Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999     

Interaction between mafic and felsic magmas in subvolcanic environment (Tastau igneous complex,eastern Kazakhstan)

The paper reports the results of integrated geological, petrological, geochemical, and geochronological studies of the Tastau igneous ring complex in the Zaisan orogen of eastern Kazakhstan. Interaction between felsic and mafic magmas has been studied. Hybrid rocks are represented by gabbros and diorites injected into a granitic magma chamber. They occur as dikes and pillow-like and globular mafic bodies variously disintegrated and mixed with host granitoids. The age of synplutonic rocks is 242 ± 20 Ma (U/Pb zircon dating), which is, with regard to analytical error, substantially younger than it was presumed.Mechanisms of interaction between felsic and mafic magmas have been studied. They include mechanical (mingling) and chemical (mixing) interaction, which produce composite mixtures and hybrid rocks. The ratios of mafic to felsic components involved in the formation of intermediate rocks were calculated from major elements by regression analysis and tested with regard to rare and trace elements. The model for mingling includes rapid quenching of the mafic melt when it is injected into the granitic magma chamber, decomposition of crystalline fragments, dispersion of fragments and crystals in the magma chamber under conditions of rapid turbulent flow, and enrichment of felsic magma with femic components to produce monzonitic magmas.     

Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany)

The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern unit is relatively mafic (SiO₂∼64%) and lacks cordierite. The more felsic central and southern units (SiO₂=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains. Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO₂=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma. Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only primary mafic phase (SiO₂=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams, samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high Mg#, low Na₂O, ɛ_Nd=−1.2 to −5.1, ⁸⁷Sr/⁸⁶Sr=0.7055 to 0.7080, δ¹⁸O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛ_Nd=−4.5 to −6.8, ⁸⁷Sr/⁸⁶Sr=0.7071 to 0.7115, δ¹⁸O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of Al₂O₃/(MgO+FeO_tot) and K₂O/Na₂O, in combination with low ratios of CaO/(MgO+FeO_tot), variable values of Sr/Nd, Eu/Eu*[=Eu_cn/(Sm_cn × Gd_cn)^0.5] and (Tb/Yb)_cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic enclaves (ɛ_Nd=−4.6 to −5.2; ⁸⁷Sr/⁸⁶Sr=0.7060 to 0.7073; δ¹⁸O ∼8.1‰) are similar to those of the low ɛ_Nd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial ⁸⁷Sr/⁸⁶Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high ⁸⁷Sr/⁸⁶Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones. Received: 30 April 1999 / Accepted: 6 August 1999     

Progressive hybridisation between Late Devonian mafic-intermediate and felsic magmas in the Meguma Zone of Nova Scotia, Canada

Two felsic plutons of Late Devonian (385–370 Ma) age in the Meguma Zone of southwestern Nova Scotia contain three circa 376 Ma synplutonic mafic-intermediate intrusions that collectively record progressive stages of in situ hybridisation. A 5 m wide spessartite dyke in the Port Mouton Pluton probably underwent rapid cooling and crystallisation immediately after intrusion, which heated and coarsened the adjacent tonalite. An 85 m long sheet of pillowed kersantite (also in tonalite of the Port Mouton Pluton) presumably contained residual magma after quenching and obtained K, P, Ba, Rb, more radiogenic Sr, Zr, Nb, and light REE from the tonalite during magma mingling. The third synplutonic body, a >100 m wide diorite sheet, intrudes granodiorite of the adjacent Shelburne Pluton and has a circa 45 m wide gradational contact of metaluminous hornblende-tonalite. This tonalite dominantly records magma mixing by the transfer of Ti, Mg, Fe, Ca, and V in hornblende, biotite, plagioclase, and (at least in part) apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite, probably in response to convection. Scattered data points, unusual “concave-down” variation trends for Al₂O₃, P₂O₅, and Sr, and non-hyperbolic Sr-Nd isotopic characteristics in the tonalite, apparently reflect syn- or post-mixing fractionation and accumulation of xenocrysts from residual magma. Phosphorus may have assisted diffusion of Sr, Zr, Nb, and light REE, and caused premature quenching of the hybrids at Mcleods Cove and Birchtown, during magma mingling and mixing. Received: 1 January 1996 / Accepted: 3 August 1996     

Petrogenesis of Middle-Eocene granitoids and their Mafic microgranular enclaves in central Urmia-Dokhtar Magmatic Arc (Iran): Evidence for interaction between felsic and mafic magmas

Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0 ± 0.7 Ma for the granodiorite phase. Mafic microgranular enclaves(MMEs) are common in these plutons, and have relatively low SiO_2 contents(53.04-57.08 wt.%) and high Mg#(42.6-60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous(A/CNK = 0.69-1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO_2 contents(59.7-66.77 wt.%) and lower Mg#(38.6-52.2); they are considered a product of partial melting of the mafic lower crust. Chondritenormalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies(Eu/Eu* = 0.60-0.93). The host granodiorite samples yield(⁸⁷Sr/⁸⁶Sr);ratios ranging from 0.70498 to 0.70591,positive eNd(t) values varying from +0.21 to +2.3, and TDM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves(⁸⁷Sr/⁸⁶Sr)i = 0.705111-0.705113, ε_Nd(t)= +2.14 to +2.16, T_DM2 = 697-785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantlederived mafic magmas in an active continental margin arc environment.     

Interaction of coeval felsic and mafic magmas from the Kanker granite,Pithora region,Bastar Craton,Central India

Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.     

Petrogenesis of syenites at a rifted continental margin: origin,contamination and interaction of alkaline mafic and felsic magmas in the Astrophyllite Bay Complex,East Greenland

The Astrophyllite Bay Complex in East Greenland (part of the Palaeogene North Atlantic Igneous Province) consists of an alkaline diorite plug, with detached trachyandesitic pillows, surrounded by co-magmatic syenite that was emplaced into Archaean basement. The diorite intrusion has yielded a 47.11 ± 0.68 Ma Rb-Sr isochron age. Saw-cut profiles through pillow-syenite-gneiss sections have been taken to resolve close spatial elemental and isotopic (Sr-Nd-Hf-Pb-O) variations. The diorite and syenite formed from alkaline basaltic, mantle-derived, melts with complex histories of prolonged assimilation and fractional crystallisation. Each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the Kangerlussuaq area. The diorite is dominated by lower crustal, granulite facies contamination, whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination, indicating stalling and fractionation of magmas at different levels within the crust. The syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at Astrophyllite Bay giving rise to superimposed local contamination trends between pillow/syenite and syenite/gneiss, respectively.     

Multiple cosmogenic nuclides document complex Pleistocene exposure history of glacial drifts in Terra Nova Bay (northern Victoria Land, Antarctica)

Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (¹⁰Be, ²⁶Al, and ²¹Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 Ma (pre-dating the build-up of the Mt. Melbourne volcanic field). Since then, summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm/Ma. Complex older drifts rest on deglaciated areas above the younger late-Pleistocene glacial drift and below the previously overridden summits. The combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion. The areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers. The exposure ages of the older drift(s) indicate multiple Pleistocene glacial cycles, which did not significantly modify the pre-existing landscape.     

Magma mixing in the Sithonia Plutonic Complex, Greece: evidence from mafic microgranular enclaves

Summary ¶Mafic microgranular enclaves occur in most calc-alkaline granitoids, and it is widely accepted that they represent the remnants of basic magmas that interacted with more acid magmas. In this work we present new data on mafic microgranular enclaves occurring in the granodiorites of the Sithonia Plutonic Complex (Northern Greece). Enclave properties have been studied using different methods. Quantitative textural analysis has been carried out in order to decipher the crystallization history of enclaves once they have been entrained in the more acid and cooler host magma. In particular, the nucleation density (C), the mode (M) and the crystal index (n) of enclaves has been measured. Along with textural analysis, the size of enclaves has also been estimated using a method that, based on two-dimensional sections of enclaves, allows the estimation of volume of enclaves. Geochemical analyses have been performed to investigate the degree of chemical interaction that enclaves suffered from the host acid magma. The different data sets have been utilized to furnish a general evolutionary model of the magmatic interaction process between the basic and the acid magmas that led to the formation of the granodioritic host rock and related mafic microgranular enclaves. It is concluded that, as the magmatic interaction process proceeded, the crystallization of enclaves involved the nucleation of apatite and epidote (first stage of crystallization) followed by biotite, ± hornblende, plagioclase, and titanite (second stage of crystallization); the last minerals that nucleate were quartz and K-feldspar. During crystallization enclaves underwent contamination by the host acid magma through flow channels opened during the transfer of mineral phases from the host magma to the enclaves. When the two magmas attained similar rheological behaviour a two-end member mixing process was favoured inducing progressively more vigorous mixing dynamics. Volumetric analysis of enclaves indicates that the smaller ones suffered a more intense geochemical interaction compared to the larger ones. We interpret this evidence as being strictly related to the kinematics of the mixing process, the latter governed by chaotic dynamics. Enclaves are viewed as portions of the basic magma that did not mix completely with the acid host magma and survived the mixing process. Host rocks are considered as volumes of the magmatic system where the more efficient mixing dynamics produced different, generally higher, degrees of hybridisation.Received May 22, 2002; revised version accepted November 5, 2002 Published online February 24, 2003     

大陆碰撞造山带榴辉岩相的熔体活动:来自南阿尔金基性麻粒岩中长英质脉体的证据

本文通过对南阿尔金巴什瓦克基性麻粒岩中长英质脉体的岩相学、锆石U-Pb年代学、Lu-Hf同位素及全岩主微量地球化学的综合研究,首次限定了该区基性麻粒岩中长英质脉体的形成时代为491±2 Ma(MSWD=0.91),此年龄与寄主麻粒岩高压—超高温阶段(榴辉岩相)的变质时代在误差范围内近一致,表明长英质脉体形成于榴辉岩相的...     

The age of young intrusions of Tsana Complex (Greater Caucasus) and isotope-geochemical evidence for their origin from hybrid magmas

This paper presents isotope-geochronological and petrological study of granitoids of the potentially ore-bearing (Au–As–Sb–Sn–Mo) Early Pliocene Tsana Complex, which are confined to the Main Caucasus fault zone (upthrow fault) in the central part of the Greater Caucasus Range. The Tsurungal and Karobi groups of magmatic bodies are distinguished based on spatial criterion. The Tsurungal group includes three small granite—granodiorite massifs (Tsurungal, Chorokhi, and Toteldash) and numerous acid and intermediate dikes in the upper reaches of the Tskhenistsqali River (Kvemo Svaneti, Georgia). The Karobi group comprises three subvolcanic rhyodacite bodies located in the upper reaches of the Chashuri River (Zemo Racha, Georgia) and numerous N–S-trending trachyandesite dikes near the axial zone of the Main Caucasus Range. The K-Ar and Rb-Sr isotope dating shows that the granitoid massifs and dike bodies of the Tsana Complex were formed in two different-age pulses of the Pliocene magmatism: phase I at 4.80 ± 0.15 and phase II at 4.15 ± 0.10 Ma. All hypabyssal rocks of the Karobi group, unlike those of the Tsurungal Group, were formed during the first pulse. Petrographic studies in combination with geochemical data indicate that most of the granitoids of the Tsana Complex are hybrid rocks (I-type post-collisional granites) and were derived through mixing of deep-seated mantle magmas with acid melts obtained by the upper crustal anatectic melting in the Main Caucasus fault zone. The granitoids of the Tsurungal Group define basic to acid evolution (diorite–granodiorite–granite–two-mica granite) possibly caused by both crystallization differentiation and increasing role of crustal contamination in the petrogenesis of the parental magmas of these rocks. This conclusion is also confirmed by the differences in the Sr isotope composition between granitoids of the early (⁸⁷Sr/⁸⁶Sr = 0.7053) and late (⁸⁷Sr/⁸⁶Sr = 0.7071) phases of the Tsana Complex. Main trends in spatiotemporal migration of magmatic activity in the central part of the Greater Caucasus in the Pliocene–Quaternary time were established using obtained and earlier published isotope-geochronological data.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

Petrochemical evidence of magma mingling and mixing in the Tertiary monzogabbroic stocks around the Bafra (Samsun) area in Turkey: implications of coeval mafic and felsic magma interactions

Miocene aged calc-alkaline mafic host stocks (monzogabbro) and felsic microgranular enclaves (monzosyenite) around the Bafra (Samsun) area within Tertiary volcanic and sedimentary units of the Eastern Pontides, Northeast Turkey are described for the first time in this paper. The felsic enclaves are medium to fine grained, and occur in various shapes such as, elongated, spherical to ellipsoidal, flame and/or rounded. Most enclaves show sharp and gradational contacts with the host monzogabbro, and also show distinct chilled margins in the small enclaves, indicating rapid cooling. In the host rocks, disequilibrium textures indicating mingling or mixing of coeval mafic and felsic magmas are common, such as, poikilitic and antirapakivi textures in feldspar phenocrysts, sieve textured-patchy-rounded and corroded plagioclases, clinopyroxene megacrysts mantled by bladed biotites, clinopyroxene rimmed by green hornblendes, dissolution in clinopyroxene, bladed biotite, and acicular apatite. The petrographical and geochemical contrasts between the felsic enclaves and host monzogabbros may partly be due to a consequence of extended interaction between coeval felsic and mafic magmas by mixing/mingling and diffusion. Whole-rock and Sr-Nd isotopic data suggests that the mafic host rocks and felsic enclaves are products of modified mantle-derived magmas. Moreover, the felsic magma was at near liquidus conditions when injected into the mafic host magma, and that the mafic intrusion reflects a hybrid product formed due to the mingling and partial (incomplete) mixing of these two magmas.     

Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel,FRG)

The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle. Most cores are greenish pleochroic and moderately resorbed (fassaitic augite). Some are pale green and strongly resorbed (acmitic augite). Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare. The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements. The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr. A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route. It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites. The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized. Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975).     

Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, cross-cutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.     

Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite

The solubility of fluorapatite in 17 silica-rich melts in the system Na₂O-K₂O-Al₂O₃-SiO₂ (with and without CaO or CaF₂) was determined at 1 kbar water pressure and 750 900°C. Apatite saturation occurs at levels of dissolved P₂O₅ ranging between 0.04 (± 0.02) and 0.28 (± 0.13) wt%. with only 4 values outside the 0.09–0.20 wt% range.The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that, under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P₂O₅ (the mean of the experimental values) a source containing as little as 0.05% P₂O₅ must be 35% melted before apatite is lost from the residue and no longer buffers the melt P₂O₅ concentration at the saturation value. Higher abundances of P₂O₅ in the source postpone the loss of residual apatite to still higher degrees of melting, and if the source P₂O₅ content exceeds 0.14 wt%, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds.The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. Fractional crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.     

Role of fluids in migmatites: CO2-H2O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and x_H2Oc. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO₂-H₂O fluid inclusions occur only in graphite-free leucosomes; (2) CO₂–CH₄± H₂O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO₂–N₂–CH₄ fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO₂–H₂O mixtures (41% CO₂, 58% H₂O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–a_H2O conditions of migmatization (c. 750° C, c. 4 kbar, x_H2Oc. 0.5). CO₂-H₂O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO₂–CH₄ fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H₂O = Grt + Crd + Kfs + L + CO₂+ CH₄. We conclude that an externally derived oxidizing CO₂-H₂O fluid was present in the middle crust and initiated anatexis. High-density CO₂-rich fluid with traces of CH₄ characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO₂–N₂–CH₄ fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.     

Monazite solubility and dissolution kinetics: implications for the thorium and light rare earth chemistry of felsic magmas

A series of monazite dissolution experiments was conducted in a hydrous (1–6 wt.%) granitic melt at 8 kbar over the temperature range 1,000–1,400° C. A polished cube of monazite was immersed in a natural obsidian melt and allowed to partially dissolve. Electron microprobe traverses perpendicular to the crystal-melt interface revealed concentration gradients in the LREEs and P. Diffusivities of the LREEs and P were calculated from these profiles, yielding the following Arrhenius relations for the LREEs: D=0.23 exp(–60.1 kcal mol^–1/RT) at 6% water D=2.30×10⁷ exp(–122.1 kcal mol^–1/RT) at 1% water These results demonstrate the importance of dissolved water on REE diffusion. Phosphorus diffusivities are nearly identical to those of the rare-earths, suggesting that P diffusion charge-compensates REE diffusion. The concentration of LREEs required for monazite saturation in these melts is given by the level of dissolved LREEs at the crystal-melt interface. These values also show a dependence on dissolved water, with LREE_sat=60 ppm at 6% H₂O when extrapolated down to 700° C, and LREE_sat=30 ppm at 1% H₂O. Calculated dissolution rates based on the above parameters indicate that minute (<30 m diameter) monazite crystals will be readily digested by an enclosing anatectic magma under reasonable geologic conditions (i.e., T=700–800° C and >2% H₂O), whereas larger (> 50 m) crystals will likely be residual over the duration of an anatectic event. The low solubility of monazite in this melt suggests that the LREE depletion observed in some felsic differentiation suites may be the result of monazite crystallization. Limited experimental and geochemical/petrologic evidence indicates that compositional changes in the melt accompanying differentiation decrease the solubility of monazite drastically. Kinetic and chemical constraints may also lead to localized monazite saturation and inclusion in major phases or even other accessories. Variations in the REE composition of monazite from different parageneses probably reflects the REE pattern of the parent melt, and may be due to gradational differences in the stability of individual or subgroup REE-complexes as a function of melt composition. Particularly important in this regard seems to be the lime+alkali/alumina balance of the melt and its volatile content.     

Microstructural evidence for crystallization regimes in mafic intrusions: a case study from the Little Minch Sill Complex,Scotland

The magma forming the 20 m thick crinanitic/picrodoleritic Dun Raisburgh sill, part of the Little Minch Sill Complex of NW Scotland, comprised a mafic carrier liquid with a crystal cargo of plagioclase and olivine (1 vol%). The olivine component of the cargo settled on the floor of the intrusion while the more buoyant plagioclase component remained suspended during solidification, resulting in a relatively high plagioclase content in the centre of the sill. The settled olivine grains form a lower fining-upwards sequence overlain by a poorly sorted accumulation formed of grains that grew within the convecting magma. The accumulation of olivine on the sill floor occurred over 5–10 weeks, synchronous with the upwards-propagation of a solidification front comprising a porous (~?70 vol% interstitial liquid) plagioclase-rich crystal mush.