Mathematics – Logic
Scientific paper
Apr 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997e%26psl.148..299a&link_type=abstract
Earth and Planetary Science Letters, Volume 148, Issue 1, p. 299-316.
Mathematics
Logic
8
Scientific paper
Sm-Nd, Rb-Sr and U-Pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the Chaya Massif, Baikal-Muya ophiolite belt, eastern Siberia, in order to determine the relationship between mantle ultramafic and crustal mafic sections. Geological relations in the Chaya Massif indicate that the mafic magmas were emplaced into, and interacted with older solid peridotite. Hand picked, acid-leached, primary rock-forming and accessory minerals (olivine, orthopyroxene, clinopyroxene and plagioclase) from the two harzburgite samples show coherent behavior and yield147Sm/144Nd-143Nd/144Nd and238U/204Pb-206Pb/204Pb mineral isochrons, corresponding to ages of 640 ± 58 Ma (95% confidence level) and 620 ± 71 Ma, respectively. These values are indistinguishable from the crystallization age of the Chaya mafic units of 627 ± 25 Ma (a weighted average of internal isochron Sm-Nd ages of four mafic cumulates). The Rb-Sr and Sm-Nd isotopic systems in the harzburgite whole-rock samples were disturbed by hydrothermal alteration. These alteration-related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence, thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution. On the basis of initial Sr and Nd values, ultramafic and mafic rocks of the Chaya Massif can be divided into two groups: (1) harzburgites and the lower mafic unit gabbronorites withɛNd = +6.6 to +7.1 andɛSr = -11 to -16; and (2) websterite of the lower unit and gabbronorites of the upper mafic unit:ɛNd = +4.6 to +6.1 andɛSr = -8 to -9. Initial Pb isotopic ratios are identical in all rocks studied, with mean values of206Pb/204Pb= 16.994 ± 0.023 and207Pb/204Pb= 15.363 ± 0.015. The similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre-existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence. The isotopic data agree with a hypothesized formation of the Chaya Massif in a suprasubduction-zone environment.
Amelin Yuri V.
Neymark Leonid A.
Ritsk Eugeni Yu.
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