Preliminary geochemical study of volcanic rocks in the Pang Mayao area, Phrao, Chiang Mai, northern Thailand: tectonic setting of formation

The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai–Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin.     

俄罗斯贝加尔裂谷带中呼兰霍博克火山岩岩石学及地球化学研究

位于贝加尔裂谷带通京盆地中的呼兰霍博克火山火山锥由火山弹、火山灰等火山碎屑岩和基性熔岩(橄榄玄武岩)组成.橄榄玄武岩中橄榄石可分为具有较高Mg#值的捕虏晶和Mg#值相对较低的斑晶.部分斜长石斑晶具有核.幔.边结构,且幔部发生减压分解,一些单斜辉石晶体(俘虏晶)边部发生了减压分解.根据岩石的化学成分,该玄武岩属于橄榄粗玄岩系列,轻稀土强烈富集,重稀土相对亏损,轻重稀土之间分异较大,具有与OIB相似的微量元素和同位素地球化学特征.岩石学和元素地球化学研究表明,该橄榄玄武岩的源区和岩浆的形成可能与地幔柱活动有关;岩浆演化经历了压力骤减的过程,在岩浆快速上升过程中,深部形成的矿物(可能是地幔矿物的俘虏晶)减压分解.快速上升的岩浆几乎未受大陆地壳的混染,仅捕获了少量流纹质熔体.     

Commingling and mixing of S-type peraluminous, ultrapotassic and basaltic magmas in the Cayconi volcanic field, Cordillera de Carabaya, SE Peru

The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K₂O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.     

Petrology of the mafic sill of Narshingpur-Lakhnadon section,Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.     

The significance of corona textured inclusions from a high pressure fractionated alkalic lava: North Otago, New Zealand

Rare gabbroic inclusions within a lherzolite-nodule bearing, fractionated, alkalic lava are of two types: olivine-two pyroxene-spinel-metagabbro and amphibole-two pryoxene-spinel-metagabbro. The metagabbros represent cumulates which have crystallized from alkalic basalt magma at high temperature. Metamorphic aggregates and coronas consisting of clinopyroxene-orthopyroxene and spinel with or without amphibole are attributed to complex subsolidus reactions between olivine and plagioclase; olivine, clinopyroxene and plagioclase; olivine and clinopyroxene; olivine, clinopyroxene, plagioclase and ilmenite in response to decreasing temperature as the rocks cooled at pressures of around 11 Kb (35–40 km) and temperatures in the range 1000–1150°C. The lower crust and upper mantle below East Otago must contain bodies of fractionated alkalic basalt showing granulitic mineralogy.     

Post-Collisional Miocene Alkaline Volcanism in the Oglakçi Region,Turkey: Petrology and Geochemistry

Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwestern Central Anatolia, Turkey, is represented by basaltic and trachytic groups of rocks. Samples of both groups have been investigated using mineral-chemical data together with whole-rock major-, trace-element, and radiogenic Sr-Nd isotopic data. The basaltic volcanic rocks consist of mugearites and shoshonites, whereas the trachytic rocks include trachytes, latite, and rhyolite. Both groups are of alkaline character. The basaltic rocks contain plagioclase (An_29-63), alkali feldspar (Or_12-74), olivine, orthopyroxene (En_64-67), clinopyroxene (Wo_43-48), biotite (Mg#_82-88), and Fe-Ti oxide phenocrysts, whereas the trachytic rocks contain plagioclase (An_21-64), alkali feldspar (Or_10-53), clinopyroxene (Wo_41-49), amphibole (Mg#_64-83), biotite (Mg#_79-85), Fe-Ti oxide, titanite, apatite, and quartz phenocrysts. The measured ⁸⁷Sr/⁸⁶Sr ratios of basaltic samples range from 0.7045 to 0.7048, and those of trachytic samples from 0.7054 to 0.7056. The basaltic samples have ¹⁴³Nd/¹⁴⁴Nd ratios ranging from 0.512753 to 0.512737, and those of trachytic samples are 0.512713 to 0.512674. Isotopic, major-, and trace-element data suggest that the Oglakci volcanic rocks are products of postcollisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization processes following the demise of Neotethys. Trace-element characters also are consistent with an OIB-like mantle source. These volcanic rocks probably were associated with extensional tectonics, which occurred within the Anatolian plate as a result of collision of the Eurasian and Afro-Arabian plates during the neotectonic evolution of Turkey.     

云南哀牢山蛇绿岩的矿物学研究

云南哀牢山蛇绿岩由变橄榄岩、堆晶杂岩、火山熔岩和硅质岩等四个单元组成,其主要矿物有橄榄石,斜方辉石、单斜辉石、尖晶石、斜长石、角闪石、石榴子石等,这些矿物均已不同程度地遭到蚀变、橄榄石、斜方辉石的化学成分显示蛇绿岩中的二辉橄榄岩为原始地幔岩;是石的化学特征表明蛇绿岩中的橄榄岩为深海橄榄岩;单斜辉石的成分反映二辉橄榄岩经历过熔融作用,堆晶杂央才基性熔岩具有火山弧和洋底玄武岩的特征。     

Evolution of a small intraplate basaltic lava field: Jerrabattgulla Creek,upper Shoalhaven River catchment,southeast New South Wales

The Jerrabattgulla Creek basalts are in the upper catchment of the Shoalhaven River of southeastern New South Wales. The basalts erupted into a narrow, north-draining valley and modified the local drainage system, re-routing the paleo-Jerrabattgulla Creek, preserving a series of sub-basaltic quartzose gravels with silcretes in the paleovalley. The paleovalley indicates that a north-flowing drainage existed in this place in the Miocene. The high-relief, narrow valley has preserved a volcanic stratigraphy allowing the magmatic evolution of this small lava field to be determined. The lavas have a large compositional range from olivine nephelinite through to quartz tholeiite, which is unusual in such a small lava field. They represent three distinct magma batches, most likely from an amphibole–apatite metasomatised sub-continental lithospheric mantle, and underwent fractional crystallisation of olivine, clinopyroxene and plagioclase and assimilated upper crust. The lava field underwent temporal change from dominantly alkaline, to mixed alkaline and subalkaline, to dominantly alkaline magmatism over the course of its evolution.     

Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.     

鲁西青山组火山岩形成的构造背景及其成因探讨:主元素和微量元素证据

研究区内火山岩从基性—中性—中酸性都有出露，包括橄榄玄武岩、安山岩和英安岩，且都属于非碱性系列。通过对主元素和微量元素的研究，认为本区火山岩为滞后型弧(陆缘弧)火山作用的产物。源区由于存在大量的因俯冲作用进入地幔的陆壳物质以及流体的交代作用，从而出现富含金云母和不相容元素的交代富集型地幔源，并具有壳源的元素组成特征。火山岩的形成是富集地幔部分熔融的结果，但在成岩过程中可能存在单斜辉石、斜长石、橄榄石和Ti—Fe氧化物等矿物的分离结晶作用，以及橄榄石的堆晶作用。     

Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif)

Corona and inclusion textures of a metatroctolite at the contact between felsic granulite and migmatites of the Gföhl Unit from the Moldanubian Zone provide evidence of the magmatic and metamorphic evolution of the rocks. Numerous diopside inclusions (1–10 μm, maximum 20 μm in size) in plagioclase of anorthite composition represent primary magmatic textures. Triple junctions between the plagioclase grains in the matrix are occupied by amphibole, probably pseudomorphs after clinopyroxene. The coronae consist of a core of orthopyroxene, with two or three zones (layers); the innermost is characterized by calcic amphibole with minor spinel and relicts of clinopyroxene, the next zone consists of symplectite of amphibole with spinel, sapphirine and accessory corundum, and the outermost is formed by garnet and amphibole with relicts of spinel. The orthopyroxene forms a monomineralic aggregate that may contain a cluster of serpentine in the core, suggesting its formation after olivine. Based on mineral textures and thermobarometric calculations, the troctolite crystallized in the middle to lower crust and the coronae were formed during three different metamorphic stages. The first stage relates to a subsolidus reaction between olivine and anorthite to form orthopyroxene. The second stage involving amphibole formation suggests the presence of a fluid that resulted in the replacement of igneous orthopyroxene and governed the reaction orthopyroxene + anorthite = amphibole + spinel. The last stage of corona formation with amphibole + spinel + sapphirine indicates granulite facies conditions. Garnet enclosing spinel, and its occurrence along the rim of the coronae in contact with anorthite, suggests that its formation occurred either during cooling or both cooling and compression but still at granulite facies conditions. The zircon U–Pb data indicate Variscan ages for both the troctolite crystallization (c. 360 Ma) and corona formation during granulite facies metamorphism (c. 340 Ma) in the Gföhl Unit. The intrusion of troctolite and other Variscan mafic and ultramafic rocks is interpreted as a potential heat source for amphibolite–granulite facies metamorphism that led to partial re‐equilibration of earlier high‐ to ultrahigh‐P metamorphic rocks in the Moldanubian Zone. These petrological and geochronological data constrain the formation of HP–UHP rocks and arc‐related plutonic complex to westward subduction of the Moldanubian plate during the Variscan orogeny. After exhumation to lower and/or middle crust, the HP–UHP rocks underwent heating due to intrusion of mafic and ultramafic magma that was generated by slab breakoff and mantle upwelling.     

Coronas from the Thessaloniki gabbros (North Greece)

Coronas which have been developed between olivine and plagioclase in Precambrian gabbroic rocks from Thessaloniki, Greece, have been studied. These consist commonly of (olivine), clinopyroxene, amphibole (plagioclase) and rarely of (olivine) orthopyroxene, clinopyroxene amphibole (plagioclase) assemblages. The results of the electron-probe microanalysis of the above constituent minerals are presented. The formation of these coronas by regional or thermal metamorphism is unlikely, but an origin by a two-way diffusion of material across the olivine and plagioclase interface is proposed.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Petrology of mafic-ultramafic rocks along North Puruliya Shear zone,West Bengal

The North Puruliya Shear zone (NPSZ) is characterized by occurrence of mafic-ultramafic rocks aligned parallel to the shear zone, intruding the high grade Proterozoic rocks of Chhotanagpur Gneissic Complex. The ultramafic rocks occur as small lenses, pockets, veins, thin dykes and are intimately associated with mafic (gabbro, norite) rocks. Pyroxenites (viz. olivine websterite, websterite, plagioclase websterite) and hornblendite are the two important members of the ultramafic rocks containing clinopyroxene, orthopyroxene, olivine, plagioclase, amphibole, phlogopite and ilmenite. The mafic-ultramafic rocks show evidence of shearing and retrogressive metamorphism. Linear correlation of chemical attributes suggests fractionation-controlled magmatic differentiation. Enrichment of LILE and LREE in the mafic-ultramafic suite suggests an enriched mantle source and pronounced negative Eu-anomalies in all the rock types except hornblendite suggest fractionation of plagioclase under low f_O2 condition. Progressive iron enrichment trend in rocks of the mafic-ultramafic suite also indicate magmatic differentiation under low f_O2 condition. Early fractionation and accumulation of clinopyroxene and plagioclase from a basaltic magma may have given rise to the ultramafic rocks of the area. Little change in the Nb/Zr and Ce/Zr ratios of ultramafic and mafic rocks (except alkali norite) strongly support low crustal contamination. A few samples of norite and gabbro-norites appeared to be variably contaminated by a crustal component or affected by late granitic intrusion resulting in enrichment of alkali in the former.     

松辽盆地东南隆起区营城组珍珠岩斑晶矿物研究

松辽盆地早白垩世营城组火山岩系含有丰富的珍珠岩,为松辽盆地重要的油气勘探目的层位。在松辽盆地边缘东南隆起九台地区出露有比较新鲜的珍珠岩,内部斑晶保存完好,裂隙发育;斑晶矿物主要有斜长石、黑云母、角闪石,还有镁铁质的单斜辉石和斜方辉石。对含斑晶珍珠岩和剔除斑晶后的珍珠岩玻璃质分别进行地球化学分析,斑晶矿物进行单矿物电子探针成分研究。其结果表明：该珍珠岩属于酸性高钾钙-碱性系列,微量和稀土元素特征显示与上部地壳相似;斑晶矿物单斜辉石为斜紫苏辉石和次透辉石,斜方辉石为紫苏辉石,均显示变质成因,属于捕虏晶;与珍珠岩相邻层位玄武岩的斑晶斜长石为拉长石,单斜辉石为铁次透辉石,属于岩浆成因。这些特征表明,珍珠岩原岩可能来自于上部地壳,其辉石等捕虏晶为岩浆上侵运移过程与围岩发生混染作用的结果。     

The investigation of petrology and geodynamic mafic magmatism in Bijar area, west of Iran (Arabia- Eurasia collision zone)

In northeastern Sanandaj-Sirjan structural zone, the Takab-Ghorveh belt comprises a volcanic province which related to the collision between the Eurasian and Arabian continents. It contains almost Quaternary andesitic basalt to alkali basalt. These alkali basaltes show Strombolian type eruptions. The volcanic rocks in Bijar area represent a range of mafic magmas, re-vealed by mingling and mixing textures. A variety of features suggest that the lava flows before eruption from magma chambers, contaminated by continental crust.     

An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.     

The genesis of the Archaean Welcome Well volcanic complex,Western Australia

The Welcome Well volcanic complex east of Leonora, Western Australia, is interpreted to be the eroded remnant of an Archaean stratovolcano. Andesitic flows and intercalated mudflow deposits comprising the volcanic centre give way to coarse, poorly-sorted lithic wackes that were deposited in alluvial fans skirting the lower slopes or base of the subaerial volcanic edifice. These deposits are succeeded both laterally and vertically by fine-grained, subaqueous, turbiditic sediments that are intercalated with pillowed, tholeiitic basalts.There is a complete petrographic and geochemical gradation from porphyritic basalt through porphyritic andesite to porphyritic dacite. In general, the rocks show calc-alkaline patterns of elemental behaviour, consistent with fractionation of variable proportions of the modal minerals amphibole, plagioclase, clinopyroxene and Ti-magnetite. Among these minerals, amphibole appears to have assumed a major role in producing the geochemical characteristics of the high-Si andesites and dacites as evidenced by the behaviour of Zr, Nb, Y and REE. In order to account for the geochemical variability of the basalts and low-Si andesites, it is proposed that they differentiated from primitive basic parents which had a range of major and LIL element contents. The most plausible origin for the primary magmas involves shallow, hydrous melting of a LIL element-enriched mantle source over a significant pressure range.     

On the origin of multi‐layer coronas between olivine and plagioclase at the gabbro–granulite transition,Valle Fértil–La Huerta Ranges,San Juan Province,Argentina

Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H₂O, Na₂O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.     

Ocellar lamprophyre dyke bearing mineralization,Wadi Nugrus,Eastern Desert,Egypt: Geology,mineralogy and geochemical implications

The ocellar lamprophyre dyke (ENE-WSW) is recorded at Wadi Nugrus, Eastern Desert, Egypt. It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length. The lamprophyre dyke has been altered, and it is characterized by porphyritic and panidiomorphic textures with plagioclase, olivine, and augite constituting the porphyritic phase in a fine groundmass of the same composition. Rutile, titanite, apatite, fluorite, graphite, calcite, allanite, autunite and Fe-Ti oxides are accessory minerals. Kaolinite, chlorite and epidote are secondary minerals. Carbonitization and hematitization are common. Rounded to sub-rounded porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and represent physical traps for mineralization. The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids.