Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±högbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region.

The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone (P>8–10 kbar) to the sillimanite–cordierite zone (P<5 kbar) at T750 °C, with maximum recorded temperatures of 800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±högbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks.

Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.     

龙岗金垅顶子近代活动火山的岩石学与地球化学

距今１６００ 年前喷发的金龙顶子火山是龙岗火山群中一座最年轻的近代活动火山, 它的喷发物包括金龙顶子火山锥体 （海拔９９９．４ｍ ）、熔岩流及其广布的火山碎屑席 （四海火山渣层）。金龙顶子火山岩为单一的粗面玄武岩, 有着极为相似的ＲＥＥ和不相容元素分布模式, ８７Ｓｒ／８６Ｓｒ和１４３Ｎｄ／１４４Ｎｄ 分别变化于０．７０４８４６～０．７０４９２１ 和０．５１２６１９～０．０．５１２６４６, 这反映了它们直接来自地幔源区, 岩浆上升过程极少演化和壳源混染, 具有较为原始的岩浆特点。年轻火山岩中的幔源捕虏体提供了该区存在相对低温 （～９００℃） 含水的地幔。     

Structure,petrology and geochemistry of the Abu Dom alkaline ring-complex,Bayuda Desert,Sudan

The Abu Dom complex is one of a cluster of Younger Granite ring-complexes piercing the Proterozoic gneisses of the Bayuda Desert of Sudan. Intrusive rocks predominate and range from early metaluminous syenites to later peralkaline syenites and granites. They are disposed as ring-dykes, nested intrusions, and cone-sheet swarms. Field evidence and geochemical variation indicate that the peralkaline rocks evolved from a metaluminous syenitic parent magma as a result of high-level fractionation. The suite carries a clear within-plate signature exemplified by marked enrichments in HFS elements such as Zr and Nb.     

华北龙岗第四纪玄武岩：岩石成因和源区性质

华北克拉通东北缘龙岗第四纪玄武岩的地球化学研究为大陆碱性玄武岩的成因以及源区的性质提供了重要的依据。龙岗第四纪玄武岩为碱性玄武岩,具有类似 OIB 的 REE 和微量元素分配特征。岩石的 Sr-Nd 同位素轻度亏损(~(87)Sr/~(86)Sr=0.7044～0.7048,ε_(Nd)=0.6～2.1),具有 Dupal 异常的高放射性成因 Pb 同位素组成(~(206)Pb/~(204)Pb:17.734～18.194,~(207)Pb/~(204)Pb=15.553～15.594,~(208)Pb/~(204)Pb=38.322～38.707)。这种地球化学特征指示了原始岩浆起源于<70km 深度的地幔,并经历了一定程度的橄榄岩、单斜辉石和钛-铁氧化物的结晶分异。岩浆源区中以来类似 MORB 软流圈物质的熔体为主,另外有少量来自 EM Ⅰ性质的富集岩石圈地幔以及俯冲流体/熔体的物质贡献,显示了深部岩石圈-软流圈一定程度的相互作用以及太平洋板块俯冲的影响。岩浆源区多种端元组分的存在表明该地区岩石圈的减薄/置换受到多种因素的影响。     

The metamorphic evolution of the high pressure mafic granulites of the Bacariza Formation (Cabo Ortegal Complex, Hercynian belt, NW Spain)

The high pressure mafic granulites of the Bacariza Formation outcrop in the two uppermost structural units of the Cabo Ortegal Complex (La Capelada unit and Cedeira unit) were separated by a Variscan thrust. In both cases, they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks, also affected by catazonal metamorphism. The main difference between the mafic granulites in the two units is the degree of deformation, which is higher in the underlying Cedeira unit. Petrologic and mineralogical data indicate that the high-pressure (HP) granulites (Gt-Cpx±Amp-Pl±Qtz±Scp-Rt±Ilm-Czo) are already retrograde (M2 Stage), post-dating an earlier eclogite facies metamorphism (M1 Stage) characterised by the mineral associations: Gt-Cpx±Amp±Ky±Qtz-Rt and Gt-Cpx±Amp±Qtz±Zo-Rt. The main structure related to the exhumation processes is the development of a general mylonitic foliation that, although initiated in granulite facies conditions, was mainly equilibrated in amphibolite facies (M3 Stage). This foliation was affected by isoclinal folds, which led to the formation of the Variscan thrusts responsible for the present stacking position. Thrust conditions were transitional between amphibolite and greenschist facies (M4 Stage). Thermobarometric data point to different P–T exhumation paths in the two units. Estimated P–T conditions were higher in La Capelada unit during M1 (P≥13 kbar; 860°C) and M2 (15 kbar; 800°C) than in the Cedeira unit (M1: P≥11 kbar, 770°C; M2: 12 kbar; 750°C). Temperatures for the M3 stage were comparable (720°C) in both units but rocks from the Cedeira unit show a much bigger drop in pressure. This resulted in an isothermal decompression type path for the Cedeira unit, while both P and T decreased more steadily in La Capelada rocks. These were always located at deeper level than the Cedeira rocks before the Variscan stacking. The difference in the two paths is related to different exhumation rates; higher in rocks from the Cedeira unit than in those from La Capelada. Exhumation processes coeval with underthrusting, and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths.     

兴蒙造山带东南缘黄松群中压型变质作用及其构造意义:岩石学、矿物学和年代学证据

本文报道了兴蒙造山带东南缘黄松群代表性岩石的岩相学、矿物化学成分和黑云母40Ar/39Ar定年结果,以确定它们的变质作用类型、变质温压条件、变质时代及其构造意义。岩相学研究表明黄松群主要由黑云斜长片麻岩、二云斜长片岩、斜长角闪片岩等代表性岩石以及不同类型的糜棱岩所组成。前者往往呈规模较小的面状分布,反映了主期区域动热变质作用,后者主要呈条带状分布,反映了后期低温动力变质作用的叠加与改造。此外,在与侵入体接触处产出的红柱石角岩反映了局部接触变质作用的存在。对黄松群代表性岩石的温压条件计算结果表明,该群主期变质作用的温压范围分别为525~597℃和5.8~7.5kb,地热梯度集中在21~27℃/km,揭示出黄松群中压型(绿帘角闪岩相-低角闪岩相)变质作用的存在。对含石榴二云斜长片岩中黑云母40Ar/39Ar定年结果为239.22±3.02Ma,而黑云母糜棱岩中黑云母的40Ar/39Ar定年结果为193.91±2.16Ma,结合黑云母的封闭温度和研究区已有的年代学资料,认为前者反映黄松群中压型变质作用的时代为晚二叠世-早三叠世,而后者反映黄松群经历的后期低温动力变质作用发生在早侏罗世。结合区域上同时代变质作用、岩浆作用以及沉积建造特征和区域构造演化历史,认为黄松群晚二叠世-早三叠世中压型变质作用与华北板块与西伯利亚板块碰撞拼合的地球动力学背景有关,该期变质作用标志着古亚洲洋东段的最终闭合已经完成。     

内蒙古西乌旗梅劳特乌拉蛇绿岩的识别

新识别出的内蒙古西乌旗梅劳特乌拉蛇绿岩位于中朝板块与西伯利亚板块之间的兴蒙造山带北部,从空间展布看,为二连浩特-贺根山蛇绿岩带的东延部分,通过对梅劳特乌拉蛇绿岩进行详细的野外地质调查和岩石学、地球化学、年代学的研究,发现梅劳特乌拉蛇绿岩带呈ENE-NE向展布,宽约6~11km,延伸约24km,蛇绿岩各单元出露较齐全,岩性主要为蛇纹石化方辉辉橄岩、层状-块状辉长岩、辉绿岩脉(墙)、枕状玄武岩、辉斑玄武岩、及硅质岩。蛇纹石化方辉辉橄岩具有低SiO 2、高MgO,稀土配分模式显示宽缓的U型,具SSZ(supra-subduction zone)型蛇绿岩的地幔橄榄岩特征。枕状玄武岩和辉绿岩脉(墙)具低K2O(平均0.82%)和TiO2(0.8%~1.2%)的特征,球粒陨石标准化显示LREE弱亏损的平坦型;微量元素MORB标准化蛛网图显示:枕状玄武岩和辉绿岩脉(墙)的大离子亲石元素Sr、K、Rb和Ba含量明显富集,而高场强元素Nb、Th、Ta、Zr、Ti明显亏损,为典型的SSZ构造背景成因的熔岩特征。获得块状辉长岩的LA-ICP-MS锆石U-Pb年龄为308.5±2.2Ma,据此确定梅劳特乌拉蛇绿岩形成于晚石炭世,为约束兴蒙造山带的构造演化提供了新佐证。     

Petrology,phase equilibria modelling,noble gas chronology and thermal constraints of the El Pozo L5 meteorite

We present the results of physical properties, petrography, bulk chemistry, mineral compositions, phase relations modelling and Noble gases study of the meteorite El Pozo. The petrography and mineral compositions indicate that the meteorite is an L5 chondrite with a low shock stage of S2-S3. Heterogenous weathering was preferentially along shock structures. Thermobarometric calculations indicate thermal equilibrium conditions between 768?°C and 925?°C at ～4 to 6?kb, which are substantially consistent with the petrological metamorphism type 5. A pseudosection phase diagram is relatively consistent with the mineral assemblage observed and PT conditions calculated. Temperature vs. fO₂ diagram shows that plagioclase compositional stability is very sensitive to Tschermack substitution in orthopyroxene, clinopyroxene and X_An plagioclase during the high temperature metamorphic process. Based on noble gases He, Ne, Ar and K contents a cosmogenic exposure age CRE of 1.9?Myr was calculated. The ²¹Ne would be totally cosmogenic, with no primordial Ne. The ²¹Ne/²²Ne value (0.97) is higher than solar value. According to the cosmogenic Ne content, we argue that El Pozo chondrite originally had a pre-atmospheric mass of 9–10?kg, which would have been produced by a later collision after the recognized collision of the L-chondrite parent body ～470?Ma ago.     

Permo-Triassic high-pressure metamorphism in the central western Korean Peninsula,and its link to Paleo-Tethyan Ocean closure: Key issues revisited

Permo-Triassic high-pressure(HP) mafic granulites, together with the Bibong retrogressed eclogite,preserved along the central western Korean Peninsula provide important insights into the Late Permian to Triassic collisional orogeny in northeast Asia. The metamorphic pressureetemperatureetime(P-T-t)paths of these rocks, however, remain poorly constrained and even overestimated, owing to outdated geothermobarometers and inaccurate isopleth techniques. Here we evaluate the metamorphic Pe T conditions of Triassic HP mafic granulites including those in Baekdong, Sinri and Daepan and the Bibong Triassic retrogressed eclogite in the Hongseong area, and the Permo-Triassic Samgot mafic granulite in the Imjingang Belt of the central western Korean Peninsula through the application of modern phase equilibria techniques. The Baekdong and Samgot mafic granulites and the Bibong retrogressed eclogite yield a range of 12.0 -16.0 kbar and 800 -900℃, representing HP granulite facies conditions. The Sinri and Daepan granulites from the Hongseong area show relatively lower grade metamorphic conditions between HP granulite and normal granulite facies, and are characterized by sub-isothermal decompression during exhumation. The similarities in the metamorphic ages and the post-collisional igneous activity from the central western Korean Peninsula indicate that the Triassic ages represent the retrograde stage of the metamorphic Pe T paths. In contrast, the Late Permian metamorphic ages, which are older than protolith ages of the post-collisional igneous rocks, correspond to the possible prograde stage of metamorphism. The P-T-t paths presented in this paper, together with the metamorphic ages and post-orogenic igneous events reported from these areas suggest trace of the subduction, accretion and exhumation history, and indicate a tectonic linkage among the northeast Asian continents during the Paleo-Tethyan Ocean closure.     

Early Mesozoic metamorphism and tectonic significance of the eastern segment of the Lhasa terrane,south Tibet

The metamorphic belt in the Basongco area, the eastern segment of Lhasa terrane, south Tibet, occurs as the tectonic blocks in Paleozoic sedimentary rocks. The Basongco metamorphic rocks are mainly composed of paragneiss and schist, with minor marble and orthogneiss, and considered previously to be the Precambrian basement of the Lhasa terrane. This study shows that the Basongco metamorphic belt experienced medium-pressure amphibolite-facies metamorphism under the conditions of T = 640–705 °C and P = 6.0–8.0 kbar. The inherited detrital zircon of the metasedimentary rocks yielded widely variable ²⁰⁶Pb/²³⁸U ages ranging from 3105 Ma to 500 Ma, with two main age populations at 1150 Ma and 580 Ma. The magmatic cores of zircons from the orthogneiss constrain the protolith age as ca. 203 Ma. The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 192–204 Ma. The magmatic cores of zircons in the orthogneiss yielded old Hf model ages (T_DM2 = 1.5–2.1 Ga). The magmatic zircons from the mylonitized granite yielded a crystallization age of ca. 198 Ma. These results indicate that the high-grade metamorphic rocks from the Basongco area were formed at early Jurassic and associated with coeval magmatism derived from the thickening crust. The Basongco metamorphic belt, together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts, formed a 400-km-long tectonic unit, indicating that the central segment of the Lhasa terrane experienced the late Paleozoic to early Mesozoic collisional orogeny.