Phlogopite Phenocrysts Research Articles

Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province, Canada

Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # ⩾ 60; SiO2 47–54%, mean K2O/Na2O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO255–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous ɛNd, 1840 Ma (−6 to −11) but highly variable ES., 1840 Ma (−40 to + 100); samples classified as lamproites have higher average ɛSr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic “superprovince”.

Potassic alkaline lamprophyres with affinities to lamproites from the Karinya Syncline, South Australia

Most newly discovered unmetamorphosed lamprophyres from the Karinya Syncline, South Australia, occur as dykes with alkaline geochemistry (TiO 2 up to 2.36 wt.%; Zr up to 1135 ppm), high fluorine (F up to 5800 ppm) and high potassium contents (K 2O up to 9.81 wt.%). The majority of samples are ultrapotassic. Two samples, one of which occurs in a diatreme, have the very low SiO 2 contents (39 and 40 wt.%) and high MgO contents (> 16 wt.%), which are more typical of olivine lamproites. Dyke emplacement occurred during the Ordovician after the Delamerian orogeny and was controlled by deep-seated crustal lineaments which are detectable by satellite imagery. The lamprophyres have porphyritic textures, with phlogopite phenocrysts and apatite microphenocrysts within a groundmass of orthoclase, leucite, plagioclase and minor quartz. Two petrographic subgroups can be distinguished: (I) phlogopite-phyric lamprophyres and (II) apatite-phlogopite-phyric lamprophyres. Type (I) lamprophyres have KAr ages of 458±2 Ma, while those from petrographic type (II) have older KAr ages of 480±3 Ma. Some phlogopite phenocrysts are zoned, with pale yellow Mg-rich cores and brown Fe-rich biotitic rims. Both mica phenocrysts, which are characterized by high F and BaO concentrations (up to 3.79 wt.% and 0.65 wt.%, respectively), and groundmass feldspars have a mineral chemistry transitional between lamprophyres and lamproites. Apatite microphenocrysts contain very high halogen concentrations (up to 3.04 wt.% F and 0.13 wt.% Cl). One lamprophyre contains alkali-amphibole (riebeckite). Whole-rock geochemistry, especially the very high HFSE concentrations of the lamprophyres (e.g. Zr, Nb and Hf up to 1135 ppm, 70 ppm and 28 ppm, respectively) are typical for potassic magmatic rocks intruded in a continental within-plate tectonic setting. This is consistent with tectonic interpretations of the area and the fact, that no subduction-indicators, such as blueschists, ophiolites and melanges have been recorded in the Karinya Syncline. Several samples have elevated precious metal contents. Gold concentrations up to 23 ppb, Pt up to 19 ppb and Pd up to 49 ppb may be attributed to late-magmatic hydrothermal fluids. The highest precious metal concentrations are present in a sample affected by secondary carbonate replacement.

Spessartites in the Montagne Noire, France: mineralogical and geochemical data

Spessartites occur as sills in the Lower Cambrian formations on the northern slope of the Montagne Noire, France. They show an unusual mineralogy with phenocrysts of amphibole (magnesio-hastingsite to pargasite) and phlogopite, both including chromian rutiles (up to 3.5 wt.% Cr 2 O 3 ), zincian chromites and Ni-Co-Fe sulfarsenides with pyrrhotite. The lack of magnetite and ilmenite together with the presence of sulphides and sulfarsenides would suggest high fS 2 for fO 2 values slightly above the NNO buffer. The magmatic evolution is firstly marked by a partial cumulation process of early magmatic phases (olivine and associated chromite and sulfarsenides) as suggested by the very high concentrations of MgO (15.5-16 wt%), Cr (1400-1500 ppm), Ni (400 ppm) and Co (75-110 ppm) measured in these rocks. As crystallization proceeds, amphibole and phlogopite become more and more Mg-rich; olivine is destabilized to phlogopite. The latest phases to crystallize are plagioclases and quartz. These rocks are calc-alkaline, SiO 2 saturated with a K/Na ratio higher than 1. They display a strong enrichment of LFS (low-field-strength) and light rare earth elements, with very low concentrations of heavy REE, Ta and Ti. Their chemical and mineralogical features suggest that the primary magma can be generated by relatively small degrees of partial melting from a metasomatically enriched phlogopite + gamet ± amphibole lherzolite. The occurrence of these sills seems to indicate transcrustal fractures allowing a rapid ascent without magma differentiation, consistent with an extensional continental zone. Their magmatic sulfarsenides suggest the possible occurrence of deep-level gold-bearing As-S mineralizations such as found in the Salsigne district

Argon isotope and halogen chemistry of phlogopite from South African kimberlites: a combined step-heating, laser probe, electron microprobe and TEM study

Argon isotopic analyses were undertaken on phlogopite from the Swartruggens kimberlite dyke and a Premier kimberlite (1200 Ma) peridotite xenolith. Groundmass phlogopite from Swartruggens yields a plateau age of 145.0± 0.4 Ma, consistent with previous age determinations. Phlogopite phenocrysts from Swartruggens and macrocrysts from the Premier xenolith yield complex age spectra, with anomalously old ages, attributed to incorporation of excess radiogenic argon. Laser probe analyses on single phlogopite grains reveal systematic zonations in excess Ar and CI concentrations across (001) cleavage surfaces. One Premier macrocryst exhibits ages in excess of 2.3 Ga and CI levels of 1600 ppm at its centre. These values decrease systematically to 1.2 Ga and 1300 ppm Cl along grain margins. Similar results were obtained from a single Swartruggens phenocryst, which exhibits a range in values of 340—800 Ma and 300—1300 ppm Cl. A second Swartruggens phenocryst is characterised by smaller variations in age (140–230 Ma) and CI content (390–470 ppm ). Fluorine concentrations, determined by electron microprobe, are relatively constant or increase slightly towards grain edges. The laser probe profiles cannot be reconciled with the step-heating results, probably due to phlogopite degradation during invacuo furnace heating. Transport of Ar and CI in kimberlitic phlogopite appears to be dominated by radial diffusion (cylindrical geometry). The variety of laser probe profiles obtained suggests that Ar and Cl diffusion is governed by factors such as lattice diffusion, diffusion anisotropy, and structural defects, which reduce the effective radii of diffusion and may impart a component of pipe diffusion. It is suggested that the xenolith phlogopite entrapped excess 40Ar and halogens in the mantle lithosphere, in response to elevated Ar and halogen fluid pressures. Swartruggens phenocrysts appear to have crystallised from a volatile-rich kimberlite melt. Subsequent magma devolatilisation prior to emplacement reduced Ar partial pressure and CI content. Possible reasons for enhanced F levels after devolatilisation include increased F solubility in the kimberlite melt, extraction of F from infiltrating hydrothermal fluids and local heterogeneities in fluid composition. The final distributions of Ar, Cl and F in kimberlitic phlogopite are variably dependent on several parameters, including local fluid composition, timing of melt devolatilisation, diffusion/ exchange mechanisms, and mineral composition.

Origin of leucite‐rich and sanidine‐rich flow layers in the Leucite Hills Volcanic Field, Wyoming

Two types of orendite (sanidine‐phlogopite lamproite) and wyomingite (leucite‐phlogopite lamproite) intraflow layering are present in the ultrapotassic Leucite Hills Volcanic Field, Wyoming. In large‐scale layering, wyomingites are confined to the base of the flow, while in centimeter‐scale layering, orendite and wyomingite alternate throughout the flow. The mineralogy of the orendites and wyomingites are the same; only the relative amount of each mineral vary substantially. The chemical compositions of adjacent layers of wyomingite and orendite are almost identical except for water. The centimeter‐scale flow layering probably represents fossil streamlines of the lava and therefore defines the path of circulation of the viscous melt. Toward the front of the flow, the layers are commonly folded. Structures present which are indicative that the flows may have possessed a yield strength are limb shears, boudinage, and slumping. Phlogopite phenocrysts are poorly aligned in the orendite layers, while they are often in subparallel alignment in the wyomingite layers; and they are used as a measure of shearing intensity during emplacement of the flow. Vesicle volumes are concentrated in the orendite layers. In the large‐scale layering, a discontinuous base rubble zone of autobreccia is overlain by a thin platy zone followed by a massive zone which composes more than the upper 75% of the flow. Consequently, we feel that the origin of the layering may be related to shearing. Two extremes in the geometry of shearing are proposed: closely spaced, thin, densely sheared layers separated by discrete intervals throughout a lava flow as in the centimeter‐scale layering and classical plug flow where all the shearing is confined to the base as in the large‐scale layering. A mechanism is proposed which causes thixotropic behavior and localizes shearing: the driving force is the breakdown of molecular water to form T‐OH bonds which establishes a chemical potential gradient for water in the melt. The higher activity of water in the nonsheared regions allows sandine to crystallize, whereas the lower activity of water in the areas of active shearing causes leucite to crystallize.

Serpentinization of phlogopite phenocrysts from a micaceous kimberlite

Phenocrysts of phlogopite from a micaceous kimberlite contain finely interlayered serpentine. These phenocrysts occur in the kimberlite groundmass and are altered along the mica layers and are slightly deformed. Lizardite is the predominant serpentine mineral, but chrysotile and mixed structures also occur. The lizardite occurs as lamellae within phlogopite, oriented such that (001) layers of the two minerals are parallel and the [100] direction of lizardite is parallel to the [100] or 〈110〉 directions of phlogopite. The serpentinized regions of phlogopite are localized and extensive along the (001) layers. Chrysotile fibers and chrysotile-like curled serpentine occur within regions of disrupted material, and polygonal structures occur in folded lizardite lamellae. Textural relations suggest three events: 1) replacement of phlogopite by lizardite, 2) deformation of the phenocrysts, and 3) partial transformation of the lizardite to chrysotile-like structures. Deformation features include openings along (001), folds, and regions of disrupted or broken material. The folded and broken material consists of lamellar lizardite and phlogopite, indicating that lamellar replacement preceded deformation. Intergrowths of lizardite and curled serpentine are associated with cleavage openings and voids in disrupted material, suggesting that a partial transformation of lizardite to chrysotile occurred within openings created by deformation. Clay minerals also occur within phlogopite as either a minor product of serpentinization or of late-stage alteration.

Middle Proterozoic ultramafic lamprophyre dykes in the Archaean of the Atâ area, central West Greenland

During the mapping of gneisses and supracrustal rocks around Eqe in the eastern part of the Atâ area in 1988 a series of ultramafic dykes of lamprophyric affinity were discovered by the authors. The dykes are conspicuous in the field because of their lithology, with often high contents of carbonate, and the occurrence of brownish phlogopite phenocrysts. Their ductile type of deformation and aggressive aiteration of the wall-rock are distinctive. The purpose of this paper is to give a preliminary account of the mode of occurrence and lithology of the dykes based on the mapping results and a provisional inspection of a dozen thin sections. Their possibie age is discussed on the basis of their spatial relation to a large NNW-SSE trending dolerite dyke dated at 1645 ± 35 Ma by Kalsbeek & Taylor (1986).

Minette lavas and associated leucitites from the western front of the Mexican Volcanic Belt: petrology, chemistry, and origin

During the late Pliocene, K-rich minette and leucitite lavas erupted in the western Mexican Volcanic Belt near the town of Los Volcanes, a region which is located much closer to the Middle America Trench than the main line of currently active andesite stratovolcanoes. During this period the tectonic regime in western Mexico was highly complex due to the simultaneous occurrence of active subduction of the young Rivera Plate, and rifting caused by crustal extension. Most of these basic lavas contain phenocrysts of phlogopite, augite and apatite, along with microphenocrysts of leucite and Fe-Ti oxide. Olivine is absent from all but two of the flows: one an olivine leucitite, and the other a felsic minette. The phlogopite phenocrysts and high whole rock Fe2O3/FeO ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities. All of these rock types are highly enriched in the large ion lithophile and light rare earth elements, with Sr≤5100 ppm, Ba≤4800 ppm and Ce≤330 ppm. They are also mildly enriched in the high field strength elements (e.g. Zr 260–700 ppm) and display the strong relative enrichment of the LILE over the HFSE that is characteristic of magmas erupted in convergent margins. Consideration of high pressure phase equilibria in the Mg2SiO4-CaMgSi2O6-KAlSiO4-SiO2 system shows that the minettes from this region are not related through fractional crystallization to the more MgO-rich, olivinebearing minettes which have erupted in other parts of the western Mexican Volcanic Belt during the Quaternary. This conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models. Instead, the data suggests that these high-K magmas were derived from a source region which consists predominantly of phlogopite, clinopyroxene and apatite, and which has formed through hydrous enrichment of the subarc mantle in response to subduction.

An absarokite from a phlogopite lherzolite source

An absarokite (SiO2 47.72 wt %, K2O 3.41 wt %) occurs in the Katamata volcano, SW Japan. The rock carries phenocrysts of olivine, phlogopite, clinopyroxene, and hornblende. Chemical compositions of bulk rock (FeO*/ MgO 0.73) and minerals (Mg-rich olivine and phlogopite, Cr-rich chromite) suggest that the absarokite is not differentiated. Melting experiments at high pressures on the Katamata absarokite have been conducted. The completely anhydrous absarokite melt coexists with olivine, orthopyroxene, and clinopyroxene at 1310° C and 1.0 GPa. The melt with 3.29 wt % of H2O also coexists with the above three phases at 1230° C and 1.4 GPa; phlogopite appears at temperatures more than 80° C below the liquidus. On the other hand, the melt is not saturated with lherzolite minerals in the presence of 5.13 wt % of H2O and crystallizes olivine and phlogopite as liquidus phases; the stability limit of phlogopite is little affected at least by the present variation of H2O content in the absarokite melt. It is suggested that the absarokite magma was segregated from the upper mantle at 1170° C and 1.7 GPa leaving a phlogopite lherzolite as a residual material on the basis of the above experimental results and the petrographical observation that olivine and phlogopite crystallize at an earlier stage of crystallization sequence than clinopyroxene. The contribution of phlogopite at the stage of melting processes is also suggested by the geochemical characteristics that the absarokite is more enriched in Rb, K, and Ba and depleted in Ca and Na than a typical alkali olivine basalt from the same volcanic field.

中国山地，世羅台地のＫに富むランプロファイヤー

Highly potassic mafic rocks are rarely found in Southwest Japan Arc. Recently, we found highly potassic lamprophyre (minette) from the northeastern part of the Sera Plateau in the Chugoku Mountains. It consists of phenocrysts of phlogopite (mg 85.6, TiO2 4.08-4.18), clinopyroxene (diopside-salite), olivine (mg 84.5) and apatite set in a groundmass of poikilitic sanidine ?? anorthoclase, phlogopite, clinopyroxene, apatite and titanomagnetite. The minette is high in K2O content (5.88 wt%) and K2O/Na2O ratio (3.51). The rock shows relatively high concentration of MgO, Ni and Cr, and is strikingly enriched in incompatible elements, particularly Rb, Sr, Ba, Zr and P. The MORB-normalized pattern of the minette is most closely matched by certain alkali basalts of island arcs and continental margins, having negative Nb-anomalies. However, it is highly enriched in incompatible elements compared with island arc basalts. This suggests that the source for the minette is similar to that for the island arc alkali basalt but spiked with K, Rb, Sr, Ba, Zr and P prior to melting.

Relationships between mineralogical, chemical, and isotopic properties of some North American kimberlites

Late Precambrian to Cretaceous kimberlites from North America have initial Sr and Nd isotopic compositions (εSr0 = −4 to 37, εNd0 = −4 to 6) which form a trend falling roughly within the mantle array. Two groups can be defined on the basis of mineralogical and geochemical characteristics: group A encompasses most of the northeast American kimberlites whose rare earth element (REE) concentrations are moderately fractionated, relative to chondrites. Nd isotopic ratios (εNd0 = 3 to 6) fall within the field of common ocean island basalts. They are also distinctive by the presence of phlogopite phenocrysts and high‐temperature garnet peridotite xenocrysts. Group B includes kimberlites from the Colorado‐Wyoming, Kansas and British Columbia areas and one sample from Pennsylvania. They lack the above quoted peridotite suite and abundant phlogopite. Their light REE concentrations show a strong enrichment, relative to chondrites, correlated with relatively unradiogenic Nd (εNd0 = −4 to 2). Kimberlites of both groups have radiogenic Pb (206Pb/204Pb = 18.17–19.41) and a wide range in 207Pb/204Pb. In both cases, melts were probably generated in the sublithospheric mantle, but group B melts interacted with an enriched component, resulting mainly from magmatic processes, and probably located within the deep lithosphere. By contrast, the Prairie Creek lamproite (Arkansas), as other ultrapotassic rocks, received its distinctive geochemical properties from a crustal component present in the mantle source.

On the origin of some mica-lamprophyres: experimental evidence from a mafic minette

Experimental melting relationships for a mafic minette (mica-lamprophyre) from Buell Park, Arizona were determined under fO2 conditions equivalent to the ironwustite-graphite and quartz-fayalite-magnetite buffers, at pressures of 10–20 kbar. A comparison between experimental products and phenocrysts in the most primitive minettes indicates that those lavas preserve a near-liquidus assemblage of olivine, diopside and Ti-rich phlogopite crystallized in the upper mantle under fO2≥QFM and in the presence of an H2O-bearing fluid phase. It is suggested that micalamprophyric (minette) magmas may originate from a metasomatized, garnet-bearing peridotitic source at deeper levels in the mantle (P≥20 kbar) but can also be in equilibrium with a phlogopite-bearing wehrlite (±opx) source at pressures of 17–20 kbar, under reducing or oxidizing mantle conditions. Owing to their rapid crystallization rate and high liquidus temperatures, a series of potassic daughter melts (potassic latites and felsic minettes) can be formed by segregation from mafic minette parents during their ascent through the cooler continental crust. The preservation of olivine in equilibrium with phlogopite phenocrysts in primitive minettes precludes a petrogenetic process dominated by assimilation/fractional crystallization in a shallow magma chamber and supports a model by which some lamprophyric magmas are brought to near surface conditions at temperatures in the range of 1,000–1,200° C and chilled rapidly.

The Blue Ball, Arkansas Kimberlite: Mineralogy, Petrology, and Geochemistry

A study of the mica peridotite in Blue Ball, Scott County, Arkansas reveals that, on the basis of mineralogy and whole-rock chemistry including the rare earth elements, this occurrence is a micaceous kimberlite. Micas occur as groundmass and phenocryst phases; both are compositionally similar and characterized by reversely pleochroic rims and patches on cores with intermediate pleochroism. Reverse pleochroism is associated with sharp increases in FeO (total Fe) and $$\Delta T [8 - (Si + Al)]$$, and a sharp decrease in $$Al_{2}O_{3}$$, features consistent with the presence of tetrahedral $$Fe^{3} +$$ . Reverse pleochroism in the phlogopite phenocrysts is associated with resorption features. Mineralogical and compositional similarities between these phlogopites and those of the cumulate glimmerites, including the MARID nodules, allows us to suggest a genetic relationship between them.

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France)

Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuee ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities. Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.

Kimberlite and lamproite dykes from Holsteinsborg, West Greenland

Numerous kimberlite and lamproite dykes occur to the south and east of Holsteinsborg in Central West Greenland. This paper gives details of the petrography, mineral chemistry, age relations and geochemistry of the dykes. The kimberlites are composed of macrocysts of olivine, phlogopite, rare ilmenite and garnet in a matrix of olivine, phlogopite, diopside, perovskite, spinel, serpentine, carbonate and apatite. They can mostly be classified as clinopyroxene-phlogopite hypabyssal kimberlites. Mantle-derived inclusions are found in some of the dykes and include lherzolites, wehrlites, harzburgites and, most commonly, dunites. Both coarse and porphyroclastic inclusions occur. Garnet-granulites and eclogites, although rare, are present. The lamproites have variable mineral assemblages and textures but the main constituents are phenocrysts of pseudoleucite, olivine, phlogopite and clinopyroxene set in a groundmass of phlogopite, potassic richterite, diopside, pseudoleucite and potassium feldspar. The mineralogy of these dykes is a reflection of unusual ultrapotassic, magnesian whole-rock compositions.

The Colima volcanic complex, Mexico: Part II. Late-quaternary cinder cones

Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO2, 7.4–15.3% MgO, 2.5–4.4% K2O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba≦ 4,200 ppm, Sr≦3,100 ppm, Zr≦ 550 ppm, Ce≦190 ppm, Hf ≦18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes. Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO2, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO2). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend. The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events. During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcan Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcan Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.

An occurrence of ultrapotassic dikes in the neighbourhood of Holsteinsborg, West Greenland

A brief description is presented here of a group of highly potassic, highly magnesian dikes, originally called kersantites but better-named as lamproites. These rocks may contain phenocrysts of forsteritic olivine, diopside, phlogopite, amphibole (a unique potassic magnesioarfvedsonite) and pseudoleucite set in a groundmass rich in phlogopite, amphibole and carbonate. A K-Ar date on phlogopite indicates an age of ea. 1200 m.y. which is similar to that of a number of other alkaline rocks in the North Atlantic region.

Qal’eh hasan ali maars, central Iran

A group of craters 120 km southeast of Kerman, the largest 1200 m across and 300 m deep, are typical maars, excavated depression with rims of bedded pyroclastic debris. Most of the crater rims are composed entirely of country rock clasts, but the largest crater yields tephrite, composed of phenocrysts of phlogopite, clinopyroxene, and olivine in a groundmass of anorthoclase, analcime, hauyne, clinopyroxene and magnetite, and a cumulate of phlogopite and clinopyroxene with highly potassic glass. The occurrence of hauyne as a ground mass mineral is a rare or unique feature. The maar field lies in the projection to the south of the Nayband fault and, with some flows near that fault, represents a province of Quaternary alkali basalt volcanism.