Geochemistry and petrogenetic evolution of the diatexites of Central Kerala,India

The hornblende-biotite gneisses of Central Kerala which cover approximately 490km² exhibit schlieric and nebulitic structures, tending towards a homophanous nature and are classified here as diatexites. Mafic protoliths and restite biotite, each representing the refractory residuum of two independent partial melting episodes are widely present in the gneisses. The general mineral assemblage of the gneisses comprise quartz, K-feldspar, oligoclase, biotite and hornblende. Chemically, they are dominantly adamellitic and the behaviour of major and trace elements is consistent with a magmatic parentage. Based on petrochemical criteria, a two-stage evolution model is proposed here, which involves (i) partial melting of mafic granulites under high Archean geothermal gradients and generation of tonalite/trondhjemite through amphibole and plagioclase fractionation and (ii) partial melting and subsequent quartz-alkali feldspar fractionation of the tonalite/trondhjemite under amphibolite facies conditions with synchronous K-enrichment resulting in the diatectic adamellites.     

Geochemistry and petrogenesis of anorogenic basic volcanic-plutonic rocks of the Kundal area,Malani Igneous Suite,western Rajasthan,India

The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gabbro intrusives are associated with rhyolite. Both the basic rocks consist of similar mineralogy of plagioclase, clinopyroxene as essential and Fe-Ti oxides as accessories. Basalt displays sub-ophitic and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic and intergrannular textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible elements as compared to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE patterns with least significant positive Eu anomalies. Petrographical study and petrogenetic modeling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and they probably derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is concluded that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate degree (< 30%) partial melting of source similar to picrite/komatiitic composition. Within plate, anorogenic setting for the basic rocks of Kundal area is suggested, which is in conformity with the similar setting for Malani Igneous Suite.     

Proterozoic anorogenic magmatism in the Central Amazonian Province,amazonian araton: Geochronological,petrological and geochemical aspects

Summary In the Central Amazonian Province, the anorogenic granites are older in the eastern block (1.88 Ga, U-Pb; 1.8 - 1.6 Ga Rb-Sr and K-Ar) and in the central block (1.75 -1.7 Ga, Rb-Sr) than in the western one (1.55 Ga, U-Pb). The country rocks are of Archaean age in the eastern block, and in the western block they are of Lower Proterozoic age (Trans-Amazonian Event). There is a minimum difference of 200 Ma between the last metamorphic event and the formation of the anorogenic granites. Metaluminous to peraluminous subsolvus granites are largely dominant but hypersolvus granites, sometimes peralkaline, as well as syenites and quartz-syenites also occur. Wiborgites and pyterlites are found only in the western block but rapakivi-like textures are described in the province as a whole. Mineralizations include large deposits of Sn, as well as small occurrences of F, Zr, REE, Y, and W. The granites are generally rich in Si, K, Fe, Zr, Ga, Nb, Y, and REE and show very high K/Na, Fe/(Fe + Mg) and Ga/Al₂O₃ ratios. They are geochemically similar to A-type and within-plate granites and more particularly to the Proterozoic rapakivi granites of the Fennoscandian shield and the metaluminous granites of the North American anorogenic province. Petrographic, geochemical and Sr isotopic data indicate crustal sources for the granite magmas. Differences in the sources should explain the contrast observed in some of the granites. The model of crustal anatexis induced by underplating or intrusion of mantle-derived basic magmas is preferred to explain the generation of the crustal granitic magmas.

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Proterozoischer, anorogener Magmatismus in der zentralen Amazonas-Provinz, Amazonas Kraton: Geochronologie, petrologische und geochemische Aspekte

Zusammenfassung In der zentralen Amazonas-Provinz sind die anorogenen Granite im östlichen (1.88 Mia, U-Pb; 1.8-1.6 Mia, Rb-Sr und K-Ar) und im zentralen Block (1.75-1.7 Mia,Rb-Sr) älter als im westlichen Block (1.55 Mia, U-Pb). Die Nebengesteine sind im östlichen Block archaiischen Alters, während sie im westlichen Block im unteren Proterozoikum (Transamazonas Event) gebildet worden sind. Dies ergibt eine minimale Altersdifferenz von 200 Mio zwischen dem letzten metamorphen Ereignis und der Intrusion der anorogenen Granite. Es dominieren metaluminöse bis peraluminöse Subsolvus-Granite, jedoch treten auch Hypersolvus-Granite, stellenweise Peralkaline, wie auch Syenite und Quarz-Syenite auf. Wiborgite und Pyterlite kommen lediglich im westlichen Block vor, Rapakivi-ähnliche Texturen werden jedoch aus der gesamten Provinz beschrieben. An Mineralisationen treten große Sn-Lagerstätten und kleinere Vorkommen von F, Zr, SEE, Y und W auf. Die Granite sind generell reich an Si, K, Fe, Ga, Nb, Y und SEE und zeigen sehr hohe K/Na, Fe/(Fe + Mg) und Ga/Al₂O₃ Verhältnisse. Sie zeigen geochemische Ähnlichkeiten mit A-Typ und Intraplatten-Graniten, besonders jedoch mit den proterozoischen Rapakivi Graniten des fennoskandischen Schildes und mit den metaluminösen Graniten der nordamerikanischen anorogenen Provinz. Petrographische, geochemische und Sr-Isotopendaten deuten krustale Ausgangsgesteine der granitischen Magmen an. Unterschiede im Ausgangsmaterial sollen die verschiedenen Granittypen erklären. Als Modell für die Entstehung krustaler granitischer Magmen wird krustale Anatexis, hervorgerufen durch underplating oder Intrusion von aus dem Mantel stammenden, basischen Magmen angenommen.

     

Geochemistry and petrogenesis of Neoproterozoic Mylliem granitoids,Meghalaya Plateau,northeastern India

The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km² and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An₂₀–An₂₉). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification.     

Geochemistry and petrogenesis of the Palampur metavolcanics, Lesser Himachal Himalaya, India

Summary The Palampur metavolcanics (PV) in the northwest Himalaya are part of the Late Archaean (2.5 Ga) Rampur flood basalt province (RFBP) which represents one of the oldest manifestation of worldwide mafic magmatism. The volcanics occur as mafic lava flows with evidence of two phases of deformation. The first phase resulted in recrystallisation which almost completely obliterated the primary mineralogy, and the second phase was of weak cataclasis. Immobile trace element ratios as well as cation percent Al - (Fe - Ti) - Mg indicate that the volcanics are tholeiitic in composition. The chemical characteristics, such as the decoupling between HFS and LIL elements i.e., distinct negative Sr, Nb and Ti anomalies in the double normalisation ratios spiderdiagram together with low Ti/Y and Zr/Y ratios, testify the rocks as low-Ti continental flood basalts. The chemical variations in the volcanics can be related to varying extents of partial melting of the mantle source(s), followed by fractional crystallisation (predominantly olivine and clinopyroxene over plagioclase). Positive correlation between LREE and Fe abundances, Ce-Nd and Y/Nd-Zr/Y data preclude any significant role of crustal contamination in the evolution of their bulk chemistry. The REE data and [Mg]-[Fe] relations rather suggest that the parental magma of the PV derived from non-pyrolitic source(s) which was heterogeneous with respect to enrichment in Zr, LREE and Fe/Mg ratios. Mantle metasomatism appears to be the main process of such source enrichment, possibly caused by the addition of a volatilerich silicate melt phase.[/ p]

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Geochemie und Petrogenese der Palampur Metavulkanite, Lesser Himachal Himalaya, Indien

Zusammenfassung Die Palampur Metavulkanite (PV), im nordwestlichen Himalaya, gehören zur spätarchaiischen (2.5 Ga) Rampur Plateau-Basalt-Provinz (RFBP), die einen der ältesten, weltweiten Phasen von mafischem Magmatismus manifestiert. Die Vulkanite treten als mafische Lavaströme auf und zeigen zwei Phasen von Deformation. Die erste resultierte in einer Rekristallisation, die nahezu den gesamten primären Mineralbestand überprägt hat. Die zweite Phase ist durch schwache Kataklase repräsentiert. Immobile Spurenelementverhältnisse, wie auch die Kationenprozent von Al- (Fe- (-Ti) -Mg, deuten eine tholeiitische Zusammensetzung der Vulkanite an. Die chemischen Charakteristika, die im unterschiedlichen Verhalten der HFS und LIL Elemente deutlich werden, deutlich negative Sr, Nb und Ti Anomalien in den zweifach normalisierten Spiderdiagrammen, niedrige Ti/Y und Zr/Y Verhältnisse, bezeugen, dass es sich um Ti-arme, kontinentale Plateaubasalte handelt. Die chemischen Unterschiede innerhalb der Vulkanite können auf unterschiedliche Grade von Aufschmelzung des (der) Mantelmaterials(e), gefolgt von fraktionierter Kristallisation (vor allem Olivin und Klinopyroxen, untergeordnet Plagioklas), zurückgeführt werden. Positive Korrelation der LREE mit Fe, wie auch die Ce-Nd und Y/Nd-Zr/Y Daten schließen eine signifikante Rolle von Krustenkontamination in der Evolution der Gesteinschemie aus. Die REE Daten und die /Mg/-/Fe/ Verhältnisse lassen eher vermuten, dass das Ausgangsmagma der PV von einem nicht-pyrolitischen Material stammt, welches bezüglich Anreicherung an Zr, LREE und Fe/Mg Verhältnis heterogen gewesen ist. Mantelmetasomatose scheint der Hauptprozeß für diese Anreicherng des Ausgangsmaterials gewesen zu sein, die möglicherweise durch Zufuhr einer silikatischen Schmelzphase, reich an flüchtigen Bestandteilen, hervorgerufen wurde.[/ p]

     

Geochemistry and petrogenesis of Biabanak-Bafq mafic magmatism: Implication for the evolution of central Iranian terrane

Precambrian magmatism in the Biabanak-Bafq district represents an extensive sequence of mafic magmatic rocks. Major, trace and rare earth elements reveal that the low-Ti basement mafic rocks are magnesium tholeiite and low-Ti cover a mafic rock belongs to Fe-tholeiite, whereas, the high-Ti alkaline mafic rocks, as well as dolerites, show much more Fe–Ti enrichment. Primitive mantle normalized trace element patterns show a relative enrichment of LREE and LILE and depletion of HFSE, but have an equally distinct continental signature reflected by marked negative Nb, Sr, P, and Ti anomalies. The composition of the intrusive rocks is consistent with fractional crystallization of olivine ± clinopyroxene ± plagioclase, whereas variations in the Sr and Nd isotope compositions suggest heterogeneous sources and crustal contamination. Low-Ti group samples contain a crustal signature in the form of high La/Yb, Zr/Nb, and negative \(\varepsilon \hbox {Nd}\) values. In contrast, high-Ti mafic magmatic rocks display an increase in La/Yb with a decrease in Proterozoic alkaline rocks recognized across the central Iran. The presence of diverse mafic magmatic rocks probably reflects heterogeneous nature of sub-continental lithospheric mantle (SCLM) source. The mafic magmatism largely represents magmatic arc or rift tectonic setting. It is suggested that the SCLM sources were enriched by subduction processes and asthenospheric upwelling.     

云南个旧似长石正长岩类及其岩石成因

云南个旧碱性岩体主要的岩石类型有碱性正长岩和似长石正长岩,其中,似长石正长岩中出现大量似长石矿物霞石、方钠石和碱性暗色矿物。本文根据矿物成分及特征,将这些似长石正长岩进一步划分为黑榴霞石方钠正长岩、霞石方钠正长岩、霞石正长岩及方钠霞石正长岩4类。岩石地球化学结果表明,4类岩石的地球化学行为整体表现出过碱质岩的特征,K2O+Na2O含量很高,为钾玄岩系列,同时表现出钾质的特点。分异指数高,呈现高度分异演化特点。稀土元素变化大,轻重稀土元素分异明显,富集轻稀土元素,亏损重稀土元素。微量元素富集大离子亲石元素Th、U及Zr、Hf等高场强元素,亏损Ba、Sr大离子亲石元素,而P和高场强元素Ta、Ti亏损,同时Cr、Co、Ni含量非常低,具有中等的负Eu异常和微弱的负Ce异常。研究表明,似长石正长岩在岩浆演化过程中表现出明显的分离结晶作用特征,且岩浆起源温度较高,约为835℃,起源较深。个旧似长石正长岩为A型岩套A1亚型,结合构造判别图解,认为其可能来自角闪石或者金云母相矿物存在的富集地幔,形成于燕山晚期伸展的构造背景,岩浆在较高温度下高度结晶分异,并在侵位过程中伴随陆壳成分的混染。     

Sedimentation assessment in a multipurpose reservoir in Central Kerala,India

Dams and their reservoirs, constructed to manage the water scarcity problems of a region, sometimes lose whole or part of their functionality due to sedimentation. This issue, seen as a negative impact as far as reservoir life and its purpose is concerned, can be a boon to the construction industry, by providing a highly demanding construction material in the form of sand dredged from the reservoirs. Malampuzha reservoir, a multipurpose reservoir in the South Indian state of Kerala, is also losing considerable part of its storage due to siltation. This paper assesses the rate of sedimentation in Malampuzha reservoir, through bathymetric survey and suggests measures for utilization of the removable sediment. Our analysis has shown that the reservoir capacity is reduced from 226 to 205.19 Mm³; a reduction in capacity of 20.81 Mm³ in 55 years. The rate of sedimentation of the reservoir is estimated as 16.95 mm/year. The dead storage capacity of the reservoir has reduced to 47.5 % from the original at present. The composition of deposited sediments is also identified, based on which its productive use is recommended.     

内蒙古查干花钼矿区成矿花岗岩地球化学、年代学及成岩作用

查干花钼矿床是内蒙古中西部的一个大型斑岩钼矿床。矿床成矿期花岗岩为似斑状黑云母花岗岩。2个花岗岩样品锆石SHRIMP U-Pb定年结果分别为253.3±2.8Ma(MSWD=1.17)和253.8±3.7Ma(MSWD=1.6),显示成矿期花岗岩形成于253～254Ma,为晚二叠世。与前人辉钼矿Re-Os测年研究(～243Ma)对比显示,矿床的成岩成矿时间差约为10Ma,这与矿床控矿构造及成矿期花岗岩的结构构造特征相符,也与国内外较多的斑岩型矿床成岩可以对比,反映出查干花钼矿床是成矿岩体经历了长时间演化以后岩浆-热液体系的产物。元素地球化学研究显示,成矿期花岗岩具有高硅、高碱、准铝质至过铝质和高钾钙碱性的特征。其源区是受到早期陆缘弧俯冲作用改造及地壳混染的岩石,再经过部分熔融作用所形成。其微量元素继承了早期陆缘弧成因岩石的一些特征。岩浆在形成以后,在深部岩浆房内经历了以斜长石和钾长石为主导的分离结晶作用,并在上侵到地壳浅部区域以后受到了远古宇宝音图群的混染。其形成的构造环境为古生代末期向中生代转换的后碰撞环境。     

Alkali granite-syenite-carbonatite association in Munnar Kerala,India; implications for rifting,alkaline magmatism and liquid immiscibility

Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO₂, Al₂O₃ and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K₂O, K₂O/Na₂O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.     

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of felsic magmatism in the Son valley

Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intrabasinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.     

Geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites, Central Anatolian Crystalline Complex, Turkey

Plagiogranites associated with the Sarikaraman ophiolite of the Central Anatolian Crystalline Complex, Turkey, closely resemble other plagiogranites from supra-subduction zone-type ophiolites of Neotethys. The ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite (ca. 10% by volume) than is usually associated with such bodies. The Sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex. The plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite. Field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber. Both the deep-seated trondhjemites and the volcanic rhyolites constitute the Sarikaraman plagiogranite suite. Geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites, which are characterised by (MORB-normalized) low HFS element contents with small negative Nb---Ta anomalies and variably enhanced LIL element abundances. Unlike other plagiogranites, however, the Sarikaraman suite is not characterized by consistently low K₂O contents; a feature that reflects the variable mobilization of the LIL elements under lower greenschist facies conditions. The REE are uniformly enriched relative to the basic components of the complex, but have similar normalized patterns exhibiting mild light REE depletion. In terms of their origin, the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization (70–85% of clinopyroxene-feldspar ± amphibole) or partial melting (5–15% batch melting) of a gabbroic ‘source material’, although only the first process can produce most of the range of the plagiogranite compositions. As a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole.     

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km²) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al₂O₃ from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).     

Geology and geochronology of neoarchean anorogenic magmatism of the Keivy structure, Kola Peninsula

Anorogenic magmatic complexes were formed during protoplatformal evolution of the Keivy structure. This evolution ended with development of aluminous schists, which were derived by deep disintegration and redeposition of the rocks from the lower parts of the sequence and surrounding of the structure. The anorogenic rocks of the region are represented by the following magmatic complexes: gabbro-labradorite-latite-monzonite-granites; ophitic gabbro and gabbrodiabases; quartz syenite-alkaline granites; alkaline and nepheline syenites. The magmatic activity of this period, starting from the emplacement of gabbrolabradorite massifs and ending with alkaline and nepheline syenite bodies, was caused by ascent of mantle asthenolith, which destructed the Earth’s crust basement in this area. The anorogenic magmatism of the Keivy structure lasted for no more than few or few tens of million years. The granitoid subcomplex of the gabbro-labradorite-latite-monzonite-granite complex is dated at 2674 ± 6 Ma, which is comparable with an age of alkaline granites of the Ponoy and Beliye Tundry massifs (2673 ± 6 Ma). The considered complexes are separated in time by intrusion of amphibole-biotite plagiomicrocline granites with an age of 2667 ± 8 Ma. Gabbrolabradorites of the Shchuch’e Ozero and Tsaga massifs have close ages (2663 ± 7 and 2668 ± 10 Ma, respectively, Bayanova, 2004), but were formed earlier than granitoids (Bayanova, 2004). Formation of alkaline syenites of the Sakharijok I Massif, which finalized the Neoarchean anorogenic magmatism of the region, falls in the same interval. During Paleoproterozoic transformations, the rocks of the Keivy structure were sheared and uranium was introduced in the contact zones of the alkaline granite massifs, which caused formation of palingenetic melts and subsequent formation of pegmatites in the outer contact zones of the granite bodies.     

The petrology and petrogenesis of the Tertiary anorogenic mafic lavas of Southern and Central Queensland,Australia — Possible implications for crustal thickening

The Miocene-Oligocene volcanism of this region is part of the larger Tertiary volcanic province found throughout E. Australia. Within the S.E. Queensland region, the volcanism is strongly bimodal, and has emanated from six major centres, and many additional smaller centres. The mafic lavas (volumetrically dominant) range continuously from ne-normative through to Q-normative and are predominantly andesine-normative; Mg/Mg+Fe (atomic ratios range from 30–60; K₂O ranges from 0.42–2.93%, and TiO₂ from 0.81–3.6%.Phenocryst contents are low (averaging 6.7 vol.%), and comprise olivine (Fa_18–75; Cr-spinel inclusions occur locally in Mg-rich phenocrysts), plagioclase (An_25–68), and less commonly augite, which is relatively aluminous in lavas of the Springsure volcanic centre. Very rare aluminous bronzite occurs in certain Q-normative lavas. Groundmass minerals comprise augite, olivine (Fa_33–77), feldspar (ranging from labradorite through to anorthoclase and sanidine), Fe-Ti oxides, and apatite. Within many of the Q-normative lavas, extensive development of subcalcic and pigeonitic pyroxenes occurs, and also relatively rarely orthopyroxene. Mineralogically, the ne- and ol-normative lavas, and some of the Q-normative lavas are indistinguishable, and in view of the gradations in chemistry, the term hawaiite has been extended to cover these lavas. The term tholeiitic andesite is used to describe the Q-normative lavas containing Ca-poor pyroxenes as groundmass phases.Megacrysts of aluminous augite, aluminous bronzite, olivine, ilmenite, and spinel sporadically occur within the lavas, and their compositions clearly indicate that they are not derived from the Upper Mantle. Rare lherzolite xenoliths are also found.The petrogenesis of these mafic lavas is approached by application of the thermodynamic equilibration technique of Carmichael et al. (1977), utilizing three parental mineral assemblages that could have been in equilibrium with the magmas at P and T. The models are: (a) standard upper mantle mineralogy; (b) an Fe-enriched upper mantle model (Wilkinson and Binns 1977); (c) lower crust mineralogy, based on analysed megacryst compositions. The calculations suggest that these mafic magmas were not in equilibrium with either mantle model prior to eruption, but show much closer approaches to equilibrium with the lower crust model. Calculated equilibration temperatures and pressures (for the lower crust model) range from 995°–l,391° C (average 1,192), and 7.2–16.3 kb (average 12.4). These results are interpreted in terms of a model of intrusion and magma fractionation within the crust-mantle interface region, with consequent crustal underplating and thickening beneath the Tertiary volcanic regions. Some support for the latter is provided by regional isostatic gravity anomalies and physiographic considerations.     

Geochemistry and petrogenesis of Early Carboniferous volcanic rocks in East Junggar,North Xinjiang: Implications for post-collisional magmatism and geodynamic process

Early Carboniferous volcanic rocks in the Batamayineishan Formation overlie unconformably the molasse deposits and the ophiolitic mélanges and are restricted in narrow zones along both sides of the Kalamaili orogenic belt in North Xinjiang, southern Central Asian Orogenic Belt. These rocks demonstrate the post-collisional setting in East Junggar commenced in Tournaisian and also mark an important transitional period from the final amalgamation to late Paleozoic voluminous juvenile granitoids in East Junggar. The volcanic rocks are composed of basalt, basaltic andesite, andesite, trachyte and rhyolite. Both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements, light rare earth elements and depletion in Nb and Ta, low initial ⁸⁷Sr/⁸⁶Sr and high, positive ɛ_Nd(t). Three groups of mafic rocks have been identified: Shoshonitic group 1 has the highest MgO, CaO, Ni and Cr and the lowest Na₂O, Al₂O₃, La, Ba, La/Yb and Ba/Th with primary magma features; group 2 calc-alkaline and high-K calc-alkaline mafic rocks have the lowest K₂O, P₂O₅, Th and Th/Nb, and the highest TiO₂; and group 3 (shoshonitic to potassic alkaline) has the highest K₂O, P₂O₅, La, Ba, La/Yb and Th/Nb, and the lowest TiO₂. The A-type-like felsic rocks were derived from the differentiation of the mafic magma. Geological and geochemical evidences indicate that the Batamayineishan Formation was generated from the process of slab breakoff (detachment). Group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet (50:50) lherzolite which has been enriched by overlying metasomatized lithosphere during ascent. Group 2 is derived from 5–10% partial melting of shallower spinel-bearing lithospheric mantle induced by the hot rising asthenosphere, where the contribution of slab-derived fluid is predominant. Low partial melting (3–5%) of the mantle wedge and/or thickened lithospheric mantle enriched by slab-derived components generates group 3. Slab breakoff as an important geodynamic process accounts for the post-collisional magmatism between 343.5 Ma–330 Ma, providing a model for post-collisional crust–mantle interaction in the CAOB.     

Sanukite（赞岐岩）的地球化学特征、成因及其地球动力学意义

赞岐岩 (サヌカイト ,sanukite)是指发现于日本四国北部的一种富Mg的火山岩 ,主要产于日本中新世 (11～14Ma)Setouchi火山岩带 ,是一种黑色玻璃质的火山岩。其化学成分以富Si质 (安山英安质 )、具很高的Mg# 值 (>0 .6 )、高的Cr、Ni丰度和K/Na值 (0 .33～ 0 .5 2 )为特征。赞岐岩的形成与菲律宾海板块年轻的热的岩石圈俯冲和四国盆地的张开有关 ,产于岛弧的弧前或弧后盆地环境。赞岐岩不只代表火山岩 ,也包括侵入岩。Shirey和Hanson(1984 )将该术语引入太古宙 ,将太古宙具上述赞岐岩特征 (Si过饱和、Mg# 高和Ni、Cr、LILE含量高 )的深成岩和火山岩称为sanukite岩套。赞岐岩与埃达克岩具有大体类似的地球化学特征 ,但前者更富Mg、Cr和Ni,表明赞岐岩可以直接由地幔岩部分熔融形成 ,而埃达克岩只能由玄武岩部分熔融形成。现代的赞岐岩很少见 ,而太古宙的赞岐岩比较常见 ,暗示太古宙导致板片熔融的消减的岩石圈本身或上地幔可能具有更高的温度。赞岐岩集中出现在晚太古代 (<3.0Ga) ,表明板块消减作用可能在 3.0Ga之后才成为一个重要的过程 ,晚太古代赞岐岩的出现可能标志着现代类型板块构造的开始     

Late Paleozoic anorogenic magmatism in Southern Mongolia: Evolutionary stages and structural control

Geochronological studies of anorogenic magmatic complexes in the South Mongolian Hercinide have been carried out. Series of the massifs composed of alkaline and subalkaline granitoids with some monzonite formed during the interval of 318–316 Ma have been found. Taking into account previously obtained data, two groups of Late Paleozoic A-type granitoids different in age have been identified in the region: Late Carboniferous and Early Permian. These two occupy different structural positions. The location the early magmatic massifs is controlled by the extended Trans–Altai fault zone of northwest strike: the massifs cluster in areas of the fault intersection with boundaries of structural blocks formed by the pull-apart mechanism. The later Early Permian igneous complexes are associated with the Gobi–Tienshan rift zone of sublatitudinal strike.     

Geochemistry and petrogenesis of Neoproterozoic A-type granites at Nakora in the Malani Igneous Suite, Western Rajasthan, India

The Nakora Ring Complex(NRC)(732 Ma) occurs as a part of Malani Igneous Suite(MIS) in the West-ern Rajasthan,India.This complex consists of three phases(volcanic,plutonic and dyke).Geochemically,the Na-kora granites are peralkaline,metaluminous and slightly peraluminous.They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content.The peralkaline granites show higher concentrations of SiO2,total alkalies,TiO2,MgO,Ni,Rb,Sr,Y,Zr,Th,U,La,Ce,Nd,Eu and Yb and lower concen-trations of Al2O3,total iron,Cu and Zn than metaluminous granites.AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites.Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths(16-28 km and 480-840℃) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites.The primitive mantle normalized multi-element profiles suggest that Nakora granites(peralkaline,metaluminous and peraluminous) are characterized by low La,Sr and Eu and relatively less minima of Ba,Nb and Ti which suggests the aspects related to crustal origin for Nakora magma.The Nakora granites are characterized as A-type granites(Whalen et al.,1987) and correspond to the field of "Within Plate Gran-ite"(Pearce et al.,1984).Geochemical,field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.     

太行山南段洪山矿化正长斑岩LA-ICP-MS锆石U-Pb年龄、Hf同位素组成及地球化学特征

太行山南段是我国东部重要的金属成矿区。本文首次报道了太行山南段洪山岩体的金属成矿作用,对含矿的正长斑岩开展了系统的年代学、岩石地球化学及Hf同位素的分析工作。详细的野外观察表明,洪山岩体内具有典型的斑岩型矿化特点,并厘定出含矿正长斑岩。LA-ICP-MS锆石U-Pb定年结果表明,洪山正长斑岩成岩年龄为130.45~131.4Ma,晚于洪山正长岩形成年龄(132~135Ma),成矿时代略晚于本区矽卡岩型铁矿的成矿年龄(133~137Ma),处在太行山地区中生代侵入岩活动高峰期内(120~140Ma),属于太行山地区后碰撞构造伸展阶段的产物。详细的岩石地球化学研究表明,洪山正长斑岩具有高硅(Si O_2=63.72%~67.63%)、高钾(K_2O/Na_2O=0.73~1.36)和富碱(K_2O+Na_2O=12.34%~12.73%)的特点,属于钾玄岩系列岩石。岩体轻稀土元素富集(LREE/HREE=9.32~12.43),不具铕异常(δEu=0.93~1.05),富集大离子亲石元素(Rb、Th、U、K)而亏损高场强元素(Nb、Ta、Ti)。2件样品24个测点的锆石εHf(t)值具有较大的变化范围(-24.4~-10.3),对应的地壳模式年龄集中于1.6~2.7Ga。综合分析表明,正长斑岩是"EMⅠ型"富集地幔部分熔融的产物,岩浆在上涌的过程中受到下地壳物质的混染,形成壳幔混源的富钾含矿岩浆,并最终导致洪山斑岩型Cu矿化的发生。