假玄武玻璃

Chapter3|PseudotachylytesPseudo­tachylytes273PseudotachylytesPseudotachylytesarecohesiveglassyorveryfine-grainedfaultrocksthatcharacteristicallyoccurinveins.Theyarecomposedofanextremelyfine-grainedorglassymatrixwithinclusionsofwall-rockfragments.Commonly,astraightmainfaultvein(Sibson1975;Spray1992)represent-ingagenerationsurface(Fig.1.1)ispresent,fromwhichsmallerinjectionveinsbranchout.Themaincriteriatodistinguishpseudotachylytesfromcataclasitesare:Thepresenceofinjectionveinsbranchingfromastraightmainfaultvein(generationsur-–face).Asharptransitionbetweenpseudotachylyteveinsandthewallrock,withcharacteristic–embaymentsatthesiteofmicaorhornblendecrystals;incataclasitesthetransitionsaremorecommonlygradual.Thereispotentialconfusion,however,wherepseudotachylyteisincontactwithcataclasiteratherthanwithintactwallrock.Othertypicalpseudotachylytemicrostructuresareapreferentialoccurrenceofclastsofquartzandabsenceofmicaclastsinthematrix,finepowderysulphiteinquartzclasts,andthepresenceoflargerclastsinahomogenousfine-grainedmatrix.Incataclasite,theretendstobeamoregeneraltransitonbetweenmatrix,smallclastsandlargerclasts.Pseudotachylytesarethoughttoformbygenerationandsubsequentmeltingoffine-grainedcataclasticrockalongfrictionsurfaces.Shearstressesbuilduptohighlevelsuntiltheysurpasstheshearresistanceoftherock,andstoredelasticenergyisthensuddenlyreleasedbyrapidmotiononthefault(aprocessoftenreferredtoas“stick-slip”)andreleasedasheatandseismicwaves,producinganearthquake.Thegeneratedheatmeltstherockandcataclasitealongthefrictionsurfacesinamatterofseconds.However,thisheatisrestrictedtoaverynarrowzoneandisthereforequicklydissipatedafterthemovementstops,causingchillingandsolidificationoftheliquidtoaglassoraphaniticcrystallinematerial.Pseudotachylytesformatthesamedepthandtemperaturerangeascataclasite.Atdeeperlevelsofthecrusthighertemperaturefavoursductiledeformationbycrystal-plasticprocesses,generatingmylonitesalongshearzones(Fig.1.1).Herethedeformationtendstobemorecontinu-ouswithtime.Pseudotachylytescanalsobeassociatedwithmeteoriteimpactstructures,andallpseudo-tachylyteveinsofmorethantwocentimetersthickshouldbeinvestigatedwiththispossibilityinmind.Insomecasespseudotachylytessufferductiledeformationaftertheirsolidification.Thiscausesdeformationoftheinclusionstoproduceapreferredshapeorientation.Someexamplesofthisprocessareshownbelow.Fig.3.2AsFig.3.1.Widthofview17mm.CPL.Fig.3.1Pseudotachylyteveincuttingaprotomylonitichornblendebearinggranodiorite.Notethesharpcontacts,thepresenceofinclusionsandtheblackglassymatrix.Nosenseofshearcanbeestablishedformthisstructure.Roraima,northernBrazil.Widthofview17mm.PPL.Chapter328Fig.3.3Pseudotachylytebandingranite.Notethesharpcontactsandthefeldsparinclusionsinthedarkglassymatrix.Nosenseofshearisapparent.Roraima,northernBrazil.Widthofview5mm.CPL.Fig.3.4Contactofpseudotachylyte(upperpart)andgraniticcountryrock(lowerpart).Notethesharpcontactwithsmallinjectionveinsinthecenter.Roraima,northernBrazil.Widthofview5mmPPL.Pseudo­tachylytes29Fig.3.5Pseudotachylyteveinindeformedgranodiorite.Amainfaultveinorgenerationsurfacecutsthehostrockfromupperrighttolowerleftwithaninjectionveinpropagatingupwards.Thedarkpseudotachylytematrixcontainssomewhatroundedinclusions,mainlyofquartzandopaqueminerals.Theboundariesbetweenthepseudotachylyteandthewallrockaresharp.TorresdePaine,southernChile.Widthofview20mm.PPL.Fig.3.6AsFig.3.5.Widthofview20mm.CPL.Chapter330Fig.3.7DetailofFig.3.5showingthelobateshapeofthecontactoftheinjectionveinofpseudotachylytewiththegranodioriticwallrock.Roundedinclusionsofquartzandopaquemateriallieinthematrix.Widthofview3mm.PPL.Fig.3.8Pseudotachylyteveininagranitichostrock.Thestraightlowercontactcorrespondsprobablytothegenerationsurface.Noteseveralminorinjectionveins(e.g.atlowerright)andtheflowstructure.Leidencollec­tion.Widthofview20mm.CPL.Pseudo­tachylytes31Fig.3.9Typicalcontactofapseudotachylytevein(upperpart)andagranitichostrock(lowerpart).Notetheorthogonalinjectionveinbelowthecenterandtheroundedinclusionswithinthepseudotachylyte.Leidencollection.Widthofview4mm.PPL.Fig.3.10Detailofpseudotachylyteveinshowingtypicallobateandcircular“devitrification”structuresaroundgasbubblesinafine­grainedmatrix.Thegasbubblesarenowfilledwithopaquematerialandcarbonate.Leidencollection.Widthofview2.5mm.PPL.Chapter332Fig.3.11Pseudotachylytevein(black)inahostrockofcataclasticprotomyloniticgranite.Thepseudotachylyteveinhassharpcontactswithcataclasiteinthecenterandalongthelowercontact.Thecataclasitewasprobalytheprecursorrockfromwhichthepseudotachylyteformed.Leidencollection.Widthofview20mm.PPL.Fig.3.12AsFig.3.11.Widthofview20mm.CPL.Pseudo­tachylytes33Fig.3.13Pseudotachylytevein(dark)inahostrockofprotomyloniticcataclasticorthogneiss.Aninjectionveinintrudesupwards,leftofthecenter.Manyminorfaultscanbeobservedinthehostrock.Inclusionsinthepseudotachylyteveinarestretchedupperlefttolowerright(comparabletoobliquefoliation),indicativeofductiledeformationsuperposedonthepseudotachylyteformation,probablywithasinistralsenseofmotion.Roraim