,并分析其地球化学特征及所揭示的地质意义。结果表明：①Sr含量、Sr/Cu、Rb/Sr、Lan/Ybn比值指示干热气候;Li、Sr、Ni、Ga微量元素含量和Sr/Ba比值均指示淡水—半咸水的水体环境,整体以淡水环境为主,偶有咸水注入盆地,局部为半咸水环境;而V/（V+Ni）、V/Cr、Ni/Co、U/Th、δCe指示氧化—弱还原的水体环境。②Co/Th—La/Sc、La/Th—Hf判别图解和稀土元素配分模式,表明阜宁组三段沉积岩应来自于上地壳的长英质物质。③La—Th—Sc、Th—Co—Zr/10和Th—Sc—Zr/10构造背景判别图解反映了大陆岛弧与活动大陆边缘的构造背景,且与Bhatia不同构造背景杂砂岩的微量、稀土元素特征值的对比分析结果一致。     

苏北东台坳陷古新统阜宁组三段微量、稀土元素地球化学特征及其地质意义

沉积岩的微量、稀土元素对沉积环境的水介质变化有着较高的敏感度,对于研究古环境、沉积物源性质和构造背景方面具有重要的意义。前人对于东台坳陷元素地球化学方面的研究相对薄弱,利用元素地球化学资料解释古环境和源区性质方面的研究尚属空白。本文以苏北东台坳陷古新统阜宁组三段20件泥岩样品为研究对象,对其进行微量、稀土元素测定,并分析其地球化学特征及所揭示的地质意义。结果表明:(1)Sr含量、Sr/Cu、Rb/Sr、La_n/Yb_n比值指示干热气候;Li、Sr、Ni、Ga微量元素含量和Sr/Ba比值均指示淡水—半咸水的水体环境,整体以淡水环境为主,偶有咸水注入盆地,局部为半咸水环境;而V/(V+Ni)、V/Cr、Ni/Co、U/Th、δCe指示氧化—弱还原的水体环境。(2)Co/Th—La/Sc、La/Th—Hf判别图解和稀土元素配分模式,表明阜宁组三段沉积岩应来自于上地壳的长英质物质。(3)La—Th—Sc、Th—Co—Zr/10和Th—Sc—Zr/10构造背景判别图解反映了大陆岛弧与活动大陆边缘的构造背景,且与Bhatia不同构造背景杂砂岩的微量、稀土元素特征值的对比分析结果一致。     

北淮阳东段徐家湾岩体地质和地球化学特征及LA-ICP-MS锆石U-Pb年龄

徐家湾二长花岗岩体位于北淮阳构造带内桐柏-桐城与郯城-庐江两大断裂的交汇处,岩体呈岩株状侵入新元古界老变质岩层中。本文利用原子吸收光谱和ICP-MS法测定了岩体主量和微量元素的含量,表明岩体具SiO₂和Al₂O₃较高、富碱、过铝质、Mg^#小等特征。大离子亲石元素(LILE)Rb、Ba富集,Sr亏损;高场强元素(HFSE)Y、Th、Nb、Hf、U富集,Ta、P、Ti亏损;岩体整体亏损HFSE,富集LILE;La_N/Yb_N与LREEs/HREEs值均较大,具较弱的δCe负异常,显示该岩体为过铝质A型花岗。LA-ICP-MS锆石U-Pb定年获得徐家湾二长花岗岩侵位年龄在128.0±0.9~129.6±1.4 Ma之间,是早白垩世岩浆活动的产物。研究认为徐家湾二长花岗岩体形成于造山后的伸展环境,形成岩体的岩浆源于岩石圈地幔,并受到地壳物质的混染。     

北羌塘盆地油页岩形成环境及其油气地质意义

最近在北羌塘长蛇山地区新发现了晚侏罗世—早白垩世大套油页岩及巨厚膏盐层,该油页岩与近年来作者在胜利河、长梁山地区发现的多处油页岩连成一片,构成了目前我国规模最大的中生代海相油页岩带—北羌塘油页岩带。沉积岩相、古地理研究表明,晚侏罗世—早白垩世时期,北羌塘盆地总体上为一个向北西开口的相对闭塞的巨大海湾,发育了河流-三角洲、海湾(潮坪-潟湖)和浅海-陆棚等沉积相组合。古气候、古环境研究表明,油页岩(段)沉积期,胜利河地区处于潮湿的热带-亚热带环境,大量淡水的注入以及高的生产力等综合因素,控制了油页岩的形成,其分布主要位于局限海湾-潟湖的边缘带;膏盐层(段)沉积则发育于相对干旱、炎热气候条件下的闭塞环境。北羌塘晚侏罗世—早白垩世油页岩及其上覆膏盐层是重要的烃源岩和区域性封盖层。油页岩及含膏盐层沉积组合的发现,对于重新认识羌塘盆地生烃条件及保存条件,具有十分重要的意义。     

北大西洋ODP1049C孔Aptian-Albian期高频旋回大洋红层的成因:矿物学证据

北大西洋ODP171B航次1049C孔Aptian-Albian期沉积以出现大洋红层与灰色、白色沉积物高频旋回为特征。为了探讨大洋红层的成因,本文进行了矿物学、地球化学、沉积学等方面的研究。漫反射光谱、磁化率和活性铁数据表明,赤铁矿、针铁矿的出现是导致样品由白色向红色转变的矿物学原因。棕色和橙色样品中出现赤铁矿和针铁矿的特征峰,FeR/FeT平均值分别为0.23和0.24,明显区别于其他颜色的样品。磁化率与红层成良好的正相关关系,说明铁氧化物矿物含量的变化是导致磁化率变化的原因。X射线衍射结果表明,不论颜色如何,样品中均含伊利石、高岭石、蒙脱石、绿泥石等粘土矿物,其分布与样品颜色没有直接关系,很可能反映当时物源区气候稳定。ODP1049C孔岩芯沉积物出现橙色、棕色、白色、灰色等颜色的高频变化,颜色过渡接触界线清晰,说明导致红色变化的赤铁矿和针铁矿是沉积期低温氧化的产物。棕色样品中CaCO3含量最低,推测红层所对应的氧化条件是由于较低的有机质堆积速率造成的。     

华北晚侏罗世—早白垩世风成砂沉积及其古地理和古生态学意义*

华北晚侏罗世—早白垩世风成砂主要分布于鲁西蒙阴盆地上侏罗统三台组、辽西金—羊盆地上侏罗统—下白垩统土城子组、冀西北尚义盆地上侏罗统—下白垩统土城子组/后城组（原阎家窑组）、鄂尔多斯盆地下白垩统志丹群和甘肃下白垩统河口群。各地风成砂岩均具高角度大型—巨型板状、楔状交错层理及平行层理,分选较好—好,磨圆次棱角状—次圆状等沉积特征。对华北晚侏罗世—早白垩世风成砂赋存层位以及风成砂岩形成时代进行了对比,将风成砂主要划分为3个时期,即基末利期—贝利阿斯期、凡兰吟期—欧特里夫期和欧特里夫期—阿普特期,且从鲁西到甘肃风成砂岩形成时代逐渐变新。通过上述5个地区风成砂的古风向研究发现,当时西北风盛行,古地磁研究显示风成砂岩发育于N25°—N45°之间区域。根据当前全球风带分布特征,认为华北晚侏罗世—早白垩世处于西风带上,为行星风系所控制。通过对比南半球同期风成砂岩古风向研究,提出全球南北半球中纬度地区均处于西风带上。结合风成砂及相应层位沉积特征、沉积环境的研究,初步推测晚侏罗世—早白垩世,华北N30°以北地区为干旱寒冷气候,而N30°以南地区则干旱炎热,华北北部整体处于海拔较高的山地环境,山间盆地发育,火山活动频发。燕辽生物群与热河生物群的演替过程与风成砂沉积相耦合,体现了古地理、古环境对生物群发展的制约作用。     

Late Devensian environments in the Vale of Mowbray, North Yorkshire, UK: evidence from palynology

Two radiocarbon-dated Lateglacial pollen diagrams from the Vale of Mowbray (northern Vale of York) are presented from sites in the lowlands between the washlands courses of the rivers Swale and Ure in North Yorkshire, an area with little previous palynological research despite its proximity to the Devensian glacial advance limits in eastern England. The profiles, from Snape Mires and Nosterfield, include the Loch Lomond Stadial (Younger Dryas) and the Holocene transition, while that from Snape Mires also includes the period from the early part of the Lateglacial Interstadial. This profile differs from most published Interstadial diagrams from the Yorkshire region in having a long-delayed expansion of tree and shrub taxa. Juniperus (juniper) remains important after vegetation development takes place and the pollen record includes evidence of two cold climate oscillations before the maximum development of Betula (birch) woodland near the end of the Lateglacial Interstadial. At both profiles Artemisia (mugwort) frequencies are lower during the Loch Lomond Stadial than at many regional sites, probably due to edaphic factors in these lowland locations. The two sites provide valuable environmental data that enable comparison between the more wooded Lateglacial vegetation to the south in the Vale of York and Humberside and the more open contemporaneous vegetation to the north in the Durham and Northumberland lowlands.     

华北地台晚寒武世层序地层及其与北美地台海平面变化的对比

华北地台在晚寒武世为一个较为典型的缓坡型台地。在其北部地层中,以发育风暴砾屑灰岩及生物丘灰岩闻名,包括崮山组、长山组和凤山组。在该套地层中,深缓坡相泥岩和泥灰岩与浅缓坡相颗粒灰岩和泥粒灰岩一起构成若干潮下型碳酸盐米级旋回,这些米级旋回本身即为有序的异成因岩相序列而成为基本工作单元。在长周期三级层序中,米级旋回形成有序垂直叠加形式,反映了三级层序本身是一个与三级相对海平面变化相关的、环境加深和变浅过程中所形成的沉积相序列。根据这些特征,在华北地台北部晚寒武世地层中可识别出4个三级层序。从北向南,相变为一套白云岩,其中三级层序的高水位体系域则发育较多的潮坪相白云岩以及由其构成的环潮坪型米级旋回。这些层序的典型特征是:层序界面为淹没不整合面,层序的相序序列以"深缓坡相-浅缓坡相"为特征,从而构成特殊的"CS(?)+HST"序列;而在华北地台南部,层序界面多为暴露间断面,层序的相序组构多以"缓坡相-潮坪相"为特征。从北向南的变化,形成了一个富有规律性的层序地层格架。从旋回到层序、从岩相序列到沉积相序列的层序地层研究表明:作为基本工作单元,米级旋回实际上是由"间断-加积作用"旋回机制所形成的岩相序列,由米级旋回的有序叠加形式所构成的三级层序则是一个沉积相序列。由各剖面点的古水深变化曲线,可以定性地综合出华北地台晚寒武世的海平面变化曲线。与北美晚寒武世海平面变化曲线相对比,既存在相似性更存在差异性,说明了长周期海平面变化既受全球性海平面变化的控制,同时也受区域因素的影响。     

北美西部内陆海盆Niobrara组的钙质超微化石及其环境意义——Ⅰ：钙质超微化石及其分带

北美西部内陆海盆上白垩统Niobrara组中的钙质超微化石十分丰富,且多保存良好,呈现典型的晚白垩世远洋钙质超微化石组合面貌,经系统鉴定,计有60属100余种和亚种。为适应不同的环境,Kansas西部和South Dakota东部的钙质超微化石组合面貌稍有不同。经与白垩纪颗石藻化石带对比,可将Kansas西部的Niobrara组划分为6个化石带(CC13～CC18)和8个亚带,其中,根据本文研究地区的化石序列,CC15和CC16带被进一步划分。根据与同一剖面所建立的无脊椎动物化石带对比,钙质超微化石CC17带的时代被重新厘定,即该带应始于中Santonian的晚期,结束于晚Santonian的早期,并据此将Santoniatr／Campanian的界线划在CC18带之内。     

Post-glacial relative sea level, isostasy, and glacial history in Icy Strait, Southeast Alaska, USA

We use the radiocarbon ages of marine shells and terrestrial vegetation to reconstruct relative sea level (RSL) history in northern Southeast Alaska. RSL fell below its present level around 13,900 cal yr BP, suggesting regional deglaciation was complete by then. RSL stayed at least several meters below modern levels until the mid-Holocene, when it began a fluctuating rise that probably tracked isostatic depression and rebound caused by varying ice loads in nearby Glacier Bay. This fluctuating RSL rise likely reflects the episodic but progressive advance of ice in Glacier Bay that started around 6000 cal yr BP. After that time, RSL low stands probably signaled minor episodes of glacier retreat/thinning that triggered isostatic rebound and land uplift. Progressive, down-fjord advance of the Glacier Bay glacier during the late Holocene is consistent with the main driver of this glacial system being the dynamics of its terminus rather than climate change directly. Only after the glacier reached an exposed position protruding into Icy Strait ca. AD 1750, did its terminus succumb - a century before the climate changes that marked the end of the Little Ice Age - to the catastrophic retreat that triggered the rapid isostatic rebound and RSL fall occurring today in Icy Strait.     

Evolution and strike variability of early post-rift deep-marine depositional systems: Lower to Mid-Cretaceous, North Viking Graben, Norwegian North Sea

The controls and development of early-post-rift, deep-water depositional systems are poorly understood due to their commonly deeply-buried nature. As a consequence, in the subsurface there is usually a lack of well penetrations and/or weak seismic imaging. At outcrop, early post-rift strata have commonly been deformed beyond reasonable recognition by later inversion tectonics. In contrast to these systems, the North Viking Graben shows a well-imaged Cretaceous early post-rift package with good well control and little effect from inversion. Therefore, this paper examines the early post-rift, deep-water depositional systems of the North Viking Graben to determine the controls on their stratigraphic position, geometry and evolution, and thus provide an analogue for comparable systems. Greater understanding of such systems will allow for the enhanced prediction of reservoir units in the subsurface and development of new play models since post-rift intervals are generally under-explored.The basin configuration inherited by the Cretaceous early post-rift in the northern North Sea was set up by Permo-Triassic and Late Jurassic rifting. In the North Viking Graben this established considerable along-strike variability, resulting in a northern basin segment surrounded by steep slopes and faulted-bounded structural highs and a southern basin segment margined by slopes with noticeably gentler gradients. Associated with the Cretaceous post-rift is an overall transgressional trend, which drowned local source areas, resulting in prevalent carbonate and hemipelagic mudstone deposition in the basins. In the North Viking Graben, the uplifted Oseberg fault-block provided the sub-aerial clastic source area until it was submerged in the early Upper Cretaceous.The early post-rift infill of the North Viking Graben was divided into four key seismic stratigraphic units (K1, K2, K3 and K4) using an integration of seismic and well data. Inside this stratigraphic framework, the depositional systems within each K-unit were resolved from characteristic seismic facies, amplitude anomalies, relationship with adjacent reflections, and geomorphologies. In the northern basin segment, the early post-rift stratigraphy contains basin-floor fans, a channel complex and a shoreline-like geometry, whereas the southern basin segment is solely characterised by hemipelagic and carbonate deposition. This spatial variability indicates that one of the dominant controls on the development of the early post-rift depositional systems in the North Viking Graben was the inherited syn-rift fault-controlled topography. The steep slopes bounding the northern basin segment aided the delivery of sediment from the sub-aerial Oseberg source area to the graben whereas the submerged, gentle slopes in the southern basin segment were relatively sediment-starved.Long- and short-term changes in relative sea-level also heavily influenced the evolution of the early post-rift basin stratigraphy. Short-term relative sea-level fall allowed basin-floor fan emplacement whereas short-term relative sea-level stand-still favoured deposition of a channel complex. Deposition of the shoreface-like geometry is associated with a short-term relative sea-level rise. This temporal difference in the style and scale of the depositional systems is also interpreted to reflect the gradual denudation and drowning of the Oseberg source area. Regional short-term trangressive and anoxic events in the northern North Sea further influenced the early post-rift strata, resulting in the deposition of stratigraphic units that can be correlated across the North Sea.     

河南汝阳地区-巨型蜥脚类恐龙股骨化石的发现及其地层学意义

报道了首次在河南省汝阳县发现的大型蜥脚类恐龙股骨化石。从股骨近端的大小来判断,该股骨的实际长度可能超过2m,应属于巨型蜥脚类恐龙的股骨无疑。虽然化石破碎,种属的归属存在一定的困难,但是它的发现无论从晰脚类恐龙的分布、演化还是从地层学上都具有重要意义。该恐龙股骨化石的发现,将该地区原认为属于古近系蟒川组的时代向前推至早白垩世晚期或晚白垩世早期,同时其下伏地层陈宅沟组也应划归白垩纪时期的沉积。     

粤北长塘盆地晚白垩世早期流纹岩的成因:地球化学和Sr-Nd-Pb-Hf-O同位素制约

长塘流纹岩为"大量流纹岩-少量玄武岩"构成的不对称双峰式火山岩组合的酸性端元,前人对年代学研究认为是目前华南识别的燕山期最年轻的流纹岩(SHRIMP年龄96.8 Ma),但在岩石地球化学特征及成因方面尚未深入研究。本文通过全岩主微量元素、Sr-Nd-Pb-O和锆石Hf同位素研究得到:流纹岩为亚碱性弱过铝质岩石,稀土元素富集,轻重稀土分馏明显,Eu负异常;具有较小的(~(87)Sr/~(86)Sr)_i、较高的εNd(t)和较低的(~(206)Pb/~(204)Pb)_i、(~(207)Pb/~(204)Pb)_i、(~(208)Pb/~(204)Pb)_i值;锆石的εHf(t)值较高且Hf模式年龄和全岩基本一致,均显示为新生下地壳岩浆;全岩的δ18O值较高。结合区域地质资料,本文认为长塘流纹岩是在晚白垩世早期拉张环境下,源于EMⅡ富集地幔的岩浆混染少量下地壳物质形成新元古代新生下地壳,发生熔融产生的岩浆在上升过程中经历结晶分异演化后喷出地表的产物。晚白垩世早期流纹岩成因对华南陆壳拉张减薄到正常厚度时间节点提供了证据。     

华北克拉通新太古代末期的克拉通化: 冀东昌黎碱性花岗质岩石的岩石学、锆石年代学和元素地球化学

冀东地区是华北克拉通早前寒武纪地质研究的经典地区, 赋存有丰富的地球早期地壳演化和太古宙板块构造信息。本文对出露于冀东昌黎地区的一套新太古代碱性花岗质岩石进行了系统的研究。其主要矿物组成为碱性长石、石英和霓辉石, 以及榍石、磷灰石等副矿物, 偶见紫苏辉石。LA-ICP-MS锆石U-Pb同位素测年显示其成岩年龄为2531~2518Ma。该套碱性花岗质岩石具有高SiO₂(70.2%~73.0%)、K₂O+Na₂O(10.1%~11.0%)、Sr/Y值(44.3~103.5), 低CaO(0.48%~0.94%)、MgO(0.14%~0.33%)、Cr (4.96×10^-6~10.9×10^-6)、Ni(2.46×10^-6~4.62×10^-6)的特征, 稀土元素配分模式与冀东-辽西地区闪长质及二长-正长花岗质岩石相似, 同时显示Eu的正异常(Eu/Eu^*=1.02~1.49)以及岩浆分异的稀土元素分布特征。根据以上元素地球化学特征以及微量元素的模拟计算, 本文认为这套岩石来自增厚镁铁质下地壳的部分熔融, 并经历了以磷灰石为主同时包括榍石、褐帘石等副矿物的分离结晶。结合前人对区域内同时代变质表壳岩岩石地球化学以及变质作用的研究, 认为增厚下地壳的部分熔融与岩石圈地幔镁铁质岩浆的底垫和侵入有关。陆块发生碰撞后, 岩石圈包括下地壳不断增厚, 岩石圈地幔下部沉入软流圈, 引起了软流圈的上涌以及残余岩石圈地幔的部分熔融。昌黎碱性花岗质岩石形成于新太古代末期碰撞后的拉张环境, 暗示了华北克拉通新太古代末期的克拉通化。