Canadian Minemlogist !o1,26, pp. 109-116 (1988) THE ORIGIN OF COMPLEX MANTLING RELATIONSHIPS IN CLINOPVROXENE FROM THE NEW ENGLAND SEAMOUNTS GEORGIA PE-PIPER Department of Geologst, St.Mary's university, Halifax' Nova Scotia B3H 3C3 LUBOMIR F. JANSA Atlantic Ceoscience Centre, Geological Suney of Canada, BedfordInstitute of Oceanography' P.O. Box 1006, Darnouth, Nova Scotia B2Y 4A2 AssrRAcr €tre due tr une circulation dans une chambre maguattque h6t6rogEne. Electron-microprobe dataarereported for clinopyrox- enes in basanites andtephrites recovered by the Deep sea (Traduit par la R6daction) DrillingProject from theNew England Seamounts, western North Atlantic. Manyof thelarger phenocrysts st o* .i- Mo*-clls:.clinooyrgfne, guyots de la Nouvelle-Angleterre' tiplezoning, with partial r.ro.dtiooindicaiing a complei basanite' tephrite' surcroissance' petrogenetic history. High \\AllIvAl values suggest that cores of pyroxene phenocrysts of both Al-Ti augite and a diopsidiwith aniron-rictr salite composition are of nigh- INTRoDUCTION pressure origin.Augite and titanian salile occur as rimson ihe phenocrysts andas microphenocrysts; these pyroxenes On Leg 43 of the Deep SeaDrilling Project, the show low-prlssure characteristics. Origin of thephenocrysts flanks of two volcanoes of the New England Sea- bydisae;eregationof mantlexenoliths orbycrystallization mount Chain were drilled (Fig. 1). Both drillholes from highly-evolved, alkaline magmas is inconsistent witl bofiomed in mafic volcaniclastic breccias apparently the mineral chemistry of the iron-rich salites. The occur- deposited as debris flows from the crest and sides rence of an iron-rich satte corein Al-Ti ausite and vrc"e ;iih;;;;;;t, in p.n.*ntemporaneity with the venra suggests alternating conditionsof crystallizatiot perhaps due to circulation wit6inan inho-o'*oo, **oll volcanism(Tucholke, VogI et al' 1979)'At Nashville chamber. directly by Campanian sediments; at Vogel Sea- Keywords: clinopyroxene, New England Seamounls, mount (Site 385), the volcanic rocks are overlain by 'basanite, tephnite, mantling. unfossiliferous $ediments of Maestrichtian or older age. Houghton (1979) described five types of basalt SoMMAIRE from Sites 382 and 385' Type t has clinolyroxene and olivine phenocrysts in a groundmass of on decdt, aumoyen de donn€es obtenues pax microsonde plagioclase and augite' In type 2' large hornblende 6lectronique, le cfinbpyroxbne dc basanites & tephrites pr€- phenocrystsare present, but olivine is absent. Type lev€esdesguyotsdelaNouvelle-Angleterre, lorsduprojet 3, the most cotnmon, has plagioclase' clinopyrox- deforages sous-marins (DSDP) dans la partieoccidentale ene. hornblende and minor kaersutite phenocrysts. de l'oc€an Atlantique-nord. Plusieurs des plus grosph6- Type 4 is devoid of phenocrysts and is highly attered. nocristaux possldent unezonation compos€e et manifes- i"i. S,t oo"" from a singlesample,hasolivine and i:T":ffiil:f3"T&fr9'i3ffiS'$Xii?Tffi ;f;;;!i4e.ri^.pt'""o".v'ts. rvpes I to4appear vrAl/vAl font penser que les noyaux deptrenocristaux ij to be alkaline mafic rocks, whereas type 5 is a pyroxenedechacund,augrtericheenAl-Tietundiopsii! tholgiile 9gi-9hed in large-ion lithophile elements au composition salite ricfre enfer ont uneorigine e haut; GILE). All five types are representedat Site 382' pression. Augiteettitansalite forment un liser6 autourdes whereas basalts at Site 385 consist entirely of types phdnocristaux, ainsi que des microph6nocristaux; ces com- 3 and 4. positions sonttypiques d'unecristallisation d basse pres- Complexly zoned, phenocrystic pyroxene is sion. Une originedesphdnocristaux par d6sagr6gation widespieadintheserocks.Inthispaperwedescribe d'enclaves deroches du manteau ou par cristallisation d'un the pyroxene, interpret its petrogeneticsignificance, ffi:f|ffin|#igfi[Si:ffi*atrf;ffi'#; ;+'A;;; lF ;;Gt" ir tr'1 zon.d pvroxene rifbre dans uneaugrte riche enA1et Ti et vice urrro r"*i phenocry.sts. The pyroxeneterminology usedin this plutdt le resultat d'unecristallisation enalternance, pei- paper follows that of Deer, HOwie & Zussman 109