Nd and Sr Age and Isotope Patterns from Variscan Eclogites of the Eastern Bohemian Massif

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Neues Jahrbuch Fur Mineralogie-Abhandlungen
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High-pressure, high-temperature eclogite-facies assemblages within metamorphic crustal rocks of the Bohemian Massif indicate that the Carboniferous eastern Variscides evolved through continent-continent collision at some stage of its history. Eclogites within the Snieznik Metamorphic Massif in the Polish Sudetes occur as boudins or lenses directly within granulite and amphibolite-facies gneisses ("country rock eclogites") and are believed to have converted to eclogites in situ. Sm-Nd garnet - whole-rock - clinopyroxene tie lines from these rocks define "ages" of 329, 337, 341 and 352 Ma (all ages +/- 6 Ma or less) and initial epsilon-Nd values of -0.05 to +8.7. Clinopyroxene Sr-87/Sr-86 ratios (assumed equal to Sr-87/Sr-86initial) are 0.70387 to 0.71047.Eclogites within the Moldanubian zone of Czechoslovakia occur as country rock eclogites and as layers and lenses within peridotites ("internal eclogites"): the internal eclogites are believed to have recrystallized in the mantle before emplacement into the Variscan Orogen. Sm-Nd "ages" from internal eclogites are 324, 338, and 341 Ma, epsilon-Nd values range from -0.3 to +3.5, and clinopyroxene Sr-87/Sr-86 ratios are 0.70395 to 0.70567. Thus, country rock and internal eclogites define similar ages and overlapping initial epsilon-Nd and Sr-87/Sr-86 ratios that show no systematic relationship with origin, history or geographic location. A garnet pyroxenite defines a significantly older "age" of 373 +/- 7 Ma, a low epsilon-Nd of -2.7, and a high Sr-87/Sr-86initial of 0.70666.Six of eight country rock and internal eclogites, and the garnet pyroxenite, define a linear array on a Sm-Nd whole-rock isochron diagram with a best-fit "age" of roughly 510 Ma and an epsilon-Nd value of +0.3. This array may be an artifact of a small number of analyses, or it may reflect mixing between a depleted MORB-type reservoir and either crustal or enriched mantle reservoirs. However, the correspondence of the age with a known period of igneous activity in the region (the Cadomian) raises the possibility that most eclogite protoliths are geochemically related and formed by variable degrees of partial melting of a common primitive mantle reservoir late in the Proterozoic or early in the Paleozoic.The mineral dates reflect either discrete eclogites-facies events at 373, 352(?) and 341-324 Ma or the punctuated exhumation and cooling of rocks from a high P-T environment that may have operated continuously at the base of the eastern Variscan orogen for over 50 Ma. The sequential uplift and cooling of these eclogites probably occurred through episodic extensional collapse of the orogen that had been thickened by subduction-driven underplating of oceanic crust, microntinental fragments, and, ultimately, a continent. The tight clustering of ages reflecting the youngest eclogite-facies event (341-324 Ma), granulite-facies metamorphism (345-338 Ma), shearing (338-331 Ma), granitoid intrusion (331 Ma), cooling to below the closure temperatures for Ar (330-320 Ma), exhumation and erosion (345 to 336 Ma), and cessation of compression (pre-310 Ma) indicate extremely rapid modification of the eastern Variscan tectonic edifice during the early to middle Carboniferous.


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