evidence for multiple metamorphic events in the adirondack mountains n y field evidence consisting of 1 rotated foliated xenoliths 2 country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one or more early dynamothermal event s of regional scale and high grade early metamorphism resulted in pronounced linear and planar fabric throughout the adirondacks and preceded the emplacement of the anorthosite mangerite charnockite granite alaskite amca suite which contains xenoliths of the metamorphosed rocks olivine metagabbros believed to be approximately contemporaneous with the amca suite also crosscut and contain xenoliths of strongly foliated metasediments these intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages subsequently this already complex framework underwent three phases of folding including an early recumbent isoclinical event and was metamorphosed to granulite facies p t conditions the age of the early metamor

calcic garnets as a geochronological and petrogenetic tool applicable to a wide variety of rocks abstract this contribution presents u pb geochronological data for ca fe ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from neoarchean to permian triassic cinder lake and eden lake in manitoba canada belaya zima and odikhincha in siberia russia afrikanda in the kola peninsula russia obtained using isotope dilution thermal ionization mass spectrometry id tims and their trace element compositions measured by laser ablation inductively coupled plasma mass spectrometry la icpms the studied garnets yield u pb concordant or subconcordant ages obtained with a precision of

debris flows as a factor of hillslope evolution controlled by a continuous or a pulse process flood effectiveness observations imply that two families of processes describe the formation of debris flow volume one is related to the rainfall erosion relationship and can be seen as a gradual process and one is related to additional geological geotechnical events those named hereafter extraordinary events in order to discuss the hypothesis of coexistence of two modes of volume formation some methodologies are applied firstly classical approaches con sisting in relating volume to catchments characteristics are considered these approaches raise questions about the quality of the data rather than providing answers concerning the controlling processes secondly we consider statistical approaches cumulative number of events distribution and cluster analysis and these suggest the possibility of having two distinct families of processes however the quantitative evaluation of the threshold differs from the one that could be obtained from the first approach but they all agree in the sense of the coexistence of two families of events thirdly a conceptual model is built exploring how and why debris flow volume in alpine catch ments changes with time depending on the initial condition sediment production the model shows that large debris flows i e with important volume are observed in the beginning period before a steady state is reached during this second period debris flow volume such as is observed in the beginning period is not observed again integrating the results of the three approaches two case studies are presented showing 1 the possibility to observe in a catchment large volumes that will never happen again due to a drastic decrease in the sediment availability sup porting its difference from gradual erosion processes 2 that following a rejuvenation of the sediment storage by a rock avalanche the magnitude frequency relationship of a torrent can be differentiated into two phases the beginning one with large and frequent debris flow and a later one with debris flow less intense and frequent supporting the results of the conceptual model although the results obtained cannot identify a clear threshold between the two families of processes they show that some debris flows can be seen as pulse of sediment differing from that expected from gradual erosion

oxygen isotope evidence for shallow emplacement of adirondack anorthosite oxygen isotopic analysis of wollastonites from the willsboro mine adirondack mountains new york reveals a 400 ft wide zone of 18o depletion at anorthosite contacts values of 18o vary more sharply with distance and are lower to 1 3 than any yet reported for a granulite fades terrain exchange with circulating hot meteoric water best explains these results and implies that the anorthosite was emplaced at relatively shallow depths 10 km in marked contrast to the depth of granulite fades metamorphism 23 km these 18o depletions offer the first strong evidence for shallow emplacement of anorthosite within the grenville province and suggest that regional metamorphism was a later and tectonically distinct event

ti rich hydroandradites from the sanbagawa metamorphic rocks of the shibukawa area central japan ti rich hydroandradite often occurs though usually as a minor constituent in serpentinized ultramafic rocks and associated gabbroic rocks of the sanbagawa metamorphic belt the chemistry of the host rocks is commonly characterized by undersaturation with sio2

origin and age of adirondack anorthosites re evaluated with nd isotopes we report the sm nd systematics in seven whole rock anorthosites in plagioclase and garnet mineral separates from one anorthosite and in five mangerite norite charnockite rocks from the snowy mountain dome in the south central adirondacks the plagioclase two garnet separates and the whole rock anorthosite define a four point internal isochron of age 1 098 7 2 m y the six whole rock anorthosites and the anorthositic gabbro in conjunction with the above internal isochron define a similar age of 1 095 7 2 m y and an initial 143nd 144nd ratio of 0 51135 1 2 this age of 1 100 m y is considered to be the time of crystallization of the anorthositic rocks the initial nd isotope ratio of the combined whole rock and mineral isochron indicates an nd value of 2 5 which suggests a light rare earth element depleted source for the anorthositic parent magma in the upper mantle the trace element and isotopic data also support the view that gabbroic anorthosite is the parent magma for the anorthosites five whole rock samples of norite mangerite charnockite found in close association with the anorthosites do not define a precise isochron the initial nd values of these rocks at 1 100 m y show a range from 2 7 to 5 1 which demonstrates that the anorthositic rocks and the mangerite charnockite are not cogenetic the isotopic data further demonstrate that the mangerite charnockite rocks are mantle derived meta igneous rocks and not of sedimentary derivation

pristine surface growth features on 100 ma garnet phenocrysts interference imaging results microstructures of crystal surfaces can reveal growth mechanisms however original surfaces of phenocrysts are rare because they are commonly covered with glass or rock matrix in an unusual occurrence microscopic growth features are clearly visible on original surfaces of 100 ma melanite garnet phenocrysts of the crowsnest formation alberta canada such details as the steps of growth pyramids are clearly resolved at the 100 nm scale vertically in reflected light nomarski differential interference contrast ndic however because of the small step size

comparison of the rainy ridge analcime phonolite sill and the crowsnest volcanics alberta canada a 5 7 m thick analcime phonolite sill occurs in the middle of the proterozoic gateway formation in southwest alberta the sill consists of sanidine aegirine augite magnesian hastingsite melanite with hydrogrossular rims titanite and minor biotite apatite and opaque minerals mineralogical and chemical similarities to the analcime rich phases of the cretaceous crowsnest formation found some 20 km to the north suggest a genetic relationship major differences are the presence of amphibole and hydrogrossular minerals not reported in the crowsnest formation the presence of amphibole as a primary hydrous phase in the rainy ridge sill indicates crystallization from a hydrous magma microprobe studies indicate a progressive enrichment of sodium in amphiboles and pyroxenes an apparent difference in chemical composition and alteration behavior of primary analcime phenocrysts and groundmass analcime is interpreted to reflect crystallization of analcime from a hydrous melt at depth followed by rapid trans

bebedourite from its type area salitre i complex a key petrogenetic series in the late cretaceous alto paranaiba kamafugite carbonatite phoscorite association central brazil abstract bebedourite is a cumulate rock composed of variable but roughly equant amounts of diopside apatite magnetite phlogopite and a ca ti phase mostly perovskite more rarely titanite and or ti garnet other minerals may be modally important such as olivine and k feldspar in the least and most evolved members of the bebedourite series respectively the magmatic evolution in bebedourites is accompanied by a progressive increase in sio2 activity which results in the transformation of perovskite into titanite and titanite into ti garnet although the sio2 increase may in some cases result from crustal contamination it seems to be a localized effect and cannot account for the evolution of the whole bebedourite series crystal fractionation is supported by the chemical variation of key mineral phases such as pyroxene and phlogopite the salitre complex is an ultrapotassic carbonatite and phoscorite bearing plutonic complex belonging to the late cretaceous alto paranaiba igneous province apip and consisting of three main bodies salitre i ii and iii the complex is composed mainly of bebedourite with lesser amounts of carbonatite and phoscorite in its central north portion a particular type of bebedourite where the main ca ti phase is ti garnet dominates the southern part of salitre i and also occurs as dikes crosscutting older bebedourites suggesting that ti garnet bebedourites form an independent intrusion sr and nd isotopic data indicate that the parental magmas to the bebedourites in the salitre complex originated in a metasomatized sub continental lithospheric mantle similar to that involved in the origin of the rest of the apip

evidence for pyroclastic emplacement of the crowsnest formation alberta the middle cretaceous crowsnest formation west of coleman alberta is composed of bedded alkaline volcanic deposits containing heterolithic volcanic rock fragments and crystal clasts comparison with modern examples of subaerial pyroclastic rocks suggests that pyroclastic flows surges fallout of material from vertical eruption columns and minor mud flows emplaced the deposits textural evidence in the form of plastically deformed volcanic fragments chilled deposit margins baked rock fragment margins recrystallization and the presence of charred wood and charred wood molds indicate emplacement at elevated temperature massive deposits containing a fine grained basal zone are interpreted as the product of pyroclastic flows whereas deposits characterized by a block rich base overlain by a thin layer of block depleted stratified material are interpreted as the product of density stratified surges deposits exhibiting pronounced stratification were emplaced by ash cloud surges thickly bedded breccias

quartz garnet isotope thermometry in the southern adirondack highlands grenville province new york quartz garnet oxygen isotope thermometry of quartz rich metasedimentary rocks from the southern adirondack highlands grenville province new york yields metamorphic temperatures of 700 800 c consistent with granulite facies mineral assemblages samples from the irving pond quartzite record d 18 o qtz grt 2 68 0 21 1 s d n 15 corresponding to peak metamorphic conditions of 734 38 c this agrees well with the estimates from garnet biotite exchange thermometry similar temperature estimates are obtained from swede pond 682 47 c n 3 and king s station c 700 c the whitehall area records higher temperatures 798 25 c n 3 all of these temperatures are higher than previous regional temperature estimates the c 800 c temperatures near whitehall are consistent with preservation of pre granulite contact temperatures adjacent to anorthosite the preservation of peak metamorphic temperatures in garnet of all sizes is consistent with slow oxygen diffusion in garnet and closure temperatures of at least 730 c peak metamorphic fractionations are preserved in rocks with varying quartz feldspar ratios indicating that the modal percentage of feldspar does not affect retrograde oxygen exchange in these rocks the lack of this correlation suggests slow rates of oxygen diffusion in quartz and feldspar consistent with the results of anhydrous oxygen diffusion experiments

mineral chemistry of igneous melanite garnets from analcite bearing volcanic rocks alberta canada the mineral chemistry of melanite garnets from the crowsnest volcanic rocks of sw alberta canada has been investigated by using electron microprobe scans quantitative analyses and multivariate statistical analysis the garnets occur with aegirine augite sanidine analcite and rare plagioclase as phenocrysts in trachyte and phonolite flows agglomerates and tuffs wavelength dispersive microprobe scans reveal complex zonation patterns both normal and oscillatory the results of fifty quantitative analyses were subjected to r mode factor analysis to delineate the chemical exchanges producing the zonation the chemical zonation of the garnets may be attributed to four independent binary exchanges al fe3 si ti ca mn and mg fe2 the stoichiometry of these garnets based on microprobe and wet chemical fe analyses combined with the strongly antithetic behavior of si and ti lead us to infer that the ti in these garnets is dominantly tetravalent it is clear from this study that quantitative modelling of the processes of crystal growth and zonation of melanite garnets in alkaline undersaturated igneous rocks should be aimed at simulating the four chemical exchanges listed above

mesozoic alkaline rocks of the averill plutonic complex the 150 ma averill alkaline plutonic complex is situated in southern british columbia within the intermontane belt of the canadian cordillera it comprises concentrically arranged phases of pyroxenite monzogabbro monzodiorite monzonite and syenite gradational changes in the modal composition of the phases contrast with an abrupt change in crystallinity from euhedral mafic minerals in the ultramafic mafic phases to an anhedral interstitial habit for mafic minerals in the syenite whole rock compositions have clear alkaline affinities e g feldspathoid normative and indicate a chemical discontinuity between the ultramafic mafic phases and the late syenite phases melanite garnet is an important accessory mineral of the syenite and is characteristic of silica undersaturated alkaline intrusions clinopyroxene feldspar amphibole biotite melanite and titanite compositions compare closely with those of other alkaline complexes in the western cordillera

timing of anatexis in the eastern adirondack highlands implications for tectonic evolution during ca 1050 ma ottawan orogenesis u pb sensitive high resolution ion microprobe shrimp analyses of zircons from migmatitic metapelites in the eastern adirondack highlands demonstrate that substantial anatexis took place ca 1050 ma during the late but still high grade ca 1090 1050 ma ottawan orogeny these results contrast with data from metapelites of the southwestern adirondack highlands and adirondack lowlands which indicate that anatexis occurred ca 1200 1160 ma during the shawinigan orogeny and subsequent ca 1155 ma anorthosite mangerite charnockite granite amcg magmatism ca 1180 1150 ma zircons from this western regime do not contain ca 1050 ma ottawan metamorphic overgrowths the absence of ca 1050 ma ottawan anatexis and overgrowths in the adirondack lowlands is explained by the observation that consistent with titanite cooling ages post 1150 ma temperatures did not exceed 700 c in the southwestern adirondack highlands the absence of ca 1050 ma anatexis can be accounted for by earlier dehydration of metapelites during ca 1180 1150 ma shawinigan amcg metamorphism the occurrence of ca 1050 ma anatexis and formation of metamorphic zircons in the eastern adirondacks cannot be explained by higher temperatures because geothermometry indicates that all of the adirondack highlands reached a nearly uniform 750 800 c during this period accordingly the occurrence of ca 1050 ma ottawan anatexis in the eastern regime is interpreted to be the result of 1 infl uxes of hydrous fl uids at ca 1050 ma or 2 decompression melting during late extensional exhumation a recently recognized low angle late ottawan ca 1050 ma fault system may have provided both fl uid conduits and footwall decompression the sense of displacement along the shear zone has not yet been unequivocally determined but preliminary investigations of kinematic indicators demonstrate normal displacement thus this structure may be an analogue of the ca 1050 ma northwest dipping carthage colton zone normal fault system that juxtaposed the adirondack lowlands against the adirondack highlands in this context these fault zones form a quasi symmetrical core complex or gneiss dome within which the high grade core of the adirondack highlands was tectonically exhumed a similar east dipping along strike normal fault in quebec tawachiche shear zone exhumed the eastern morin and lac taureau terranes at ca 1050 ma and may merge with the eastern adirondack shear zone described here

sidewall differentiation in an alkalic magma chamber evidence from syenite xenoliths in tuffs of the latera caldera italy feldspathoidal and quartz bearing syenite xenoliths in c 235 to 155 ka tuffs surrounding the latera caldera have textures and mineral assemblages that indicate an origin from the crystalline margins of a shallow magma chamber this lithic suite exhibits a diversity of plutonic fabrics unaltered glass bearing nodules with undeformed sanidine frameworks coexist with completely crystallized clasts many of which show evidence of subsolidus modification the syenites comprise eutectic mineral assemblages with high percentages of titanite apatite and melanite garnet as accessory minerals ubiquitous reaction textures in syenite accompany progressive changes in mineral assemblages and they show the decreasing influence of limestone contamination with distance from the contact of syenite and skarn wallrock diagnostic mineral assemblages include nepheline rich in calcite inclusions coexisting titanite and metamorphic perovskite zircon with baddeleyite inclusions fluorapatite mantled by a possible carbonate bearing apatite francolite melanite garnet intergrown with clinopyroxene and interstitial hauyne rich in pyrrhotite inclusions at latera pumice fragments in the same deposit can exhibit up to ten fold differences in vesicularity and crystal content from 50 vol phenocrysts these clasts in conjunction with glass bearing syenite and skarn xenoliths may represent a range of progressively crystallized magmas that were quenched at the instant of their eruptive entrainment the major element abundances of pumices and syenite reflect the fractionation of plagioclase and sanidine with lesser amounts of fassaitic diopside leucite biotite apatite and alkali amphibole trace element ratios e g rb sr nb ta zr hf lree hree on the other hand are highly variable for crystalline and pumiceous ejecta and some element variations e g li ta nb th cannot be entirely reconciled with simple crystal liquid fractionation the syenite clasts span a range of compositions from strongly fractionated assemblages rich in accessory minerals to compositions that more closely approximate quenched reservoir liquids these features reflect processes that range from pure fractional crystallization to in situ crystallization where fractionated liquid was trapped in the pores of sidewall cumulates along the chamber roof and walls

unusual occurrences of aegirine augite fassaite and melanite in oceanic basalts dsdp hole 504b abstract dsdp hole 504b located on the south flank of the costa rica rift shows the most complete reference section through the upper oceanic crust 274 5 m into the sediment and 1075 5 m into the basement have been penetrated very unusual secondary parageneses occur in some basaltic massive flows recovered at dsdp hole 504 b replacement of magmatic augite by secondary ti and or na rich augite and aegirine augite was observed in five veins between the 298 and 685 m sub basement depth one of these veins also contains melanite ti rich andradite and another one contains fassaite these minerals seem independent of and prior to other alteration stages the conditions of formation and origin of the fluids responsible for the crystallization of these unusual minerals are discussed several arguments suggest that their origin is not hydrothermal but possibly deuteric 1 these minerals probably formed at higher temperatures than hydrothermal minerals from dsdp hole 504 b 300 400 c for melanite 800 1000 c for fassaite 400 c for aegirine augite 2 the chemical composition of the studied minerals are not compatible with the chemical composition of hydrothermal fluids responsible for the alteration at dsdp hole 504 b 3 the distribution and chemical composition of these minerals seem to be independent of the alteration zonation and thus of depth and 4 these minerals are anhydrous in contrast to mostly hydrated minerals from dsdp hole 504 b known to be hydrothermal in origin moreover since these minerals occur in subophitic or coarse grained basalts their crystallization may have been enhanced by the long standing stability of the physico chemical conditions existing in the veins

age of xenolith bearing olivine metagabbro eastern adirondack mountains new york a u pb zircon age of 1143 6 pm 7 ma has been obtained on a xenolith bearing olivine metagabbro in the eastern adirondack highlands thus fixing a minimum age for metamorphism represented by truncated foliation in the xenolith and the igneous crystallization age of the rock the igneous age predates commonly accepted metamorphic ages of 1000 1100 ma for the adirondacks and documents that more than one high grade metamorphic event affected the region

geology mineral chemistry and formation conditions of calc silicate minerals of astamal fe lree distal skarn deposit eastern azarbaijan province nw iran the astamal fe lree skarn deposit is the largest iron skarn in nw iran it is a unique case as a distal skarn deposit which crops out approximately 600 m from the associated oligo miocene granodioritic pluton this deposit formed in the south southwest of the pluton where fractures and faults within the upper cretaceous volcano sedimentary host rocks acted as conduits for the mineralizing fluids the deposit contains three iron ore bodies southern northern and eastern the main mineral assemblage within the ore zones is characterized by magnetite pyrrhotite and pyrite with lesser quantities of chalcopyrite hematite goethite and limonite the skarn minerals predominantly comprise garnet epidote actinolite calcite quartz clinopyroxene and chlorite in order of abundance retrograde alteration is strongly developed in the skarn zone where most of the garnet has been pervasively altered to secondary minerals e g epidote calcite and quartz both in the rims and the cores whereas the majority of the clinopyroxene has been replaced by a hydrous retrograde mineral assemblage e g tremolite actinolite and chlorite garnets with andradite grossular compositions are the dominant mineral in the skarn zone which are generally isotropic with a narrow compositional range along the growth lines adr94 3 64 5grs21 9 2 7alm11 1 0 2 these garnets are fe rich and have high fe fe al ratios between 0 96 and 0 78 cu and ni are enriched in the garnets this suggests that these elements were enriched in the hydrothermal fluids from which the garnet precipitated this is supported by the presence of chalcopyrite and ni bearing massive magnetite in the study area which also suggests that cu and ni were enriched in the late stage ore bearing hydrothermal fluids clinopyroxene with a hedenbergitic composition is generally homogenous and has particularly high fe fe mg ratios between 0 99 and 0 86 and is poor in tio2 mno and cr2o3 high zn concentrations were also detected in the clinopyroxenes up to 1044 ppm despite an absence of significant zn mineralization such as sphalerite in the district therefore it is believed that the proportion of zn in the hydrothermal fluids decreased significantly from the time of clinopyroxene formation to the period of the sulfide deposition phase allanite and lree bearing epidotes are the main lree bearing minerals in this deposit the epidote is also fe rich with high fe fe al ratios between 0 32 and 0 44 due to a lack of replacement texture between both garnet and clinopyroxene and garnet and actinolite which is formed by alteration of clinopyroxene it is believed that these two minerals have grown simultaneously and are coexisting minerals in the astamal skarn these minerals can be stable and coexisting at temperatures between 490 and 560 c and log o2 16 to 31

almandine garnet phenocrysts in a 1 ga rhyolitic tuff from central india we report on the newly discovered almandine garnet phenocrysts in rhyolitic ignimbrites sukhda tuff in the precambrian churtela shale formation of the chhattisgarh supergroup in central india shrimp ages of igneous zircon from the ignimbrites range from 990 ma to 1020 ma these ignimbrites exhibit characteristic eutaxitic texture with compacted curvilinear glass shards with triple junctions quartz commonly embayed bluish cathodoluminescence and albite altered but retaining ghosts of twinning are common phenocrysts others are apatite ilmenite rutile magnetite zircon monazite and garnet there are no metamorphic or granitic xenoliths in the ignimbrites garnet grains occur as isolated broken isotropic crystals with sharp or corroded boundaries in a very fine grained groundmass of volcanic ash that consists principally of albite quartz magnetite and glass they do not have any systematically distributed inclusions a few have penetratively intergrown phenocrysts of apatite ilmenite rutile and zircon which we interpret as subophitic texture extensive sem bse imaging of more than 100 grains and electron microprobe traverses across about 30 grains showed no zoning or systematic compositional variability common metamorphic garnets are usually zoned with respect to fe mg mn and typically have mineral inclusions we infer therefore that these observed garnets are not metamorphic xenocrysts the average major oxide composition of analysed garnets from five different horizons within the sukhda tuff spanning approximately 300 m of the stratigraphic section have very small standard deviation for each element which is suggestive of a single magmatic source phenocrysts of quartz including those in contact with coexisting garnets show blue scanning electron cl indicating rapid cooling from high temperature this suggests that adjacent coexisting garnets are not slowly cooled restites we conclude on the basis of texture mineral chemistry and absence of any indicative xenoliths or xenocrysts that these almandine garnets al 78 7 py 12 3 gr 7 4 sp 1 6 are phenocrysts within the sukhda tuff almandine of such composition is stable under high pressure we infer that almandine crystallized at lower crustal depths in a magma that ascended very rapidly and may have erupted explosively

genesis of metapelitic migmatites in the adirondack mountains new york oxygen isotope ratios and rare earth element ree concentrations provide independent tests of competing models of injection v anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the adirondack mountains new york values of 18o and ree profiles were measured by ion microprobe in garnet zircon pairs from 10 sample localities prior u pb sims dating of zircon grains indicates that inherited cores 1 7 1 2 ga are surrounded by overgrowths crystallized during the grenville orogenic cycle 1 2 1 0 ga cathodoluminescence imaging records three populations of zircon i featureless rounded whole grains interpreted as metamorphic or anatectic and rhythmically zoned igneous cores truncated by rims that are either ii discordant rhythmically zoned igneous or iii unzoned metamorphic or anatectic these textural interpretations are supported by geochronology and oxygen isotope analysis in both the amphibolite facies nw adirondacks and the granulite facies se adirondacks 18o zrc values in overgrowths and whole zircon are highly variable for metamorphic zircon 6 1 13 4 n 95 10 m spot in contrast garnet is typically unzoned and 18o grt values are constant at each locality differing only between leucosomes and corresponding melanosomes none of the analysed metamorphic zircon garnet pairs attained oxygen isotope equilibrium indicating that zircon rims and garnet are not coeval furthermore ree profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth thus petrological estimates from garnet equilibria e g p t cannot be associated uncritically with ages determined from zircon the unusually high 18o values 10 in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks 18o zrc 10 indicating that these leucosomes are not injected magmas derived from known igneous rocks surrounding melanosomes have similarly high 18o zrc values suggesting that leucosomes are related to surrounding melanosomes and that these migmatites formed by anatexis of high 18o metasedimentary rocks

nature and timing of anatexis in the eastern and southern adirondack highlands metapelitic rocks in the eastern and southern adirondack highlands contain two feldspar quartz leuco somes that pre date most deformational fabric as well as granulite facies recrystallization whole rock xrf analyses of these leucosomes yield cipw norms which correspond closely with experimental melts in the system ab or qt an h 2 o thus indicating that the leucosomes are anatectic in origin derived p t values for anatexis indicate high temperatures and low pressures suggestive of widespread contact metamorphism the heat source is believed to have been provided by anorthositic gabbroic magmas postulated to have been anorogenically emplaced prior to the grenville orogeny

complexly zoned ti rich melanite schorlomite garnets from ambadungar carbonatite alkalic complex deccan igneous province gujarat state western india abstract ti rich garnet phenocrysts from a tephrite nephelinite plug in the ambadungar complex situated in the chhota udaipur alkalic subprovince show concentric zoning based on paired orthogonal traverses across three selected crystals total 81 step scan point analyses andradite content ranges from 55 to 86 mol the remainder being almost entirely schorlomite corresponding to the following wt oxide variations tio2 5 5 15 8 cao 29 6 32 5 mgo 0 3 1 6 fe2o3 20 26 al2o3 0 7 3 5 mno v2o3 na2o and zro2 each rarely exceeds 0 5 zoning patterns in individual grains from this one rock differ considerably in several ways 1 there may be 2 5 alternating pale and dark zones the pale generally being more andradite rich enriched in ti mg and usually zr but impoverished in al 2 the two orthogonal traverse may or may not be mirror images 3 monotonic trends decreasing ca al increasing mg zr from core to rim may or may not be present 4 oscillatory zoning varies in amplitude and 5 apparent substitutions differ although all crystals clearly show the si ti substitution inferred for ti rich garnets elsewhere an idealized case is developed from an observed complexly zoned phenocryst population to piece together a history of the alkaline host magma that experienced several events such as polybaric differentiation magma mixing and kinetic effects