Chilled margin

Abstract: The Sept Iles layered intrusion (Quebec, Canada; 564 Ma) is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro, and an upper border series with anorthosite, capped by cupolas of A-type granite. Chilled margin compositions suggest a ferrobasaltic parental magma close to that of the Skaergaard intrusion, but much richer in iron and titanium. Samples from drill-cores and surface sampling of the 4·7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 Fe-Ti oxide layers cm-to m-thick and many anorthositic autolithic blocks. The sequence of crystallization in the layered series is: plagioclase (An72-34) and olivine (Fo75-21) followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. An olivine gap is observed between Fo66 and Fo59. The saturation of Fe-Ti oxides before Ca-rich pyroxene is interpreted to be the result of the high FeOt and TiO2 contents and the low CaO content of the parental magma. Contamination by old continental crust has occurred during crystallization of the layered series, as indicated by Sr isotopic compositions (87Sr/86Sr564=0·70360-0·70497). The differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases, Cr content of magnetite and lower Sr isotope ratios. These reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma. © The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org.

Abstract: The undeformed 564 Ma Sept Iles layered intrusion (Quebec, Canada) is a large igneous body of c. 20 000 km 3 . From the base to the top, it consists of a Layered Series dominated by troctolite and gabbro, an anorthositic Upper Border Series and a dominantly granitic Upper Series. The parent magma of the Layered Series is inferred to be an iron-rich tholeiitic basalt (48 wt % SiO 2 ; 15 wt % FeO t ). Whole-rock compositions from the chilled margin, dykes cross-cutting the Layered Series and silicic rocks from the Upper Series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous A-type granites (77 wt % SiO 2 ). Initial 143 Nd/ 144 Nd (0·51201–0·51207) and 87 Sr/ 86 Sr (0·70353–0·70548) indicate a juvenile-mantle source and minimal contamination by old crust (1–2%) during crystallization. Geochemical modeling, using the MELTS thermodynamic calculator combined with equations predicting mineral–melt equilibria from experiments on tholeiitic basalts, indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the Layered Series. Fe–Ti oxides saturate early in the Layered Series, after 14% fractionation of plagioclase–olivine cumulates. Further fractionation of Fe–Ti oxide-bearing gabbros drives the residual liquids toward silica enrichment and iron depletion. Major and trace element modeling indicates that the A-type granites from the Upper Series were produced by protracted fractional crystallization of an iron-rich basaltic parent magma, at a fraction of residual liquid of only 8%. The observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations. Most of the intermediate compositions correspond to magmatic mafic enclave-bearing granitoids and display geochemical evidence of hybridization. Intermediate compositions produced by fractional crystallization are scarce and a Daly gap occurs from 57 to 67 wt % SiO 2 . This gap could result either from the fractional crystallization process or from silicate–liquid immiscibility during that compositional interval.

Abstract: A variety of rock types is developed in the Shonkin Sag laccolith, with extreme compositions represented by the porphyritic pseudoleucite-bearing chilled margin, shonkinitic in composition, and by the final differentiate, chemically a nepheline syenite During differentiation the pyroxenes changed in composition from Ca-rich varieties through aegirine-augite to acmite; there is no evidence of an immiscibility gap between Ca-rich and Na-rich pyroxenes Olivine compositions changed from approximately Fa20 to Fa40, but in marked contrast the coexisting biotites exhibit a more extensive compositional range, from annite24 to annite100; the crystal margins of annites in the most evolved rocks are manganophyllite-rich Titanomagnetites are TiO2-poor varieties Arfvedsonite and melanite occur in the most evolved syenites