Petrological and source region characteristics of ophiolitic hornblende gabbros from the Aksaray and Kayseri regions,central Anatolian crystalline complex,Turkey

Many Neo-Tethyan ophiolitic bodies are well exposed as thrust-slices in Central Anatolia and are predominantly represented by massive hornblende gabbros, most of which are cut by Supra Subduction Zone (SSZ) plagiogranites. The allochthonous gabbros are distinct from their autochthonous counterparts, with their mineralogy including both igneous hornblende, relict diopside rimmed by replacement hornblende and their chemical composition corresponding mostly to gabbro rather than diorite.The results of major and trace element analyses of forty-two samples, and REE analyses of nine samples, indicate that the hornblende gabbros are SSZ-type and formed from a wet magma by high-degree partial melting of peridotite possibly coupled with contamination by predominantly neighbouring-slab derived fluids within an intra-oceanic back-arc basin. The mafic magmas then underwent high-level fractional crystallization involving titaniferous magnetite, diopside, tschermakite and possibly olivine. Emplacement was followed by extensive ocean–floor metamorphism, which has induced crystallization (or recrystallization) of chlorite, biotite, amphiboles and mobilisation of most of the major elements such as alkali and alkali earth elements, and some LREE.     

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite,central Eastern Desert,Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P₂O₅ anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.     

Hornblende gabbros of Wadi Az Zarib area,Central Eastern Desert,Egypt: opaque mineralogy,geochemistry, tectonic setting,and petrogenesis

A small intrusive fresh gabbroic mass intrudes the Neoproterozoic metasediments and Dokhan volcanics of Wadi Az Zarib area, Central Eastern Desert. It is composed of hornblende gabbros and leuco-hornblende gabbros. Their petrography, opaque mineralogy, and geochemistry are addressed to elucidate their tectonic setting and petrogenesis. They represent a subduction-related calc–alkaline magma that evolved in an island arc setting. In terms of maturity, the supposed arc represents an intermediate stage between continental arc and active continental margin. Thermobarometry and physical–chemical data of the parent magma as deduced from compositions of amphiboles, biotite, and plagioclase indicate crystallization temperatures of 931–825 °C at pressures of 6.16–4.01 kbar and H₂O_melt of 6.4–5.2 wt%. Data, as presented, argue in favor of fractional crystallization mechanism to be accounted to the present suite to interpret the observed variations. The evolution of the suite from hornblende gabbros to leuco-hornblende gabbros was accompanied by decreasing of MgO, CaO, Cr, and Ni with simultaneous increasing of Al₂O₃, TiO₂, Na₂O, K₂O, Ba, Rb, Sr, La, and Ce. Residuals calculated during mass balance fractional crystallization modeling suggest that brown and green hornblendes are the main fractionated phases which derived the melt composition towards the leuco-hornblende gabbros.     

Important role of hornblende fractionation in generating the adakitic magmas in Tongling,Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

ABSTRACT

Tongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of ?3.00 to ?4.52 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.     

Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain,Medicine Lake volcano,California: implications for fractionation mechanisms and the formation of composition gaps

The rhyolite of Little Glass Mountain (73–74% SiO₂) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO₂) and partially crystalline cumulate hornblende gabbro (53–55% SiO₂). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO₂) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO₂) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO₂). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An₇₄ at the core to around An₃₀. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions.     

Geochemical modelling of the tonalitic and trondhjemitic granulites from the Itabuna-Salvador-Curaçá Block,Bahia, Brazil

The studied tonalitic and trondhjemitic granulites are located in the SSE granulitic domain of the São Francisco craton, Bahia, Brazil, where they represent most of the southern part of the Archean and Paleoproterozoic Itabuna-Salvador-Curaçá Block (ISCB). Chemically, the tonalitic and trondhjemitic granulites belong to a low-K calc-alkaline suite; their REE patterns are steep with strong LREE/HREE fractionation and no significant Eu anomaly. Garnet-bearing mafic granulites that occur as enclaves in the tonalitic and trondhjemitic granulites were derived from basalts and/or gabbros of tholeiitic affinity. Geochemical modelling showed that the tonalitic and trondhjemitic granulites were produced by moderate fractional crystallization of an assemblage of hornblende and plagioclase, with subordinate amounts of magnetite, apatite, allanite and zircon. The garnet-bearing mafic granulites would be the source of the magmas that generated these rocks. Partial melting left a residue made up of plagioclase, garnet, orthopyroxene and hornblende.     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.     

冀东晚古生代东湾子岩体的岩石成因研究

冀东晚古生代东湾子岩体由角闪石岩、少量辉石岩和辉长岩组成.典型的堆晶结构、全岩和镁铁质矿物(透辉石、角闪石)的上凸型稀土分布模式、相容元素含量低且变化范围大(如:角闪石岩中V=296×10-6～673×10-6)的特征表明了岩体的堆晶成因.计算得到的与辉石岩中的透辉石相平衡的熔体具有很高的稀土含量,轻重稀土分馏较为明显,富集大离子亲石元素(如:Sr,Ba,K),亏损高场强元素(如:Nh,Zr,Ti),具有典型的弧岩浆特征.透辉石和角闪石的矿物成分也具有弧岩浆的特征.高钙透辉石、大量的角闪石与黑云母的存在说明母岩浆富水.透辉石在高PH2O的状态下与熔体反应,生成角闪石的结构特征也证明了这一点.结合岩体的球化学特征,认为岩浆来源于富集的含有角闪石的尖晶石橄榄岩的部分熔融,母岩浆具有富水的特征(>3%).考虑到岩体形成时代(～300Ma;Zhao et al.,2007)和地质背景,认为东湾子岩体与位于华北北缘的其它晚石炭-早二叠世的岩体形成于同一构造背景下,都是晚古生代时期古亚洲洋向华北板块之下俯冲的产物.     

白云鄂博辉长岩的稀土含量

本区辉长岩沿宽沟背斜南翼分布,多被海西期花岗岩切割或包裹。在主矿以南和东部接触带都有分布。本次研究中,选择了三种不同类型的辉长岩作了稀土总含量和单一稀土元素含量的分析。第一种是主矿南部铁路附近距花岗岩最近的矿物颗粒较细的边缘相辉长岩(B₂);第二种是距花岗岩较远的属于中心相的辉长岩(B₁);第三种是东部接触带炸药库附近斜长石含量增多的较粗粒辉长岩。     

The evolution of young silicic lavas at Medicine Lake Volcano,California: Implications for the origin of compositional gaps in calc-alkaline series lavas

At Medicine Lake Volcano, California, the compositional gap between andesite (57–62 wt.% SiO₂) and rhyolite (73–74 wt.% SiO₂) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO₂ andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO₂ andesite.Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H₂O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions.     

Rare-earth elements,Co, Sc and Hf in the Steens Mountain basalts

The concentrations of the rare-earth elements (REE) in 52 of 70 consecutive lava flows from Steens Mountain, Oregon, are reported. The concentrations of Co, Sc and Hf were measured in 17 of the flows. Logarithmic partitioning theory is used to correlate the concentrations of the trace elements with the major element and mineralogical compositions of the samples.Production of the basalts by partial melting requires a parent material that has concentrations of the REE that are several times those of chondrites. Those samples enriched in alumina have positive Eu anomalies compared to chondrites and those depleted in alumina have negative Eu anomalies. Production of these samples by partial melting is unlikely because the Eu anomalies would require that plagioclase was stable under the conditions of melting.All of the samples can be related to a single parent magma by fractional crystallization processes. The concentrations of the trace elements are controlled by the addition or removal of plagioclase and the removal of clinopyroxene. Samples with high concentrations of alumina are interpreted as plagioclase cumulates while those with low concentrations of alumina are residual liquids produced by crystallization of plagioclase. During fractional crystallization, the concentrations of the light REE are increased selectively in the residual liquid. The material that must be crystallized and removed during fractional crystallization has characteristics that are similar to those reported for gabbros.     

内蒙古白云鄂博地区呼和恩格尔杂岩体的地球化学特征及其构造意义

内蒙古白云鄂博地区白音珠日和苏木以南呼和恩格尔附近出露一套杂岩体,由中心向外依次出现角闪橄榄岩、角闪辉长岩和闪长岩,具有环状分布的特征,并以闪长岩为主。主要造岩矿物为橄榄石、辉石、斜长石及角闪石等。根据K-Ar同位素表观年龄,呼和恩格尔杂岩体形成于242～287Ma之间。电子探针数据分析表明,角闪辉长岩中的辉石为透辉石,斜长石An值平均为90.6,属倍长石。杂岩体的各类岩石均表现为大离子亲石元素(LILE)K,Rb,Sr,Ba明显富集,高场强元素(HFSE)Nb,Ta,Zr,Hf相对亏损。根据AlZ-TiO2,An-Mg,Hf/3-Th-Ta以及Hf/3-Th-Nb/16图解,结合其微量元素地球化学特征,可以判断呼和恩格尔杂岩体形成于活动大陆边缘,是二叠纪古亚洲洋板块向华北板块北缘下部俯冲事件的直接反映。     

Petrological and Geochemical Characteristics of the Cerro Colorado Plutonic Complex in the Precordillera of Northern Chile

Abstract. The Cerro Colorado intrusive stock in the northeastern Chilean Precordillera is a plutonic complex formed during Late Cretaceous (64–72 Ma), and consists predominantly of pyroxene-bearing biotite monzogabbro (Colorado Unit), with lesser amounts of pyroxene-bearing hornblende biotite diorite (Pucaquisca Unit) and biotite hornblende monzonite (Pabellón Unit). Compositional variations of major and trace elements suggest that the Cerro Colorado complex is composed of shoshonitic alkali granitoids generated at the active continental margin. The basic to intermediate rocks of the Colorado Unit are characterized by high contents of A1₂O₃ (>20 wt%), CaO and LIL elements (K, Sr, Ba), high Fe/Mg ratio and fairly low contents of Cr, Ni and Y. These characteristics suggest that the Colorado Unit was formed by plagioclase-free source magma originated from asthenospheric mantle or mafic lower crust. All the Cerro Colorado rocks generally display linear compositional trends, and the latest Pabellón Unit rocks are richer in SiO₂ than the Colorado Unit and Pucaquisca Unit rocks. These indicate that the Pabellón Unit rocks were produced by assimilation-fractional crystallization process of the basic to intermediate magma genetically related to the voluminous Colorado Unit.     

Geochemistry of Neoproterozoic mafic intrusions in the Panzhihua district (Sichuan Province,SW China): Implications for subduction-related metasomatism in the upper mantle

Metasomatism above subduction zones is an important process that produces heterogeneous mantle and thus a diversity of igneous rocks. In the Panzhihua district, on the western margin of the Yangtze Block (SW China), two Neoproterozoic mafic intrusions, one olivine gabbro and one hornblende gabbro, have identical ages of 746 ± 10 and 738 ± 23 Ma. Both of the gabbros are tholeiitic in composition and have arc-like geochemical compositions. The hornblende gabbros have K₂O concentrations ranging from 0.70 to 1.69 wt.% and show enrichment of Rb, Ba, U, Th and Pb and depletion of Nb,Ta and Ti. They have variable ⁸⁷Sr/⁸⁶Sr ratios (0.7045–0.7070) with constant ɛNd(t) values (−0.12 to −0.93). The olivine gabbros have relatively low K₂O (0.19–0.43 wt.%), are depleted in Rb and Th relative to Ba and U, and have obvious negative Nb–Ta and Zr–Hf anomalies on primitive mantle-normalized trace element diagrams. Their ɛNd(t) values range from −0.64 to −1.73 and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7070 to 0.7075. Both types of gabbro experienced fractional crystallization of clinopyroxene, plagioclase, amphibole and minor Fe–Ti oxide. The parental magmas of the olivine and hornblende gabbros were formed by about 20% partial melting of garnet–spinel lherzolite and spinel lherzolite, respectively. According to trace elemental ratios, the hornblende gabbros were probably derived from a source strongly modified by subducted slab fluids, whereas the olivine gabbros came from a mantle source modified by subducted slab melts. The close association of the olivine gabbros and hornblende gabbros suggests that a steep subduction zone existed along the western margin of the Yangtze Block during Neoproterozoic time. Thus, the giant Neoproterozoic magmatic event in South China was subduction-related.     

内蒙古狼山地区早石炭世角闪辉长岩、花岗闪长岩的岩石成因及构造意义

内蒙古狼山山脉西侧分布有大面积的晚古生代岩浆岩,时代集中在早石炭世—晚二叠世,早石炭世角闪辉长岩、花岗闪长岩体出露于潮格温都尔镇西侧。角闪辉长岩体呈岩滴状产出,被花岗闪长岩体侵入,LA-ICP-MS锆石U-Pb年龄显示,角闪辉长岩的~(206)Pb/~(238)U加权平均年龄为329.0±2.3 Ma,花岗闪长岩的~(206)Pb/~(238)U加权平均年龄为331.1±0.9 Ma~330.0±4.2 Ma。花岗闪长岩暗色矿物以角闪石为主,富钠(Na2O=3.48%~4.46%),高钠钾比值(Na2O/K2O=1.03~2.39),钙碱性系列,P2O5-SiO_2之间存在较好的负相关性,岩石地球化学特征具Ⅰ型花岗岩的特点。Hf同位素及元素地球化学特征指示了角闪辉长岩及花岗闪长岩均来自于受地壳混染的亏损地幔,为同源岩浆演化的产物。角闪辉长岩及花岗闪长岩稀土元素配分型式一致,均为轻稀土元素富集,重稀土元素亏损,具弱的负Eu异常;角闪辉长岩富集Ba、Sr,亏损Nb、Ta、Zr、Hf;花岗闪长岩富集大离子亲石元素Rb、K、Pb、Sr,不同程度地亏损高场强元素Nb、Ta、P、Ti,总体反映了岩浆弧的地球化学特征。结合区域地质背景,早石炭世狼山地区侵入岩岩石组合为角闪辉长岩(闪长岩)+石英闪长岩+花岗闪长岩,认为狼山地区早石炭世处于大陆边缘弧构造背景。     

阿拉斯加型岩体的基本特征、成岩过程及成矿作用

阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体,常呈链状分布于汇聚板块边缘。其形成时代跨度较大,从元古代到新生代均有分布,以中生代最为发育。大部分阿拉斯加型岩体规模较小,出露面积约12～14 km~2或更小,平面上呈近似同心环状结构,垂直剖面上呈管道状。岩体中心为纯橄岩,向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等,副矿物为铬铁矿、磁铁矿、钛铁矿等,超镁铁质岩石中少或无斜方辉石,斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物,含量最高达15%～20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量,且富集大离子亲石元素,亏损高场强元素。矿物化学特征上,橄榄石富镁且Fo值变化较大;单斜辉石主要为富Ca的透辉石,其成分变化具有弧堆晶趋势;角闪石主要是镁角闪石和韭角闪石;铬铁矿富集Fe-Al,贫Cr。这些特征揭示,该类岩体成因明显不同于层状岩体和阿尔卑斯型岩体。综合岩石学、矿物学和地球化学分析表明,阿拉斯加型岩体形成于与板块俯冲作用有关的岛弧或者活动大陆边缘背景下,其母岩浆为受到熔/流体交代的地幔楔部分熔融产生的含水玄武质岩浆。各岩相为未受明显地壳混染的同源母岩浆在地壳深度结晶分异的产物。阿拉斯加型岩体的岩浆体系具有含水且高氧逸度的特征,其通常为铂族元素和铬铁矿矿床的重要载体,无或少铜镍矿化。     

An archaean granulite-grade tonalite-trondhjemite-granite suite from scourie,NW Scotland: Geochemistry and origin

The Lewisian complex of the Scourie-Badcall area is composed predominantly of banded tonalitic gneiss which intrudes layered gabbro-ultramafic complexes. Intrusive into both gabbro and tonalitic gneiss are homogeneous acid sheets which are trondhjemitic to granitic in composition. All rocks were subjected to granulite facies metamorphism. Smooth continuous trends on chemical variation diagrams suggest that the evolution of these rocks was dominated by fractional crystallisation. A scheme is proposed whereby a tonalitic melt was parental to trondhjemite and granite. Variation within tonalites was a function of the fractional crystallisation of hornblende and plagioclase, and trondhjemite was derived from tonalite by the fractional crystallisation of hornblende and/or plagioclase. Granite and granodiorite represent residual liquids which evolved along the quartz-feldspar cotectic surface; they were derived by the fractional crystallisation of plagioclase from a trondhjemite liquid. Some trondhjemitic sheets are quartz-plagioclase residues from which a granitic melt was removed. The associated gabbros and ultramafic rocks are not directly related to the proposed fractional crystallisation scheme and are not crystal residues removed from the tonalitic melt. Tonalites were probably derived from a basaltic source by partial melting or fractional crystallisation with either hornblende and/or garnet as residual phases.     

华北克拉通南缘中条山-嵩山地区1.78Ga基性岩墙群的地球化学特征及构造环境

幔源岩浆侵位产生的基性岩墙群是地壳伸展裂解的重要标志。华北克拉通南缘中条山-嵩山地区出露有大量的中元古代基性岩墙群,对其研究将有助于深入了解华北克拉通中元古代构造演化特征。其岩石类型以辉绿岩为主,少量辉长辉绿岩和辉绿玢岩;主要造岩矿物为斜长石和单斜辉石,其它矿物包括角闪石、Fe-Ti氧化物、磷灰石、黑云母、碱性长石和石英。锆石SHRIMPU-Pb年龄为1785±18Ma,代表岩墙的结晶年龄。岩石K2O+Na2O含量为3.63%～6.18%,K2O/Na2O比值为0.73～1.38,FeOT含量较高(10.03%～13.59%),属于拉斑玄武岩系列。岩石的稀土元素含量高(142×10-6～381×10-6),亏损Nb、Ta、Zr、Hf等高场强元素,富集Rb、Ba、La等大离子亲石元素,全岩εNd(t)值为-6.2～-8.1。岩墙的固结指数(SI)和MgO含量呈明显的正相关关系,说明幔源岩浆发生过明显的结晶分异作用。地球化学分析表明,研究区基性岩墙属于板内拉斑玄武岩系列,与熊耳群火山岩有相似的地球化学特征;基于两者紧密的产出关系,我们认为该区岩墙可能是熊耳群火山岩的通道岩墙,共同代表了华北克拉通东、西陆块碰撞后伸展事件。     

张广才岭地块早-中二叠世镁铁质侵入岩体的年代学、地球化学及岩石成因

双凤山基性侵入岩体位于松嫩-张广才岭地块南缘,其岩石组成主要为橄榄辉长岩.锆石LA-ICP-MS U-Pb定年显示该岩体形成于279±4 Ma.岩石矿物组成主要为橄榄石、单斜辉石、斜方辉石、高An值（80.1~87.9）斜长石以及以反应边形式存在的角闪石,矿物学特征指示其母岩浆为经历过充分流体交代的富水岩浆.全岩主微量元素组成揭示其源区物质组成为石榴子石二辉橄榄岩,部分熔融程度约20%,岩浆演化过程中经历了斜长石堆晶作用以及斜方辉石的结晶分异作用.全岩Sr-Nd同位素及锆石Hf同位素研究显示其（87Sr/86Sr）_i=0.705 2~0.706 1,并具有正的ε_Nd（t）值（0.9~1.3）和ε_Hf（t）值（0~10.2）,表明其来源于亏损的地幔源区,并且岩浆上升过程中没有经历明显的地壳混染.微量元素和同位素特征综合反映岩浆源区经历了古亚洲洋俯冲沉积物熔体和流体交代作用的改造,但起主导作用的是流体.其地球化学特征总体显示MORB以及弧型玄武岩过渡的特征,暗示其形成于弧后伸展环境.研究区基性侵入岩地幔源区的不均一性主要受到古亚洲洋多期次俯冲作用析出熔/流体对上覆地幔楔不同程度的改造.      

Trace element and isotopic variations in a zoned pluton and associated volcanic rocks,Unalaska Island,Alaska: A model for fractionation in the Aleutian calcalkaline suite

Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu^*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial ⁸⁷Sr/⁸⁶Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.LDGO Contribution no. 2964