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Metamorphic Petrology of Xenoliths from Kenya and Northern Tanzania and Implications for Geotherms and Lithospheric Structures 总被引:1,自引:4,他引:1
Pyroxenitic and peridotitic xenoliths from the Quaternary volcanicfield of Marsabit (northern Kenya) bear strong evidence of decompressionand cooling. Pyroxenites are mostly garnet (grt) websteritesand grt clinopyroxcnites with some olivine (ol) and amphibole(amph). Grt is mostly rimmed by kelyphitic reaction zones butotherwise appears to have been in stable association with thepyroxenes. Along contacts between grt and rare ol, medium-grainedsymplectites consisting of orthopyroxene (opx), clinopyroxene(cpx), and spinel (spl) occur. Garnets do show significant compositionalvariations from core to rim. Primary pyroxenes are strained,have exsolution lamellae, and are chemically zoned. Integratedcore compositions of pyroxenes and grt compositions yield temperaturesof 1065950 C and pressures of 2823 kb (stage1). Pyroxene rims in contact with grt or kelyphite show Ca concentrationssimilar to, but Al concentrations higher than pyroxene rimsremote from garnet. Grt-opx contacts yield pressures of 11.59.0kb, and temperatures of 860770C are obtained from pyroxenerims (stage 2). Peridotites from Marsabit show various stages of transformationfrom the garnet peridotite to the spinel peridotite stabilityfield. On the basis of differences in textures and mineral compositionsthey can be grouped into four types. Type I has a granular textureand contains fine-grained opx-cpx-spl symplectites frequentlysurrounding kelyphite which, in turn, may enclose relict grt.Rare matrix spl has higher Cr/(Cr + Al) ratios (0.250.32)than symplectitic spl (0.09). As in grt pyroxenites, matrixpyroxenes are strained, show exsolution lamellae, and have rimcompositions which are dependent on their positions relativeto former garnet. Integrated core compositions of matrix pyroxenessuggest former equilibration temperatures between 1050 and 880Cand pressures between 25 and 19 kb (opxgrt barometryusing composition of relict grt; stage 1). Pyroxene rims yieldsignificantly lower temperatures of 920785 C (stage2). These PT estimates and the occurrence of one compositexenolith consisting of type I peridotite and grt pyroxenitepoint to a common PTevolution of both grt pyroxenitesand type I peridotites. Granular type II peridotites are characterizedby medium-grained clusters of opx + cpx + spl amph and containmatrix spl, too. Pyroxenes are never strained and are free ofexsolution lamellae. All minerals are homogeneous and thereare no compositional differences between pyroxenes and spinelsof the matrix and those of the splopxcpx clusters.Cr/(Cr+Al) ratios of spl are between 007 and 0.11. Two-pyroxenetemperatures are relatively uniform (970925 C at anassumed pressure of 12 kb; stage 2). Type III peridotites arecoarse-grained granular spl peridotites without any indicationof the former presence of grt. Cr/(Cr + Al) ratios of spl aresimilar to those of peridotite type II. Pyroxenes show minorchemical zoning with Ca increasing in opx but decreasing incpx from core to rim indicating temperatures of 960900C for pyroxene cores and of up to 1000C 相似文献
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MEREDITH A. KELLY SUSAN IVY-OCHS PETER W. KUBIK FRIEDHELM VON BLANCKENBURG CHRISTIAN SCHLÜCHTER 《Boreas: An International Journal of Quaternary Research》2006,35(4):634-643
Surface exposure dating, using in situ produced cosmogenic 10 Be, is applied to determine the time since deglaciation of bedrock surfaces in the Grimsel Pass region. Nine 10 Be dates from bedrock surfaces corrected for cover by snow are minimum ages for deglaciation of the pass. Four 10 Be dates from surfaces below 2500 meters above sea level (m a.s.l.) on Nägelisgrätli, east of Grimsel Pass, yield ages that range from about 14 000 to 11 300 years. Three 10 Be dates from locations above 2600 m a.s.l. on Nägelisgrätli are between about 11 700 and 10 400 years. Two 10 Be dates from locations at 2560 m a.s.l. below Juchlistock are about 12 100 and 11 000 years. The geographical distribution of 10 Be dates on Nägelisgrätli either may show the timing of progressive deglaciation of Grimsel Pass or may reflect differences in subglacial erosion of bedrock in the pass region. All dates are discussed in the context of deglaciation of the late Würmian Alpine ice cap and deglaciation from Last Glacial Maximum (LGM) ice extents in other regions. 相似文献
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