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Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians |
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| Authors: | WHALEN JOSEPH B; JENNER GEORGE A; LONGSTAFFE FREDERICK J; ROBERT FRANCINE; GARIPY CLMENT |
| Institution: | 1GEOLOGICAL SURVEY OF CANADA 601 BOOTH STREET, OTTAWA, ONT., CANADA KIA OE8 2DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND ST. JOHN'S NFLD., CANADA AIB 3X5 3DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF WESTERN ONTARIO LONDON, ONT., CANADA N6A 5B7 4DEPARTMENT DES SCIENCES DE LA TERRE AND GEOTOP, UNIVERSIT DU QUBEC MONTRAL P. O. BOX 8886, STATION ‘A’, MONTRAL, QUE., CANADA H3C 3P8 |
| Abstract: | The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites.  Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive Nd(T) values(+1.1 to +3.9). Granitoid initial 87Sr/86Sr and most  18 O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/ ). Restricted 207Pb/204Pbvariation is accompanied by significant dispersion of 206Pb/204Pband 208Pb/204Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negativeNd(T) and a large range in 207Pb/204Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland
*Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca |
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