Implications of experimental petrology to the evolution of ultrapotassic rocks

The contributions of experimental studies pertinent to ultrapotassic rocks of Groups I (lamproites) and II (kamafugites and related rocks) are discussed in terms of synthetic systems, ultrapotassic rock compositions, experiments on characteristic minerals in these rocks and experiments designed to model mantle metasomatism. These studies indicate that the majority of ultrapotassic magmas are derived by partial melting of a metasomatically enriched mantle source at depths of 100 km or greater, and under fluid conditions represented by the C---O---H system with fluorine that may be reduced or oxidized relative to other compositions. Many lamproitic magmas may be derived from a phlogopite-harzburgite with volatiles that are predominantly H₂O and F₁ whereas kamafugitic type ultrapotassic magmas may be products of partial melts of a more wehrlitic mantle source in which the main volatiles are H₂O, CO₂ and possibly F. Experimental and theoretical considerations of mantle metasomatism suggest that it occurs at of f_O2 in the range of the FMQ buffer. Metasomatism involves low density mantle fluids (melts?) in which H₂O and CO₂ are the important volatiles, buffered by amphibole, phlogopite and carbonates. Results of recent experiments suggest that the reactions causing metasomatism may be decoupled and cyclic and occur at different depths.     

Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas

A model is developed for the origin of ultrapotassic melts by melting of veined lithosphere; the veins are rich in clinopyroxene and mica, whereas the wall-rocks consist principally of peridotites. The veins originate by solidification of low-degree melts which are themselves the results of earlier, deeper, multistage processes ultimately due to the presence of a transition zone between large-scale channelled and porous flow regimes. The melting event producing the ultrapotassic magma begins in the veins due to the concentration of hydrous phases and incompatible elements, but spreads to include the surrounding wall-rocks by a combination of two mechanisms. The alkaline magma composition is thus a hybrid of vein (V) and wall-rock (W) components.

The melt hybridization mechanisms are: (i) Solid-solution melting: Minerals which from extensive solid-solutions are abundant in the vein assemblages (Cr/Al spinel, F/OH mica, amphibole and apatite). The breakdown of these phases take place over a temperature range between the solidus of the vein assemblage and the elimination of the more refractory end-members. This process bridges the temperature gap between the solid of vein and wall-rock, so that a melt component from the wall-rock is added to that from the vein before elimination of all vein minerals. Phlogopite forms the most effective of these sliding reactions, resulting in its stability at near-liquids temperatures in experiments. (ii) Dissolution of wall-rock minerals: The initial melt fraction in the vein infiltrates the surrounding wall-rock due to the dominance of surface energy minimization on melt flow at the intergranular scale. Following infiltration, dissolution of wall-rock minerals occurs at temperatures lower than their melting temperatures, thus imparting a refractory wall-rock component to the melt composition. Dissolution of olivine and/or orthopyroxene occurs preferentially, since these minerals are furthest from equilibrium with the strongly alkaline, vein-derived melt.

Remobilisation of several generations of veins explains the occurrence within a restricted space and time of rocks bearing chemical characteristics which are generally thought to indicate contrasting tectonic settings (e.g. central Italy). The ultrapotassic rocks are explained as being dominanyly vein-derived (i.e. high V/W ratio): further dilution of the V-component by wall-rock, supplemented by asthenospheric melt in advanced cases, leads to the production of more voluminous basaltic rocks bearing incompatible element signatures reminiscent of those of ultrapotassic rocks.     

The role of fluorine and oxygen fugacity in the genesis of the ultrapotassic rocks

The effects of H₂O, CO₂, CH₄ and HF on partial melting of a model phlogopite harzburgite mantle are considered with regard to the production of ultrapotassic magmas. Fluorine has a polymerising effect in H₂O-poor conditions, but in the presence of abundant H₂O where HF rather than F is dominant, the overall effect is depolymerisation. Methane also dissolves by forming (OH)^– groups, and so has a depolymerising effect. Group I ultrapotassic rocks (lamproites) probably originate from primary magmas with SiO₂ contents ranging from around 40 wt% to at least 52 wt%. This range can be explained by differing depths of origin from a similar source with a similar reduced H₂O-CH₄-HF volatile mixture. The formation of silica-rich initial melts from a model phlogopite harzburgite is assisted by the presence of CH₄ and HF. Dissociation of less than 0.1 wt% H₂O, driven by H₂ loss, is sufficient to cause oxidation during emplacement to observed oxidation states. Silica-poor ultrapotassic rocks could be produced at higher pressures in a reduced environment, or in an oxidised environment with high CO₂/(CO₂ + H₂O) ratios.Group II (African Rift) potassic rocks may originate in H₂O-poor conditions in which fluorine will maintain a large phlogopite phase field, so that initial melts will be magnesian and silica-undersaturated.     

西藏查孜地区中新世正长岩的锆石U-Pb年代学、地球化学及岩石成因

查孜正长岩体是拉萨地块中段新发现的中新世钾质-超钾质侵入岩,岩性主要为中粗粒石英角闪正长岩和斑状石英正长岩,有暗色包体发育.对两种正长岩进行了岩石学、锆石U-Pb年代学和地球化学研究.结果显示,两种岩石的锆石U-Pb年龄分别为10.37±0.24 Ma、11.06±0.39 Ma,代表它们形成于中新世,是拉萨地块后碰撞岩浆作用的产物.查孜正长岩具有相对高钾（K₂O=6.75%~7.39%）、低镁（MgO=1.44%~2.97%）的特征,K₂O/Na₂O>1,属钾质岩;具有与超钾质岩石相似的微量元素特征,强烈富集Rb、Th、U、K等大离子亲石元素（LILE）和轻稀土元素（LREE）,相对亏损Nb、Ta、Zr、Hf、Ti、P等高场强元素（HFSE）和重稀土元素（HREE）,但Cr（22.7×10-6~64.6×10-6）、Ni（18.9×10-6~46.6×10-6）含量明显偏低.其中,斑状石英正长岩的SiO₂含量相对较高,但MgO、K₂O、Cr、Ni、REE和Y等元素含量比石英角闪正长岩的低.综合分析认为,查孜正长岩主要形成于岩浆混合作用,是富集岩石圈地幔部分熔融形成的超钾质岩浆与地壳物质部分熔融形成的酸性岩浆混合的结果,两种岩石的地球化学差异主要是岩浆混合的程度和比例不同导致的;它的形成可能与岩石圈地幔的减薄作用有关.      

Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO₂, Al₂O₃, Na₂O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al₂O₃, CaO and variable enrichment in K₂O and incompatible elements. LMP rocks are saturated in silica and have high SiO₂, K₂O, K₂O/Na₂, MgO, Ni and Cr and low Al₂O₃, CaO, Na₂O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS.

All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K₂O, whereas different trends of Sr and HFSE vs. K₂O are defined by LMP-TRANS and KS-HKS-KAM. ⁸⁷Sr/⁸⁶Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar ⁸⁷Sr/⁸⁶Sr values clustering around 0.710. LMP and TRANS rocks have the highest ⁸⁷Sr/⁸⁶Sr values.

Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes.

Different trends of incompatible elements vs. K₂O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na₂O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.     

A perspective on potassic and ultrapotassic rocks: Constraints on the Paleoproterozoic late to post-collisional event in the São Francisco paleocontinent

The late- to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts. In this paper, we present new whole rock geochemical analyses combined with zircon and titanite U–Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur within the Archean Itacambira-Monte Azul block (BIMA), to discuss their petrogenesis and the tectonic implications for the São Francisco paleocontinent. The new U–Pb ages range from ca. 2.06 Ga to 1.98 Ga and reveal long-lasting potassic magmatism within the BIMA, which is within the late- to- post-collisional stage of the São Francisco paleocontinent evolution. The ultrapotassic rocks are compatible with a fluid-related metasomatized mantle source enriched by previous subduction events, whereas the potassic rocks are bimodal and have a transitional shoshonitic to A-type affinity. These rocks have a hybrid nature, possible related to the mixing between the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust. Our results also show an increase of within-plate signature towards the younger potassic magmas. The participation of an important Archean crustal component in the genesis of these rocks is highlighted by the common and occasionally abundant occurrence of Archean inherited zircons. The Hf isotopic record shows that most of the zircon inheritance has dominantly subchondritic ε_Hf(t) values, which fits a crustal reworking derivation from a similar Eo- to Paleoarchean precursor crust. However, the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.     

THE HISTORY OF TETHYS IN THE LATE PALEOZOIC-EARLY MESOZOIC AND THE RECONSTRUCTION OF PANGEA

The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms.

Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO₂ saturated, and Al₂O₃ poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO₂ and low Al₂O₃ (0.20 to 2.00%) and TiO₂ (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the early crystallization of K-feldspar.

The syenitic-facies rocks are mostly peralkaline, whereas the granites are metaluminous. The syenites have high concentrations of incompatible elements (Ba, Sr, and Rb) and light-rare-earth-element (LREE) concentrations lower than for typical ultrapotassic rocks, with chondrite-normalized Ce/Yb ratios of 10 to 20 and wide variation in the La/Ta ratios (40 to 250). The granites have lower incompatible-element contents and La/Ta ratios (20 to 60) than do the syenites.

Syenites from the dike swarm exhibit high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7106), whereas εNd, values for the ultrapotassic mafic enclaves range from -1.1 to -3.7, suggesting that the enclaves and the syenites have different sources.

Field evidence, combined with geochemical data, shows that the granites and the syenites alternate in space and time, suggesting that syenites and granites cannot be associated by either fractional crystallization or partial melting of the same source. The syenites probably represent partial melting of a metasomatized lithospheric mantle, modified by subduction-zone fluids and crustal metasediments during the early stages of a Brasiliano (Pan-African) collisional event. Low-degree partial melting of a metasomatized lower crust appears to be the source of the granites.     

Mineralogy and petrology of cretaceous subsurface lamproite sills, southeastern Kansas, USA

Cores and cuttings of lamproite sills and host sedimentary country rocks in southeastern Kansas from up to 312 m depth were analyzed for major elements in whole rocks and minerals, certain trace elements in whole rocks (including the REE) and Sr isotopic composition of the whole rocks. The lamproites are ultrapotassic (K₂O/Na₂O = 2.0–19.9), alkalic [molecular (K₂O/Na₂O)/Al₂O₃ = 1.3-2.8], enriched in mantle-incompatible elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homogeneous initial Sr isotopic compositions (0.707764-0.708114).

These lamproites could have formed by variable degrees of partial melting of harzburgite country rock and cross-cutting veins composed of phlogopite, K-Ti richterite, titanite, diopside, K-Ti silicates, or K-Ba-phosphate under high H₂O/CO₂ ratios and reducing conditions. Variability in melting of veins and wall rock and variable composition of the metasomatized veins could explain the significantly different composition of the Kansas lamproites.

Least squares fractionation models preclude the derivation of the Kansas lamproites by fractional crystallization from magmas similar in composition to higher silica phlogopite-sanidine lamproites some believe to be primary lamproite melts found elsewhere. In all but one case, least squares fractionation models also preclude the derivation of magmas similar in composition to any of the Kansas lamproites from one another. A magma similar in composition to the average composition of the higher SiO₂ Ecco Ranch lamproite (237.5–247.5 m depth) could, however, have marginally crystallized about 12% richterite, 12% sanidine, 7% diopside and 6% phlogopite to produce the average composition of the Guess lamproite (305–312 m depth).

Lamproite from the Ecco Ranch core is internally fractionated in K₂O, Al₂O₃, Ba, MgO, Fe₂O₃, Co and Cr most likely by crystal accumulation-removal of ferromagnesian minerals and sanidine. In contrast, the Guess core (305–312 m depth) has little fractionation throughout most of the sill except in several narrow zones. Lamproite in the Guess core has large enrichments in TiO₂, Ba, REE, Th, Ta and Sc and depletions in MgO, Cr, Co and Rb possibly concentrated in these narrow zones during the last dregs of crystallization of this magma.

The Ecco Ranch sill did not show any evidence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproite which did apparently lose H₂O-rich fluid to the country rock. Perhaps a greater confining pressure and lesser amount of H₂O-rich fluid prevented it from escaping.     

Sr-Nd-Pb isotopes from the Radicofani Volcano, Central Italy: constraints on heterogeneities in a veined mantle responsible for the shift from ultrapotassic shoshonite to basaltic andesite magmas in a post-collisional setting

The Radicofani Volcano is characterised by few lava flows, a cinder cone and a denudated neck, and is part of the Tuscan Magmatic Province, the northernmost volcanic region of the Italian peninsula. In spite of the short time span of activity, a large time-dependant chemical and isotopic variability is observed. Most of the rocks of the Radicofani volcano are ultrapotassic shoshonites associated to younger basaltic andesites, found at the bottom of the neck. K₂O contents are positively correlated with trace element and isotopic variations. Shoshonitic and high-K calc-alkaline rocks of the Radicofani volcano are significantly different from shoshonites occurring in association with leucite-bearing ultrapotassic rocks in the southernmost portion of the Roman Magmatic Province. The studied rocks are characterised by high, but variable, levels of incompatible trace elements with a subduction-related signature, with troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.71333 to 0.71588, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.512050 to 0.512183, while the lead isotope ratios vary between 18.672 and 18.716 for ²⁰⁶Pb/²⁰⁴Pb, 15.665 to 15.696 for ²⁰⁷Pb/²⁰⁴Pb, and 39.981 to 39.081 for ²⁰⁸Pb/²⁰⁴Pb. Ultrapotassic shoshonites show the highest incompatible trace element contents coupled with the highest ⁸⁷Sr/⁸⁶Sr and the lowest ¹⁴³Nd/¹⁴⁴Nd. On the basis of geochemical and isotopic signatures it is argued that magmas were generated in a modified lithospheric peridotitic source containing metasomatic veins generated by K-rich melts from recycled sediments within the mantle via subduction. A further metasomatic event generated by slab-derived fluids pervasively enriched the peridotitic source. Partial melting of the veins produced leucite-free ultrapotassic magmas (i.e. lamproite), and was triggered by rising of the isotherms after the orogenic front migrated eastward in the Italian Peninsula. Further rise of the isotherms induced larger degrees of partial melting inducing melting of the surrounding wall peridotite. The variation of the degree of partial melting of such a heterogeneous peridotitic source produced a wide spectrum of magma compositions, which mimic a mixing line between two components: ultrapotassic magma from partial melting of the metasomatic vein and a basaltic andesitic magma from partial melting of the surrounding peridotite.     

西藏拉萨地块西部扎布耶茶卡火山岩的成因与意义

近年来在青藏高原南部拉萨地块不断发现的碰撞后钾质和超钾质岩石,对于揭示印度与亚洲大陆碰撞以来高原岩石圈的深部作用与过程发挥了重要作用。分布在拉萨地块西部扎布耶茶卡东岸的钾质和超钾质火山岩主体喷发时代为中新世（约16Ma）,出露面积约为400km2,火山岩持续喷发0.45Ma,估算的喷发速率约为0.26×10-3km3/a。岩石包括3种类型,第一类（约16Ma）为超钾质的粗面安山岩,SiO2低（55%~58 %）,高Fe2O3、MgO、TiO2;第二类（约27Ma）为钾玄质的响岩和粗面岩;第三类是高SiO2的钾玄质—超钾质粗面岩（SiO2＝59%~64％）和流纹岩（SiO2＝69％）。岩石显示轻稀土元素、大离子亲石元素高度富集和部分高场强元素亏损的特征,部分中酸性岩石显示高Sr低Y的埃达克岩的属性。岩石的Sr-Nd-Pb-O同位素组成与拉萨地块典型的超钾质岩石明显不同,显示亲青藏高原北部地球化学省的地球化学特征。扎布耶茶卡不同类型的岩浆代表了碰撞后高原南部岩石圈减薄作用导致的岩石圈不同层次的岩石部分熔融的产物。     

Ultrapotassic Volcanism of the Valagin Ridge,Kamchatka

New isotopic-geochemical data are reported on the Late Cretaceous–Paleocene ultrapotassic volcanic rocks of the alkaline–ultrabasic complex of the Valagin Ridge, Eastern Kamchatka. The high Mg, low Ca and Al contents at high K/Na ratios in these rocks make them similar to the Mediterranean-type lamproites and ultrapotassic rocks. The low contents of high-field strength (HFSE) and heavy rare-earth (HREE) elements relative to the MORB composition, and the low Sr and high Nd isotopic ratios indicate the formation of their primary melts from a depleted mantle source. The enrichment of the ultrapotassic rocks in the large-ion lithophile elements (LILE) can be explained by the fluid influx in melts during melting of subsided oceanic crust.     

The S-type Ladybird leucogranite suite of southeastern British Columbia: Geochemical and isotopic evidence for a genetic link with migmatite formation in the North American basement gneisses of the Monashee complex

The 62–52 Ma Ladybird granite (LBG) suite is a peraluminous, leucocratic, S-type, quartz monzonitic to granitic suite which occurs as batholiths, stocks, dikes, sills, and pegmatite veins predominantly in the high-grade rocks of the Shuswap complex, in southeastern British Columbia. The emplacement of the LBG was synchronous with the production of abundant migmatites within Thor–Odin dome of the Monashee complex, an exposure of North American basement, exhumed from depths of ca. 26–33 km by Eocene extensional faults. The LBG and the leucosome in migmatites from Thor–Odin dome have similar major and trace element patterns, and are both characterized by zircons which have inherited Precambrian cores. Whole rock Nd isotope compositions show a range of values for the LBG with εNd_{(55 Ma)} values from − 5.0 to − 17.2. The εNd_{(55 Ma)} for the leucosome samples range from − 9.5 to − 23.6, overlapping with those of the granitic suite. These data support the interpretation of a genetic link between formation of the LBG suite and melting of North American basement rocks, such as those exposed in the core of Thor–Odin dome. The leucosome samples have lower high field strength element (HFSE) concentrations and positive Eu anomalies, whereas the LBG samples have higher HFSE concentrations and negative Eu anomalies. The similar trace element characteristics suggest that the leucosome from the migmatites and the LBG are related, whereby most of the leucosome samples are cumulates and the LBG samples represent evolved or residual melts. The initial ⁸⁷Sr/⁸⁶Sr isotope values for both the LBG and leucosome samples have a large range. However, the initial Sr isotopic ratios for the LBG suite are lower than those of the leucosome samples, with ⁸⁷Sr/⁸⁶Sr_{(55 Ma)} ranging from 0.70603 to 0.73688 and 0.74256 to 0.76593, respectively. This isotopic discrepancy suggests either: a) isotopic disequilibrium during partial melting in the mid- to lower crust where the leucosome formed, b) the distribution of Sr during partial melting was controlled by different melt-producing reactions, and/or c) isotopic heterogeneity in the source rocks. At least part of the LBG suite likely formed via melting of North American basement rocks that were dominantly of sedimentary origin. Melting of the Proterozoic supracrustal metasedimentary rocks overlying North American basement may also have contributed to the formation of the different phases of the suite found at the regional scale. However, the abundant leucosomes in the basement rocks of Thor–Odin dome may mark the paths along which anatectic melt migrated in the structurally overlying Ladybird granites of the South Fosthall pluton.     

青藏高原新生代两类超钾质岩石的成因——实验岩石学和地球化学约束

青藏高原分布有羌塘—囊谦—滇西和冈底斯两条新生代钾质-超钾质火山岩带。羌塘—囊谦—滇西超钾质岩浆活动的峰值时间为40～30Ma,主体岩石具有Ⅰ型超钾质岩的高MgO和低CaO、Al2O3含量特征;30～24Ma期间羌塘中、西部出现Ⅲ型钾质-超钾质岩浆活动,主体岩石以贫SiO2、高CaO、Al2O3和低MgO/CaO为特征。冈底斯新生代超钾质火山岩也显示I型超钾质岩的高MgO和低CaO、Al2O3含量特征,其形成时间为25～12Ma。综合超钾质岩石的实验资料,可知区内I型超钾质岩的源区以富硅、富钾流(熔)体交代形成的金云母方辉橄榄岩为主;Ⅲ型钾质-超钾质岩浆源区则以斜辉橄榄岩地幔为主。囊谦—滇西Ⅰ型超钾质岩带空间上严格受红河走滑构造带所控制,40～28Ma出现I型超钾质岩浆活动,16Ma转变为OIB型钾质火山岩。岩浆源区从岩石圈地幔向软流圈演变,暗示大型走滑断裂引起的岩石圈地幔减薄和软流圈上涌是导致交代岩石圈地幔金云母分解熔融产生区内I型超钾质岩浆的主控因素。羌塘中部35～34Ma有软流圈来源为主的钠质碱性玄武岩岩浆的喷发,30～24Ma转变为以岩石圈地幔为主要来源的Ⅲ型钾质-超钾质岩浆活动,岩浆源区从软流圈向岩石圈迁移,指示软流圈上涌伴随的富CO2流(熔)体活动是导致古交代岩石圈地幔升温熔融产生Ⅲ型钾质-超钾质岩浆的主控因素,软流圈上涌可能是俯冲板片断离或岩石圈地幔拆沉作用的结果。     

Aluminous granulites from the Pulur complex, NE Turkey: a case of partial melting, efficient melt extraction and crystallisation

In the Pulur complex, NE Turkey, a heterogeneous rock sequence ranging from quartz-rich mesocratic gneisses to silica- and alkali-deficient, Fe-, Mg- and Al-rich melanocratic rocks is characterized by granulite-facies assemblages involving garnet, cordierite, sillimanite, ilmenite, ±spinel, ±plagioclase, ±quartz, ±biotite, ±corundum, rutile and monazite. Textural evidence for partial melting in the aluminous granulites, particularly leucosomes, is largely absent or strongly obliterated by a late-stage hydrothermal overprint. However, inclusion relations, high peak P–T conditions, the refractory modes, bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting. The melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica-rich, bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with SiO₂. Peak conditions are estimated to be ≥800 °C and 0.7–0.8 GPa. Subsequent near-isothermal decompression to 0.4–0.5 GPa at 800–730 °C is suggested by the formation of cordierite coronas and cordierite–spinel symplectites around garnet and in the matrix. Sm–Nd, Rb–Sr and ⁴⁰Ar/³⁹Ar isotope data indicate peak conditions at 330 Ma and cooling below 300 °C at 310 Ma.     

中国东部燕山期斑岩钼-热液型铅锌(银)成矿系统的成矿物质来源

中国是世界第一大钼生产国和资源国,同时也是铅、锌的重要资源国。中国东部燕山期斑岩型钼矿床及热液脉型、夕卡岩型铅锌(银)矿床是中国钼、铅、锌的主要来源。前人基于斑岩钼和热液型铅锌(银)矿床的地质、地球化学研究,提出了中国东部燕山期斑岩型钼-热液型铅锌(银)成矿系统的新认识,但对该成矿系统的岩浆起源、成矿物质来源等仍存在认识上的分歧。近年来,越来越多的地质、地球化学证据表明,斑岩钼矿的成矿可能与幔源岩浆活动有关,成矿斑岩的Sr、Nd、Pb同位素组成也显示有幔源物质的贡献。碳酸岩作为典型的幔源岩浆岩,是研究地幔物质组成的探针岩石,源自俯冲交代富集地幔的碳酸岩是已知Mo含量最高的岩浆岩,同时其Pb、Zn、Ag含量也很高,并具有富水、富F、富S、富CO₂的特征。中国东部与斑岩钼矿同期的碳酸岩、基性岩的地球化学研究表明,中国东部中生代地幔为经历了俯冲交代的富集地幔,富集地幔的部分熔融可能为斑岩钼-热液型铅锌(银)成矿系统提供了成矿岩浆、成矿金属,同时还可能提供了S、F和成矿流体。     

饶阳凹陷沙一下亚段优质烃源岩地球化学特征及成藏贡献

为了研究饶阳凹陷沙一下亚段优质源岩的地球化学特征及其成藏贡献.基于有机碳、热解、生烃模拟实验评价源岩生烃潜力;基于源岩GC-MS实验和生烃潜力评价结果,描述源岩分子地球化学及有机相特征,利用多元数理统计方法,建立油-岩关系并定量计算原油中各源岩贡献率,总结了沙一下亚段成藏贡献规律.研究表明:沙一下亚段源岩的生烃潜力在凹...     

江南造山带基底变质沉积岩中锂元素分布和富集机制及对锂成矿的制约

内容提要:江南造山带是我国重要的稀有金属成矿带,已发现多处与锂相关的花岗伟晶岩型稀有金属矿床。锂等稀有金属元素在源区基底岩石中的富集是花岗伟晶岩型锂稀有金属矿床成矿的物质基础,但是江南造山带基底岩石中锂的分布及其富集机制仍不清楚。本文详细调查研究了江南造山带东段新元古代冷家溪群、双桥山群、溪口岩群和板溪群变质沉积岩和星子杂岩。这些基底变质岩的岩石类型包括变质砂岩、泥质板岩、千枚岩和云母片岩以及少量片麻岩。地球化学分析结果显示,冷家溪群、双桥山群和溪口岩群变质沉积岩具有相似的成分,变质砂岩和云母片岩-片麻岩整体上比泥质板岩和千枚岩具有较高的Si O₂含量,较低的Ti O₂、Al₂O₃、K₂O、Mg O和TFe₂O₃含量。泥质板岩和千枚岩含有更高的稀有金属元素含量,其中锂含量达到61.8×10^-6,而变质砂岩的锂丰度为44.9×10^-6。板溪群具有最低的稀有金属元素含量(Li=30.8×10     

Major,minor, and trace element compositions of peridotitic and basaltic komatiites from the precambrian crust of Southern Africa

Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earth's crust (3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue.     

拉萨地块西段中新世赛利普超钾质火山岩富集地幔源区和岩石成因:Li同位素制约

作为一种“非传统稳定同位素”,锂同位素地球化学研究已经成为近年来国际上研究的热点之一.文章成功应用锂同位素对青藏高原西南部赛利普超钾质火山岩进行了示范研究.研究表明,赛利普超钾质火出岩的w(Li)为11.2×10-6～22.9× 10-6,同位素组成δ7Li为1.2‰～+3.5‰,平均值为0 2‰,与平均上地壳的相当.超钾质火山岩的锂同位素组成与岩浆结晶分异程度参数之间不存在任何相关性,这表明在超钾质火山岩结晶分异过程中没有发生明显的锂同位素分馏,锂同位素组成特征反映了其形成时的源区特征.超钾质火山岩的锂同位素组成变化范围达4.7‰,并且与pb-Sr-Nd同位素和岩浆结晶分异参数之间亦无任何相关性,表明锂同位素异常可能反映了不均匀源区岩石特征.通过计算模拟以及与前人的类似研究成果进行对比,笔者认为俯冲印度地壳而不是特提斯洋壳(包括沉积物)的流体/熔体参与了超钾质火山岩的源区富集,并在此基础上提出了超钾质火山岩成因模式.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).