阿尔泰南缘中泥盆统北塔山组火山岩的矿物学研究

阿尔泰山南缘中泥盆统北塔山组火山岩极为发育。本文对富辉橄玄岩和玄武岩中的单斜辉石和橄榄石斑晶进行了矿物化学分析,并结合矿物的地质温压计和岩浆结晶模拟,约束了两种岩石中矿物的形成过程,推测了原始岩浆的演化过程。提出富辉橄玄岩和玄武岩的原始岩浆属于拉斑玄武系列,富辉橄玄岩中的高压单斜辉石来源于深部（40～50 km）,其结晶早于橄榄石;玄武岩中单斜辉石大多属于岩浆房结晶产物（30～40km）,其结晶晚于橄榄石;两种岩石中橄榄石均属于岩浆房结晶产物（30～40 km）。     

阿尔泰南缘中泥盆世北塔山组火山岩中单斜辉石的矿物学研究及其地质意义

阿尔泰南缘晚古生代火山岩十分发育,其中中泥盆世北塔山组为一套以中基性火山岩为主的火山—沉积岩系,并且其下部发育少见的苦橄岩和富辉橄玄岩。北塔山组富辉橄玄岩和玄武岩中存在着三种类型的单斜辉石：大斑晶辉石、小斑晶辉石和基质辉石。这三种类型的辉石在成分上有明显的差异,从大斑晶辉石—小斑晶辉石核部—小斑晶辉石边部和基质辉石,成分由富镁、钙向富铁方向演化,显示出拉斑玄武岩系列的特点。根据单斜辉石与熔体平衡原理计算了富辉橄玄岩和玄武岩中斑晶相辉石形成的温度和压力,前者变化范围主要集中在1345～1366℃和1.75～2.05GPa,其深度相当于58～67km,平均地温梯度为20～24℃/km,该区这种异常高的地温梯度可能是导致本区形成斑岩铜矿的主要原因之一;后者的温度和压力都相对较低,分别为1129～1191℃和1.19～1.60GPa,而相应的深度为39～53km,暗示着玄武岩是在相对低温低压的环境中形成的演化岩浆岩。单斜辉石的成分指示了其构造背景为岛弧环境。     

阿尔泰山南缘中泥盆世苦橄岩中单斜辉石的成分特征及其地质意义

岛弧苦橄岩非常稀少,但在阿尔泰山南缘却分布有世界上罕见的中泥盆世厚大苦橄岩层。本文对该地区苦橄岩及其共生的玄武岩中单斜辉石的矿物化学成分进行了电子探针分析,结果表明,苦橄岩中的单斜辉石全部为透辉石,玄武岩中的单斜辉石种属有普通辉石和透辉石两类,但玄武岩中的单斜辉石成分呈连续演化关系。根据单斜辉石-熔体平衡原理计算了苦橄岩中单斜辉石斑晶形成的温度和压力,其变化范围主要集中在1350～1387℃和16.6～21.3kPa,相应的深度为55～70km,大致代表了苦橄岩岩浆房的深度。与此不同的是,玄武岩中单斜辉石斑晶结晶的深度较苦橄岩浅,为37～41km,暗示玄武岩浆是在相对低压条件下分离结晶后形成的演化岩浆。基于单斜辉石的成分和演化,确定了该地区火山岩属亚碱性系列,其构造背景为岛弧环境。     

藏南桑单林地区晚白垩世—始新世砂岩物源区分析

阿尔泰山南缘晚古生代火山岩十分发育,其中中泥盆世北塔山组为一套以中基性火山岩为主的火山－沉积岩系,并且其下部发育少见的苦橄岩和富辉橄玄岩。北塔山组富辉橄玄岩和玄武岩中存在着三种类型的单斜辉石：大斑晶辉石、小斑晶辉石和基质辉石。这三种类型的辉石在成分上有明显的差异,从大斑晶辉石—小斑晶辉石核部—小斑晶辉石边部和基质辉石,成分由富镁、钙向富铁方向演化,显示出拉斑玄武系列的特点。根据单斜辉石-熔体平衡原理计算了富辉橄玄岩和玄武岩中斑晶相辉石形成的温度和压力,前者变化范围主要集中在1345～1366℃和1.75～2.05GPa,其深度相当于58－67km,平均的地温梯度大约20～24℃/km,说明该区具有异常的热或地温梯度,这可能是导致本区斑岩铜矿形成的主要原因之一;后者的温度和压力都相对较低,分别为1129～1191℃和1.19～1.60Gpa,而相应的深度为39～53km,暗示着玄武岩是在相对低温低压的环境中形成的演化岩浆。单斜辉石的成分指示了其构造背景为岛弧环境。     

峨眉山大火成岩省二滩高钛玄武岩和白马层状岩体中单斜辉石的成分特征及其成因指示

本文通过研究峨眉山大火成岩省白马层状岩体和二滩高钛玄武岩中单斜辉石的主量和微量元素组成,反演与单斜辉石平衡的熔体成分,以查明层状岩体和高钛玄武岩之间的成因联系。结果显示,不同岩石中单斜辉石的稀土配分型式基本相似,在微量元素配分图解中均表现出Sr、Zr、Hf、Ti、Zn、Co和Ni负异常。二滩高钛玄武岩中单斜辉石斑晶环带核部的成分与高钛苦橄岩中单斜辉石斑晶的成分类似。模拟结果显示,白马岩体中与单斜辉石平衡的熔体成分与峨眉山高钛玄武岩成分类似,单斜辉石的Zr-Hf负异常与Zr和Hf在单斜辉石和熔体之间的分配系数较低有关。因此,高钛玄武岩和白马岩体分别代表深部岩浆房高钛苦橄质岩浆经分离结晶作用产生的高钛玄武质岩浆的喷出和侵入产物。     

滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶环带结构的成因：岩浆补给-混合过程

滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶普遍出现正环带结构、反环带结构或韵律环带结构，少量为具绿色核部的单斜辉石(“绿核辉石”)。反环带斑晶和“绿核辉石”的幔部与正环带斑晶的核部具有相似并且相对较窄的成分范围，相对高Mg#（0.83 ~ 0.90）,低TiO2（0.13 % ~ 0.29 %）,Al2O3（0.73 % ~ 1.68 %）和Na2O （0.22 % ~ 0.42 %），为钾质岩浆平衡结晶的产物。反环带斑晶的核部相对低Mg#（0.77 ~ 0.84），但与反环带斑晶的幔部、正环带斑晶的核部均具有相似的Ti/Al比值（0.06 ~ 0.16）；韵律环带结构斑晶的成分变化均在正、反环带斑晶的成分范围之内。莴中高镁富钾火山岩中的这些环带结构单斜辉石斑晶应来源于相似的岩浆体系，反 环带结构表明在岩浆房存在较原始岩浆对较演化岩浆再补给的岩浆混合过程，而韵律环带结构特征揭示曾多次发生这种岩浆混合过程。“绿核辉石”的核部明显低Mg# （0.50 ~ 0.74）,相对富Al2O3（1.66 % ~ 3.63 %）和Na2O（0.87 % ~ 2.17 %），具有明显较低的Ti/Al比值（< 0.05）和较高的AlVI/AlIV比值（0.38 ~ 0.76），为下地壳捕虏晶 来源，证实了在滇西晚始新世富钾岩浆演化过程中存在少量地壳混染作用。     

南海新生代玄武岩中单斜辉石矿物化学及成因意义

从南海南大陆坡底至北大陆坡底采集的玄武岩样品,基本可以代表南海新生代岩浆活动的代表性样品.本文中对这些玄武岩中单斜辉石的矿物化学成分进行了研究,研究的单斜辉石以顽透辉石为主(占3/4),还有少量普通辉石及次透辉石等,也出现少量较为罕见的富钙辉石,位于深绿辉石区域内.基质微晶辉石比辉石斑晶更富钙、钛、铁,可能反映了岩浆演化向着富钙、钛、铁方向发展,且碱度逐渐降低.与碱性系列的演化趋势基本一致.由单斜辉石-熔体平衡温压计计算的岩浆房深度分别为:碱玄岩岩浆房约49km左右,粗面玄武岩岩浆房约25km;玄武岩岩浆房约15km.由碱玄岩→粗面玄武岩→玄武岩,平衡温度(K)依次降低:从1535～1498→1429→1369.南海新生代玄武岩为板内碱性玄武岩.碱性玄武岩系列可能是地幔柱在上升过程中在不同深度处发生部分熔融并伴随有分离结晶作用等物理化学过程的连续演变的产物.     

延吉地区古新世埃达克岩成因的岩浆混合证据

本文通过年代学、矿物学、主量元素、微量元素和同位素的研究,指出延吉地区古新世富镁埃达克岩起源于被俯冲板片熔体交代的地幔楔。单斜辉石和角闪石斑晶所显示的反环带结构为幔源富镁埃达克熔体于壳源中酸性岩浆混合的结果,而非前人普遍认为的熔体—地幔的反应产物;其地球化学特征表现出的岩浆演化趋势可以很好地用地壳混染/AFC过程来解释。延吉古新世埃达克岩形成于俯冲后岩石圈伸展作用背景,与太平洋板块的俯冲后撤作用有关。因此,我们认为富镁埃达克岩的形成不必强调同期洋脊的俯冲作用,只要早先或同时有俯冲板片熔体交代,在合适的p-t条件下交代地幔并发生熔融作用,就能形成这一类岩石。     

喀喇沁玻基橄辉岩中的磷酸盐质熔体及磷灰石

磷酸盐在本文讨论的坡基橄辉岩中以多种形式产生,有早期结晶的磷灰石斑晶;有熔离的磷酸盐质熔浆液滴;也有晚期穿入橄榄石斑晶的磷酸盐质细脉,表明元素磷活跃在岩浆演化的每一个阶段。磷酸盐质熔浆液滴的存在,说明磷酸盐质熔体可能是由于岩浆的熔离作用而形成的。沿着橄榄石晶格生长石出现的磷酸盐质玻璃细脉则进一步证实地幔中含磷流体不但存在,而且具有很强的活动性。     

雅鲁藏布江缝合带西段普兰蛇绿岩中地幔橄榄岩的岩石学研究

西藏雅鲁藏布江缝合带西段普兰蛇绿岩以出现面积约600余平方千米的特大型地幔橄榄岩体而引人注目.该地幔橄榄岩以方辉橄榄岩为主体,含有少量的二辉橄榄岩和纯橄榄岩,岩体中另有一些橄榄单斜辉石岩、辉长岩和辉绿岩等侵入体.地幔橄榄岩的主要造岩矿物橄榄石的Fo 90～93,其中呈包裹体的橄榄石的Fo略高,斜方辉石为顽火辉石(En 88～90),单斜辉石主要为顽透辉石和透辉石,以低铝(0.48％～3.96％)和高Mg#(91～96)为特征,铬尖晶石的Cr#值为18～69,其中方辉橄榄岩和二辉橄榄岩中的铬尖晶石属富铝型尖晶石,而纯橄岩中为富铬型尖晶石.橄榄单斜辉石岩的橄榄石Fo值一致较低,平均为88.4,斜方辉石En平均87,单斜辉石以透辉石为主,铬尖晶石的Cr#值为45～69.普兰地幔橄榄岩及橄榄单斜辉石岩都具有相似的稀土元素和微量元素配分模式,表现为LREE相对富集,Eu亏损不明显,微量元素中大离子亲石元素含量较低,部分样品高场强元素亏损,另一些则相对富集,显示地幔橄榄岩具有亏损地幔源区特征,但也具有俯冲带流体的交代特征,表明普兰岩体可能经历了MOR和SSZ两种构造环境,该特征与雅鲁藏布江缝合带东段的罗布莎地幔橄榄岩的特征可以对比.     

冀西北姚家庄超镁铁岩-正长岩杂岩体中辉石的环带特征及意义

冀西北姚家庄存在一套晚三叠世的超镁铁岩-正长岩杂岩体,岩体内发育具有环带特征的单斜辉石。辉石的环带有两种:简单环带和复杂环带。简单环带一般为正环带,辉石核部的MgO和Cr2O3含量高,Si O2、Fe O和Na2O含量低;边部的主要氧化物含量与核部刚好相反。简单正环带可以分为两类,其中核边接触带平滑、由核到边化学成分具有渐变特征的正环带辉石可能是岩浆在分离结晶或地壳混染过程中形成。而核边接触带有熔蚀结构、由核到边化学成分突变的正环带辉石可能是早期结晶的辉石颗粒受到晚期低镁岩浆的溶蚀改造而成的。复杂环带具有核-幔-边结构,其中,核部低镁高铁、幔部高镁低铁、边部与核部相似,但其Mg#更低,这些特征暗示了岩浆混合作用的存在,形成辉石核部的母岩浆可能来自富集的岩石圈地幔,幔部高Mg#的特征指示了软流圈地幔物质的贡献,其边部低Mg#的特征则指示了地壳物质的加入。具有韵律环带的复杂辉石是在岩浆多期侵入的过程中形成的。辉石环带的组成特征表明,姚家庄杂岩体是由岩浆多期侵位形成的,后期侵入的岩浆与前期就位的岩浆不断反应,形成了具有多种不同环带特征的辉石,并最终形成了空间上由外到内依次为辉石岩、辉石正长岩和正长岩的环状杂岩体。结合前人的研究成果,推测形成姚家庄岩体的岩浆主要来源于富集的岩石圈地幔,并由少量地壳组分和软流圈物质的贡献。     

Late Cretaceous magmatic activity in the southern Lhasa terrane: insights from the Dazhuqu hornblende gabbro and the Xietongmen granite porphyry

ABSTRACT

The mechanisms triggering the emplacement of Cretaceous magmatic rocks related to Neo-Tethyan subduction in the southern Lhasa subterrane (SLT) remain controversial because geochronological, geochemical, and isotopic data from the Cretaceous magmatic rocks in the western portion of the SLT are lacking. This paper provides the first report indicating that the Dazhuqu hornblende gabbro and Xietongmen granite porphyry were generated in the Late Cretaceous (ca. 100 Ma and ca. 68 Ma, respectively) in the western portion of the SLT. The Dazhuqu hornblende gabbro is characterized by high MgO, Na₂O, Cr, Co, and Mg^# values, enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depletion in high field strength elements with no Eu anomalies. The Xietongmen granite porphyry displays high SiO₂, low MgO, Mg^#, Cr, Co, and Ni, enrichment in LREEs and LILEs, flat heavy rare earth element (HREE) patterns and negative Eu anomalies. Zircons from the Dazhuqu hornblende gabbro and Xietongmen granite porphyry display high positive ε_Hf(t) ranges from 11.79 to 14.91 and from 9.05 to 12.38, respectively. The Dazhuqu hornblende gabbro was produced by the partial melting of 12–15% garnet peridotite in the depleted mantle that was metasomatized by fluids released from the dehydrating subducted Neo-Tethyan oceanic slab. This magma was then emplaced at an upper crustal depth of 5.3–7.0 km. The Xietongmen granite porphyry was generated by the partial melting of underplated basaltic crust during the subduction of the Neo-Tethyan oceanic crust and formed in the presence of pyroxene, plagioclase, and minor hornblende in its magmatic source. In combination with previously published data from Cretaceous magmatic rocks in the SLT, our data suggest that the petrogenesis of the Dazhuqu hornblende gabbro was related to flat or low-angle slab subduction of the Neo-Tethyan oceanic crust during 109–97 Ma and that the formation of the Xietongmen granite porphyry was related to lithospheric delamination in the western portion of the SLT after ca. 68 Ma.     

Possible manifestations of slab window magmatisms in Cretaceous southwest Japan

The slab window system related to ridge subduction is studied to explain the conspicuous activity and the along-arc migration of the magmatisms in the eastern margin of Eurasian continent in the late Mesozoic. The trend of the granite ages is scrutinized with systems approach and analyzed by the statistical method in Cretaceous southwest Japan, a reference field of the Eurasian eastern margin. The magmatic distribution of simultaneous activity was determined to be the V-shape of the slab window, excluding dating errors from the located age data. The slab window magmatism has the most dominant zone of the acidic rocks at or near the centerline of the window (the spreading center of two plates), and also has the subdominant zone of the acidic to intermediate rocks on either side of the window.As the slab window opens due to the plate motions, the upwelling current is adiabatically induced in the asthenosphere to fill the window gap. The partial melt is generated in the upwelling flux and transports heat to the lower crust to make greater granitic magma chambers by heat conduction to, and assimilation with, the crustal matter. The younger and hotter slabs outside the window also play a role in partial slab melting and/or dehydration to the asthenosphere and generate the subdominant granodioritic and adakitic and/or high-magnesian andesitic magmatisms as the pre- and post-activities of the main granite genesis.The slab window formed by the subducted ridge between the Kula and Pacific plates was the strong heat source of the active magmatisms and migrated at the rate of 2.8 cm/year from southwest to northeast about 8000 km along the continental margin. The growth of the continent associated with the active acidic to intermediate magmatisms including the adakitic and/or high-magnesian andesitic activities is modeled by the slab window system.     

Appinite suites and their genetic relationship with coeval voluminous granitoid batholiths

ABSTRACT

Appinite complexes preserve evidence of mantle processes that produce voluminous granitoid batholiths. These plutonic complexes range from ultramafic to felsic in composition, deep to shallow emplacement, and from Neo-Archean to Recent in age. Appinites are a textural family characterized by idiomorphic hornblende in all lithologies, and spectacular textures including coarse-grained mafic pegmatites, fine-grained ‘salt-and-pepper’ gabbros, as well as planar and linear fabrics. Magmas are bimodal (mafic-felsic) in composition; ultramafic rocks are cumulates, intermediate rocks are hybrids. Their geochemistry is profoundly influenced by a mantle wedge extensively metasomatized by fluids/magmas produced by subduction. Melting of spinel peridotite sub-continental lithospheric mantle (SCLM) produces appinites whose geochemistry is indistinguishable from coeval low-K calc-alkalic arc magmatism. Coeval felsic rocks within appinite complexes and adjacent granitoid batholiths are crustal magmas. When subduction terminates, asthenospheric upwelling (e.g. in a slab window, or in the aftermath of slab failure) induces melting of metasomatized garnet SCLM to produce K-rich sho shonitic magmas enriched in large ionic lithophile and light relative to heavy rare earth elements, whose asthenospheric component can be identified by Sm-Nd isotopic signatures. Coeval late-stage Ba-Sr granitoid magmas have a ‘slab failure’ geochemistry, resemble TTG and adakitic suites, and are formed either by fractionation of an enriched (shoshonitic) mafic magma, or high pressure melting of a meta-basaltic protolith either at the base of the crust or along the upper portion of the subducted slab. Appinite complexes may be the crustal representation of mafic magma that underplated the crust for the duration of arc magmatism. They were preferentially emplaced along fault zones around the periphery of the granitoid batholiths (where their ascent is not blocked by overlying felsic magma), and as enclaves within granitoid batholiths. When subduction ceases, appinite complexes with a more pronounced asthenospheric component are preferentially emplaced along active faults that bound the periphery of the batholiths.     

河南省安林矽卡岩型铁矿的成岩时代和成矿物质来源探讨

位于华北克拉通中部的河南省安林铁矿是典型的邯邢式矽卡岩型铁矿,矿体产于闪长质岩石和中奥陶统灰岩的接触带。LA-ICPMS锆石U-Pb定年结果表明安林闪长岩体的侵位年龄为123.38±0.81Ma,略晚于华北克拉通东部地区的含矿岩体,形成于岩石圈大规模减薄伸展时期。但其中含有古老的锆石说明岩浆经历了地壳的混染。闪长质岩石具有相对低SiO2、高Mg#、高碱,富集Ba、Sr和LREE大离子亲石元素,亏损Nb、Ti、Ta等高场强元素的特点,暗示了其形成于岩石圈地幔。岩相学特征以及Harker图解指示了岩浆经历了较强的分离结晶作用,因此推断安林闪长岩可能是软流圈地幔上涌导致富集的岩石圈地幔发生部分熔融形成原始的辉长质岩浆在上升过程中或岩浆房中发生了以铁镁矿物为主的分离结晶作用,同时受到地壳物质的混染的结果。安林地区矿石具有和闪长岩体相似的稀土元素地球化学特征,反映铁的成矿作用与岩浆作用密切相关。矿床中闪长岩体、矽卡岩、矿石和碳酸盐围岩的主量元素对比研究表明钠交代作用引起Na、K、Fe、Si等元素在各岩类间的迁移,其中迁移出的铁为成矿提供了物质基础。安林地区初始岩浆富含水,分离结晶作用使残留岩浆水饱和而发生出溶;且岩浆在演化过程中受到区内膏盐层和碳酸盐地层的混染,促进了岩浆中流体的出溶并使出溶的流体富含Cl-,为有利于铁质活化的富Cl-岩浆流体的形成创造了条件。     

Magnesian andesites in north Xinjiang,China

Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan and west- and east-Tianshan in the north Xinjiang region. These MAs are andesitic rocks with 53–65% SiO₂,<1% (0.21–1.08%; average of 0.72%) TiO₂, and ≥50 Mg^#. Magnesian dacites and diorites, with 52.38–66.91% SiO₂, <0.30% TiO₂ and ≥42 Mg^# commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na. They have characteristic flat chondrite-normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65–1.15), low (La/Yb)_N (0.98–6.4, mostly 4±) and low total REE contents (15–95 ppm). They also have high contents of compatible elements Cr and Ni (72–790 and 29–276 ppm, respectively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams. If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high Y contents (>15 ppm), low Sr/Y ratios (4.4–6.2), low (La/Yb)N, and high Mg^# of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle wedge peridotite. New petrographic, chemical and isotopic [(¹⁴³Nd/¹⁴⁴Nd)I = 0.51221–0.51255 (εNd(t) +0.28 to +7.2); (⁸⁷Sr/⁸⁶Sr)I = 0.7029–0.7065] data suggest that the petrogenesis of the MAs in the north Xinjiang region may have involved: (1) multiple source materials including subducted oceanic slab, juvenile crustal materials (mainly volcanic-volcanoclassic rocks with low maturity and clear mantle geochemical signatures) coming from the forearc accretionary prism and mantle wedge peridotite; (2) a combination of different petrogenetic processes including partial melting of subducted oceanic slab and juvenile crustal materials, followed by interaction of slab melts with the mantle wedge peridotite; (3) high geothermal gradient creating a high temperature (>1,000°C) environment in a volatile-rich source region; (4) unique tectonic settings including oblique subduction, slab break off resulting in slab window formation and asthenosphere upwelling, and subduction erosion resulting in transfer of forearc accretionary materials into the source region of MA magma.     

http://www.sciencedirect.com/science/article/pii/S1674987112001533

The collision of a divergent ocean ridge may evolve into two end cases:in the continuity of ocean-floor subduction.or in the detachment of the subducted plate.The northern Patagonia active plate margin has the unique situation that in Cenozoic time it has been subjected to two divergent ridge collisions,each one representing one of the end members.The Neogene Antarctica-Nazca divergent ridge collision evolved as a continuous ocean-floor subduction system,promoting a magmatic hiatus at the arc axis,the obduction of part of the ridge ocean-floor in the fore-arc.and basaltic volcanism in the back-arc.In contrast,the Paleogene Farallon-Aluk divergent ridge collision evolved into a transform margin,with the detachment and sinking of the Aluk plate and the development of a large slab window.As in the previous case,this collision promoted a magmatic hiatus at the arc axis,but the tectono-magmatic scenario changed to postorogenic synextensional volcanism that spread to the former fore-arc（basalt,andesite,rhyolite） and former back-arc（bimodal ignimbrite flare-up,basalt）.Geochemistry of this slab window synextensional volcanism shows more MORB-like basalts towards the former fore-arc,and MORB-OIB-like basalts towards the former back-arc.Instead,an isolated undeformable crustal block in the former back-arc,with an "epeirogenic" response to the slab window and extensional regime,was covered by OIB-type basalts after uplift.Major elements show that slab window basalts reach TiCh values up to 3 wt%,as compared with the top value of 1.5 wt%of arc magmas.Besides,the MgO with respect to（FeO^t + Al₂O₃） ratio helps to distinguish slab window magma changes from the former fore-arc to the former back-arc and also with respect to the "epeirogenic" block.Higher contents of HFS elements such as Nb and Ta also help to distinguish this slab window from arc magmas and also,to distinguish slab window magma changes from the former fore-arc to the former back-arc and "epeirogenic" block settings.The isotope compositions of slab window magmatism show a disparate coeval array from MORB to crustal sources,interpreted as a consequence of the lack of protracted storage and homogenization due to the extensional setting.     

Extensional collapse of the Gondwana orogen: Evidence from Cambrian mafic magmatism in the Trivandrum Block,southern India

The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show ²⁰⁶ Pb/²³⁸ U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent.     

藏南努日铜-钨-钼矿床晚白垩世石英闪长岩U-Pb定年及其地球化学特征

藏南努日矿床位于冈底斯成矿带南缘,前人获得的辉钼矿Re-Os同位素年龄为23 Ma,与明则和程巴矿床成矿时代一致,但矿区内至今未发现与矿化有关的成矿斑岩体。本文报道了努日矿区新发现的与矿化关系密切的石英闪长岩的地球化学特征,获得石英闪长岩的LA-ICP-MS锆石U-Pb年龄为93.42±0.76 Ma,与同一成矿带内桑布加拉和克鲁铜金矿成矿时代一致(90~93 Ma),表明矿区可能存在两期成矿事件。石英闪长玢岩的主量微量元素SiO_2含量为57.19%~58.23%,A1_2O_3含量为15.78%~16.03%,MgO含量为4.74%~5.32%,Mg~#指数为65.2~67.3;富集大离子亲石元素(Rb、Sr、Ba、U等)及轻稀土元素,亏损高场强元素,显示出埃达克岩特征。研究表明石英闪长玢岩形成于洋壳俯冲阶段的弧岩浆岩,洋壳熔融形成的母岩浆侵入近地表形成早期铜多金属矽卡岩矿化。晚白垩世成矿事件的发现进一步佐证了研究区存在两期矿化叠加事件,拓展了研究区找矿方向。     

南羌塘增生过程的中-晚三叠世岩浆记录:藏北玛依岗日-角木日地区基性岩墙

古特提斯洋向北俯冲形成增生杂岩,它们向南增生构成了南羌塘增生地体,且增生期内发育的岩浆岩可以用来研究南羌塘的增生过程。在南羌塘增生杂岩带内,由北向南,发育有玛依岗日(MG)辉长岩墙、角木日(JM)辉长辉绿岩墙。锆石U-Pb测年结果显示,两者分别形成于237. 1±2. 3Ma和230. 7±1. 8Ma,为南羌塘增生期内岩浆岩。它们的岩石地球化学特征均介于OIB与E-MORB之间,富集Ti、Nb,以及LREE和LILE。Nd-Pb同位素结果显示两者均起源于富集地幔源区。Sm/YbLa/Yb图解显示,两者均起源于尖晶石-石榴石二辉橄榄岩源区,相对于JM辉长辉绿岩MG辉长岩部分熔融程度较高。两者Mg#和Ni、Cr特征表明,MG辉长岩分异程度高于JM辉长辉绿岩。Th/Nb-La/Nb图解显示,相对于JM辉长辉绿岩MG辉长岩经历了一定程度的地壳混染。综合研究认为,MG辉长岩和JM辉长辉绿岩为古特提斯洋壳俯冲过程中软流圈上涌的两次岩浆活动的产物,并受俯冲洋壳和地幔楔影响。MG辉长岩和JM辉长辉绿岩记录了南羌塘的增生过程,它们为相关研究提供了岩浆岩证据。