Geological and Geochemical Characteristics and Petrogenesis of the Anqing—Lujiang Quartz Syenite Rock Belt,East China

The Anqing-Lujiang quartz syenite rock belt consists of the Huangmeijian, Chengshan and Dalongshan composite batholiths which intruded into the Mesozoic strata. The country rocks were subjected to thermal contact metamorphism with little sign of folding and regional metamorphism. The rock belt is Late Yenshanian in age with a Rb-Sr isochron age of 135 Ma. Major rock types are quartz syenite and, to a lesser extent, syenite porphyry and alkali feldspar granite. Rock-forming minerals are dominantly potash feldspar (more than 50%) and lesser amounts of plagioclase and quartz. Mafic minerals, mostly Mg-biotite with lesser amounts of amphibole and pyroxene, are rare. Occasionally, alkalic mafic minerals (aegirine, riebeckite) are found. Characteristic accessary minerals of the earlier intrusives are magnetite, sphene and apatite and those of the later intrusives are ilmenite and zircon. Typologic distribution and evolutionary trend of zircon population are very similar to those of granites of mantle origin as suggested by Pupin, J. P. Petrochemically, the rock belt is poor in Ca but rich in alkali and Al with Na₂O+K₂O > 10%. It belongs to the K-Na transitional series, with a high alkalinity ratio (A. R. = 3–7) and a K /Na (atom) ratio close to unity. Rocks in the belt are rich in REE which tends to decrease from the early to the late stage, belonging to LREE type. The initial Sr ratio is 0.7078–0.7064. The rock belt is the Mesozoic anorogenic product of alkalic magmatic activity from a deep-seated source. According to a mantle-crust mixing model for Sr and Pb isotopes, it is estimated that 60.2–53.8% of the materials has been derived from the mantle. Additionally, its rich alkali, poor water content and anorogenic characteristics suggest that the belt is similar to the A-type granites.     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

Aureoles of Songshugou Alpine-type ultramafic massif

The Songshugou Alpine-type ultramafic massif intruded into the surrounding plagioclase-amphibole schists where high-temperature aureoles are found well exposed but not distroyed. From the outer part inwards, four aureoles are recognized: the oligoclase-hornblende zone, the andesine-hornblende zone, the garnet zone and the pyroxene zone. Toward the massif, systematic variations are noticed in the rocks from one zone to another, i.e., the rocks vary from fine-grained, fibrous, crystalloblastic texture to medium coarse-grained, granular, crystalloblastic texture and from schistose structure to massive structure; hornblende varies from blue-green to brofn; plagioclases vary in composition from An_12–17 to An₃₇, even to An₆₀ near the contact; garnet and pyroxene become more and more abundant; sphene is replaced progressively by titanomagnetite; and Na(K), Al and Ti in the hornblende tend to increase near the contact. Studies have shown that the typical rocks in the four contact metamorphic zones are generally similar in major and trace elements. But different mineral assemblages and mineral chemistry indicate that the rocks were formed under different physical conditions.     

http://www.sciencedirect.com/science/article/pii/S1674987113000893

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.     

塔里木巴楚小海子正长岩杂岩体的岩石成因探讨

巴楚小海子正长岩杂岩体是二叠纪塔里木大火成岩省的重要组成部分.SIMS锆石U-Pb定年显示其形成于279.7±2.0Ma,与本区辉绿岩脉和石英正长斑岩岩脉近于同时侵位.根据矿物学特征,小海子正长岩体可分为铁橄榄石正长岩和角闪正长岩两类.前者主要由碱性长石、铁橄榄石、单斜辉石、角闪石和少量石英、斜长石组成,后者主要由碱性长石、角闪石、黑云母和少量的石英、斜长石组成.小海子正长岩体为铁质、碱性系列,轻稀土相对富集,重稀土亏损,具有明显的Eu正异常,无Nb、Ta负异常,相对低的(87Sr/86Sr);(0.7033 ～0.7038)和正的εNd(t)值(+3.1～+3.8),暗示它们来自亏损的地幔源区,没有地壳物质的加入.主微量和同位素地球化学分析,暗示巴楚小海子正长岩的母岩浆为碱性的幔源玄武质岩浆经橄榄石、单斜辉石分离结晶后的残余熔体,并且含有堆晶的碱性长石.这种含有碱性长石堆晶的熔体,在相对还原的条件下结晶,形成铁橄榄石正长岩;在相对氧化的条件下结晶,并经过不同程度斜长石的分离结晶形成角闪正长岩.     

西南三江甭哥金矿床磷灰石地球化学特征及地质意义

甭哥金矿床位于西南三江造山带北段的义敦弧南缘,属于与富碱侵入岩有关的金矿床。目前,对其成矿机理认识仍较为薄弱,制约了资源评价和找矿勘查进展。本文选取甭哥金矿床强矿化的正长斑岩和弱矿化的黑云辉石正长岩中的磷灰石作为研究对象,详细剖析磷灰石的地球化学特征,探讨其记录的成岩成矿信息。结果表明,磷灰石的稀土元素含量特征及配分模式显示富碱岩浆主要来自于壳幔混合的源区,黑云辉石正长岩中磷灰石的(La/Sm)N、(La/Yb)N、(Sm/Yb)N值和Sr含量呈正相关,说明长石结晶对岩浆结晶分异有重要的影响;正长斑岩中磷灰石具有高Sr/Y、Ce/Pb值,而Th/U、(Sm/Yb)N值较低,指示强烈的流体活动参与了岩浆结晶过程;磷灰石挥发分(F、Cl)含量及比值特征指示金矿成矿流体主要来自地幔源区,成矿与富碱、高氯的成矿流体有关。磷灰石Mn氧逸度计估算结果显示,甭哥富碱侵入岩具有高氧逸度特性,但两种不同岩性岩石的氧逸度具有差异性。其中,正长斑岩的logf_(O_2)值为-12～-10. 3,黑云辉石正长岩的logf_(O_2)值为-15. 5～-11. 1,磷灰石中SO_3含量及Ga含量也暗示正长斑岩的氧逸度高于黑云辉石正长岩的特征;结合磷灰石低Mn、Ga含量和高的Cl、SO_3含量,反映甭哥金矿床金的成矿是在高氧逸度条件下金氯络合物迁移、富集而沉淀的结果。因而,磷灰石的地球化学特征对金矿床成矿过程示踪和勘查评价具有重要的指示意义。     

Petrology of nepheline syenite gneiss from Amazonian Brazil

Nepheline syenite gneiss is exposed in three localities near the village of Boca Nova (47·04′W, 1°51′S) in northeastern Pará, Brazil. Isotopic and chemical evidence for an igneous premetamorphic history include: (1) bulk compositions which plot in the Q–Ne–Ks system on a peralkaline fractionation curve; (2) bulk compositions which fall on the Hviddal igneous trend in the SiO₂–Al₂O₃–Na₂O + K₂O molecular diagram; (3) an average K/Rb ratio identical to the average for nepheline syenites of the Khibina massif; (4) K/Rb ratios which lie on the ideal igneous fractionation curve; (5) Nb(Ta) contents far in excess of average crustal abundance; and (6) a low Sr₈₇/Sr⁸⁶ value (0·7034) characteristic of mantle derivation. Post-kinematic lit-par-lit injections, discordant veins, and concordant pods of leucocratic nepheline–alkali feldspar pegmatite partially replaced the metamorphic assemblage and destroyed any microscopic equilibrium texture that may have been present. The final disequilibrium assemblage consists of Fe-rich biotite and two generations each of sodic plagioclase, microcline, and nepheline. Some minor metasomatic chemical effects may be present in the rocks but a case for nephelinization of crustal rocks cannot be documented. Field, petrographic, and chemical constraints indicate that the Boca Nova gneiss is an early-stage migmatite formed by anatexis of metamorphosed igneous nepheline syenite or its volcanic equivalent. Regional metamorphism and partial melting opened the Rb-Sr system so that the whole-rock Rb/Sr isochron age of 724 m.y. has no unambiguous interpretation. A K/Ar age of 580 ± 10 m.y. from biotite denning the metamorphic foliation is regarded as a minimum date for the regional metamorphism and anatexis. No other alkaline rocks of comparable age are known in the Sao Luis Craton.     

Mafic microgranular enclaves in Late Paleozoic granitoids in the Burgasy quartz syenite massif, western Transbaikalia: Composition and petrogenesis

The paper presents original data on the inner structure, mineralogy, and geochemistry of the Late Paleozoic Burgasy quartz syenite massif in western Transbaikalia and mafic microgranular enclaves (MME) in its rocks. The composition of the mafic microgranular enclaves is close to that of phase-1 monzonitoids of this pluton, but the enclaves are not xenoliths of these rocks but were produced by the crystallization of an individual portion of dispersed hybridized basalt melt. The basaltoid nature of the enclaves follows, first of all, from the relict assemblage of calcic plagioclase (An _73–60) and clinopyroxene and from the magmatic dolerite and microgabbro textures of the rocks. The monzonitoid composition of the enclaves was caused by hybridism, which was responsible for the crystallization of quartz, potassic feldspar, and sodic plagioclase due to the introduction of silica, potassium, and some other components. Hybridism was restricted to a boundary crystallization layer in the deep portion of the magmatic chamber (near its bottom). The scatter of the enclaves throughout the whole volume of the pluton is explained by the density inversion of the hybrid layer and material transfer by convective flows. The mafic enclaves crystallized from basaltic melt of within-plate geochemical type. In spite of intense hybridism, the enclaves preserved typical compositional signatures of mafic magma related to the generation of granites in western Transbaikalia in the Late Paleozoic. The basaltoid nature of the mafic enclaves of the Burgasy Massif testifies that magma was simultaneously generated in the mantle and crust during the development of the Late Paleozoic province in the area.     

Crustal reworking in a shear zone: transformation of metagranite to migmatite

This study uses field, microstructural and geochemical data to investigate the processes contributing to the petrological diversity that arises when granitic continental crust is reworked. The Kinawa migmatite formed when Archean TTG crust in the São Francisco Craton, Brazil was reworked by partial melting at ~730 °C and 5–6 kbar in a regional‐scale shear zone. As a result, a relatively uniform leucogranodiorite protolith produced compositionally and microstructurally diverse diatexites and leucosomes. All outcrops of migmatite display either a magmatic foliation, flow banding or transposed leucosomes and indicate strong, melt‐present shearing. There are three types of diatexite. Grey diatexites are interpreted to be residuum, although melt segregation was incomplete in some samples. Biotite stable, H₂O‐fluxed melting is inferred via the reaction Pl + Kfs + Qz + H₂O = melt and geochemical modelling indicates 0.35–0.40 partial melting. Schlieren diatexites are extremely heterogeneous; residuum‐rich domains alternate with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO₂ and K₂O contents and are coarse‐grained, leucocratic rocks. Homogeneous diatexites, quartzofeldspathic domains from the schlieren diatexites and the leucosomes contain both plagioclase‐dominated and K‐feldspar‐dominated feldspar framework microstructures and hence were melt‐derived rocks. Both types of feldspar frameworks show evidence of tectonic compaction. Modelling the crystallization of an initial anatectic melt shows plagioclase appears first; K‐feldspar appears after ~40% crystallization. In the active shear zone setting, shear‐enhanced compaction provided an essentially continuous driving force for segregation. Thus, Kinawa migmatites with plagioclase frameworks are interpreted to have formed by shear‐enhanced compaction early in the crystallization of anatectic melt, whereas those with K‐feldspar frameworks formed later from the expelled fractionated melt. Trace element abundances in some biotite and plagioclase from the fractionated melt‐derived rocks indicate that these entrained minerals were derived from the wall rocks. Results from the Kinawa migmatites indicate that the key factor in generating petrological diversity during crustal reworking is that shear‐enhanced compaction drove melt segregation throughout the period that melt was present in the rocks. Segregation of melt during melting produced residuum and anatectic melt and their mixtures, whereas segregation during crystallization resulted in crystal fractionation and generated diverse plagioclase‐rich rocks and fractionated melts.     

Crystal fractionation in the petrogenesis of an alkali monzodiorite–syenite series: the Oshurkovo plutonic sheeted complex, Transbaikalia, Russia

The Oshurkovo Complex is a plutonic sheeted complex which represents numerous successive magmatic injections into an expanding system of subparallel and subvertical fractures. It comprises a wide range of rock types including alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, in the proportion of about 70% mafic rocks to 30% syenite. We suggest that the variation within the complex originated mainly by fractional crystallization of a tephrite magma.

The mafic rocks are considered as plutonic equivalents of lamprophyres. They exhibit a high abundance of ternary feldspar and apatite, the latter may attain 7–8 vol.% in monzodiorite. Ternary feldspar is also abundant in the syenites. The entire rock series is characterized by high Ba and Sr concentrations in the bulk rock samples (3000–7000 ppm) and in feldspars (up to 1 wt.%). The mafic magma had amphibole at the liquidus at 1010–1030 °C based on amphibole geothermometer. Temperatures as low as this were due to high H₂O and P₂O₅ contents in the melt (up to 4–6 and 2 wt.%, respectively). Crystallization of the syenitic magmas began at about 850 °C (based on ternary feldspar thermometry). The series was formed at an oxygen fugacity from the NNO to HM buffer, or even higher.

The evolution of the alkali monzodiorite–syenite series by fractional crystallization of a tephritic magma is established on the basis of geological, mineralogical, geochemical and Sm–Nd and Rb–Sr isotope data. The geochemical modeling suggests that fractionation of amphibole with subordinate apatite from the tephrite magma leaves about 73 wt.% of the residual monzonite melt. Further extraction of amphibole and plagioclase with minor apatite and Fe–Ti oxides could bring to formation of a syenite residuum. Rb–Sr isotopic analyses of biotite, apatite and whole-rock samples constrain the minimum age of basic intrusions at ca. 130 Ma and that of cross-cutting granite pegmatites at ca. 120 Ma. Hence the entire evolution took place in an interval of ≤10 My. Initial ⁸⁷Sr/⁸⁶Sr ratios for the mafic rocks range from 0.70511 to 0.70514, and for syenites from 0.70525 to 0.70542. Initial _Nd (130 Ma) values for mafic rocks vary from −1.9 to −2.4, and for syenites from −2.9 to −3.5. In a _Nd(T) vs. (⁸⁷Sr/⁸⁶Sr)_i diagram, all rock types of the complex fall in the enriched portion of the Mantle Array, suggesting their derivation from a metasomatized mantle source. However, the small but distinguishable difference in Sr and Nd isotopic compositions between mafic rocks and syenites probably resulted from mild (10–20%) crustal contamination during differentiation. Large negative Nb anomalies are interpreted as a characteristic feature of the source region produced by Precambrian fluid metasomatism above a subduction zone rather than by crustal contamination.     

Geochemical specifics of ongonites in the Ary-Bulak Massif, eastern Transbaikalia

A genetic model for magmatic rocks of the Ary-Bulak Massif is discussed based on a detailed geological map of the massif (prepared by the authors) and on original data of the authors on the petrography of the massif, its compositional zoning, trace-element geochemistry, physicochemical parameters of its crystallization, and melt inclusions in its minerals. The Ary-Bulak Massif was determined to be zonal, with the predominance (approximately 70% by area) of porphyritic topaz ongonites (central facies), which grade toward contacts into weakly porphyritic ongonites bearing topaz and, occasionally, fluorite (margin facies). Aphyric rocks with fluorite (inner-contact facies) occur as a stripe 50–80 m wide at the southwestern inner contact of the massif. Analysis of petrographic and geochemical data indicates that subvolcanic rocks of the Ary-Bulak Massif differ from typical elvanes (as they occur in the Cornwall province) but are similar to classic ongonites in the central and marginal facies of the massif. Rocks in the southwestern inner-contact zone are unusual high-F and high-Ca varieties, whose analogues have never been found in any rare-metal provinces with ongonites and which provide evidence of a complicated evolutionary history of the Ary-Bulak Massif. The geochemical evolution of this massif was determined to be characterized by the enrichment of the older inner-contact facies rocks in CaO, K₂O, F, and Rb, Cs, B, Ba, Sr, Sn, and Ta, whose concentrations decrease in the ongonites of the central facies. The central-facies ongonites thereby have much higher Na₂O and Li concentrations than those in the inner-contact facies rocks. It is demonstrated that the intense heating and melting of crustal material in this region at the Jurassic-Cretaceous boundary could have been induced by subalkaline basaltic magma. The chemical composition of the rocks, which is unusual for typical ongonites in, for example, high Ca and Sr concentrations, could be caused by the possible assimilation by the magma of limestones, which occur in the territory at a certain depth in the Ust’-Borzya Formation that hosts the Ary-Bulak Massif. The genesis of most rocks in the massif was controlled by the magmatic differentiation of crustal granitic magma, with the residual melts forming Li-F granites enriched in several trace elements (Li, Rb, Cs, B, Ba, Sr, etc.) and ongonites as their subvolcanic analogues.     

Petrogenesis of the Sabongari alkaline complex,cameroon line (central Africa): Preliminary petrological and geochemical constraints

The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene–amphibole (the most abundant), amphibole–biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole–pyroxene and amphibole–biotite quartz syenites; (iii) pyroxene–amphibole–biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite–trachyte–granite (Group 1) bimodal series (SiO₂ gap: 44 and 63 wt.%) and a basanite–microdiorite–monzodiorite–syenite–granite (Group 2) less pronounced bimodal series (reduced SiO₂ gap: 56–67 wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO₂-undersaturated (modal analcite in the groundmass and 11.04 wt.% normative nepheline); its Ni (240 ppm) and Cr (450 ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO₂-saturated (1.56 wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line.     

Petrogenesis,zircon U–Pb age,and geochemistry of the A-type Mogou syenite,western Henan Province: Implications for Mesozoic tectono-magmatic evolution of the Qinling Orogen

The Mogou syenite intruded into the Mesoproterozoic Xiong’er Group is the main lithostratigraphic unit, along the southern margin of the North China Craton (NCC). This paper reports zircon LA-ICP-MS data, whole-rock major and trace element compositions of late Triassic magmatic rocks in the Mogou syenite, in order to constrain the formation age of the Mogou syenite, research the origin and evolution of the magma and analyse the geodynamic setting of the Qinling Orogen (QO) in Late Triassic. These rocks consist of medium- to coarse-grained syenite and fine-grained quartz syenite. Zircon U–Pb dating yields a crystallization age of 226.5±2.7 Ma. The syenites are characterized by high SiO₂ (63.49–72.17%), alkali (K₂O+Na₂O of 11.18–15.38%) and potassium (K₂O/Na₂O of 2.88–28.11), are peralkaline or metaluminous (molar A/CNK of 0.87–1.02) and belong to shoshonite series. The syenites have ΣREE of 33.01–191.30 ppm, LREE/HREE of 14–20, (La/Yb)_N of 11–24, with LREE-rich distribution pattern and obvious differentiation between HREE and LREE. Eu anomalies are positive for the medium- to coarse-grained syenite and weakly negative for the fine-grained quartz syenite. In addition, the syenites are enriched in large-ion lithophile elements (Ba, K, Sr, and Pb) but depleted in high strength field elements (Ti, Ta, Nb, Zr, and Hf), and have high differentiation indices of 91.69–97.06. These geochemical features indicate that the primary magma of the Mogou syenite most likely originated from a mantle source with minor crustal component, and underwent a fractional crystallization process during its emplacement. The late Triassic A-type Moguo syenite along the southern margin of the NCC was generated in the late stage of the syn-collision event of QO, recording a transition period from compression to extension at around 227 Ma.     

小秦岭东吉口印支期辉石正长岩年代学及其地质意义

东吉口辉石正长岩是小秦岭地区此前唯一获得加里东期年龄的岩体,呈不规则卵圆状侵入于古元古代太华群高级变质岩中,发育弱片麻理。详细的岩相学观察发现,岩体内部可以分为两种具不同结构特征的岩性,从野外接触关系来看:细粒辉石正长岩形成较早,发育烘烤边;中粒含斑辉石正长岩略晚,发育冷凝边。二者的矿物均由斜长石、钾长石、辉石、角闪石、石英等组成,斑晶主要为辉石和斜长石,辉石和角闪石均发生退变质作用。细粒辉石正长岩LA-ICP-MS锆石U-Pb测年结果为(214.8±3.1) Ma（MSWD=0.48）,中粒含斑辉石正长岩为(212.9±3.1) Ma（MSWD=5.6）,均形成于晚三叠世印支期,而不是此前认为的加里东期。这一组年龄表明,小秦岭地区加里东期岩浆活动可能极其微弱,或者不存在该期岩浆活动。总结前人资料,发现东吉口辉石正长岩的形成时代与小秦岭印支期成矿事件同时,印支期岩浆活动与洋陆俯冲转为陆陆碰撞作用有关,强烈的壳幔相互作用导致了深部物质的上涌,岩浆活动为金的富集提供了热驱动力。     

Geochemical evolution of the Zadoi Alkaline-Ultramafic Massif,Cis-Sayan area,southern Siberia

The unaltered magmatic rocks of the Zadoi Massif were analyzed for Sr isotopic composition and concentrations of major oxides and trace elements by ICP MS. The evolution of the massif involved four phases: (i) perovskite and ilmenite clinopyroxenites, (ii) ijolites, (iii) nepheline syenites, and (iv) carbonatites. The perovskite clinopyroxenites have anomalously high Ce/Pb (223–1132) and Pr/Sr × 1000 (70–360) ratios at a low initial Sr isotopic ratio (⁸⁷Sr/⁸⁶Sr)₀ = 0.70247–0.70285. The ilmenite clinopyroxenites have Ce/Pb and Pr/Sr × 1000 ratios approaching those in basalts of oceanic islands (OIB) (decreasing to 39 and 30, respectively) at a simultaneous increase in the (⁸⁷Sr/⁸⁶Sr)₀ ratios (0.7030–0.7036). The ijolites and nepheline syenites have patterns of incompatible trace elements similar to those in OIB and the highest (⁸⁷Sr/⁸⁶Sr)₀ ratios (0.70346–0.70414). The carbonatites are complementarily enriched in incompatible elements of the nepheline syenites and have (⁸⁷Sr/⁸⁶Sr)₀ = 0.7029–0.7034, which is comparable with the range of analogous ratios for the ilmenite clinopyroxenites. Our geochemical data indicate that the carbonatites were formed as an immiscible liquid or fluid, which separated from the ijolite-nepheline syenite melt during its interaction with the source material of the perovskite and ilmenite clinopyroxenites.     

Perpotassic granulites from southern Bohemia

Garnetiferous, perpotassic granulites associated with voluminous, felsic calc-alkaline garnet-kyanite granulites in the Blanský les massif are described as a new rock type. A K₂O content near 13 wt%, Zr content ranging from 17 to 5877 ppm, primary Ca-Mg-Fe garnet (pyrope up to 33 mol %), and the absence of primary plagioclase and quartz characterize the new type of granulite. Calcium is incorporated only in garnet and apatite. Owing to the content of garnet the bulk syenitic compositions are metaaluminous to peraluminous. The perpotassic granulites are accompanied by subordinated melanocratic perpotassic granulite and alkali-feldspar melanocratic granulite containing up to 20 vol % quartz. These rock types also contain highly variable amounts of zircon. High-pressure, noneutectic partial melting of crustal rocks and crystallization under upper mantle conditions is suggested as the main process in generation of perpotassic granulites. A polyphase, ductile to brittle deformation and recrystallization in pyroxene granulite and later in amphibolite facies accompanied obduction of the granulite massif which carries numerous enclaves of pyrope peridotite. Euhedral equant zircon crystals up to 4 mm long enclosed in large garnet crystals and in the alkali feldspar matrix of perpotassic granulite represent a uniform, high-temperature population with a nearly concordant U-Pb system (Aftalion et al. 1989). The Hercynian age of this zircon indicates the role of the Hercynian anatexis of crustal rocks under upper mantle conditions in generation of granulites in the Moldanubian zone of the Bohemian massif.     

Transition from I‐type to A‐type magmatism in the Sanandaj–Sirjan Zone,NW Iran: an extensional intra‐continental arc

The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO₂ content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A‐type affinity with high Na₂O + K₂O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO₂ levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na₂O + K₂O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO₂ content (48.5%–55.9 wt.%) with metaluminous and calc‐alkaline compositions, relatively lower Na₂O + K₂O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geochemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic subduction of the Neo‐Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley & Sons, Ltd.     

Model systems for anatexis of pelitic rocks

Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An₂₅-An₄₀) the equilibria are rarely extended by more than 30 ° C above those in KA1O₂-NaAlO₂-Al₂O₃-SiO₂-H₂O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.     

Petrology,geochemistry and source characteristics of the Burpala alkaline massif,North Baikal

The Burpala alkaline massif contains rocks with more than 50 minerals rich in Zr,Nb,Ti,Th,Be and rare earth elements(REE).The rocks vary in composition from shonkinite,melanocratic syenite,nepheline and alkali syenites to alaskite and alkali granite and contain up to 10%LILE and HSFE,3.6%of REE and varying amounts of other trace elements(4%Zr,0.5%Y,0.5%Nb,0.5%Th and 0.1%U).Geological and geochemical data suggest that all the rocks in the Burpala massif were derived from alkaline magma enriched in rare earth elements.The extreme products of magma fractionation are REE rich pegmatites,apatite-fiuorite bearing rocks and carbonatites.The Sr and Nd isotope data suggest that the source of primary melt is enriched mantle(EM-Ⅱ).We correlate the massif to mantle plume impact on the active margin of the Siberian continent.