青藏高原新生代火车头山碱性及钙碱性两套火山岩的地球化学特征及其物源讨论

藏北羌塘火车头山新生代火山岩可区分为钙碱性及碱性两个不同的系列。钙碱性火山岩主要岩石组合为玄武岩-安山岩-英安岩,其SiO_2介于49%～70%之间,Al_2O_3>10%,Na_2O/K_2O>1;其中玄武岩具平坦型稀土配分型式,LREE/HREE为1.3～1.8,(La/Yb)_N为2.87～4.45,无明显铕异常,δEu为0.96～1.09;该套岩石的Mg~#与SiO_2相关关系以及La/sm-La等亲岩浆元素与超亲岩浆元素协变关系表明,它们应为幔源岩浆经分离结晶演化的产物,其岩石组合类型以及低的Sm/Yb值(Sm/Yb=1.53～5.35)表明它们的原始岩浆应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融。本区碱性火山岩为一套典型的钾质岩石系列,主要岩石组合类型为碱玄岩-碱玄质响岩-响岩,其SiO_2介于44%～59%之间,Al_2O_3>14%,Na_2O/K_2O介于0.47～1.51之间;岩石轻稀土强烈富集,LREE/HREE为13.20～15.76,(La/Yb)_N=50.44～91.99;其岩石组合类型以及Mg~#与SiO_2相关关系以及La/Sm-La协变关系同样表明它们为共源岩浆分离结晶演化的产物;然而,其较高的Sm/Yb值(Sm/ Yb=2.63～13.98)表明它们并非地幔橄榄岩直接局部熔融的产物,岩石弱的负Eu异常(δEu=0.77～0.85)以及Th、U的强烈富集和Nb、Ta的相对亏损,又反映了原始岩浆中有显著的地壳物质的贡献;该套钾质碱性系列岩石在La/Co-Th/Co同分母协变图上呈直线型分布,而在La/Co-Sc/Th异分母协变图上呈显著的双曲线分布,从而表明其源区为二源混合型,是青藏高原特殊的壳幔混合层局部熔融的产物,这些特征是新生代青藏高原壳幔层圈物质交换的重要岩石学证据。     

Geochemistry of the Pliocene basalts erupted along the Malatya-Ovacik fault zone (MOFZ), eastern Anatolia, Turkey: Implications for source characteristics and partial melting processes

The study area is located on the middle sector of the Malatya-Ovacık Fault Zone (MOFZ) in the eastern Anatolia. Four basaltic flows from bottom to top, which are tholeiitic in character and intercalated with Pliocene sedimentary rocks, were erupted along this fault zone. Chemical compositions of these flows reveal some differences between the first flow and others in terms of high-field strength elements (HFSEs) (e.g. Ti, Zr, Nb). Limited variations in compositions within the first flow and upper flows suggest a limited fractionation range. Trace-element patterns exhibit that all the flows have similar and OIB-like patterns without positive peak at Pb and a trough at Nb—Ta, indicating minimal or no crustal contribution. Rare-earth element (REE) patterns indicate that the first flow has flat patterns with negative Eu anomaly, whereas the upper flows have variable enrichments in LREE and depletions in HREE. La/Sm_N, Dy/Yb_N and Zr/Y ratios exhibit that the degree of partial melting decreases from the first flow to upper flows. Higher values of La/Yb_N ratio for the upper flows and depletions at Y and Yb on the trace-element patterns suggest the presence of garnet as a residual phase, which imply that the depth of partial melting took place solely in the garnet-stability field. OIB-like trace-element patterns and trace-element ratios (e.g. La/Nb, Ce/Y and Zr/Nb) emphasize that the melts forming the Arguvan basalt were originated from the asthenospheric mantle rather than the lithospheric mantle.     

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting     

Magma Genesis and Mantle Dynamics at the Harrat Ash Shamah Volcanic Field (Southern Syria)

A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± Fe–Ti oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na₂O and SiO₂ variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 30–40% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: ⁴⁰Ar/³⁹Ar ages; intraplate volcanism; mantle plume; partial melting; Syria     

元素地球化学场及其地学意义

有关岩浆花岗岩成因的原地重熔说,揭示了各种化学元素在内生过程中的迁移规律。据此并结合元素在周期表中的位置,得出了元素地球化学场的概念,元素地球化学场不但总结了元素在原地重熔过程中的聚散趋势,而且揭示了元素自身组构与其空间分布规律的关系,展示出三个不同层次的地质断面,即花岗岩体上部和顶部盖层断面,大陆地壳断面及地球球体断面。     

Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu–Bonin – Mariana arc

The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO₂. They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K₂O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc.     

Magma traps: Part II,application

A model of the lithostatic control of the ascent of magma, described in Part I (this volume), is tested against data from the Upper Cretaceous-Lower Tertiary sedimentary and volcanic region of central West Greeland: the thickness of sedimentary rock; the thickness of the pillow breccias; the total thickness of the lava pile; the depth of the post volcanic paleosurface. The local development is largely determined by a single parameter, the proportion of crustal thinning, and requires a magma source at 75 km depth with differentiation at 11 km depth. The model is applied in outline to the development of continental and orogenic volcanism in New Zealand.     

陆面过程模型中垂直非均匀土壤的水分传输及相变的模拟

土壤湿度在陆气相互作用中的重要性体现在它既能影响陆地和大气之间水循环的速率, 又能改变地表的能量分配。本文针对陆面过程模型中描述土壤湿度变化的方程进行了理论分析, 指出在非均匀土壤和冻土中采用土壤水势梯度描述垂直非均匀土壤水分流动的合理性。基于描述土壤内部水热传输的统一土壤模型, 并利用推广的表征土壤水分特征的Clapp-Hornberger关系式, 研究了非冻结和冻结的土壤湿度对于垂直非均匀土壤的敏感性。结果表明, 由土壤质地决定的土壤水势和导水率对土壤湿度的模拟有重要的影响。具体地, 在决定土壤性质的Clapp-Hornberger关系式中, 与土壤质地有关的饱和水势、饱和导水率以及土壤孔隙大小分布指数B, 对土壤湿度的模拟起到了关键作用。参数B的重要性尤为突出, 它的增加会引起导水率的大大下降, 从而对水分在土壤中的垂直分布产生重要影响。饱和水势的绝对值和参数B的增加会使得土壤水势绝对值增加明显, 使土壤的结冰(融化)过程延迟, 土壤温度因为没有结冰(融化)释放(吸收)的潜热加热(冷却)而持续下降(上升), 因此在冻融时期土壤温度会比观测值振幅偏大。上述结果揭示了考虑土壤垂直非均匀性并采用有效的土壤特性参数对于陆面过程模型的重要性。     

Chalcophile element partitioning between sulfide phases and hydrous mantle melt: Applications to mantle melting and the formation of ore deposits

Understanding the geochemical behavior of chalcophile elements in magmatic processes is hindered by the limited partition coefficients between sulfide phases and silicate melt, in particular at conditions relevant to partial melting of the hydrated, metasomatized upper mantle. In this study, the partitioning of elements Co, Ni, Cu, Zn, As, Mo, Ag, and Pb between sulfide liquid, monosulfide solid solution (MSS), and hydrous mantle melt has been investigated at 1200 °C/1.5 GPa and oxygen fugacity ranging from FMQ−2 to FMQ+1 in a piston-cylinder apparatus. The determined partition coefficients between sulfide liquid and hydrous mantle melt are: 750–1500 for Cu; 600–1200 for Ni; 35–42 for Co; 35–53 for Pb; and 1–2 for Zn, As, and Mo. The partition coefficients between MSS and hydrous mantle melt are: 380–500 for Cu; 520–750 for Ni; ∼50 for Co; <0.5 for Zn; 0.3–6 for Pb; 0.1–2 for As; 1–2 for Mo; and >34 for Ag. The variation of the data is primarily due to differences in oxygen fugacity. These partitioning data in conjunction with previous data are applied to partial melting of the upper mantle and the formation of magmatic-hydrothermal Cu–Au deposits and magmatic sulfide deposits.I show that the metasomatized arc mantle may no longer contain sulfide after >10–14% melt extraction but is still capable of producing the Cu concentrations in the primitive arc basalts, and that the comparable Cu concentrations in primitive arc basalts and in MORB do not necessarily imply similar oxidation states in their source regions.Previous models proposed for producing Cu- and/or Au-rich magmas have been reassessed, with the conclusions summarized as follows. (1) Partial melting of the oxidized (fO₂ > FMQ), metasomatized arc mantle with sulfide exhaustion at degrees >10–14% may not generate Cu-rich, primitive arc basalts. (2) Partial melting of sulfide-bearing cumulates in the root of thickened lower continental crust or lithospheric mantle does not typically generate Cu- and/or Au-rich magmas, but they do have equivalent potential as normal arc magmas in forming magmatic-hydrothermal Cu–Au deposits in terms of their Cu–Au contents. (3) It is not clear whether partial melting of subducting metabasalts generates Cu-rich adakitic magmas, however adakitic magmas may extract Cu and Au via interaction with mantle peridotite. Furthermore, partial melting of sulfide-bearing cumulates in the deep oceanic crust may be able to generate Cu- and Au-rich magmas. (4) The stabilization of MSS during partial melting may explain the genetic link between Au-Cu mineralization and the metasomatized lithospheric mantle.The chalcophile element tonnage, ratio, and distribution in magmatic sulfide deposits depend on a series of factors. This study reveals that oxygen fugacity also plays an important role in controlling Cu and Ni tonnage and Cu/Ni ratio in magmatic sulfide deposits. Cobalt, Zn, As, Sn, Sb, Mo, Ag, Pb, and Bi concentrations and their ratios in sulfide, due to their different partitioning behavior between sulfide liquid and MSS, can be useful indices for the distribution of platinum-group elements and Au in magmatic sulfide deposits.