GeoScienceWorld
GeoRef Record 2015-072886

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia Block of south China

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia Block of south China

Latitude & Longitude N26°00′00″ – N30°00′00″ and E119°00′00″ – E121°00′00″N41°00′00″ – N48°00′00″ and E129°00′00″ – E136°00′00″

Abstract

Major and trace element and the H (sub 2) O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting < 4%. The H (sub 2) O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390-590 ppm, 160-330 ppm and approximately 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H (sub 2) O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H (sub 2) O content within single pyroxene grains, and (2) the equilibrium H (sub 2) O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H (sub 2) O between the clinopyroxene and olivine, the calculated whole-rock H (sub 2) O contents range from 90 to 220 ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H (sub 2) O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H (sub 2) O abundance in these rocks. Nevertheless, the variations in whole-rock H (sub 2) O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H (sub 2) O between peridotite and melt. The lack of coherent variations between the H (sub 2) O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H (sub 2) O contents after the melting event(s). Based on the similarities in major and trace elements, and of H (sub 2) O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H (sub 2) O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100 ppm vs. 20 ppm for whole rock H (sub 2) O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different. Abstract Copyright (2014) Elsevier, B.V.