Geochemistry of high-Mg andesites and adakitic andesite from the Sanchazi block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China: Evidence of partial melting of the subducted Paleo-Tethyan crust.

Major and trace element and Nd and Pb isotopic compositions of volcanic rocks from the Sanchazi (SCZ) block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China were analyzed to provide insights into the subduction processes during the Late Paleozoic along the margins of the paleo-Tethyan ocean. All igneous rocks from the SCZ block show characteristics typical of arc volcanic rocks such as depletion in high field-strength elements (HFSE) and enrichment in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalts (N-MORB). Included among these arc rocks are high-Mg andesites (HMAs) and an adakitic andesite. The adakitic andesite has a steep REE pattern, low Y and Yb, and high La/Yb values, which is similar to those of typical adakites derived by partial melting of subducted basaltic crust. The HMAs from the SCZ block also have adakitic affinities except that they have lower La/Yb (∼10) and Al2O3 (<15 wt.%). Both the adakitic andesite and HMAs have higher MgO, Ni, and Cr contents than the normal andesites and basalts. The age-corrected Nd-Pb isotopic compositions of the HMAs and adakitic andesite show an enriched isotopic signature relative to the MORB-type rocks in the Mianlue area, suggesting that the former probably contain components from sediments or continental crust materials. The data reveal that the adakitic andesite and HMAs in the SCZ block were most probably produced by partial melting of an eclogitic oceanic crust and/or subducted sediments, followed by interaction of the melt with the overlying mantle wedge, and finally by a possible contamination of the melt by the crustal materials. Our result infers that seafloor spreading was active in the paleo-Tethyan ocean, and an ancient island arc system was present along the proto-Qinling area in the Late Paleozoic. The paleo-Tethys ocean separated the South China and North China blocks in Late Paleozoic. The final integration between above two blocks is suggested to have occurred in the Triassic.