广东三水盆地玄武岩源区特征与南海早期演化

南海在扩张前是否经历了陆内裂谷阶段是南海成因研究中一个重要的问题。三水盆地位于南海北部陆缘,其新生代以来喷发的双峰式火山岩具备大陆裂谷的岩石组合特征。通过对其中玄武岩主微量元素分析认为三水盆地玄武岩可以分为亚碱性和碱性玄武岩系列,两者均显示出明显的Nb、Ta正异常,相对于大陆地壳具有较低的Th/Sc、La/Nb和U/Al×1000,陆壳混染程度低;首次对盆地内玄武岩进行40Ar-39Ar测年,结合前人年代学结果表明玄武质岩浆强烈喷发的时段为61～54 Ma,其中亚碱性玄武岩喷发时间（60 Ma）早于碱性玄武岩（56 Ma）;通过熔融柱模型反演得到亚碱性岩浆源区起止熔融温压分别为1 517 ℃（3.03 GPa）和1 471 ℃（2.25 GPa）,深度为101～76 km,碱性岩浆源区起止熔融温压分别为1 555 ℃（3.33 GPa）和1 506 ℃（2.48 GPa）,深度为110～84 km,整体为石榴石-尖晶石橄榄岩过渡区且呈逐渐变深的趋势。综合岩浆源区特征以及岩石组合特征认为三水盆地在古新世具备大陆裂谷特征。通过对比三水盆地与南海扩张期岩浆活动的分布时段及源区特征,发现三水盆地与南海扩张期岩浆活动时间分布存在较长间隔,深部过程差异较大,三水盆地岩浆活动与南海扩张并无直接因果联系。     

南海及其围区新生代岩浆活动时序与成因研究

南海及其围区新生代岩浆活动具有相似的特征,根据与南海扩张时间(32~15.5 Ma)的先后关系,将岩浆活动分为三期：扩张前(>32 Ma)、扩张期(32~15.5 Ma)和停止扩张后(< 15.5 Ma)。从岩相学特征可知：扩张前的岩石以三水盆地双峰式岩浆岩组合为代表;根据最新的IODP 349航次的钻孔样品资料,南海海盆扩张期岩浆岩为典型的MORB,而少量的陆缘岩石资料显示,该期岩浆岩为碱性玄武岩;停止扩张后岩浆活动可分为两期,早期以大陆-大洋中脊过渡型拉斑玄武岩为主,晚期以碱性玄武岩为主。除了扩张期南海海盆的MORB,整个新生代的岩浆岩地球化学特征大体一致,与OIB相似,显示地幔源区具有不均一性,由一个DMM和一个EM端元组成,且岩石均表现出明显的Dupal异常特征。对于富集端元的性质和来源、Dupal异常的成因、海南地幔柱是否存在等问题目前存在较大争议。具OIB特征的岩浆除来自地幔柱作用外,地幔交代作用也是重要的产生机制。本文认为碳酸盐流体对地幔源区的交代作用可能在南海地区新生代岩浆活动中扮演了重要的角色。另外,岩浆岩成岩过程的研究也不够全面深入,这些都是在今后的研究工作中需要关注的地方,有待后来进一步的研究。     

南马里亚纳海槽岩石学特征

南马里亚纳海槽岩性比较复杂：有玄武岩，根据标准矿物组合，进一步划分为碱性玄武岩，橄揽拉斑玄武岩和石英拉斑玄武岩；火山碎屑岩，包括凝灰岩和角砾凝灰岩；以及方辉橄榄岩．大部分玄武岩样品是距今五百万年以后形成的，个别岩石样品的形成年龄为八百万年左右．不同类型的玄武岩是含Ｈ２Ｏ的地幔岩块在底辟上升过程中，局部熔融、分离的产物；火山碎屑岩是火山喷发的火山灰、火山砾被水化学分解的物质胶结而成；方辉橄榄岩是地幔岩浆底辟上升直接侵入结晶的产物．     

南海及其周缘中新生代火山活动时空特征与南海的形成模式

根据南海海区、华南和中南半岛的地面露头、钻井、拖网及地球物理资料,分析了南海地区火山活动的时空分布特点。在南海陆缘和周边陆区中生代末期花岗岩分布非常广泛。新生代火山岩活动规模较小,主要是海底扩张之后在洋盆扩张脊、北部陆缘的陆洋边界附近、雷琼地区和中南半岛南部的玄武岩。在南海北部陆缘的深部地震调查中发现,在地壳下部存在小规模的高速异常体,结合浅部的晚第三纪一第四纪火山活动,认为该高速体形成于南海扩张之后。这些特征表明,在南海的拉张过程中岩浆供应不丰富,在陆缘未形成大规模的侵入和喷出岩。南海陆缘属于岩浆匮乏型被动大陆边缘。南海海区残留多个刚性断裂陆块,反映了裂谷拉张过程中脆性破裂。根据这些特征,南海形成难以用印藏碰撞引起的软流圈物质上涌导致岩石圈破裂这样的模式来解释。     

晚中新世南海珍贝-黄岩海山岩浆活动及其演化:岩石地球化学和年代学证据

南海海盆15°N附近呈东西向展布的珍贝-黄岩海山被认为是32~17Ma前南海海盆的残留扩张中心.对采自黄岩海山的两个火山岩样品(9DG,9DG-2)进行了岩石学、地球化学和年代学研究.两个样品的SiO₂含量分别为60.3%和63.6%,Al₂O₃含量分别为17.56%和17.55%,TiO₂含量分别为0.48%和0.31%,碱度率分别为3.88和3.62.根据岩石学和岩石化学分类,样品属碱性系列的粗面岩.对稀土元素和微量元素分析表明岩石具有洋岛玄武岩(OIB)型配分型式,轻重稀土总量比(∑c(LREE)/∑c(HREE))和球粒陨石标准化镧镱比((La/Yb)N)分别高达17.22和27.23,并具有铕负异常和锶、磷、钛亏损的特点.样品9DG的锶-钕-铅同位素分析结果为锶-87的含量与锶-86的含量之比值为0.704183,钕-143的含量与钕-144的含量之比值为0.512827,铅-206的含量与铅-207的含量之比值为18.68668,铅-207的含量与铅-204的含量之值为15.67962,铅-208的含量与铅-204的含量之比值为39.00261,表明初始岩浆来自软流圈地幔,具有与珍贝海山玄武岩相似的同位素组成.经钾-氩法测年,粗面岩的年龄为(7.77±0.49)Ma,略晚于珍贝海山玄武岩的年龄[(9.1±1.29)~10.0±1.80Ma],属于南海扩张期后晚中新世火山活动的产物.对比珍贝海山玄武岩的地球化学和同位素特征,认为两者有相同的岩浆源区,但是它们经历了不同程度的结晶分异过程,在晚中新世期间珍贝-黄岩海山可能有地幔柱活动.     

西太平洋采薇海山群基岩特征

对西太平洋采薇海山群基岩的类型、分布及岩石学特征进行了研究,同时对该海山群玄武岩主量元素和微量元素地球化学特征进行了初步分析。研究结果表明,采薇海山群的基岩类型主要为火山碎屑岩、灰岩、玄武岩和磷块岩;海山群的基岩以火山碎屑岩为主,其次为灰岩和玄武岩,磷块岩相对不发育。镜下鉴定结果显示,火山碎屑岩以玄武质火山角砾岩为主,灰岩主要为泥晶生屑灰岩或亮晶生屑灰岩,磷块岩多由生物碎屑灰岩或火山碎屑岩发生磷酸盐化而形成。采薇海山群玄武岩主量元素特征与夏威夷岛碱性玄武岩类似,属碱性玄武岩;微量元素分析结果显示其具有洋岛和碱性玄武岩特征,反映其形成于碱性洋岛构造环境。     

莺歌海盆地新生代构造沉积演化及动力学机制分析

南海西北陆缘构造演化极其复杂,受到红河断裂、海南地幔柱和南海形成演化等多种因素的控制。莺歌海盆地位于南海西北部,发育了巨厚的新生代沉积物,详细记录了南海西北陆缘新生代的演化历史。但是莺歌海盆地新生代以来主要受到何种构造因素的控制目前还不太清楚。本文在莺歌海盆地较为均匀地选择了7口钻井和23口模拟井,通过空盆构造沉降方法重建了莺歌海盆地的构造沉降量、构造沉降速率和沉积速率,同时运用重力反演方法模拟了莺歌海盆地深部地壳结构,并结合前人研究成果进行了综合分析。结果发现莺歌海盆地在裂陷期（45～23 Ma BP）,盆地北部和中部沉降速率较大,南部沉降速率较小;在裂后期（23～0 Ma BP）, 盆地北部和中部沉降速率存在两期“台阶式”上升,分别为23～11.7 Ma BP和11.7 Ma BP至今,北部裂后期构造沉降速率最大可达80 m/Ma,中部最大可达110 m/Ma;南部地堑和隆起裂后期分别在11.7～5.7 Ma BP和15.9～11.7 Ma BP构造沉降速率最大可达70 m/Ma。莺歌海盆地新生代整体上表现为沉降速率与沉积速率变化基本一致,说明构造沉降对沉积速率具有显著的控制作用。重力反演发现莺歌海盆地可能存在下地壳高密度异常体,结合盆地沉积物内部钻遇玄武岩,我们推测下地壳高密度异常体为基性侵入体。通过与南海周边其他沉积盆地沉降速率对比发现,几乎所有盆地都在中中新世−晚中新世（15.9～11.7 Ma BP）发生了加速沉降事件,我们认为这可能跟南海海盆停止扩张导致大陆边缘次生地幔对流消失有关。莺歌海盆地5.7 Ma BP至今的加速沉降则可能与红河断裂右旋走滑活动有关。     

Geochemistry and petrogenesis of Quaternary volcanism from the islets in the eastern Beibu Gulf: evidence for Hainan plum

Some of the islets in the eastern Beibu Gulf are covered by Quaternary volcano strata. The rock samples from these islets mainly consist of quartz tholeiites （at Shenjiandao）, olivine tholeiites （at Linshidao and Xieyang- dao） and alkali basalts （at Yangpubi and Jianshidao）, and basically represent four periods of the Quaternary volcanism of Hainan Island and its adjacent regions. Except for the samples from Shenjiandao, most of the Quaternary volcanics of these islets belong to alkali magma series. The trace element characteristics of all of these samples show they are OIB （oceanic island basalt） -like, which implies that their deep geodynamic setting may be related to a mantle plume. The Sr-Nd-Pb isotopic compositions show that the mantle source beneath the Quaternary strata can be regarded as a result of binary mixing between a depleted, DMM （de- pleted MORB mantle）-like source and an enriched mantle type 2 （EM2）. The EM2 may be originated from the Hainan mantle plume, and has been metasomatized by carbonaceous fluids released from ancient re- cycled oceanic crust at an asthenospheric mantle level. These features, together with typical trace element ratios, reflect that the parent magma was not subjected to crustal contamination during its ascent to the surface. This study provides further petrological and geochemical evidence for the existence of the Hainan mantle plume.     

南海北部深水区渐新-中新世沉积地球化学特征及物源演变

Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values(110–115 Ma for U1435 and L21; 150 Ma for H1), indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River(Pearl River), as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.     

渤海海域黄河口凹陷渤中29-6构造火山岩锆石U-Pb年龄及其地质意义

为了厘定渤海海域黄河口凹陷北部渤中29-6构造钻遇的90.5 m火山岩系地层层位,对该套火山岩进行了系统的岩石学、锆石U-Pb定年分析。薄片结果表明,该套火山岩可分为3段,自上而下分别是顶部玄武岩(2 880.5~2 910.5 m)、中部煌斑岩(2 910.5~2 959.5 m)和底部凝灰岩(2 959.5~2 971 m)。锆石U-Pb年龄结果显示,中部煌斑岩和底部凝灰岩均形成于早白垩世时期,地层年龄晚于124 Ma,而顶部玄武岩形成于新生代岩浆活动,地层年龄晚于45.2 Ma,结合玄武岩上部和玄武岩中泥岩的古生物分析结果,认为该套玄武岩指示了沙河街组三段沉积期的岩浆喷出事件。本研究落实了渤中29-6构造底部连续钻遇的火山岩系地层实为中生界、新生界2套不同时期的火山岩系,对进一步理解、认识黄河口凹陷的中生代、新生代火山活动及相应的火山岩油气勘探具有重要意义。     

Basalts from the Solomon and Bismarck Seas

Volcanic rocks collected from the Solomon Sea Basin are mostly ferrobasalt lavas similar to evolved MORB; an exception is a single sample of basaltic crystal tuff invaded by later basalt. The rocks contain labradorite, aluminous diopsidic augite, and titanomagnetite, with olivine and pigconite in the more vitrophyric samples, and segregation vesicles in some. Cobbles dredged from Gudaraba Canyon, south of the Solomon Sea Basin, include both MORB-like glassy lava and K-metasomatised basalt and andesite(?). Two small pieces of volcanic glass from the southern Bismarck Sea are more primitive MOR-type basalts.     

南海中南—司令断裂带的延伸特征及其与南海扩张演化的关系

为了确定中南—司令断裂带在南海海盆及其在南部陆缘的延伸位置,并探讨其与南海扩张的关系,本文利用重磁异常、地震、莫霍面深度、P波速度特征、钻井拖网资料,对中南—司令断裂带的延伸位置进行了综合地质和地球物理研究,厘定了中南—司令断裂带在东部次海盆与西南、西北次海盆之间呈NS向延伸,并南延至南海南部陆缘之上,深度上切割至莫霍面。根据南海海盆中磁异常条带走向的变化,及磁异常条带、走滑/转换断裂、扩张方向的印证关系,结合前人对古南海“剪刀状”碰撞闭合、南海扩张演化、构造应力场的研究,提出在32~25 Ma,伴随着南海东部次海盆的NNW向扩张,南海海盆及南沙地块整体发生顺时针旋转,使中南—司令断裂走向由形成初期的NNW向转变为N—S向;23.5 Ma之后,顺时针旋转停止,南海东部次海盆继续NNW向扩张,西南次海盆呈NW—SE向渐进式扩张。作为一条切穿地壳的深大断裂,中南—司令断裂与红河-越东断裂、马尼拉海沟断裂三条深大断裂一起组成区域“滑线场”,制约南海海盆的扩张与南沙地块的南移。     

南海北缘琼南缝合带的锆石U-Pb年龄制约

为了获得南海北缘琼南缝合带南北两侧地块的碰撞时间,对九所陵水东段采集的中高级变质石榴石石英片岩和花岗质片麻岩进行了LA-ICP-MS锆石U-Pb定年。实验结果发现:原岩为沉积岩的石榴石石英片岩形成于晚泥盆纪;3个锆石变质壳谐和/近谐和年龄表明碰撞事件发生在(233±4) Ma左右;花岗质片麻岩原岩为同构造侵位的花岗岩,其8个锆石增生年龄显示的是深熔作用的时间,为(206±3) Ma。这些研究成果对重建南海北缘琼南缝合带在研究区内的演变历史具有重要意义。     

南海大洋钻探及海洋地质与地球物理前沿研究新突破

南海是西太平洋地区规模最大且具有代表性的边缘海盆地之一。经过近几十年的研究积累,尤其是通过实施5个国际大洋钻探航次(1999–2018年)与国家自然科学基金委“南海深海过程演变”重大研究计划(2011–2019年),我国科学家获得了大量宝贵的第一手资料,取得了一系列创新进展与重大突破,标志着南海海洋地质与地球物理研究正走向国际前沿。重要研究成果包括:(1)新提出南海是“板缘张裂”盆地,与经典的大西洋型陆缘模式不同;(2)大洋钻探首次获取了基底玄武岩样品,结合中国在南海首次深拖地磁测量实验,精确测定了南海海盆玄武岩年龄,揭示南海海盆从东向西分段扩张;(3)大洋钻探结果发现南海陆缘岩石圈减薄之初岩浆迅速出现,未发现缓慢破裂造成的蛇纹岩出露;(4)发现南海扩张结束后仍存在大量岩浆活动,可能受控于多种构造与地幔因素;(5)地球化学证据与地球动力学模拟都显示南海岩浆的形成受到周边俯冲带的影响。目前我国的海洋地球科学正在进入崭新的发展阶段,有望以南海为基点,开始拓展到周边大洋,通过主导大型研究计划以及建设我国大洋钻探平台,以提升我国在南海、西太平洋与印度洋海洋地质科学研究的实质性影响力与引领地位。     

南海扩张过程及海陆变迁沉积记录

南海南、北共轭大陆边缘盆地的对比研究是深入了解南海扩张过程及古地理格局的重要途径。由于历史原因,目前对南海南缘盆地构造—沉积演化研究还非常薄弱,极大地限制了对南海扩张及海陆变迁等基础地质问题的整体认识。综合南海及其周缘盆地沉积地层和沉积环境的研究进展,对南海扩张过程和古地理格局演化进行了分析。南海南、北缘盆地破裂不整合面存在着明显的穿时性,从NE向SW逐渐变年轻,对应南海海底扩张从NE向SW渐进式打开。台湾新生代地层破裂不整合面位于33~39 Ma之间,暗示南海洋壳开始形成的时间可能在33~39 Ma之间,有部分较老的洋壳可能已经向东俯冲消减掉。南海经历了从早期"北陆南海"逐渐演变为现今"北海南陆"的过程,南海北缘早期存在一个向东开口的海湾,可能为古南海的一部分。伴随南海的扩张,海侵范围由东向西逐渐扩展,从一个狭窄的海湾形成今日的形貌。南海北缘盆地物源在~25 Ma左右发生明显的改变,早期主要为华南沿海的近源剥蚀沉积。在~25 Ma后,来自扬子地块的沉积物逐渐增多。南海南缘盆地物源在~25 Ma前与南海北缘盆地具有相似的物质来源,~25 Ma后南海洋盆阻挡扬子地块的物源向南输送,南海南缘仍以陆块内部中生代花岗岩及火山岩为主要物质来源。     

Petrology, Geochemistry and Nd-Sr-Pb Isotopic Properties of Volcanic Rocks in Daheishan Island, Penglai, Shandong Province

The major elements, trace elements, K-Ar age and Sr-Nd-Pb isotopic systems of the Cenozoic volcanic rocks in Daheishan Island and Cishan, Penglai, Shandong Province are measured. The volcanic rocks ( olivine-nephelinite and nepheline-basanite ) in DaheishanIsland erupted periodically in an interval of 0.32 Ma, from 8.72 Mm 8.39 Ma, 8.08 Ma to 7.73Ma. The volcanic rocks are all rich in light REEs. They are similar to the OIB-type alkali basalt in the trace elements normalized model by primordial mantle: rich in high field elements such as Nb and Ta, and imcompatible elements such as Cs, Rb, Ba, Th, U. The volcanic rocks show a depletion of K and Rb elements. It is suggested by the trace elements that the olivine-nephelinite in Daheishan Island is originated fi‘om deep resources under the continental mantle, ε Nd (0) values of the volcanic rocks in Daheishan Island and Cishan are 5.31 - 8.51 and 7.33 respectively, suggesting that the volcanic rocks are from the depleted mantle resources, which have higher Sm/Nd ratios than the CHUR. ^143Nd /^144Nd ratios of Daheishan Island olivine-nephelinite and Cishan alkali basalts are 0.512 910 - 0.513 074 and 0.513 014 resoeetivelv. The ^87Sr /^86Sr of Daheishan lsland volcanic rocks are lower than that of Cishan, 0.703 427 - 0.703 482 and 0.703 895 respectively. The Daheishan Island olivinenephelinite has the Pb isotopic values as follows: ^206Pb /^204pb= 18.028 9 ～17.972 8,^207Pb /^204Pb= 15,435 8 - 15.402 2 and ^208Pb /^204Pb=38.087 6 - 37.997 5, lower than those of Cishan basanite. The Cishan basanite has ^206Pb / ^204Pb = 18.240 1, ^208Pb /^204Pb = 15.564 5 and ^208Pb /^204Pb=38.535. The authors suggest that the olivine-nephelinite in Daheishan Island is similar to the E-type MORB or Hawaii OIB, and the alkali basalts in Cishan similar to the Kerguelen OIB. The dominant mantle components of DM PREMA and perhaps DM( Dupal type ) are the dominant mantle components for volcanic rocks in Daheishan Island and Cishan. The PREMA component plays an important role.     

A 33 Ma lithostratigraphic record of tectonic and paleoceanographic evolution of the South China Sea

The 853 m thick sediment sequence recovered at ODP Site 1148 provides an unprecedented record of tectonic and paleoceanographic evolution in the South China Sea over the past 33 Ma. Litho-, bio-, and chemo-stratigraphic studies helped identify six periods of changes marking the major steps of the South China Sea geohistory. Rapid deposition with sedimentation rates of 60 m/Ma or more characterized the early Oligocene rifting. Several unconformities from the slumped unit between 457 and 495 mcd together erased about 3 Ma late Oligocene record, providing solid evidence of tectonic transition from rifting/slow spreading to rapid spreading in the South China Sea. Slow sedimentation of 20–30 m/Ma signifies stable seafloor spreading in the early Miocene. Dissolution may have affected the completeness of Miocene–Pleistocene succession with short-term hiatuses beyond current biostratigraphical resolution. Five major dissolution events, D-1 to D-5, characterize the stepwise development of deep water masses in close association to post-Oligocene South China Sea basin transformation. The concurrence of local and global dissolution events in the Miocene and Pliocene suggests climatic forcing as the main mechanism causing deep water circulation changes concomitantly in world oceans and in marginal seas. A return of high sedimentation rate of 60 m/Ma to the late Pliocene and Pleistocene South China Sea was caused by intensified down-slope transport due to frequent sea level fluctuations and exposure of a large shelf area during sea level low-stands. The six paleoceanographic stages, respectively corresponding to rifting (33–28.5 Ma), changing spreading southward (28.5–23 Ma), stable spreading to end of spreading (23–15 Ma), post-spreading balance (15–9 Ma), further modification and monsoon influence (9–5 Ma), and glacial prevalence (5–0 Ma), had transformed the South China Sea from a series of deep grabens to a rapidly expanding open gulf and finally to a semi-enclosed marginal sea in the past 33 Ma.     

海南岛鹿回头珊瑚礁与全新世高海平面

通过实地考察和资料分析,海南岛南部全新世以来以沉降活动为主要特征,鹿回头珊瑚礁能够反映全新世以来的海平面变化,鹿回头岸礁礁坪面上的原生珊瑚礁是全新世高海平面时期的产物。全新世以来南海北部至少存在过 4 期相对高海平面阶段:7300~6000、4800~4700、4300~4200和3100~2900cal.aBP。其中 7300~6000aBP 是整个全新世最高海平面时期,也是鹿回头珊瑚礁发育的繁盛期,基本形成了现代珊瑚礁的地貌格局,后来不同时期发育的珊瑚礁是在此时期形成的礁塘或礁坑等低洼地中形成,并在鹿回头半岛两侧向外发展,现代珊瑚礁则发育于全新世珊瑚礁的外礁坪和礁前斜坡带。另外,鹿回头珊瑚礁反映的南海北部全新世高海平面时间与南海其他地区高海平面时间可以衔接或对应,说明南海地区全新世高海平面具有全球背景,至少在南海是一致的,且与气候变暖紧密相联。     

Preliminary K-Ar geochronology of lavas from southern Lau Basin

In this short note, we report the ages of five lava samples from a segment of the Eastern Lau Spreading Center (ELSC) and three samples from the Valu Fa Ridge (VFR) in the southern Lau Basin. These samples were collected in situ from the axes and flanks of the spreading centers in the basin. These ages provide a key to better understanding the spreading mode, crustal formation and overall tectonic evolution of the basin. Except for two basaltic andesites and one andesite, the lavas analyzed are basalts. The ages of the lavas from ELSC range from (1.45 ± 0.15) Ma to (0.74 ± 0.04) Ma whereas those from VFR range from (0.50 ± 0.06) Ma to (0.32 ± 0.27) Ma, and the basalts give the oldest ages. The relatively younger ages of the VFR lavas are consistent with proposed tectonic evolution of the southern Lau Basin, i.e., VFR is a propagating extension of ELSC. The occurence of older lavas close to or on spreading axes in the southern Lau Basin implies the complex tectonic evolution of the basin. These results underscore a need for further detailed geophysical and geological studies in the southern Lau Basin, in order to better clarify the crustal accretion tectonic evolution in this area.     

白垩纪以来太平洋上地幔组成和温度变化

The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling(DSDP/ODP/IODP) sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge(the East Pacific Rise). The Ontong Java plateau(125–90 Ma) basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.