TY - JOUR
T1 - Evolution of spinel-pyroxene symplectite in spinel-lherzolites from the Horoman Complex, Japan
AU - Morishita, Tomoaki
AU - Arai, Shoji
PY - 2003/2
Y1 - 2003/2
N2 - We examined the textural and geochemical characteristics of spinel-pyroxene symplectites in spinellherzolites collected from the lowest, middle, and upper parts (LZI, MZ, and UZI, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZI satisfies garnet stoichiometry and those of the MZ and UZI are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZI symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomally. Thus, the LZI symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZI symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZI) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZI) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the symplectite minerals. Selective enrichment of Sr and LREE in the symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of symplectite.
AB - We examined the textural and geochemical characteristics of spinel-pyroxene symplectites in spinellherzolites collected from the lowest, middle, and upper parts (LZI, MZ, and UZI, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZI satisfies garnet stoichiometry and those of the MZ and UZI are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZI symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomally. Thus, the LZI symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZI symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZI) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZI) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the symplectite minerals. Selective enrichment of Sr and LREE in the symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of symplectite.
UR - http://www.scopus.com/inward/record.url?scp=0037330620&partnerID=8YFLogxK
U2 - 10.1007/s00410-002-0417-y
DO - 10.1007/s00410-002-0417-y
M3 - Article
SN - 0010-7999
VL - 144
SP - 509
EP - 522
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 5
ER -