山东临朐山旺新生代玄武岩中超镁铁岩包体的研究

山旺新生代玄武岩中的超镁铁质包体分为五类:尖晶石纯橄榄岩、尖晶石二辉橄榄岩、尖晶石方辉橄榄岩、尖晶石石榴石二辉岩和石榴石二辉橄榄岩。对它们的地质学,岩相学、岩石化学,造岩矿物的化学成分,稀土配分模式及热力学计算的研究表明,前三种岩石属原始地幔岩,后二种是地幔中岩浆作用的产物。     

青藏高原羌塘南部晚古生代地幔柱？——来自基性—超基性岩的地球化学证据

羌塘南部地区大面积出露基性—超基性火成岩。基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世—早二叠世（284～318Ma）,反映出岩浆的快速侵位-喷溢的过程。基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体。基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点。另外,苦橄质岩石多与基性岩墙和玄武岩一起产出,主要为单斜辉石橄榄岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩。由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物。     

云南中甸峨眉山玄武岩中超基性－基性岩包体元素地球化学分析

峨眉山大火成岩省于滇西地区广泛发育有一系列的镁铁-超镁铁质岩体,但对于其中地幔包体的报道还相对较少。本次研究在云南中甸地区峨眉山玄武岩中发现了岩性表现为由纯橄岩→辉橄岩→橄辉岩→苦橄岩→辉长岩→辉绿辉长岩→辉绿岩变化系列的超基性-基性岩包体。通过对包体岩石开展元素地球化学分析研究,结果表明:形成包体的原始超基性岩浆属于峨眉山地幔柱活动产物,且与主岩形成背景相似,其岩浆源区位于难熔的方辉橄榄岩区与石榴二辉橄榄岩区的过渡部位,受岩石圈地壳混染程度较低。结合丽江苦橄岩性质,认为滇西地区上地幔存在早阶段峨眉山地幔柱活动。     

藏南亚东地区片麻状黑云花岗闪长岩LA-ICP-MS锆石U-Pb测年青藏高原羌塘南部晚古生代地幔柱?——来自基性—超基性岩的地球化学证据

羌塘南部地区大面积出露基性—超基性火成岩。基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世—早二叠世(284~318Ma),反映出岩浆的快速侵位-喷溢的过程。基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体。基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点。另外,苦橄质岩石多与基性岩墙和玄武岩一起产出,主要为单斜辉石橄榄岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩。由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物。     

新疆北山地区罗东镁铁-超镁铁质岩体地球化学特征及找矿潜力分析

罗东镁铁-超镁铁质岩体位于新疆东部北山地区,呈眼球状出露于白地洼-淤泥河断裂北侧,主要岩石类型包括辉长岩、橄榄岩、辉石橄榄岩和橄榄辉石岩等,具有明显的堆晶结构。岩相学、全岩主微量地球化学研究表明罗东岩体属于拉斑玄武岩系列且经历了早期橄榄石、辉石和斜长石的分离结晶作用,以及岩浆演化过程中上地壳混染作用,具有良好的铜镍矿形成条件。罗东岩体的母岩浆应该为高镁高温苦橄质岩浆,是地幔软流圈在尖晶石矿物稳定区域较高程度部分熔融后产物。岩石MgO为7%～33.7%,属含铜镍中等镁铁质岩石,镁铁比(m/f)为1.70～6.80,主体属于铁质超镁铁岩,综合分析罗东地区地、物、化信息显示岩体具有良好的找矿潜力,综合找矿模型的构建为北山地区经济、快速地从众多镁铁质-超镁铁质岩体中筛选出有利的赋矿岩体提供了经验和方向。     

北大巴山笔架山—铜洞湾碱性镁铁质熔岩的岩石学研究

笔—铜碱性镁铁质火山岩体由碎屑岩和熔岩组成。熔岩的主要岩石类型有苦橄岩(富橄辉玄岩)和碱性玄武岩。所有样品都富集不相容元素,REE显示出高度分离的分配型式,其(La/Yb)_(CN)比值多数在15和20之间,而相容元素(Co、Cr和Ni)则明显亏损,计算表明,碱性镁铁质熔岩不可能由球粒陨石型地幔分离出的橄榄岩部分熔融产生,而可能是由交代地幔适度部分熔融产物。     

北大巴山笔架山—铜洞湾碱性镁铁质熔岩的岩石学研究

笔-铜碱性镁铁质火山岩体由碎屑岩和熔岩组成。熔岩的主要岩石类型有苦橄岩(富橄辉玄岩)和碱性玄武岩。所有样品都富集不相容元素,REE显示出高度分离的分配型式,其(La/Yb)_(ON)比值多数在15和20之间,而相容元素(Co、Cr和Ni)则明显亏损,计算表明,碱性镁铁质熔岩不可能由球粒陨石型地幔分离出的橄榄岩部分熔融产生,而可能是由交代地幔适度部分熔融产物。     

青藏高原羌塘南部晚古生代地幔柱?——来自基性-超基性岩的地球化学证据

羌螗南部地区大面积出露基性一超基性火成岩.基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世-早二叠世(284～318Ma),反映出岩浆的快速侵位-喷溢的过程.基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体.基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点.另外,苦橄质岩石多与基性岩墙和玄武岩一起产出.主要为单斜辉石橄税岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩.由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物.     

宕昌好梯第三纪超镁铁质岩中巨晶及其形成条件

分布于好梯第三纪陆相地层中的超镁铁质岩属碱性超基性岩，为超镁铁煌斑岩。该岩石系中心式超基性火山爆发—喷溢产物。其产出与南北向幔型断裂活动和地幔部分熔融岩浆贯入密切相关。根据岩石组合及产状，好梯陆相超基性火山岩可分为火山颈相、喷发相和次火山岩相。其中火山颈相和喷发相内的碎屑熔岩含有大量的二辉橄榄岩包体，并与单斜辉石和橄榄石巨晶共生。研究结果表明，巨晶系岩浆在上地幔条件下结晶的高压产物     

四川阿布郎当超镁铁质侵入体成岩机制的地球化学约束

阿布郎当超镁铁质侵入体位于扬子地台西缘,康滇地轴中段,安宁河深大断裂之西侧。该岩体呈似同心环状相带分布,基性程度很高,岩体的中心为含长橄榄岩,向外依次过渡为含长辉橄岩、辉橄岩及斜长辉橄岩,边缘带斜长橄辉岩。在岩体的边缘带附近存在明显的Cu-Ni-PGE矿化。随着近年来矿产价格的走高和国家对地质普查工作的力度加大,阿布郎当岩体又重新引起人们的关注。该岩体在地质勘探方面已经积累了丰富的资料,但在地球化学方面的研究还很薄弱。本文对阿布郎当超镁铁岩体进行了系统的主要造岩矿物成分、主量元素、微量元素及铂族元素含量的分析,讨论了该岩体的原始岩浆和地幔部分熔融程度,并对成岩过程进行了探讨。研究认为,阿布郎当超镁铁质岩属拉斑玄武岩系列,是峨眉山大火成岩省构造-岩浆活动产物,成岩原始岩浆为苦橄质岩浆,由类似于洋岛玄武岩岩浆源区成分的地幔经18%左右的部分熔融形成。在岩浆上升过程中,最先结晶的镁铁矿物由于岩浆的流动而集中于岩浆管道的中央,于是形成各种岩石的环带分布。当岩浆上升侵入阿布郎当岩浆房以后,发生了以橄榄石为代表的镁铁矿物的堆积,与此同时,岩浆在岩浆房内继续进行着结晶分异并且还可能与围岩发生了混染,导致岩体边缘相附近出现了硫化物的熔离。在后期的地壳运动中,阿布朗当岩浆房露出地表,即今天所见到的阿不朗当超镁铁质岩体。     

A greenstone belt in southeast Tibet: An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (～272 Ma), followed by picritic ultramafic rocks (～270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (ε_Hf(t) = +13.1–+16.9; ε_Nd(t) = +6.9–+7.1), transitional mafic rocks (ε_Hf(t) = +1.8–+16.9; ε_Nd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (ε_Hf(t) = +0.5–+5.4; ε_Nd(t) = ?1.5 to +1.9) and picritic ultramafic rocks (ε_Hf(t) = +14.9–+17.2; ε_Nd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.     

Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: A review

Igneous rocks of the Devonian Kola Alkaline Carbonatite Province (KACP) in NW Russia and eastern Finland can be classified into four groups: (a) primitive mantle-derived silica-undersaturated silicate magmas; (b) evolved alkaline and nepheline syenites; (c) cumulate rocks; (d) carbonatites and phoscorites, some of which may also be cumulates. There is no obvious age difference between these various groups, so all of the magma-types were formed at the same time in a relatively restricted area and must therefore be petrogenetically related. Both sodic and potassic varieties of primitive silicate magmas are present. On major element variation diagrams, the cumulate rocks plot as simple mixtures of their constituent minerals (olivine, clinopyroxene, calcite, etc). There are complete compositional trends between carbonatites, phoscorites and silicate cumulates, which suggests that many carbonatites and phoscorites are also cumulates. CaO / Al₂O₃ ratios for ultramafic and mafic silicate rocks in dykes and pipes range up to 5, indicating a very small degree of melting of a carbonated mantle at depth. Damkjernites appear to be transitional to carbonatites. Trace element modelling indicates that all the mafic silicate magmas are related to small degrees of melting of a metasomatised garnet peridotite source. Similarities of the REE patterns and initial Sr and Nd isotope compositions for ultramafic alkaline silicate rocks and carbonatites indicate that there is a strong relationship between the two magma-types. There is also a strong petrogenetic link between carbonatites, kimberlites and alkaline ultramafic lamprophyres. Fractional crystallisation of olivine, diopside, melilite and nepheline gave rise to the evolved nepheline syenites, and formed the ultramafic cumulates. All magmas in the KACP appear to have originated in a single event, possibly triggered by the arrival of hot material (mantle plume?) beneath the Archaean/Proterozoic lithosphere of the northern Baltic Shield that had been recently metasomatised. Melting of the carbonated garnet peridotite mantle formed a spectrum of magmas including carbonatite, damkjernite, melilitite, melanephelinite and ultramafic lamprophyre. Pockets of phlogopite metasomatised lithospheric mantle also melted to form potassic magmas including kimberlite. Depth of melting, degree of melting and presence of metasomatic phases are probably the major factors controlling the precise composition of the primary melts formed.     

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The ε_Hf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding T_DM1 values are in the range of 919–523 Ma. The ε_Hf(t) values of zircon from the DSC I-type granite are between –1 and 3, with T_DM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ¹⁸O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.     

藏南夏瓦地区基性脉岩锆石U-Pb定年、地球化学特征及地质意义

西藏夏瓦地区位于特提斯喜马拉雅构造带中东部,广泛发育大量近东西向的基性脉岩,这些基性脉岩对于研究该区域地质构造演化具有重要的意义。本文对夏瓦地区基性脉岩开展了系统的岩石学、年代学、地球化学等研究。夏瓦基性脉岩的岩石类型以辉绿岩和辉长玢岩为主。锆石U-Pb年代学揭示基性脉岩结晶年龄为146～145 Ma。夏瓦基性脉岩具有低SiO₂(47.20%～50.54%)和高Mg~#值(39.78～53.79)特征,富集Ti、Fe、P元素,属碱性系列,富集轻稀土元素(LREE),高场强元素(Nb、Ta、Zr、Hf、Th)相对富集,显示出似OIB的地球化学特征,指示夏瓦基性脉岩来源于软流圈地幔的部分熔融,形成于大陆边缘裂谷背景下。结合区域大火成岩省基性岩的发育,认为夏瓦地区基性脉岩是大陆边缘裂谷背景下Kerguelen地幔柱作用的产物。夏瓦基性脉岩的结晶年龄虽然早于Kerguelen地幔柱活动的峰期(132 Ma),但可能属于地幔柱峰期之前的小规模岩浆活动。     

U-Pb Zircon Age,Geochemical, and Sr-Nd-Pb Isotopic Constraints on the Age and Origin of Mafic Dykes from Eastern Shandong Province,Eastern China

U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).     

Ultramafic Xenoliths From a Kamafugite Lava in Cenozoic Volcanic Field of West Qinling, China and Its Geological Implication

ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

The limited extent of plume-lithosphere interactions during continental flood-basalt genesis: geochemical evidence from Cretaceous magmatism in southern Brazil

It has been suggested that large areas of the Earth's lithospheric mantle undergo pervasive dehydration melting during the impact of mantle plumes and the Early-Cretaceous Paraná-Etendeka continental flood-basalt (CFB) province has repeatedly been cited as evidence of this phenomenon. During the Cretaceous, however, southern Brazil experienced two phases of mafic magmatism. These igneous events occurred ～50?Ma apart and therefore represent distinct episodes of melt genesis in the underlying mantle. The first phase of magmatism, in the Early Cretaceous, included the emplacement of lava flows associated with the Paraná-Etendeka CFB province and also the intrusion of small-volume mafic alkaline magmas (e.g. Anitápolis, Jacupiranga and Juquiá) in the Dom Feliciano and Ribeira mobile belts. During the Late Cretaceous, both sodic and potassic mafic magmas were emplaced on the margin of the adjacent Luis-Alves craton and intrude the flood-basalts at Lages. On the basis of variations in incompatible trace-element concentrations (e.g. Ba?=?1000 to 2000?ppm), initial ⁸⁷Sr/⁸⁶Sr ratios (0.7048–0.7064) and ?_Nd values (?3 to ?12), we suggest that all of the Late-Cretaceous mafic potassic magmas were derived from the subcontinental lithospheric mantle (SCLM) which was metasomatically enriched during the Proterozoic. We propose that these relatively low temperature, volatile-rich, mafic melts provide direct evidence that the underlying SCLM did not melt wholesale during the previous Early-Cretaceous Paraná-Etendeka CFB event. Late-Cretaceous melting of the SCLM beneath southern Brazil may have been caused by heat conduction from either: (1) ponded ～132?Ma Tristan plume-head material; or (2) ～85?Ma Trindade plume-head material channelled southwards between the thick cratonic keels of the Amazonas and São Francisco cratons. The Late-Cretaceous magmatism appears to have been contemporaneous with uplift across southern Brazil and Paraguay; we suggest that both of these phenomena represent the widespread effects of the impact of the Trindade mantle plume on the base of the SCLM. Plate margin stresses and lithospheric extension associated with the opening of the South Atlantic may also have changed the geothermal gradient beneath southern Brazil and contributed to mantle melting.     

Orogenic Volcanism in Eastern Kazakhstan: Composition,Age, and Geodynamic Position

Studies of volcanic rocks in orogenic troughs of Eastern Kazakhstan were carried out. The troughs were formed at late-orogenic stages of evolution of Hercynian Altai collision system. Volcanic rocks are represented by basalts, andesites, dacites and rhyolites. Based on geochemical and isotopic data, the basalts and andesites derived from mafic magmas that formed as a result of partial melting of garnet peridotites in the upper mantle under the orogen. U–Pb zircon data prove two volcanic stages: more-scaled Middle Carboniferous (~311 Ma) and less-scaled Early Permian (297–290 Ma). Basalts and andesites in lower parts of the orogenic troughs and independent dacite-rhyolite structures were formed at the Middle Carboniferous stage. Parental mafic magmas were formed as a result of partial melting of mantle substrates in local transtensional zones along large shear faults. The formation of dacites and rhyolites could have been caused by partial melting of crustal substrates under effect of mafic magmas. Transtensional movements in the lithosphere of orogenic belts may indicate the beginning of collapse of orogens. A smaller volume of basalts and andesites formed at the Early Permian stage. Geochemical data prove the independent episode of partial melting in upper mantle. Synchronous basalts and andesites also appeared at wide territory in Tian Shan, Central Kazakhstan, and Central and Southern Mongolia. Early Permian volcanism indicates general extension of the lithosphere at the postorogenic stages. Large-scaled Early Permian mafic and granitoid magmatism in Central Asia has been interpreted in recent years as the Tarim Large Igneous Province caused by Tarim mantle plume activity. Thus, the extension of the lithosphere and associated volcanism in the Early Permian can be an indicator of the onset of the plume–lithosphere interaction process.

     

The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet

The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO₂ = 42 %) to felsic (SiO₂ = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO₂, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo₈₇ to Fo₉₃, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO₂ and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO₂ and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO₂, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO₂ contents and Mg/Fe ratio to values dictated by equilibrium with the mantle.