The radiogenic isotope characteristics of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada

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• 作者：Pete Hollings ; Mark Smyk ; Brian Cousens
• 关键词：Midcontinent Rift ; Radiogenic isotope ; Dike ; Mantle plume
• 刊名：Precambrian Research
• 出版年：2012
• 出版时间：September, 2012
• 年：2012
• 卷：214-215
• 期：Complete
• 页码：269-279
• 全文大小：1389 K

文摘

New radiogenic isotope data are presented for intrusive rocks of the Mesoproterozoic Midcontinent Rift from the north shore of Lake Superior. These mafic rocks are amongst the older portions of the rift sequence and offer insights into the early development and evolution of the rift. Two sill suites are recognized within the study area, an earlier, spatially restricted ultramafic unit termed the Riverdale sill and the predominant Logan sills, as well as three dike sets, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt Mollie dike. In addition an enigmatic extrusive unit termed the Devon basalts is also present. The Logan sills are characterized by ⁸⁷Sr/⁸⁶Sr_i values of 0.7048-0.7119, with ¹⁴³Nd/¹⁴⁴Nd values from 0.512012 to 0.512187, whereas the Riverdale sill is characterized by ⁸⁷Sr/⁸⁶Sr_i values ranging from 0.7037 to 0.7078 and ¹⁴³Nd/¹⁴⁴Nd of 0.511938-0.511984. Both the Cloud River and Pigeon River dikes show a range of isotope data with the Cloud River dikes having ⁸⁷Sr/⁸⁶Sr_i values ranging from 0.7042 to 0.7064 and ¹⁴³Nd/¹⁴⁴Nd values from 0.512222 to 0.512379 whereas the Pigeon River dikes have ⁸⁷Sr/⁸⁶Sr_i values from 0.7041 to 0.7243 and ¹⁴³Nd/¹⁴⁴Nd from 0.511640 to 0.512383. The Mt. Mollie dike is characterized by ⁸⁷Sr/⁸⁶Sr_i of 0.7041-0.7074 and ¹⁴³Nd/¹⁴⁴Nd values range from 0.512014 to 0.512371. The majority of samples are characterized by negative ?Nd_T, which are interpreted to be the result of contamination of a plume-related mantle source by older crustal material. The lack of correlation with other indices of fractionation (e.g., SiO₂) for the majority of samples implies interaction with subcontinental lithospheric mantle, either through assimilation or melting of the mantle, rather than contamination during emplacement. However, a small number of samples appear to have assimilated material during emplacement resulting in strongly negative ?Nd_T values. The radiogenic isotope data are consistent with a suite of rocks that have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth, in large magma chambers, and during emplacement.