K‐Ar age determination,whole‐rock and oxygen isotope geochemistry of the post‐collisional Bizmişen and Çaltı plutons,SW Erzincan,eastern Central Anatolia,Turkey

Post‐collisional granitoid plutons intrude obducted Neo‐Tethyan ophiolitic rocks in central and eastern Central Anatolia. The Bizmişen and Çaltı plutons and the ophiolitic rocks that they intrude are overlain by fossiliferous and flyschoidal sedimentary rocks of the early Miocene Kemah Formation. These sedimentary rocks were deposited in basins that developed at the same time as tectonic unroofing of the plutons along E–W and NW–SE trending faults in Oligo‐Miocene time. Mineral separates from the Bizmişen and Çaltı plutons yield K‐Ar ages ranging from 42 to 46 Ma, and from 40 to 49 Ma, respectively. Major, trace, and rare‐earth element geochemistry as well as mineralogical and textural evidence reveals that the Bizmişen pluton crystallized first, followed at shallower depth by the Çaltı pluton from a medium‐K calcalkaline, I‐type hybrid magma which was generated by magma mixing of coeval mafic and felsic magmas. Delta ¹⁸O values of both plutons fall in the field of I‐type granitoids, although those of the Çaltı pluton are consistently higher than those of the Bizmişen pluton. This is in agreement with field observations, petrographic and whole‐rock geochemical data, which indicate that the Bizmişen pluton represents relatively uncontaminated mantle material, whereas the Çaltı pluton has a significant crustal component. Structural data indicating the middle Eocene emplacement age and intrusion into already obducted ophiolitic rocks, suggest a post‐collisional extensional origin. However, the pure geochemical discrimination diagrams indicate an arc origin which can be inherited either from the source material or from an upper mantle material modified by an early subduction process during the evolution of the Neo‐Tethyan ocean. Copyright © 2005 John Wiley & Sons, Ltd.     

花岗岩体定位机制研究进展综述

花岗岩体的定位机制及其派生的构造是当代花岗岩地质学研究的前沿。本文从花岗岩体定位机制研究的意义出发,综述了花岗岩体定位的几种主要机制的研究进展、花岗岩体定位机制的控制因素、以及花岗岩体定位机制的研究方法等。最后,提出了加强我国花岗岩体定位机制及花岗岩体构造研究的建议。     

马鬃山地区侵入岩体渗透性能分析

文章根据马鬃山地区的水文地质普查资料和近年来开展的钻孔水文地质试验结果,阐述了该区侵入岩体渗透系数的范围,分析了侵入岩体渗透性在空间上的分布特征。     

Nd、Sr同位素组成对中国大陆科学钻探工程主孔0～2000米片麻岩原岩成因的制约

中国大陆科学钻探工程主孔0～2000岩芯段的片麻岩包括二长花岗质片麻岩和斜长片麻岩两类,其中的Sr同位素组成与元素Rb的丰度关系显示原岩侵位后发生过强烈的Rb加入事件,说明Rb-Sr同位素体系在超高压变质和随后的退变质作用过程中受到了明显的扰动.相反,Sm-Nd同位素体系尚保持着封闭状态,可有效地示踪原岩的成因.二长花岗片麻岩的εNd(t)值较低,变化范围也不大(-8.2～-13.0),它们的Nd同位素模式年龄(TDM)主要介于2.25到2.54 Ga之间,远大于其原岩的实际侵位年龄,指示其原岩可能是由古元古代的地壳物质在新元古代部分熔融形成的.而斜长片麻岩的εNd(t)值变化范围广,从 1.3到-9.4.其中采自大套斜长片麻岩段的样品其εNd(t)值从-3.0到-9.4,它们的Nd同位素模式年龄介于2.1～2.4 Ga,与二长花岗片麻岩的TDM值类似,该类岩石的地球化学特征总体类似于扬子地台北缘新元古代双峰式火山岩的酸性端元,指示它们的原岩可能是在新元古代由地幔来源的玄武岩浆上升到地壳层位,引起成分不均一的古元古代地壳物质发生部分熔融形成的,其中可能混入了少量地幔来源的物质.而采自变镁铁质岩"夹层"中的斜长片麻岩显示出接近球粒陨石的Nd同位素特征(εNd(t)= 1.3～-2.6),类似或略低于邻近的榴辉岩和斜长角闪岩围岩的Nd同位素组成及大别山-苏鲁造山带内大多数榴辉岩的Nd同位素组成,指示它们与这些变镁铁质岩之间可能有成因联系.它们的Nd同位素模式年龄(TDM=1.28-1.60 Ga)虽然早于其原岩的侵位年龄,但明显小于花岗片麻岩及大套斜长片麻岩的TDM值.我们倾向于认为这些"薄层"状的斜长片麻岩的原岩是由玄武岩浆派生的,伴有一定程度的地壳混染.     

Cambro-Ordovician magmatism in the Araçuaí Belt (SE Brazil): Snapshots from a post-collisional event

The focus of the present work is the rebound of the magmatism at the late stages of orogenic collapse in the Araçuaí Belt, in Brazil. At the end of the orogen bimodal inversely zoned plutons were emplaced along a tectonic corridor following the Atlantic coast of Brazil. This event culminates around 500 Ma. We review geology, petrography, geochemistry and geochronology of these plutonic structures in the region of Espírito Santo. New geochemical modeling of trace element data together with new whole rock Sm–Nd and Rb–Sr isotopic data depict a gradual process of mantle contamination during different stages of the orogen. In the post-orogenic stage contamination reaches extreme values. Basic rocks reach εNd_(0.5Ga) values below −10. Gabbros to peridotites are more enriched in Ba, Zr, LREE and Sr than the surrounding metasediments. Four new U–Pb data sets from determinations in zircon and monazite crystals, (central northern part of the belt) confirm peak magma production around 500 ± 15 Ma, ranging up to 525 ± 3 Ma in one of the structures. Based on the pictured data, we discuss the significance of this magmatism and present a possible model: a process of gradational delamination of the subcontinental lithospheric mantle and adjacent deep continental crust through underplating. As a consequence, an already enriched mantle was dramatically further contaminated. At the end of this event the intrusion of alkaline melts, with less negative εNd (−5) and further enrichment in incompatible elements, point towards a new input of deeper juvenile mantle magma. Our data suggest the possible onset of a hotspot due to the destabilization of the asthenospheric mantle after orogenic collapse.     

Zircon U–Pb and Lu–Hf systematics of the major terranes of the Western Superior Craton,Canada: Mantle-crust interaction and mechanism(s) of craton formation

Paleo-to Neoarchean granitoid gneisses (ca. 3.30 to 2.49 Ga) are well preserved in the Western Superior Craton. Protoliths of these gneisses are mainly I-type granitoids characterized by high Sr/Y and La/Yb ratios and low Mg#, consistent with Archean tonalite-trondhjemite-granodiorites. Zircons from granitoid gneisses commonly contain three growth phases: inherited cores (zircon I), magmatic rims (zircon II) and outer rims that have undergone Pb-loss (zircon III). The 3.12 Ga to 2.86 Ga zircon I represent early crustal material, that was captured in younger zircons; zircon II preserve crustal re-working and younger crustal additions that are constrained between 2.85 to 2.72 and 2.69 to 2.65 Ga.Zircon II contains both positive and negative εHf(t) values (−6.3 to +8.1), with both depleted-mantle and older crustal signatures. Half of the magmatic rims (II) are characterized by depleted mantle signatures with positive εHf(t) values representing juvenile crust-forming events, whereas the other half are characterized by recycled crustal signatures with negative εHf(t) values. εHf(t) results show that the North Caribou and the Island Lake terranes and the northern Uchi domain are isotopically more enriched than the southern Uchi, English River, Wabigoon and Winnipeg River terranes, suggesting the northern Uchi margin represents a major terrane boundary.Based on mass balance calculations, large volumes of juvenile material at circa 3.0 Ga mixed with smaller amounts of older crust. The vast majority of the granites were derived from a source with about 50% mantle material during the peak crust formation events after 2.8 Ga. The decline in the volume of felsic magmatism in the later Archean is coeval with a reduced supply of both heat and material from depleted mantle sources. Combined with previously published geochemical, geochronological and isotopic data, this suggests an evolution in felsic magma sources consistent with crustal thickening.     

垭口片麻岩的主要岩石化学特征

出露于川西滇中地区的垭口片麻岩及相应岩石是一套英云闪长质-奥长花岗质(为主)-花岗闪长质片麻岩(灰色片麻岩即TTG岩套)和花岗质片麻岩组合,后者可能是前者活化时遭受钾长石化的结果。其常量元素、过渡金属元素和稀土元素特征与早前寒武纪低铝型灰色片麻岩一致,是本区晚太古代结晶基底(花岗地体)。     

Geochemical evolution of peraluminous plutons in southern Nova Scotia, Canada—a pegmatite-poor suite

Three southern Nova Scotia plutons crystallized rapidly at 375 Ma from magma containing both mantle and crustal components. Isotopic and chemical data suggest that the crustal contribution included both lower crustal material and Cambro-Ordovician turbidites of the host Meguma Group. Despite local evidence of mixing and mingling of magmas, the bulk of the plutons evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differentiated. Like parental plutons, pegmatites fall into biotite+plagioclase and muscovite+potassium feldspar assemblages. The latter locally contain Mn-rich garnet+biotite, giving calculated P–T conditions of pegmatite crystallisation of 620°C, 0.44 GPa under water-saturated conditions. Host rocks at the time of emplacement experienced P–T conditions varying from <500° to 620°C at 0.44 GPa. Beryl-bearing pegmatites occur only where host rocks were below sillimanite-grade, and pegmatites of any kind are scarce in sillimanite-grade host rocks. Rarity of pegmatites can be ascribed to a combination of insufficient amounts of F, Li, and B in the magma with rapid cooling which prevented extensive fractionation and undercooling. Our data suggest rather that peraluminous suites, produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks, and typified by biotite-rich, cordierite-bearing plutons, are not favorable locales for major rare-metal-enriched pegmatite fields. Such fields appear to require relatively prolonged anatexis resulting from crustal thickening, or an areally extensive mafic underplate,     

冀北大滩盆地铀(钼)成矿流纹岩-花岗斑岩SHRIMP锆石U-Pb定年、地球化学及Sr-Nd同位素特征

大滩火山岩盆地位于华北克拉通北缘,燕辽多金属(Mo-U-Ag-Pb-Zn)成矿带西段,沽源-红山子铀成矿亚带西南段。高精度SHRIMP锆石U-Pb定年表明,盆地内张家口组流纹岩和侵入其中的花岗斑岩分别形成于140.2±1.9Ma(2σ,MSWD=1.7)和131.7±1.1Ma(2σ,MSWD=0.8)。全岩主、微量元素特征显示,流纹岩和花岗斑岩均富集Si、Fe、Ga、Rb、Pb、Th、U、Zr、Hf、Nb、Ta和LREE,亏损Al、Mg、Ti、Ba、Sr、Eu、Cr、Ni和HREE,为典型的A型花岗岩特征。全岩Nd同位素研究表明,二者具有相似的ε_(Nd)(t)(分别为-15.9和-16.5)和t_(DM2)值(分别为2267Ma和2220Ma),暗示二者具有相同的岩浆来源。元素相关性分析表明,母岩浆从流纹岩演化至花岗斑岩经历了一定程度的钾长石和黑云母的结晶分异,导致了后者Si、U、HREE的进一步富集,以及Fe、Mg、Ti、Al、Sr、Ba、Eu、Zr、LREE等元素的相对亏损。结合前人工作,我们认为流纹岩和花岗斑岩形成于华北克拉通岩石圈减薄及破坏的板内伸展背景,来源于古元古代(ca.2.2~2.3Ga)火成地壳物质(英云闪长质、花岗闪长质岩石)的部分熔融。结合全岩Rb-Sr等时线年龄及前人获得的矿石U-Pb年龄推断,该区主要铀(钼)矿化时代介于105~110Ma,明显晚于流纹岩和花岗斑岩的成岩时代。通过与区域上的产铀盆地(沽源)对比,以及对流纹岩和花岗斑岩的U含量等特征分析认为,两者(尤其是花岗斑岩)是很好的铀矿成矿物质来源,暗示了大滩盆地具有良好的成矿前景。     

花岗岩结晶分离作用问题——关于花岗岩研究的思考之二

岩浆结晶分离作用是一个古老的话题,很早就有学者指出,地球内部生成的岩浆大多是玄武质岩浆,大多数花岗岩是由玄武岩结晶分离形成的.本文在考察了岩浆结晶分离作用的制约因素、比较了不同性质岩浆结晶分离作用的特征之后指出：玄武质岩浆可以发生结晶分离作用,因为有与其相关的堆晶岩产出;安山质岩浆也可以发生结晶分离作用,因为也有与其相关的堆晶岩产出.但是,花岗质岩浆似乎不大可能发生结晶分离作用,因为,很少见到有与（富硅的）花岗质岩浆相伴的堆晶岩产出.花岗质岩浆之所以不大可能发生结晶分离作用的原因在于：（1）岩浆的黏性大,它不仅阻滞了矿物的结晶作用（使斜长石不能发育为自形晶）,而且阻止了密度大的矿物（例如角闪石）下沉;（2）主要造岩矿物（例如斜长石）的密度与花岗质岩浆的密度相差无几,使结晶分离作用难以进行.本文详细考察了花岗质岩浆中斜长石的行为,指出在花岗质岩浆中斜长石结晶分离几乎是不可能的.那么,文献中大量充斥的花岗岩结晶分离作用的说法是依据什么呢？作者认为,文献中的许多说法可能主要是根据哈克图解得出的,而不是根据实际观察和理论研究得出的.作者认为,玄武岩和花岗岩不仅来源不同,成分不同,而且解释也不同.哈克图解中许多适合玄武岩的解释未必适合花岗岩.由于鲍文反应原理是结晶分离作用的理论基础,因此,文中也对鲍文反应原理进行了评述,并指出文献中存在的一些需要认真对待的问题,例如,从玄武岩-安山岩-英安岩-流纹岩的连续演化序列是不可能的;单元-超单元填图方法是不科学的;中国东部中生代大规模花岗岩不可能是玄武质岩浆结晶分离形成的等等.本文还以Ajaji et al.（1998）报道的摩洛哥Tanncherfi花岗岩为例,指出结晶分离作用的解释是不可能的.作者认为,花岗岩类的成分变化大,主要可能与源区组成、温度、压力、挥发分、部分熔融程度和过程、混合作用、岩浆分异及结晶分离作用有关.其中,源区组成可能是花岗岩多样性的最重要的原因,而结晶分离作用的影响可能是微乎其微的.本文认为,花岗岩结晶分离作用对于花岗岩成因的意义已经被大大地夸大了,我们应当重新思考结晶分离作用对于花岗质岩浆的意义.由于花岗岩的极端复杂性,许多问题还得不到比较合理的解释,本文的认识只是初步的.