大洋岩石圈拆沉与大陆下地壳拆沉：不同的机制及意义——兼评“下地壳＋岩石圈地幔拆沉模式”

拆沉作用(delamination)是地球科学中一个重要的科学问题。本文认为,大洋岩石圈拆沉和大陆下地壳拆沉是不一样的:(1)拆沉的物质不同。大洋岩石圈拆沉的物质包括大洋地壳、岩石圈地幔甚至一部分软流圈地幔,它们共同进入地幔深部;而大陆下地壳拆沉仅仅限制在下地壳,不包括岩石圈地幔。(2)拆沉的动力不同。大洋岩石圈拆沉是由板块俯冲引起的,是地幔对流的产物,因此是一种快速的主动的拆沉;而下地壳拆沉是由于下地壳加厚使下地壳密度增加引起的,还要求其下刚性的岩石圈地幔转变成塑性的软流圈地幔才有可能发生。因此下地壳拆沉要克服许多阻力才能实现,使拆沉成为一个漫长的过程,是慢速的和被动的拆沉。(3)拆沉的过程不同。大洋岩石圈拆沉是由板块俯冲触发的,俯冲导致碰撞,大洋岩石圈从根部断裂,拆沉进入地幔。大陆下地壳拆沉由地壳加厚开始,使下地壳转变为榴辉岩相;随后,岩石圈地幔减薄,直至全部转化为软流圈地幔;下地壳发生部分熔融,形成大规模的(埃达克质)岩浆,使下地壳榴辉岩的密度大于下伏的地幔,从而引发拆沉。大陆下地壳拆沉不大可能是整体进行的,可能是一块一块地被蚕食、被拆沉的。(4)拆沉后的效应不同。大洋岩石圈地幔拆沉,使热的软流圈地幔上涌,从而引发了一系列地质效应:如岩浆活动、地壳抬升、构造松弛以及随后的造山带垮塌等。而下地壳拆沉只引起地壳减薄,高原和山脉垮塌,并不伴有大规模的岩浆活动和地壳抬升等过程。(5)拆沉与岩浆活动的关系不同。主动拆沉导致大规模岩浆活动,而被动拆沉是在大规模岩浆活动的基础上开始的。此外,文中还对"下地壳 岩石圈地幔拆沉"模式提出了质疑,认为该模式有许多难以理解的问题和太多推测的成分,而且与现在保存的地质事实不符。     

中国东部壳-幔、岩石圈-软流圈之间的相互作用带：特征及转换时限

中国东部中—新生代,下部岩石圈存在壳与幔、岩石圈与软流圈两个相互作用带,它们是重要的岩浆源区,在层圈相互作用中,热和物质的交换及其动力学过程是引起中、新生代岩石圈内部层圈间的厚度调整、岩石圈不均匀减薄以及区域构造-岩浆-成矿作用的重要机理。大陆内部的壳-幔作用有3种类型:地幔来源的底侵熔体与下地壳的作用;下地壳拆沉进入弱化(weakening)了的岩石圈地幔二者发生的作用以及陆-陆碰撞深俯冲带的壳-幔相互作用。它们形成的火山岩组合有一定的差别,但源区都含有地壳组分。岩石圈-软流圈的作用带也是重要的岩浆源区,源区是以软流圈地幔为主,基本不含地壳组分。东部岩石圈的减薄时间大体与出现大规模软流圈来源的玄武岩喷发的时间一致,也与上述两类层圈作用转换的时间一致,大约在100Ma以后。     

东南沿海地区古近纪大陆岩石圈地幔特征及成因

东南沿海地区新生代玄武岩中的橄榄岩包体来自岩石圈地幔 ,上地幔橄榄岩包体的岩石学及地球化学特征都记录了地幔演化的历史。普宁橄榄岩包体斜方辉石含量与太古宙克拉通地幔类似 ,但在矿物学、REE、痕量元素和Sr Nd同位素上又与太古宙岩石圈地幔不同。橄榄岩包体的岩相学、矿物学、REE、痕量元素特征都提供了含H2 O富Si流体交代橄榄岩的证据 ,这种流体可能主要是洋壳物质局部熔融而成。流体交代使橄榄岩富Si,同时富Sr、Pb和强不相容元素等大洋岩石圈物质。这表明普宁大陆岩石圈地幔既保留太古宙岩石圈地幔的特征 ,又具有大洋俯冲地幔的特征 ,它是古老岩石圈地幔向大洋岩石圈地幔转换的一部分 ,这种转换可能是大洋岩石圈与大陆岩石圈地幔相互作用的结果。     

云南金宝山铂钯矿Nd-Os同位素特征及成因意义

金宝山铂钯矿是峨眉山大火成岩省具有典型意义的岩浆Ni-Cu-PGE矿床,成矿岩体岩矿石Sm-Nd、Re-Os同位素特征表明为地幔柱成因并受到岩石圈地幔和地壳混染作用的影响.∈Nda=+0.01～+0.89,显著高于大陆岩石圈地幔,而低于软流圈地幔,介于地幔柱与岩石圈地幔之间但明显靠近地幔柱的同位素组成.γOs=20～60,高于任何端员类型的地幔储集库,反映地壳混染作用.混合模型分析表明,成矿岩浆演化经历了10%左右的大陆岩石圈地幔熔体混合并受到下地壳5%～10%的混染.     

华北太古代上地幔钕同位素组成、演化及对该区岩石圈地幔不均一性的制约

近年所获得的华北各地区太古代变质火山岩系9个较准确的Sm-Nd同位素年龄及初始Nd同位素比值表明,华北太古代(3500—2500Ma)上地幔都是亏损的,但在1000Ma范围內其ε_(Nd)值一直恒定在+3左右,与全球太古代亏损地幔Nd同位素演化规律一致。这表明华北太古代的构造体制有可能使大量地壳物质重新进入地幔从而保持ε_(Nd)值的恒定。如果华北大陆岩石圈地幔的主体是在太古代时从对流上地幔中分离出来的,利用该区太古代上地幔Nd同位素组成及其可能的sm/Nd值进行计算,可获得该区岩石圈非交代地幔应是与N型大洋中脊玄武岩(以下简称MORB)源类似或更亏损的高亏损地幔,其现代ε-(Nd)值最大变化范围在+7—+23之间。至今在华北我们尚未发现由这种高亏损非交代大陆岩石圈地幔产生的大陆玄武岩。如果某些大陆玄武岩可能产生于大陆岩石圈地幔,则其源区必定是经过地幔交代作用再富集了的地幔。     

陆壳的垂向增生

陆壳的垂向增生是幔源物质进入到下地壳和下地壳物质进入到地幔的双向过程 ,前者主要表现为壳下底侵作用 ,后者主要表现为岩石圈规模的拆沉作用 ,其中拆沉作用往往诱发了陆壳下大规模的底侵作用。下地壳部分熔融残余的超镁铁质岩沉入到岩石圈地幔的过程称为陆壳沉没作用 ,它可能是陆壳物质进入地幔的一种重要方式。     

拆沉作用(delamination)及其壳—幔演化动力学意义

拆沉作用导致下地壳和岩石圈地幔下沉，相应软流圈上涌至壳—幔边界，使下地壳、岩石圈地幔和软流圈三者发生物质交换，引起岩浆作用、山脉隆升、伸展、垮塌，形成坳陷盆地，并最终使大陆地壳向长英质方向演化，产生与其它行星不同的、独一无二的中性安山质或英云闪长质成分。拆沉作用是对经典板块构造理论的重要补充与完善     

地幔富硅交代与大陆岩石圈的演化

富硅交代是弧下地幔中熔体岩石相互作用的主要表现形式 ,是造成古老克拉通陆下岩石圈地幔富硅的主要机制。在弧下地幔捕掳体中 ,橄榄岩被来自俯冲洋壳物质部分熔融生成的含水富硅熔体交代后 ,斜方辉石含量的增加使全岩富集SiO2 ,斜方辉石显示异常低的Al2 O3和Cr2 O3,微量元素上表现为强烈富集LILE ,强烈亏损Nb ,Ta和Ti。在古老克拉通地幔岩样品中 ,方辉橄榄岩具过剩的斜方辉石 ,橄榄石的Ni含量与斜方辉石的组成含量成正比 ,而和橄榄石的x(Mg) /x(Mg +Fe2 +)值没有正比关系 ,被解释为亏损的地幔橄榄岩和来自俯冲板片的富硅熔体相互作用的结果。熔体岩石相互作用最终导致了陆下岩石圈地幔富集SiO2 ,这种被含水富硅熔体改造后的地幔岩石的部分熔融可能是造成陆壳富硅富镁的主要原因。含水富硅熔体对岩石圈地幔的影响程度也可能是大陆岩石圈增生或裂解、增厚或减薄的关键因素之一。     

前寒武纪地球动力学(Ⅷ):华北克拉通太古宙末期地壳生长方式

地球早期大陆地壳的生长方式和壳幔动力学机制一直是国际前寒武纪研究的热点问题。尽管太古宙是大陆地壳生长的主要时期已基本获得共识,但是对于太古宙时期地壳生长的具体方式和壳幔动力学过程仍然存在很大的争议。部分学者提出地幔柱或者与拆沉相关的垂向构造体制,而其他学者主张与俯冲相关的侧向增生模式或者地幔柱-岛弧的联合作用体制。研究表明,太古宙末期科马提岩明显减少、富钾花岗质岩石普遍发育、地壳再循环速度显著增强,反映地壳演化的壳幔动力学机制发生了明显的转变。华北克拉通以发育大规模2.5~2.6Ga构造岩浆活动为特征,是探讨太古宙末期地壳生长方式和壳幔动力学机制转变的关键地区。本文系统总结了近些年来华北克拉通东部陆块西北缘早前寒武纪研究的最新进展,特别是对辽西、冀东、辽北和五台地区的新太古代晚期(约2.5~2.6Ga)表壳岩变质火山岩系进行了系统的岩石成因和壳幔作用探讨。研究表明,上述地区的变质铁镁质岩石可以划分为3个岩石成因系列:MORB型、IAT(岛弧拉斑玄武岩)型和CAB(岛弧钙碱性玄武岩)型,它们的原岩分别起源于洋中脊软流圈地幔以及受到不同程度俯冲流体交代的地幔楔的部分熔融。变质安山质-英安质火山岩分别具有类似高镁安山岩和埃达克岩的地球化学特征,它们的原岩形成过程可能与俯冲板片的部分熔融以及板片熔体与地幔楔的相互作用有关。结合整个东部陆块早前寒武纪的研究进展,我们提出华北克拉通在太古宙末期(约2.5和约2.7Ga)经历了强烈的地壳生长过程,其中新太古代早期(约2.7Ga)地壳生长以地幔柱-岛弧联合作用体制为主,而新太古代末期(约2.5~2.6Ga)以洋内俯冲和弧陆增生作用体制占主导地位。新太古代末期与俯冲增生相关的构造岩浆活动在Tarim克拉通、印度南部、南极洲Vestfold Hills地体以及南澳Gawler克拉通被广泛报道,这可能与类似显生宙板块构造体制的启动以及太古宙末期Kenorland超大陆的汇聚过程有关。     

从地壳上地幔构造看大陆岩石圈伸展与裂解

本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。     

北京燕山地区薛家石梁杂岩体特征、成因、源区性质及岩石圈减薄方式

薛家石梁杂岩体位于北京北山地区,在平面上呈北西向的椭圆状,主要由辉长岩、二长辉长岩、二长岩、正长岩和花岗岩组成。根据锆石SHRI MP定年结果为132·8~123·3Ma,形成于早白垩世早期。野外地质特征、矿物学特征、岩石学特征及地球化学特征表明,薛家石梁杂岩体中二长岩是二长辉长岩岩浆与正长岩岩浆混合作用的产物,从辉长岩岩浆到二长辉长岩岩浆经历了结晶分异作用。薛家石梁杂岩体中正长岩具高Sr,低Y及Eu正异常特征,推测其可能来源于加厚陆壳的底部。薛家石梁杂岩体中辉长岩中Mg#值为65,w(Nb)/w(U)值为37·8,这些特征暗示其可能为原生岩浆。辉长岩中ε(Nd)值为-6·5,表明其源区岩石不具亏损地幔特征;而辉长岩具富集Pb、Ti、Nb正异常,Hf的负异常,与EMI型富集地幔特征(具Nb、Hf正异常及Pb的负异常)不一致;辉长岩中Rb、Th、Nb、U、La、Ce元素含量比EMI型富集地幔低一个数量级;杂岩体中N(87Sr)/N(86Sr)与N(206Pb)/N(204Pb)值具正相关关系也表明不具交代富集型地幔特征。因此,我们认为辉长岩岩浆源区应为软流圈地幔,而不是富集型地幔(EMI)。辉长岩中ε(Nd)的负值是辉长岩岩浆与太古宙下地壳相互作用的结果。因此,我们认为中国东部岩石圈减薄的主要机制是岩石圈的拆沉作用。     

U-Pb Zircon Age,Geochemical, and Sr-Nd-Pb Isotopic Constraints on the Age and Origin of Mafic Dykes from Eastern Shandong Province,Eastern China

U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).     

Contrasting Archean and Proterozoic lithospheric mantle: isotopic evidence from the Shonkin Sag sill (Montana)

²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb model ages using Shonkin Sag data and published analyses for magmas of the Cenozoic Wyoming-Montana alkaline province (WYMAP) provide evidence of an Archean age for the subcontinental lithospheric mantle (SLM) associated with the Wyoming craton. The SLM imprint on magmas is expressed as Ba, Ta, Nb and Ti "anomalies" which correlate with radiogenic isotopic data, and it resembles a subduction imprint on Cenozoic south-western USA basalts (SWUSAB). However the latter give Proterozoic Pb isotope model ages. Although the Archean and Proterozic model ages may represent mixing lines, the fact that they resemble the ages for continental crust cut by WYMAP and SWUSAB respectively indicates that the age of the underlying SLM helped control the "isochron" slopes and inferred "ages". Lower ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb but comparable ⁸⁷Sr/⁸⁶Sr for WYMAP suggest that SLM associated with Archean cratons has lower Sm/Nd, U/Pb and Rb/Sr ratios than SLM associated with SWUSAB Proterozic terranes, regardless of when the subduction imprint or imprints developed. WYMAP magmas have high Pb/Zr ratios indicating that Archean SLM, like Archean continental crust, is enriched in Pb compared to Proterozoic SLM. If the enrichment was Archean, it implies that higher Archean heat flow enhanced Pb transfer from the subducting slab to overlying lithospheric mantle and crust. A subducted sediment imprint on the SLM is also consistent with high i¹⁸O values for the Shonkin Sag. Low TiO₂ in WYMAP may reflect a residual mantle TiO₂ phase. If so, the Nb "missing" from crustal and oceanic mantle reservoirs may reside in rutile of Archean SLM. Isotopic similarities between WYMAP and EM1 oceanic island basalts may reflect the presence of delaminated, Archean SLM in the oceanic mantle, although low Pb/Zr ratios and a lack of Ti, Nb and Ta anomalies in oceanic island basalts deserve further investigation.     

The composition and formation of Miocene tholeiites in the Central European Cenozoic plume volcanism (CECV)

Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low ¹⁴³Nd/¹⁴⁴Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks. Received: 10 December 1999 / Accepted: 23 June 2000     

Petrogenesis and mantle source characteristics of Cenozoic alkaline diabase,Jiangxi Province,southeastern China

Major element, trace element, and Sr–Nd–Pb isotopic compositions of Cenozoic diabase in southeastern China provide insights into the nature of their mantle sources and processes. The diabases are alkaline in lithochemistry (Na₂O + K₂O = 4.37–5.19 wt.%) and have overall oceanic island basalt-like trace element patterns, without negative Nb–Ta anomalies. In addition, they are characterized by lower La/Nb (<1.5) and La/Ta (<22), and higher Ce/Pb (>15) and Nb/U (>30) ratios, indicating an origin in the asthenospheric mantle. The relatively lower ¹⁴³Nd/¹⁴⁴Nd (0.512632–0.512648) and ²⁰⁶Pb/²⁰⁴Pb (18.20–18.22), but intermediate ⁸⁷Sr/⁸⁶Sr (0.7061–0.7063) ratios of the diabases are similar to enriched mantle type 1, suggesting crustal contamination or mixing with metasomatized lithsopheric mantle. However, the low Th and U contents and lack of correlations of Nd isotope compositions and MgO preclude significant crustal contamination. Alternatively, the moderate TiO₂ contents (2.01–2.09 wt.%) and high Cr concentrations (>240 ppm) suggest interaction between asthenosphere-derived melts and metasomatized lithospheric mantle. Petrological modelling suggests that the diabases were generated from a low degree (~3–5%) of partial melting of lherzolite with ~2–3% garnet. Jiangxi diabase was generated in a within-plate extensional regime, probably related to the far effect of the Himalaya–Tibetan orogen.     

A modified recycle model for Sr-Nd isotope system

A recycling model in conjunction with the theory of partial melting has been proposed. In contrast to other models, we divided the mantle and crust into the homogeneous recycling systems and the heterogeneous, isolated system, respectively. Inaccourdance with various partition coefficients for Sm, Nd and Rb and Sr, recycling rates and mantle/crust ratios, a series of numerical solutions are presented on the diagrams of f_Rb/Sr, f_Sm/Nd and Nd(t), Sr(t) v. s. time. In the model contrains that the recycling rates must be limited in the range of 1–4 km/yr and a medium-size depleted mantle, i.e., mantle/crust=20–60 can be adopted. The calculated ? Nd- ? Sr correlation curve is quite consistent with the observed values of Nd and Sr isotopes from young volcanic rocks and crustal materials.     

The evolution of the Arabian lower crust and lithospheric mantle — Geochemical constraints from southern Syrian mafic and ultramafic xenoliths

Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma.Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections.The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.     

Origin of anorogenic ‘lamproite-like’ potassic lavas from the Denizli region in Western Anatolia Extensional Province, Turkey

The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low ⁸⁷Sr/⁸⁶Sr, 0.703523–0.703757; high ¹⁴³Nd/¹⁴⁴Nd, 0.512708–0.512784; high ²⁰⁶Pb/²⁰⁴Pb, 19.079–19.227, ²⁰⁷Pb/²⁰⁴Pb, 15.635–15.682, ²⁰⁸Pb/²⁰⁴Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. La_N?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.     

浙东南晚中生代玄武岩的地球化学特征及其对源区物质的指示意义

浙江东南部晚中生代上、下火山岩系（以下简称上、下岩系）中均有玄武岩产出,本文对这些玄武岩分别进行了元 素地球化学和Sr-Nd-Pb同位素研究。不同岩系玄武岩的主量元素均表现出富碱、富Al等特征。但微量元素存在差异,下岩 系天台和青田样品具有轻稀土富集以及Ba, Pb和Sr富集,Eu负异常,Nb, Ta, Zr和Hf亏损的特征。上岩系玄武岩的元素特征 也有差别,永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山等地样品的元素特征表现出的性质与下岩系样品相似,武义玄 武岩样品没有Ta, Nb亏损特征,金衢盆地玄武岩的元素特征则介于两者之间。对应的,这些玄武岩样品的同位素组成也有 明显差异,下岩系玄武岩的初始同位素组成范围为 I Sr=0.70850~0.70897,εN（d t） = -5.6~-4.1,（206Pb/204Pb） i =18.21~18.38,（207Pb/204Pb） i =15.55~15.58,（208Pb/204Pb） i =38.26~38.49,接近下岩系中酸性岩浆岩的范围,反映了下地壳物质对其岩浆源区的显著影响。上岩系玄武岩有明显差异,表现出与元素特征对应的分组现象。其中永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山样品 I Sr = 0.70734~0.70936, εN（d t）= -7.1~-2.1,（ 206Pb/204Pb） i =18.01~18.40,（ 207Pb/204Pb） i = 15.54~15.62,（ 208Pb/204Pb） i=37.99~38.62, 具有富集特征, 可能来自活动大陆边缘; 而武义和金衢盆地样品的 I Sr=0.70533~0.70589, εNd（ t） =0.4~3.3,（206Pb/204Pb） i =17.23~18.11,（ 207Pb/204Pb） i =15.46~15.53,（ 208Pb/204Pb） i =36.91~38.43,具有类似OIB特征,趋向亏损地幔端元。上下岩系玄武岩的元素和同位素组成的研究表明,玄武岩的物质来源有较明显的差别,且表现出随时间变化的特征。其中下岩系玄武岩源区中可能有古老岩石圈地幔、下地壳物质和俯冲蚀变洋壳物质的贡献,而上岩系中玄武岩源区有可能是类似下岩系玄武岩性质的岩石圈、软流圈地幔和下地壳物质等的贡献。浙东南晚中生代岩石圈演化的动力学过程可能与太平洋板块俯冲有关,但不能排除岩石圈地幔拆沉的影响,具体的讨论还需要更多的岩石学和/或地幔包体资料的补充。     

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity,fractional crystallization and crustal assimilation

At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO₂ andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower ⁸⁷Sr/ ⁸⁶Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.