再论兴隆沟组火山岩成因

本文通过对辽西北票地区兴隆沟组安山质火山岩的组分和岩相学研究,发现作为捕虏晶的斜方辉石存在反环带和韵律环带.根据具反环带结构的斜方辉石的微量元素丰度研究,提出兴隆沟组火山岩是岩浆混合作用的产物而非下地壳拆沉作用的产物.对比华北克拉通北缘不同时代,不同成因的单斜辉石,推测辉石有可能来源于早中生代下地壳的堆晶岩.     

Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV, USA

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from 45–75 wt.% SiO₂. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate.

Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting.     

Magma-reservoir crystallization processes: small-scale dikes in cumulate gabbros, Mauna Kea Volcano, Hawaii

 Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale (<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene, and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo_71–66, cpx-Mg # 79–77, An_72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo₆₈ vs Fo₇₁₎. Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO₂<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage liquids (e.g., <5 wt.% MgO, based on <Fo₇₀) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo₇₀) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification. Received: 25 February 1997 / Accepted: 14 June 1997     

花岗岩可否分离结晶，有堆晶花岗岩吗？——评学术界最近的一些争论

花岗岩能否分离结晶?世界上有没有堆晶花岗岩？这是学术界长期争论的问题。从近期发表的文献判断,上述争论已经从国际学术界转入国内学术界。国际学术界一直坚持花岗岩能够分离结晶,笔者一直持反对意见。最近,关于堆晶花岗岩的说法突然盛行起来。什么是堆晶岩?堆晶与堆积是一回事吗?斜长石是堆晶矿物吗?这些问题都是需要认真讨论的。众所周知,玄武岩有堆晶岩,花岗岩则主要呈晶粥状态出现,这是两种岩浆粘性不同的表现。花岗岩由于粘性大,晶出的矿物不能下沉,与其晶出的岩浆一起构成类似粥状的面貌,故称为“晶粥”。玄武岩粘性低,晶出的矿物可以下沉离开其结晶的部位在岩浆房底部堆积形成堆晶岩,故玄武岩不形成晶粥。花岗岩形成晶粥后,由于冷却、固结而不可能再次升温熔融造成“晶”与“粥”分离的可能。因此,关于上有高分异花岗岩（或流纹岩）,下有堆晶的花岗岩的说法是不可能成立的。堆晶岩、堆晶结构等,统统是外国学者创造的。毋庸置疑,外国学者对花岗岩研究有很大的贡献,现今花岗岩理论的成果基本上都是外国学者提出来的,我们只是在学习、仿效和跟踪。然而,花岗岩研究中也有许多错误的理论、模型、假说,毫无疑问也是外国学者提出来的。面对上述情况,我们只有一个办法：独立思考。学习是必须的,但是,要批判地学,要仔细地鉴别,取其精华,去其糟粕。这样,我们才能更快地进步,才能不至于跟着外国学者犯错误。

     

塔里木板块东北部坡一镁铁质-超镁铁质层状侵入体岩石成因

坡一侵入体位于塔里木板块东北部坡北岩体内,是该岩体第三阶段岩浆活动形成的十几个小侵入体中的一个,锆石U-Pb年龄为278±2Ma。该侵入体属于以超镁铁质岩石为主的层状岩系,堆晶结构与韵律性堆晶层理非常发育。岩浆分异充分,形成了从纯橄岩到石英闪长岩的多种岩石类型。在超镁铁质岩石中,所有的橄榄石和大部分斜方辉石是堆晶相,少量斜方辉石是填隙相,大部分单斜辉石、褐色普通角闪石和黑云母是填隙相。在镁铁质岩石中,橄榄石和斜方辉石全部是堆晶相,单斜辉石与斜长石既可以是堆晶相,也可以是填隙相;褐色普通角闪石、黑云母和石英均为填隙相。超镁铁质岩石属拉斑玄武岩系列,镁铁质岩石属钙碱性系列。侵入体中大量存在的捕掳体、微量元素地球化学、Nd-Sr同位素组成的EMⅡ型演化趋势,充分证明了同化混染作用伴随岩浆演化过程而逐渐增强,并不断促进了岩浆的分异,而且导致了岩石化学系列的转化。PGE和亲硫元素地球化学以及硫同位素组成证明,硫主要来自于岩浆,硫化物形成于岩浆阶段,岩浆未经历过早期硫化物熔离作用,硫化物熔离起始于橄榄岩相结晶的晚期阶段,并伴随着此后的岩浆演化过程而继续熔离。硫化物熔离是岩浆自身演化和同化混染共同作用的结果。橄榄石Fo分子含量和全岩FeO含量显示,原生岩浆是苦橄质岩浆;源区物质应该是石榴石辉石岩;岩浆生成于地幔柱轴部。在塔里木板块东北部还存在分别来自于软流圈和亏损型大陆岩石圈地幔的二叠纪岩浆岩,它们都应该是塔里木大火成岩省的组成部分。     

Mineral chemistry of ultramafic and mafic cumulates as an indicator of the arc-related origin of the Mersin ophiolite (southern Turkey)

The Mersin ophiolite, represented by approximately 6-km-thick oceanic lithospheric section on the southern flank of the Taurus calcareous axis, formed in the Mesozoic Neo-Tethyan ocean some time during Late Cretaceous in southern Turkey. The ultramafic and mafic cumulates having over 3 km thickness consist of dunite ± chromite, wehrlite, clinopyroxenite at the bottom and pass into gabbroic cumulates in which leucogabbro, olivine-gabbro and anorthosite are seen. Crystallization order is olivine (Fo₉₁₋₈₀) ± chromian spinel (Cr# 60-80), clinopyroxene (Mg#₉₅₋₇₇), plagioclase (An_95.6−91.6) and orthopyroxene (Mg#₆₈₋₇₇). Mineral chemistry of ultramafic and mafic cumulates suggest that highly magnesian olivines, clinopyroxenes and absence of plagioclase in the basal ultramafic cumulates are in good agreement with products of high-pressure crystal fractionation of primary basaltic melts beneath an island-arc environment. Major, trace element geochemistry of the cumulative rocks also indicate that Mersin ophiolite was formed in an arc environment. Coexisting Ca-rich plagioclase and Forich olivine in the gabbroic cumulates show arc cumulate gabbro characteristics. Field relations as well as the geochemical data support that Mersin ophiolite formed in a supra-subduction zone tectonic setting in the southern branch of the Neo-Tethys in southern Turkey.     

北秦岭松树沟橄榄岩与铬铁矿矿床的成因关系

松树沟橄榄岩体是秦岭造山带中规模最大的赋存铬铁矿床的超基性岩体。松树沟橄榄岩主要由细粒橄榄岩质糜棱岩和中粗粒橄榄岩组成。本文通过对松树沟橄榄岩的岩相学、主微量、稀土元素地球化学的系统研究,认为松树沟细粒方辉橄榄岩为洋脊扩张过程中地幔岩减压-近分离熔融产生的残留体,细粒纯橄岩主要由地幔橄榄岩熔融残留橄榄石、消耗辉石的减压熔融反应:aCpx+bOpx+cSpl=dOl+1Melt生成的橄榄石和少量的地幔方辉橄榄岩残留体组成,但均受到了后期渗滤熔体的再富集作用;中粗粒纯橄岩和方辉橄榄岩主要为上述反应产生的渗滤熔体被圈闭在迁移通道或减压扩容带内在热边界层(TBL)通过反应:MeltA=Ol+MeltB冷凝结晶而成,属堆晶橄榄岩。Pb-Sr-Nd同位素地球化学的证据显示,松树沟橄榄岩与基性岩具有共同的地幔源区,二者同为松树沟蛇绿岩的重要组成部分。通过矿床地质特征及铬铁矿电子探针测试研究,认为松树沟铬铁矿床是产于中粗粒堆晶纯橄岩中的层状铬铁矿床,形成于格林威尔期松树沟洋盆的扩张过程中,是中粗粒纯橄岩在热边界层(TBL)的冷凝结晶过程中岩浆分异作用的产物。     

The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe

The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo₆₀). The troctolite is composed of about 50% olivine (Fo_50-54), 40% plagioclase (An_53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product.