Peridotite Xenoliths from Tariat, Mongolia: Geochemical Characteristics and Heterogeneity in Fertile Lithospheric Mantle
Date Issued
2011
Date
2011
Author(s)
Chieh, Chia-Ju
Abstract
In the Tariat region, mantle-derived xenoliths of Tertiary basalts provide direct information on the nature of the lithospheric mantle. Most Tariat spinel lherzolites, with up to 15 vol.% of modal clinopyroxene (cpx), have neither hydrous minerals nor “melt pocket”, implying that some parts of the subcontinental lithospheric mantle (SCLM) are relatively fertile and anhydrous. Other cpx-rich (15-20 vol.%) lherzolites contain apatites, melt pockets and fluid inclusions in orthopyroxene (opx) and cpx porphyroblasts. Such mineralogical characteristis reflect modal metasomatism. Using the Ca-in-opx thermometry of Brey and Kohler (1990), the equilibration temperatures of Tariat spinel lherzolites range from 862°C to 1041°C, similar to that reported by Ionov (2007). Whole-rock Mg# (0.89-0.91; mainly between 0.89 and 0.90) and Fo contents of olivines in spinel lherzolites suggest that part of the SCLM beneath this region is fertile and similar to the estimated composition of primitive or fertile asthenospheric mantle. The mineral and whole-rock chemical compositions demonstrate that melt extraction was responsible for the geochemical variation of the Tariat lherzolites. Trace element (Y-Yb, Zr-Ti) modeling of cpx indicates that most Tariat spinel lherzolites had experienced less than 10% of melt extraction, except one lherzolite which requires a higher degree of melt extraction ( > 10%). Rare earth element (REE) patterns of cpx can be divided into three types: depleted, enriched and transitional types. The depleted patterns are typical of unmetasomatised, relatively fertile lithospheric mantle. The enriched and transitional patterns suggest that lherzolites had undergone enrichment. In the REE-depleted group of lherzolites, complementary Ti and Zr anomalies in opx (positive) and cpx (negative) have been observed as reported on mantle peridotites worldwide. In some lherzolites, cpx maybe REE-enriched, but other HFSE’s depleted, suggesting that the metasomatism could have occurred in a subduction zone. Plots of (Pb/Nb)N vs Ce/Pb, Ce/Pb vs Ce and (La/Yb)N vs Ti/Eu of cpx suggest that some lherzolites in the REE-enriched and transitional groups are most likely products of hydrous fluid, silicate and carbonatitic metasomatism. Therefore, at least three episodes of metasomatism are recognized in the SCLM beneath the Tariat region. Sr-Nd analysis in the Tariat lherzolites reveals ancient ages for the lithospheric mantle formation and suggests that some Proterozoic mantle domains have survived, and probably metasomatic enrichment.
Subjects
mantle xenoliths
spinel lherzolites
fertile lithospheric mantle
metasomatism
Tariat volcanic zone
Mongolia
Type
thesis
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