Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.     

华北克拉通晚前寒武纪镁铁质岩墙群K－Ar年龄及地质意义

华北克拉通腹地山西地块内广泛发育大规模的晚前寒武纪镁铁质岩墙群。通过对晋北地区新平堡镁铁质岩墙群９个样品的Ｋ－Ａｒ年龄等时线分析，这些岩墙可能形成于１４３０Ｍａ．从山西各地所采集的６４个镁铁质岩墙群样品的Ｋ－Ａｒ表观年龄统计分析可见，华北克拉通腹地内广泛发育的镁铁质岩墙群主要形成于中元古代１０００～１８００Ｍａ间，活动高峰期在１２００～１６００Ｍａ。     

A Clinopyroxene-Orthopyroxene-Plagioclase Symplectite Formed by Garnet Breakdown in Granulite Facies, Guaxupé, Minas Gerais, Brazil

In mafic granulites, garnet can form by reactions such as Opx + Pl = Cpx + Grt + Qtz; Opx + Pl = Grt + Qtz. As a result of isothermal decompression (ITD), garnet can then break down to a characteristic orthopyroxene-plagioclase symplectite. Mafic, iron-rich garnet-pyroxene granulite from the Guaxupé Massif has symplectite that formed by near-isothermal decompression, as a consequence of uplift of the granulite facies terrane. This symplectite was found to consist of vermicular clinopyroxene-orthopyroxene-plagioclase, with clinopyroxene clearly growing from the garnet that is breaking down, modal amounts of clinopyroxene being less than orthopyroxene. Electron probe analyses show clear differences between core (Cpx₁), rim, and symplectite clinopyroxene (Cpx₂). Considering also the presence of magnetite in the symplectite texture, garnet breakdown is thought to be better represented by a reaction such as Cpx₁ + Grt + O₂ = Cpx₂ + Opx + Pl +Mt + Qtz.     

广西田东布兵石炭纪水下沉积灰岩脉及其地质意义

笔者等首次在德保台地边缘上泥盆统灰岩中发现大量多期次下石炭统杜内阶之灰岩岩脉;通过调查,查明岩脉的形态、沉积物特征、时代及其分布规律,首次在沉积岩脉中采获早石炭世早期牙形刺及Gnathodus与Siphnodella大量混生的现象。经过调查研究,对岩脉的成因有新的认识: 其形成是构造与地震相结合的产物,地震造成台地的隆升、沉降,使上泥盆统已成岩的刚性灰岩发生断裂,形成较深层的张性（拉张）裂隙,早石炭世未成岩的软沉积物及碎屑灌入裂隙中,形成灰岩脉。多期次的构造运动、拉张,造成张性裂隙的多次的张裂,多期次灌入则形成似直立产状的岩墙。这一成因模式既能合理解释岩脉中具有地震造成的液化脉之现象,又能解释形成如此深度岩脉的动力因素,也能解释台地边缘相中有深水相的沉积物及生物之物质来源。通过对岩脉的调查及研究,对理解右江盆地的构造演化具有重要的意义。     

华北克拉通南缘小秦岭金矿区基性脉岩时代及地质意义

产于太古代-古元古代变质地体中的石英脉型金矿是世界上最重要的金矿类型之一, 大多数金矿区内基性脉岩非常发育, 空间上与含金石英脉密切相关.但目前对基性脉岩与石英脉型金矿的成因联系尚未取得一致认识.以华北克拉通南缘小秦岭金矿区为例, 对典型矿区(东闯、大湖、枪马)采矿巷道内的基性脉岩开展精确的U-Pb年代学研究.4个脉岩样品给出一致的锆石207Pb/206Pb加权平均年龄(1 819±10 Ma, 1σ); 1个样品中的黑云母给出了略微年轻的40Ar/39Ar坪年龄(1 719.0±21.0 Ma, 2σ).定年结果表明: (1)小秦岭金矿区内大量基性脉岩形成于古元古代晚期, 是华北克拉通东西块体在1.85 Ga左右发生碰撞后伸展作用的产物; (2)前人获得的基性脉岩K-Ar和Rb-Sr年龄(187.6~75.9 Ma)可能并非脉岩真实侵位年龄, 而是代表这些脉岩遭受中生代构造-热事件影响的扰动年龄(或冷却年龄).最新的成矿年代学研究结果表明, 小秦岭地区绝大多数金矿床形成于早白垩世130~120 Ma, 大大晚于上述古元古代基性脉岩的侵位时代, 因此两者之间没有成因联系(尽管它们的空间关系十分密切).小秦岭地区是否存在与金矿床同时的晚中生代基性岩浆活动, 对于讨论本区金矿床的矿床成因和成矿构造环境非常重要, 但本文工作尚不能明确回答这一问题, 今后需要对矿区内各期基性脉岩进行系统的同位素定年.      

内蒙古喀喇沁堆晶岩捕虏体和寄主闪长岩的同位素年龄

喀喇沁地区们于大兴安岭中生代构造岩浆岩带与华北克拉通交汇部位。研究区的闪长岩侵入到太古宙基底中。闪长岩中含有二辉岩、辉长岩、苏长岩、角闪岩、斜长岩等不同组分的镁铁质－超镁铁堆晶岩捕虏体和基性麻粒岩捕虏体,它们是幔源玄武质岩浆底侵作用的产物。分别用Ｒｂ－Ｓｒ法、Ｋ－Ａｒ法和Ａｒ－Ａｒ法,对堆晶岩捕虏体及其寄主岩全岩或单矿物进行了详细的同位素地质年代学研究,共获得各种年龄数据８件,橄榄辉石岩的全岩Ｒｂ     

铂族元素矿物共生组合(英文)

由于铂族元素能有效地降低汽车尾气的污染 ,其需求量日益增加 ,对铂族元素矿床的寻找已是当务之急。着重从矿物矿床学角度对铂族元素的矿物共生特点进行了探讨。铂族元素可呈独立矿床产出 ,主要产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中。铂族元素也伴生于铜镍矿床中 ,该类铜镍矿床主要与苏长岩侵入体、溢流玄武岩及科马提岩有关。产于基性超基性层状侵入体中的铂族矿物有铂钯硫化物、铂铁合金、钌硫化物、铑硫化物、铂钯碲化物、钯砷化物及钯的合金。这些铂族矿物可与硫化物矿物共生 ,也可与硅酸盐矿物共生 ,还可与铬铁矿及其他氧化物矿物共生。产于蛇绿岩套中的铂族矿物主要是钌铱锇的矿物 ,而铂钯铑的矿物则较少出现 ,这些铂族矿物可呈合金、硫化物、硫砷化物以及砷化物 4种形式出现。产于阿拉斯加式侵入体中的铂族矿物主要有铂铁合金、锑铂矿、硫铂矿、砷铂矿、硫锇矿及马兰矿等少数几种 ,其中铂铁合金与铬铁矿及与其同时结晶的高温硅酸盐矿物共生 ,而其他的铂族矿物则与后来的变质作用及蛇纹岩化作用中形成的多金属硫化物及砷化物共生。产于铜镍矿床中的铂族矿物主要是铂和钯的矿物。产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中的铂族矿物的共同特点是它们均与铬铁矿?     

恒山中元古代早期基性岩墙群的单颗粒锆石U—Pb年龄及其克拉通构造演化意义

华北克拉通中北部变质基底发育大规模的前寒武纪基性岩墙群,由于基性岩墙同位素定年技术存在问题,因此一直未获得岩墙群的确切年龄数据。本文利用单颗粒锆石U-Pb化学法,首次获得恒山地区北西向基性岩墙群的U-Pb年龄为1769.1±2.5Ma,并证明它代表基性岩墙群的侵位年龄,这一年龄数据也是华北克拉通大规模分布的未变质基性岩墙群已知的最古老年龄。综合分析同期岩浆活动的同位素年龄记录,本文提出中元古代早期华北的岩浆侵位发生于伸展构造背景下。     

青藏高原东北缘多福屯第三纪钠质基性火山岩及构造启示

位于青藏高原东北缘的多福屯第三纪火山岩,为钠质基性火山岩系。该火山岩(La/Yb)N值在6~11,ΣREE平均为117,样品普遍具有不相容元素富集、Pb亏损以及弱的Nb、Ta正异常,Nb/U和Ce/Pb平均值分别为30和17;样品的87Sr/86Sr值为0.7041~0.7069,Nd初始值为0.5129(εNd(t)=6),Δ207Pb/204Pb和Δ208Pb/204Pb分别在11~19和73~84之间,结合高的Sr初始值,表现了特征的Dupal(高放射成因铅)异常。地球化学特征表明,该火山岩属于似OIB性质的陆内火山岩,源区具有DM与EMⅡ混合特点。个别岩石表现出明显陆壳混染的迹象。火山岩源区特征反映了OIB类岩石的复杂性,可能由区域原、古特提斯地幔继承而来,因而是原地的和固有的,并非与青藏高原物质向东挤出有关。通过与邻区同时代火山岩的对比推断,整个高原东北缘新生代火山岩幔源区具有小尺度的不均一性。高原东北缘新生代火山岩的发育可能与较大规模的贺兰-川滇南北复合构造有关。