Crystal-Mush Compaction and the Origin of Pegmatitic Segregation Sheets in a Thick Flood-Basalt Flow in the Mesozoic Hartford Basin, Connecticut

Where the Holyoke flood-basalt flow in the Mesozoic HartfordBasin in Connecticut is thick and contains coarse-grained, horizontalsegregation sheets in its central part, the lower part of theflow is strongly depleted in incompatible elements; where theflow is thin and contains no segregation sheets it is homogeneousthroughout. This chemical variation can be explained only throughcompaction of the partly crystallized basalt. The compositionof the segregation sheets shows that they separated from thebasalt following only 33% crystallization. The segregation sheets,however, are clearly intrusive into the basalt, which must thereforehave already formed a crystal mush with finite strength at thislow degree of crystallinity. The incompatible element concentrationsindicate that the partly crystallized basalt underwent as muchas 28% compaction in the lowest 60 m of the flow. Between 60and 130 m above the base of the flow, the crystal mush becamedilated, and eventually ruptured with formation of the segregationsheets. No segregation sheet has a composition indicating separationafter more than 33% crystallization of the basalt. This is interpretedto indicate that compaction ceased at this stage because ofthe increasing strength of the mush and the increasing densityof the fractionating interstitial liquid KEY WORDS: crystal-mush compaction; segregation shtets; flood basalt; tholeiitie; Connecticut ^*e-mail: philpotts{at}geol.uconn.edu     

Silicate Liquid Immiscibility in Tholeiitic Basalts

Immiscible silicate liquids, preserved as fresh glasses, constituteat least 32 per cent of a tholeiitic basalt in the Triassic-Jurassicbasin of Southbury, Connecticut. This apparently uncommon abundanceof immiscible liquids cannot be attributed to an unusual magmacomposition, for the rock is similar to many quartz-normativetholeiites associated with other basins of similar age in easternNorth America. Nor can peculiar conditions of fractional crystallizationbe involved, for experiments carried out at one atmosphere totalpressure reveal that two silicate liquids coexist over a widetemperature interval during crystallization, and that this intervalis increased by raising the oxygen fugacity. Experiments onthe standard diabase, W-1, which differs only slightly in compositionfrom the Connecticut basalt, indicates the presence of immiscibleliquids during the crystallization of this magma compositionas well. It is concluded that liquid immiscibility is to beexpected during the crystallization of many tholeiitic magmas,and at a sufficiently early stage of solidification to playa role in igneous differentiation.     

Origin of the Anorthosite-Mangerite Rocks in Southern Quebec

Rocks of the anorthosite-mangerite suite were intruded in manyplaces in the southern part of the Grenville province of theCanadian Shield. These rocks, known as the Morin series in southernQuebec, are described from two contrasted areas, one in whichthere has been intense deformation during the emplacement ofthese rocks and in the other only mild deformation. Differencesin the mineralogy and chemistry of these rocks in the two areascan be related to the degree of deformation. Although many of the rocks of the series still retain theirigneous textures, others, especially the mangerites, are largelyrecrystallized and commonly exhibit a gneissic foliation. Thefeldspars and pyroxenes which are by far the most abundant mineralsof the Morin series are dealt with in detail. From a study ofthe coexisting pyroxenes and the amount of iron in the plagioclases,evidence is given for a difference in the degree of metamorphismbetween the two areas. All of the pyroxenes from the more highlydeformed area contain considerably more alumina than those fromthe other area. Coexisting lime-rich and lime-poor pyroxenesare characteristic of almost all of the rocks of the series.The coexistence of subcalcic ferro-augite and inverted pigeoniteas iron-rich as Ca_25.6Mg_22.4Fe_52.0 and Ca₉Mg₂₃Fe₆₃, respectively,in the unmetamorphosed rocks suggests very high temperaturesof crystallization. Analyses of the rocks are compared with those from other anorthositeassemblages. Variation diagrams indicate that the anorthosite-mangeriterocks form a very definite series which is not exactly comparablewith any other known rock series. Although the anorthosite-mangeritesuite shows strong iron enrichment and is therefore similarto many tholeiitic suites, the basic members of the series aremore akin to calc-alkaline rocks. It is concluded that the anorthosite-mangerite rocks are derivedfrom a calc-alkaline parental magma that underwent differentiationin a very dry environment and hence gave rise to the strongiron enrichment trend that is characteristic of tholeiitic suites.Early accumulation of plagioclase from the parental magma toform anorthosites may be due to a probable increase in the sizeof the field of crystallization of intermediate and sodic plagioclasesunder high pressures. The fact that intrusion took place duringan orogenic period is of great importance, for the rocks inthe more deformed area are more differentiated than those inthe less deformed area.     

Wallrock Melting and Reaction Effects along the Higganum Diabase Dike in Connecticut: Contamination of a Continental Flood Basalt Feeder

The question of whether continental flood basalts are contaminatedwith crustal material during ascent through the lithosphereis addressed through a study of the feeder dike to the firstof the Mesozoic flood basalts in the Hartford basin of Connecticut.Faulting associated with basin formation exposed the dike, notonly where it connects with the flood basalt, but also at levelsthat may have been as deep as 10 km at the time of crystallization.Wallrocks at all levels of exposure and along the entire 250-kmlength of this 50–60 m wide dike show evidence of partialmelting and contamination of the diabase magma. Melting of the wallrocks took place on grain boundaries betweenquartz and feldspar, especially where these boundaries werefluxed with water released from the breakdown of biotite. Themelts were simple mixtures of quartz and the particular feldsparinvolved rather than equilibrium minimum compositions, probablybecause heating rates were faster than melting rates. The meltscrystallized to form granophyre. Feldspars in direct contactwith the diabase partly melted, with plagioclase developingthe fingerprint texture and orthoclase being converted to anexceptionally fine-grained mixture of plagioclase and quartz. Melts generated in the wallrocks were forced into the marginsof the dike by the volume expansion caused by melting. The earliestmelts to enter the dike are preserved as felsic wisps in thechilled margins, but at distances of more than a few centimetersfrom the edge of the dike they appear to have been assimilatedby the diabase magma. Melts that entered the dike after themargins had solidified were intruded as crosscutting granophyreveins. Chemical profiles across the dike reveal that the contaminantswere not restricted to narrow marginal zones but entered themain body of the dike. Thus, by the time this magma had risento the surface and erupted as the Talcott basalt it had assimilateda significant amount of crustal material, which accounts forthe change from olivine normative compositions in the deepestlevel exposures of the dike to quartz normative ones in thebasalt. Mass balance calculations indicate that the Talcottbasalt may have had {small tilde}6% granophyre added to it duringthe magma's ascent through the last 10 km of the crust. Theamount assimilated in traversing the entire continental crustwould therefore be greater than this.     

Quantifying the Building Blocks of Igneous Rocks: Are Clustered Crystal Frameworks the Foundation?

Most phenocryst populations in volcanic rocks, and those preservedin shallow-level igneous intrusions, are clustered (variouslyreferred to as clots, clumps or glomerocrysts). These clustersof crystals are the building blocks that accumulate to formthe high-porosity, touching crystal frameworks from which igneouscumulates form. Examination of touching crystal frameworks inolivine- (komatiite cumulates and experimental charges) andplagioclase-dominant crystal populations (Holyoke flood basalt,Connecticut, USA) reveal complex, high-porosity, clustered crystalarrangements. Olivine touching frameworks in komatiite flowsare interpreted to form in hundreds of days. Plagioclase frameworksare calculated to have formed in less than 17 years for a crystalgrowth rate of 1 x 10^-10 mm/s to less than 3 years for a growthrate of 5 x 10^-10 mm/s based on crystal size distributions.The origin of crystal clusters is likely to involve either (ora combination of) heterogeneous nucleation, remobilization ofcumulate mushes or crystals sticking together during settlingand/or flow. The spatial distribution pattern of clustered crystalframeworks from both natural and experimental examples constrainsfields on spatial packing diagrams that allow the identificationof touching and non-touching crystal populations, and furtherimprove our understanding of crystal packing arrangements andcluster size distributions. KEY WORDS: cumulates; CSD; komatiite; basalt; spatial packing; textural analysis