کد مقاله کد نشریه سال انتشار مقاله انگلیسی ترجمه فارسی نسخه تمام متن
5784650 1639573 2017 71 صفحه PDF سفارش دهید دانلود رایگان
عنوان انگلیسی مقاله ISI
Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland
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موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
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Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland
چکیده انگلیسی
We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO2 and Al2O3, coupled with a decrease in MgO, total iron (FeOT) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeOT, low Al2O3/TiO2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeOT, high Al2O3/TiO2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb∗) and LREE-enrichment (La/Sm)N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Precambrian Research - Volume 302, November 2017, Pages 50-73
نویسندگان
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