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Late-Stage Mafic Injection and Thermal Rejuvenation of the Vinalhaven Granite, Coastal Maine 总被引：11，自引：0，他引：11
WIEBE R. A.; MANON M. R.; HAWKINS D. P.; McDONOUGH W. F. 《Journal of Petrology》2004,45(11):2133-2153
The Vinalhaven intrusive complex consists mainly of coarse-grainedgranite, inward-dipping gabbro–diorite sheets, and a fine-grainedgranite core. Small bodies of porphyry occur throughout thecoarse-grained granite. The largest porphyry body (roughly 0·5km by 2·5 km) occurs with coeval gabbro, hybrid rocks,and minor fine-grained granite in the Vinal Cove complex, whichformed during the waning stages of solidification of the coarse-grainedVinalhaven granite. Porphyry contacts with surrounding coarse-grainedgranite are irregular and gradational. Compositions of wholerocks and minerals in the porphyry and the coarse-grained graniteare nearly identical. Neighboring phenocrysts in the porphyryvary greatly in degree of corrosion and reaction, indicatingthat the porphyry was well stirred. Thermal rejuvenation ofa silicic crystal mush by a basaltic influx can explain thecomposition and texture of the porphyry. Comparable rejuvenationevents have been recognized in recent studies of erupted rocks.Weakly corroded biotite phenocrysts in the porphyry requirethat hydrous interstitial melt existed in the granite duringremelting. Field relations, along with thermal calculations,suggest that cooling and crystallization of coeval mafic magmacould have generated the porphyry by thermal rejuvenation ofgranite crystal-mush containing about 20% melt. Field relationsalso suggest that some of the porphyry matrix may representnew felsic magma that was emplaced during remelting. KEY WORDS: granite; magma chamber; mafic replenishment; rejuvenation 相似文献
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Late-stage hydrous fluids, which evolved during the cooling of the Cadillac Mountain granite, Maine, USA, produced narrow veins that transect the pluton. The vein margins contain microstructures transitional between granite and the vein centre. They preserve the vestige shapes of original Na-rich alkali feldspar crystals that have been pseudomorphed by cordierite+quartz+K-rich alkali feldspar. Closer to the centres of the veins, the minerals change from an outer zone with cordierite, hercynitic–galaxite spinel, minor corundum, K-feldspar and plagioclase to an inner zone with remnant cordierite, hercynitic gahnite, strongly zoned almandine–spessartine garnet, chlorite and quartz. Three allochemical reactions are inferred to have occurred with the influx of hydrous fluids during the replacement process. Reaction (1) is Na-rich alkali feldspar+iron ions in solution=Fe-cordierite+quartz+K-feldspar+sodium ions in solution. Reactions (2) and (3) occurred during desilication. Reaction (2) is Fe-cordierite=hercynite+silica in solution, and reaction (3) is Fe-cordierite+water=corundum+iron hydroxide in solution+silicic acid in solution. Two independent techniques, experimental silica-solubility data and spinel–cordierite thermobarometry, constrain the conditions of vein formation to c. 1.0 kbar and both methods indicate that the progressive mineral reactions occurred during cooling of the hydrous fluids from c. 775° to 400–340 °C. This cooling trend is consistent with the petrographic evidence, which demonstrates that reactions occurred before desilication, during desilication, and then diminished with a final phase of resilication. Although the veins are minor features of the Cadillac Mountain granite, they provide insight into the conditions that prevailed during cooling of the pluton, and similar features may be important for constraining the cooling history of shallow-level plutonic complexes elsewhere. 相似文献
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Fractionation and Liquid Immiscibility in an Anorthositic Pluton of the Nain Complex, Labrador 总被引：1，自引：1，他引：1
Fine-grained anorthositic dikes are associated with a massiveleuconorite pluton (Cl = 15) which is exposed over an area ofabout 200 km2. Internally, the pluton shows little compositionalvariation; average plagioclase composition ranges from An52to An48. The dikes are nearly uniform in composition and similarto the estimated bulk composition of the pluton (55 per centSiO2). They therefore appear to represent the parental magmaof the leuconorite pluton. A small body of granite (10 km2) was emplaced within and priorto the complete solidification of the leuconorite. The graniticintrusion caused local deformation of the leuconorite and filter-pressingof its late stage interstitial liquids. These liquids occurin the younger hydrous granite as very finegrained, chilledpillows of nearly anhydrous Fe-rich diorite and granite. Mostof the pillows are diorites with approximately 55 per cent SiO2.On oxide plots these lie approximately on a plagioclase controlline passing through the composition of the leuconorite dikes.The entire group of chilled pillows ranges in composition from45 to 71 per cent SiO2 with a gap between 57 and 63 per centSiO2. On oxide plots they produce a smooth trend which is obliqueto and truncates the plagioclase control line. Variation inthe pillows can best be explained by late-stage liquid immiscibility. Fractionation in the interstitial magma was controlled earlyby crystallization of plagioclase and later by plagioclase pluspyroxene. Very late stage differentiation was controlled mainlyby liquid immiscibility and produced FeO- and SiO2-rich liquids. 相似文献
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Replenishment, Crystal Accumulation and Floor Aggradation in the Megacrystic Kameruka Suite, Australia 总被引：3，自引：0，他引：3
Detailed field evidence indicates that the Kameruka Suite plutonsof the Bega Batholith, eastern Australia, grew by crystal accumulationon the floor of a magma chamber. Depositional features in theplutons, including mafic enclave channels, asymmetric enclavepillows and exotic rafts, load casts and flame structures, andgraded and trough cross-beds, indicate that the pluton builtprogressively upward. The general eastward dip of depositionalfeatures in the main pluton implies a lower western and uppereastern contact, consistent with a basal granite–migmatitecontact in the west and a sharp hornfelsic sidewall contactin the east. Mafic, felsic and composite dykes, most commonnear and below the basal western contact, are interpreted asconduits for magma chamber replenishment and imply open-systembehaviour during pluton construction. Textural relations arealso consistent with an open-system, cumulate origin. Typically,centimetre-scale grains of quartz, plagioclase and megacrysticalkali feldspar form a touching framework with interstices filledwith smaller biotite flakes and smaller intercumulus quartzand feldspar crystals. Alkali feldspar megacrysts vary fromeuhedral and unzoned, to mantled and partially replaced by plagioclase,to ovoid and completely pseudomorphed by quartz–albiteaggregates. The common occurrence of mantled and pseudomorphedalkali feldspar in mafic enclaves, and in hybrid tonalitic rocksforming the matrix to enclave swarms, suggests that replacementor resorption of granitic primocrysts was associated with maficreplenishments. The occurrence of all megacryst types at outcropscale implies extended alkali feldspar crystallization in differentparts of the chamber, thorough stirring during, or after, periodicreplenishment, and final settling in a cumulate mush. The bulkcomposition of the cumulate mush, represented by granodiorite,cannot represent the emplaced magma. Compositional variationcan be modelled by variable degrees of crystal accumulationfrom a parental, silica-rich melt represented by the silicicdykes. As dykes periodically fed the magma chamber, crystalsaccumulated on the floor, and more evolved melts probably eruptedfrom its roof. Thus, the average composition of the magma, andthe cumulus minerals, may have remained relatively constant,and the sublinear chemical trends that typify the Kameruka Suitesimply reflect differing proportions of melt and cumulate material.Sublinear chemical trends can also be explained by a restitemodel; however, the distinctive Ba, light rare earth elementand Zr spikes at high silica can be explained only by a cumulatemodel, which also explains why the low-silica granites of thesuite share the same chemical characteristics as the high-silicagranites. KEY WORDS: crystal accumulation; magma chamber; open system; granitoids; Kameruka; Australia 相似文献
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Nodules and xenocrysts dominated by high-A1 orthopyroxene occurin Proterozoic basaltic dikes that cut the Nain anorthositecomplex, Labrador. This pyroxene (En73–68, Al2O3 = 6.5–4.5)lacks exsolution and occurs both as anhedral xenocrysts up to10 cm in diameter and with euhedral plagioclase (An55) in ophiticnodules. Rarely, olivine (Fo70) occurs with orthopyroxene andAl-spinel with plagioclase. Scarce, more Fe-rich nodules containtwo pyroxenes (orthopyroxene + pigeonite and pigeonite+augite)and coarse intergrowths of ilmenite and Ti-rich magnetite. Pyroxenepairs yield temperatures of 1250? to 1170 ?C; coexisting oxidelamellae yield temperatures between 1145? and 1120 ?C. The highsubsolidus temperatures of the nodules contrasts with the lowtemperature of the host anorthosite at the time of dike emplacementand indicates a deep source for the nodules. Coexisting olivine(Fo70) and plagioclase (An54) suggest a maximum pressure ofabout 11 kb.The dominant orthopyroxene in these nodules is nearlyidentical in composition to the high-Al orthopyroxene megacrystswith exsolved plagioclase (HAOM) found in most Proterozoic anorthosites,and the ophitic nodules have textures similar to ophitic occurrencesof HAOM in anorthosite. Rafting of cotectic nodules from thelower crust can explain occurrences of HAOM in shallow levelanorthosites.The nodules and xenocrysts have compositions consistentwith crystallization from magmas that were parental to the anorthosites.They lend support to models which derive anorthosites by fractionalcrystallization of basaltic magma near the base of the crust. 相似文献
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The Newark Island Layered Intrusion occurs in the ProterozoicNain anorthosite complex of Labrador. It contains an exceptionalsuite of cumulates ranging from troctolites and gabbros to quartzmonzonites and intermediate hybrid rocks. These layered rocksformed in a chamber that was periodically fed by a wide rangeof basic and acid magmas, the compositions of which are preservedin numerous feeder dikes. Where basic magmas commingled withcooler granitic magma, they commonly formed chilled pillows.Because of periodic injections of both acid and basic magmasthe magma chamber was compositionally stratified for much ofits existence. At times, granitic cumulates formed along thechamber walls while mafic to intermediate hybrid cumulates formedon the floor. Stratigraphic and structural relations indicatethat the magma chamber grew upward during deposition, and thatit evolved from a west-dipping sheet to a north-plunging synform.Three major episodes of expansion can be linked to injectionsof large (e.g., 20km3) volumes of acid magma. The entry of thisacid magma into the chamber disrupted previously formed cumulates,creating enlarged feeders down which resident basic magma collapsed.The resultant structures (troughs) contain strongly chilledpillows of resident basic magma that existed near the bottomof the chamber at the time of acid replenishment. 相似文献
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The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber 总被引：11，自引：1，他引：11
The Pleasant Bay layered gabbro–diorite intrusion, locatedon the coast of Maine between Bar Harbor and Machias, is roughlyoval in plan, measuring 12 km by 20 km. Gravity data, contactrelations, and internal layering suggest that it is basinformin structure with a maximum thickness of {small tilde}3 km.Its roof and upper parts have been lost through erosion. Whereit is in contact with underlying granite, the base of the intrusiontypically consists of strongly chilled gabbro with convex-downwardlobate forms, suggesting that the granite was incompletely solidifiedwhen the gabbro was emplaced. Roughly 90% of the exposed rocksare weakly layered gabbro and mafic diorite, both of which varywidely in grain-size and texture. Layers and lenses of medium-grainedleucocratic diorite to granodiorite are widely intercalatedwith the chilled mafic rocks and commonly contain partly digestedmafic inclusions; they also commonly contain zones of pillow-likebodies of gabbro chilled on all margins. The dioritic rocksare consistently topped by gabbroic layers with chilled lobatebases and commonly appear to feed granitic pipes and diapirsinto overlying gabbro. Much of the intrusion can be subdividedinto hundreds of macrorhythmic units (from 1 to 100 m thick)consisting of basally chilled gabbro that grades upward to dioriteor highly evolved leucocratic silicic cumulates. Basaltic dikesare abundant both in the underlying granite and in the layeredgabbro–diorites; they have appropriate compositions tobe feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundredsof basaltic injections into a chamber with resident silicicmagma. Small injections produced chilled gabbroic layers andpillows within silicic cumulates. Larger infusions of basalticmagma produced temporary compositional stratification and episodesof double-diffusive convection within the chamber. Althoughfractional crystallization produced compositional variationin much of the gabbro, units that grade from chilled gabbroat the base to highly silicic cumulates at the top provide cumulaterecords of magma stratification and hybridization along a double-diffusiveinterface between basaltic and silicic magmas. The intrusionprovides a superb plutonic record of events that have oftenbeen inferred for silicic eruptive centers. Mafic–siliciclayered intrusions comparable with the Pleasant Bay are morewidespread than has generally been appreciated. 相似文献
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