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Lower-crustal Xenoliths from Jurassic kimberlite diatremes, Upper Michigan (USA): Evidence for proterozoic orogenesis and plume magmatism in the lower crust of the southern superior province

Zartman, R E; Kempton, Pamela D; Paces, J.B.; Downes, Hilary; Williams, Ian; Dobosi, Gabor; Futa, Kiyoto

Description

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P-T estimates for granulites indicate peak temperatures of 690-730°C and pressures of 9-12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites...[Show more]

dc.contributor.authorZartman, R E
dc.contributor.authorKempton, Pamela D
dc.contributor.authorPaces, J.B.
dc.contributor.authorDownes, Hilary
dc.contributor.authorWilliams, Ian
dc.contributor.authorDobosi, Gabor
dc.contributor.authorFuta, Kiyoto
dc.date.accessioned2015-12-13T22:16:34Z
dc.identifier.issn0022-3530
dc.identifier.urihttp://hdl.handle.net/1885/70926
dc.description.abstractJurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P-T estimates for granulites indicate peak temperatures of 690-730°C and pressures of 9-12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/204Pb vs. 207Pb/204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm-Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/204Pb vs. 206Pb/204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a unique peraluminous composition. It has the lowest ε Nd and ε Hf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3-1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.
dc.publisherOxford University Press
dc.sourceJournal of Petrology
dc.subjectKeywords: Archean; garnet; geochronology; granulite; isotopic composition; Jurassic; kimberlite; lead-lead dating; lower crust; orogeny; P-T conditions; Proterozoic; protolith; trace element; xenolith; Michigan; United States Continental lithosphere; Geochemistry; Radiogenic isotopes; Xenolith
dc.titleLower-crustal Xenoliths from Jurassic kimberlite diatremes, Upper Michigan (USA): Evidence for proterozoic orogenesis and plume magmatism in the lower crust of the southern superior province
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume54
dc.date.issued2013
local.identifier.absfor040313 - Tectonics
local.identifier.absfor040304 - Igneous and Metamorphic Petrology
local.identifier.absfor040303 - Geochronology
local.identifier.ariespublicationf5625xPUB2473
local.type.statusPublished Version
local.contributor.affiliationZartman, R E, Massachusetts Institute of Technology
local.contributor.affiliationKempton, Pamela D, Natural Environment Research Council
local.contributor.affiliationPaces, J.B., U.S. Geological Survey
local.contributor.affiliationDownes, Hilary, Birbeck University of London
local.contributor.affiliationWilliams, Ian, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationDobosi, Gabor, Budapest University
local.contributor.affiliationFuta, Kiyoto, US Geological Survey
local.description.embargo2037-12-31
local.bibliographicCitation.issue3
local.bibliographicCitation.startpage575
local.bibliographicCitation.lastpage608
local.identifier.doi10.1093/petrology/egs079
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciences
dc.date.updated2016-02-24T08:59:07Z
local.identifier.scopusID2-s2.0-84874093689
local.identifier.thomsonID000315216300006
CollectionsANU Research Publications

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